Section 10. Foundations PDF

SECTION 10: FOUNDATIONS TABLE OF CONTENTS 10 10.1—SCOPE............................................................................................................................................................. 10-1 10.2—DEFINITIONS................................................................................................................................................. 10-1 10.3—NOTATION..................................................................................................................................................... 10-3 10.4—SOIL AND ROCK PROPERTIES................................................................................................................... 10-7 10.4.1—Informational Needs.............................................................................................................................. 10-7 10.4.2—Subsurface Exploration.......................................................................................................................... 10-8 10.4.3—Laboratory Tests.................................................................................................................................. 10-11 10.4.3.1—Soil Tests................................................................................................................................... 10-11 10.4.3.2—Rock Tests................................................................................................................................. 10-11 10.4.4—In-Situ Tests......................................................................................................................................... 10-11 10.4.5—Geophysical Tests................................................................................................................................ 10-12 10.4.6—Selection of Design Properties............................................................................................................. 10-13 10.4.6.1—General....................................................................................................................................... 10-13 10.4.6.2—Soil Strength.............................................................................................................................. 10-15 10.4.6.2.1—General............................................................................................................................ 10-15 10.4.6.2.2—Undrained Strength of Cohesive Soils............................................................................. 10-15 10.4.6.2.3—Drained Strength of Cohesive Soils................................................................................. 10-16 10.4.6.2.4—Drained Strength of Granular Soils................................................................................. 10-16 10.4.6.3—Soil Deformation....................................................................................................................... 10-18 10.4.6.4—Rock Mass Strength................................................................................................................... 10-21 10.4.6.5—Rock Mass Deformation............................................................................................................ 10-26 10.4.6.6—Erodibility of Rock.................................................................................................................... 10-29 10.5—LIMIT STATES AND RESISTANCE FACTORS....................................................................................... 10-29 10.5.1—General................................................................................................................................................. 10-29 10.5.2—Service Limit States............................................................................................................................. 10-29 10.5.2.1—General....................................................................................................................................... 10-29 10.5.2.2—Tolerable Movements and Movement Criteria.......................................................................... 10-30 10.5.2.3—Overall Stability......................................................................................................................... 10-30 10.5.2.4—Abutment Transitions................................................................................................................ 10-30 10.5.3—Strength Limit States........................................................................................................................... 10-31 10.5.3.1—General....................................................................................................................................... 10-31 10.5.3.2—Spread Footings......................................................................................................................... 10-31 10.5.3.3—Driven Piles............................................................................................................................... 10-31 10.5.3.4—Drilled Shafts............................................................................................................................. 10-31 10.5.3.5—Micropiles.................................................................................................................................. 10-32 10.5.4—Extreme Events Limit States................................................................................................................ 10-32 10.5.4.1—Extreme Events Design.............................................................................................................. 10-32 10.5.4.2—Liquefaction Design Requirements............................................................................................ 10-32 10.5.5—Resistance Factors................................................................................................................................ 10-39 10.5.5.1—Service Limit States................................................................................................................... 10-39 10.5.5.2—Strength Limit States................................................................................................................. 10-39 10.5.5.2.1—General............................................................................................................................ 10-39 10.5.5.2.2—Spread Footings............................................................................................................... 10-40 10.5.5.2.3—Driven Piles..................................................................................................................... 10-41 10.5.5.2.4—Drilled Shafts................................................................................................................... 10-47 10.5.5.2.5—Micropiles........................................................................................................................ 10-49 10.5.5.3—Extreme Limit States................................................................................................................. 10-50 10.5.5.3.1—General............................................................................................................................ 10-50 10.5.5.3.2—Scour................................................................................................................................ 10-50 10.5.5.3.3—Other Extreme Limit States............................................................................................. 10-51 10.6—SPREAD FOOTINGS................................................................................................................................... 10-51 10.6.1—General Considerations........................................................................................................................ 10-51 10.6.1.1—General....................................................................................................................................... 10-51 10.6.1.2—Bearing Depth............................................................................................................................ 10-51 10.6.1.3—Effective Footing Dimensions................................................................................................... 10-52 10-i © 2017 by the American Association of State Highway and Transportation Officials. 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Duplication is a violation of applicable law. 10-ii AASHTO LRFD BRIDGE DESIGN SPECIFICATIONS, EIGHTH EDITION, 2017 10.6.1.4—Bearing Stress Distributions....................................................................................................... 10-52 10.6.1.5—Anchorage of Inclined Footings................................................................................................. 10-53 10.6.1.6—Groundwater.............................................................................................................................. 10-53 10.6.1.7—Uplift.......................................................................................................................................... 10-53 10.6.1.8—Nearby Structures....................................................................................................................... 10-53 10.6.2—Service Limit State Design................................................................................................................... 10-53 10.6.2.1—General....................................................................................................................................... 10-53 10.6.2.2—Tolerable Movements................................................................................................................ 10-53 10.6.2.3—Loads.......................................................................................................................................... 10-54 10.6.2.4—Settlement Analyses................................................................................................................... 10-54 10.6.2.4.1—General............................................................................................................................. 10-54 10.6.2.4.2—Settlement of Footings on Cohesionless Soils................................................................. 10-55 10.6.2.4.3—Settlement of Footings on Cohesive Soils....................................................................... 10-58 10.6.2.4.4—Settlement of Footings on Rock....................................................................................... 10-63 10.6.2.5—Overall Stability......................................................................................................................... 10-64 10.6.2.6—Bearing Resistance at the Service Limit State............................................................................ 10-64 10.6.2.6.1—Presumptive Values for Bearing Resistance.................................................................... 10-64 10.6.2.6.2—Semiempirical Procedures for Bearing Resistance.......................................................... 10-65 10.6.3—Strength Limit State Design................................................................................................................. 10-66 10.6.3.1—Bearing Resistance of Soil......................................................................................................... 10-66 10.6.3.1.1—General............................................................................................................................. 10-66 10.6.3.1.2—Theoretical Estimation..................................................................................................... 10-67 10.6.3.1.2a—Basic Formulation................................................................................................... 10-67 10.6.3.1.2b—Considerations for Punching Shear......................................................................... 10-70 10.6.3.1.2c—Considerations for Footings on Slopes.................................................................... 10-71 10.6.3.1.2d—Considerations for Two-Layer Soil Systems—Critical Depth................................ 10-76 10.6.3.1.2e—Two-Layered Soil System in Undrained Loading................................................... 10-76 10.6.3.1.2f—Two-Layered Soil System in Drained Loading....................................................... 10-78 10.6.3.1.3—Semiempirical Procedures............................................................................................... 10-78 10.6.3.1.4—Plate Load Tests............................................................................................................... 10-79 10.6.3.2—Bearing Resistance of Rock....................................................................................................... 10-80 10.6.3.2.1—General............................................................................................................................. 10-80 10.6.3.2.2—Semiempirical Procedures............................................................................................... 10-80 10.6.3.2.3—Analytic Method.............................................................................................................. 10-80 10.6.3.2.4—Load Test......................................................................................................................... 10-80 10.6.3.3—Eccentric Load Limitations........................................................................................................ 10-80 10.6.3.4—Failure by Sliding....................................................................................................................... 10-81 10.6.4—Extreme Event Limit State Design....................................................................................................... 10-83 10.6.4.1—General....................................................................................................................................... 10-83 10.6.4.2—Eccentric Load Limitations........................................................................................................ 10-83 10.6.5—Structural Design.................................................................................................................................. 10-83 10.7—DRIVEN PILES............................................................................................................................................. 10-83 10.7.1—General................................................................................................................................................. 10-83 10.7.1.1—Application................................................................................................................................. 10-83 10.7.1.2—Minimum Pile Spacing, Clearance, and Embedment into Cap.................................................. 10-84 10.7.1.3—Piles through Embankment Fill.................................................................................................. 10-84 10.7.1.4—Batter Piles................................................................................................................................. 10-84 10.7.1.5—Pile Design Requirements.......................................................................................................... 10-84 10.7.1.6—Determination of Pile Loads...................................................................................................... 10-85 10.7.1.6.1—General............................................................................................................................. 10-85 10.7.1.6.2—Downdrag........................................................................................................................ 10-85 10.7.1.6.3—Uplift Due to Expansive Soils.......................................................................................... 10-86 10.7.1.6.4—Nearby Structures............................................................................................................ 10-86 10.7.2—Service Limit State Design................................................................................................................... 10-86 10.7.2.1—General....................................................................................................................................... 10-86 10.7.2.2—Tolerable Movements................................................................................................................ 10-86 10.7.2.3—Settlement.................................................................................................................................. 10-87 10.7.2.3.1—Equivalent Footing Analogy............................................................................................ 10-87 © 2017 by the American Association of State Highway and Transportation Officials. 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TABLE OF CONTENTS 10-iii 10.7.2.3.2—Pile Groups in Cohesive Soil........................................................................................... 10-89 10.7.2.4—Horizontal Pile Foundation Movement...................................................................................... 10-90 10.7.2.5—Settlement Due to Downdrag..................................................................................................... 10-92 10.7.2.6—Lateral Squeeze.......................................................................................................................... 10-92 10.7.3—Strength Limit State Design................................................................................................................. 10-92 10.7.3.1—General....................................................................................................................................... 10-92 10.7.3.2—Point Bearing Piles on Rock...................................................................................................... 10-93 10.7.3.2.1—General............................................................................................................................ 10-93 10.7.3.2.2—Piles Driven to Soft Rock................................................................................................ 10-93 10.7.3.2.3—Piles Driven to Hard Rock............................................................................................... 10-93 10.7.3.3—Pile Length Estimates for Contract Documents......................................................................... 10-94 10.7.3.4—Nominal Axial Resistance Change after Pile Driving................................................................ 10-95 10.7.3.4.1—General............................................................................................................................ 10-95 10.7.3.4.2—Relaxation........................................................................................................................ 10-96 10.7.3.4.3—Setup................................................................................................................................ 10-96 10.7.3.5—Groundwater Effects and Buoyancy.......................................................................................... 10-97 10.7.3.6—Scour.......................................................................................................................................... 10-97 10.7.3.7—Downdrag.................................................................................................................................. 10-98 10.7.3.8—Determination of Nominal Bearing Resistance for Piles........................................................... 10-99 10.7.3.8.1—General............................................................................................................................ 10-99 10.7.3.8.2—Static Load Test............................................................................................................. 10-100 10.7.3.8.3—Dynamic Testing............................................................................................................ 10-101 10.7.3.8.4—Wave Equation Analysis................................................................................................ 10-101 10.7.3.8.5—Dynamic Formula.......................................................................................................... 10-102 10.7.3.8.6—Static Analysis............................................................................................................... 10-103 10.7.3.8.6a—General.................................................................................................................. 10-103 10.7.3.8.6b—-Method.............................................................................................................. 10-104 10.7.3.8.6c—-Method.............................................................................................................. 10-105 10.7.3.8.6d—-Method.............................................................................................................. 10-106 10.7.3.8.6e—Tip Resistance in Cohesive Soils.......................................................................... 10-107 10.7.3.8.6f—Nordlund/Thurman Method in Cohesionless Soils............................................... 10-107 10.7.3.8.6g—Using SPT or CPT in Cohesionless Soils............................................................. 10-112 10.7.3.9—Resistance of Pile Groups in Compression.............................................................................. 10-115 10.7.3.10—Uplift Resistance of Single Piles............................................................................................ 10-116 10.7.3.11—Uplift Resistance of Pile Groups............................................................................................ 10-117 10.7.3.12—Nominal Lateral Resistance of Pile Foundations................................................................... 10-119 10.7.3.13—Pile Structural Resistance...................................................................................................... 10-120 10.7.3.13.1—Steel Piles.................................................................................................................... 10-120 10.7.3.13.2—Concrete Piles.............................................................................................................. 10-120 10.7.3.13.3—Timber Piles................................................................................................................. 10-120 10.7.3.13.4—Buckling and Lateral Stability..................................................................................... 10-120 10.7.4—Extreme Event Limit State................................................................................................................. 10-121 10.7.5—Corrosion and Deterioration.............................................................................................................. 10-122 10.7.6—Determination of Minimum Pile Penetration..................................................................................... 10-123 10.7.7—Determination of Rndr Used to Establish Contract Driving Criteria for Nominal Bearing Resistance10-123 10.7.8—Drivability Analysis........................................................................................................................... 10-124 10.7.9—Probe Piles......................................................................................................................................... 10-125 10.8—DRILLED SHAFTS..................................................................................................................................... 10-126 10.8.1—General............................................................................................................................................... 10-126 10.8.1.1—Scope....................................................................................................................................... 10-126 10.8.1.2—Shaft Spacing, Clearance, and Embedment into Cap............................................................... 10-126 10.8.1.3—Shaft Diameter and Enlarged Bases......................................................................................... 10-126 10.8.1.4—Battered Shafts......................................................................................................................... 10-127 10.8.1.5—Drilled Shaft Resistance........................................................................................................... 10-127 10.8.1.6—Determination of Shaft Loads.................................................................................................. 10-128 10.8.1.6.1—General.......................................................................................................................... 10-128 10.8.1.6.2—Downdrag...................................................................................................................... 10-128 © 2017 by the American Association of State Highway and Transportation Officials. 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Duplication is a violation of applicable law. 10-iv AASHTO LRFD BRIDGE DESIGN SPECIFICATIONS, EIGHTH EDITION, 2017 10.8.1.6.3—Uplift.............................................................................................................................. 10-129 10.8.2—Service Limit State Design................................................................................................................. 10-129 10.8.2.1—Tolerable Movements.............................................................................................................. 10-129 10.8.2.2—Settlement................................................................................................................................ 10-129 10.8.2.2.1—General........................................................................................................................... 10-129 10.8.2.2.2—Settlement of Single-Drilled Shaft................................................................................. 10-129 10.8.2.2.3—Intermediate Geo Materials (IGMs)............................................................................... 10-132 10.8.2.2.4—Group Settlement........................................................................................................... 10-132 10.8.2.3—Horizontal Movement of Shafts and Shaft Groups.................................................................. 10-132 10.8.2.4—Settlement Due to Downdrag................................................................................................... 10-133 10.8.2.5—Lateral Squeeze........................................................................................................................ 10-133 10.8.3—Strength Limit State Design............................................................................................................... 10-133 10.8.3.1—General..................................................................................................................................... 10-133 10.8.3.2—Groundwater Table and Buoyancy........................................................................................... 10-133 10.8.3.3—Scour........................................................................................................................................ 10-133 10.8.3.4—Downdrag................................................................................................................................. 10-133 10.8.3.5—Nominal Axial Compression Resistance of Single Drilled Shafts........................................... 10-134 10.8.3.5.1—Estimation of Drilled Shaft Resistance in Cohesive Soils............................................. 10-135 10.8.3.5.1a—General.................................................................................................................. 10-135 10.8.3.5.1b—Side Resistance..................................................................................................... 10-135 10.8.3.5.1c—Tip Resistance....................................................................................................... 10-137 10.8.3.5.2—Estimation of Drilled Shaft Resistance in Cohesionless Soils....................................... 10-137 10.8.3.5.2a—General.................................................................................................................. 10-137 10.8.3.5.2b—Side Resistance..................................................................................................... 10-138 10.8.3.5.2c—Tip Resistance....................................................................................................... 10-139 10.8.3.5.3—Shafts in Strong Soil Overlying Weaker Compressible Soil.......................................... 10-139 10.8.3.5.4—Estimation of Drilled Shaft Resistance in Rock............................................................. 10-140 10.8.3.5.4a—General.................................................................................................................. 10-140 10.8.3.5.4b—Side Resistance..................................................................................................... 10-141 10.8.3.5.4c—Tip Resistance....................................................................................................... 10-142 10.8.3.5.4d—Combined Side and Tip Resistance...................................................................... 10-142 10.8.3.5.5—Estimation of Drilled Shaft Resistance in Intermediate Geo Materials (IGMs)............. 10-143 10.8.3.5.6—Shaft Load Test.............................................................................................................. 10-144 10.8.3.6—Shaft Group Resistance............................................................................................................ 10-144 10.8.3.6.1—General........................................................................................................................... 10-144 10.8.3.6.2—Cohesive Soil................................................................................................................. 10-145 10.8.3.6.3—Cohesionless Soil........................................................................................................... 10-145 10.8.3.6.4—Shaft Groups in Strong Soil Overlying Weak Soil........................................................ 10-146 10.8.3.7—Uplift Resistance...................................................................................................................... 10-146 10.8.3.7.1—General........................................................................................................................... 10-146 10.8.3.7.2—Uplift Resistance of Single Drilled Shaft....................................................................... 10-146 10.8.3.7.3—Group Uplift Resistance................................................................................................. 10-147 10.8.3.7.4—Uplift Load Test............................................................................................................. 10-147 10.8.3.8—Nominal Horizontal Resistance of Shaft and Shaft Groups..................................................... 10-147 10.8.3.9—Shaft Structural Resistance...................................................................................................... 10-147 10.8.3.9.1—General........................................................................................................................... 10-147 10.8.3.9.2—Buckling and Lateral Stability....................................................................................... 10-147 10.8.3.9.3—Reinforcement................................................................................................................ 10-147 10.8.3.9.4—Transverse Reinforcement............................................................................................. 10-148 10.8.3.9.5—Concrete......................................................................................................................... 10-148 10.8.3.9.6—Reinforcement into Superstructure................................................................................ 10-148 10.8.3.9.7—Enlarged Bases............................................................................................................... 10-148 10.8.4—Extreme Event Limit State................................................................................................................. 10-148 10.9—MICROPILES.............................................................................................................................................. 10-149 10.9.1—General............................................................................................................................................... 10-149 10.9.1.1—Scope........................................................................................................................................ 10-149 10.9.1.2—Minimum Micropile Spacing, Clearance, and Embedment into Cap....................................... 10-150 10.9.1.3—Micropiles through Embankment Fill...................................................................................... 10-150 © 2017 by the American Association of State Highway and Transportation Officials. 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TABLE OF CONTENTS 10-v 10.9.1.4—Battered Micropiles................................................................................................................. 10-150 10.9.1.5—Micropile Design Requirements.............................................................................................. 10-150 10.9.1.6—Determination of Micropile Loads........................................................................................... 10-150 10.9.1.6.1—Downdrag...................................................................................................................... 10-150 10.9.1.6.2—Uplift Due to Expansive Soils....................................................................................... 10-150 10.9.1.6.3—Nearby Structures.......................................................................................................... 10-151 10.9.2—Service Limit State Design................................................................................................................ 10-151 10.9.2.1—General..................................................................................................................................... 10-151 10.9.2.2—Tolerable Movements.............................................................................................................. 10-151 10.9.2.3—Settlement................................................................................................................................ 10-151 10.9.2.3.1—Micropile Groups in Cohesive Soil............................................................................... 10-151 10.9.2.3.2—Micropile Groups in Cohesionless Soil......................................................................... 10-151 10.9.2.4—Horizontal Micropile Foundation Movement.......................................................................... 10-151 10.9.2.5—Settlement Due to Downdrag................................................................................................... 10-151 10.9.2.6—Lateral Squeeze........................................................................................................................ 10-151 10.9.3—Strength Limit State Design............................................................................................................... 10-151 10.9.3.1—General..................................................................................................................................... 10-151 10.9.3.2—Ground Water Table and Bouyancy........................................................................................ 10-152 10.9.3.3—Scour........................................................................................................................................ 10-152 10.9.3.4—Downdrag................................................................................................................................ 10-152 10.9.3.5—Nominal Axial Compression Resistance of a Single Micropile............................................... 10-152 10.9.3.5.1—General.......................................................................................................................... 10-152 10.9.3.5.2—Estimation of Grout-to-Ground Bond Resistance.......................................................... 10-153 10.9.3.5.3—Estimation of Micropile Tip Resistance in Rock........................................................... 10-154 10.9.3.5.4—Micropile Load Test...................................................................................................... 10-155 10.9.3.6—Resistance of Micropile Groups in Compression.................................................................... 10-155 10.9.3.7—Nominal Uplift Resistance of a Single Micropile.................................................................... 10-155 10.9.3.8—Nominal Uplift Resistance of Micropile Groups..................................................................... 10-155 10.9.3.9—Nominal Horizontal Resistance of Micropiles and Micropile Groups..................................... 10-156 10.9.3.10—Structural Resistance............................................................................................................. 10-156 10.9.3.10.1—General........................................................................................................................ 10-156 10.9.3.10.2—Axial Compressive Resistance.................................................................................... 10-156 10.9.3.10.2a—Cased Length...................................................................................................... 10-157 10.9.3.10.2b—Uncased Length.................................................................................................. 10-157 10.9.3.10.3—Axial Tension Resistance............................................................................................ 10-158 10.9.3.10.3a—Cased Length...................................................................................................... 10-158 10.9.3.10.3b—Uncased Length.................................................................................................. 10-159 10.9.3.10.4—Plunge Length Transfer Load...................................................................................... 10-159 10.9.3.10.5—Grout-to-Steel Bond.................................................................................................... 10-160 10.9.3.10.6—Buckling and Lateral Stability..................................................................................... 10-160 10.9.3.10.7—Reinforcement into Superstructure.............................................................................. 10-160 10.9.4—Extreme Event Limit State................................................................................................................. 10-160 10.9.5—Corrosion and Deterioration.............................................................................................................. 10-160 10.10—REFERENCES.......................................................................................................................................... 10-160 APPENDIX A10—SEISMIC ANALYSIS AND DESIGN OF FOUNDATIONS................................................ 10-168 A10.1—INVESTIGATION.................................................................................................................................... 10-168 A10.2—FOUNDATION DESIGN......................................................................................................................... 10-168 A10.3—SPECIAL PILE REQUIREMENTS......................................................................................................... 10-172 © 2017 by the American Association of State Highway and Transportation Officials. All rights reserved. Duplication is a violation of applicable law. 10-vi AASHTO LRFD BRIDGE DESIGN SPECIFICATIONS, EIGHTH EDITION, 2017 This page intentionally left blank. © 2017 by the American Association of State Highway and Transportation Officials. All rights reserved. Duplication is a violation of applicable law. SECTION 10 FOUNDATIONS 10.1—SCOPE C10.1 Provisions of this Section shall apply for the design The development of the resistance factors provided of spread footings, driven piles, drilled shaft, and in this Section are summarized in Allen (2005), with micropile foundations. additional details provided in Appendix A of Barker et The probabilistic LRFD basis of these al. (1991), in Paikowsky et al. (2004), in Allen (2005), Specifications, which produces an interrelated and in D’Appolonia (2006). combination of load, load factor resistance, resistance The specification of methods of analysis and factor, and statistical reliability, shall be considered calculation of resistance for foundations herein is not when selecting procedures for calculating resistance intended to imply that field verification and/or reaction other than that specified herein. Other methods, to conditions actually encountered in the field are no especially when locally recognized and considered longer needed. These traditional features of foundation suitable for regional conditions, may be used if design and construction are still practical considerations resistance factors are developed in a manner that is when designing in accordance with these Specifications. consistent with the development of the resistance factors for the method(s) provided in these Specifications, and are approved by the Owner. 10.2—DEFINITIONS Battered Pile—A pile or micropile installed at an angle inclined to the vertical to provide higher resistance to lateral loads. Bearing Pile—A pile or micropile whose purpose is to carry axial load through friction or point bearing. Bent—A type of pier comprised of multiple columns or piles supporting a single cap and in some cases connected with bracing. Bent Cap—A flexural substructure element supported by columns or piles that receives loads from the superstructure. Bond Length—The length of a micropile that is bonded to the ground and which is conceptually used to transfer the applied axial loads to the surrounding soil or rock. Also known as the load transfer length. Casing—Steel pipe introduced during the drilling process to temporarily stabilize the drill hole. Depending on the details of micropile construction and composition, this casing may be fully extracted during or after grouting, or may remain partially or completely in place as part of the final micropile pile configuration. Centralizer—A device to centrally locate the core steel within a borehole. Column Bent—A type of bent that uses two or more columns to support a cap. Columns may be drilled shafts or other independent units supported by individual footings or a combined footing; and may employ bracing or struts for lateral support above ground level. Combination Point Bearing and Friction Pile—Pile that derives its capacity from contributions of both point bearing developed at the pile tip and resistance mobilized along the embedded shaft. Combined Footing—A footing that supports more than one column. Core Steel—Reinforcing bars or pipes used to strengthen or stiffen a micropile, excluding any left-in casing. CPT—Cone Penetration Test. CU—Consolidated Undrained. Deep Foundation—A foundation that derives its support by transferring loads to soil or rock at some depth below the structure by end bearing, adhesion or friction, or both. DMT—Flat Plate Dilatometer Test. 10-1 © 2017 by the American Association of State Highway and Transportation Officials. All rights reserved. Duplication is a violation of applicable law. 10-2 AASHTO LRFD BRIDGE DESIGN SPECIFICATIONS, EIGHTH EDITION, 2017 Drilled Shaft—A deep foundation unit, wholly or partly embedded in the ground, constructed by placing fresh concrete in a drilled hole with or without steel reinforcement. Drilled shafts derive their capacity from the surrounding soil and/or from the soil or rock strata below its tip. Drilled shafts are also commonly referred to as caissons, drilled caissons, bored piles, or drilled piers. Effective Stress—The net stress across points of contact of soil particles, generally considered as equivalent to the total stress minus the pore water pressure. ER—Hammer efficiency expressed as percent of theoretical free fall energy delivered by the hammer system actually used in a Standard Penetration Test. Free (Unbonded) Length—The designed length of a micropile that is not bonded to the surrounding ground or grout. Friction Pile—A pile whose support capacity is derived principally from soil resistance mobilized along the side of the embedded pile. Geomechanics Rock Mass Rating System—Rating system developed to characterize the engineering behavior of rock masses (Bieniawski, 1984). Geotechnical Bond Strength—The nominal grout-to-ground bond strength. GSI—Geologic Strength Index IGM—Intermediate Geomaterial, a material that is transitional between soil and rock in terms of strength and compressibility, such as residual soils, glacial tills, or very weak rock. Isolated Footing—Individual support for the various parts of a substructure unit; the foundation is called a footing foundation. Length of Foundation—Maximum plan dimension of a foundation element. Load Test—Incremental loading of a foundation element, recording the total movement at each increment. Micropile—A small-diameter drilled and grouted non-displacement pile (normally less than 12.0 in.) that is typically reinforced. OCR—Over Consolidation Ratio, the ratio of the preconsolidation pressure to the current vertical effective stress. Pile—A slender deep foundation unit, wholly or partly embedded in the ground, that is installed by driving, drilling, auguring, jetting, or otherwise and that derives its capacity from the surrounding soil and/or from the soil or rock strata below its tip. Pile Bent—A type of bent using pile units, driven or placed, as the column members supporting a cap. Pile Cap—A flexural substructure element located above or below the finished ground line that receives loads from substructure columns and is supported by shafts or piles. Pile Shoe—A metal piece fixed to the penetration end of a pile to protect it from damage during driving and to facilitate penetration through very dense material. Piping—Progressive erosion of soil by seeping water that produces an open pipe through the soil through which water flows in an uncontrolled and dangerous manner. Plunge Length—The length of casing inserted into the bond zone to effect a transition between the upper cased portion to the lower uncased portion of a micropile. Plunging—A mode of behavior observed in some pile load tests, wherein the settlement of the pile continues to increase with no increase in load. PMT—Pressuremeter Test. Point-bearing Pile—A pile whose support capacity is derived principally from the resistance of the foundation material on which the pile tip bears. © 2017 by the American Association of State Highway and Transportation Officials. All rights reserved. Duplication is a violation of applicable law. SECTION 10: FOUNDATIONS 10-3 Post Grouting—The injection of additional grout into the load bond length of a micropile after the primary grout has set. Also known as regrouting or secondary grouting. Primary Grout—Portland cement-based grout that is injected into a micropile hole, prior to or after the installation of the reinforcement to provide the load transfer to the surrounding ground along the micropile and afford a degree of corrosion protection for a micropile loaded in compression. Reinforcement—The steel component of a micropile which accepts and/or resists applied loadings. RMR—Rock Mass Rating. RQD—Rock Quality Designation. Shallow Foundation—A foundation that derives its support by transferring load directly to the soil or rock at shallow depth. Slickensides—Polished and grooved surfaces in clayey soils or rocks resulting from shearing displacements along planes. SPT—Standard Penetration Test. Total Stress—Total pressure exerted in any direction by both soil and water. UU—Unconsolidated undrained. VST—Vane Shear Test (performed in the field). Width of Foundation—Minimum plan dimension of a foundation element. 10.3—NOTATION A = steel pile cross-sectional area (ft2) (10.7.3.8.2) Ab = cross-sectional area of steel reinforcing bar (in.2) (10.9.3.10.2a) Ac = cross-sectional area of steel casing (in.2) (10.9.3.10.2a) Act = cross-sectional area of steel casing considering reduction for threads (in.2) (10.9.3.10.3a) Ag = cross-sectional area of grout within micropile (in.2) (10.9.3.10.3a) Ap = area of pile or micropile tip or base of drilled shaft (ft2) (10.7.3.8.6a) (10.8.3.5) (10.9.3.5.1) As = surface area of pile shaft; area of grout to ground bond surface of micropile through bond length (ft2) (10.7.3.8.6a) (10.9.3.5.1) Au = uplift area of a belled drilled shaft (ft2) (10.8.3.7.2) A′ = effective footing area for determination of elastic settlement of footing subjected to eccentric loads (ft2) (10.6.2.4.2) asi = pile perimeter at the point considered (ft) (10.7.3.8.6g) B = footing width; pile group width; pile diameter (ft) (10.6.1.3) (10.7.2.3.2) (10.7.2.4) B′ = effective footing width (ft) (10.6.1.3) C = correction factor for concrete-soil interference (10.6.3.4) C = secondary compression index, void ratio definition (dim) (10.4.6.3) C = secondary compression index, strain definition (dim) (10.6.2.4.3) Cc = compression index, void ratio definition (dim) (10.4.6.3) Cc = compression index, strain definition (dim) (10.6.2.4.3) CF = correction factor for K when  is not equal to f (dim) (10.7.3.8.6f) CN = overburden stress correction factor for N (dim) (10.4.6.2.4) Cr = recompression index, void ratio definition (dim) (10.4.6.3) Cr = recompression index, strain definition (dim) (10.6.2.4.3) Cwq, Cw = correction factors for groundwater effect (dim) (10.6.3.1.2a) C′ = bearing capacity index (dim) (10.6.2.4.2) c = cohesion of soil taken as undrained shear strength (ksf) (10.6.3.1.2a) cv = coefficient of consolidation (ft2/yr) (10.4.6.3) c1 = undrained shear strength of the top layer of soil as depicted in Figure 10.6.3.1.2e-1 (ksf) (10.6.3.1.2e) c2 = undrained shear strength of the lower layer of soil as depicted in Figure 10.6.3.1.2e-1 (ksf) (10.6.3.1.2e) c′1 = drained shear strength of the top layer of soil (ksf) (10.6.3.1.2f) © 2017 by the American Association of State Highway and Transportation Officials. All rights reserved. Duplication is a violation of applicable law. 10-4 AASHTO LRFD BRIDGE DESIGN SPECIFICATIONS, EIGHTH EDITION, 2017 c* = reduced effective stress soil cohesion for punching shear (ksf) (10.6.3.1.2b) c′ = effective stress cohesion intercept (ksf) (10.4.6.2.3) c′i = instantaneous cohesion at a discrete value of normal stress (ksf) (C10.4.6.4) D = depth of pile embedment; pile width or diameter; diameter of drilled shaft (ft) (10.7.2.3) (10.7.3.8.6g) (10.8.3.5.1c) DD = downdrag load per pile (kips) (C10.7.3.7) D′ = effective depth of pile or micropile group (ft) (10.7.2.3.2) (10.9.2.3.2) Db = depth of embedment of pile into a bearing stratum (ft) (10.7.2.3.2) Dest = estimated pile length needed to obtain desired nominal resistance per pile (ft) (C10.7.3.7) Df = foundation embedment depth taken from ground surface to bottom of footing (ft) (10.6.3.1.2a) Di = pile width or diameter at the point considered (ft) (10.7.3.8.6g) Dp = diameter of the bell on a belled drilled shaft (ft) (10.8.3.7.2) Dr = relative density (percent) (C10.6.3.1.2b) Dw = depth to water surface taken from the ground surface (ft) (10.6.3.1.2a) db = grouted bond zone diameter (ft) (10.9.3.5.2) dq = correction factor to account for the shearing resistance along the failure surface passing through cohesionless material above the bearing elevation (dim) (10.6.3.1.2a) E = modulus of elasticity of pile material (ksi) (10.7.3.8.2) Ed = developed hammer energy (ft-lb) (10.7.3.8.5) Ei = modulus of elasticity of intact rock (ksi) (10.4.6.5) Em = rock mass modulus (ksi) (10.4.6.5) Ep = modulus of elasticity of pile (ksi) (10.7.3.13.4) ER = hammer efficiency expressed as percent of theoretical free fall energy delivered by the hammer system actually used (dim) (10.4.6.2.4) Es = soil (Young’s) modulus (ksi) (C10.4.6.3) e = void ratio (dim) (10.6.2.4.3) eB = eccentricity of load parallel to the width of the footing (ft) (10.6.1.3) eL = eccentricity of load parallel to the length of the footing (ft) (10.6.1.3) eo = void ratio at initial vertical effective stress (dim) (10.6.2.4.3) FCO = base resistance of wood in compression parallel to the grain (ksi) (10.7.8) f ′c = 28-day compressive strength of concrete or grout, unless another age is specified (ksi) (10.6.2.6.2) (10.9.3.10.2a) fpe = effective prestressing stress in concrete (ksi) (10.7.8) fs = approximate constant sleeve friction resistance measured from a CPT at depths below 8D (ksf) (C10.7.3.8.6g) fsi = unit local sleeve friction resistance from CPT at the point considered (ksf) (10.7.3.8.6g) fy = specified minimum yield strength of steel (ksi) (10.7.8) (10.9.3.10.2a) H = horizontal component of inclined loads (kips) (10.6.3.1.2a) Hc = height of compressible soil layer (ft) (10.6.2.4.2) Hcrit = minimum distance below a spread footing to a second separate layer of soil with different properties that will affect shear strength of the foundation (ft) (10.6.3.1.2d) Hd = length of longest drainage path in compressible soil layer (ft) (10.6.2.4.3) Hs = height of sloping ground mass (ft) (10.6.3.1.2c) Hs2 = distance from bottom of footing to top of the second soil layer (ft) (10.6.3.1.2e) hi = length interval at the point considered (ft) (10.7.3.8.6g) I = influence factor of the effective group embedment (dim) (10.7.2.3.2) Ip = influence coefficient to account for rigidity and dimensions of footing (dim) (10.6.2.4.4) Iw = weak axis moment of inertia for a pile (ft4) (10.7.3.13.4) ic, iq, i = load inclination factors (dim) (10.6.3.1.2a) j = damping constant (dim) (10.7.3.8.3) Kc = correction factor for side friction in clay (dim) (10.7.3.8.6g) Ks = correction factor for side friction in sand (dim) (10.7.3.8.6g) K = coefficient of lateral earth pressure at midpoint of soil layer under consideration (dim) (10.7.3.8.6f) L = length of foundation; pile length (ft) (10.6.1.3) (10.7.3.8.2) Lb = micropile bonded length (ft) (10.9.3.5.2) Li = depth to middle of length interval at the point considered (ft) (10.7.3.8.6g) Lp = micropile casing plunge length (ft) (10.9.3.10.4) L′ = effective footing length (ft) (10.6.1.3) LL = liquid limit of soil (percent) (10.4.6.3) © 2017 by the American Association of State Highway and Transportation Officials. All rights reserved. Duplication is a violation of applicable law. SECTION 10: FOUNDATIONS 10-5 N = uncorrected Standard Penetration Test (SPT) blow count (blows/ft) (10.4.6.2.4) N 160 = average corrected SPT blow count along pile side (blows/ft) (10.7.3.8.6g) N1 = SPT blow count corrected for overburden pressure ′v (blows/ft) (10.4.6.2.4) N160 = SPT blow count corrected for both overburden and hammer efficiency effects (blows/ft) (10.4.6.2.4) (10.7.2.3.2) Nb = number of hammer blows for 1.0 in. of pile permanent set (blows/in.) (10.7.3.8.5) Nc = cohesion term (undrained loading) bearing capacity factor (dim) (10.6.3.1.2a) Ncm, Nqm, Nm = modified bearing capacity factors (dim) (10.6.3.1.2a) Nm = modified bearing capacity factor (dim) (10.6.3.1.2e) Ns = slope stability factor (dim) (10.6.3.1.2c) Nq = surcharge (embedment) term (drained or undrained loading) bearing capacity factor (dim) (10.6.3.1.2a) Nu = uplift adhesion factor for bell (dim) (10.8.3.7.2) N′ = alternate notation for N1 (blows/ft) (10.6.2.4.2) N ′q = pile bearing capacity factor from Figure 10.7.3.8.6f-8 (dim) (10.7.3.8.6f) N = unit weight (footing width) term (drained loading) bearing capacity factor (dim) (10.6.3.1.2a) N1 = number of intervals between the ground surface and a point 8D below the ground surface (dim) (10.7.3.8.6g) N2 = number of intervals between 8D below the ground surface and the tip of the pile (dim) (10.7.3.8.6g) N60 = SPT blow count corrected for hammer efficiency (blows/ft) (10.4.6.2.4) n = porosity (dim); number of soil layers within zone of stress influence of the footing (dim) (10.4.6.2.4) (10.6.2.4.2) nh = rate of increase of soil modulus with depth (ksi/ft) (10.4.6.3) PL = plastic limit of soil (percent) (10.4.6.3) Pf = probability of failure (dim) (C10.5.5.2.1) Pm = p-multiplier from Table 10.7.2.4-1 (dim) (10.7.2.4) Pt = factored axial load transferred to ground along micropile plunge length (kips) (10.9.3.10.4) Pu = factored axial load on uncased micropile segment adjusted for plunge length load transfer (10.9.3.10.4) pa = atmospheric pressure (ksf) (Sea level value equivalent to 2.12 ksf or 1 atm or 14.7 psi) (10.8.3.5.1b) Q = load applied to top of footing, shaft, or micropile (kips); load test load (kips) (C10.6.3.1.2b) (10.7.3.8.2) (10.9.3.10.4) Qf = load at failure during load test (kips) (10.7.3.8.2) Qg = bearing capacity for block failure (kips) (C10.7.3.9) Qp = factored load per pile, excluding downdrag load (kips) (C10.7.3.7) QT1 = total load acting at the head of the drilled shaft (kips) (C10.8.3.5.4d) q = net foundation pressure applied at 2Db/3; this pressure is equal to applied load at top of the group divided by the area of the equivalent footing and does not include the weight of the piles or the soil between the piles (ksf) (10.7.2.3.2) qc = static cone tip resistance (ksf) (C10.4.6.3) qc = average static cone tip resistance over a depth B below the equivalent footing (ksf) (10.6.3.1.3) qc1 = average qc over a distance of yD below the pile tip (path a-b-c) (ksf) (10.7.3.8.6g) qc2 = average qc over a distance of 8D above the pile tip (path c-e) (ksf) (10.7.3.8.6g) qL = limiting unit tip resistance of a single pile from Figure 10.7.3.8.6f-9 (ksf) (10.7.3.8.6f) qℓ = limiting tip resistance of a single pile (ksf) (10.7.3.8.6g) qn = nominal bearing resistance (ksf) (10.6.3.1.1) qn-sloping ground = nominal bearing resistance of footings constructed on or adjacent to slopes (ksf) (10.6.3.1.2c) qo = applied vertical stress at base of loaded area (ksf) (10.6.2.4.2) qp = nominal unit tip resistance of pile or micropile (ksf) (10.7.3.8.6a) (10.9.3.5.1) qR = factored bearing resistance (ksf) (10.6.3.1.1) qs = unit shear resistance (ksf); unit side resistance of pile or micropile (ksf) (10.6.3.4) (10.7.3.8.6a) (10.9.3.5.1) qsbell = nominal unit uplift resistance of a belled drilled shaft (ksf) (10.8.3.7.2) qu = uniaxial compression strength of rock (ksf) (10.4.6.4) qult = nominal bearing resistance (ksf) (10.6.3.1.2e) q1 = nominal bearing resistance of footing supported in the upper layer of a two-layer system, assuming the upper layer is infinitely thick (ksf) (10.6.3.1.2d) q2 = nominal bearing resistance of a fictitious footing of the same size and shape as the actual footing but supported on surface of the second (lower) layer of a two-layer system (ksf) (10.6.3.1.2d) © 2017 by the American Association of State Highway and Transportation Officials. All rights reserved. Duplication is a violation of applicable law. 10-6 AASHTO LRFD BRIDGE DESIGN SPECIFICATIONS, EIGHTH EDITION, 2017 RC = factored micropile structural axial compression resistance (kips) (10.9.3.10.2) RCBC = reduction coefficient for bearing resistance of footings due to slope effects (dim) (10.6.3.1.2c) RCC = factored structural axial compression resistance of cased micropile segments (kips) (10.9.3.10.2a) RCU = factored structural axial compression resistance of uncased micropile segments (kips) (10.9.3.10.2b) Rep = nominal passive resistance of soil available throughout the design life of the structure (kips) (10.6.3.4) Rn = nominal resistance of footing, pile, shaft, or micropile (kips) (10.6.3.4) Rndr = nominal pile driving resistance including downdrag (kips) (C10.7.3.3) Rnstat = nominal resistance of pile from static analysis method (kips) (C10.7.3.3) Rp = nominal pile or micropile tip resistance (kips) (10.7.3.8.6a) (10.9.3.5.1) RR = factored nominal resistance of a footing, pile, micropile, or shaft (kips) (10.6.3.4) (10.9.3.5.1) Rs = pile side resistance (kips); nominal uplift resistance due to side resistance (kips); nominal micropile grout-to-ground bond resistance (kips) (10.7.3.8.6a) (10.7.3.10) (C10.9.3.5.1) Rsbell = nominal uplift resistance of a belled drilled shaft (kips) (10.8.3.5.2) Rsdd = skin friction which must be overcome during driving (kips) (C10.7.3.7) RT = factored structural axial tension resistance (kips) (10.9.3.10.3) RTC = factored structural axial tension resistance of cased micropile segments (kips) (10.9.3.10.3a) RTU = factored structural axial tension resistance of uncased micropile segments (kips) (10.9.3.10.3b) Rug = nominal uplift resistance of a pile group (kips) (10.7.3.11) R = nominal sliding resistance between the footing and the soil (kips) (10.6.3.4) r = radius of circular footing or B/2 for square footing (ft) (10.6.2.4.4) Sc = primary consolidation settlement (ft) (10.6.2.4.1) Sc(1-D) = single dimensional consolidation settlement (ft) (10.6.2.4.3) Se = elastic settlement (ft) (10.6.2.4.1) Ss = secondary settlement (ft) (10.6.2.4.1) St = total settlement (ft) (10.6.2.4.1) Su = undrained shear strength (ksf) (10.4.6.2.2) Su = average undrained shear strength along pile side (ksf) (10.7.3.9) s = pile permanent set (in.) (10.7.3.8.5) s, mb, a = fractured rock mass parameters (10.4.6.4) s c , s q, s  = shape factors (dim) (10.6.3.1.2a) sf = pile top movement during load test (in.) (10.7.3.8.2) T = time factor (dim) (10.6.2.4.3) t = time for a given percentage of one-dimensional consolidation settlement to occur (years) (10.6.2.4.3) t1, t2 = arbitrary time intervals for determination of secondary settlement, Ss (years) (10.6.2.4.3) U = percentage of consolidation (10.6.2.4.3) V = total vertical force applied by a footing (kips); pile displacement volume (ft3/ft) (10.6.3.1.2a) (10.7.3.8.6f) Wg = weight of block of soil, piles and pile cap (kips) (10.7.3.11) WT1 = vertical movement at the head of the drilled shaft (in.) (C10.8.3.5.4d) X = width or smallest dimension of pile group (ft) (10.7.3.9) Y = length of pile group (ft) (10.7.3.9) Z = total embedded pile length; penetration of shaft (ft) (10.7.3.8.6g) z = depth below ground surface (ft) (C10.4.6.3)  = adhesion factor applied to su (dim) (10.7.3.8.6b) b = nominal micropile grout-to-ground bond stress (ksf) (10.9.3.5.2) E = reduction factor to account for jointing in rock (dim) (10.8.3.5.4b) t = coefficient from Figure 10.7.3.8.6f-7 (dim) (10.7.3.8.6f)  = reliability index; coefficient relating the vertical effective stress and the unit skin friction of a pile or drilled shaft (dim) (C10.5.5.2.1) (10.7.3.8.6c) m = punching index (dim) (10.6.3.1.2e) z = factor to account for footing shape and rigidity (dim) (10.6.2.4.2)  = unit density of soil (kcf) (10.6.3.1.2a) p = load factor for downdrag (C10.7.3.7) Hi = elastic settlement of layer i (ft) (10.6.2.4.2)  = elastic deformation of pile (in.); friction angle between foundation and soil (degrees) (C10.7.3.8.2) (10.7.3.8.6f) v = vertical strain of over consolidated soil (in./in.) (10.6.2.4.3) © 2017 by the American Association of State Highway and Transportation Officials. All rights reserved. Duplication is a violation of applicable law. SECTION 10: FOUNDATIONS 10-7  = shaft efficiency reduction factor for axial resistance of a drilled shaft or micropile group (dim) (10.7.3.9)  = empirical coefficient relating the passive lateral earth pressure and the unit skin friction of a pile (dim) (10.7.3.8.6d) c = reduction factor for consolidation settlements to account for three-dimensional effects (dim) (10.6.2.4.3) f = angle of internal friction of drained soil (degrees) (10.4.6.2.4) ′f = drained (long term) effective angle of internal friction of clays (degrees) (10.4.6.2.3) ′i = instantaneous friction angle of the rock mass (degrees) (10.4.6.4) ′1 = effective stress angle of internal friction of the top layer of soil (degrees) (10.6.3.1.2f) ′s = secant friction angle (degrees) (10.4.6.2.4) * = reduced effective stress soil friction angle for punching shear (degrees) (10.6.3.1.2b)  = resistance factor (dim) (10.5.5.2.3) b = resistance factor for bearing of shallow foundations (dim) (10.5.5.2.2) bl = resistance factor for driven piles or shafts, block failure in clay (dim) (10.5.5.2.3) C = structural resistance factor for micropiles in axial compression (dim) (10.9.3.10.2) CC = structural resistance factor for cased micropiles segments in axial compression (dim) (10.9.3.10.2a) CU = structural resistance factor for uncased micropiles segments in axial compression (dim) (10.9.3.10.2b) da = resistance factor for driven piles, drivability analysis (dim) (10.5.5.2.3) dyn = resistance factor for driven piles, dynamic analysis and static load test methods (dim) (10.5.5.2.3) ep = resistance factor for passive soil resistance (dim) (10.5.5.2.2) load = resistance factor for shafts, static load test (dim) (10.5.5.2.4) qp = resistance factor for tip resistance (dim) (10.8.3.5) (10.9.3.5.1) qs = resistance factor for shaft side resistance (dim) (10.8.3.5) stat = resistance factor for driven piles or shafts, static analysis methods (dim) (10.5.5.2.3) T = structural resistance factor for micropiles in axial tension (dim) (10.9.3.10.3) TC = structural resistance factor for cased micropiles segments in axial tension (dim) (10.9.3.10.3a) TU = structural resistance factor for uncased micropiles segments in axial tension (dim) (10.9.3.10.3b) ug = resistance factor for group uplift (dim) (10.5.5.2.3) up = resistance factor for uplift resistance of a single pile or drilled shaft (dim) (10.5.5.2.3) upload = resistance factor for shafts, static uplift load test (dim) (10.5.5.2.4) (10.9.3.5.1)  = resistance factor for sliding resistance between soil and footing (dim) (10.5.5.2.2) 10.4—SOIL AND ROCK PROPERTIES 10.4.1—Informational Needs C10.4.1 The expected project requirements shall be The first phase of an exploration and testing analyzed to determine the type and quantity of program requires that the Engineer understand the information to be developed during the geotechnical project requirements and the site conditions and/or exploration. This analysis should consist of the restrictions. The ultimate goal of this phase is to identify following: geotechnical data needs for the project and potential methods available to assess these needs.  Identify design and constructability requirements, Geotechnical Engineering Circular #5—Evaluation e.g., provide grade separation, support loads from of Soil and Rock Properties (Sabatini et al., 2002) bridge superstructure, provide for dry excavation, provides a summary of information needs and testing and their effect on the geotechnical information considerations for various geotechnical applications. needed.  Identify performance criteria, e.g., limiting settlements, right of way restrictions, proximity of adjacent structures, and schedule constraints.  Identify areas of geologic concern on the site and potential variability of local geology.  Identify areas of hydrologic concern on the site, e.g., potential erosion or scour locations.  Develop likely sequence and phases of construction and their effect on the geotechnical information needed. © 2017 by the American Association of State Highway and Transportation Officials. All rights reserved. Duplication is a violation of applicable law. 10-8 AASHTO LRFD BRIDGE DESIGN SPECIFICATIONS, EIGHTH EDITION, 2017  Identify engineering analyses to be performed, e.g., bearing capacity, settlement, global stability.  Identify engineering properties and parameters required for these analyses.  Determine methods to obtain parameters and assess the validity of such methods for the material type and construction methods.  Determine the number of tests/samples needed and appropriate locations for them. 10.4.2—Subsurface Exploration C10.4.2 Subsurface explorations shall be performed to The performance of a subsurface exploration program provide the information needed for the design and is part of the process of obtaining information relevant for construction of foundations. The extent of exploration the design and construction of substructure elements. The shall be based on variability in the subsurface elements of the process that should precede the actual conditions, structure type, and any project exploration program include a search and review of requirements that may affect the foundation design or published and unpublished information at and near the site, construction. The exploration program should be a visual site inspection, and design of the subsurface extensive enough to reveal the nature and types of soil exploration program. Refer to Mayne et al. (2001) and deposits and/or rock formations encountered, the Sabatini et al. (2002) for guidance regarding the planning engineering properties of the soils and/or rocks, the and conduct of subsurface exploration programs. potential for liquefaction, and the groundwater The suggested minimum number and depth of borings conditions. The exploration program should be are provided in Table 10.4.2-1. While engineering sufficient to identify and delineate problematic judgment will need to be applied by a licensed and subsurface conditions such as karstic formations, experienced geotechnical professional to adapt the mined out areas, swelling/collapsing soils, existing fill exploration program to the foundation types and depths or waste areas, etc. needed and to the variability in the subsurface conditions Borings should be sufficient in number and depth to observed, the intent of Table 10.4.2-1 regarding the establish a reliable longitudinal and transverse substrata minimum level of exploration needed should be carried profile at areas of concern such as at structure out. The depth of borings indicated in Table 10.4.2-1 foundation locations and adjacent earthwork locations, performed before or during design should take into account and to investigate any adjacent geologic hazards that the potential for changes in the type, size and depth of the could affect the structure performance. planned foundation elements. As a minimum, the subsurface exploration and testing This Table should be used only as a first step in program shall obtain information adequate to analyze estimating the number of borings for a particular foundation stability and settlement with respect to: design, as actual boring spacings will depend upon the project type and geologic environment. In areas  geological formation(s) present, underlain by heterogeneous soil deposits and/or rock  location and thickness of soil and rock units, formations, it will probably be necessary to drill more  engineering properties of soil and rock units, such frequently and/or deeper than the minimum guidelines as unit weight, shear strength and compressibility, in Table 10.4.2-1 to capture variations in soil and/or rock type and to assess consistency across the site area.  groundwater conditions, For situations where large diameter rock socketed  ground surface topography, and shafts will be used or where drilled shafts are being  local considerations, e.g., liquefiable, expansive or installed in formations known to have large boulders, dispersive soil deposits, underground voids from or voids such as in karstic or mined areas, it may be solution weathering or mining activity, or slope necessary to advance a boring at the location of each instability potential. shaft. Even the best and most detailed subsurface exploration programs may not identify every important subsurface problem condition if conditions are highly variable. The goal of the subsurface exploration program, however, is to reduce the risk of such problems to an acceptable minimum. Table 10.4.2-1 shall be used as a starting point for In a laterally homogeneous area, drilling or determining the locations of borings. The final advancing a large number of borings may be redundant, exploration program should be adjusted based on the since each sample tested would exhibit similar variability of the anticipated subsurface conditions as engineering properties. Furthermore, in areas where soil well as the variability observed during the exploration or rock conditions are known to be very favorable to the © 2017 by the American Association of State Highway and Transportation Officials. All rights reserved. Duplication is a violation of applicable law. SECTION 10: FOUNDATIONS 10-9 program. If conditions are determined to be variable, the construction and performance of the foundation type exploration program should be increased relative to the likely to be used, e.g., footings on very dense soil, and requirements in Table 10.4.2-1 such that the objective of groundwater is deep enough to not be a factor, obtaining establishing a reliable longitudinal and transverse fewer borings than provided in Table 10.4.2-1 may be substrata profile is achieved. If conditions are observed justified. In all cases, it is necessary to understand how to be homogeneous or otherwise are likely to have the design and construction of the geotechnical feature minimal impact on the foundation performance, and will be affected by the soil and/or rock mass conditions previous local geotechnical and construction experience in order to optimize the exploration. has indicated that subsurface conditions are homogeneous or otherwise are likely to have minimal impact on the foundation performance, a reduced exploration program relative to what is specified in Table 10.4.2-1 may be considered. If requested by the Owner or as required by law, Borings may need to be plugged due to boring and penetration test holes shall be plugged. requirements by regulatory agencies having jurisdiction Laboratory tests, in-situ tests, or both shall be and/or to prevent water contamination and/or surface performed to determine the strength, deformation, and hazards. permeability characteristics of soils and/or rocks and Parameters derived from field tests, e.g., driven pile their suitability for the foundation proposed. resistance based on cone penetrometer testing, may also be used directly in design calculations based on empirical relationships. These are sometimes found to be more reliable than analytical calculations, especially in familiar ground conditions for which the empirical relationships are well established. © 2017 by the American Association of State Highway and Transportation Officials. All rights reserved. Duplication is a violation of applicable law. 10-10 AASHTO LRFD BRIDGE DESIGN SPECIFICATIONS, EIGHTH EDITION, 2017 Table 10.4.2-1—Minimum Number of Exploration Points and Depth of Exploration (modified after Sabatini et al., 2002) Minimum Number of Exploration Points and Application Location of Exploration Points Minimum Depth of Exploration Retaining Walls A minimum of one exploration point for each Investigate to a depth below bottom of wall at least to a retaining wall. For retaining walls more than depth where stress increase due to estimated foundation 100 ft in length, exploration points spaced every load is less than ten percent of the existing effective 100 to 200 ft with locations alternating from in overburden stress at that depth and between one and two front of the wall to behind the wall. For times the wall height. Exploration depth should be great anchored walls, additional exploration points in enough to fully penetrate soft highly compressible soils, the anchorage zone spaced at 100 to 200 ft. For e.g., peat, organic silt, or soft fine grained soils, into soil-nailed walls, additional exploration points competent material of suitable bearing capacity, e.g., at a distance of 1.0 to 1.5 times the height of the stiff to hard cohesive soil, compact dense cohesionless wall behind the wall spaced at 100 to 200 ft. soil, or bedrock. Shallow For substructure, e.g., piers or abutments, Depth of exploration should be: Foundations widths less than or equal to 100 ft, a minimum of one exploration point per substructure. For  great enough to fully penetrate unsuitable substructure widths greater than 100 ft, a foundation soils, e.g., peat, organic silt, or soft fine minimum of two exploration points per grained soils, into competent material of suitable substructure. Additional exploration points bearing resistance, e.g., stiff to hard cohesive soil, should be provided if erratic subsurface or compact to dense cohesionless soil or bedrock ; conditions are encountered.  at least to a depth where stress increase due to estimated foundation load is less than ten percent of the existing effective overburden stress at that depth; and  if bedrock is encountered before the depth required by the second criterion above is achieved, exploration depth should be great enough to penetrate a minimum of 10.0 ft into the bedrock, but rock exploration should be sufficient to characterize compressibility of infill material of near-horizontal to horizontal discontinuities. Note that for highly variable bedrock conditions, or in areas where very large boulders are likely, more than 10.0 ft or rock core may be required to verify that adequate quality bedrock is present. Deep For substructure, e.g., bridge piers or In soil, depth of exploration should extend below the Foundations abutments, widths less than or equal to 100 ft, a anticipated pile or shaft tip elevation a minimum of 20 ft, minimum of one exploration point per or a minimum of two times the minimum pile group substructure. For substructure widths greater dimension, whichever is deeper. All borings should than 100 ft, a minimum of two exploration extend through unsuitable strata such as unconsolidated points per substructure. Additional exploration fill, peat, highly organic materials, soft fine-grained points should be provided if erratic subsurface soils, and loose coarse-grained soils to reach hard or conditions are encountered, especially for the dense materials. case of shafts socketed into bedrock. For piles bearing on rock, a minimum of 10.0 ft of To reduce design and construction risk due rock core shall be obtained at each exploration point to subsurface condition variability and the location to verify that the boring has not terminated on a potential for construction claims, at least one boulder. exploration per shaft should be considered for For shafts supported on or extending into rock, a large diameter shafts (e.g., greater than 5.0 ft in minimum of 10.0 ft of rock core, or a length of rock core diameter), especially when shafts are socketed equal to at least three times the shaft diameter for into bedrock. isolated shafts or two times the minimum shaft group dimension, whichever is greater, shall be extended below the anticipated shaft tip elevation to determine the physical characteristics of rock within the zone of foundation influence. Note that for highly variable bedrock conditions, or in areas where very large boulders are likely, more than 10.0 ft or rock core may be required to verify that adequate quality bedrock is present. © 2017 by the American Association of State Highway and Transportation Officials. All rights reserved. Duplication is a violation of applicable law. SECTION 10: FOUNDATIONS 10-11 10.4.3—Laboratory Tests 10.4.3.1—Soil Tests C10.4.3.1 Laboratory testing should be conducted to provide Laboratory tests of soils may be grouped broadly the basic data with which to classify soils and to into two general classes: measure their engineering properties. When performed, laboratory tests shall be  Classification or index tests. These may be conducted in accordance with the AASHTO, ASTM, or performed on either disturbed or undisturbed Owner-supplied procedures applicable to the design samples. properties needed.  Quantitative or performance tests for permeability, compressibility and shear strength. These tests are generally performed on undisturbed samples, except for materials to be placed as controlled fill or materials that do not have a stable soil-structure, e.g., cohesionless materials. In these cases, tests should be performed on specimens prepared in the laboratory. Detailed information regarding the types of tests needed for foundation design is provided in Geotechnical Engineering Circular #5—Evaluation of Soil and Rock Properties (Sabatini et al., 2002). 10.4.3.2—Rock Tests C10.4.3.2 If laboratory strength tests are conducted on intact Rock samples small enough to be tested in the rock samples for classification purposes, they should be laboratory are usually not representative of the entire considered as upper bound values. If laboratory rock mass. Laboratory testing of rock is used primarily compressibility tests are conducted, they should be for classification of intact rock samples, and, if considered as lower bound values. Additionally, performed properly, serves a useful function in this laboratory tests should be used in conjunction with field regard. tests and field characterization of the rock mass to give Detailed information regarding the types of tests estimates of rock mass behavioral characteristics. When needed and their use for foundation design is provided performed, laboratory tests shall be conducted in in Geotechnical Engineering Circular #5—Evaluation of accordance with the ASTM or Owner-supplied Soil and Rock Properties, April 2002 (Sabatini et al., procedures applicable to the design properties needed. 2002). 10.4.4—In-Situ Tests C10.4.4 In-situ tests may be performed to obtain Detailed information on in-situ testing of soils and deformation and strength parameters of foundation soils rock and their application to geotechnical design can be or rock for the purposes of design and/or analysis. In- found in Sabatini et al. (2002) and Wyllie (1999). situ tests should be conducted in soils that do not lend Correlations are in some cases specific to a themselves to undisturbed sampling as a means to geological formation. While this fact does not preclude estimate soil design parameters. When performed, in- the correlation from being useful in other geologic situ tests shall be conducted in accordance with the formations, the applicability of the correlation to those appropriate ASTM or AASHTO standards. other formations should be evaluated. Where in-situ test results are used to estimate For further discussion, see Article 10.4.6. design properties through correlations, such correlations should be well established through long-term widespread use or through detailed measurements that illustrate the accuracy of the correlation. © 2017 by the American Association of State Highway and Transportation Officials. All rights reserved. Duplication is a violation of applicable law. 10-12 AASHTO LRFD BRIDGE DESIGN

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