Unit 1 - Site Investigation PDF

SITE INVESTIGATION (Unit-1) 1  Foundation Earth Structure Design CE 438 Site Investigations Contents – Introduction – Program of site investigation – Planning – Implem...

SITE INVESTIGATION (Unit-1) 1  Foundation Earth Structure Design CE 438 Site Investigations Contents – Introduction – Program of site investigation – Planning – Implementation – Reporting  Introduction  What is Site Investigation (SI)?  Why Site Investigation?  Objectives of Site Investigation Introduction  What is site investigation (SI)? The design of foundations of structures (such as buildings, bridges, and dams) generally requires information about: Structure Structure Ground Ground  Introduction  What is site investigation (SI)? Site investigation (SI) or soil exploration is the process of gathering information, within practical limits, about the stratification (layers) and engineering properties of the soils underlying the proposed construction site. The principal engineering properties of interest are the strength, deformation, and permeability characteristics. Drilling rig Structure Site Investigation Ground Borehole Layers  Introduction  Why site investigation (SI)? Many engineering failures could have been avoided if a proper site investigation had been carried out. The site has a sinkhole risk which might have been discovered in a proper site investigation Sinkhole  Introduction  Why site investigation (SI)? The success or failure of a foundation depends essentially on the reliability of the knowledge obtained from the site investigation. Sophisticated theories alone will not give a safe and sound design.  Introduction  Objectives of site investigation The knowledge about the ground of the proposed construction site is obtained by Site Investigation, and used to determine: Design Suitability: of site parameters: such for the proposed as strength, Effect of compressibility, construction? changes: How will permeability & Site Investigation the design affect other parameters adjacent properties used for and the ground geotechnical water? design Type of design Geo-materials: solution: e.g. available on site type of which can be re- foundation: used? shallow or deep. Ground or Ground-water conditions: that would affect the design and construction? e.g. expansive soil, collapsible soil, high ground water…  Introduction  Objectives of site investigation Suitability: of site for the proposed construction? Effect of Design Site Investigation changes: Ho parameters: such w will the as strength, design affect compressibility, adjacent permeability & Manage the other parameters properties and the ground geotechnical used for water? risk geotechnical Type of design design solution: Geo- e.g. type of materials: foundation: available on shallow or deep. site which can be re-used?  Program of site investigation  Before Site Investigation  The sequence of Site Investigation  Program  Before Site Investigation Site Investigation is usually carried out as part of Subsurface Exploratory program. Before conducting the Site Investigation, the program usually include: Desk Study and Site Reconnaissance. Site Reconnaissance Visual inspection of the site. Desk Study Collect and review preliminary information about the site, and the structure to be built. Program  Before Site Investigation Desk Study Collecting general information about the structure, from the architectural and structural design: Information about the Structure; – Type, dimensions, and use of the structure, and any special architectural considerations. Structure – the load that will be transmitted by the superstructure to the foundation system Ground – the requirements of the local building code (e.g. allowable settlement) Program  Before Site Investigation Desk Study Collecting general information about the ground, from already existing data such as: geological maps, seismic maps, Ariel Photography, Services records (Gas, Water, Electricity), Previous geo- environmental or geotechnical reports, … etc. at or near site. Information about the ground: – the geological conditions of the ground (e.g. layers, Geological features, Ground water, Flood & Earthquake risk in the area, Structure..). – the historical use of the site – if Ground previously used as quarry, agricultural land, industrial unit with contamination issue, man-made fill/slope, etc. Ariel Photograph taken for a site – shows a possible sinkhole Program  Before Site Investigation Site Reconnaissance The Site Reconnaissance is normally in the form of a walk-over survey of the site. What things do I need to look for? Engineer during Site Visit Program  Before Site Investigation Site Reconnaissance Important evidence to look for is: 1. Stratification of soil: from deep cut, such as those made for the construction of nearby highway or other projects – if any. 2. Slope: signs of slope instability include bent trees, shrinkage cracks on the ground and displaced fences or drains. Stratification of soil Signs of slope instability Program  Before Site Investigation Site Reconnaissance Important evidence to look for is: 3. Structures: type of buildings in the area and the existence of any cracks in walls or other problems. You may need to ask local people. Indication of Tipping settlement Differential settlement possible ground- (often without cracks) (with cracks) related problem Program  Before Site Investigation Site Reconnaissance Other important evidence to look for is: 4. Mining: The presence of previous mining is often signs of subsidence and possibly disused mine shafts. Open cast mining is indicated by diverted streams replaced or removed fence/hedge lines. 5. Hydrogeology: Wet marshy ground, springs or seepage, ponds or streams and Wells. 6. Topography: possible existence of drainage ditches or abandoned debris or other man-made features. 7. Vegetation: may indicate the type of soil. 8. Access: It is essential that access to the site can be easily obtained. Possible problems include low overhead cables and watercourses. Program  The sequence of Site Investigation Soil exploration is a requirement for the Sequence of Site Investigation design of foundations of any project. In large construction projects, 2 site Planning investigations (SI) are carried out: – Preliminary SI, followed by – Detailed SI. Whether investigation is preliminary or Implementation detailed, there are three important phases: Reporting  Planning,  Implementation and  Reporting.  Sequence of Site Investigation Planning Planning  Why planning Implementation  Depth of investigation Reporting  Spacing of boreholes  Planning  Why planning? How many borings do we need? How deep the borings should be? The more the better, but what about the cost? Borehole  Planning  Why planning? Planning for site investigation is required to: Minimize cost of explorations and yet give reliable data. Decide on quantity and quality depending on type, size and importance of project and whether investigation is preliminary or detailed. Decide on minimum depth and spacing of Depth of Borehole Borehole Spacing exploration.  Planning  Depth of investigation In general, depth of investigation should be such that any/all strata that are likely to Depth of Borehole experience settlement or failure due to loading. The estimated depths can be changed during the drilling operation, depending on the subsoil encountered. To determine the approximate minimum depth of boring, engineers may use the following rules:  Planning  Depth of investigation Determination of the minimum depth of boring 1. Determine the net increase of stress,  under a foundation with depth as shown in the Figure. 2. Estimate the variation of the vertical effective stress, ‘0 , with depth. 3. Determine the depth, D = D1, at which the stress increase ’ is equal to (1/10) q (q = estimated net stress on the foundation). 4. Determine the depth, D = D2, at which /'0 = 0.05. 5. Unless bedrock is encountered, the smaller of the two depths, D1 and q D2, is the approximate minimum depth of boring required. D Example A 1.25 m square footing is subjected to a contact pressure of 300 kN/m². The unit weight of the cohesive soil supporting the footing is 18.5 kN/m³ and groundwater is known to be at a depth. Determine the minimum depth of boring that should be carried out in  0’ the site  ’  Depth of Boring For hospitals and office buildings, the following rule could be use to determine boring depth. The approximate required minimum depth of the borings should be predetermined. The estimated depths can be changed during the drilling operation, depending on the subsoil encountered. To determine the approximate minimum depth of boring, engineers may use the following rule: Db=3S0.7 (for light steel or narrow concrete buildings) Db=6S0.7 (for heavy steel or wide concrete buildings) Where: Db = Depth of Boring, in meters S = Number of Stories When deep excavations are anticipated, the depth of boring should be at, least 1.5 times the depth of excavation. Sometimes subsoil conditions are such that the foundation load may have to be transmitted to the bedrock. The minimum depth of core boring into the bedrock is about 3m. If the bedrock is irregular or weathered, the core borings may have to be extended to greater depths.  Exercise Planning Site investigation is to be made for a structure of 100  Depth of investigation m length and 70 m width. The soil profile is shown below, if the structure is subjected to 200 kN/m2. Determine the approximate depth of borehole (Assume γw = 9.81 kN/m3). Table shows the minimum depths of borings for buildings based on the preceding rule. Building Number of Stories width (m) Depth of Boring What do you notice about this table? Planning  Spacing of boreholes There are no strict or no hard and fast rules rules for the spacing of the boreholes. The following table gives some general guidelines for borehole spacing. These spacing can be increased or decreased, depending on the subsoil condition. If various soil strata are more or less uniform and predictable, the number of boreholes can be reduced. Type of project Spacing (m) What do you notice about this table? 27  Implementation Sequence of Site Investigation  Overview Planning  Boring  Sampling Implementation  Testing Reporting  Implementation  Overview The implementation phase of site investigation usually includes three important aspects: 1 2 3 Boring Sampling Testing Trial pits Soil In-situ Sampling tests Rock Laboratory Boreholes Sampling tests  Implementation  Boring 1 2 3 Boring Sampling Testing Trial pits Soil In-situ Sampling tests Rock Laboratory Boreholes Sampling tests Implementation  Boring Trial pits Trial pits are shallow excavations - less than 6m deep. The trial pit is used extensively at the Pick and surface for block sampling and shovel Backhoe detection of services prior to borehole excavation. For safety ALL pits below a depth of 1.2m must be supported. Trial Pit Depth Excavation Method 6m > depth 0-2m By Hand 2-4m Wheeled Back Hoe 4-6m Hydraulic Excavator  Implementation  Boring Boreholes Boreholes may be excavated by one of these methods: 1. Auger Boring 2. Wash Boring 3. Rotary Drilling 4. Percussion Drilling Borehole The right choice of method depends on: – Ground condition: presence of hard clay, gravel, rock. – Ground-water condition: presence of high ground-water table (GWT). – Depth of investigation – Site access Implementation  Boring Boreholes 1. Auger Boring Power driven augers This is the simplest of the methods. Hand operated or power driven augers may be used. Suitable in all soils above GWT but only in cohesive soil below GWT. Hand operated augers Post hole auger Helical auger Implementation  Boring Boreholes 2. Wash Boring A casing is driven with a drop hammer. A hollow drill rod with chopping bit is inserted inside the casing. Soil is loosened and removed from the borehole using water or a drilling mud jetted under pressure. Wash boring is a very convenient method for soil exploration below the ground water table provided the soil is either sand, silt or clay. The method is not suitable if the soil is mixed with gravel or boulders. Implementation  Boring Boreholes 3. Percussion Drilling In this method a heavy drilling bit is alternatively raised and dropped in such a manner that it powders the underlying materials which form a slurry with water and are removed as the boring advances. Possibly this is the only method for drilling in river deposits mixed with hard boulders of the quartzitic type.  Implementation  Boring Boreholes 4. Rotary Drilling In this method a rapidly retaining Movement drilling bit (attached to a drilling rod) transmitter cut the soil and advance the borehole. When soil sample is needed the drilling rod is raised and Rotary Head the drilling bit is replaced by a sampler. Drilling rod This method is suitable for soil and rock. Drilling bit  Implementation  Sampling 1 2 3 Boring Sampling Testing Trial pits Soil In-situ Sampling tests Rock Laboratory Boreholes Sampling tests Implementation  Sampling Soil sampling Samples from each type of soils are required for laboratory testing to determine the engineering properties of these soils. Soil samples are recovered carefully, stored properly to prevent any change in physical properties, and transferred to laboratory for testing. Soil Sampling equipment? Disturbed vs Undisturbed? Implementation  Sampling Soil sampling Soil Sampling equipment There is a wide range of sampling methods such as Split-spoon sampler, Thin-walled Tube. The choice of method depends on: The requirement of disturbed or undisturbed samples Type of soil discovered at site (Gravel, Sand, Silt, Clay) Split-spoon Sampler Soil Sample advancement Implementation  Sampling Soil sampling Soil Sampling equipment  Implementation  Sampling Soil sampling Disturbed vs Undisturbed Samples Two types of soil samples can be obtained during sampling: disturbed and undisturbed. The most important engineering properties required for foundation design are strength, compressibility, and permeability. These tests require undisturbed samples. Disturbed samples can be used for determining other properties such as Moisture content, Classification & Grain size analysis, Specific Gravity, and Plasticity Limits.  Implementation  Sampling Soil sampling Disturbed vs Undisturbed It is nearly impossible to obtain a truly undisturbed sample of soil. The quality of an "undisturbed" sample varies widely between soil laboratories. So how is disturbance evaluated? Quality of samples is evaluated by calculating Area Ratio AR: soil The thicker the wall of the sampling tube, the greater the disturbance. Good quality samples AR < 10%. Sampling tube  Implementation  Sampling Soil sampling Disturbed vs Undisturbed Samples Samples collected in Split-spoon Sampler is usually classified as “disturbed”. What is the Area Ratio? Area Ratio AR = ----------------- = Implementation  Sampling Rock Sampling (Coring) Rock samples are called “rock cores”, and they are necessary if the soundness of the rock is to be established. Core drilling equipment? Core recovery parameters?  Implementation  Sampling Rock Sampling (Coring) Core drilling equipment Drill rod Coring is done with either tungsten carbide or diamond core bits. Inner Rock sampler is called “core Core barrel barrel” which usually has a barrel Outer Rock Rock single tube. barrel Rock Rock Double or triple tube core barrel is used when sampling Rock of weathered or fractured core rock. Coring bit Diamond Drill Bit (a) (b) Core barrel: 45 (a) Single-tube; (b) double-tube Implementation  Sampling Rock Sampling (Coring) Core drilling equipment Cores tend to break up inside the drill barrel, especially if the rock is soft or fissured. Core recovery parameters are used to describe the quality of core. Length of pieces of core are used to determine: – Core Recovery Ratio (Rr) – Rock Quality Designation (RQD) Rock cores  Implementation  Sampling Rock Sampling (Coring) Core drilling equipment Assuming the following pieces for a given core run: Core recovery (lengths of intact pieces of core) Rr Recovery Ratio, Rr (Core run) Rock Quality Designation, RQD 10 L i = 100% ( L i ≥ 10 cm ) (Core 47 run) L  Implementation  Sampling Rock Sampling (Coring) Core recovery parameters So Rock Quality Designation (RQD) is the percentage of rock cores that have length ≥ 10 cm over the total drill length (core run). RQD may indicate the degree of jointing or fracture in a rock mass. e.g. High-quality rock has an RQD of more than 75%. RQD is used in rock mass classification systems and usually used in estimating support of rock tunnels. Implementation  Sampling Rock Sampling (Coring) Core recovery parameters Class Example Work out Rr and RQD for the following core recovery (intact pieces), assuming the core run (advance) is 150 cm. What is the rock mass quality based on RQD?  Implementation  Sampling Rock Sampling (Coring) Core recovery parameters Solution: Total core recovery L = 125 cm Core recovery ratio: Rr = 125/150 = 83% ? On modified basis (for pieces ≥ 10cm), 95 cm are counted, thus: L i RQD = 100% = 95/150 = 63 % L RQD = 50% - 75%  Rock mass quality is “Fair” L= ? L i ? Implementation  Testing 1 2 3 Boring Sampling Testing Trial pits Soil In-situ Sampling tests Rock Laboratory Boreholes Sampling tests Implementation  Testing In-situ tests Introduction Plate Load Test (PLT) Groundwater measurements Pressure-meter Test (PMT) Standard Penetration Test (SPT) Flat Dilatometer Test (DMT) Cone Penetration Test (CPT) Vane shear test (VST) The Borehole Shear Test (BST). PLT Piezometer In Borehole 52 Implementation  Testing : In-situ tests Introduction Definition: In-situ tests are carried out in the field with intrusive testing equipment. If non-intrusive method is required, then it is better to use geophysical methods which use geophysical waves – i.e. without excavating the ground. Advantage of in-situ testing (against lab testing) It avoids the problems of sample recovery and disturbance some in-situ tests are easier to conduct than lab tests In-situ tests can offer more detailed site coverage than lab testing. Testing standards American Society for Testing and Materials (ASTM) British Standard (BS) Implementation  Testing : In-situ tests Groundwater measurements Why Groundwater: Standpipe Groundwater conditions are fundamental Ground factors in almost all geotechnical analyses water level and design studies. Types of Groundwater measurements: Determination of groundwater levels (GWT) and pressures. Borehole instrumented with Piezometer is used for this purpose. Measurement of the permeability of the subsurface materials, particularly if seepage Piezometer analysis is required. The test called Pumping test.  Implementation  Testing : In-situ tests Standard Penetration Test (SPT) Definition This empirical test consists of driving a split- Falling spoon sampler, with an outside diameter of 50 Hammer mm, into the soil at the base of a borehole. 760 mm Drivage is accomplished by a trip hammer, weighing 65 kg, falling freely through a distance Drive head of 760 mm onto the drive head, which is fitted at the top of the rods. The split-spoon is driven three times for a distance of 152.4 mm (6 in) into the soil at the bottom of the borehole. The number of blows required to drive (only) the last two 152.4 mm Slit are recorded. The blow count is referred to as spoon the SPT-N. 152.4 mm (6 in) x 3 times The first one does not count Implementation  Testing : In-situ tests Standard Penetration Test (SPT) Advantage Relatively quick, simple, reasonably cheap, and suitable for most soils. Good correlation between SPT-N and soil properties. Provides a representative soil sample for further testing. Disadvantage SPT does not typically provide continuous data Limited applicability to soil containing cobbles and boulders. Samples obtained from the SPT are disturbed. SPT N blow require correction  Implementation  Testing : In-situ tests Standard Penetration Test (SPT) Corrections for energy and equipment Corrections are normally applied to the SPT blow count (N) to account for: – Energy loss: during the test (about only 60% of energy remains) – Equipment differences: hammer, sampler, borehole diameter, rod The following equation is used to compensate for these factors: (usually 0.50-0.80) (1.0-1.15) 60% (0.8-1.0) (0.75-1.0) Usually this correction is made by the Site Investigation operator. Implementation  Testing : In-situ tests Standard Penetration Test (SPT) Corrections for overburden pressure In granular soil (sand, gravel) the SPT blows are influenced by the effective overburden pressure at the test depth: CN = overburden pressure correction factor Many equations have been suggested for CN – see Page 86, (Das’s text book). For example: Correlation between CN, o’ and Pa Correlation between CN, o’ and Pa Correlation between CN, o’ and Pa  Implementation  Testing : In-situ tests Standard Penetration Test (SPT) Correlation between N and friction angle There are many equations suggested. The figure shows the correlation with the angle of shearing resistance of sand (according to Pecks, 1974). Corrected SPT N blow Angle of shearing resistance ’ (degree) Implementation  Testing : In-situ tests Standard Penetration Test (SPT) Class example The following are the recorded numbers of SPT blows required for spoon penetration of three 152.4cm (6 in) in a sand deposit: Depth from ground 1.5 3 4.5 6 7.5 surface (m) SPT blows (blow/ 6 in) 3, 4, 5 7, 9, 10 7, 12, 8, 13, 10, 14, 11 14 15 Note. Assume the above SPT blows are corrected for energy and equipment. The ground water table (GWT) is located at a depth of 4.5m. The wet unit weight of sand above GWT is 18 kN/m3, and the saturated unit weight of sand below GWT is 19.81 kN/m3. Draw a sketch of the foundation showing the given details of the soil. Determine the standard penetration number (SPT-N) at each depth. What is the corrected (SPT-N) value? (use Seed’s equation). Determine the friction angle at depth 4m below the footing. (Use Peck’s Equation or Chart).  Implementation  Testing : In-situ tests Standard Penetration Test (SPT) Solution Z, SPT m blow N60 ’ (kPa) CN N ’ 2 1.5 3, 4, 5 4+5=9 1.5x18 =27 0.27 1.7 15.3 29.79o 3 7, 9, 10 9+10=19 54 0.54 =18 kN/m3 7, 12, 4 4.5 11 23 8, 13, sat=19.8 kN/m3 6 14 ? 10, 14, 7.5 15 Z Only the last 2 sets of blows count Corrected  Implementation  Testing : In-situ tests Standard Penetration Test (SPT) Correlation between N and undrained shear strength The corrected SPT N blow can be approximately correlated to many important engineering properties of soil such as shear strength & compressibility. This equation shows the correlation with undrained shear strength Su (or Cu) of clay. (also with OCR = Over Consolidation Ratio). In Clay  Implementation  Testing : In-situ tests Standard Penetration Test (SPT) Correlation between N and undrained shear strength The table shows the correlation corrected SPT-N with undrained shear strength Su (or Cu) of clay (according to Terzaghi et al. 1996) Correlation between N60 and Relative Density of Granular Soil Correlation between N60 and Relative Density of Granular Soil Correlation between N60 and Relative Density of Granular Soil Correlation between Angle of Friction and Standard Penetration Number Correlation between Angle of Friction and Standard Penetration Number 71 Correlation between Modulus of Elasticity and Standard Penetration Number Implementation  Testing : In-situ tests Standard Penetration Test (SPT) Class Example the Figure shown below Implementation  Testing : In-situ tests Standard Penetration Test (SPT) Solution Z, m N60 ’ (kPa) Cu (kPa) ’ (MPa) OCR 1.5x16.5+ 100x0.29 38.5/1000= 0.193x(5/ 3 5 1.5x(19-9.81) = 38.5 x50.72 =92.3 0.0385 0.038)0.689 = 5.5 38.5+1.5x(16.5- 4.5 8 9.81) = 48.5 129.6 0.0485 6 8 7.5 9 9 10 Cu -av = OCRav =  Implementation  Testing : In-situ tests Standard Penetration Test (SPT) Correlation between N and Relative Density Dr correlation between N60 and Relative Density of Granular Soil General For Clean sand only Implementation  Testing : In-situ tests Standard Penetration Test (SPT) Correlation between N and Relative Density Dr Very loose Loose Medium Dense  Implementation  Testing : In-situ tests Standard Penetration Test (SPT) Correlation between Modulus of Elasticity and Standard Penetration Number The modulus of elasticity of granular soils (Es) is important parameter in estimation the elastic settlement of foundation. An approximate estimation for Es was given by Kulhawy and Mayne (1990) as: Principles of Foundation Engineering, 8th edition Das Vane Shear Test The vane shear test may be used during the drilling operation to determine the in situ undrained shear strength (c )of clay soils— u particularly soft clays. Torque is applied at the top of the rod to rotate the vanes at a standard rate of 0.1o/sec. Rotation will induce failure in a soil of cylindrical shape surrounding the vanes. The maximum torque, T, applied to cause failure is measured. T Cu = C u = in situ undrained shear strength K K = vane constant 78 © 2016 Cengage Learning Engineering. All Rights Reserved. Principles of Foundation Engineering, 8th edition Das Vane Shear Test For rectangular Vanes For tapered vanes 2 pd d K= (h + ) d d 2 d 2 3 K (   6h) 3 12 cos iT cos iB If h/d = 2 then K = 7p d iB and i are angles defined in the shear T Advantages: 6 vane picture. Moderately rapid Economical Gives good results in soft and medium-stiff clays Excellent in determining the properties of sensitive clays Errors: Poor calibration of torque measurement Damaged vanes. If rate of rotation of the vane is not properly controlled, other errors can be introduced 79 © 2016 Cengage Learning Engineering. All Rights Reserved. Principles of Foundation Engineering, 8th edition Das Vane Shear Test Undrained shear strength values from vane shear tests are too high. Values are corrected according to the equation ' 0.83 s = 7.04[Cu( field ) ] c l = correction factor Most common equation for correct factor l = 1.7 - 0.54log[PI%] Relationship for estimating the preconsolidation pressure of a natural clay deposit C = lC u(corrected ) u ' s c = Preconsolidation pressure (kN /m2 ) Cu( field ) = Field vane shear strength (kN /m2 ) 80 © 2016 Cengage Learning Engineering. All Rights Reserved. Principles of Foundation Engineering, 8th edition Das Vane Shear Test Relationship for overconsolidation ratio (OCR). Cu( field ) OCR = b s o' The value of b is determined by any of the equations -0.48 b = 22[PI(%)] 1 222 b= b= 0.08 + 0.0055(PI) w(%) 81 © 2016 Cengage Learning Engineering. All Rights Reserved. Principles of Foundation Engineering, 8th edition Das 82 © 2016 Cengage Learning Engineering. All Rights Reserved. Principles of Foundation Engineering, 8th edition Das Cone Penetration Test Formally known as Dutch cone penetration test. Also currently referred to as static penetration test. Versatile sounding method Used to determine the materials in a soil profile. Estimates material engineering properties. No boreholes are necessary. Penetrometers measure Cone resistance (qc) to penetration Equal to vertical force applied to the cone, divided by its horizontally projected area. Frictional resistance ( f c ) Resistance measured by a sleeve located above the cone with the local soil surrounding it. Equal to the vertical force applied to the sleeve, divided by its surface area (the sum of friction and adhesion). 83 © 2016 Cengage Learning Engineering. All Rights Reserved. Principles of Foundation Engineering, 8th edition Das Cone Penetration Test Two types of penetrometers Mechanical friction-cone penetrometer Electric friction-cone penetrometer 84 © 2016 Cengage Learning Engineering. All Rights Reserved. Principles of Foundation Engineering, 8th edition Das Cone Penetration Test Mechanical friction-cone penetrometer The tip of this penetrometer is connected to an inner set of rods. Tip is first advanced about 40 mm, giving the cone resistance. Further thrusting engages the friction sleeve. As inner rod advances, the rod force is equal to the sum of the vertical force on the cone and sleeve. Subtracting the force on the cone gives the side resistance. Electric friction-cone penetrometer The tip is attached to a string of steel rods. The tip is pushed into the ground at the rate of 20 mm/sec. Wires from the transducers are threaded through the center of the rods and continuously measure the cone and side resistances. 85 © 2016 Cengage Learning Engineering. All Rights Reserved. Principles of Foundation Engineering, 8th edition Das Cone Penetration Test Several correlations useful in estimating the properties of soils for the point resistance and the friction ratios. fc Fr = (frictional resistance)/(cone resistance)= qc Fr (%) = 1.45 - 1.36log D50 (electric cone) Fr (%) = 0.7811 - 1.611logD50 (mechanical cone) D50 = size through which 50% of soil will pass through (mm) – Size ranges 0.001 mm to 10 mm 86 © 2016 Cengage Learning Engineering. All Rights Reserved.  Principles of Foundation Engineering, 8th edition Das Correlation between Relative Density(Dr )and qc for Sand OCR = Overconsolidation Ratio qc pa = atmospheric pressure 1 pa Dr  [ ' ] 305Qc OCR 1.8  o 0.5 ( ) Qc = compressibility factor pa Recommended Values of Qc qc Dr (%) = 68[log( ) - 1] Highly compressible sand = 0.91 pa.s 0' Moderately compressible sand = 1.0 Low compressible sand = 1.09 pa = Atmospheric Pressure ' 87 s o = Vertical effective stress © 2016 Cengage Learning Engineering. All Rights Reserved. Principles of Foundation Engineering, 8th edition Das ' Correlation between qc and Drained Friction Angle for f Sand ' -1 qc f = tan [0.1 + 0.38log( ' )] s o ' -1 qc f = tan [0.38 + 0.27og( ' )] so Correlations using horizontal ' effect stress ( s h) ' qc 0.1714 f = 15.575( ' ) sh 88 © 2016 Cengage Learning Engineering. All Rights Reserved. Principles of Foundation Engineering, 8th edition Das Correlation between q and N c 60 qc a Values for C and a are found in the table. ( )/ N60 = cD50 pa 89 © 2016 Cengage Learning Engineering. All Rights Reserved. Principles of Foundation Engineering, 8th edition Das Correlations for Undrained Shear Strength ( C ), Preconsolidation s ' u Pressure ( c), and Overconsolidation Ratio (OCR) for Clays Undrained shear strength is determined by the equation qc - s o Cu = Nk s o = total vertical stress Nk = bearing capacity factor The value of N can vary form 11 to 19. k 90 © 2016 Cengage Learning Engineering. All Rights Reserved. Principles of Foundation Engineering, 8th edition Das Correlations for Undrained Shear Strength ( C u ), Preconsolidation Pressure (s c'), and Overconsolidation Ratio (OCR ) for Clays ' Correlations for preconsolidation pressure (s ) and overconsolidation ratio c (OCR) ' 0.96 s = 0.243(qc ) c qc - s o 1.01 qc OCR = 0.37( ' ) ' f = 15.575( )0.1714 s o s h'  oand ' o= total and effective stress, respectively © 2016 Cengage Learning Engineering. All Rights Reserved. 91 Implementation  Testing : In-situ tests Cone Penetration Test (CPT) Advantages: Borehole is not necessary Almost continuous data (reading every 10mm) Elimination of operator error (automated) Reliable, repeatable test results Disadvantages: Inability to penetrate through gravels and cobbles Newer technology = less populated database than SPT Lack of sampling Implementation  Testing : In-situ tests Cone Penetration Test (CPT) Class example: Correlation with shear strength Use equation proposed 93 by Ricceri et all’s. 2002.  Implementation  Testing: In-situ tests Cone Penetration Test (CPT) Solution: Depth, qc (MPa) m ’ (kPa) qc /’ ’ (Rad) ’ (deg) 1.5 2.06 1.5 x 16 =24 2060 / 24 = 85.8 0.69 0.69x180/=40o 3 4.23 48 88.1 4.5 6.01 6 8.18 7.5 9.97 9.0 12.42 ’av = 94 Note. tan -1 is inverse tangent, the angle returned is in Radian. ’av = ’ / 6  Implementation  Testing : In-situ tests Plate Load Test (PLT) Plate load test is a field test to determine the ultimate bearing capacity of soil. The test essentially consists in loading a rigid steel plate at the foundation level and determining the settlement corresponding to each load increments. The ultimate bearing capacity is then taken as the load at which the plate starts sinking at a rapid rate. 95 Student Practice Principles of Foundation Engineering by Braja M. Das Page 126-130. 96 Implementation  Testing Laboratory tests Basic physical properties tests (Moisture content, Specific gravity, Soil Indexes,..) Particle size test (sieving, Sedimentation) Direct shear box test Unconfined compression test Triaxial test Consolidation test Permeability test Other lab tests: Chemical test (pH, contamination,..) 97  Reporting Sequence of Site Investigation Planning  Preparation of Borehole  Site Investigation Report Implementation Reporting Reporting  Preparation of Boring Logs Initial information: Name and address of the drilling company, Driller’s name, Job description and reference number, boring information (number, type, and location of, and date of boring). Example of a typical boring log  Reporting  Preparation of Boring Logs Subsurface stratification: which can be obtained by visual observation of the soil brought out by auger, split- spoon sampler, and thin- walled Shelby tube sampler. Groundwater: Elevation of water table and date observed, use of casing and mud losses, and so on Reporting  Preparation of Boring Logs In-situ tests: Standard penetration resistance and the depth of SPT Samples: Number, type, and depth of soil sample collected; in case of rock coring, type of core barrel used and, for each run, the actual length of coring, length of core recovery, and RQD. Reporting  Preparation of Boring Logs Class example The following borehole is part of a site investigation (SI) carried out over a proposed location of a bridge. Assess the subsoil conditions and ground-water conditions based on the borehole data. In particular write about: Soil layers: types, description, depth… Soil properties: shear strength properties -based on SPT. Ground water depth 103  Reporting  Site Investigation Report When: After the completion of all of the field and laboratory work, a site investigation report is prepared. Why: for the use of the design office and for reference during future construction work. The report is also called soil exploration report or Geotechnical Factual report. What should be included in the site investigation report?  Reporting  Site Investigation Report The report should contain descriptions of the followings: Purpose & Scope of the investigation Site & Structure: site location, existing structures, drainage conditions, vegetation,… and information about the structure. Factual Details of field exploration: boreholes, samples, and testing. Usually given in another report (Geotechnical Design Report) For each type, quantities, method, tools should be presented. Geological setting of the site (variation of depth and thickness of layers as interpreted from the borings) Subsoil and water-table conditions, (soil parameters as interpreted from the testing results). Design analysis & recommendations: type of foundation, allowable bearing pressure, settlement estimation, and any special construction procedure; alternatives design solution. Conclusions and limitations of the investigations Reporting  Site Investigation Report The following graphical presentations must be attached to the report: 1. General map showing site location 2. A plan view of the location of the borings with respect to the proposed structures and those nearby 3. Boring logs (including in-situ tests results and samples) 4. Laboratory test results 5. Other graphical presentations (geotechnical cross section based on the boring logs, photos of the field work and soil samples,…) Reporting  Site Investigation Report Geotechnical cross section based on the boring logs