Bahir Dar University Foundation Engineering Chapter 1 PDF

Bahir Dar University School of Civil & Water Resource Eng. Department of Water Engineering Foundation Engineering. 1 Foundation engineering I course code: 3152 Credit points: 5(2hr lecture, 3hr tutor) Academic year: 2018/2019 prerequisite...

Bahir Dar University School of Civil & Water Resource Eng. Department of Water Engineering Foundation Engineering. 1 Foundation engineering I course code: 3152 Credit points: 5(2hr lecture, 3hr tutor) Academic year: 2018/2019 prerequisite: soil mechanics II and Reinforced concrete I Course objective: To equip students with a sound knowledge about site exploration methods, selection of foundation type, analysis and design of shallow foundations and retaining structures. 2 Course outline Chapter one 1. Site exploration: Chapter two 2. Types of foundations and their selection criteria. Chapter three 3. Design of shallow foundations: 3.1. Isolated or spread footings, 3.2. Combined footings, 3.3. Strap or cantilevered footings, 3.4. Mat foundations, 3.5. Eccentrically and inclined loaded foundations. Chapter four 4. Analysis and design of retaining structures Reference: 1. Bowles, J. E., Foundation Analysis and Design, McGraw-Hill. 2. Das, B. M., Principles of Foundation Engineering, PWS pub. Co. 3. Tefera, A., Principles of Foundation Engineering, AAU. 3 1. Soil Exploration “ The process of exploring to characterize or define small scale properties of substrata at construction sites is unique to geotechnical engineering. In other engineering disciplines, material properties are specified during design, or before construction or manufacture, and then controlled to meet the specification. Unfortunately, subsurface properties cannot be specified; they must be deduced through exploration.” Charles H. Dowding (1979). 4 Purpose of exploration The primary objective is to analyze the nature of the soil in all aspects, basically is to find out strength characteristics of the sub-soil over which the structure has to be built. Purpose of Soil exploration includes:  Selection of alternative construction sites or the choice of the most economical sites,  Selection of alternative types or depth of foundation,  Selection of alternative methods of construction,  Evaluation of the safety of existing structure,  Location and selection of construction materials. 5 The soil exploration should provide the following data:  Soil parameters and properties of different layers (e.g. for classification, bearing capacity or settlement calculation),  Thickness of soil layers and depth to bedrock (stratification of soil),  Location of ground water level and important groundwater related issues,  Special problems and concerns. 6  Planning an exploration program The planning of a program for soil exploration depends upon: – The nature of sub-soil – The type of structure – The importance of structure The actual planning of a subsurface exploration program includes some or all of the following steps: i. Desk-study: Assembly of all available information on type and use of the structure, and also of the general topographic and geological character of the site. 7 ii. Reconnaissance of the area: Consists of walking the site and visually assessing the local condition, inspection of behavior of adjacent structures, rock outcrops, cuts, etc. Reconnaissance includes the assessment of; Any previous development on site, Any previous grading on site, Any potential landslide or other stability problems, Condition of nearby structure 8 iii. A preliminary site investigation: This is usually in the form of a few borings or a test pit to establish the types of materials, Stratification (vertical profile) of the soil, and possibly the location of the ground water level.  For small projects this step may be sufficient to establish foundation criteria, in which case the exploration program is finished. iv. A detailed site investigation: For complex projects or where the soil is of poor quality and/or erratic, a more detailed investigation may be undertaken. This may involve sinking several boreholes, taking soil samples for laboratory investigations, conducting sounding and other field tests. 9 Methods of exploration It is important to investigate the surface condition, but we basically rely on subsurface exploration soil and rock samples obtained by drilling vertical holes known as Borings or by digging exploratory trenches or test pits. Methods of determining the stratification and engineering characteristics of sub-surface are  Test pits  Boring and sampling  Field tests  Geophysical methods  Laboratory tests Test Pits The simplest and cheapest method of shallow soil exploration is to sink test pit to depths of 3 to 4 m. Test pits enable the in-situ soil conditions to be examined visually. It is relatively easy to obtain disturbed or undisturbed soil samples: 10 Soil Boring and Sampling Soil Boring: This is the most widely used method. It provides samples from shallow to deeper depths for visual inspection as well as laboratory tests. The most commonly used methods of boring are:  Auger boring  Wash boring  Percussion drilling  Rotary drilling 11 Auger boring: Operated by hand or by power. Hand operated augers, = 15 to 20cm, are of two types. Post-hole and helical augers This boring method provides highly disturbed soil samples. Power operated augers (helical) can be used to great depths, even to 30m, and used in almost all types of soils above water table. 12 Hand Augers a) helical and b) post hole 13 This image compares solid- stem (left) and hollow-stem (right) auger flights. 14 Wash boring: Power operated. Hole is advanced by chopping, twisting action of a light chopping bit and jetting action of drilling fluid, usually water, under pressure.  Loosened soil particles rise as suspended particles through the annular space between casing and drill rod. This method best suits in sandy and clayey soils and not in very hard soil strata (i.e. boulders) and rocks.  Depth of boring could be up to 60m or more. Changes in soil strata are indicated by changes in the rate of progress of boring, examination of out coming slurry and cutting in the slurry. Undisturbed samples whenever needed can be obtained by use of proper samplers. 15 Wash boring 16 Different drill bits 17 Percussion drilling: Power operated. Hole is advanced by repeated blows of a heavy chisel into the bottom of the hole. The resulting slurry formed at bottom of borehole is removed by bailer or sand pump. Because of the deep disturbance of the soil this method of boring is not favored. Casing is generally required. Maximum depth of boring is 60m. 18 Percussion Drilling at Site 19 Rotary drilling: Power operated. Hole is advanced by a rapidly rotating bit. This is the most rapid method for penetrating highly resistant materials (e.g. bed rock). In this method undisturbed samples can be obtained at desired depths by using suitable samplers. Maximum depth of drilling is 80 to 150m. 20 Rotary Drilling & Drill bits 21 Drilling in soils prone to caving or squeezing Open hole methods encounter problems in soils prone to caving (i.e., the sides of the boring fall in) or squeezing (the soil moves inwards, reducing the boring diameter). Caving is most likely in loose sands and gravels, especially below the groundwater table, while squeezing is likely in soft saturated silts and clays. In such cases, it becomes necessary to provide some type of lateral support inside the hole during drilling. 22 Drilling in soils prone to caving or squeezing One method of supporting the hole is to install casing (see Figure ), which is temporary lining made of steel pipe. This method is especially useful if only the upper soil are prone to caving, because the casing does not need to extend for the entire depth of the boring. The other method is to fill the boring with drilling mud or slurry, which is a mixture of bentonite or attapulgite clay and water. This material provides a hydrostatic pressure on the walls of the boring, thus preventing caving or squeezing. 23 24 Soil Sampling Laboratory test results are mainly dependent on the quality of soil samples. There are two main types of soil samples which can be recovered from bore holes or trial pits.  Disturbed and  Undisturbed samples 25 Soil Sampling for Geo-stratification 26 Disturbed Samples These are samples where the structure of the natural soil has been disturbed to a considerable degree by the action of the boring tolls or excavation equipment. However, these samples represent the composition and the mineral content of the soil. Disturbed samples are satisfactory for performing classification tests such as, sieve analysis, Atterberg limits etc. 27 Undisturbed Samples These are samples, which represent as closely as is practicable, the true in-situ structure and water content of the soil. Undisturbed samples are required for determining reliable information on the shearing resistance and stress- deformation characteristics of a deposit. It is virtually impossible to obtain totally undisturbed samples. This is due to that: The process of boring, driving the coring tool, raising and withdrawing the coring tool and extruding the sample from the coring tool, all conspire to cause some disturbance. In addition, samples taken from holes may tend to swell as a result of stress relief. 28  Samples should be taken only from a newly- drilled or newly extended hole, with care being taken to avoid contact with water. As soon as they are brought to the surface: Core tubes ends should be sealed with wax and capped to preserve the loss of moisture content. Core tubes should properly be labeled to indicate the number of bore holes and the depth at which they are taken and then stored away from extremes of heat or cold and vibration. 29 Types of tube samplers  Split Spoon Sample  Thin-Walled Tube Sampler  Piston Samplers 30  Field [in-situ] tests Assignment-1(Max.Mark (30%) write on the following field tests i. Method of operation ii. Limitation of tests iii. How to correlate to bearing capacity and settlement from test result. 1. Penetration or sounding tests 2. Vane shear test 3. Plate loading test 4. Pile loading test (Min.10page; Max. 15page; font size 12 ) Submission date:- after one week (late submission is not allowed). Every student should submit his/her own work. 31 Field [in-situ] tests These tests are valuable means of determining the relative densities; shear strengths and bearing capacities of soils directly without disturbing effects of boring and sampling. The most commonly used field tests are: Penetration or sounding tests Vane shear test Plate loading test Pile loading test 32 Penetration Tests They are conducted mainly to get information on the relative density of soils with little or no cohesion. The tests are based on the fact that the relative density of a soil stratum is directly proportional to the resistance of the soil against the penetration of the drive point. From this, correlations between values of penetration resistance versus angle of internal friction (Φ), bearing pressure, density and modulus of compressibility have been developed. Penetration tests are classified as: Static and dynamic penetration tests. 33 Static Penetration Tests 1) Swedish Weight Sounding Test: This method of testing is widely used in Scandinavia and here in Ethiopia. The depth of penetration is measured for each loading after which the number of half-turns is counted by 100kg load; the penetration depth is then measured after 25 half-turns. If the penetration after 25 half-turns is less than 5cm the rod is unloaded and driven down by a 5 to 6kg hammer. 34  50 25 HT/20cm penetration 100 75 Depth Swedish weight sounding equipment, penetration diagram 35  The correlation between density of frictional soils and consistency of cohesive soils and ht/m (half-turns per meter) are as given below. Frictional Soils Density (kN/m3) Very loose 500ht/m 21 – 24 Cohesive Soils Density (kN/m3) Soft 0 ht/m 16 –19 Firm 0 – 100 ht/m 17.5 – 21 Stiff 100-200 ht/m Very stiff 200 - 500 ht/m 19 – 22.5 Hard >500 ht/m 36 2. Static Cone Penetration Test (Dutch Cone Penetrometer Test): This method is widely used in Europe. The test consists of a cone (apex angle 600, overall diameter 35.7mm, end area 10cm2, rods (⅝” ), casing pipe ( ¾”). The rod is pushed hydraulically into the ground at a rate of 10mm/sec. The pressure exerted on the rod is measured with a proving ring, manometer or a strain gauge. The cone is 1st pushed into the ground. The force required to push the cone 20cm into the soil is recorded. The casing pipe is then advanced to join the cone. The force required to push the pipe is also recorded. The readings thus taken are plotted against depth. 37 38 Correlation between Cone (Point) Resistance and Relative Density of Frictional Soils Relative Density Point Resistance (kN/m2) Very loose soil < 2500 Medium dense 5000 – 10,000 Dense 10,000 – 15,000 Very dense > 15,000 According to Meyerhof: N = ¼ (Ckd) where: N = Standard penetration number Ckd = Static Cone resistance (kg/cm2) For sand, modulus of compressibility (ES) can be estimated from cone resistance from the following relationship. ES =3/2(Ckd) 39 Dynamic Penetration Tests 1) Standard Penetration Test (SPT): This is the most common of the field tests and measures the resistance of the soil to dynamic penetration by a 50mm diameter split spoon sampler which is driven into the soil at the bottom of a borehole (sometimes cased). The sampler is attached to drill rods and the dynamic driving force is a 63.5kg mass falling through a height of 76cm onto the top of the rods. The sampler is initially driven 15cm below the bottom of the borehole. It is then further driven 30cm. The number of blows required to drive the last 30cm is termed as the standard penetration value denoted by N. 40 41 42  Correlation between Number of blows (N), Angle of Internal Friction and Relative Density of Frictional Soils(Terzaghi and Peck). N 0-4 4 -10 10-30 30 - 50 > 50  420 Relative Very loose Loose Medium Dense Very dense Density Correlation between Number of blows (N), Unconfined Compressive Strength and Consistency of Cohesive Soils. (Terzaghi and Peck). N 0 -2 2-4 4-8 8 -15 15-30 >30 qu(kN/m2) 0 -25 25 -50 50 -100 100 -200 200-400 >400 Consistency Very soft Soft Medium Stiff Very stiff Hard 43  The relationship between  and Dr may be expressed approximately by the following equation (Meyerhof): For granular soil, containing more than 5 percent fine sand and silt. 0=25+0.15Dr For granular soil, containing less than 5 percent fine sand and silt. In the equations Dr is expressed in percent. 0=30+0.15Dr 44 Correction to be applied to measured values of SPT: The N values of SPT as measured in the field may need to be corrected. When SPT is made in fine saturated sands, saturated silty sands, or saturated silts, correction is usually made for possible build up of pore water pressure. The SPT values, greater than 15 are modified as follows N = 15 + ½ (N’ –15) Suggested by Terzaghi and peck where: N= corrected value N’= Recorded value 45  The other type of correction is known as correction for overburden pressure. This correction is applied only to cohesionless soils (dry, moist or wet). The correction suggested by Gibbs and Holtz and widely used is as follows. N =  2N’, o’ = effective overburden pressure in kN/m2 46 2) Dynamic Cone Penetration Test: This is another useful test, which is normally used to determine the relative resistance offered by the different soil layers. The cone is fixed to the bottom of a rod and driven into the ground in the same way as a SPT is performed. The number of blows required to penetrate 30cms depth is called as Nc value. In the case of dynamic cone penetration test no borehole is used. 47  Experiments carried out indicate that beyond about 6m depth, frictional resistance on the rod increases which gives erroneous results for Nc value. The maximum depth suggested for this test is about 6 m. If the test has to be conducted beyond 6 m depth, one has to use drilling mud (bentonite slurry) under pressure forced through the pipe and the cone. The mud solution coming out of the cone rises above along the drill rod eliminating thereby the frictional resistance offered by the soil for penetration. The former method is called as dry method and the latter wet method. 48  To judge the consistency of soil from Nc values, the general practice is to convert Nc to N values of SPT: Nc = N/C Where: N = blow count for SPT Nc = blow count for dynamic cone C = Constant, lies between 0.8 and 1.2 when bentonite is used. Nc = 1.5N for depths up to 3m Nc = 1.75N for depths between 3m and 6m Nc Values need to be corrected for overburden pressure in cohesionless soils like SPT 49 Vane Shear Test It is used to determine the undrained shear strength of soft clays soils. The apparatus consists of a vertical steel rod having four thin stainless steel blades (vanes) fixed at its bottom ends. Vane head (torsion head), complete with pointer, stop pin, circumferential graduated scale, calibrated torsion spring. 50 51  In most cases a hole is drilled to the desired depth, where the vane shear test is planned to be performed and the vane is carefully pushed into the soil. A torque necessary to shear the cylinder of soil defined by the blades of the vane is applied by rotating the arm of the apparatus with a constant speed of 0.5 degree/sec. The maximum torque is then measured from which the shearing strength is determined. From the measured maximum torque one may estimate the shearing resistance of the tested clay from the following formula. 52  T  Cu =  2H D  3  D    2 12  where : T = Torque D = Diameter of Vane H = Height 53 Plate Loading Test In this test a gradually increasing static load is applied to the soil through a steel plate, and readings of the settlement and applied load are recorded, from which a relationship between bearing pressure and settlement for the soil can be obtained. The test procedure: 1. Pit for the test must be at least 5 times the size of the plate. 2. The plate should be properly placed in the soil. In the case of cohesionless soil (to prevent early displacement of soil under the edges of the plate), the plate must be positioned in cast in-situ concrete. 3. Loading platform should be properly erected. 54 Dead Weight Load Loaded platform Pressure gauge Hydraulic jack Short block Settlement dial gauge Bearing pressure (kPa) Bp Settlement, Sp (cm) 55 56 4. Loading of the soil is conducted in steps (loading increment is kept constant). 5. Once completion of the test, the plate is unloaded in the same incremental steps (to draw the expansion curve). Bearing capacity of non-cohesive soil is determined from settlement consideration. If the maximum permissible settlement, S, of a footing of width Bf is given, the settlement, Sp, of a plate of width Bp under the same intensity of loading is given by. Sp (2 Bf ) 2 S (B f  B p ) 2 Using the value Sp, computed from the above equation, the loading intensity under the footing could be read from the load settlement curve. 57  The settlement of footing in clay is normally determined from principles of consolidation. However from plate load test, the approximate settlement of footing of width B can be determined using the following expression. Bf S  Sp Bp 58 Limitation of Plate Loading Test: Plate loading test is of short duration. Hence consolidation settlement does not fully occur during the test. For settlement consideration, its use is restricted to sandy soils, and to partially saturated or rather unsaturated clayey soils. Plate loading test can give very misleading information of the soil is not homogeneous within the effective depth (depth of stress influence) of the prototype foundation. Plate loading test should not be recommended in soils which are not homogeneous at least to depth of 1½ to 2 times the width of the prototype foundation. 59 Pile Loading Test: This is the most reliable means for determining the load carrying capacity of a pile. The load arrangement and testing procedure are more or less similar to the plate-loading test. From the results of this test, the allowable bearing capacity and load- settlement relationship of a group of friction piles can be estimated. 60 61  Geophysical methods: This method can be used for the location of different strata and for a rapid evaluation of the sub-soil characteristics. However, these methods are very approximate. These method can be broadly divided into two categories; 1. Seismic Refraction method and 2. Electrical Resistivity method. 62 1. Seismic Refraction Method:  These methods are based on the principle that the elastic shock waves have different velocities in different materials. i.e. sound (shock) waves travel faster through rocks than through soils.  The shock wave is created by a hammer blow or by a small explosive charge at point P in fig. below. 63 64  The geophones convert the ground vibration into electrical impulses and transmit them to a recording apparatus.  The basic equations of the refraction survey are derived based on the assumption that the velocity of the shock wave increases as the depth increases. i.e. V3 > V2 > V1  At geophones located close to the point of impact, such as point A, the direct waves with velocity V1 reach first.  At point B, the refracted waves reach earlier than the direct waves. 65  At points further away from the point of impact, such as point C, the waves which are refracted twice, once at the interface of the layers Ι and ΙΙ, and once at the interface of the layers ΙΙ and ΙΙΙ, reach earlier.  For the determination of the thickness of different layers, a distance-time graph is plotted. 66 Up to certain distance X1, the direct waves in the layer Ι reach first. At this point, the first two lines in Fig. above intersect, which indicates that the direct wave travelling a distance X1 with a velocity V1 and the refracted wave travelling with a velocity V1 in distance 2H1 and with a velocity V2 in distance X1 reach simultaneously, where H1 is the thickness of the layer I. 67 Thus: The above Eq. gives reliable results when the waves are produced by a sinusoidal force and not by impact. The following empirical equation gives reliable results for impact shock. Likewise, the thickness of the second layer (H2) is obtained from the distance X2 corresponding to the point of intersection of the second and the third line in Fig. above It is give by the relation 68 The procedure is continued if there are more than three layers Limitation of the seismic methods 1. The methods cannot be used if a hard layer with a greater seismic velocity overlies a softer layer with a smaller seismic velocity. 2. The methods cannot be used for the areas covered by concrete, asphalt, pavements or any other artificial hard crust, having a high seismic velocity. 69 3. If the area contains some underground features, such as buried conduits, irregularly dipping strata and irregular water table, the interpretation of the results becomes very difficult. 4. If the surface is layer is frozen, the method cannot be successfully used, as it corresponds to a case of harder overlying a softer layer. 5. The methods require sophisticated and costly equipment. 6. For proper interpretations of the seismic survey results, the services of an expert are required. 70 Electrical Resistivity: 3. In this method four metallic spikes to serve as electrodes are driven in to the ground at equal intervals along a line. A known potential is then applied between the outermost electrodes and potential drop is measured between the innermost electrodes. 4. The resistivity method makes use of the fact some soils (e.g. soft clays) have low electrical resistivity than others (e.g. sand or gravel). 5. Seismic and resistivity methods are normally employed as preliminary or supplementary to other methods of exploration. 71 ρ = mean resistivity Ι = current supplied d = spacing of electrodes V = voltage drop 72 Laboratory tests The common laboratory tests that concern the foundation engineers are – Grain size analysis – Atterberg limits – Natural moisture content – Unit weight – Unconfined compression test – Direct shear test – Triaxial compression test – Consolidation test – Compaction test – Chemical analysis 73 Ground Water Measurement Ground water affects many elements of foundation design and construction. Because of this its location should be determined in each job with reasonable accuracy. The depth of water table is measured by lowering a chalk- coated steel tape in the borehole. The depth can also be measured by lowering the leads of an electrical circuit. As soon as the open ends of the leads touch the water in the borehole, the circuit is completed. It is indicated by glow of the indicator lamp. 74 Depth and number of borings Depth of Boring The depth to which boreholes should be sunk is governed by the depth of soil affected by foundation bearing pressures. According to Tomlinson the following depths of boreholes for various foundation conditions may be used. 1. For widely spaced strip of pad foundations, boring depth should be deeper than 1.5 times the width of the foundation. 2. For raft foundations, boring depth deeper than 1.5 times width of raft should be used. 75 3. For closely spaced strip or pad foundations where there is overlapping of the zones of pressure, boring depth deeper than 1.5 times width of building should be used. 4. For group of piled foundation on soil, boring depth should be deeper than 1.5 times width of pile group, the depth being measured from a depth of two- thirds of the length of the piles. 5. For piled foundation on rock, boring depth should be deeper than 3.0m inside bedrock. 6. According to Teng, for high ways and airfields minimum depth of boring is 1.5m, but should be extended below organic soil, fill or compressible layers such as soft clays and silts. 76 2. Number of Borings: From experience Teng has suggested the following guideline for preliminary exploration: Project Distance between boring (m) Minimum number of Horizontal stratification of soil boring for each Uniform Average Erratic structure Multi-story building 45 30 15 4 One or two story 60 30 15 3 building Bridge piers, - 30 75 1-2 for each abutments, television foundation unit towers, etc Highways 300 150 30 77 Data presentation The results of borings, samplings, penetration tests and laboratory tests of a site are usually plotted graphically on a sheet of drawing paper. The graphical presentation should include. 1. A plot plan, showing the location of all boreholes, test pits, etc and their identification number. 2. A separate plot, showing the soil profile as established from the drillings or test pits records. 3. Soil profiles along given lines in the ground surface, showing the boundaries between identifiable soil layers, variation of thickness of firm bottom layer, thickness of soft clay layers etc. 78 d. The penetration number, the unconfined compression strength, Atterberg limits, natural moisture content, and other appropriate laboratory data may be shown on each boring on the soil profile. e. The location of ground water table should also be shown on the soil profile. 79  Depth Legend Soil type N qu  L.L p.I 0.00m Top soil 1.00m W.T Sandy Silt 4.00 Dense sand 8.00 Gravel 12.00 Hard 15.00 rock 80 81 Soil Exploration Report Most reports have the following contents: 1. Introduction: - Purpose of investigation, type of investigation carried out. 2. General description of the site: - general configuration and surface features of the site. 3. General geology of the area. 4. Description of soil conditions found in bore holes (and test pits) 5. Laboratory test results. 6. Discussion of results of investigation in relation to foundation design and constructions. 7. Conclusion: recommendations on the type and depth of foundations, allowable bearing pressure and methods of construction. 82