Geotechnical Parmeters- soil lecture-Solanki

GEOTECHNICAL INVESTIGATIONS & PARAMETERS Dr C H Solanki , Associate Professor, Applied Mechanics Department SVNIT, Surat: 

GEOTECHNICAL INVESTIGATIONS & PARAMETERS Dr C H Solanki , Associate Professor, Applied Mechanics Department SVNIT, Surat 1

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The geotechnical investigation program should be such that it can be ascertained that foundation system is safe against: Shear Failure of Soil Excessive Settlement of Soil Liquefaction 2

Geotechnical Project Sequence: 

Geotechnical Project Sequence Field Exploration Laboratory Investigations Geotechnical Interpretations and Analysis Report of Exploration

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Some Common Objectives Identify & Describe pertinent surface conditions Determine location and thickness of soil and rock strata (subsurface soil profile) Determine location of groundwater table Recover samples for laboratory testing Conduct lab and/or field testing Identify special problems and concerns

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Geotechnical investigation would consist of: Number of test bores Depth of test bores Field tests Laboratory tests Correlations of field and laboratory tests data for determination of safe bearing capacity (SBC) 5

Preliminaries: How Many Borings & How Deep?: 

Reference : Braja M. Das, Principles of Geotechnical Engineering , 6 th Edition Preliminaries: How Many Borings & How Deep? “No hard-and-fast rule exists for determining the number of borings or the depth to which borings are to be advanced.”

Preliminaries: How Many Borings?: 

Conventional Wisdom The number (density) of borings will increase: As soil variability increases As the loads increase For more critical/significant structures Thumb Rules Soft Soils - Space 100’ to 200’ As soils become harder, spacing may be increased up to 500’ Preliminaries: How Many Borings?

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Preliminaries: How Many Borings? Source: Sowers 1979 Structure or Project Subsurface Variability Spacing of Borings (ft) Highway Subgrade Irregular 100-1000 (200, typical) Average 200-2000 (500, typical) Uniform 400-4000 (1000, typical) Multistory Building Irregular 25-75 Average 50-150 Uniform 100-300

Number of Test Bores: 

Number of Test Bores As per IS:1892,Clause 2.3.1 “For a compact building site covering an area of 0.4 hectare (4000m 2 ), one borehole or trial pit in each corner and one in center should be adequate. For smaller and less important buildings even one borehole or trial pit in the centre will sufficient”. 9

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10 If soil is highly heterogeneous, some reference books suggests as under:

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However, for ordinary buildings investigation can be limited to economize and time and two test bores may be taken across the site. If highly variable strata are encountered additional test bores may be taken. 11

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When area to be investigated is large and plant layout is not finalized, the area may be divided into grid of suitable size and test points taken on grid corners. Where structural layout has been finalized, test points may be taken to correspond with all the important building units. 12

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For various structures the number of test points may be considered as under: 13

Depth of Test Bores: 

Depth of Test Bores The depth of test bores should extend up to the point at which the vertical stress due to proposed structure is equal to or less than 10% of original effective stress at the point before the structure is constructed . Normally, it should be one and half times the width of footing below the foundation level. 14

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How Deep?

Preliminaries: How Deep (Bridges)?: 

Preliminaries: How Deep (Bridges)? Boring depth is governed by various factors, including: Foundation type Foundation load etc. Generally speaking, 50ʹ- 80ʹ is reasonable Local experience is helpful Look at nearby structures if available If no experience or other info available, plan for long first hole, then adjust.

Preliminaries: How Many Borings & How Deep?: 

Reference : George F. Sowers, Introductory Soil Mechanics and Foundations: Geotechnical Engineering , 4 th Edition Preliminaries: How Many Borings & How Deep? “The final engineering can be no better than the data upon which it is based.”

Field Tests: 

Field Tests The most widely used method of subsurface investigation is boring holes in the ground and simultaneously conducting standard penetration tests and collecting undisturbed samples. However, sometimes due to site constraints, time constraints, importance of the project, erratic subsurface strata or to supplement the data obtained from test bores additional field tests are required. 18

Tests in Test Bores: 

Tests in Test Bores 19

Tests in Trial Pits: 

Tests in Trial Pits 20

In-situ Testing: 

In-situ Testing When it is difficult to obtain “undisturbed” samples Cohesion less soils, Sensitive clays In-situ Test Methods Standard Penetration Test (SPT) Cone Penetration Test (CPT) Vane Shear Test (VST)

Standard Penetration Test (SPT): 

Standard Penetration Test (SPT)

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Specifications 140 lb (63.5 kg) Hammer 30in (76 cm) free fall Drive sampler over 18 inches Record no. of blows per each 6 inch penetration SPT blow count=blows for 2 nd 6 inch penetration + blows for 3 rd 6inch penetration

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Standard Split Spoon Sampler Thick wall (0.25in) cylinder Sampling tube is split along the length Hammered into the ground

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Types of Samples : Disturbed Sample ( DS ): Obtained by direct excavations, augers & thick wall samplers. Used for mechanical analysis, water content determination , index properties tests, compaction tests, …etc. Undisturbed Sample ( UDS ): Obtained by forcing a thin wall sampler (75 cm dia shelby tube) into the soil at the bottom of the borehole by hand or by jacking. For a hard strata sample may be driven by blows from monkey. In soft clayey / silty soil, below water table UDS is obtained by piston sampler.

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In sandy soil, below water table UDS is obtained by compressed air sampler. Used for shear, consolidation, & permeability test. Split Spoon Sampler ( SPT ): ( IS 2132) A thick wall split – tube sampler, 50.8 mm OD & 35 mm ID, is driven into soil at the bottom of the hole under the blows of a 65 kg hammer with 75 cm free fall. The number of blows required to drive each 15cm penetration is recorded. The first 15 cm penetration is termed as a seating value . The last 30 cm penetration termed as ‘ N value ’. If the stratum consists of fine sand & silt below water table, the corrected N value obtained by, N’=15+½(N–15) For cohesion less soil N Value is also corrected for Overburden pressure.

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It should be noted that Standard Penetration Test is currently most popular and economical means to obtain subsurface information (both on land and offshore). It is estimated that 85 to 90 percent of conventional foundation design is made using the standard penetration test N-value . The N-value has been used in correlation for unit weight (γ), relative density (D r ), angle of internal friction (  ) and undrained compressive strength (q u ). 27 N Value

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28 N Value Correlations

Cone Penetration Test (CPT): 

Cone Penetration Test (CPT) Originally Developed in Netherlands 1930s Further developments in 1950s “Dutch Cone” ASTM D 3441 Types of CPT devices mechanical cone electric cone piezocone

Mechanical Cone: 

Mechanical Cone

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Cone Penetrometer

Cone Penetration Test (CPT): 

Cone Penetration Test (CPT) Measures: Cone Resistance, q c Sleeve Resistance, f sc Typical CPT results

Typical CPT Data: 

Typical CPT Data

Use of CPT Data: 

Use of CPT Data

Vane Shear Test: 

Vane Shear Test Specially suited for soft, sensitive clays Quick test, used to determine undrained shear strength

Vane Shear Test: 

Vane Shear Test Drill test hole Insert vane Rotate head Measure torque Relate resistance to soil shear strength

Vane Shear Test: 

Vane Shear Test Relationship between S u and applied Torque:

Pressure meter: 

Pressure meter

Pressuremeter Test: 

Pressuremeter Test

Truck-Mounted Drill Rig: 

Truck-Mounted Drill Rig Typical Equipment Used for Geotechnical Drilling Truck Mounted Drill Rig & Support Truck (Water Tank)

Field Drilling and Sampling: 

Field Drilling and Sampling Air or Mud Rotary Drilling

Congested Busy Sites: 

Congested Busy Sites Traffic control is a must Large percentage of effort is in the planning Special ordinances/ regulations may apply

Drilling: Continuous Flight Auger: 

Drilling: Continuous Flight Auger

Drilling & Sampling Hollow Stem Auger: 

Drilling & Sampling Hollow Stem Auger

Drilling & Sampling Shelby Tube Sampler: 

Drilling & Sampling Shelby Tube Sampler Suitable for Soil Thin-wall Steel Tubes 3.0" OD, 2.875" ID, 30.0" long, 7.2 lbs

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Groundwater Monitoring Groundwater level must be determined during geotechnical exploration Measure at time of drilling and later (24 hrs, 1 week, etc.) Can be accomplished by leaving selected soil borings open Or, install a piezometer

SPT: Automatic Trip Hammer: 

SPT: Automatic Trip Hammer

CPT Versus SPT: 

CPT Versus SPT CPT: Advantages over SPT provides much better resolution, reliability versatility; pore water pressure, dynamic soil properties CPT: Disadvantages Does not give a sample Will not work with soil with gravel Need to mobilize a special rig

Sounding Test: 

Sounding Test 50

Reliability & Validity of Field Penetration Test Data: 

Reliability & Validity of Field Penetration Test Data Do you know you have reliable results? Correlations with other test methods

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Field CBR Test : IS 2720 (Part 31 – 1990) Guiding Parameter for the design of Flexible Pavements. Evaluate the strength of sub grade & bases for roads , runway pavements . The ratio of the force per unit area required to penetrate a soil mass with a standard circular piston at the rate of 1.25 mm/min to that required for corresponding penetration of a standard material. California Bearing Ratio = Pt/Ps x 100 , Where Pt = Corrected unit load read from load penetration curve. Ps = Unit Standard load taken from table given.

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Modulus of Sub grade Reaction : IS (9214 – 1979) Guiding Parameter for the design of Rigid Pavements & Conventional design of Raft Foundation. Evaluate the strengths of sub grade & bases for roads & runway pavements. The ratio of load per unit area of horizontal surface of a mass of soil to corresponding settlement of the surface. K – Value is taken as the slope of the line passing through the origin & the point on the curve corresponding to 1.25 mm settlement. K - Value = p/0.125 Mpa / cm, Where P = load intensity corresponding to settlement of plate of 1.25 mm

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Plate Load Test : IS (1888 – 1982) Determination of ultimate bearing capacity of soil in place which assumes that soil strata is reasonably uniform. It is also used to find modulus of sub grade reaction. Limitation : The test results reflect only the character of the soil located within a depth of less than twice the width of bearing plate. Since the foundations are generally larger then the test plates, the settlement & shear resistance will depend on the properties of a much thicker stratum.

Balancing Cost & Risk: 

Balancing Cost & Risk “ The [scope of a subsurface exploration] for any particular site is a difficult problem which is closely linked with the relative cost of the investigation and the project for which it is undertaken.” VNS Murthy: Geotechnical Engineering: Principles and Practices of Soil Mechanics and Foundation Engineering

Ex-Situ (Laboratory) Tests: 

Ex-Situ (Laboratory) Tests Laboratory testing is the most common method for measuring soil and rock properties. Numerous examples... Moisture content Unit weight Sieve analysis Atterberg limits Compaction Hydraulic conductivity Consolidation Direct shear Triaxial shear Unconfined compression

Volume Weight Characteristics: 

Volume Weight Characteristics Parameters Application Moisture content (w) Classification & in volume weight relationship Classification & pressure computation Parameters used to represent relative volume of solids in given volume of soil Volume computations Density (  ) Porosity (n) Void ratio (e) Specific Gravity (G)

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Plasticity Characteristics Parameter Application Liquid limit ( w L ) Classification & property correlation ships Study of field behavior Plastic limit ( w p ) Liquidity Index (I L ) Consistency Index (I C ) Shrinkage limit ( w s ) Identification of clay mineral & swelling potential,swelling pressure Activity (A)

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Gradation Characteristics Parameters Application Effective Diameter ( D 10 ) Classification, Permeability, Filter Design Percent Grain Size (D 15 , D 30 , D 50 , D 60 , D 85 ) Classification and Filter Design Uniformity coefficient (C u ) Coefficient of Curvature (C c ) Clay size fraction Classification &Property correlation

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Consolidation Characteristics Parameters Application Coefficient of compressibility ( a v ) Computation of magnitude of settlement under loading Coefficient of volume ( m v ) Compression index(c c ) Recompression index ( c r )

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Parameters Application Coefficient of Consolidation ( c v ) Computation of time rate of settlement Coefficient of secondary compression (c α' ) Computation of secondary compression

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Strength Characteristics Parameters Application Angle of Internal Friction ( Φ ) Stability Analysis Load Carrying Capacity based on Strength Cohesion Intercept (C) Unconfined compression strength ( q u ) In-Situ shear strength

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Parameters Application Blow count from SPT N value Empirical relationship of strength & compressibility Bearing capacity factor N c , N q , N γ Bearing capacity Sensitivity Estimating effect of disturbance of structure on strength

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CHARACTERISTICS OF COMPACTED SOILS Parameters Application Maximum unit weight  dmax Optimum moisture content Compaction control Computation of stresses Relative density (I D ) Compaction control, To estimate strength parameter California bearing ratio Pavement Design

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67

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Ex-Situ (Laboratory) Tests

Data Presentation Scope of Information: 

Data Presentation Scope of Information Log of Boring Soil Test Boring Records Test Pit Records Data Included Field Laboratory Software Based Programs

Maximum total settlement & differential settlement of building: 

Criterion For isolated foundation Rafts Angular distortion 1/300 1/300 Greatest differential settlements in clays Sands cm 4.5 3.25 cm 4.5 3.25 Maximum settlements in Clays sands cm 7.5 5.0 cm 10.0 6.25 Maximum total settlement & differential settlement of building

Typical values for compression index of soil: 

Soil type General range of c c value Clay, Plastic 0.15-1.0 Clay, stiff .06-0.15 Clay, medium hard 0.03-0.06 Sand, loose 0.025-0.05 Sand, dense 0.0005-0.01 Typical values for compression index of soil

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Fine Grained Soil : ML – Silt of Low Plasticity MI – Silt of Intermediate Plasticity MH – Silt of High Plasticity CL – Clay of Low Plasticity CI – Clay of Intermediate Plasticity CH – Clay of High Plasticity

IS CLASSIFICATION : IS (1498) Coarse Grained Soils : 

IS CLASSIFICATION : IS (1498) Coarse Grained Soils Notations Description GW Well Graded Gravel SW Well Graded Sand GP Poorly Graded Gravel SP Poorly Graded Sand SP-SM Poorly graded Silty Sand SM Silty sand SC Clayey Sand

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Specific Gravity : IS 2720 ( Part 5 – 1985 ) Inherent material properties. Co - related with the consistency & particle size. Related to soil composition i.e. void ratio, porosity, density Shear Parameters i.e. C - f : IS 2720 ( Part 12 – 1981 ), IS 2720 ( Part 13 – 1981 ) Determines the shear strength capacity, stability of slopes, the bearing capacity of foundations, the lateral earth pressure exerted by soil on retaining walls & similar structures. Determine either by two procedures based on soil composition, density, specific gravity. --- Triaxial Shear Test, --- Direct Shear Test (Box Shear)

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Unconsolidated Undrained Condition (UU – Test) Consolidated Undrained Condition ( CU – Test ) Consolidated Drained Condition ( CD – Test )

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Swelling Parameters : IS 2720 ( Part 40 & 41 – 1977 ) Causes soil re – arrangement, increases compressibility of soils & instability to the founding structures & retaining wall. Expansive clays increase in their volume when they come in contact with water due to its surface properties. --- Free Swell Index, --- Swelling Pressure Test

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SBC Based on Shear : IS (6403 – 1981) The ultimate net bearing capacity is evaluated after taking into consideration of shape factor, depth factor & inclination factor of the foundation in accordance with I.S. 6403-1981. The net bearing capacity worked out using the following equation. For General Shear Failure : Q = C Nc s c d c i c + q (Nq -1) s q d q i q + 0.5 B g Nr s r d r i r W’ For Local Shear Failure : Q = C N’c s c d c i c + q (N’q -1) s q d q i q + 0.5 B g s r N’r d r i r W’ Where, C = Cohesion, q = Overburden Pressure g = Density, B = Width of the Footing Nc,Nq,Nr, N’c,N’q,N’r = Bearing capacity Factor s c ,s q ,s r = Shape Factor & d c ,d q ,d r = Depth Factor i c ,i q ,i r = Inclination factor

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SBC Based on Shear : IS (6403 – 1981 ) Depth Factors : Inclination Factors : Effect of Water Table: Shape Factors :

CONCLUSION: 

CONCLUSION Information regarding type of structure and total loads coming on the foundation should be given to the Agency carrying out Geotechnical Investigation so that depth of exploration may be modified during course of investigation if sufficient bearing capacity is not available at required/estimated depth. No. of test bores should be adequate to get horizontal as well as vertical profile of subsoil strata. For each strata encountered, following data should be available: N-value, Bulk Density, Natural Water Content, Shear Parameters, Consolidation Parameters, Swelling Parameters, Classification of Soil. Undisturbed samples and SPT should be staggered in test bores. i.e. If SPT is conducted at 1m depth in Test Bore BH1, then Undisturbed Sample shall be collected at 1m depth in Test Bore BH2 and so forth. 81

Problem: 

Problem Industrial building in central India 3 storeyed structure Type of foundation : Raft Size of foundation : 12m X 7.5m Depth = 1.5m

Soil Investigation : 

Soil Investigation Bore Log Detail

Influence zone below foundation level: 

Influence zone below foundation level Influence zone below foundation level = 7.5 X 1.5 = 11.25m G. W. T = 5.5m below ground level Strata below foundation level 3.5m CL 1.5m ML – CL 4. 0 m SM 2.25m CL C = 3 t/m 2  = 8 0 G = 2.65 w = 8.75%  d = 1.6 gm / cc  bulk = 1.749 gm/cc C c = 0.11, e = 0.656

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Φ (Degrees) Nc Nq N γ 0 5.14 1.00 0.00 5 6.49 1.57 0.45 10 8.35 2.47 1.22 15 10.98 3.94 2.65 20 14.83 6.40 5.39 25 20.72 10.66 10.88 30 30.14 18.40 22.40 35 46.12 33.30 48.03 40 75.31 64.20 109.41 45 138.88 134.88 271.76 50 266.89 319.07 762.89 Bearing Capacity Factors NOTE : For obtaining values of N’c, N’q and N’ γ calculate Φ ’=tan -1 (0.67 tan Φ ).

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Sr.No Relative Density ( Density Index) Void Ratio Condition Method of Analysis i Greater than 70 percent Less than 0.55 Dense General Shear ii Less than 20 percent Greater than 0.75 Loose Local Shear (as well punching shear) iii 20 to 70 percent 0.55 to 0.75 Medium Interpolate between ( i )and (ii) Method of Analysis Based on Relative Density & Void ratio

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Sr No. Shape of Base Shape Factor sc sq s γ i Continuous strip 1.00 1.00 1.00 ii Rectangle 1+0.2 B/L 1+0.2 B/L 1-0.4 B/L iii Square 1.3 1.2 0.8 vi Circle 1.3 1.2 0.6 Shape Factors Note :- The net ultimate bearing capacity on fairly saturated homogeneous cohesive soils ( Φ = 0) shall be calculated by following relationship ( q d ) net ult = c N c. S c. d c .i c Where c= q u/2 and Nc = 5.14

Depth Factors : 

Depth Factors The depth factors shall be calculated as under: d c = 1 + 0.2(Df/B) √N Φ d c = d γ = 1 for Φ < 10° d q = d γ = 1 + 0.1 (Df/B) √N Φ for Φ > 10° Df = Depth of foundation below ground level B = width of foundation N Φ = tan 2 ( 45 + Φ /2) The inclination factor shall be as under : i c = i q = [ 1 – ά /90] 2 where ά = Inclination of load to the vertical (degrees) i γ = [ 1 – ά / Φ ] 2

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Bearing Capacity Factors Values of bearing capacity factors Interpolated values Φ = 8 ° Φ ’ = 5.4 ° Nc 7.61 6.64 6.64 + 7.61 – 6.64 * 0.094 = 7.09 0.2 Nq 2.11 1.64 1.64 + 2.11 – 1.64 * 0.094 = 1.86 0.2 N γ 0.91 0.51 0.51 + 0.91 – 0.51 * 0.094 = 0.70 0.2

General shear failure (e<0.55): 

General shear failure (e<0.55) q nf = C N c S c d c i c + q (N q -1) S q d q i q + ½  BN  S  d  i  w’ Local Shear failure (e > 0.75) q nf = 2/3 c N c ’ S c d c i c + q (N q -1) S q d q i q + ½  BN  S  d  i  w’ Ultimate net bearing capacity on the basis of shear criteria as per IS 6403 – 1981 e 0 = 0.656 e o > o.55 but less than 0.75 Interpolation General shear failure / Local shear failure q nf = 25.34, F.S. = 2.5 q s = 10.14 t/m 2

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Settlement Computations Criteria A load placed on a soil mass causes stress changes within the soil below. The stress distribution ( σ z ) due to a concentrated load ( Q ) on the soil surface can be computed using ( i ) Boussinesq equation, i.e. σ z = Q/Z 2 .3/2 π [1 + (r/z) 2 ] 5/2 ( ii ) New mark chart ( Based on Boussinesq analysis ) ( iii ) Influence factors

Settlement computation TABLE: 

Settlement computation TABLE

Stress distribution curve (Borehole): 

Stress distribution curve (Borehole)

Excessive stress intensity computation (p) at CL of first strip i.e. 0.5m depth below foundation level: 

Excessive stress intensity computation ( p) at CL of first strip i.e. 0.5m depth below foundation level M = 2z / B n = L/B

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p at centre of 2nd strip i.e. at 2m depth below foundation level

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S1 = 41.0mm S2 = 57.3 mm S3 = 36.1mm

Settlement in 4m thick SM Soil: 

Settlement in 4m thick SM Soil Ncor = 15, B = 7.5m Settlement per unit pressure from N Value S = 23mm but ground W.T. table is above SM, 23/0.5 = 46mm

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 P 3 = 7.66 t/m 2 ,  P 4 = 6.64 t/m 2  P 5 = 3.57 t/m 2  P 6 = 3.34 t/m 2  P 7 = 2.84 t/m 2

Correction for Water Table (Ref.: IS:8009 Part -1): 

Correction for Water Table (Ref.: IS:8009 Part -1)

Settlement per Unit Pressure from Standard Penetration Test N-value (Ref.: IS:8009 Part-1): 

Settlement per Unit Pressure from Standard Penetration Test N-value (Ref.: IS:8009 Part-1)

Determination of Depth Factor (Ref. IS:8009 Part-1): 

Determination of Depth Factor (Ref. IS:8009 Part-1)

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S4 at 10 t/m 2 – 46mm S4 = 13.9mm for Average p 6.74 & 3.57 t/m 2 S5 = 6mm S6 = 6mm S = S1 + S2 + S3 + S4 + S5 + S6 = 160.3mm Applying Rigidity and depth factors S = 123mm 100 mm permissible settlement, the allowable bearing capacity is computed as =

References:: 

References : IS:1892-1979: C.O.P. for site investigations for foundations. IS:1904-1978: C.O.P. for structural safety of buildings-shallow foundations. IS:6403-1981: C.O.P. for determination of bearing capacity of shallow foundations. IS:8009 (Part 1)-1976: C.O.P. for calculations of settlements of foundations. J. E. Bowles: Foundation Analysis and Design, McGraw-Hill Companies Inc, New York. Alam Singh: Soil Engineering in Theory and Practice, Asia Publishing House, Mumbai N. V. Nayak: Foundation Design Manual, Dhanpat Rai and Sons, New Delhi Vora Mihir : Geotechnical Investigations for Various Structures , Structural Engineering Digest, May – June 2008 Vora Mihir : Bearing Capacity of Shallow foundations for Cohesionless Soils , Structural Engineering Digest Mittal S : Soil testing for Engineers, Khanna Publishers 103

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