Soil Bearing Capacity of soil and foundationSBC.pptx

Soil Bearing Capacity of soil and foundation

1. Introduction: The load carrying capacity of foundation soil or rock which enables it to bear and transmit loads from a structure is know as bearing capacity Factors Affecting Bearing Capacity: Nature of soil and its physical and engineering properties. Nature of foundation and other details such as the size, shape, depth and rigidity of the structure Location of the ground water table relative to the foundation level. Mode of shear failure i.e. General, Local or Punching shear failure SAFE BEARING CAPACITY OF SOIL

2.  Ultimate bearing capacity ( q u ) The gross pressure at the base of the foundation at which soil fails is called ultimate bearing capacity. Net ultimate bearing capacity ( q nu ) By neglecting the overburden pressure from ultimate bearing capacity we will get net ultimate bearing capacity. Where       = unit weight of soil,  D f  = depth of foundation Net safe bearing capacity ( q ns ) I t is that net pressure which can be applied safely at the base of footing without the risk of shear failure. N et ultimate bearing capacity is divided by certain factor of safety will give the net safe bearing capacity. q ns   = q nu / F Where F = factor of safety = 2 to 3 (usual value use) Gross safe bearing capacity ( q s ) When ultimate bearing capacity is divided by factor of safety it will give gross safe bearing capacity. q s  = q u /F Important Definitions

3. Mode of Shear Failure: General shear failure:  General shear failure, usually associated with medium to dense or stiff soils of relatively low compressibility. Local shear failure:  Local shear failure is an intermediate failure mode characterized by well defined slip surface immediately below the footing but extending only a short distance into the soil mass, as shown in figure b. Punching shear failure:  Punching shear failure, usually associated with pile footings in loose sand or soft clays.  The soil below the foundation is highly stressed which gets separated by vertical shear from adjacent soil.

4. Methods to Determine Bearing Capacity Analytical Methods Terzaghi's method Rankine's theory Meyerhoff's method Pauker's theory Bell's theory Skempton's method I.S code method Field methods Standard penetration test (SPT) Cone penetration test (CPT) Plate load test (PLT)

5. Terzaghi's Bearing Capacity Theory Terzaghi’s bearing capacity theory is useful to determine the bearing capacity of soils under a strip footing. This theory is only applicable to shallow foundations. He considered some assumptions which are as follows.Assumptions : Footing is located at a depth Df below the ground surface such that Df ≤ B where B is the width of the footing. This means he considered the foundation to be shallow. Base of foundation is rough Footing is continuous, this makes the analysis two-dimensional. The angle of roughness between base of footing and soil is considered equal to friction angle of the soil. Failure is general shear failure and at the time of failure soil reaches into plastic state. He assumed that the length of the footing is infinite. He considered Mohr-coulomb equation as a governing factor for the shear strength of soil.

6. Standard Penetration Test (SPT) Standard penetration test can be used to determine:- Unconfined compressive strength of clays Ultimate bearing capacity on the basis of shear criteria Allowable bearing pressure on the basis of settlement criteria. Load carrying capacity of pile Angle of internal friction (ꬾ)

7. Procedure: This test is conduct at regular interval of 2-5 m or at change of stratum. To conduct this test, a bore hole of 55-150 mm is made and a split spoon sampler is used which has tubular shape with 35 mm inner dia. 50.5 mm outer dia and 650 mm length. The sampler is driven in the soil by a standard hammer of weight 63.5 kg and height of free fall 75 cm. The number of blows required for penetration of 300 mm is designated as standard penetration value. The test is conducted in three stages having 150 mm penetration each. The number of blows for first 150 mm penetration are ignored and No. of blows in last 300 mm penetration are noted and called observed SPT Number ( N ).

8. 50 blows are required for any 150mm penetration. 100 blows are required for 300mm penetration. NOTE: Corrections in Standard Penetration Test: Before the SPT values are used in empirical correlations and in design charts, the field ‘N’ value have to be corrected as per  IS 2131 – 1981.  The corrections are: Dilatancy or water table correction (C 2 ): In case of fine sand or silt below water table , apparently high values may be noted for N 0 . The values of SPT , corrected for overburden pressure i.e N 1 . N= 15+1/2(N 1 -15) Where , N1 = corrected value after overburden correction. N= Finally corrected value Overburden Pressure Correction (C 1 ): The corrected SPT No. of over burden pressure is given by N1 = C 1 N = N X 350/( Where, N0 = observed value of SPT σ = effective overburden pressure in KN/m 2  

9. Net ultimate bearing capacity of footing using N values: Teng (1962) has developed the following empirical equations for the net ultimate bearing capacity of footings on granular soils. For continuous or strip footings: q nu = 1/6 [3N 2 BR’ w + 5(100+ N²) D f R w ] For square and circular footings: q nu = 1/3 [N² BR +3 (100+ N²) D f R w ] Where, q nu = net ultimate bearing capacity in kN /m² N = averagqe N value corrected for overburden pressure B = depth of footing in m. If Df > B, use Df = B R’ w and R w correction factors for water table The net allowable bearing pressure or safe settlement pressure in kN /m² is given by qa net= 1.4(N-1)(B+0.3)S-CW-CD kN /m²1.4(N-3)2 B where, B Width of footing in m S permissible settlement in mm N = Final corrected SPT No. C = Water table correction factor = 0.5(1+)25(+1) = where, D Depth of water table below foundation D, Depth of foundation base

10. Engineer objectives : The design engineer wants to know what is under the surface to support the structures. To that end, the engineer needs to know several critical soil properties 1.Soil boring equipment Recommendations for construction methods and slopes Soil boring logs Soil description (classification such as silty-sand) Soil properties (hard, stiff, dense, loose, etc.) Boring depth Boring refusal Blow counts Soil moisture content Soil dry density Particle distribution curves (sieve analysis) Atterburg limits (liquid limits and plasticity index) Compaction test (optimum moisture for compaction) Direct shear tests Seismic velocity lines Permeability

11. Boring Methods for Soil Sampling 1. Auger Boring It can be used for almost all types of soil apart from gravelly soil and rocks. 2. Rotary Drilling Rotary drilling method of boring is suitable for all types of soil including rocks 3. Wash Boring Wash boring method is used to collect disturbed and undisturbed samples in almost all types of soils except rocks 4. Percussion Drilling It is used for all types of soils and rocks including stiff soils and rocks

12. The basic clues that must be understood to make an educated judgement of the soil conditions are: Soil boring equipment Recommendations for construction methods and slopes Soil boring logs Soil description (classification such as silty-sand) Soil properties (hard, stiff, dense, loose, etc.) Boring depth Boring refusal Blow counts Soil moisture content Soil dry density Particle distribution curves (sieve analysis) Atterburgs limits (liquid limits and plasticity index) Compaction test (optimum moisture for compaction) Direct shear tests Seismic velocity lines Permeability

13. Engineers objectives : The design engineer wants to know what is under the surface to support the structures. To that end, the engineer needs to know several critical soil properties: What is the allowable soil bearing pressure? What is the expected foundation settlement? What is the active soil load? What is the passive soil loading? What is the sliding friction factor? What is the potential for differential settlement? What is the soil liquefaction potential during an earthquake ? What are the seismic design accelerations? Where is the groundwater table? What is a permanent stable slope? Will piling be required? Can the native soil be used for backfill? What are the criteria for the pavement sections? Are hazardous wastes present? How corrosive are the soils? Will there be voids, obstructions or unstable soils ?