Unit-3.pptx Foundation engineering unit - 3

Foundation engineering unit - 3

1. FOUNDATION ENGINEERING SVECW Shri Vishnu Engineering College for Women Dr. B. Sudhir Kumar (Assistant Professor) Civil Engineering

2. UNIT – 03 Shallow foundations – bearing capacity criteria : Types of foundations and factors to be considered in their location – Bearing capacity – criteria for determination of bearing capacity factors influencing bearing capacity - analytical methods to determine bearing capacity – Terzaghi’s theory –IS Methods. Shallow foundations – settlement criteria : Safe bearing pressure based on N- value allowable bearing pressure; safe bearing capacity and settlement from plate load test, Types of foundation settlements and their determination allowable settlements of structures.

3. Bearing capacity is the ability of soil to safely carry the pressure/load placed on the soil from any engineered structure without undergoing shear failure and excessive large settlements . Gross pressure intensity (q) Total pressure at the base of the footing due to weight of superstructure, self weight of the footing and the weight of the earth-fill/soil. Net Pressure Intensity (q n ) Difference in intensities of the gross pressure after the construction of the structure and the original overburden pressure qn = q – γD

4. F - Factor of safety usually taken as 2.00 -3.00

5. TYPES OF BEARING CAPACITY FAILURES Distinct failure patterns are developed depending on failure mechanism . Vesic (1973) classified shear failure of soil under a foundation base into three categories depending on the type of soil & location of foundation General Shear failure Local Shear failure Punching Shear failure 11

6. In low compressibility (dense sand or stiff clay ) soils Heaving on both sides of foundation Final slip (movement of soil) on one side only causing structure to tilt BEARING CAPACITY FAILURES- GENERAL SHEAR FAILURE 1

7. Clay Sand

8. It has well defined failure surface reaching to ground surface There is considerable bulging of sheared mass of soil adjacent to footing Failure is accompanied by tilting of footing Failure is sudden Ultimate bearing capacity is well defined BEARING CAPACITY FAILURES- GENERAL SHEAR FAILURE

9. In highly compressible soils Only slight heaving on sides Significant compression of soil under footing but no tilting BEARING CAPACITY FAILURES- LOCAL SHEAR FAILURE 15

10. In soils of high compressibility and in sands having relative density between 35 and 70 percent Failure pattern is clearly defined only immediately below the footing Failure surface do not reach ground surface Only slight bulging of soil around the footing Failure is not sudden and there is no tilting of the footing Failure is defined by large settlements Ultimate bearing capacity not well defined BEARING CAPACITY FAILURES- LOCAL SHEAR FAILURE 17

11. In loose, uncompacted soils Vertical shearing around edges of footing High compression of soil under footing, hence large settlements No heaving, no tilting BEARING CAPACITY FAILURES- PUNCHING SHEAR FAILURE 18

12. No failure pattern observed No bulging of soil around the footing No tilting of footing Failure is characterised in terms of very large settlements Ultimate bearing capacity not well defined BEARING CAPACITY FAILURES- PUNCHING SHEAR FAILURE 20

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14. Terzaghi’s Bearing capacity equation for determining ultimate bearing capacity of strip footing (Only) ɣ = Unit weight of soil 2 C= Cohesion B= Width of foundation Df= Depth of foundation

15. Determine the ultimate bearing capacity of a square footing of 1.5 m size, at a depth of 1.5 m, in a pure clay with an unconfined strength of 150 kN/m 2 . φ = 0° and γ = 17 kN/m 3 . N c = 5.7, N q = 1, and N γ = 0. A continuous footing of width 2.5 m rests 1.5 m below the ground surface in clay. The unconfined compressive strength of the clay is 150 kN/m 2 . Calculate the ultimate bearing capacity of the footing. Assume unit weight of soil is 16 kN/m3. Nc = 5.7, Nq = 1, and Nγ = 0. What is the ultimate bearing capacity of a rectangular footing, 1 m × 2 m, on the surface of a saturated clay of unconfined compression strength of 100 kN/m 2 .

16. A foundation, 2.0 m square is installed 1.2 m below the surface of a uniform sandy gravel having a density of 19.2 kN/m 3 , above the water table and a submerged density of 10.1 kN/m 3 . The strength parameters with respect to effective stress are c′ = 0 and φ′ = 30°. Find the gross ultimate bearing capacity for the following conditions: ( i ) Water table is well below the base of the foundation (i.e., the whole of the rupture zone is above the water table); (ii) Water table rises to the level of the base of the foundation; and (iii) the water table rises to ground level. (For φ = 30°, Terzaghi gives Nq = 22 and Nγ = 20)