Bearing Capacity Of Foundations

Bearing Capacity of Foundations: Exercises 1. A strip foundation has to carry a load of 58 kN/m. The foundation has to be installed at a depth of 500 mm in soil with the following properties: c = 30 kPa;  = 180;  = 18.7 kN/m3. The water table is at a depth of 300 mm below the ground surface. Use a factor of safety of 2.5 and determine the required width of the foundation 2. A light industrial factory is being constructed at the edge of an estuary on an extensive area of deposits comprising gravely sand. A site investigation has determined that the effective angle of friction for the granular deposits is    20 0 , the soil unit weight to be 18 kNm -3 above the water table and 20 kNm -3 below the water table and c  0 . The water table is predicted to rise to within 1 m depth below the ground surface. a) Calculate the required dimensions for a square footing founded 1 m below ground surface to support a load of 240 kN/running metre from an external column for the factory. Use a factor of safety of 2.5 3. A square footing 2m by 2m is to be constructed 1.22 m below the ground surface as shown in Fig Q3. The centric column load on the footing is 225 kN. The unit weight of the soil is 18.84 kN/m 3, unit weight of concrete is 24 kN/m3. The soil is cohesive with unconfined compressive strength of 144 kN/m2. Assume that the contact pressure can be  Q M x y M yx  computed by using the flexural formula q    . The value of A I xx I yy   NC = 5.157 in the Terzaghi equation if  = 0. Determine the following: a) Soil contact pressure b) Safe bearing capacity for a factor of safety of 3

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Fig Q3 4. A square footing is to be constructed as shown in Fig Q4. For a factor of safety of 3, determine the safe load (in kN) that the footing can carry.

Fig Q4 5. A concrete pile of a square cross-section with side length of 0.36 m is driven 10.5 m in to a clayey soil. The clay is of the following characteristics:  = 18.1 kNm-3; C = 60 kNm-2. Neglecting the self weight of the pile, calculate the design capacity of the pile if the factor of safety is 2 6. A circular concrete pile 300 mm in diameter is driven 4.6 m into a sandy soil with  = 280. The water table is at a depth of 1.4 m below the ground surface. The sand above the water table is saturated (due to capillary fringe) and has a density of 1830 kg/m 3. For a factor of safety of 2, determine the safe bearing capacity of the pile

7.

A cylindrical ribbed steel pile, 400 mm in diameter is driven 10 m into a 350 mm diameter hole. The soil is composed of two layers whose properties are a given in Table Q7. The water table is at a depth of 3.6 m below the surface. Calculate the maximum safe bearing capacity of the pile if the factor of safety is taken as 2.5

Table Q7 Type of soil Sand Sand Clay

Thickness (m) 0 – 3.6 3.6 – 7.0 7.0 – 10

 (kN/m3) 18 19 (sat)

 (0) 35 35

C (kN/m2)

100

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8. A 12 m long 350 mm diameter circular smooth steel pile has been driven into a layered deposit as shown in Fig Q8. The water table is at a depth of 2 m below the ground surface. For a factor of safety of 2.5, determine the ultimate and safe axial load capacity of the pile.

Fig Q8 9. A wooden pile 20 m long with a cross section 381 mm x 381 mm is driven 3 into a hole 375 mm x 375 mm in a sandy soil for which  sat  18.9kN / m and   38 0 . The water table is at a depth of 2 m from the surface. For a FOS of 2.5 determine the safe bearing capacity of the pile

10.

A pile group consists of nine friction-concrete piles in clay soil. The diameter of each pile is 300 mm and the centre-to-centre spacing is 0.75 m. The embedded length is 10 m. The soil conditions are as shown in Fig Q10. Take Nc = 5.14 for  = 0 for clay and determine the following: a) The block capacity of the pile group using a factor of safety of 3; b) The allowable group capacity based on individual pile failure using a factor of safety of 2 along with the Converse-Labarre equation for pilegroup efficiency; and c) The design capacity of the pile group

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Fig Q10

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