INDIAN INSTITUTE OF TECHNOLOGY MADRAS, CHENNAI Department of Civil Engineering CE 3350 Geotechnical Engineering
Assignment # 4: Pile Foundations Note: Make suitable assumptions wherever necessary. 1. (a) Pile driving in loose sands without predrilling tends to densify these soils. What effect does this densification have on the load bearing capacity of such piles? (b) A 400 mm × 400 mm reinforced concrete pile is driven through a deposit of fine loose sand and soft clay 20 m thick, to a depth of 1 m into an underlying stratum of dense sand. The water table is located close to the ground surface. In submerged state, the angle of shearing resistance of sand was 35º and unit weight 10 kN/m 3. Calculate the point bearing resistance of the pile. 2. (a) In what circumstances would you expect caving or squeezing conditions to be a problem? What construction construction methods could a contractor use to overcome these problems? (b) A 300 mm diameter concrete pile, 9 m long, is driven in a deposit of cohesionless soil. The average corrected standard penetration resistance, N for the deposit is 15. Using the correlations, estimate the safe load capacity of the pile. Adopt Adopt a factor of safety 2.5 and assume unit unit weight of 16 kN/m kN/m 3 for the sand. 3. The following data was obtained in a vertical pile load test on a 300 mm diameter pile: Load (kN) Settlement (mm)
50 2.5
100 4.0
200 9.5
300 16.5
400 27.0
500 40.5
600 61.0
Plot the load-settlement load-settlement curve and determine the allowable load as per IS Code. 4. A group of nine piles arranged in a square pattern is to be proportioned in a deposit of soft clay. Assuming the piles to be square (with side 300 mm) and 10 m long, work out the spacing for 100 per cent efficiency of the pile group (η ( η = 1). Neglect bearing and assume adhesion factor of 0.8. 5. Design a friction pile group to carry a load of 3000 kN including the weight of pile cap at a site where the soil is uniform clay to a depth of 20 m, underlain by rock. Average unconfined compressive strength of clay is 70 kN/m2. The clay may be assumed to be of normal sensitivity and normally consolidated. A factor of safety of 3 is required against shear failure. 6. A symmetrical 16 pile group in soft clay, with unconfined compressive strength of 40 kN/m 2, is to be used as foundation for a column. The piles are 300 mm in diameter and 10 m long and spaced at 900 mm centres. Determine the maximum load the group can carry with the piles failing (i) individually, and (ii) as a block. 7. A concrete pile 15.24 m long having a cross section of 406 mm × 406 mm is fully embedded in a saturated clay layer for which γsat = 19.02 kN/m 3, = 0, and c u = 76.7 kN/m 2. Determine the allowable load that the pile can carry. (Let FS = 3). Use the α method to estimate the skin friction and Vesic's method for point load estimation. 8. A concrete pile is 18 m long and has a cross section of 0.406 m × 0.406 m. The pile is embedded in a sand having γ = 16 kN/m3 and ' = 37º. The allowable working load is 900 kN. If 600 kN are contributed by the frictional resistance and 300 kN are from the point load, determine the elastic 6 2 3 2 settlement of the pile. Given: E p = 2.1 × 10 kN/m , Es = 30 × 10 kN/m , νs = 0.38 and ξ = 0.57. 9. A concrete pile measuring 0.406 m × 0.406 m in cross section is 18.3 m long. It is fully embedded in a layer of sand. The following is an approximation of the mechanical cone penetration resistance (q c) and the friction ratio (Fr ) for the sand layer. Estimate the allowable bearing capacity of the pile. Use FS = 4. Depth below ground surface (m) qc (kN (kN/m ) Fr (%) 0 - 6.1 2803 2.3 6.1 - 13.7 3747 2.7 13.7 - 19.8 8055 2.8
4.1
10. The plan of a group pile is shown in Fig. 1. Assume that the piles are embedded in a saturated homogeneous clay having a cu = 86 kN/m2. Given: diameter of piles (D) = 316 mm, center to center spacing of piles = 600 mm and length of piles = 20 m. Find the allowable load carrying capacity of the pile group. Use FS = 3.
Fig. 1 11. A concrete pile of 450 mm diameter is driven to a depth of 16 m through a layered system of sandy soil (c = 0) as shown in Fig. 2. Assume that the value of δ in all the layers of sand is equal to 0.75. The value of ̅ for each layer as equal to half of the passive earth pressure coefficient. The water table is at ground level. Calculate the values of Qu and Qa with FS = 2.5 by the conventional method for Q f and Berezantsev's method for Q b. Fig. 2 12. A group of 9 piles with 3 piles in a row were driven into a soft clay extending from ground level to a great depth. The diameter and length of the piles were 300 mm and 10 m respectively. The unconfined compressive strength of clay is 70 kPa. If the piles were spaced at 900 mm centre to centre, compute the allowable load on the pile group on the basis of shear failure criteria for a factor of safety of 2.5. 13. A 350 mm diameter closed-end steel pipe pile with 10 mm thick walls and a 20 mm thick bottom plate is to be driven to a depth of 18.5 m into a soil that has f s = 50 kPa for downward loads and 40 kPa for upward loads and q t' = 9000 kPa. This pile will be constructed with the benefit of a wave equation analysis and onsite dynamic testing . Compute the allowable downward and upward load capacities. 14. An office building is to be supported on a series of 700 mm diameter, 12 m long piles that will be built using the open hole method. The soil profile at this site is as follows: Depth (m) Soil Classification Undrained Shear Strength, su (kPa) 0 - 2.2 Stiff clayey silt (CL) 70 2.2 - 6.1 Stiff silty clay (CL) 85 6.1 - 11.5 Very stiff sandy clay (CL) 120 11.5 - 30.0 Very stiff sandy clay (CL) 180 15 (a) What is "block failure" in a group of piles? (b) A 500 mm square prestressed concrete pile (f c' = 40 MPa) is to be driven 20 m into a clay. The ultimate side friction capacity, Σf sAs is 1450 kN and the ultimate net toe bearing capacity, q t'At is 300 kN. Using Equations (14.46) to (14.48) (See Coduto book), develop a load-settlement curve, then determine the allowable load for a factor of safety of 2.5 and the corresponding settlement. 16. A building column carrying a dead load of 1100 kN and an imposed load of 300 kN is to be supported by a single bored pile installed in firm to stiff fissured London Clay (Fig. 3). Select suitable dimensions and penetration depth to obtain a safety factor of 2 in total pile resistance, or safety factors of 3 in end bearing and unity in shaft friction. Calculate the immediate settlement at the working load. Fig. 3 4.2
