II Semester M.E. (Civil) (Geotechnical Engg.) Degree Examination, July 2014 (2K8 Scheme) GT – 203 : DESIGN OF DEEP FOUNDATIONS Time : 3 Hours
Max. Marks : 100
Instructions : 1) Answer any five full questions. 2) Missing data may be suitably assumed. 1. a) Discuss the advantages and disadvantages of displacement piles as compared to replacement piles. 12 b) With a neat sketch, explain the load transfer mechanism of a single pile subjected to vertical load.
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2. a) Discuss how the subsoil conditions influence in the selection of the type of pile.
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b) A concrete pile of square section 0.35 m × 0.35 m is driven into a layered sandy strata up to a depth of 13 m as shown in figure – 1. Determine the allowable load by making use of critical depth concept. Take factor of safety = 2.5. 14
Figure – 1, Q.No. 2(b)
P.T.O.
JEP – 1022
*JEP1022*
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3. a) Explain how to separate the load on a pile into skin friction and end bearing resistance from the result of a cyclic plate load test.
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b) A single underreamed pile of length 18 m is to be installed in a deep hard clay deposit. The undrained strength obtained from a series of vane shear test at different depth have shown a linear relationship cu = 50 + 8D in KN/m2, where ‘D’ is the depth in meters. The diameter of the pile shaft and the bulb are respectively 1 and 3 m. The centre of the underream is located at 15 m from the ground surface. Determine the allowable load on the pile to ensure an overall factor of safety of 2.0.
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4. a) A concrete pile of section 0.35 m × 0.35 m is driven into normally consolidated clay up to a depth of 10 m. A static cone penetration test was conducted at the site with an electric cone penetrometer. The values of cone resistance and sleeve friction with respect to depth obtained in the test are tabulated as follows. Determine the safe load on the pile by Schmertmann’s method. Take factor of safety = 2.5 and correction factor for sleeve friction, K = 0.8. 14
Depth. m Cone resistance (Qc), KN/m2 Sleeve friction, (fc), KN/m2
1
2
180 375
13
25
3
4
5
6
7
8
9
10
11
12
360 625 750 875 1125 1190 1375 1625 2100 2180
35
40
50
75
80
85
95
100
115
120
b) Discuss the suitability of the dynamic pile driving formulae. 5. a) What are the salient points to be observed in the design of pile caps ?
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b) Explain the method of extimating load carrying capacity of group of piles in cohesionless soils.
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c) A group of 9 piles with three piles in each row passes through a recently placed fill. The depth of fill is 3 m, the diameter of the pile is 0.3 m and they are spaced at 0.9 m apart. If the soil is cohesive with q u = 60 KN/m 2 , γ = 15 KN/m 3. Compute the negative frictional load on the pile group.
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*JEP1022*
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JEP – 1022
6. a) What do you understand by group action and group efficiency of piles ? Discuss briefly. 10 b) For a column in a uniform deposit of medium clay, 0.3 m diameter, 8 m long piles are used as foundations. The unconfined compressive strength of clay is 100 KN/m2. The spacing between the piles is 0.5 m centre to centre. There are 9 piles in a group arranged in a square pattern. Calculate the ultimate pile load capacity of the group. Assume adhesion factor = 0.9. 10 7. a) Explain the procedure adopted in sinking of well and bring out problems that are encountered in open sinking. 10 b) Explain the limit equilibrium method of determining the grip length of wells in cohesive soils.
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8. Write short notes on the following : a) Underreamed piles b) Spacing of piles c) Negative skin friction d) Failure of piles.
sandy strata up to a depth of 13 m as shown in figure â 1. Determine the. allowable load by making use of critical depth concept. Take factor of. safety = 2.5. 14.
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