Geotechnical Engineering - 2 - May 2015

MU Civil Engineering (Semester 6)

Total marks: --
Total time: -- INSTRUCTIONS
(1) Assume appropriate data and state your reasons
(2) Marks are given to the right of every question
(3) Draw neat diagrams wherever necessary 1 (a) Draw the free body diagram of forces for an embankment in c-ϕ soil with circular failure surface and define the notations used. Also determine the factor of safety with respect to cohesion for a submerged embankment 25 m high and having a slope of 40°. The properties of soil are ϕ=10°, c=40 kN/m², γ_sat=19 kN/m³, Taylor's stability number, S_n=0.097. 5 marks

1 (b) An unsupported excavation is to be made in soil having ϕ=12°, c=19 kN/m², γ=19 kN/m³. What is the maximum depth of unsupported excavation in soil? And draw the active pressure distribution diagram. 5 marks

1 (c) Considering Coulomb's theory of lateral earth pressure, draw and define the forces acting on the cohesionless backfill at failure under active state behind a retaining wall. Also, why are retaining walls usually designed for active earth pressure? 5 marks

1 (d) Explain the basic difference in the theoretical bearing capacity computation of shallow and deep foundations. 5 marks

1 (e) Define allowable bearing capacity of soil and what is the effect of increase in width and depth of a footing on the bearing capacity and settlement behaviour of footing resting on i) sand and ii) clay? 5 marks

1 (f) In what respect does the design of braced cuts differ from that of a retaining wall and what are apparent earth pressure diagrams used in the design of braced cuts? 5 marks

2 (a) Derive an expression for factor of safety of an infinite slope in a c-ϕ soil when there is a steady seepage parallel to the slope. 5 marks

2 (b) A deep cut of 12 m depth is made soil for the construction of a road. The properties of soil are: cohesion=30 kN/m², angle of internal friction =15° and a unit weight=20 kN/m³. The slope angle of the cut is 35°. Consider a trial slip circle of radius 20 m passing through the toe and cutting the top ground surface at a distance 5 m from top edge. Determine the factor of safety with respect to cohesion for the given trial slip circle by friction circle method. Assume factor of safety w.r.t friction as 1.5. 5 marks

2 (c) Critically compare Rankine's theory with the Coulomb'd lateral earth pressure theory. 5 marks

3 (a) Explain Culmann's method for the determination of active earth pressure on retaining wall considering the effect of line load on backfill. 5 marks

3 (b) A 7 , retaining wall with a smooth vertical back face has a stratified back fill and a surcharge load of 10 kpa. The properties of soil are as follows up to 3.5 m height from top: unit weight=15 kN/m³, angle of shearing resistance =10° and cohesion=10 kpa. Draw the lateral active earth pressure diagram and estimate the resultant thrust on the wall and its position. 5 marks

3 (c) What are the different types of conduits and the factor that affect the load on a conduit? 5 marks

4 (a) For the cantilever sheet pile wall shown in Figure 1, compute the embedment depth below the dredge line.
5 marks

4 (b) Differentiate between general, local and punching shear failure of shallow foundations. 5 marks

4 (c) Describe how the bearing capacity of soil for shadow foundation can be determined from standard penetration test. 5 marks

5 (a) Check the stability of the wall shown in Figure 2. Unit weight of concrete=24 kN/m³. Foundation sail: Y = 19kN/m³, Ï• = 20°, C=40 kpa, &=23°
Safe bearing capacity = 500 kpa 5 marks

5 (b) A circular foundation of 2.5 m diameter carries a load of 2500 kN. The soil has following properties :γ=19kN/m³, Φ =30°, c=4kN/m², N_c=37.2, N_q=22.5, N_γ=19.7. Using Terzaghi's theory,
(a) Find the depth at which the foundation should be loaded to provide a factor of safety of 3.
(b) What is the depth of location of foundation if there exists a water table close to the ground surface? 5 marks

6 (a) Explain the internal stability of mechanically stabilized retaining wall. 5 marks

6 (b) Explain the method of computation of settlement of pile groups in clayey soil. 5 marks

6 (c) A precast concrete pile of diameter 450 mm is driven into stiff clay. The unconfined compressive strength of the clay is 200 kN/m². Determine the length of pile required to carry a safe load of 400 kN with factor of safety=2.5. Assume adhesion factor=0.55. 5 marks

6 (d) Following data was obtained in a vertical pile load test on a 300 m diameter pile. Plot the load-settlement curve and determine the allowable load as per IS code.