Civil Engineering Department, ISM Dhanbad Geotechnical Engineering – I (CEC 15102)

Home Assignment – II 1. Determine the amount of settlement given the following data:

i. Thickness of compressible medium = 3m. ii. Coefficient of volume decrease = 0.002cm2/N.

iii. Pressure increment at the centre of the compressible medium = 75kN/m2. 2. The subsurface consists of 6m of sandy soil (γ = 18.4kN/m3) underlain by a deposit of clay (γ = 19.4kN/m3). The water table is at 4.2m below the ground surface. Given the information (in the following table) from a consolidation test of an undistributed clay sample obtained from a depth of 9m from the ground surface, find Cc and pre-consolidation pressure. Explain with reason whether this is normally or over-consolidated clay.

Pressure, kN/m2

0 25 50 100 200 400 800 1600

Void ratio 0.960 0.950 0.942 0.932 0.901 0.870 0.710 0.540

3. The co-ordinates of two points on a straight line section of a semi-logarithmic plot of compression diagram are: e1 = 2.50, 1σ = 150kN/m2; and e2 = 1.75, 2σ = 600kN/m2. Calculate the compression index. 4. The void ratio of clay is 1.56, and its compression index is found to be 0.8 at the pressure 180kN/m2. What will be the void ratio if the pressure is increased to 240kN/m2? 5. The compression diagram for a pre-compressed clay indicates that it had been compressed under pressure of 240kN/m2. The compression index is 0.9; its void ratio under a pressure of 180kN/m2 is 1.5. Estimate the void ratio if the pressure is increased to 360kN/m2. 6. In a clay stratum below the water table, the pore pressure is 36kN/m2 at a depth of 3m. Is the clay fully consolidated under the existing pressure? Explain. 7. A saturated clay specimen is subjected to a pressure of 240kN/m2. After the lapse of a time, it is determined that the pore pressure in the specimen is 72kN/m2. What is the degree of consolidation? 8. A compressible stratum is 6m thick and its void ratio is 1.70. If the final void ratio after the construction of building is expected to be 1.61, what will be the probable ultimate settlement of the building? 9. The total anticipated settlement due to consolidation of a clay layer under a certain pressure is 150mm. If 45mm of settlement has occurred in 9 months, what is expected settlements in 18 months? 10. A stratum of a clay 5m thick is sandwiched between highly permeable sand strata. A sample of this clay, 25mm thick, experienced 50% of ultimate settlement in 12 minutes after the application of a certain pressure. How long will it take for a building proposed to be constructed at this site, and which is expected to increase pressure to a value comparable to that applied in the laboratory test, to settle 50% of the ultimate value? 11. A 30mm thick oedometer sample of a clay reached 30% consolidation in 15 minutes with drainage at top and bottom. How long would it take the clay layer from which this sample was obtained to reach 60% consolidation? The clay layer had one-way drainage and was 6m thick. 12. A clay stratum is 4.5m thick and rests on a rock surface. The coefficient of consolidation of a sample of this clay was found to be 4.5x10-8 m2/s in the laboratory. Determine probable period of time required for the clay stratum to undergo 50% of the ultimate settlement expected under a certain increment of pressure.