DHARMCOMPRESSIBILITY AND CONSOLIDATION OF SOILS 251
7.16. Determine the amount of settlement given the following data:
Thickness of compressible medium = 3 m
Coefficient of volume decrease = 0.002 cm^2 /N
Pressure increment at the centre of the compressible medium = 75 kN/m^2.
(S.V.U.—B.Tech. (Part-time)—June, 1981)
7.17. The subsurface consists of 6 m of sandy soil (γ = 18.4 kN/m^3 ) underlain by a deposit of clay
(γ= 19.4 kN/m^3 ). The water table is at 4.2 m below the ground surface. Given the information (in
the following table) from a consolidation test of an undisturbed clay sample obtained from a
depth of 9.0 m from the ground surface, find Cc and preconsolidation pressure. Explain with
reason whether this is a normally or overconsolidated clay.
Pressure, 0 25 50 100 200 400 800 1600
kN/m^2Void ratio 0.960 0.950 0.942 0.932 0.901 0.870 0.710 0.540(S.V.U.—B.E., (R.R.)—Nov., 1975)
7.18. The co-ordinates of two points on a straight-line section of a semi-logarithmic plot of compres-
sion diagram are: e 1 = 2.50, σ 1 = 150 kN/m^2 ; and e 2 = 1.75, σ 2 = 600 kN/m^2. Calculate the
compression index.
7.19. The void ratio of a clay is 1.56, and its compression index is found to be 0.8 at the pressure 180
kN/m^2. What will be the void ratio if the pressure is increased to 240 kN/m^2?
7.20. The compression diagram for a precompressed clay indicates that it had been compressed
under a pressure of 240 kN/m^2. The compression index is 0.9; its void rato under a pressure
of 180 kN/m^2 is 1.5. Estimate the void rato if the pressure is increased to 360 kN/m^2.
7.21. In a clay stratum below the water table, the pore pressure is 36 kN/m^2 at a depth of 3 m. Is the
clay fully consolidated under the existing pressure? Explain.
7.22. A saturated clay specimen is subjected to a pressure of 240 kN/m^2. After the lapse of a time, it is
determined that the pore pressure in the specimen is 72 kN/m^2. What is the degree of consolida-
tion?
7.23. A compressible stratum is 6 m thick and its void rato is 1.70. If the final void rato after the
construction of a building is expected to be 1.61, what will be the probable ultimate settlement of
the building?
7.24. The total anticipated settlement due to consolidation of a clay layer under a certain pressure is
150 mm. If 45 mm of settlement has occurred in 9 months, what is the expected settlement in 18
months?
7.25. A stratum of a clay 5 m thick is sandwiched between highly permeable sand strata. A sample of
this clay, 25 mm thick, experienced 50% of ultimate settlement in 12 minutes after the applica-
tion 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 the pressure to a value comparable to that applied in the
laboratory test, to settle 50% of the ultimate value?
7.26. A 30 mm thick oedometer sample of 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 6 m. thick.
7.27. A clay stratum is 4.5 m thick and rests on a rock surface. The coefficient of consolidation of a
sample of this clay was found to be 4.5 × 10–8 m^2 /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.