Soil Mechanics

(a) Define $D_{10},D_{30}$ and $D_{60}$ with the help of gradation curve. Why are $C_u$ and $C_c$ important? What is the value of $C_u$ for a soil with only one grain size? [2] (b) An undisturbed sample of clay brought from field was noted to have a volume of 18 cc and weight of 30.8 gm. On oven drying the weight of sample was reduced to 20.5 gm. The volume of dried sample as obtained by displacement of mercury was 12.5 cc. Calculate shrinkage limit, specific gravity and shrinkage ratio. [4] (c) Three soils A, B and C are mixed in proportion of 20%, 30% and 50% by mass respectively. The specific gravity of soils A, B and C are 2.5, 2.6 and 2.7 respectively. If the bulk density of mixed soil is 2.0 g/cc at 20% water content, determine void ratio and degree of saturation. [4] (d) Describe field identification procedure of soil.

(a) Describe different laboratory and field methods of determination of permeability and their significance. [4] (b) An earth dam is built on $a^n$ impervious foundation with a horizontal filter at the base near toe. The permeability of the soil in the horizontal and vertical directions are $4\times 10^{-2}\text{ mm/s}$ and $2\times 10^{-2}\text{ mm/s}$ respectively. The height of water and free board are 35m and 5m respectively. If top width is 10m, side slope is 2H:1V and the length of drainage is 15m. Estimate the seepage loss per meter length of dam.

(a) What are the factors affecting effective stress, and explain one of them, showing a stress diagram? [3] (b) Determine the vertical stress at point P due to strip load as shown in figure. [3] c) Describe Newmark's chart and its application.

(a) Write the assumption of Terzaghi's one-dimensional consolidation theory. A soil sample $20\text{ mm}$ thick takes $20\text{ minutes}$ to reach $20\%$ consolidation. Find the time taken for a $4\text{ m}-thick$ clay layer to reach $40\%$ consolidation. Assume double drainage for both cases. [1+3] (b) Why is it necessary to accelerate consolidation? Should it be done before or after construction? How can it be done? [2] (c) Explain the method to determine preconsolidation pressure.

Draw a typical stress-strain and volume change characteristics curves for loose sands and dense sands from consolidated drained test. An unconfined compression test was carried out on a saturated clay sample. The maximum load the clay sustained was $127\text{ N}$ and the vertical displacement was $0.8\text{ mm}$. The size of the sample was $38\text{ mm}$ diameter and $76\text{ mm}$ long. Determine the undrained shear strength. Draw Mohr's circle of stress for the test and locate undrained shear strength ($c_u$).

(a) Discuss the effects of compaction on soil characteristics. [2] (b) In a standard proctor test on a soil having a specific gravity of $2.6,the$ following results were obtained.

Plot the dry density versus water content curve in a graph. Also, show the zero-air void line on the same curve. Determine the values of void ratio, porosity, and degree of saturation for the soil at the optimum moisture content condition.