JNTU
JNTU B.Tech III SOIL MECHANICS Previous Question Paper (January/February - 2021 - R16)
Answer any five questions. All questions carry equal marks.
Main
Draw the phase diagram for soils and derive from fundamentals:
Answer
A soil phase diagram consists of three components: Air, Water, and Solids. By convention, the volume of solids (Vs) is taken as 1, the volume of water is Vw, and the volume of air is Va. The weights are Ws for solids and Ww for water (weight of air is negligible). From the definitions: Void ratio e = Vv / Vs = (Va + Vw) / 1, so Vv = e. Degree of saturation S = Vw / Vv, so Vw = S × e. Water content w = Ww / Ws, so Ww = w × Ws. Specific gravity G = Ws / (Vs × γw), so Ws = G × Vs × γw. Since Vs = 1, Ws = G × γw. Substituting these into the weight of water relation: Ww = w × G × γw. Also, Ww = Vw × γw = (S × e) × γw. Equating the two expressions for Ww: w × G × γw = S × e × γw. Simplifying gives the fundamental relationship: e × S = w × G.
The dry unit weight of a sand sample in the loosest state is 13.50 kN/m3 and in the densest state, it is 20.0 kN/m3. Determine the density index of this sand when it has a porosity of 35%. Assume the grain specific gravity as 2.65.
Answer
Given: γd(min) = 13.50 kN/m3, γd(max) = 20.0 kN/m3, n = 35% (0.35), G = 2.65. First, calculate the void ratio in the current state (e) using n = e / (1 + e): 0.35 = e / (1 + e) ⇒ 0.35 + 0.35e = e ⇒ 0.65e = 0.35 ⇒ e = 0.5385. Next, calculate the void ratios for the loosest and densest states using γd = (G × γw) / (1 + e), taking γw = 9.81 kN/m3. For loosest state: 13.50 = (2.65 × 9.81) / (1 + emax) ⇒ 1 + emax = 26.0 / 13.50 = 1.926 ⇒ emax = 0.926. For densest state: 20.0 = (2.65 × 9.81) / (1 + emin) ⇒ 1 + emin = 26.0 / 20.0 = 1.30 ⇒ emin = 0.30. The Density Index (ID) is calculated as: ID = (emax - e) / (emax - emin) = (0.926 - 0.5385) / (0.926 - 0.30) = 0.3875 / 0.626 = 0.619. The density index is 61.9%.
Why is classification of soils required? Draw the IS plasticity chart and label the symbol of various soils.
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A soil sample has a liquid limit of 20% and plastic limit of 12%. Classify the soil approximately according to Indian Standard Classification for the following sieve analysis data:
| Sieve Size (mm) | 2.0 | 0.425 | 0.075 |
|---|---|---|---|
| % Passing | 100 | 85 | 38 |
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Write short notes on neutral and effective stress and discuss the role of effective stress in soil engineering.
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Define permeability and discuss any four factors that affect the permeability of soil.
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For a homogeneous earth dam having water head 28m, a flow net was constructed with four flow channels. The number of potential drops was 18. The dam has a horizontal filter at the base near the toe. The coefficient of permeability of the soil was 2×10-2 mm/s. Determine the anticipated seepage, if the length of the dam is 120m.
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Explain Westergaard’s theory for determination of the vertical stress at a point. How is it different from Boussinesq’s solution?
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Write a brief critical note on the concept of pressure bulb and its use in soil engineering practice.
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Define preconsolidation pressure and explain the Casagrande’s graphical procedure for preconsolidation pressure in soil.
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Discuss the following: (a) UU test (b) critical void ratio and (c) stress – strain plots for normally consolidated and over consolidated soils.
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