JNTU
JNTU B.Tech III GEOTECHNICAL ENGINEERING Previous Question Paper (March - 2021 - R18)
Answer any five questions. All questions carry equal marks.
Main
Define and distinguish the following in terms of their use in soil engineering: (i) Consistency index, (ii) Relative Density.
Answer
The Consistency Index (Ic) is defined as the ratio of the difference between the liquid limit and the natural water content to the plasticity index of the soil. It is expressed as
The Relative Density (Dr), also known as the density index, is defined as the ratio of the difference between the void ratio in the loosest state and the natural void ratio to the difference between the void ratios in the loosest and densest states. It is expressed as
A soil has water content 10%, Specific gravity 2.7 and degree of saturation 35%. Find the void ratio, porosity, bulk unit weight and dry unit weight of soil.
Answer
Given: w = 0.10, G = 2.7, Sr = 0.35. Assuming unit weight of water γw = 9.81 kN/m³.
- Using the relationship Se = wG: e = (wG) / Sr = (0.10 × 2.7) / 0.35 = 0.771.
- Porosity (n) = e / (1 + e) = 0.771 / (1 + 0.771) = 0.435 or 43.5%.
- Bulk unit weight (γ) = [G(1 + w)γw] / (1 + e) = [2.7(1 + 0.10) × 9.81] / (1 + 0.771) = 16.36 kN/m³.
- Dry unit weight (γd) = Gγw / (1 + e) = (2.7 × 9.81) / (1 + 0.771) = 14.87 kN/m³.
Discuss the importance of consistency limits.
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A sieve analysis on a soil sample gave the following results:
| Sieve size (mm) | 4.75 | 2.0 | 0.84 | 0.42 | 0.25 | 0.15 | 0.075 |
|---|---|---|---|---|---|---|---|
| % Finer | 65 | 55 | 44 | 30 | 24 | 15 | 9 |
Sketch the grain size distribution curve and determine the % of sand fractions as per the IS nomenclature. Also determine effective size and uniformity coefficient.
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Discuss the constant head method to determine coefficient of permeability of soils.
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Determine the average horizontal and vertical permeability of a soil mass made up of three horizontal strata. The thicknesses of each layer are 2m, 3m, and 2.5m and their respective coefficient of permeabilities are 2 × 10-3 mm/s, 4 × 10-3 mm/s, and 9 × 10-3 mm/s.
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Discuss the effective stress concept of Terzaghi and define (i) neutral stress and (ii) effective stress.
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For a homogeneous earth dam having water head 26m, a flow net was constructed with four flow channels. The number of potential drops was 12. The dam has a horizontal filter at the base near the toe. The coefficient of permeability of the soil was 2 × 10-4 mm/s. Determine the anticipated seepage, if the length of the dam is 300m. Assume no tail water level at the downstream.
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Define pressure bulb. Discuss the variation of vertical stress due to point load along horizontal and vertical plane.
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An annular circular raft has outer and inner diameters as 12m and 8m respectively. If the load intensity on the raft is 150 kPa, estimate the increase in vertical stress at a depth of 4m and 6m from the ground surface and exactly below the centre of the raft.
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Discuss the following: (i) Zero air void line, (ii) Degree of compaction.
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Discuss the factors affecting compaction.
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Explain the following: (a) Primary consolidation settlement, (b) Degree of consolidation, (c) Compression index and (d) Double drainage system.
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Elaborate the following: (a) UU test, (b) Critical void ratio and (c) Stress-strain curves for normally and over consolidated clays.
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