JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III Year GEOTECHNICAL ENGINEERING Previous Question Paper (September - 2021 - R13)
Answer any five questions. All questions carry equal marks.
Main
What is the purpose of soil classification? Explain how soils are classified according to IS soil classification system.
Answer
The primary purpose of soil classification is to organize soils into groups with similar engineering properties, providing a common language for engineers to predict soil behavior. It facilitates the systematic evaluation of soil suitability for construction projects such as foundations, embankments, and pavements, allowing for the estimation of permeability, compressibility, and shear strength without exhaustive testing for every project.
The Indian Standard (IS) Soil Classification System (IS: 1498) classifies soils based on grain size distribution and plasticity characteristics. The system uses a two-letter symbol to represent the soil type, where the first letter denotes the primary soil component and the second letter denotes its secondary characteristic.
The classification steps are as follows:
- Coarse-grained soils: These are soils where more than 50% of the material is retained on the 75-micron IS sieve. They are further classified as Gravel (G) if more than 50% of the coarse fraction is retained on the 4.75 mm sieve, or Sand (S) if more than 50% passes the 4.75 mm sieve. Sub-groups are determined by the percentage of fines (less than 5% for clean soils like GW/SW, or greater than 12% for silty/clayey soils like GM/SM/GC/SC).
- Fine-grained soils: These are soils where more than 50% of the material passes the 75-micron IS sieve. They are classified based on their plasticity using the Plasticity Chart (Casagrande's chart), which plots the Plasticity Index (Ip) against the Liquid Limit (WL).
- Classification categories: Soils are categorized as Silt (M) or Clay (C) based on their position relative to the 'A-line' (Ip = 0.73(WL - 20)). Soils plotting above the A-line are classified as clays, while those below are silts.
- Compressibility: Fine-grained soils are further subdivided based on the Liquid Limit into low (L), medium (I), or high (H) compressibility, resulting in symbols such as CL, CI, CH, ML, MI, or MH.
- Organic soils: Soils containing significant organic matter are classified with the symbol O (e.g., OL, OI, OH), while highly organic soils like peat are designated as Pt.
Explain the salient features of plasticity chart.
Answer
The plasticity chart is a graphical tool used in the Unified Soil Classification System (USCS) to classify fine-grained soils based on their plasticity index (Ip) and liquid limit (wL). The chart plots Ip on the y-axis against wL on the x-axis.
Key features include:
- The A-line: Defined by the equation Ip = 0.73(wL - 20), it acts as the primary separator between clays (plotted above the line) and silts or organic soils (plotted below the line).
- The U-line: An empirical upper boundary defined by Ip = 0.9(wL - 8), representing the upper limit of experimental data for natural soils.
- Classification zones: Soils are categorized based on their position relative to the A-line and specific liquid limit values: wL < 35% (low plasticity, L), 35% < wL < 50% (intermediate plasticity, I), and wL > 50% (high plasticity, H).
- Vertical line at wL = 50%: This divides soils into low/intermediate plasticity and high plasticity groups.
Derive the relation between void ratio and porosity for dry soil mass.
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What are the limitations in the application of Darcy’s law to flow through soil media.
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Show that the seepage force per unit volume is given by the product of hydraulic gradient and unit weight of water.
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Enumerate and briefly explain the factors affecting permeability of a soil.
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A uniform soil deposit has a void ratio 0.6 and specific gravity of 2.65. The natural groundwater is 2.5m below natural ground level. Due to capillary moisture, the average degree of saturation above the groundwater table is 50%. Determine the neutral pressure, total pressure and effective pressure at a depth of 6m. Draw a neat sketch.
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A point load of 100 kN acts on the ground surface. Using Boussinesq analysis, find the maximum vertical pressure on a vertical plane distant 2 meters from the loading.
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Briefly describe the significant features of the compaction and how it would influence the soil.
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Discuss the variation of vertical stress under point load along with vertical and horizontal plane.
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Explain the effects of compaction on soil properties.
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Settlements computed are normally higher than the actual settlements. Discuss the statement critically.
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How do you differentiate between the compactive effort imparted in the laboratory and that in field.
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Classify shear test based on the drainage conditions. How will these drainage conditions realized in the field?
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In a vane apparatus test, a torque of 45 N-m was applied at which failure took place. Subsequently, vane instrument rotated rapidly, so as to get remolded soil sample. This remolded soil was sheared at a torque of 18N-m. Calculate sensitivity of clayey soil.
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