Jawaharlal Nehru Technological University Hyderabad B.Tech IV Soil Mechanics Previous Question Paper (September - 2021 - R16)

Soil structure refers to the geometric arrangement of soil particles and the nature of the forces acting between them. The primary types of soil structures found in nature are:

1. Single-Grained Structure: Typical of coarse-grained soils like sands and gravels. Particles are deposited individually due to gravitational forces. The structure is relatively stable, and the behavior is dominated by particle-to-particle contact and friction.
2. Honeycombed Structure: Occurs in fine silts and rock flour. During deposition, surface forces (attraction) become significant relative to gravity, causing particles to form a loose, arch-like network resembling a honeycomb. This structure is metastable and prone to collapse under heavy loads or vibration.
3. Flocculated Structure: Common in clay minerals deposited in saline water. The net inter-particle force is attractive, causing particles to bond edge-to-face or edge-to-edge. This results in a loose, open structure with high void ratios and high shear strength at low pressures, but it is highly compressible.
4. Dispersed Structure: Occurs when repulsive forces dominate, often due to chemical changes or high mechanical remolding. Clay particles align in a face-to-face, parallel orientation. This structure is dense, has low permeability, and exhibits lower shear strength compared to flocculated clays.
5. Coarse-Grained Skeleton Structure: A composite structure where larger particles form a primary load-bearing skeleton, while smaller particles (fines) occupy the voids between them. The stability depends on the contact between the coarse grains.

These structures significantly influence engineering properties such as permeability, compressibility, and shear strength, determining the soil's suitability for construction foundations.

To determine the volume of soil to be excavated, we use the principle of conservation of mass for the solid particles of the soil. The mass of solids remains constant during the excavation and compaction process.

Given data:

Dry density of soil in the borrow pit (ρ_d1) = 1.62 g/ml = 1.62 t/m³

Dry density of compacted embankment (ρ_d2) = 1.72 g/ml = 1.72 t/m³

Volume of the compacted embankment (V₂) = 1000 m³

Water content in the borrow pit (w) = 10% = 0.10

The mass of solids (M_s) required for the embankment is calculated as:

Since the same mass of solids must be excavated from the borrow pit, the volume of soil to be excavated (V₁) is determined by the dry density of the borrow pit material:

Thus, the volume of soil to be excavated from the borrow pit to construct 1000 m³ of the embankment is 1061.73 m³.