Summary Soil Mechanics - Civil Engineering SSC JE (Technical) - Civil Engineering

Introduction

• Soil properties describe characteristics like texture, structure, density, moisture content, permeability, and nutrient composition that control fertility, stability and erosion.

Formation of Soils

• Parent rocks: Igneous, Sedimentary, Metamorphic.
• Soil origin depends on: breakdown of parent rock, transportation (gravity, water, ice, wind), environment of deposition (flood plain, terrace, moraine, lacustrine, marine), and subsequent loading/drainage conditions.

Weathering and Classification

• Physical weathering: reduces rock size without changing composition; driven by water, frost, temperature, wind, ice; produces coarse particles.
• Chemical weathering: alters original minerals by processes like hydration, oxidation, carbonation; forms new compounds and finer particles.
• Effects of weathering and transport determine particle size, shape, composition, distribution, density, moisture and fabric of the soil.

Types by Origin and Deposition

• Residual soil: formed and remains at the original site; typical depth 5-20 m; wide range of particle sizes.
• Transported soil: moved from origin and deposited elsewhere; common deposits: alluvial (river), lacustrine (lake), marine (sea), glacial (from glaciers).
• Notable soils: Black cotton soil (high montmorillonite clay; high swelling/shrinkage, low shear strength); Bentonite (montmorillonite clay); Loam (sand+silt+clay).

Phase Diagram and System

• Soil is a multi-phase mass of solids, liquid and gas; the phase diagram represents these phases.
• 3-phase system applies to partially saturated soil; 2-phase system applies to saturated or dry states.

Volume Relations

• Total volume: V = Vs + Vv (Vs = volume of solids, Vv = volume of voids).
• Water content (w) = weight of water / weight of solids; 0% for dry soil; can exceed 100%.
• Void ratio (e) = volume of voids / volume of solids; no fixed upper limit, generally higher for fine-grained soils.
• Porosity (n) = volume of voids / total volume; 0 < n < 100%; porosity and void ratio are related but void ratio is used more frequently in practice.
• Percentage air voids = volume of air / total volume.
• Degree of saturation (S) = volume of water / volume of voids; S = 0 for dry soil, S = 100% for fully saturated soil.
• Air content (ac) = volume of air / volume of voids.

Weight Relations

• Bulk unit weight = total weight / total volume (Wt = Ws + Ww).
• Dry unit weight = weight of solids per unit total volume; indicates soil denseness.
• Saturated unit weight = total weight of fully saturated soil / total volume (S = 100%, air voids = 0).
• Submerged (buoyant) unit weight = submerged weight per unit volume when below groundwater table.

Inter-relations

• Water content and unit weight determine soil condition; values may change during transportation and storage.
• Dry unit weight can be calculated from bulk unit weight and water content.

Index Properties and Classification

• Common index properties: water content, specific gravity, particle distribution, consistency limits, in-situ density, density index.

Specific Gravity

• Specific gravity (Gs) = weight of a given volume of solids / weight of same volume of water at 4°C.
• Unit weight of water ≈ 9.8 kN/m3. Typical Gs: granular soils ≈ 2.65, cohesive soils ≈ 2.80.

Particle Distribution and Consistency Limits

• Particle distribution describes percentages of gravel, sand, silt, clay and is shown by a grain-size distribution curve; it is used for soil classification.
• Consistency limits include liquid limit, plastic limit and shrinkage limit; they describe soil plasticity with moisture change and guide engineering use.

Methods to Determine Water Content

• Oven Drying Method: most accurate; dry sample at controlled temperature (organic soils ≈ 60°C), typical drying 24 hrs (sandy soils 4-6 hrs). Formula: w = (W2 - W3) / (W3 - W1) × 100% where W1 = container, W2 = container + moist sample, W3 = container + dried sample.
• Pycnometer Method: quick; suitable for cohesionless soils when specific gravity is known; uses pycnometer weights W1..W4 and formula w = [((W2-W1)/(W3-W4)) × ((G-1)/G)] - 1.
• Calcium Carbide (Rapid Moisture Meter): fast (5-7 min); uses CaC2 reaction with water; small sample (4-6 g); gives quick wet-basis reading but may be less accurate.
• Sand Bath Method: field alternative to oven; dry sample in heated sand; use oven formula for w.
• Torsion Balance Moisture Meter: laboratory quick method using infrared; drum display reads moisture% on wet basis as sample dries on balance.

Determination of Specific Gravity of Soil Solids

• Pycnometer method (or density bottle 50 ml / flask 500 ml) is used; record weights W1..W4 (empty pycnometer, + oven-dried soil, + soil+water, + water).
• Notes: specific gravity often reported at 27°C; corrections provided for test temperature and for use of kerosene as wetting agent.