Soil Mechanics Fundamentals – Study Notes
1. Introduction to Soil Mechanics
| Concept |
Description |
| Soil |
A natural, unconsolidated material composed of mineral particles, organic matter, water, and air. |
| Soil Mechanics |
Branch of civil engineering that studies the behavior of soil under various loading conditions and its interaction with structures. |
| Why It Matters |
Determines bearing capacity, settlement, stability of foundations, slopes, and earth‑retaining structures. |
2. Soil Classification
| Classification System |
Main Criteria |
Typical Uses |
| Unified Soil Classification System (USCS) |
Grain size, plasticity, and behavior |
Geotechnical reports, design of foundations, embankments |
| AASHTO Soil Classification |
Grain size & plasticity (for highways) |
Road and pavement design |
| ASTM Soil Classification |
Similar to USCS, with additional specifications |
Laboratory testing standards |
2.1 Grain‑Size Distribution
- Sieve analysis – for particles > 0.075 mm (coarse fraction).
- Hydrometer or pipette analysis – for particles ≤ 0.075 mm (fine fraction).
Key parameters
- D₁₀, D₅₀, D₆₀ – particle diameters at 10 %, 50 %, 60 % passing.
- Coefficient of Uniformity (Cu) = D₆₀ / D₁₀.
- Coefficient of Curvature (Cc) = (D₃₀)² / (D₁₀·D₆₀).
| Soil type |
Cu |
Cc |
| Well‑graded sand/gravel |
> 4 (sand) / > 6 (gravel) |
1–3 |
| Poorly graded (uniform) |
< 4 (sand) / < 6 (gravel) |
– |
| Silts & clays |
– |
– |
2.2 Atterberg Limits (Fine‑grained soils)
| Limit |
Definition |
Typical Range |
| Liquid Limit (LL) |
Moisture content at which soil changes from plastic to liquid behavior. |
20–80 % |
| Plastic Limit (PL) |
Moisture content at which soil changes from semi‑solid to plastic. |
5–30 % |
| Plasticity Index (PI) |
PI = LL – PL; measures plasticity. |
0–50 % |
| Shrinkage Limit (SL) |
Moisture content below which volume no longer changes on drying. |
0–15 % |
Interpretation
- PI < 7 → non‑plastic (silt).
- PI > 7 → plastic (clay).
- High LL & PI → highly compressible, low shear strength.
3. Soil Properties
| Property |
Symbol |
Units |
Typical Test |
| Unit Weight (γ) |
γ = w / V |
kN/m³ |
Moisture‑density (Proctor) test |
| Specific Gravity (Gₛ) |
– |
– |
Pycnometer test |
| Porosity (n) |
n = Vᵥ / Vₜ |
% |
Derived from γ and Gₛ |
| Void Ratio (e) |
e = Vᵥ / Vₛ |
– |
e = n / (1‑n) |
| **Water Content ( |
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