Soil Mechanics - Soil Classification & Phase
Geotechnical Laboratory Suite
ASTM D2487
USCS Group Symbol
CL
Lean Clay
Moisture Content (w)
0.0 %
Void Ratio (e)
0.00
Porosity $n$: 0.0%
Saturation (S)
0.0 %
Casagrande Plasticity Chart (Fines Fraction)
Soil Sample
A-Line
U-Line
Particle Size Fraction Breakdown
| Gravel Fraction (> No. 4) | 0.0 | % |
| Sand Fraction (No. 4 to No. 200) | 0.0 | % |
| Fines Fraction (< No. 200) | 0.0 | % |
| Grading (if applicable) | Cu = -, Cc = - | |
Volumetric 3-Phase Diagram
Phase & Density Relationships
| Bulk Density ($\rho$) | 0.00 | g/cm³ |
| Dry Density ($\rho_d$) | 0.00 | g/cm³ |
| Saturated Density ($\rho_{sat}$) | 0.00 | g/cm³ |
| Buoyant / Submerged ($\rho'$) | 0.00 | g/cm³ |
Estimated Shear Strength
| Relative State | Medium Dense | |
| Effective Cohesion ($c'$) | 0.0 | kPa |
| Effective Friction Angle ($\phi'$) | 30.0 | ° |
| Derived empirically from N₆₀ and USCS. | ||
Soil Mechanics Theory
1. USCS Classification (ASTM D2487)
The Unified Soil Classification System uses a logic tree based on the percentage of material passing the No. 200 sieve (0.075 mm) to separate Coarse-Grained soils (< 50% fines) from Fine-Grained soils ($\ge$ 50% fines).
- Coarse Soils: Classified as Gravel (G) or Sand (S). Further modified as Well-graded (W), Poorly-graded (P), Silty (M), or Clayey (C).
- Fine Soils: Classified by plotting Atterberg Limits on the Plasticity Chart as Silt (M) or Clay (C), and modified by High (H) or Low (L) plasticity.
2. Phase Relationships
Soil is a three-phase material consisting of Solids, Water, and Air. By knowing the total volume ($V$), total mass ($M$), dry mass ($M_s$), and Specific Gravity ($G_s$), all other volumetric and gravimetric properties are derived.
$$V_s = \frac{M_s}{G_s \cdot \rho_w} \quad ; \quad M_w = M - M_s \quad ; \quad V_w = \frac{M_w}{\rho_w}$$
$$e = \frac{V_v}{V_s} \quad ; \quad S = \frac{V_w}{V_v} \quad ; \quad w = \frac{M_w}{M_s}$$
3. Empirical Shear Strength
If direct shear or triaxial lab tests are unavailable, strength parameters can be estimated from field or lab indices.
- SPT Method ($N_{60}$): For coarse soils, $\phi' = \sqrt{20 N_{60}} + 20$ (Hatanaka & Uchida). For fine soils, Undrained Shear Strength $S_u \approx 6 N_{60}$ kPa (Terzaghi & Peck).
- Lab Index Method: For fine soils, the Liquidity Index $LI = (w_n - PL) / PI$ estimates consistency ($LI < 0$ is Hard/Stiff, $LI \ge 1$ is Liquid/Very Soft). For coarse soils, $\phi'$ is conservatively assigned based on the USCS classification (e.g., GW $\approx 38^\circ$, SP $\approx 32^\circ$).
Standard Geotechnical Reference Tables
Typical Soil Parameters
| Soil Description | Eff. Friction φ' (°) | Eff. Cohesion c' (kPa) | γ (kN/m³) |
|---|---|---|---|
| Loose Sand / Gravel | 28 - 30 | 0 | 14 - 16 |
| Med Dense Sand / Gravel | 32 - 36 | 0 | 17 - 19 |
| Dense Sand / Gravel | 36 - 40+ | 0 | 19 - 22 |
| Soft Clay | 0* | 10 - 25 | 14 - 17 |
| Medium Clay | 0* | 25 - 50 | 17 - 19 |
| Stiff / Hard Clay | 0* | 50 - 100+ | 19 - 21 |
* For Undrained (Short-Term) total stress analysis. Drained clay has $\phi' > 0$ and $c' \approx 0$.
Typical Void Ratio & Porosity
| Soil Description | Void Ratio ($e$) | Porosity ($n$) % |
|---|---|---|
| Loose Uniform Sand | 0.80 | 44 |
| Dense Uniform Sand | 0.45 | 31 |
| Soft Spongy Clay | 1.20 - 2.40 | 55 - 70 |
| Stiff Glacial Clay | 0.60 | 37 |
Plasticity Index (PI) States
| Plasticity State | PI Range | Typical Soil |
|---|---|---|
| Non-Plastic | 0 | Sand, Silt |
| Low Plasticity | < 7 | Silt, Lean Clay |
| Medium Plasticity | 7 - 17 | Clayey Silt, Silty Clay |
| High Plasticity | > 17 | Fat Clay (CH) |
USCS Major Divisions (ASTM D2487)
| Major Division | Classification Criteria | Group Symbols |
|---|---|---|
| Coarse-Grained Soils | More than 50% retained on No. 200 sieve | GW, GP, GM, GC, SW, SP, SM, SC |
| Fine-Grained Soils | 50% or more passes No. 200 sieve | ML, CL, OL, MH, CH, OH |
| Highly Organic Soils | Primarily organic matter (Peat) | PT |
USCS Group Symbols Glossary
| Coarse-Grained Soils | |
|---|---|
| GW | Well-graded Gravel |
| GP | Poorly-graded Gravel |
| GM | Silty Gravel |
| GC | Clayey Gravel |
| SW | Well-graded Sand |
| SP | Poorly-graded Sand |
| SM | Silty Sand |
| SC | Clayey Sand |
| Fine-Grained & Highly Organic Soils | |
|---|---|
| ML | Silt (Low Plasticity) |
| CL | Lean Clay (Low Plasticity) |
| OL | Organic Silt/Clay (Low Plasticity) |
| MH | Elastic Silt (High Plasticity) |
| CH | Fat Clay (High Plasticity) |
| OH | Organic Silt/Clay (High Plasticity) |
| PT | Peat |
Prefixes: G = Gravel, S = Sand, M = Silt, C = Clay, O = Organic, Pt = Peat.
Suffixes: W = Well-graded, P = Poorly-graded, M = Silty, C = Clayey, L = Low Plasticity (LL < 50), H = High Plasticity (LL ≥ 50).
Suffixes: W = Well-graded, P = Poorly-graded, M = Silty, C = Clayey, L = Low Plasticity (LL < 50), H = High Plasticity (LL ≥ 50).
References
- ASTM D2487-17. Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System). ASTM International. ASTM.org
- ASTM D4318-17. Standard Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index of Soils. ASTM International. ASTM.org
- Hatanaka, M., & Uchida, A. (1996). Empirical correlation between penetration resistance and internal friction angle of sandy soils. Soils and Foundations, 36(4), 1-9.
- Das, B. M. (2010). Principles of Geotechnical Engineering (7th ed.). Cengage Learning. Google Scholar
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