Soil Mechanics: Atterberg Limits, Soil Classification & In-situ Density

Liquid Limit Apparatus & Grooving Tools

• Casagrande cup device with crank, cam-follower, brass cup, hard-rubber base.
• Key dimensions (approx.): cup Ø ext{ ≈ }50\,\text{mm}, drop height ext{ ≈ }10\,\text{mm}, hard-rubber base thickness \sim12\,\text{mm}.
• Grooving tools supplied:
 ◦ Standard (ASTM / BS) tool for ordinary soils.
 ◦ Special narrow tool for silts or very soft clays.
• Handle, adjusting screw and brass cup identified in Fig. 2.7.

Liquid-Limit (LL) Test Procedure (Casagrande method)

• Sample preparation:
 ◦ Oven-dry soil, sieve on BS No. 36 (0.425 mm); discard coarser fraction.
 ◦ Mix \approx100\,\text{g} soil with distilled water to putty-like consistency.
• Place soil in cup (depth \approx10\,\text{mm}); strike off surface level.
• Cut groove through centre line of cam-follower with grooving tool.
• Crank at 2 rev s⁻¹; record blows (N) to close groove 12\,\text{mm} (½ in.) along bottom.
• Collect \approx15\,\text{g} of tested soil → weigh wet, oven-dry at 105!\unicode{x2013}110^\circ\text{C}, determine moisture content w\,(\%).
• Repeat at least 5 times: 3 results with 15\le N\le25 and 2–3 with 25<N\le40.
• Plot w (natural scale) vs. N (log scale); read w at N=25 ⇒ Liquid Limit.

Plastic Limit (PL) & Plasticity Index (PI)

• Plastic Limit: water content at which soil begins to crumble when rolled into 3\,\text{mm} (⅛ in.) threads.
• Plasticity Index: PI = LL - PL.
• Moisture content determination identical to LL test.

Plasticity Concepts & Indices

• Index limits:
 ◦ Low plasticity (USCS): LL

Unified Soil Classification System (USCS)

• Primary symbols:
 ◦ G = Gravel, S = Sand, M = Silt, C = Clay, O = Organic, Pt = Peat.
• Secondary symbols:
 ◦ W Well-graded, P Poorly-graded, M w/ non-plastic fines, C w/ plastic fines.
• Plasticity modifiers for fine soils: L (low plasticity), H (high plasticity).
• Coarse-grained criteria (>$50\%$ retained on No. 200, 0.063 mm):
 ◦ Well-graded gravels GW: Cu>4 and 1

British Soil Classification System (BS 5930)

• Two-stage coding: main term + qualifiers.
• Main groups:
 ◦ Coarse soils – Gravels (G), Sands (S).
 ◦ Fine soils – Silts (M), Clays (C).
 ◦ Organic (O), Peat (Pt).
• Fines content:
 ◦ Slightly silty/clayey (0–5 %), Silty/Clayey (5–15 %), Very silty/clayey (15–35 %).
• Plasticity qualifiers for fine soils: L, I, H, V, E (see ranges above).
• Grading qualifiers for coarse soils:
 ◦ W Well-graded, P Poorly-graded, Pu Uniform, Pg Gap-graded.
• Example code: G\,F\,CH ⇒ gravelly very clayey soil, high plasticity clay fraction.
• Plasticity chart for BS (Fig. 1.6) mirrors Casagrande but with BS ranges.

Particle-Size Distribution (PSD)

• Size scale (BS):
 ◦ Clay <0.002\,\text{mm}, Silt 0.002\unicode{x2013}0.06\,\text{mm}, Sand 0.06\unicode{x2013}2\,\text{mm}, Gravel 2\unicode{x2013}60\,\text{mm}, Cobbles 60\unicode{x2013}200\,\text{mm}.
• Plot: % passing vs. particle size (logarithmic abscissa) – reference curve provided (Page 7).

In-Situ Dry Density by Sand-Replacement (BS 1377 §14A/14B)

Scope & Applicability

• Soils with \ge90\% passing 25 mm sieve; compacted layers \le200\,\text{mm} thick.
• Combines BS 14A (small-hole) & 14B (medium-hole); adapted for student lab.

Apparatus

• Pouring cylinder with shutter & conical base.
• Calibration can: Ø 150\,\text{mm}, height 200\,\text{mm} (medium kit).
• Glass plate (≈50 × 50 cm), metal tray with central 150 mm hole, excavation tools.
• Oven 105!\unicode{x2013}110^\circ\text{C}, metal tins with lids, 1 g-readable balance.
• Standard dry sand: passes 0.6 mm, retained on 0.3 mm sieve; uniform & air-dry.

Calibration

1. Constant initial mass in cylinder: W.
2. Determine mean sand mass in cone: W_c.
3. Find volume of calibration can V_c (weigh water).
4. Dispense sand into can, weigh dispensed mass W_s.
5. Bulk density of sand: \gammas = \dfrac{Ws}{V_c}\,(\text{g cm}^{-3}).

Field Test Steps

1. Prepare flat test area ≈45 cm square.
2. Place tray, cut cylindrical hole Ø ≈150 mm, depth ≤200 mm; collect all excavated soil.
3. Remove tray; position filled cylinder centrally, open shutter, allow sand to flow (no vibration).
4. Close shutter, weigh cylinder ⇒ final weight W_f.
5. Mass of sand in hole: Wh = (W - Wf) - W_c.
6. Volume of hole: Vh = \dfrac{Wh}{\gamma_s}.
7. Weigh wet excavated soil W_{wet}; determine moisture content M by oven-drying (or Speedy moisture meter).
8. Dry density: \rhod = \dfrac{W{dry}}{Vh} = \dfrac{W{wet}}{V_h\,(1+M)}\,(\text{g cm}^{-3}).

Data-Sheet Headings (Page 8)

• Calibration: W,\;Wf,\;Wc,\;Ws,\;Vc,\;\gammas. • Field: W{cy\,0},\;W{cy\,1},\;Wh,\;W{wet},\;Vh,\;\rhob,\;M,\;\rhod.

Laboratory Recording – Liquid-Limit Sheet (Pages 10, 13)

• Columns: sample #, blows N, tin mass, wet mass W2, dry mass W3, water mass (W2-W3), dry-soil mass (W3-W1), moisture content w, \log N.
• Plot w vs. \log N; w at N=25 ⇒ LL.
• Calculate PI,\,CI as defined above.

Key Formulas (LaTeX form)

• Plasticity Index: PI = LL - PL.
• Consistency Index: CI = \dfrac{LL - w}{PI}.
• Coefficient of Uniformity: Cu = \dfrac{D{60}}{D{10}}. • Coefficient of Curvature: Cc = \dfrac{D{30}^2}{D{10}\,D{60}}. • Moisture content (mass basis): M = \dfrac{W{water}}{W{dry}}. • Bulk density of sand: \gammas = \dfrac{Ws}{Vc}.
• Hole volume: Vh = \dfrac{(W - Wf) - Wc}{\gammas}.
• Dry density of soil: \rhod = \dfrac{W{dry}}{Vh},  Bulk density: \rhob = \dfrac{W{wet}}{Vh}.

Practical & Interpretive Notes

• Avoid vibration during sand pouring – compaction alters \gamma_s.
• Clean & dry device after each LL trial to prevent adhesion effects.
• For LL graph, use linear scale for moisture, log for blows – yields near-straight line.
• Dual USCS symbols flag borderline gradations (e.g. SM!\unicode{x2013}SC, GW!\unicode{x2013}GM).
• Organic soils given suffix O (USCS) or ‘‘O’’ descriptor (BS). Peat always Pt.
• High LL (>90 %) in BS system classified ‘‘E’’ – extremely high plasticity.
• Sand replacement not suitable for gap-graded gravelly fills; use core cutter or nuclear gauge instead.
• Calibration must be repeated if sand batch, moisture, or apparatus changes.