CELE Geotechnical Engineering — CompactionStudy Notes

Soil compaction is the process of mechanically densifying soil by reducing air voids while maintaining or adjusting water content. The primary objectives are: • **Increase shear strength** — higher dry unit weight improves bearing capacity and slope stability. • **Reduce settlement** — denser soil exhibits lower compressibility and post-construction settlement. • **Decrease permeability** — reduced pore space restricts water flow, critical for embankments and liners. • **Improve durability** — higher density reduces void space available for water infiltration and weathering. **The Compaction Process** When mechanical force (impact, vibration, kneading) is applied to soil, air is expelled and soil particles rearrange into a denser configuration. The effectiveness of compaction depends on: 1. **Water content (w)** — acts as a lubricant; too dry (particles locked), too wet (water incompressible). 2. **Compaction energy (E)** — work per unit volume; standard Proctor vs modified Proctor. 3. **Soil type** — clay vs sand responds differently; fine-grained soils show pronounced peaks. **Key Distinction: Wet vs Dry of Optimum** • **Dry of optimum (w < OMC)** — insufficient water lubricates; high friction prevents efficient rearrangement; γd increases slowly with w. • **At optimum (w = OMC)** — water lubricates efficiently; maximum dry unit weight (γd,max) achieved. • **Wet of optimum (w > OMC)** — excess water incompressible; replaces soil solids; γd decreases with further w increase.

The Proctor test (named after R.R. Proctor, 1933) is the laboratory standard for determining the compaction characteristics of soil: **maximum dry unit weight (γd,max)** and **optimum moisture content (OMC)**. **Standard Proctor Test (ASTM D698)** Procedure: • Mold: 944 cm³ (1/30 ft³) • Hammer: 2.49 kg, fall height 305 mm • Compaction: 25 blows/layer × 3 layers = 75 blows total • Energy per unit volume: E ≈ 600 kJ/m³ Typical results: • γd,max ≈ 17–18 kN/m³ (for clay) • OMC ≈ 14–18% (for clay) **Modified Proctor Test (ASTM D1557)** Procedure: • Mold: 944 cm³ (same) • Hammer: 4.54 kg, fall height 457 mm • Compaction: 25 blows/layer × 5 layers = 125 blows total • Energy per unit volume: E ≈ 2,700 kJ/m³ (≈ 4.5 × standard) Typical results: • γd,max ≈ 18–20 kN/m³ (for clay) • OMC ≈ 10–14% (for clay) **Key Differences** | Property | Standard Proctor | Modified Proctor | |----------|------------------|------------------| | Hammer mass | 2.49 kg | 4.54 kg | | Fall height | 305 mm | 457 mm | | Blows per layer | 25 | 25 | | Layers | 3 | 5 | | Total energy | ~600 kJ/m³ | ~2,700 kJ/m³ | | γd,max | Lower | Higher (typically 5–10%) | | OMC | Higher | Lower | | Use | Older specifications, fine-grained fills | Modern road, embankment, pavement subgrades | **The Compaction Curve** A soil is compacted at 4–6 different water contents (e.g., 8%, 10%, 12%, 14%, 16%, 18%). For each w: 1. Mold + soil + water are mixed to uniform w. 2. Mold is compacted per test procedure. 3. Moist weight is measured; γ = mass/volume. 4. Sample is dried; dry unit weight γd = γ/(1+w) is calculated. 5. Points (w, γd) are plotted. The curve is **unimodal, concave, and smooth**: • Dry side (w < OMC): γd rises with w (water lubricates). • Peak: γd = γd,max at w = OMC. • Wet side (w > OMC): γd falls with w (water replaces solids). **Why Modified Proctor Lowers OMC** Higher compaction energy achieves denser packing at lower water content. The water required to lubricate particles is less when more mechanical energy is supplied. Thus, the peak shifts left (lower OMC) and up (higher γd,max).

In compaction practice, the relationship between moist unit weight (γ), water content (w), and dry unit weight (γd) is fundamental. **Derivation** A soil element has: • Mass of solids: Ms • Mass of water: Mw = w·Ms (where w is water content ratio) • Total mass: M = Ms + Mw = Ms(1 + w) • Volume: V (total, including voids) Dry unit weight: γd = Ms/V Moist unit weight: γ = M/V = Ms(1+w)/V = γd(1+w) Rearranging: $$\gamma_d = \frac{\gamma}{1+w}$$ or equivalently: $$\gamma = \gamma_d(1+w)$$ **Practical Implications** 1. **γd is independent of water content assumption** — If you measure γ and w in the field, γd follows uniquely; no ambiguity. 2. **Compaction control uses γd, not γ** — A fill at γ = 21 kN/m³ (w = 12%) and γ = 21 kN/m³ (w = 8%) have different γd values; only γd comparison is valid. 3. **Unit weight changes with w** — As water content increases, γ changes, but γd decreases if w exceeds OMC. **Saturated vs Dry Unit Weight** At **saturation (S = 100%)**, all pores filled with water: $$\gamma_{sat} = \frac{G_s\gamma_w + w_s\gamma_w}{1+e} = \frac{(G_s+e)\gamma_w}{1+e}$$ where $w_s = e/G_s$ is the **saturation moisture content** (unique for a given e and Gs). For compaction, we work with unsaturated soil (S < 100%) at the lab moisture content w. Field γd is always lower than or equal to γsat (unless soil is driven to full saturation post-compaction).

**Relative Compaction Definition** Relative compaction (RC) is the ratio of achieved field dry unit weight to laboratory maximum dry unit weight, expressed as a percentage: $$RC = \frac{\gamma_{d,\text{field}}}{\gamma_{d,\max}} \times 100\%$$ where: • γd,field = dry unit weight measured in the field (by sand cone, nuclear density, or core extraction) • γd,max = maximum dry unit weight from standard or modified Proctor test **Interpretation** • **RC = 100%** — Perfect match to lab maximum; theoretically unachievable (field conditions differ). • **RC = 90–95%** — Typical target for embankments, road subgrades, and structural fills (NSCP 2015, DPWH specifications). • **RC < 90%** — Inadequate compaction; may not meet design strength/settlement criteria; rework required. • **RC > 95%** — Excellent compaction; rare in practice due to field variability; indicates high-quality work. **Why Field RC Differs from Lab** 1. **Grain size segregation** — coarse particles migrate outward during placement and compaction. 2. **Layer thickness variation** — uneven lift height affects energy distribution. 3. **Equipment efficiency** — different compactors (drum roller, plate, hand) apply energy unevenly. 4. **Moisture loss** — water evaporates between mixing and measurement; field w may differ from intended w. 5. **Soil variation** — in-situ material may differ from lab sample (borrow pit heterogeneity). **Field Measurement Methods** 1. **Sand Cone (ASTM D1556)** — measure in-place volume using calibrated sand; most common, inexpensive. 2. **Nuclear Density Gauge (ASTM D2922)** — rapid, non-destructive; requires operator certification (ASNT LEVEL III typically required in Philippines). 3. **Core Sampling (ASTM D1433)** — extract soil sample, measure volume; destructive but accurate for verification. 4. **Balloon Method (ASTM D2167)** — alternative volume measurement; less common in modern practice. **Philippine Code Requirements (NSCP 2015)** For structural fills and road embankments: • Minimum RC = 90% (general fills) • Minimum RC = 95% for pavement subgrades and foundation fills • Test frequency: minimum 1 test per 500 m³ of fill or per 2,000 m² area • Multiple tests per lift recommended (edge, center, depth variation) **Statistical Control** When testing multiple locations per lift: • **Mean RC** — average of all tests should meet spec (e.g., ≥95%). • **Standard deviation** — limit scatter (e.g., standard deviation ≤3% to ensure consistency). • **Individual test** — each test should be within acceptable range (e.g., not below 92% if spec is 95%).

**Definition and Theory** The **zero-air-voids line (ZAV)** represents the theoretical dry unit weight of soil fully saturated (S = 100%) with no air trapped in pores at a given water content and specific gravity of solids. Formula: $$\gamma_{zav} = \frac{G_s\gamma_w}{1+w\cdot G_s}$$ where: • Gs = specific gravity of soil solids (dimensionless; typically 2.65–2.75 for silica-based soils) • γw = unit weight of water = 9.81 kN/m³ (at 4°C) • w = water content (as decimal, e.g., 0.12 for 12%) **Derivation** For a soil element: • Volume of solids: Vs = Ms / (Gs · γw) • Volume of water: Vw = w · Ms / γw • Total volume (saturated, no air): V = Vs + Vw = Ms / (Gs · γw) + w · Ms / γw = Ms(1 + w · Gs) / (Gs · γw) Dry unit weight: $$\gamma_{zav} = \frac{M_s}{V} = \frac{M_s \cdot G_s \cdot \gamma_w}{M_s(1+w \cdot G_s)} = \frac{G_s\gamma_w}{1+w \cdot G_s}$$ **Physical Meaning** The ZAV line is the **upper bound** for the compaction curve: • At any given w, the actual γd of a compacted (unsaturated) soil is **always below** the ZAV line because real soil always contains trapped air (S < 100%). • The vertical distance between the compaction curve and the ZAV line is the **air-voids content** (air volume per unit volume). • If a calculated or measured point plots **above** the ZAV line, it is impossible (violates conservation of mass and volume); recheck data. **Properties of the ZAV Line** 1. **Shape** — The ZAV line is a smooth, decreasing curve (γzav decreases as w increases). 2. **Concavity** — Lies above the compaction curve, which is concave upward. 3. **Independence from compaction energy** — ZAV line depends only on Gs and w, not on compaction method. 4. **Parallelism in practice** — For fine-grained soils, the compaction curve often approaches a shape roughly "parallel" to ZAV at higher w values. **Application in Field Control** 1. **Feasibility check** — If a specification requires RC = 100%, it is theoretically unachievable; the field point will always lie below ZAV. 2. **Density assessment** — Comparison of field γd to ZAV line (not just to γd,max) indicates air content: - Close to ZAV → low air content, well-compacted. - Far below ZAV → high air content, loose or incompletely compacted. 3. **Moisture sensitivity** — As w increases: - ZAV line shifts downward (γzav decreases). - Compaction curve shifts right (OMC increases slightly with energy). - Gap between curve and ZAV line changes, affecting achievable RC. **Example: ZAV Line at Multiple Water Contents** For Gs = 2.70, γw = 9.81 kN/m³: | w (%) | 1 + w·Gs | γzav (kN/m³) | |-------|----------|-------------| | 8 | 1.216 | 21.68 | | 10 | 1.270 | 20.81 | | 12 | 1.324 | 19.99 | | 14 | 1.378 | 19.22 | | 16 | 1.432 | 18.49 | | 18 | 1.486 | 17.81 | Note: As w increases, γzav decreases (more water, less solid per unit volume).

Compaction behavior differs significantly between sand/gravel (coarse-grained) and clay/silt (fine-grained) soils due to differences in particle interaction and water role. **Fine-Grained Soils (Clay, Silt)** *Characteristics:* • Significant water attraction (cohesion); water acts as lubricant between clay platelets. • Compaction curve shows **pronounced, distinct peak** at OMC. • Dry-side (w < OMC): water increases lubrication, γd rises steeply. • Wet-side (w > OMC): excess water replaces solids, γd falls sharply. • **Standard Proctor commonly used** for clay fills, embankments, and liners. *Typical Values:* • γd,max ≈ 17–19 kN/m³ (standard), 18–21 kN/m³ (modified) • OMC ≈ 12–18% (standard), 8–14% (modified) • OMC sensitive to soil type and compaction energy. *Control Method:* • **Relative Compaction (RC)** — field γd / lab γd,max; standard approach. • Specifications: RC ≥ 90–95% (NSCP 2015 embankment specs). **Coarse-Grained Soils (Sand, Gravel)** *Characteristics:* • Water acts as lubricant between grains, but effect is weak (no electrostatic cohesion). • Compaction curve is **flat or broad, with weak or absent peak**. • Dry condition: compaction difficult due to friction between particles. • Slight moisture (5–10%) lubricates; γd increases gently. • Further moisture increase has little effect on γd (curve flattens). • **Saturation or near-saturation** preferred for dense packing (vibration compaction). *Typical Values:* • γd,max ≈ 16–18 kN/m³ (dry sand); can reach 18–20 kN/m³ (saturated, vibrated) • OMC ≈ 5–10% (if meaningful); often water content is not critical. • Relative compaction may not be appropriate control metric. *Control Method:* • **Relative Density (Dr)**, not relative compaction: $$D_r = \frac{e_{\max} - e_{\text{field}}}{e_{\max} - e_{\min}} \times 100\%$$ where emax is void ratio in loosest state, emin in densest state, and efield in field. • Specifications: Dr ≥ 70–80% (dense sand for foundations). • Often controlled by vibratory compaction (roller, plate); water content less critical. **Comparison Table** | Property | Fine-Grained (Clay) | Coarse-Grained (Sand) | |----------|-------------------|----------------------| | Compaction curve | Sharp, distinct peak | Flat, broad, no peak | | OMC | Clear, typically 10–18% | Weak or undefined | | γd,max sensitivity to w | High | Low | | Control metric | Relative Compaction (RC) | Relative Density (Dr) | | Specification | RC ≥ 90–95% | Dr ≥ 70–80% | | Compaction method | Tamping, kneading roller | Vibration, plate compactor | | Water role | Lubricant, major effect | Lubricant, minor effect | | Field challenge | Moisture management (rain) | Particle size segregation | **Practical Considerations in the Philippines** 1. **Monsoon impact** — Heavy rainfall increases w in clay fills (wet of optimum); effectiveness decreases. In sandy fills, drainage is better; relative density control is more effective. 2. **Borrow pit variation** — Clay fills may have broad range of OMC; multiple Proctor tests recommended across borrow area (different clay types). 3. **Vibration vs tamping** — Modern embankment construction uses **vibrating roller** (effective for both clay and sand); standard or modified Proctor may not reflect actual field compaction method. Some engineers use **VRAM (Vibratory Roller Assessment Method)** or equipment-specific calibration. 4. **Tropical soils** — Laterite and residual soils common in Philippines; compaction characteristics may differ from standard clay/sand models; lab testing of local material essential.

**Pitfall 1: Confusing Compaction with Natural Consolidation** Students sometimes think compaction and consolidation are the same. They are not: • **Compaction** = rapid, mechanical expulsion of air by applied energy (minutes to hours). • **Consolidation** = slow, hydraulic expulsion of water from saturated soil due to applied load (days to years). Compaction is a **construction process**; consolidation is a **post-construction process**. **Pitfall 2: Using Moist (γ) Instead of Dry (γd) Unit Weight** A critical error in field control: • Specification: RC ≥ 95% • Student incorrectly uses: γfield / γ,max (both moist) × 100% • Correct use: γd,field / γd,max × 100% Misusing moist weight invalidates the control (compaction at different w values will have different γ even if γd is the same). **Pitfall 3: Incorrectly Calculating γd** Common algebra error: • Wrong: γd = γ − w (subtracting w as a value, not ratio). • Correct: γd = γ / (1 + w), where w is a decimal (0.12 for 12%), not percent. Always convert % to decimal before calculation; use parentheses: γd = γ / (1 + 0.12). **Pitfall 4: Assuming Modified Proctor Always Better** Modified Proctor is NOT "better"; it reflects **higher field compaction energy**: • Standard Proctor: lower γd,max, higher OMC → appropriate if field equipment applies lower energy (e.g., manual tamping). • Modified Proctor: higher γd,max, lower OMC → appropriate if field equipment applies higher energy (e.g., vibrating roller). Choose the Proctor variant that **matches the field compaction method**. Specifying modified Proctor for hand-tamped fill is unrealistic. **Pitfall 5: Plotting Point Above Zero-Air-Voids Line** If a calculated point (w, γd) plots above the ZAV line: • **This is impossible** — indicates measurement or calculation error. • Recheck: w reading, γ measurement, Gs value, ZAV formula. • Do NOT accept the data; return to field and verify. **Pitfall 6: Ignoring Soil Variation (Borrow Pit Heterogeneity)** A borrow pit may contain two soil types (clay of different origin, mixed sand/gravel): • Lab Proctor test on one composite sample may not represent all material. • Solution: Proctor test **multiple samples from different pit locations**; identify OMC range. • Field compaction specs may need **minimum and maximum OMC** to accommodate heterogeneity. **Pitfall 7: Misinterpreting RC > 100%** If field RC = 102% (field γd > γd,max from lab): • Not a "better than spec" result. • Indicates either: 1. Different soil than lab sample (higher Gs, naturally denser), 2. Lab test error (γd,max underestimated), or 3. Field measurement error (γd overestimated). • **Recommend retest** lab and field samples; investigate discrepancy. **Pitfall 8: Water Content as Percentage vs Decimal** If w = 15% (15 percent): • As decimal: w = 0.15 • Common error: using w = 15 (the number) in formula → large calculation error. • Always: γd = γ / (1 + w_decimal) = γ / (1 + 0.15) **Exam Strategy Tips** 1. **Identify the soil type immediately** — Is it clay (RC control) or sand (Dr control)? Read problem statement. 2. **List given data carefully** — Separate γ (moist), γd (dry), w (%), w (decimal), OMC, γd,max, Gs, γw. 3. **Check units and convert** — Ensure all γ in kN/m³ or consistent units; w as decimal in formulas. 4. **Sketch the compaction curve** — If given data points, plot (w, γd) and identify peak location. 5. **Calculate ZAV line points** — Check that lab and field data lie below ZAV; if not, flag error. 6. **Interpret RC qualitatively** — RC = 92% is "good," 98% is "excellent," 87% is "poor" (rework needed). 7. **Read the question twice** — Does it ask γd,max, OMC, RC, or air-voids content? Answer what is asked. 8. **Show all steps clearly** — Partial credit for method, even if arithmetic is off by ~1%; hidden calculations lose full points.