IS 2720:1980 Part 7 is the Indian Standard (BIS) for methods of test for soils - determination of water content-dry density relation using light compaction. This standard specifies the laboratory methods for determining the specific gravity of soil solids. It covers procedures for both fine-grained soils (using a pycnometer or density bottle) and coarse-grained soils (using a gas jar). This fundamental soil property is essential for calculating phase relationships like void ratio and degree of saturation.
Describes the procedure for determining the optimum moisture content and maximum dry density of soil using the light compaction (Standard Proctor) test.
Standard (light) compaction — mould, rammer, drop, blows, layers and energy used to obtain OMC / MDD.
| Reference | Value | Clause |
|---|---|---|
| Test | Standard Proctor compaction (light compaction) | Cl. 1 |
| Mould volume — small (≤ 4.75 mm) | 1000 cm³ (100 mm dia × 127.3 mm) | Cl. 4.1 |
| Mould volume — large (≤ 19 mm material) | 2250 cm³ (150 mm dia × 127.3 mm) | Cl. 4.1 |
| Rammer mass | 2.6 kg | Cl. 4.2 |
| Rammer drop | 310 mm | Cl. 4.2 |
| Rammer face diameter | 50 mm | Cl. 4.2 |
| Layers | 3 | Cl. 5.3 |
| Blows per layer — small mould | 25 | Cl. 5.3 |
| Blows per layer — large mould | 56 | Cl. 5.3 |
| Compactive energy | ≈ 595 kJ/m³ (light) | Cl. 5.3 |
| Number of points (compaction curve) | ≥ 4 (varying water content) | Cl. 5.4 |
| Water content increment (between trials) | ≈ 2 % between points | Cl. 5.4 |
| Result — OMC and MDD | Peak of dry density vs water content curve | Cl. 6 |
| Zero air voids line | γd = γw·G / (1 + w·G); G = specific gravity | Cl. 6 |
| Specimen retained for next layer — protocol | Material reused or fresh per IS guidance | Cl. 5 |
| Reporting precision — γd | 0.01 g/cc | Cl. 7 |
| Reporting precision — w | 0.1 % | Cl. 7 |
IS 2720 (Part 7):1980 is the Method of Test for Soils — Determination of Water Content–Dry Density Relation Using Light Compaction — the Indian version of the Standard Proctor compaction test. It establishes the Optimum Moisture Content (OMC) and Maximum Dry Density (MDD) of a soil for the standard compactive effort.
Use it when: - Designing earthen embankments, dams, road subgrades — fill must achieve a specified % of MDD - Setting backfill specifications for foundations, retaining walls, services trenches - Approving borrow material — different soils have different OMC and MDD; lab tests on each source - Quality control during placement — field density (sand replacement IS 2720 Part 28 or core cutter IS 2720 Part 29) compared to MDD from Part 7
For heavier compactive effort (modern road/airport pavement work, large dams), use IS 2720 Part 8:1983 (Modified Proctor) which uses ~4.5× the energy per unit volume. Part 7 (Standard) is for most building, light embankment, and pipe-bedding applications; Part 8 (Modified) is for roads, runways, dams, and heavy structures.
Apparatus (Clause 3): - Cylindrical mould — 1000 cm³ volume (100 mm dia × 127.3 mm height), or 2250 cm³ for coarser soils - Rammer — 2.6 kg mass, 310 mm drop height (this is the 'Standard' effort; Modified uses 4.9 kg + 450 mm) - Three layers of soil, 25 blows per layer (per 1000 cm³ mould)
Procedure:
1. Sample prep: air-dry the soil, break clods, sieve through 19 mm. Take 16-20 kg representative sample.
2. Add water: start at estimated 'dry side' — typically 2-3% below estimated OMC. Mix thoroughly; seal in container; let cure 30 minutes for moisture distribution.
3. Compact in 3 equal layers in the mould; 25 blows of the 2.6 kg rammer falling 310 mm per layer. Total energy ≈ 593 kJ/m³.
4. Trim the top to mould level. Weigh the mould + soil; subtract empty mould weight; divide by mould volume → bulk density γ_bulk.
5. Sample for moisture content — take ~50 g from middle of compacted mass; oven-dry at 105°C for 24 hours; compute water content w (%).
6. Dry density: γ_d = γ_bulk / (1 + w/100)
7. Repeat at progressively higher water contents (typically 2% increments) for 5-6 points spanning dry-of-OMC to wet-of-OMC.
8. Plot γ_d vs w. The curve peaks at OMC; the peak γ_d is the MDD. Also plot the zero-air-voids (ZAV) curve as a check — your compaction curve must fall below the ZAV line (you can't compact wetter than ZAV without expelling air).
Typical results for Indian soils: - Sandy soil (SP/SM): OMC ≈ 8-12%, MDD ≈ 1.85-2.00 g/cm³ - Silty/clayey sand (SC/SM): OMC ≈ 10-14%, MDD ≈ 1.80-1.95 g/cm³ - Lean clay (CL): OMC ≈ 15-22%, MDD ≈ 1.70-1.85 g/cm³ - Fat clay (CH) (black cotton): OMC ≈ 22-32%, MDD ≈ 1.45-1.65 g/cm³ (significantly lower due to plate-like particles + high water demand)
The OMC and MDD from Part 7 are the reference; field density (measured by IS 2720 Parts 28 or 29) is compared as a percentage:
Typical specifications (Indian practice): - Building backfill (granular): ≥ 95% of Standard Proctor MDD - Pipe bedding and trench backfill: ≥ 90% of Standard Proctor MDD - Embankment for highways (MoRTH 5th Revision, Section 305): ≥ 95% of Modified Proctor MDD (note: MODIFIED, not Standard — for roads) - Earthen dam core: ≥ 98% of Standard Proctor MDD with strict moisture control at OMC + 1% (for clay cores) - Earthen dam shoulders: ≥ 95% of Modified Proctor MDD - Subgrade for flexible pavement (IRC 37): MDD reference is Modified Proctor; OMC ± 2% tolerance
Why moisture control matters: - Dry of OMC: dense soil but with macro-voids; future wetting causes settlement (collapse) - At OMC: maximum dry density achieved; water films lubricate particles into densest packing - Wet of OMC: water occupies voids that should be solids; lower density; expulsion of water on loading causes consolidation - Way wet of OMC: 'pumping' — soil becomes saturated under construction traffic, loses bearing capacity
1. Wrong compactive effort — Standard Proctor (IS 2720 Part 7) gives different MDD than Modified Proctor (Part 8) for the same soil. Modified gives ~5-15% higher MDD and ~2-4% lower OMC. Confirm which Proctor your specification references before designing or testing. MoRTH and IRC use Modified; many building codes use Standard.
2. Re-using soil between compactions — each Proctor point must use fresh soil. Re-using soil that has been compacted breaks down particle structure and gives artificially higher density (false-high MDD). Discard each point's soil; mix new soil for the next.
3. No ZAV cross-check — the zero-air-voids line is the upper bound of physical possibility (γ_d at given w with all air expelled). If your test compaction curve plots ABOVE the ZAV line, the data are wrong (oven-dry weight error or volume error). Always plot ZAV alongside.
4. Insufficient points or wrong range — minimum 5 points spanning dry-to-wet of OMC. Many labs do only 3 points and miss the OMC. The curve must show a clear peak; if it doesn't, the test range is too narrow.
5. Wrong sample weight for moisture content — for fine-grained soils, ~50 g sample is OK. For coarse-grained (gravel), use at least 200-500 g — small gravel samples have high moisture-content variability. IS 2720 Part 2 (water content) prescribes minimum sample sizes by soil type.
6. Field density-to-MDD comparison without correction for over-size aggregate — if field soil has gravel > 19 mm (the mould sieve cut-off), the lab MDD applies only to the matrix. Correct field density per Bureau of Reclamation 7.4 method or AASHTO T 224 for the over-size fraction. Many sites skip this; field density appears low simply because of stones, not poor compaction.
7. Black cotton soil tested wet — fat clays are highly water-sensitive. The OMC determination requires precise moisture control (± 0.5%); ordinary site labs often can't achieve this. For black cotton subgrade and embankment compaction, use specialty geotechnical labs.
IS 2720 Part 7:1980 is 45 years old and methodologically aligned with ASTM D698 (Standard Proctor) and BS 1377 Part 4. The method is mature; revisions would be cosmetic.
Real-world testing: Standard Proctor is the routine compaction test for building work, light embankments, and pipe bedding. Modified Proctor (Part 8) dominates highway, airport, and dam work. Most Indian site labs can run Part 7 in 1-2 days per soil source; Part 8 takes the same time but is slightly more equipment-intensive.
Field-density to MDD comparison reality: site QC for compaction is usually done by nuclear density gauge (IS 11315) — faster (minutes per reading vs hours for sand replacement), allows hundreds of readings per day on highway / runway work. The gauge correlates with MDD via field calibration. Sand-replacement (Part 28) is the referee test when nuclear readings are disputed.
For black cotton soil regions (Maharashtra, MP, Karnataka, parts of TN): Standard/Modified Proctor compaction at OMC alone isn't enough — black cotton swells on wetting. Specifications typically also impose CBR ≥ 4-5% (IS 2720 Part 16) and soaked CBR check to ensure post-monsoon performance. IS 13094:1992 (foundations on expansive soils) has further guidance.
Saturated CBR vs Standard Proctor: a common confusion — these are different tests. Proctor gives MDD/OMC. CBR (IS 2720 Part 16) tests bearing capacity AT a specified compaction (typically 97% of Modified Proctor). They are complementary, not interchangeable.