IS 2720 Soil Testing Complete Guide — All Parts Quick Reference
If you’re tendering a soil-investigation BoQ, classifying a fill material, or arguing with a contractor about CBR test results, you need IS 2720. The series covers every routine soil test used in Indian civil engineering — from basic water content to advanced triaxial shear — across 41 separate parts. Most engineers know parts 4 (sieve), 5 (Atterberg), 7 (Proctor) and 16 (CBR) cold and forget the rest exists. Then a project demands consolidation testing, organic matter determination, or specific gravity of fines, and the lookup begins. This guide is the complete IS 2720 part-by-part reference: what each part covers, sample size required, equipment, and where it’s used.
The IS 2720 series at a glance
IS 2720 — Methods of test for soils — is published by BIS in 41+ separate parts. Each part covers one test or test category. The series is the canonical reference cited by:
- IS 456 for foundation design soil parameters
- IS 1893 for site classification (seismic)
- IRC 37 for flexible pavement subgrade testing
- IRC 78 for bridge foundation soil parameters
- MoRTH specifications for road construction quality control
Frequently-used parts (these cover 80% of routine work)
| Part | Test | Used for |
|---|---|---|
| 1 | Sample preparation for testing | Air-drying, quartering, sub-sampling — applies to all subsequent tests |
| 2 | Determination of water content | Daily soil compaction QA, moisture-density correlation |
| 3 | Specific gravity | Phase relationship calculations, void ratio, dry density |
| 4 | Grain size analysis (sieve + hydrometer) | Soil classification, filter design, gradation specs |
| 5 | Atterberg limits (LL, PL, SL) | Plasticity classification, swell potential assessment |
| 7 | Proctor compaction (light, 2.6 kg) | Subgrade compaction control for roads |
| 8 | Modified Proctor (heavy, 4.9 kg) | Sub-base, base course compaction control |
| 9 | Density determination — sand replacement | Field compaction acceptance test |
| 10 | Density — core cutter | Field compaction in cohesive soils |
| 11 | Density — water displacement | Cohesive samples in lab |
| 13 | Direct shear test | Shear strength of cohesionless soils |
| 14 | Standard penetration test (SPT) corrections | Subsurface investigation, foundation design |
| 15 | Consolidation test (oedometer) | Settlement prediction for clays |
| 16 | California Bearing Ratio (CBR) | Pavement subgrade strength, IRC 37 design input |
| 17 | Permeability (constant head) | Coarse soils, drainage design |
| 18 | Permeability (falling head) | Fine soils |
| 22 | Organic matter content | Subgrade quality, fill rejection criteria |
| 27 | Standard Proctor — fine soil | Variant of Part 7 for finer materials |
| 30 | Density — replacement (large hole) | Coarse-grained fills, embankments |
| 40 | Free swell index | Black cotton soil identification |
| 41 | Field swelling pressure | BC soil foundation design |
Sample sizes — how much soil do you actually need?
The most common BoQ mistake: under-quoting on disturbed sample collection because no one read IS 2720 Part 1.
| Test | IS 2720 part | Minimum sample size |
|---|---|---|
| Water content | 2 | 10–200 g (depending on max grain size) |
| Specific gravity | 3 | 30–50 g |
| Grain size (sieve only, sand) | 4 | 500 g |
| Grain size (sieve + hydrometer, mixed) | 4 | 1.5 kg |
| Atterberg limits | 5 | 250 g (passing 425 micron) |
| Proctor compaction | 7 / 8 | 30 kg (mould volume × 6 trial points × ~1.2 safety) |
| CBR | 16 | 20 kg (3 specimens × ~5 kg each + spare) |
| Direct shear | 13 | 5 kg disturbed for remoulded samples |
| Consolidation | 15 | Undisturbed 38 mm or 60 mm sample tube; 1.5x cell height min |
| Permeability (constant head) | 17 | 10 kg |
Soil classification — the IS 1498 + IS 2720 combination
Soil classification per IS 1498 requires test results from IS 2720 Parts 4, 5, and sometimes 22:
- Run Part 4 sieve analysis — determine fines content (% passing 75 micron).
- If fines > 50%, classify as fine-grained (silt or clay) using Part 5 (Atterberg limits) on the plasticity chart.
- If fines < 50%, classify as coarse-grained (sand or gravel) using Part 4 D60/D10 (uniformity) and D30² / (D60×D10) (curvature) ratios.
- If organic matter > threshold (Part 22), upgrade to organic class (OL, OH, Pt for peat).
Common classification labels:
- SP: poorly-graded sand
- SW: well-graded sand
- SC: clayey sand
- CH: high-plasticity clay (LL > 50)
- CL: low-plasticity clay (LL < 50)
- BC: black cotton (highly expansive, IS 1498 special class)
CBR test — the pavement design number
The California Bearing Ratio (Part 16) is the single most-cited soil test on Indian highway projects because IRC 37 uses it directly for pavement layer thickness:
| CBR value | Subgrade rating | Pavement design implication |
|---|---|---|
| < 2 | Very poor | Reject or stabilise; bituminous overlay needs 750+ mm |
| 2–5 | Poor | BM + DBM total thickness ~600 mm (Indian highway) |
| 5–8 | Fair | ~450 mm total bituminous thickness |
| 8–15 | Good | ~350 mm total |
| > 15 | Excellent | ~250 mm total; lower-traffic roads |
CBR is run on samples soaked for 4 days (Indian standard practice) at the OMC + 95% MDD condition (from Part 7 or 8). Both penetration values (at 2.5 mm and 5 mm) are reported; the higher governs for design.
Field tests — compaction acceptance
On site, compaction QA uses one of three IS 2720 field-density methods:
| Method | Best for | Limitation |
|---|---|---|
| Sand replacement (Part 9) | Granular soils, embankment fills | Slow; needs calibrated sand |
| Core cutter (Part 10) | Cohesive soils with no gravel | Won’t work in coarse soils |
| Nuclear gauge (not in IS 2720) | Fast, all soils | Equipment cost; calibration drift |
Acceptance criteria typically: 95% of MDD for embankments, 97% for sub-base, 98% for base course (varies by specification — MoRTH for highways, state PWD for buildings).
Black cotton soil — the special case
Black cotton (BC) soil — expansive clays found across central and western India — gets two dedicated IS 2720 parts:
- Part 40: Free swell index test. Identifies BC soil if free swell > 50%.
- Part 41: Swelling pressure test. Quantifies the pressure BC soil exerts when wetted (typically 100–500 kN/m² for severe BC).
BC soils require special foundation design: deep foundations below the active zone (typically 2–4 m), or under-reamed piles, or soil replacement / stabilisation. Plain isolated footings on BC soil routinely heave 50–150 mm seasonally, cracking superstructures.
Consolidation testing — settlement prediction
IS 2720 Part 15 (oedometer test) provides parameters for settlement calculation:
- Compression index (Cc): governs primary consolidation settlement.
- Coefficient of consolidation (Cv): governs time rate of settlement.
- Pre-consolidation pressure (Pc): stress history of the soil.
- Over-consolidation ratio (OCR): Pc / current effective stress.
For soft alluvial clays in coastal cities (Mumbai, Kolkata, Visakhapatnam), consolidation testing is mandatory before any major foundation design. Predicted settlements of 100–400 mm over 5–20 years are common.
Lab vs field equipment requirements
A minimum geotechnical lab needs:
- Drying oven (105°C) and balance (0.01 g)
- IS sieves (75 micron to 75 mm, full set)
- Hydrometer set (152H type)
- Casagrande apparatus + plastic limit threading plate
- Pycnometer (for specific gravity)
- Proctor moulds (light + heavy compaction) + rammers
- CBR test apparatus + soaking tank + load frame
- Direct shear box + load frame + dial gauges
- Oedometer (consolidation) + ring + porous stones
- Field tools: sand replacement kit, core cutter, dynamic cone penetrometer (DCP)
Total capital outlay for a basic lab: ₹8–15 lakh in 2026. Mid-sized lab with triaxial + nuclear gauge: ₹35–50 lakh.
Common mistakes in soil testing BoQ
- Under-quoting samples. A single CBR run consumes 20 kg; engineers often quote 5 kg and the lab refuses or runs partial samples.
- Mixing modified and standard Proctor. CBR samples must use modified Proctor (Part 8) MDD, not standard Proctor (Part 7), unless the spec says otherwise.
- Ignoring soaking duration. CBR with 0-day soak gives 2–3x higher values than 4-day soak. Specs mostly require 4-day soaked CBR.
- Mis-classifying BC soil. Plasticity Index alone isn’t enough; run Part 40 free swell to confirm.
- Wrong density method on coarse soil. Core cutter doesn’t work on gravel; use sand replacement or large-hole replacement (Part 30).
FAQ
How many parts does IS 2720 have?
41+ parts as of 2026. Some are revised periodically (Part 1 most recently in 1983; many parts have older base years with technical correction in supplements).
Which IS 2720 parts are mandatory for road construction?
Parts 1, 2, 4, 5, 7 or 8 (Proctor), 9 or 10 (field density), 16 (CBR), 22 (organic). For BC soil zones, also Parts 40 and 41. MoRTH specifies these explicitly for embankment, subgrade, sub-base and base course construction.
Is CBR test enough for foundation design?
No. CBR is a strength index for pavements, not foundations. Foundation design needs SPT (Part 14), specific gravity (Part 3), shear strength (Part 13 direct shear or laboratory triaxial outside IS 2720), and consolidation (Part 15) for settlement.
What is the minimum sampling depth for foundations?
Per IS 1892 (subsurface investigation), minimum depth is 1.5x the foundation footprint width or to bedrock or to depth where stress increase is < 10% of overburden — whichever is shallower. IS 2720 covers the testing methods; IS 1892 covers the sampling strategy.
Where do I find IS 2720 PDFs?
Each part has its own code page on InfraLens: IS 2720 Part 4 (sieve), Part 5 (Atterberg), Part 7 (Proctor), Part 16 (CBR), etc. Browse all IS codes at /is-codes and search “IS 2720” for the full series.