IS 2720 : Part 16 · California Bearing Ratio Test · IRC 37 | MoRTH 5th Revision | Highway Engineering India
// Table of Contents
1. What is the CBR Test? — Introduction
The California Bearing Ratio (CBR) Test is a penetration test used to evaluate the mechanical strength of subgrade soils, sub-base and base course materials for the design of flexible pavements. It was originally developed by the California Department of Transportation in the 1930s and has since been adopted worldwide as the primary empirical method for pavement thickness design.
In India, the CBR test is standardized under IS 2720 : Part 16 : 1987 and is a mandatory input for the design of flexible pavements as per IRC 37 and road construction as per MoRTH Specifications for Road and Bridge Works (5th Revision). Every National Highway, State Highway, and rural road project in India requires a CBR value before any pavement design can proceed.
Why is CBR Important for Highway Design?
Pavement design is entirely dependent on the load-bearing capacity of the soil below the road. A strong subgrade (high CBR) means thinner, cheaper pavement layers. A weak subgrade (low CBR) demands thick, expensive construction. The CBR value directly enters the IRC 37 design charts and equations to determine total pavement thickness and individual layer thicknesses. Even a 1–2% increase in CBR can save millions of rupees in material cost on a large highway project.
IS 2720 : Part 16 : 1987 — Lab Determination of CBR | IRC 37 : 2018 — Flexible Pavement Design (CBR is primary input) | IRC SP 72 : 2015 — Low Volume Rural Roads | MoRTH 5th Rev. Clause 305 — Sub-base requirement CBR ≥ 20%
Soaked CBR
Sample soaked in water for 96 hours (4 days) — simulates worst-case monsoon saturation. Used for permanent highway design. Mandatory for all Indian NH/SH projects.
Unsoaked CBR
Tested at natural moisture content — no soaking. For granular materials, dry climates, or temporary roads. Values are higher; not safe for monsoon-zone pavement design.
2. Technical Specifications & Apparatus
Apparatus Required (IS 2720 Part 16)
| # | Apparatus | Specification | IS Ref. |
|---|---|---|---|
| 1 | CBR Mould | Dia: 150 mm, Height: 175 mm, Vol: ≈ 2250 cc; with base plate & collar (50 mm) | IS 9669 |
| 2 | Cylindrical Plunger | Dia: 50 mm (Area = 1935 mm²), hardened steel | IS 2720-16 |
| 3 | Loading Frame | Capacity ≥ 50 kN; rate of penetration: 1.25 mm/min (constant) | — |
| 4 | Proving Ring / Load Cell | Capacity: 25–50 kN, LC: 0.25 kN | — |
| 5 | Penetration Dial Gauge | Travel: 25 mm, LC: 0.01 mm | — |
| 6 | Surcharge Weights | 2.5 kg annular discs (dia 150 mm, hole dia 53 mm); min 2 nos | IS 2720-16 |
| 7 | Compaction Rammer | Standard: 2.6 kg, 310 mm drop; Modified: 4.89 kg, 450 mm drop | IS 2720-7/8 |
| 8 | Soaking Tank | Min depth 150 mm — full submersion of mould assembly | — |
| 9 | Swell Dial Gauge | Mounted on tripod over soaking mould; LC 0.01 mm | IS 2720-16 |
| 10 | Weighing Balance | Capacity 15 kg, accuracy 1 g | IS 9873 |
Standard Reference Loads (IS 2720 Part 16)
| Penetration | Standard Unit Load (kN/m²) | Total Load on Ø50mm Plunger |
|---|---|---|
| 2.5 mm | 6,900 kN/m² | 13.44 kN (≈ 1370 kgf) |
| 5.0 mm | 10,350 kN/m² | 20.13 kN (≈ 2055 kgf) |
IRC 37 Minimum CBR Requirements by Layer
| Layer / Application | Min. CBR (%) | Standard | Status |
|---|---|---|---|
| Subgrade — Expressway / NH | ≥ 8% | IRC 37 | Mandatory |
| Subgrade — State Highway / MDR | ≥ 5–6% | IRC 37 | Mandatory |
| Subgrade — Low Volume Road | ≥ 3% | IRC SP 72 | Min. req. |
| Sub-base (GSB) | ≥ 20% | MoRTH Cl 401 | Mandatory |
| Base Course (WMM) | ≥ 80% | MoRTH Cl 406 | Mandatory |
| Embankment Fill | ≥ 3% | MoRTH Cl 305 | Min. req. |
3. Technical Diagrams
// Figure 1 — CBR Mould Assembly Cross-Section (IS 2720 : Part 16)
Fig. 1 — CBR Mould Assembly showing dimensions, plunger, surcharge weights, proving ring, and swell gauge as per IS 2720 Part 16.
// Figure 2 — CBR Test Process Flow Chart (Lab Procedure)
Fig. 2 — Complete CBR test flow chart with decision branch for expansive soils (swell > 5%).
// Figure 3 — Load vs. Penetration Curve & Correction Methodology
Fig. 3 — Load–Penetration curve showing normal, concave (requires origin correction), and corrected curves with read-off points at 2.5mm and 5.0mm.
// Figure 4 — Flexible Pavement Cross-Section — CBR Requirements per Layer (IRC 37)
Fig. 4 — Flexible pavement cross-section with minimum CBR requirements per layer as per IRC 37:2018 and MoRTH 5th Revision.
4. Step-by-Step Test Procedure (IS 2720 : Part 16)
Sample Preparation
Collect a representative soil sample. Air-dry and pulverise. Pass through a 20 mm IS sieve — replace material retained on 20 mm with equal mass of material passing 20 mm and retained on 4.75 mm. Determine initial moisture content. Prepare at least 5.5 kg per mould. Mix with water to achieve the Optimum Moisture Content (OMC) from the Proctor test (IS 2720 Part 7 or 8). Wrap in polythene for minimum 16 hours for moisture equilibration.
For design CBR, always test at OMC and MDD corresponding to the field compaction energy specified for the project (Standard or Modified Proctor as per MoRTH specification).
Mould Assembly & Compaction
Weigh the clean, oiled mould with base plate. Compact soil in 3 equal layers (Standard Proctor energy: 56 blows/layer for 150 mm dia. mould). Soil should fill to the top of the extension collar. Remove collar; trim excess soil flush with the mould rim using a straight edge. Weigh mould + soil. Invert the mould so the base plate is on top — this exposes the bottom face of the compacted sample for testing (gives a more representative in-situ condition).
Surcharge Weights
Place the perforated disc on the compacted soil surface. Add annular surcharge weights to simulate overburden from pavement layers above:
| Pavement Thickness Above | Surcharge Mass |
|---|---|
| Up to 80 mm | 2.5 kg (1 disc) |
| 80 mm to 160 mm | 5.0 kg (2 discs) |
| 160 mm to 240 mm | 7.5 kg (3 discs) |
Soaking — 96 Hours
Set up the swell dial gauge on the perforated disc. Record the initial dial reading. Submerge the entire mould assembly in water (access from both top and bottom via perforated base). Soak for 96 hours. Record swell dial gauge every 24 hours.
// Swell % Calculation
Swell (%) = [(D_final − D_initial) / H_specimen] × 100
H_specimen = typically 127 mm (3-layer compaction)
Acceptable limit: Swell < 5%
If swell exceeds 5%, the soil is highly expansive. Do not use as subgrade without stabilisation. Typical treatment: lime (3–5%) or lime + fly ash as per IRC SP 89.
Penetration Test
After soaking, remove mould; drain for 15 minutes. Place in loading frame with surcharge weights on surface. Seat the Ø 50 mm plunger with a seating load of 44 N (4.5 kgf). Zero both the penetration gauge and load dial. Apply load at 1.25 mm/min. Record load at penetrations: 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 5.0, 7.5, 10.0 mm. The primary readings are at 2.5 mm and 5.0 mm.
Plot & Correct the Load–Penetration Curve
Plot Load (kN) vs. Penetration (mm). If the curve is concave upward at the start (due to surface irregularities), apply the origin correction: draw a tangent at the steepest point → project to x-axis → this new intersection = corrected zero. Re-read loads at 2.5 mm and 5.0 mm from this shifted origin. This is mandatory per IS 2720 Part 16, Clause 7.5.
Calculate & Report CBR
Calculate CBR at 2.5 mm and 5.0 mm. Normally report CBR at 2.5 mm (higher value). If CBR₅.₀ > CBR₂.₅, repeat the test — if confirmed, report 5.0 mm value. Run minimum 3 specimens; report average. Reject any value differing from mean by more than 15% and retest.
5. Detailed Calculation — Worked Example
// Core CBR Formula — IS 2720 : Part 16
CBR (%) = (P_test / P_std) × 100
P_test = load carried by soil at specified penetration (kN)
P_std at 2.5 mm = 13.44 kN (Standard load = 6,900 kN/m² × 1935 mm²)
P_std at 5.0 mm = 20.13 kN (Standard load = 10,350 kN/m² × 1935 mm²)
Plunger area = π/4 × 50² = 1963.5 mm² (sometimes rounded to 1935 mm² in IS)
Worked Example
Given: Soil at OMC = 14.5%, MDD = 1.78 g/cc, soaked 96 hrs. Proving ring calibration: 1 division = 0.15 kN.
| Penetration (mm) | Proving Ring (div) | Load (kN) |
|---|---|---|
| 0 | 0 | 0 |
| 0.5 | 14 | 2.10 |
| 1.0 | 25 | 3.75 |
| 1.5 | 37 | 5.55 |
| 2.0 | 48 | 7.20 |
| 2.5 | 60 | 9.00 |
| 3.0 | 70 | 10.50 |
| 4.0 | 87 | 13.05 |
| 5.0 | 99 | 14.85 |
| 7.5 | 112 | 16.80 |
// CBR @ 2.5 mm
CBR₂.₅ = (9.00 / 13.44) × 100 = 66.96% ≈ 67%
// CBR @ 5.0 mm
CBR₅.₀ = (14.85 / 20.13) × 100 = 73.77% ≈ 74%
Since CBR₅.₀ (74%) > CBR₂.₅ (67%) — repeat the test. If 5.0mm value is again higher on repetition, report CBR = 74%. This is typical for granular or cemented materials.
Swell Calculation
// Swell % Example
Swell = [(12.72 − 12.55) / 127.0] × 100 = 0.17 / 127 × 100 = 0.13% ✓ (well within 5% limit)
CBR vs. Pavement Thickness — IRC 37 Interpretation
| CBR (%) | Subgrade Class | Approx. Total Thickness (80 MSA) |
|---|---|---|
| 2% | Very Weak | ~900 mm |
| 3% | Weak | ~780 mm |
| 5% | Moderate | ~650 mm |
| 8% | Fair | ~560 mm |
| 10% | Good | ~510 mm |
| 15% | Very Good | ~440 mm |
| ≥ 20% | Excellent | ~380 mm |
6. Field CBR Testing — Site Guidance
Field CBR testing verifies compaction quality during construction. It is referenced under ASTM D4429 and is used on large highway projects in India for Quality Control (QC) acceptance.
Field CBR is used for QC verification only — not pavement design. Design always uses soaked lab CBR (IS 2720 Pt.16) from specimens at OMC + MDD. Field CBR values are typically higher because in-situ conditions are drier than saturated.
Field CBR Procedure (5 Steps)
Surface Preparation
Mark test locations every 50–100 m per lane (or per specification). Level the surface; remove loose material. Test location must be ≥ 1 m from layer edge. Moisten lightly with spray if surface is dusty.
Reaction Beam Setup
Set up field CBR loading frame. Use a loaded truck or bulldozer as reaction mass — parked directly over the test point. The reaction load must be ≥ 2× the maximum test load. Mount proving ring and Ø 50 mm plunger.
Seating Load
Apply seating load of 44 N (4.5 kgf). Zero both the penetration dial gauge and proving ring. Confirm plunger is fully seated.
Load Application
Apply load at 1.25 mm/min via hand hydraulic jack. Record load at 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 5.0, 7.5, 10.0 mm. Stop at 12.5 mm or clear failure.
MoRTH Test Frequency
| Work Item | Frequency | Standard |
|---|---|---|
| Subgrade acceptance | 1 test / 500 m / lane | MoRTH Cl 305 |
| GSB layer acceptance | 1 test / 500 m / lane | MoRTH Cl 401 |
| Design CBR (new project) | Min 3 per soil type per 500 m (lab) | IRC 37 |
| Rehab projects | 1 per 250 m (Benkelman + CBR) | IRC 81 |
Always record in-situ moisture content alongside field CBR. Test after specified curing period for stabilised subgrades (7 days for lime treatment). In monsoon season, rely on soaked lab CBR. Record GPS coordinates of every test point. Verify Degree of Compaction ≥ 97% MDD alongside every CBR test. For expansive black cotton soils, stabilise with lime (3–5%) per IRC SP 89 and verify post-treatment CBR ≥ 8%.
7. Frequently Asked Questions (FAQ)
What is the difference between soaked and unsoaked CBR? Which is used for design in India?
Soaked CBR simulates worst-case monsoon saturation — the sample is submerged in water for 96 hours. This is mandatory for all permanent highway design in India under IRC 37 and MoRTH specifications. Unsoaked CBR is tested at natural moisture content without soaking and is used only for granular sub-bases, dry climates, or temporary roads. Unsoaked values are significantly higher, and using them for permanent pavement design in India’s monsoon climate would be dangerous and non-compliant with IS/IRC standards.
Why is CBR at 5.0 mm sometimes higher than at 2.5 mm, and what should I do?
Theoretically CBR₂.₅ should be greater than CBR₅.₀. However, for coarse-grained soils, gravels, or stabilised materials, the load–penetration curve may continue rising steeply past 2.5 mm. Per IS 2720 Part 16 Clause 7.6: if CBR₅.₀ > CBR₂.₅, repeat the test. If the 5.0 mm value is again higher on retesting, report CBR at 5.0 mm as the final result.
What is the purpose of surcharge weights in the CBR test?
Surcharge weights simulate the overburden stress from pavement layers above the layer being tested. Without surcharge, CBR would be underestimated because it ignores the lateral confinement from overlying layers. Per IS 2720 Part 16, annular 2.5 kg discs are used — quantity depends on total pavement thickness above the tested layer (up to 3 discs = 7.5 kg for thick pavements).
How many specimens are required for design CBR as per IRC 37?
Per IRC 37 : 2018 Appendix I: minimum 3 representative samples per homogeneous soil section (typically up to 500 m of similar soil). Report the 90th percentile CBR (or mean − 1.5σ) for high-volume roads. For low-volume rural roads, use the mean CBR. Discard any value differing from mean by more than 15% and retest.
What is curve correction in CBR and when is it required?
Curve correction is required when the initial part of the load–penetration curve is concave upward (indicates surface looseness or irregularity). To correct: draw a tangent at the steepest point → project to the x-axis → the intersection is the corrected zero/origin. Shift the curve so this new point is zero penetration; re-read loads at 2.5 mm and 5.0 mm from the shifted curve. This is mandatory per IS 2720 Part 16 Clause 7.5.
Can black cotton soil (expansive clay) be used as subgrade? What is the treatment?
Black cotton soil (BCS) has high plasticity, high swell (often >5%), and very low CBR (1–3%). Its use as subgrade is not permitted without treatment per MoRTH and IRC. Treatment options: Lime stabilisation (3–5% lime, per IRC SP 89) — raises CBR to 10–20% and reduces swell dramatically; Fly ash (10–25% Class C); Lime + Fly ash (LFA) for very high PI soils; or replacement of top 500 mm with granular fill of CBR ≥ 10%.
What is the minimum CBR for subgrade on National Highways in India?
Per IRC 37 : 2018 and MoRTH 5th Revision: Expressways and NHs require subgrade CBR ≥ 8% (soaked, at 97% MDD); State Highways/MDRs require ≥ 5–6%; Other District Roads ≥ 4%; PMGSY village roads ≥ 3% (IRC SP 72). If natural subgrade CBR is below the minimum, the engineer must propose stabilisation or increase pavement thickness accordingly.
How is CBR used in IRC 37 pavement design to calculate resilient modulus?
IRC 37 : 2018 uses a mechanistic-empirical design approach. CBR is converted to Resilient Modulus (M_R) using: M_R = 10 × CBR (for CBR ≤ 5%) or M_R = 17.6 × CBR⁰·⁶⁴ (for CBR > 5%). M_R feeds into the design equations for allowable tensile strain in bituminous layers and compressive strain on the subgrade. The design produces layer thicknesses for BC, DBM, WMM, and GSB based on design traffic (MSA) and M_R.
References: IS 2720 : Part 16 : 1987 — Methods of Test for Soils (Lab CBR) | IRC 37 : 2018 — Guidelines for Design of Flexible Pavements | MoRTH Specifications for Road & Bridge Works, 5th Revision | IRC SP 72 : 2015 — Low Volume Rural Roads | IRC SP 89 — Lime and Lime–Fly Ash Stabilisation | ASTM D1883 / D4429
