CBR Value in Road Construction: Test, Meaning, and How to Improve It

The CBR value measures how strong a subgrade soil is, as a percentage of crushed stone. It’s found by the California Bearing Ratio test, laboratory or field, and it drives pavement thickness under IRC:37. Clay sits near 2 to 5 percent, good granular soil 10 to 25, and crushed stone 100. Below about 5 percent, the subgrade needs help, and compaction, stabilization, or geosynthetic reinforcement can raise the effective value.

Every road design starts with one question about the ground it sits on. How strong is the soil under it?

The answer is the CBR value. It’s the number that sets how thick the pavement must be. Get it wrong at the start, and the error runs through the whole design.

We manufacture the geogrids, geocells, and geotextiles that lift a weak subgrade’s effective CBR. So we see this number from the reinforcement side.

This guide covers what the CBR value means, how the test works, what counts as a good number, and how to fix a poor one.

What CBR Value Means in Road Construction

Put plainly, the CBR value rates a soil’s strength against crushed stone. A soil at 10 percent CBR carries a tenth of what crushed stone would. So it’s a comparison, not an absolute strength.

That single percentage tells a designer whether the ground can carry traffic, and how much pavement it needs on top. Higher is stronger. Crushed stone, the reference, sits at 100.

The Ratio Behind the CBR Value

The test presses a plunger into the soil and measures the load. That load is divided by the load a standard crushed stone needs for the same penetration, then multiplied by 100.

So CBR is a ratio, which is why it runs from near zero for soft clay up to 100 for the reference material. It reflects strength under controlled density and moisture, not the soil on a random day.

The CBR Test: How Subgrade Strength Is Measured

The CBR test works by penetration. A 50 mm plunger presses into a compacted sample at 1.25 mm per minute, and the load is logged as it sinks. It can run in a lab or out on the subgrade itself.

The lab version gives controlled, repeatable numbers. The field version shows how the soil actually behaves in place, moisture and all.

Lab Versus Field CBR

The laboratory CBR test, to IS 2720 Part 16, compacts the soil into a mould, soaks it four days to mimic the monsoon, then loads it.

The soaking matters. A soil that tests strong when dry can collapse once it’s saturated.

The field CBR test, to IS 2720 Part 31, runs in place on the prepared subgrade. A dynamic cone penetrometer often rides alongside for a quick estimate. Lab and field numbers rarely match, though.

For design, the lab soaked value usually wins. It captures the worst the soil will face, which is the only safe basis for a road meant to last decades.

California Bearing Ratio: The Standard and the Math

The California Bearing Ratio is really just a division. Take your soil’s load over a fixed standard load, at 2.5 mm and 5.0 mm, and keep the higher figure. The arithmetic is easy; the discipline is in the sample.

The standard loads come from crushed stone: 1370 kgf at 2.5 mm and 2055 kgf at 5.0 mm. Your soil’s load at each depth, divided by these, gives the CBR.

How the CBR Value Is Calculated

Usually the 2.5 mm figure governs. If the 5.0 mm value comes out higher, recheck the test. That points to a sample or procedure problem, not a real result.

Compaction and moisture at the time of the test change the answer sharply. That’s why the standard fixes both, so numbers from different sites can actually be compared.

CBR Value of Soil: What’s Good and What’s Weak

The CBR value of soil swings from around 2 percent for soft clay to 25 or more for good granular ground. Five percent is the rough line between weak and workable. Soil type sets the ballpark; density and moisture move it.

Fine-grained clays and silts sit low because water weakens them. Granular soils sit higher because the grains interlock and drain.

Reading CBR Values by Soil Type

Below 2 percent, the subgrade is very poor and nearly always needs treatment. From 2 to 5, it’s weak and usually wants improvement. Above 10, it’s a sound foundation that keeps the pavement thin.

India’s black cotton soil is the classic problem. It swells when wet, and its soaked CBR can drop close to 2. That’s exactly why the soaked test, not the dry one, governs design here.

Much of central and western India sits on this ground, so a low starting CBR is the norm, not the exception.

CBR Test Procedure, Step by Step

The CBR test procedure runs in four moves: compact, soak, load, then calculate, all to IS 2720. Each step is fixed, which is what makes the result repeatable.

Skip a step or rush the compaction and the number drifts. The value is only as trustworthy as the procedure behind it.

The Laboratory Method

First, the soil is compacted into the mould at its target density and moisture. Then, for design work, it’s soaked for four days under surcharge weights that mimic the pavement load above.

After soaking, the plunger is driven in at 1.25 mm per minute, and the load is logged at 2.5 and 5.0 mm. The CBR is calculated from those loads, and the higher figure is reported.

Subgrade CBR and Pavement Design

One number drives pavement thickness more than any other, and that’s the subgrade CBR. IRC:37 reads it alongside the design traffic to size every layer above. A stronger subgrade means a thinner, cheaper road.

The logic is direct. A weak subgrade can’t carry much, so the layers above have to spread the load further, which means more granular material and more cost.

Why a Higher CBR Means a Thinner Road

Lift the subgrade CBR from 3 to 8 percent and the pavement above it gets noticeably thinner, saving aggregate on every kilometre. That saving is why improving the subgrade often pays for itself.

The layers above it, the granular subbase, base, and surface, are all sized from that one number. We lay out the full stack in our guide to the layers of road construction.

Get the subgrade right, and the rest follows cleanly.

How to Improve CBR Value

A weak CBR isn’t a dead end. Compact harder, fix the moisture, bind the clay with lime or cement, or reinforce the soil with geosynthetics. Which one you reach for depends on the soil and how much lift you need.

Start with the cheap wins. Proper compaction at optimum moisture is the first, and often it’s enough to move a marginal soil into a workable range.

Where the soil is beyond saving, the last resort is to dig it out and replace it with granular fill. It works, but the haulage and disposal make it the costliest route, which is why reinforcement usually wins on soft ground.

Compaction, Stabilization, and Reinforcement

When compaction alone won’t do it, the next steps are mechanical or chemical. Blending in granular material lifts a clayey soil, and lime or cement binds an expansive clay and dries it out.

For soft subgrades under real load, reinforcement does what treatment can’t. A geogrid confines the granular layer and lifts its effective CBR, turning a marginal site into a working platform. That’s where our INDODRAIN PET Geogrid earns its place.

On the very softest ground, a geocell goes further still.

That’s the job of Indokon Geocell. Its honeycomb pens the fill in place so it can’t spread, forming a stiff mattress over soil that would otherwise never hold a road.

CBR Value in Indian Practice

In India, the CBR test follows IS 2720, and the result feeds straight into IRC:37 for pavement design. The value used is the soaked one, to allow for the monsoon. The codes are built around the soils and the rains here.

This is where the soaked test earns its keep. A design based on dry-season strength would fail the first time the water table rose.

IS 2720, IRC:37, and Soaked CBR

IS 2720 Part 16 sets the laboratory method and Part 31 the field method. IRC:37 then turns the design subgrade CBR and the traffic into a layer stack, with weaker soils driving thicker or reinforced pavements.

So much of India builds on soft, water-holding ground. There, keeping the subgrade and sub base apart matters as much as raw strength.

A separation geotextile stops the soft soil pumping up into the granular layer. The effective CBR the design counted on doesn’t quietly drop over the years.

Start With the CBR Value

Get the CBR value right at the start and the whole road design rests on solid ground. Get it wrong, or skip the soaked test, and the pavement is built on a number that won’t hold once the rains come.

Read the subgrade honestly first. Then improve it where it’s weak, compaction and moisture first, reinforcement where the soil won’t carry the load on its own.

That’s where the materials we make come in, raising a weak subgrade’s effective CBR so the road above it lasts. When you’re sizing a pavement on soft ground, the team at Indonet Group can match the reinforcement to your subgrade and your design.

This guide draws on Indonet Group’s experience manufacturing and supplying geosynthetics, geogrids, geocells, and geotextiles, for subgrade reinforcement and road construction across India. Specifications reference IS 2720, IRC:37, and MoRTH.

Frequently Asked Questions

1. What is the CBR value in road construction?

The CBR value is a measure of a soil’s strength, given as a percentage of standard crushed stone. It comes from the California Bearing Ratio test and is the main input for deciding how thick a road pavement needs to be.

2. What is a good CBR value for a subgrade?

Above 10 percent is a strong subgrade that keeps the pavement thin. Between 5 and 10 is workable. Below 5 percent is weak and usually needs improvement, and below 2 percent almost always needs treatment or replacement.

3. How is the CBR value calculated?

A plunger is pressed into a compacted soil sample, and the load is recorded at 2.5 and 5.0 mm penetration. Each load is divided by a standard crushed-stone load, 1370 and 2055 kgf, and multiplied by 100. The higher result is the CBR.

4. What is the difference between soaked and unsoaked CBR?

Soaked CBR is measured after four days submerged, mimicking a saturated monsoon subgrade, and it’s used for design because it’s the worst case. Unsoaked CBR, tested at field moisture, reads higher and suits drier conditions.

5. Which Indian standard covers the CBR test?

The laboratory CBR test follows IS 2720 Part 16 and the field test follows IS 2720 Part 31. The resulting value is used in IRC:37 to design flexible pavement thickness.

6. How can a low CBR value be improved?

A low CBR can be raised by better compaction, moisture control, blending in granular material, or binding with lime or cement. On soft ground, geosynthetics such as geogrids and geocells reinforce the soil and lift its effective CBR.