Every road, however well built, ultimately rests on soil. Load passes from the tyre through each pavement layer and finally into the ground — and if that ground cannot take it, no thickness of bitumen above will save the road.
This post covers subgrade soil: what it is, the properties it must have, how its strength is measured, and the two tests that dominate this topic — the plate bearing test and the CBR test.
What Soil and Subgrade Mean
Soil is an accumulation or deposit of earth material, derived naturally from the disintegration of rocks or the decay of vegetation, that can be excavated readily with power equipment in the field or disintegrated by gentle mechanical means in the laboratory.
| Term | Meaning |
|---|---|
| Subgrade | The supporting soil beneath the pavement and its special under courses |
| Compacted subgrade | Soil compacted by controlled movement of heavy compactors |
The definition of soil is worth reading closely, because it is really a definition by ease of excavation. If power equipment can dig it, it is soil. If not, it is rock. That practical test is what separates the two in engineering terms.
Six Desirable Properties of Subgrade Soil
- Stability
- Incompressibility
- Permanency of strength
- Minimum changes in volume and stability under adverse conditions of weather and ground water
- Good drainage
- Ease of compaction
Notice how many of these concern water. Property 4 is explicitly about weather and ground water, property 5 is drainage, and properties 2 and 3 are both undermined by moisture. A soil that is strong when dry and weak when wet is useless as a subgrade, because the road must perform in every season.
Property 3 — permanency of strength — makes the point directly. It is not enough to be strong today; the strength must last.
Index Properties of Soil
1. Grain Size Distribution
Coarse-grained components are analysed by sieve analysis, and fines by sedimentation analysis. Together these make up the grain size analysis or mechanical analysis, carried out to determine the percentage of individual grain sizes in a soil sample.
2, 3 and 4 — The Consistency Limits
| Limit | Definition |
|---|---|
| Liquid Limit | The minimum water content at which the soil will flow under the application of a very small shearing force |
| Plastic Limit | The minimum moisture content at which the soil remains in a plastic state |
| Shrinkage Limit | The maximum moisture content at which further reduction in water content does not cause a reduction in volume. It is the lowest water content that can occur in a completely saturated clayey soil sample |
These three limits describe the same soil at three different moisture states. Add water to a dry soil and it passes from solid, through plastic, to liquid — and the limits mark the boundaries between those states.
Indian Standard Classification of Soil Grains
| Soil Type | Size | Grain Size (mm) |
|---|---|---|
| Gravel | — | 2.0 |
| Sand | Coarse | 2.0 |
| Medium | 0.6 | |
| Fine | 0.2 | |
| Silt | Coarse | 0.06 |
| Medium | 0.02 | |
| Fine | 0.006 | |
| Clay | Coarse | 0.002 |
| Medium | 0.0006 | |
| Fine | 0.0002 |
Read the column of numbers and the pattern is clear — each step down is roughly a factor of ten. Gravel at 2 mm, silt at 0.06 to 0.006, clay down at 0.0002 mm. Clay particles are around ten thousand times smaller than gravel, which is why they behave so differently: enormous surface area for their volume, and therefore an enormous appetite for water.
Tests to Determine Soil Strength
Three families of test exist:
- Shear tests
- Bearing tests
- Penetration tests
Shear Tests
Carried out on relatively small soil samples in the laboratory. To find the strength properties, a number of representative samples from different locations are tested.
Commonly known shear tests are the direct shear test, triaxial compression test and unconfined compression test.
Bearing Tests
Loading tests carried out on subgrade soils in situ with a load bearing area. Results are influenced by variations in the soil properties within the stressed soil mass underneath.
Penetration Tests
Penetration tests may be considered small scale bearing tests in which the size of the loaded area is much smaller and the ratio of penetration to size of loaded area is much greater than in bearing tests.
The California Bearing Ratio test and cone penetration test are commonly known penetration tests.
The Complete Classification
| Group | Tests |
|---|---|
| Shear tests | Direct shear test, triaxial test, unconfined compression test, vane shear test |
| Bearing tests | California penetration test, cone penetration test |
| Penetration tests | Standard penetration test, dynamic cone penetration test, plate bearing test |
Five Factors Affecting Soil Strength
- Soil type
- Moisture content
- Dry density
- Internal structure of the soil
- The type and mode of stress application
Factor 5 is easy to overlook but important: the same soil gives different strengths depending on how the load is applied. This is exactly why several different tests exist, and why their results are not interchangeable.
The Main Laboratory Tests
Direct Shear Test
One of the oldest shear tests. The apparatus is essentially a box divided horizontally into two halves — one half fixed, the other free to move horizontally. A vertical load is applied and a horizontal pull produces a certain rate of horizontal displacement.
Note a practical limitation: carrying out an undrained (quick) test on sandy soil is rather impracticable in the shear box.
Triaxial Compression Test
Used to determine the shear strength of soil under lateral confinement. An attempt is made to simulate the confining pressure observed in a loaded soil mass.
This is the test’s advantage. Real subgrade soil is surrounded on all sides by more soil, which confines it. The triaxial test recreates that condition; the direct shear test does not.
Unconfined Compression Test
A special case of the triaxial compression test where the confining pressure is zero and only axial compressive stress is applied to the cylindrical specimen.
Stress is applied and the deformation and load readings noted until the specimen fails.
Modulus of Subgrade Reaction (K)
The modulus of subgrade reaction is the pressure sustained per unit deformation of the subgrade at a specified deformation.
Standard plate size = 75 cm diameter
K = P / Δ = P / 0.125 (kg/cm3)
A graph is plotted of mean settlement against mean bearing pressure. The pressure corresponding to a settlement of 0.125 cm is read off, and K is calculated from it.
Here P is the pressure at unsoaked condition producing the deformation. Note the unit: pressure in kg/cm2 divided by a deflection in cm gives kg/cm3, which is why K is quoted in those units.
Plate Bearing Test
The plate bearing test evaluates the supporting power of the subgrade for pavement design, using relatively large diameter plates. It was originally devised to find the modulus of subgrade reaction.
Two Corrections
Correction for worst subgrade moisture. The field test is usually done on unsoaked soil, but the road must survive the monsoon. The modulus for the soaked condition is:
Ks = K × Ps / P
K = modulus of subgrade reaction at unsoaked condition
Ps = pressure at soaked condition to produce the same deformation
Correction for small plate size. A 75 cm plate is heavy and awkward, so a smaller one is often used and the result corrected:
K = K1 a1 / a
K = modulus for the standard plate size
K1 = modulus obtained with the smaller plate
a1 = radius of the smaller plate
a = radius of the standard plate
California Bearing Ratio (CBR) Test
The most important single test in this topic.
A penetration test developed by the California Division of Highways as a method of evaluating the stability of soil subgrade. Results have been correlated with flexible pavement thickness requirements for highways and airfields.
It may be conducted in the laboratory on a prepared specimen in a mould, or in situ in the field.
Apparatus
| Component | Specification |
|---|---|
| Mould | 150 mm diameter with base plate and collar |
| Plunger | Cylindrical, 50 mm diameter |
| Dial gauges | For measuring expansion during soaking and penetration values |
Procedure
- The plunger penetrates the material at a rate of 1.25 mm per minute.
- Loads are noted at penetrations of 0.0, 0.5, 1.0, 1.5, 2.0, 3.0, 4.0, 5.0, 7.5, 10.0 and 12.5 mm.
- The load values causing 2.5 mm and 5.0 mm penetration are recorded.
- These loads are expressed as percentages of standard load values at the same deformation levels to obtain the CBR value.
- The specimen is subjected to four days of soaking, and swelling and water absorption values are noted.
Standard Load Values
At 2.5 mm penetration: 1370 kg (70 kg/cm2)
At 5.0 mm penetration: 2055 kg (105 kg/cm2)
These are obtained from the average of a large number of tests on crushed stone. In other words, CBR compares your soil against a reference material — crushed stone scores 100 %, and your soil is rated as a percentage of it.
Correcting the Curve
Two types of load–penetration curve may be obtained:
- The normal curve, with convexity upwards. The loads at 2.5 mm and 5.0 mm are simply read off.
- A curve with initial upward concavity, which indicates that a correction is necessary. The corrected origin is established by drawing a tangent from the steepest point on the curve, and the loads at 2.5 mm and 5.0 mm are read from that corrected origin.
The concave start happens because of surface irregularities or a slightly loose top layer — the plunger seats itself before it truly begins to load the soil. Correcting the origin removes that artefact.
Formula Summary
| Quantity | Expression or Value |
|---|---|
| Modulus of subgrade reaction | K = P / 0.125 (kg/cm3) |
| Standard plate diameter | 75 cm |
| Settlement used for K | 0.125 cm |
| Soaked correction | Ks = K × Ps/P |
| Plate size correction | K = K1a1/a |
| CBR mould diameter | 150 mm |
| CBR plunger diameter | 50 mm |
| CBR penetration rate | 1.25 mm/min |
| CBR standard loads | 1370 kg at 2.5 mm; 2055 kg at 5.0 mm |
| CBR soaking period | 4 days |
Quick Revision Notes
- Subgrade = supporting soil beneath the pavement.
- Six desirable properties: stability, incompressibility, permanency of strength, minimum volume change under weather and ground water, good drainage, ease of compaction.
- Liquid limit and plastic limit are minimum water contents; shrinkage limit is a maximum.
- Three test groups: shear, bearing, penetration.
- Penetration tests are small-scale bearing tests with a much smaller loaded area and a much greater penetration-to-area ratio.
- Unconfined compression test = triaxial test with zero confining pressure.
- Five factors affecting strength: soil type, moisture content, dry density, internal structure, type and mode of stress application.
- K is measured with a 75 cm diameter plate at a settlement of 0.125 cm.
- CBR: mould 150 mm, plunger 50 mm, rate 1.25 mm/min, soaking 4 days.
- CBR standard loads: 1370 kg at 2.5 mm and 2055 kg at 5.0 mm, from tests on crushed stone.
- A concave-upward start to the CBR curve requires an origin correction by tangent from the steepest point.
Mistakes Students Commonly Make
- Swapping the mould and plunger diameters. Mould is 150 mm, plunger 50 mm.
- Swapping the CBR standard loads. 1370 kg goes with 2.5 mm, 2055 kg with 5.0 mm.
- Using 0.25 cm instead of 0.125 cm for the K settlement.
- Forgetting that the standard plate for K is 75 cm diameter.
- Calling the shrinkage limit a minimum. It is the maximum moisture content below which volume no longer reduces.
- Assuming the direct shear test and triaxial test give the same result. The triaxial test provides lateral confinement; the direct shear test does not.
- Skipping the origin correction when the CBR curve starts concave upwards.
Conclusion
Soil is the layer everything else depends on, and the whole of this topic is about measuring how much load it can take. Shear tests examine small laboratory samples, bearing tests load the ground in place, and penetration tests push a small plunger deep. Two results matter most in pavement design — the modulus of subgrade reaction, found from a 75 cm plate at 0.125 cm settlement and used for rigid pavements, and the CBR value, compared against crushed stone and used for flexible pavement thickness.
Frequently Asked Questions
What is subgrade?
The supporting soil beneath the pavement and its special under courses.
What are the desirable properties of subgrade soil?
Stability, incompressibility, permanency of strength, minimum changes in volume and stability under adverse weather and ground water conditions, good drainage, and ease of compaction.
What is the liquid limit?
The minimum water content at which the soil will flow under the application of a very small shearing force.
What is the shrinkage limit?
The maximum moisture content at which further reduction in water content does not cause a reduction in volume. It is the lowest water content that can occur in a fully saturated clayey soil.
What is the difference between a bearing test and a penetration test?
A penetration test is essentially a small scale bearing test, in which the loaded area is much smaller and the ratio of penetration to the size of the loaded area is much greater.
What is the unconfined compression test?
A special case of the triaxial compression test in which the confining pressure is zero and only axial compressive stress is applied to the cylindrical specimen.
What is the modulus of subgrade reaction?
The pressure sustained per unit deformation of the subgrade at a specified deformation, found using a standard 75 cm diameter plate and the pressure corresponding to 0.125 cm settlement.
What is the CBR test?
A penetration test developed by the California Division of Highways to evaluate the stability of soil subgrade, whose results are correlated with flexible pavement thickness requirements.
What are the standard load values in the CBR test?
1370 kg at 2.5 mm penetration and 2055 kg at 5.0 mm penetration, corresponding to 70 and 105 kg/cm2, obtained from the average of a large number of tests on crushed stone.
How long is a CBR specimen soaked?
Four days, during which swelling and water absorption values are also noted.
When is the CBR curve corrected?
When the curve shows initial upward concavity. A tangent is drawn from the steepest point on the curve to establish a corrected origin, and the loads at 2.5 and 5.0 mm are read from there.