Explain CBR test on subgrade soil. Give its signifacance

Sample preparation takes up most of the time spent in the geotechnical laboratory testing CBR samples. The materials used in the study are sampled and processed in the lab to create a collection of test specimens.

The preparation follows the procedures of ASTM & AASHTO). The dry density of the specimens may be altered at the request of the CBR test specifier. The required number of blow counts can be changed to vary the dry density. Other expansion measuring equipment, such as spacer discs, surcharge weights, and other expansion measuring equipment, is also required.
The test is mostly used to determine the strength of cohesive materials with maximum particle sizes less than 19 mm, while it can also be used to examine the strength of granular soils and aggregates (0.75 in). Prior to the penetration test, three to five samples are made and immersed for a length of time. The prepared test specimens are placed in a load frame, and controlled penetration forces from a piston with a 3*2 surface area are applied (1,935mm2). At specific penetration intervals, loads are measured and graphically shown.

Soaked CBR Test Specimens

Most CBR testing includes soaking to account for adverse moisture conditions such as predicted rain or flooding. Before the penetration test, each specimen is normally immersed in water for 96 hours.
To mimic the weight of the pavement and other loads required, a surcharge load of at least 10 lbf (4.54kg) must be put on the sample during the soaking time. The soil swell induced by soaking must be measured using expansion measuring instruments and swell plates placed on the sample prior to penetration testing. It takes a long time to prepare several compressed specimens for a single test.

Note: Unsoaked samples are fairly uncommon in locations with limited rainfall and a low local water table.

CBR Penetration Test

For penetration testing, the sample is retained in the mould with the perforated plate and required surcharge weights in place on the soil, the piston is positioned against the test sample, and loading commences at a rate of 0.05in (12.7mm) per minute. Test loads are recorded as the piston enters the earth at eleven predetermined depths up to 0.500in (13mm).

CBR Test Diagram

On a graph, penetrations vs. load values are plotted and corrected using the test standard’s procedure. Standard stresses of 1,000psi (6.9mPa) and 1,500psi (10.3mPa) are multiplied by 100 after being divided by adjusted stress values obtained from penetration depths of 0.100in (2.54mm) and 0.200in (2.54mm), respectively. CBR values usually fall as penetration rises. The CBR value measures the stress at a penetration depth of 0.100in (2.54mm). The ratio at the 0.200in (5.08mm) penetration depth is occasionally greater than the 0.100in-depth, necessitating a repetition of the test. If the check test yields similar findings, the reported CBR is the stress from a 0.200in penetration depth.

CBR plays a key role in the Project cost and pavement design.