Title: Laboratory Test - California Bearing Ratio (CBR)
Introduction
The California Bearing Ratio (CBR) test is a widely used laboratory test to determine the mechanical
strength and bearing capacity of soils and other granular materials. The CBR test provides valuable
information for designing and evaluating the performance of flexible pavements, embankments, and
other civil engineering structures. This technical report aims to provide a comprehensive overview of
the CBR test, including its purpose, procedure, equipment, and interpretation of results.
Purpose of the CBR Test
The primary purpose of the CBR test is to evaluate the strength characteristics of subgrade soils and
the suitability of the soil for road construction. It helps engineers determine the CBR value, which is a
measure of the soil's relative strength compared to a standard crushed stone material. The CBR value
is used in pavement design to estimate the thickness of the pavement layers required to support
anticipated traffic loads.
Test Procedure
The CBR test involves subjecting a compacted soil specimen to a gradually increasing load and
measuring the penetration resistance. The following steps outline the general procedure for
conducting a CBR test:
Step 1: Sample Preparation
Obtain undisturbed or disturbed soil samples from the field.
Determine the moisture content and sieve the soil to remove large particles.
Step 2: Specimen Preparation
Determine the maximum dry density (MDD) and optimum moisture content (OMC) of the soil using a
compaction test such as the Standard or Modified Proctor test.
Prepare cylindrical specimens of the soil at the desired OMC and compact them using the appropriate
compaction effort (Proctor or Modified Proctor).
Step 3: Soaking
Soak the compacted specimens in water for a specific duration (typically 4 days) to simulate field
conditions.
Step 4: Penetration Testing
Place the soaked specimen in a CBR testing machine equipped with a penetration piston and a loadmeasuring device.
Apply a static load at a constant rate and measure the penetration resistance (load vs. penetration) at
regular intervals.
Continue the test until a specified penetration value is reached (usually 2.5 mm or 5 mm).
Step 5: Calculation
Calculate the CBR value using the formula: CBR = (P/P_standard) × 100, where P is the load required
to penetrate the soil and P_standard is the load required to penetrate a standard crushed stone
material.
Equipment Required
The CBR test requires several pieces of equipment, including:
CBR testing machine: It consists of a loading frame, a penetration piston, and a load-measuring device.
Molds: Cylindrical molds to prepare the soil specimens.
Compaction equipment: Proctor or Modified Proctor compaction equipment to achieve the desired
density.
Penetration measurement device: A dial gauge or displacement transducer to measure the
penetration of the piston.
Water bath: A container to soak the specimens in water.
Interpretation of Results
The CBR value obtained from the test represents the relative strength of the soil compared to the
standard crushed stone material. Higher CBR values indicate stronger and more stable soils. The
interpretation of CBR results depends on the specific application and design requirements. For
example, CBR values above 80 are considered excellent for road construction, while values below 2
are considered very weak.
CBR test results are typically used in pavement design to determine the required pavement thickness
and the selection of appropriate materials. Engineers consider the CBR values, traffic loads, and other
factors to design pavements that can withstand the expected traffic and environmental conditions.
Limitations and Considerations
The CBR test provides a measure of the soil strength under laboratory conditions,