The Plate bearing test (PBT) and the California Bearing Ratio (CBR) are widely used in-situ testing methods to evaluate soil's vertical deformation and strength. The PBT is used when testing involves large ground particles, whereas the CBR is only suitable for areas where the particle sizes are no more than 20mm. In practice, the CBR result is commonly used by codes of practice to assess the bearing value of roads subgrades (where the material size used is smaller than 20mm as mentioned before).
Despite PBT and CBR are thought for different ground
conditions involving distinct methodologies, when conducting a PBT, a CBR
equivalent value can be obtained. Remember that both methods evaluate the
vertical deformation and strength of the soil. It could be useful to assess the
capacity of the ground when looking for CBR standards of quality.
Plate Bearing Test
A Plate Bearing Test determines the ground's ultimate
bearing capacity, the shearing strength and the likely settlement under a given
load (BS 1377 Part 9: 1990). This method covers the determination of the
vertical deformation and strength characteristics of soil in situ by assessing
the force and amount of penetration with time when a rigid plate is made to penetrate the soil. The metal plate deflection level is limited to a low value
and will be in the 1.25mm-5mm order. As mentioned before, this test is used
when the testing involves large particle sizes or solids that have a harder
texture, as can be crane pads or formation and sub-formation levels for
foundations.
From the raw data, a load-settlement curve is obtained. The modulus of subgrade reaction, the recommended maximum design pressure and the plastic-elastic deformation of the ground (yield pressure) could be obtained from this curve. Also, PBT is a quick way to get a CBR value (UK DMRB, IAN 73/06), either for small or large particle soils. The CBR test and the formulation to obtain the equivalent value from a PTB are exposed next.
California Bearing Ratio
The California Bearing Ratio test is an in-situ testing
method used to evaluate roads and pavements' subgrade strength. In road design,
the subgrade's CBR value determines the thickness of the upper layers and the pavement(Design
Manual for Roads and Bridges).
The CBR can, therefore, be mathematically expressed as:
CBR equivalent value obtained from a PBT
When conducting a PBT, an equivalent CBR value could be obtained from the following equation (UK DMRB, IAN 73/06):
The K762 is the modulus of subgrade reaction (kN/m3) for a
762mm testing plate. K762 is calculated for the stress (in Kpa or KN/m2) that
causes 1.25mm settlement. The PBT could be performed with different testing
plate sizes; therefore, to calculate the K762, a correction factor should be
applied. Thus:
CBR values
When performing a field CBR test, the results are obtained
for the existing ground conditions and the soil' saturation is not controlled. Thus,
it is not easy to make a direct correlation between field and lab CBR tests.
Field materials are often less than 80% saturated, whereas soaked lab samples
are near 100%. The soil is more resistant to
penetration under drained conditions; therefore, field test should give higher CBR values than soaked samples.
The chart below provides a rough idea of what to expect for field CBR values of
different soil types.
