CBR and Plate Bearing Tests

Both are very common geotechnical field tests. People often get confused about which test they require or how to interpret the results of the tests.

California Bearing Ratio (CBR) Testing

Originally developed in California (hence the name) this test is generally used to design pavements/roads or for concrete slab foundations. It measures the relative compaction of granular backfill such as gravel or aggregate by measuring the load required to push a small piston (approx 50mm diameter) into the ground, with measurements taken at fixed penetration increments. The results are reported as a percentage, with typical values being 3-5% for clays and 15-30% for aggregates. Because the piston is relatively small, the test is not suitable for coarse aggregates (e.g. Type II backfill or coarser).

Plate Bearing Test

Or Plate Loading Test.

This test is used throughout the world. The UK version uses circular plates, varying in size up to 760mm diameter.

The plates are loaded by hydraulic jack and their settlement measured at increasing load increments. A graph is then plotted of settlement against bearing pressure. This can then be used to determine whether the ground has sufficient bearing capacity to support a given structure such as temporary pads for crane outriggers or piling rigs. Because the circular plate is much bigger than the piston used in a CBR test, the plate bearing test is more suitable for testing larger aggregate backfills but it does require a minimum 15 tonne excavator or other plant to use as a static load.

For floor construction, it is strongly recommended that values of ‘modulus of subgrade reaction k’ are determined from a plate-loading test. Different plate sizes to match the combination of design load required and reaction weight (i.e. excavator) available. Larger plates give greater accuracy and it is preferable to use a plate of the British Standard diameter of 750 mm. If other loading plate diameters are used it is necessary to employ a conversion factor. The minimum size plate used should be 300 mm.

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