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CBR and Plate Testing

CBR Testing

The Californian Bearing Ratio (CBR) test is a penetration test used to evaluate the bearing capacity of subgrade for roads and pavements. The test is fully described in BS1377: Part 9: 1990: Soils for civil engineering purposes. It is performed by measuring the pressure required to penetrate a soil sample with a plunger of standard area. The reaction load is usually provided by a four wheel drive vehicle such as a Landrover. The measured pressure is then divided by the pressure required to achieve an equal penetration on a standard crushed rock material. The harder the surface, the higher the CBR value. The test is primarily intended for use on cohesive soils that have a maximum particle size of less than 19 mm.

Ashford CBR IMG_20200228_125421

Plate Bearing Test

Plate Bearing (or Loading) Test is used to determine the ultimate bearing capacity of the soil and the probable settlement under a given load. The results of the test are usually used for designing temporary structures such as working platforms for piling rigs or pads for crane outriggers.

The test is performed by gradually applying an increasing  load to a cylindrical plate at a specific  test depth and recording the settlement for each increment of the load. At the point that the settlement becomes rapid, the total load up to that point is calculated. The value obtained is then divided by the area of the plate to obtain the ultimate bearing capacity of soil at that depth. The ultimate bearing capacity is then divided by a safety factor (typically 2.5~3) to determine the safe bearing capacity.  The necessary load (known as kentledge) is usually a provided by a 15 tonne excavator or similar.