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The Use of the Plate Load and California Bearing Ratio (CBR) Tests in Construction

Prior to the construction of buildings and roads, soil load bearing tests are required.  Two of the most common tests are the Plate Load Test and the California Bearing Ratio (CBR) Test. 

Aims

A Plate Load Test determines the actual strength, as well as the bearing capacity of ground, and is used when the testing involves large particle sizes or soils that have a harder texture. The CBR test is commonly used for road/pavement design to assess the strength of the sub-grade, and is suitable for areas where the particle sizes are no more than 20mm.

The findings are commonly applied in the designing of temporary working stations for piling pads or rigs for crane outriggers, as well as spread foundations for building slabs, wind turbines and electrical sub stations.

Process

The Plate Load Test is carried out in accordance with the British Standard 1377 1990: Part 9 and consists of loading a steel plate and recording the settlements corresponding to each load increment. The test is usually carried out at foundation level. A load is applied in five increments with readings taken every minute until detectable movement of the plate has stopped. During the test the pressure exerted is added at a consistent rate.

Plant machinery (such as an excavator) is used as a kentledge in the test procedure. The minimum kentledge weight required is approximately 8 tonne however, to test to higher loads, heavier plant will be required to achieve higher loading pressures. There are 3 plate sizes available when testing - 300mm, 450mm and 600mm.

Results from the test consist of raw data, load-settlement curve, and modulus of subgrade reaction (K) for road design. Also, a fairly accurate California Bearing Ratio (CBR) value can be derived from the sub grade reaction in this test. The Plate Load Test method of CBR testing allows testing over a greater surface area than that of a traditional CBR plunger test, which therefore enables testing of both cohesive and granular material.

The below infographic explains the process to determine which test is most suitable. 

SOCOTEC offers both Plate Load Tests and CBR Tests nationwide as part of a comprehensive range of field testing services. Contact us for more information. 

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