Test methods can include a single sample tested at optimum water content as determined by ASTM D or D , a series of three test specimens at optimum water content tested at different maximum dry unit weights, or the testing of each sample used in the development of a compaction curve. The two latter methods may also require compaction of several samples with different blow counts to adjust the final density of the sample materials.
Selected test protocol should be clearly understood by all stakeholders, and especially the testing lab before preparation begins. Soaking accounts for adverse moisture conditions from potential rainfall or flooding, and most CBR tests use this procedure.
In addition to the compaction process, preparation usually involves soaking each specimen in water for 96 hours before the penetration test. During the soaking period, a surcharge load of 10lbf 4. The soil swell resulting from soaking must be measured using expansion measuring apparatus and swell plates placed on the sample before penetration testing. It takes a significant amount of time to prepare multiple compacted specimens for a single test. Complete CBR testing sets containing multiple CBR molds and components are available to maximize testing productivity and efficiency.
Note: Unsoaked tests are not unusual in areas where rainfall amounts and the local water table are both low. For penetration testing, the sample remains in the mold. With the perforated plate and the specified surcharge weights in place on the soil, the piston is seated against the test sample, and loading commences at a rate of 0.
As the piston penetrates the soil, test loads are recorded at eleven specified depths up to 0. Penetrations vs. Corrected stress values from penetration depths of 0. In most cases, CBR values decrease as penetration increases. The CBR value is the stress at the 0. Occasionally, the ratio at the 0. If the check test shows similar results, the stress from the 0.
Remove the perforated plate and the top filter paper. Weigh the soaked soil sample and record the weight. Draw graph between Load versus Penetration. California Bearing Ratio Test 1. Penetration of Plunger mm Standard Load kg 2. Collar A detachable extension collar of 60 mm height.
Weights Fig. The dry density for a remoulding shall be either the field density or the value of the maximum dry density estimated by the compaction test Heavy Compaction Test as per IS Part-8 - , for Railway Formation.
The water content used for compaction shall be the optimum water content or the field moisture as the case may be. Procedure for Penetration Test Place the mould assembly with test specimen on the lower plate of penetration testing machine. To prevent upheaval of soil into the hole of the surcharge weights, 2. Seat the penetration piston at the center of the specimen with the smallest possible load, but in no case in excess of 4 kg so that full contact of the piston on the sample is established.
Set the load and deformation gauges to read zero. Apply the load on the piston so that the penetration rate is about 1. Record the load readings at penetrations of 0. Raise the plunger and detach the mould from the loading equipment. Take about 20 to 50 g of soil from the top 30 mm layer and determine the moisture content. Find and record the correct load reading corresponding to each penetration. The CBR test forms part of the site investigation and is used to determine the thickness of materials needed for the proposed road construction.
The CBR test is performed by measuring the pressure required to penetrate a soil sample with a plunger of standard area. 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 CBR test is carried out on soils with a maximum particle size of 20mm.
Note: For material greater than 20mm please see Plate Bearing Tests. The technique involves driving a small cylindrical plunger approx 50mm into the ground at a uniform rate, using a four wheel drive vehicle as the reaction load to provide the force. Tests are normally carried out at surface level or at depths of between mm in m intervals along the proposed construction centreline.
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