There are various types of load-bearing tests. For description purposes we will briefly discuss the

The California bearing ratio is a measure of the shearing resistance of a soil under carefully controlled conditions of density and moisture. The CBR is determined by a penetration shear test and is used with empirical curves for designing flexible pavements. The test procedure used to determine the CBR consists of two principal steps. First, the soil test

Figure 13-8.Laboratory CBR test equipment and tools.

specimens are prepared; second, a penetration test is performed upon the prepared soil samples. Although one standardized procedure has been established for the penetration portion of the test, it is not possible to establish one procedure for the preparation of test specimens since soil conditions and construction methods vary widely. The soil test specimens are prepared to duplicate the soil conditions existing (or expected to occur later) in the field. Although penetration tests are most frequently performed on laboratory-compacted test specimens, they may also be performed upon undisturbed soil samples or in the field upon the soil in place. Detailed procedures for preparing the test samples and performing the test can be found in NAVFAC MO-330.

Figure 13-8 illustrates the equipment and tools needed to perform the CBR test. The principal piece of equipment is the CBR loading press. It is used to force the penetration piston into the compacted CBR specimen. The complete loading-press assembly (fig. 13-9) includes a penetration piston, proving ring and proving-ring dial, penetration dial, and a mechanical (or

Figure 13-9.Assembled CBR loading press.

motorized) type of jack. Three proving rings are available with capacities of 2,000, 5,000, and 7,000 pounds, respectively. The surcharge weights are used to approximate (within + 5 pounds) the expected weight of the pavement and base in the field. The tripod attachment (when fitted with a dial indicator) and the swell plate are used to measure the expansion, or swell, of the material in the CBR mold. Other items needed to perform the test are equipment and tools, such as a balance or scale, a CBR mold, a 10-pound tamper, mixing bowls, spoons, spatulas, a soaking tank or bucket, and moisture canisters.