Field in-place CBR tests are used for evaluation and design of flexible pavement components such as base and subbase course and subgrades and for other applications (such as unsurfaced roads) for which CBR is the desired strength parameter. If the field CBR is to be used directly for evaluation or design without consideration for variation due to change in water content, the test should be conducted under one of the following conditions: (a) when the degree of saturation (percentage of voids filled with water) is 80 % or greater, (b) when the material is coarse grained and cohesionless so that it is not significantly affected by changes in water content, or ( c) when the soil has not been modified by construction activities during the two years preceding the test. In the last-named case, the water content does not actually become constant, but generally fluctuates within a rather narrow range. Therefore, the field in-place test data may be used to satisfactorily indicate the average load-carrying capacity.

Any construction activities, such as grading or compacting, carried out subsequent to the bearing ratio test will probably invalidate the results of the test.

Soils and flexible pavement components at the same location may exhibit significantly different load deflection relationships. No method presently exists to evaluate the precision of a group of non-repetitive plate load tests on soils and flexible pavement components due to the variability of these materials.

Note 1-Field in-place tests are used to determine the relative strength of soils, subbase, and some base materials in the condition at which they exist at the time of testing. Such results have direct application in test section work and in some expedient construction, military, or similar operations. Also, as indicated in 4.1, field in-place tests can be used for design under conditions of nominal stability of water, density, and general characteristics of the material tested. However, any significant treating, disturbing, handling, compaction, or water change can affect the soil strength and make the prior to test determination inapplicable, leading to the need for retest and reanalysis.

Note 2-Notwithstanding the statements on precision and bias contained in this standard: The precision of this test method is dependent on the competence of the personnel performing it and the suitability of the equipment and facilities used. Agencies which meet the criteria of Practice D 3740 are generally considered capable of competent and objective testing. Users of this method are cautioned that compliance with Practice D 3740 does not in itself assure reliable testing. Reliable testing depends on many factors; Practice D 3740 provides a means of evaluating some of those factors.