![]() ![]() The advantage of the CBR method is that it can be used to find the total thickness of the pavement and that of the individual courses in addition to the thickness of the subgrade soil (provided the CBR-values of the materials of the courses are also known). ![]() The California bearing ratio (CBR) value of the subgrade soil was the basis for the method of design of flexible pavements, developed originally by the California State Highway Department, and adopted by The Road Research Laboratory, London, for developing their own design procedure and design charts. (This method is obsolete and is of historical importance.) The higher the group index, the weaker is the subgrade soil. The group-index of a soil, an arbitrary index assigned to different soil types based on the percent fines, liquid limit and plasticity index, is defined by the following equation – ![]() Highway Engineers (Highway Research Board) in 1945. The group-index method was devised by the U.S. IRC Guidelines for Low Traffic Volume Roads: (IRC: SP: 72-2007 and IRC: SP: 77-2008) 5. Hence, only the following methods will be presented here: 1. Some of the empirical and semi-empirical methods like the ‘group-index method’ (based on soil classification and traffic volume), California resistance value method (based on the results of Hveem’s stabilometer and cohesiometer tests), North Dakota core method (based on cone penetration test values), Mc Leod’s method (based on plate load test results), and even theoretical-empirical approaches like the triaxial test method and Burmister’s (two-layer) method have been practically superseded by more rational approaches based on long-term road tests on experimental stretches. ![]()
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