Civil Engineering July 2022 | Vol 30 No 6

26 July 2022 Civil Engineering I t is standard practice in South Africa to soak all California Bearing Ratio (CBR) test samples for all layers for four days or, in some cases, for one day if it is shown that this is sufficient. Soaking is either carried out on the three moulds compacted at the usual three efforts as required in the TMH1:1986 or now the SANS 3001‑GR40 procedure and/or on the moulds used to determine the com- paction characteristics of subgrade mate- rials. In the latter procedure, the so-called maximum Proctor CBR is determined, which is usually slightly higher and at a lower moisture content than the CBR at the Proctor maximum dry density (MDD) and optimum moisture content (OMC). However, there is a growing feeling that the use of soaked CBRs is unneces- sarily conservative for well-drained pavements in the drier areas of Southern Africa which almost never become soaked and where it has been shown that the equilibrium moisture contents are normally less than OMC. For such purposes it is also necessary to determine the CBR at lower moisture contents than soaked. However, as the CBR is also a test of material quality, the soaked test is also always advisable to ensure that the material is not excessively moisture sensitive. There are two procedures for ob- taining this information: unsoaked CBRs on the specimens used for the determina- tion of the compaction characteristics (MDD and OMC) and unsoaked CBRs on specially prepared specimens. The former procedure is convenient and does not re- quire the preparation of extra specimens. However, as the CBR is probably both stress (and moisture) path dependent, the latter should give more realistic results and should also be done whenever pos- sible, or in preference. UNSOAKED CBR ON MDD/ OMC SPECIMENS Q Q Seal the specimens used to determine the compaction characteristics in plastic bags, keep for at least four days to dissipate pore pressures and deter- mine the unsoaked CBR (see note 1). Q Q Plot the unsoaked CBRs on the com- paction characteristics diagram as well as the dry density (DD) and moisture content (MC). The effort to be used must be specified, but (except possibly for subgrade samples) is usually Modified AASHO (MAASHO) i.e. 2 413 kJ/m 3 in South Africa (this is less than the current heavy AASHTO T180 effort of 2 695 kJ/m 3 ). STANDARD CBR The standard CBR test involves a four-day soaked CBR and swell at the usual three efforts, all compacted at MAASHO OMC. The following information is always required, but it is not necessary to repeat the tests provided they were done on split samples: Q Q Report the CBRs calculated at the usual percentages, compaction both by the traditional method of using the MDD as the reference density and the new SANS method which will be the A compaction density (i.e. MAASHO) achieved using the same water content and effort. Q Q Report the average soaked moisture content calculated by weighing the moulded specimen and soaking the base plate before and after soaking (see note 2). STANDARD UNSOAKED CBR This test is conducted at the usual three efforts at MAASHO OMC and approxi- mately half OMC. Q Q Mix and split into two or three por- tions as required. Q Q After compaction at MAASHO OMC at the usual three efforts, test one set Dr Frank Netterberg CEng Independent researcher: pavement materials and geotechnics frank.netterberg@gmail.com Determination of unsoaked California Bearing Ratios