Predicting Four Day Sub-Grade CBR Strength from Unsoaked Laboratory Specimens
8 Pages Posted: 21 Oct 2013
Date Written: October 21, 2013
Most construction projects take place on soil and fewer projects are carried out on solid bedrock. Therefore the bearing capacity of any soil must be evaluated prior to the construction of any road. The bearing capacity or strength of granular materials for road construction is measured in terms of California Bearing Ratio (CBR). This test method has been used for the past seven decades with very limited improvements in its lifetime especially with regards to the time it takes to complete. It is considered one of the most fundamental tests of any granular material in road construction. It takes any soil laboratory a period of at least seven days to produce a comprehensive set of CBR and Indicator tests. The former is in essence a five day long test method. The waiting period means whatever progress that can be made with regards to construction on site will in the meantime be all based only on experience of site technical staff and very little scientific reliance. Therefore there is a need to make improvements on current test methods in order to expedite such a lengthy test procedure. The methodology followed in this research included extensive soil laboratory testing, particularly CBR tests on identical samples that are compacted at Optimum Moisture Content (OMC) and compressed after soaking at varying daily intervals. The results of such tests for all the specimen are then plotted on a graph to obtain a trend that will best represent the plotted data such that a formula can also be developed. An equation aimed at obtaining CBR strength of materials within a shorter timeframe than the current five day period it takes to soak and compress the soil specimen has been derived from the obtained data. Preliminary findings reveal that the equation is ideal for use on weaker gravels used as subgrade for road pavements as it has only been tested on such materials. These are materials that generally have a CBR strength ranging from 3% to 15%. Thus far, the formula has provided an impeccable correlation with the conventional four day CBR strength test method. The equation, similarly to other test methods such as DCP, provides a rapid and accurate way to determine the CBR of weaker materials. Ordinarily, the Maximum Dry Density (MDD) and CBR test alone would take five days to complete and the proposed formula shall drastically reduce that turnaround time as it manages to remove the entire four day soaking period. This means the CBR of such materials can be confirmed in two days, the same day as the actual MDD.
Keywords: CBR; soaking; maximum dry density; sub-grade; optimum moisture content
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