Our Forgotten Friend - the Dynamic Cone Penetrometer

Abstract

Investigation and testing of subgrade soils plays an extremely important part in the pavement design phase of a road rehabilitation/construction project. One of the parameters required for pavement design is to obtain a design subgrade CBR value. Pavement engineers can derive this design input parameter in a variety of ways, however common approaches in most instances is to utilise laboratory CBR test results (often soaked for periods of 4 or 10 days), insitu derived CBR values that can be estimated from DCP testing in the field, or a combination of both. One of the advantages of the laboratory CBR test is the determination of the swell characteristics of the soil sample. High swell values (eg. greater than 2.5%) can provide useful information that minimum cover requirements or treatment of the subgrade through stabilisation, replacement or incorporation of geosynthetics may be required to reduce the effects of the expansive behaviour. Often though, the quantum of laboratory CBR tests available to a designer to evaluate constitutes a very small portion of the project size. On the one hand, some designers rely solely on soaked CBR values. Other projects do not have laboratory tests conducted to establish CBR’s and reliance is placed solely on CBRs derived from the DCP test, as this test is quicker and more of these can be done in the field during the investigation phase. However the DCP test produces no direct associated swell values and the results can be subjective based on the soil type being tested and the moisture condition of the soil at the time the test was conducted. The challenge then arises in how to effectively derive a design subgrade CBR value when multiple test methods are available, yet guidance on how to actually interpret and deal with various data sets is not well documented. This paper explores the various ways a subgrade design CBR value can be obtained and provides a recommended approach based on extensive experience from the authors in both construction and design over many decades. relationship between insitu CBR values obtained from the DCP test and swell characteristics, based on hundreds of test results from the eastern states of Australia.