The Dynamic Cone Penetrometer (DCP) is modified impact hammer that has been designed to estimate the insitu CBR strength of soils and in pavement design, the subgrade of the pavement structure.
This tool is particularly beneficial to develop an understanding of the insitu support, comparatively to that of laboratory prepared sample testing which is tested at insitu moisture contents and compaction levels.
It is widely used within pavement investigations for its simplicity which involves the dropping of a nominal 9kg hammer repetitively, and measuring the vertical displacement of the calibrated rod and cone tip. The strength of the subgrade is then calibrated from empirically acquired data for materials that exhibit similar resistance to the hammer drops.
Given the nature of the test, the Dynamic Cone Penetrometer (DCP) can be used to identify bands of weaker subgrades, higher moisture contents and changes in materials and has been used historically to identify high water tables.
Although a very versatile measuring tool, the DCP has known limitations, including:
- Surcharge Loading – The strength of a subgrade material is dependent on the confining pressure and the associated surcharge loading of the pavement structure above. Typically when the DCP testing is undertaken, all surcharge loading is removed (i.e. in a borehole or test pit) and may not ideally represent the insitu strength.
- Seasonal Moisture Contents – The use of Dynamic Cone Penetrometer (DCP) CBR results in isolation can misrepresent the functional CBR of the subgrade, given that unless the test is undertaken periodically, the seasonal moisture fluctuation may not be represented. Additionally, if the pavement is subjected to inundation, the in-service moisture levels may be drastically underestimated.
- Cohesive Materials – Current methods of correlating the no. blows/100mm or mm/blow has not been accurately verified for non-cohesive materials (i.e. sand and gravels) and is not considered a reliable method when these subgrade materials are encountered.
Given the cost-efficiency of undertaking Dynamic Cone Penetrometer (DCP) testing, it is a popular approach in identifying the in-service subgrade conditions, however it must be undertaken in caution and when a diligent approach is required, it is typically coupled in with applied judgment and laboratory testing to provide confidence in the outcomes.
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