Evaluation of the electrical density gauge for in-situ moisture and density determination
Densification of soil during construction of earth structures is achieved through the process of compaction by application of mechanical energy to obtain the required engineering properties of the soil for a particular project such as hydraulic conductivity, soil strength and compressibility. These properties are dependent on attainment of high compaction densities normally achieved at specific moisture contents for a given compactive effort. The optimum moisture content and maximum dry density for a particular soil is determined by means of Proctor tests in the laboratory. A relative compaction index is then used to correlate the laboratory values with the field compaction values obtained using in-situ tests. The Sand Cone (SC) and Nuclear Density Gauge (NDG) are the common field tests used to the dry density and moisture content of the soil for purposes of quality control of the compaction process. The sand cone is a laborious test that involves excavation of part of the compacted layer and requires a 24-hour waiting period to obtain the moisture content of the soil through the laboratory oven method. The NDG on the other hand is less laborious, however it uses a radioactive source that is a potential health hazard and therefore requires strict handling, storage and maintenance of the equipment to maintain safety standards. The Electrical Density Gauge (EDG) is an alternative in-situ test that is quicker, safer and easier to maintain since it uses electric current to measure the compaction characteristics of the soil. The objective of the study was to determine the repeatability, accuracy and applicability of the EDG on South African soils by comparing its measurements for dry density and moisture content in the laboratory and in the field to the results from the sand cone and oven method. In the laboratory, a clean sand and a clayey sand were tested at the optimum moisture content and at ± 3% of the optimum moisture content. The soils were compacted to 200 mm using the RT74 rammer and the compaction values first tested using the EDG then followed by the sand cone test at the centre of the EDG test spot. The moisture content of the excavated sample from the sand cone test was determined using the oven method. For the field tests, the compaction characteristics of a sandy gravel and three uniformly graded sands were tested in-situ using the EDG followed by the sand cone test. Overall, the EDG measurements were repeatable based on test-retest comparison of the paired measurements. EDG results for moisture content were consistent with the values obtained from the laboratory oven method especially in the uniformly graded sands. However, the density measurements differed from the results of the sand cone test, which was considered the reference test for determination of field soil density. It is recommended that the EDG calibration relationship for bulk density be revised in order to improve the accuracy of the density measurements.
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