Cone penetrometer with a helical-type outer screw rod for evaluation of the subgrade condition

A weak railway subgrade may lead to instability of the railway track system. The condition of a railway subgrade istypically evaluated using trial pit excavation and a portable ballast sampler, which may severely disturb the ballast layer. Also, nondestructive geophysical techniques, such as ground penetrating radar (GPR), are used to investigate the condition of the ballast and subgrade along the track. The objective of this study is the development and application of acone penetrometer with a helical-type outer screw rod (CPH) that minimizes the disturbance of the ballast layer for the evaluation of the subgrade condition by measuring the resistance of the cone tip. The CPH consists of a helical-type outer screw rod (helical rod) and a mini cone penetrometer inside the rod. The design concerns include easy installation ofthe helical rod into the ballast layer, the reaction force of the mini cone penetrometer, installation of the strain gauge, and calculation of the cone tip resistance. The helical rod, which penetrates into the ballast layer with a rotationforce, provides the reaction force for the mini cone penetration test. The mini cone penetrometer with a diameter of 15 mm is pushed into the subgrade (sand layer) at the penetration rate of 1 mm/s. In addition, dynamic cone penetrometer (DCP) tests are carried out to compare the cone tip resistance with the DCP index (DCPI). Field application tests reveal that the cone tip resistance increases with an increase in the depth and is inversely proportional to the DCPI. On the basis of the cone tip resistance, the soil types of subgrade at three sites are classified as sandy silt to silty sand. This study suggests that the CPH may be a useful device for providing a reliable geotechnical parameter and estimating thesubgrade condition with minimal disturbance of the ballast layer.