Cone Penetrometer with a Helical-Type Outer Screw Rod for Evaluation of the Subgrade Condition

Abstract

A weak railway subgrade may lead to instability of the railway track system. The condition of a railway subgrade is typically 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 a cone 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 of the 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 rotation force, 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