Development of an instrumented portable device that measures shear, sinkage and friction properties of soil in-situ

Abstract

Cone index values are often used as indicators of soil strength in empirical traction prediction equations. One of the main advantages of the cone penetrometer is its simplicity and ease with which it can be used to obtain field data. Motion resistance of off-road vehicles is related to the compressibility of soil and that of the traction device. Therefore, cone index appears to be an adequate indicator of compressive ability of soil. However, tractive characteristics of off-road vehicles depend on shear and sinkage characteristics of soil and cone index values alone appear to be unable to represent these aspects of soil behavior adequately. Bevameter or instrumented devices that provide shear and sinkage characteristics of soil are research devices and are not as simple and easy to use as a standard cone penetrometer. There is a need to develop a simple, portable device that can measure soil shear, frictional, and sinkage characteristics and relate its outputs to fundamental engineering properties of soil and/or develop parameters that could be used to predict tractive ability of off-road vehicles. The objectives of this study were to (i) develop an instrumented portable device that can measure shear, sinkage, and frictional properties of soil in-situ; (ii) semi-automate the data collection process by retrofitting the instrumented device to a hydraulic soil penetration system that is installed on a tractor; and) conduct verification tests using a commercially available cone penetrometer and other fabricated devices. The instrumented portable device is composed of a cone tip (ASAE standard small cone) that will provide cone index values with depth and in addition, will measure soil-metal friction when the unit is rotated at desired depths. Moreover, the device also consists of a conical shear vane unit to measure soil shear characteristics at desired depths. Furthermore, it is equipped with an ultrasonic depth sensor to monitor the depth at which cone index, soil-metal friction, and soil shear characteristics measurements are obtained. The operation of the device consists of pushing the unit into the soil just like a cone penetrometer to obtain cone index values with depth. At desired depths, the whole unit is rotated to obtain soil-metal friction and shear characteristics. An ultrasonic depth sensor provides depth information. Torque acting on the cone surface is measured by a strain gage set mounted on the inner shaft near the cone tip, and another strain gage set which is also mounted on the outer tube will measure total torque needed to operate the device. Vertical loads acting on the cone tip and shear vane during operation is measured by two load washers. The device has been calibrated extensively to make sure that all sensors are working as designed. The presentation deals with the design, development, and testing of the device in the laboratory and some preliminary tests in the field.