The Combined {V, H, M} Loading Apparatus

Prof. Guy Houlsby and Byron Byrne

A general overview of the laboratory is shown below left - it is located in the general civil engineering laboratory on the ground floor of Jenkin's Building, on Parks Rd. The major components of the laboratory are the V:H:M loading rig, two large sample tanks, an intelligent stepper motor controller and a Pentium computer. A 16 bit A/D card is used to log all the necessary transducers, at speeds of up to 10 kHz. A program has been written in Visual Basic which takes care of all data logging requirements. The program also issues the required stepper motor control commands to the intelligent stepper motor controller via an RS232 communications link. The sample tanks have been designed so that it is possible to perform eight separate tests on the same sample (around the perimeter), as well as a bearing capacity test in the centre. During preparation of the saturated sand it will be necessary to apply a downward hydraulic gradient to the sample, thus the need for the large plated lid.

The V:H:M rig (shown below centre) was developed by Chris Martin during his stay in the Civil Engineering Group (Martin, 1994), to examine the performance of spud-cans on clay samples. Essentially a large plate slides vertically relative to the loading rig frame. Another plate slides horizontally relative to the first plate whilst the foundation arm rotates relative to this second plate. All movements are controlled by three stepper motors which drive the different plates and foundation arm. Typically commands for the stepper motors are issued from the control program which detail the number of steps, the velocity and acceleration of each. The rig can thus independently apply vertical, horizontal and rotational displacements to a footing. A Cambridge Insitu 'Stroud' Cell allows measurements of the corresponding footing loads. These load measurements can be utilised to provide independent load control on any of the three axes using PID control loops. The rig has been designed to apply vertical loads up to 3kN, horizontal loads up to 600 N and moments to 50 Nm, and typically footings have diameters of 100mm to 150mm. Coarse measurement of the position of the footing is made by three long stroke LVDTs. Whilst a system of three small LVDTs (10mm stroke - see below right) measures accurately (to about a few microns) the position of the footing utilising a simple trigonometric manipulation.