CAMBRIDGE SELF BORING EXPANSION PRESSUREMETERS
Mark VIIId and Mark Xd
[Click on image to obtain a large view]
Specification:
The Cambridge Self Boring Expansion Pressuremeter is a hollow cylindrical
probe, a little over one metre in length and 83 mm in diameter. In operation
the probe is jacked steadily into the ground and soil entering the shoe of
the instrument is cut into small pieces by a central rotating cutter. The
soil cuttings are brought to the surface up the inside of the instrument
by fluid, normally water, pumped down the cutter rods. The cutting shoe is
made very exactly the same size as the body of the instrument.
About one half of the outer surface of the probe is covered by a membrane expandable against the soil by gas pressure. The membrane is made from polyether urethane and is protected by a sheath of flexible stainless steel strips bonded to a rubber sleeve known as a chinese lantern. The radial expansion of the membrane is measured by pivoted levers which are kept in contact with the membrane by spring cantilevers. These springs are strain gauged and indicate the movement of the membrane, the radial strain. There are two versions of the self boring pressuremeter, the Mark VIIId has three strain sensing arms and the Mark Xd has six..
A cell in the body of the probe measures the gas pressure within the instrument. A further two cells, bolted through the membrane and moving with it as it expands into the soil, read pore water pressure directly. In the Mark Xd instrument the pore pressure cells lie between the strain arms numbers 3 and 4 and between numbers 6 and 1. The localised stiffening of the membrane due to the pore pressure cells thus occurs after the passage of the strain arms.
In the Mark VIIId the original arrangment of pore pressure cells is used between arm 1 and 180° round from this.
The signals from the electrical transducers pass into the sealed multiplexer chamber. Here they are converted into a digital serial data train and are fed to a three wire serial RS 232 port from which they are taken to the surface via a four wire armoured logging cable inside the gas pressure line. The fourth wire conveys the total strain signal for use by a Strain Control Unit.
the Interface Unit at the surface receives a 12 volt DC signal, normally from a car battery, supplies regulated power to the down hole instrument, receives the returning signals and distributes them to the on-site computer and to the Strain Control Unit. There are separate and two electrically isolated outputs for computers.
The instrument is supplied mounted on a tubular metal frame which also carries the cable drum on its windlass and the interface unit. This frame is designed for use as a workbench on site.