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Top tips for monitoring pore pressure

SITE INVESTIGATION CONTRACTS

GEOTECHNICAL OBSERVATIONS is using 23 of its modular piezometers to monitor pore pressures at hourly intervals in a 20m high railway embankment near Bristol.

They have been installed to depths of 17m, along with vertical and horizontal inclinometers to monitor slope and track deformations.

Geotechnical Observations' modular piezometer is a recoverable system for monitoring positive or negative pore pressures in earth structures (for example embankments, cut slopes and behind retaining walls). Each comprises a 60mm diameter permanent plastic stem with a porous tip at the bottom.

The tips can be fitted with three types of filter. Type I is a small bull-nosed ceramic tip (100kPa blow through) and requires a hole with a minimum diameter of 70mm.

Type II is a cylindrical Casagrande style filter made from a porous ceramic material (100kPa blow through) and requires a hole with a minimum diameter of 70mm.

Type III is a cylindrical Casagrande style filter made from porous plastic and requires a hole with a minimum diameter of 80mm.

All configurations can be installed in boreholes and drilled with window sampling equipment, cable percussion rigs or rotary rigs.

The choice of tip is determined by the depth of the installation and the ground conditions.

Each piezometer stem can be fitted with an open standpipe or a flushable electric piezometer. Open standpipes are available in sizes from 10mm to 50mm and the water level in the pipe can be monitored automatically by using a pressure sensor or manually.

The flushable electric piezometer locks into the piezometer stem just above the porous filter. It pumps water into the filter section, which removes air or gas that may be present.

When the filter section is full, the piezometer can be sealed using a hydraulically operated piston. This isolates the water in the piezometer from the water in the tubes and ensures the pressure measured (positive or negative) is that in the ground at the depth of the porous filter.

Pore pressure is recorded on a datalogger and can be downloaded to a hand-held computer for on-site examination and transferring to a desktop computer.

Within each piezometer stem the open standpipes and the flushable piezometers are interchangeable and can be exchanged or removed for recalibration or replacement at any time.

This makes the equipment suitable for long term monitoring. At the end of a project the electrical sensors can be removed and the stem can be sealed for future use.

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