Piezometers

Piezometers are used to measure the pore pressure of soils or rocks, within a defined response zone. Each type of piezometer has differing performance characteristics, and most commonly vibrating wire piezometers are used since they are very fast response devices, easy to data log and nowadays are an economical choice.
Casagrande Piezometers.
	Casagrande piezometers are a simple open tube fitted with either a ceramic or porous sintered plastic filter unit normally installed within a defined response zone. As such, they are the simplest means of measuring pore pressure. Since a finite volume of water is required to flow into or out from the tube to maintain a pressure equilibrium, they should not be used in fine grained soils since time response to pore pressure changes may be considerable. They are normally monitored manually using a Dipmeter. It is now becoming common to use a down hole datalogger (see our AquaBAT water level logger), and where hydro-dynamic response is a problem this can be mitigated using packered installations.
Hydraulic Piezometers.
	Hydraulic piezometers consist of a simple ceramic filter unit connected to twin small bore tubes which connect to a pressure measuring unit remote from the piezometer. The twin tubes are filled with deaired water to form hydraulic continuity from the piezometer to measuring unit. In practice, this device is limited to a minimum pressure at the measuring unit of minus 6 metres water head since over time air will come out of solution in the hydraulic system causing errors. Whilst this is routinely corrected by flushing of water through the system, this is particularly time consuming.
Vibrating Wire Piezometers. Vibrating wire piezometers are constructed as a electron beam sealed capsule with a stainless steel flexible diaphragm at one end connected via a high tensile steel wire anchored to the body of the capsule at the distal end. Deflection of the diaphragm, from imposed pressure, changes the tension and corresponding resonant frequency of the wire. A pair of electrical coils are used to excite the wire to vibrate and to output the frequency of the resonant vibration. There is a direct relationship between the square of the resonant frequency and the applied pressure. Since the signal output from the piezometer is in frequency form, it is not materially affected by changes in resistance of cables and the frequency is not changed if resistance changes, unlike voltage analogue signals. As such, cables from piezometers may be in excess of 1 km without adverse effects. Vibrating wire piezometers are exceptionally fast response devices, responding to pore pressure changes within milli-seconds as there is negligible volume change. Since they are easy to data log (see our AquaLOG section) and contain no electronics, they are the instrument of choice for long term monitoring for critical structures such as Dams.
Vibrating Wire Heavy duty Piezometer.
	This unit is a piezometer constructed with an extremely strong case suitable for direct burial in fill for embankments for Dams etc. This unit is suitable for site connection to cables.
Slim Vibrating Wire Piezometer for Borehole applications.
	These units are slim 20 mm diameter suitable for installation in boreholes, and also can be used to convert 25 mm ID Casagrande piezometer to automatic data capture.
Vibrating Wire Up lift Transducer.
	This unit is similar to the Heavy duty piezometer, but instead of an intake filter, it is fitted with a threaded connection. Typically in the galleries of dams, tubes are installed up and downstream of a grout curtain. These tubes are fitted with "Tee" piece onto which an Up lift transducer can be fitted and also allowing water to be released from the tube, which in normal operation is under effective artesian pressure. In this way, pore pressures in the dam can be data logged, and also by drawing off water from the tube changes in permeability of the foundation of the dam can be assessed.
Attapulgite "Salt water Bentonite"; Pellets.
	Conventional bentonite clays fail to swell in saline conditions where either the soil is saline or seawater has been used for flushing the drill hole. Increasingly, seawater is being used as a flushing medium in areas where fresh water is at a premium. In drillhole installations, bentonite clay seals are used to isolate sections of the hole for sampling, sealing and pore pressure determination. Without adequate seals, the entire monitoring system is liable to errors. Attapulgite, unlike bentonite, swells and forms impermeable gel structures in salt water by the charged nature of Palygorskite particles. Our Attapugite borehole sealing pellets are manufactured from naturally occuring Attapulgite (Smectite and Palygorskite). Pelletising of the Attapulgite allows the pellets to be placed at depth in a drill hole with minimum risk of bridging in the hole.