Vibrating wire piezometers are used for the measurement of pore water pressure and/or ground water levels and are used typically in the following applications:

 

Dams

Used mainly to control fill placement, monitor pore water pressures, up lift pressures and to indicate seepage rates.

 

ELEMENT                  MEASURAND (PARAMETER)

Foundation                  Water pressure

Embankment               Water pressure

Abutments                  Water level/ Water Pressure

Reservoir                    Water level

 

Embankment Constructed for Consolidation (surcharge)

Embankments may be created to consolidate the ground below; the aim of this method is squeeze out the pore water, so that the ground that is constructed on is less likely to settle after construction has been completed.

To consolidate the ground faster band drains are located in the ground to allow a faster route for the water to escape. Piezometers are used to monitor and identify when pore water pressures have dissipated to the required level.

It also determines the level of consolidation when dynamic compaction is used.

 

ELEMENT                    MEASURAND (PARAMETER)

Embankment                Water pressure

 

Potential Mass Movements and Landslides

Soil strength, seepage and soil mass are key issues which can result in the failure of a slope.

 

ELEMENT                    MEASURAND (PARAMETER)

Cuttings                       Water pressure

Embankments              Water pressure

 

Pumping tests

Pumping tests are carried out to determine the rate of pumping which is required to dewater the soil. The test includes measuring the flow, plot the drawdown curve and identify the radius of influence which is used to design a dewatering system. Dewatering is required where excavation is to be carried out below the water table in non-cohesive soils to ensure stability of the excavation.

 

ELEMENT                 MEASURAND (PARAMETER)

Groundwater              Water level

 

Dewatering Systems

Piezometers are used to measure groundwater levels and thus the effectiveness of the dewatering system. They can also be automated with alarms to provide an early warning system in case of pump failure.

 

ELEMENT                 MEASURAND (PARAMETER)

Groundwater              Water level

 

Groundwater recharge 

Where dewatering is carried out adjacent to structures the subsequent consolidation of the soil as the water is removed can cause damage. To minimise settlement water from the dewatering wells is pumped back into the soil (recharge) and piezometers are used to monitor groundwater levels and determine the effectiveness of recharge. They can also be used to monitor the pressures being applied to the wall.

 

ELEMENT                 MEASURAND (PARAMETER)

Groundwater              Water level

 

Retaining/Diaphragm Walls   

Retaining and diaphragm walls are used for deep excavations which are typically below groundwater tables. To confirm their stability during construction earth pressures need to be calculated from the measured water pressures.

 

ELEMENT                    MEASURAND (PARAMETER)

Foundation                    Water pressure

Retained Pressure         Water pressure

 

Fully grouted method

The Use of the Fully-grouted Method for Piezometer Installation

Although first introduced into the market by Vaughan in 1969 the use of the “Fully Grouted Method” is becoming more and more popular as it is a faster, easier way to install piezometers. It eliminates the conventional sand filter and Bentonite seal.

The piezometer is lowered into the borehole, sometimes tied to a grout pipe, and the entire borehole is backfilled with a non-shrinking, low permeability grout.  See also Multi-point vibrating wire piezometers.

There has been extensive research and testing which showed that the error in the measured pressure is significant only when the permeability of the borehole backfill is two orders of magnitude greater than the permeability of the surrounding ground. If the permeability of the cement-Bentonite grout is lower than the permeability of the surrounding ground, measured pressures will be without error. As a result, for the fully-grouted method to work, the grout mix used to backfill the borehole must meet certain permeability requirements.

Source:http://www.geosense.co.uk/products/details/vibrating-wire-piezometers/application

Link to report of this instrument: EPIE