Deep Multi-point Piezometer Installation in a Borehole

What role does Piezometer play in Geotechnical monitoring?

Pressure experienced by water contained in pores of earth materials, concrete structures or rock is generally called pore water pressure. In any instrumentation scheme for geotechnical or geo-structural study associated with large civil engineering structures like tall buildings, dams, underground tunnels etc., measurement of pore water pressure (also known as piezometric level) plays an important part.

The study of pore pressure has following main purposes:

· Effect of water in pores of soil or rock is to reduce load baring capacity of soil or rock. Effect is more pronounced with higher pore water pressure leading eventually in some cases to total failure of load bearing capacity of the soil.

· To determine level and flow pattern of ground water

· Determine flow pattern of water in earth/rock fill & concrete dams and their foundations and to delineate the phreatic line.

Piezometer Application Areas

Encardio-rite vibrating wire piezometer is used widely for geotechnical applications as its frequency output is immune to external noise, it is able to tolerate wet wiring common in geotechnical applications and it is capable of transmission of signals to long distances. It has applications in the measurement of positive or negative pore pressure in soil, concrete mass or rock including:

· Construction control, stability investigation and monitoring of earth dams, embankments, foundations, shallow underground works and surface excavations.

· Uplift and pore pressure gradients in foundations, embankments, abutments and fills.

· Hydrological investigation, ground water elevation study and water supply operations.

· Pore pressure studies in relation to waste and environmental applications

· Monitoring of pore pressure for soil improvement & stability and for slope stability

Why fully grout method better for multi-point piezometer installation in borehole?

Installing multiple piezometers in a borehole using the traditional method, which involves placement of sand pockets surrounding the piezometers, bentonite seals and grout columns to separate piezometers at different levels, is slow and complex. It also poses a risk of unintended communication between piezometers.

Installation of multi-level piezometers in a borehole is preferred with fully grouted method as it results in excellent zone isolation, accurate readings and fast response to changing pore water pressures.

This installation method is quick and easy and allows installation of piezometers in boreholes shared with other geotechnical instruments like inclinometers. This saves a considerable amount of time and cost spent on the installation.

Vibrating wire piezometers are the electrical piezometers of choice to implement this method as these require very little volume of water to register a change in pore water pressure, thus resulting in a fast response time.

Multi-point or multi-level Piezometer in a deep borehole

Encardio-rite has in past successfully implemented fully grouted method in a five level piezometer installation in a 95 m deep borehole, in one of our projects in the U.A.E. The multi-level installation in such a deep borehole posed a number of challenges that included placing of the piezometer at correct level, buoyancy effect on piezometer, protection of sensor and cable, backfilling of the borehole, etc.

Model EPP-30V vibrating wire piezometer were used in this application as they are heavy duty and more robust against any damage that may occur at deeper depth and increased pressure.

The five piezometers were attached to the outer surface of a ¾” PVC pipe at the desired depths of 90, 60, 30, 15 and 4 m, as the pipes were being attached and lowered down the borehole. Tip of the piezometers (filter side) were pointed upwards to avoid escape of de-aired water filled between the ceramic filter and the sensing diaphragm. Cables from the piezometers were secured to the pipe itself.

The borehole was fully grouted using the same ¾” PVC pipe with cement bentonite grout. Ratio of cement, bentonite and water used was 1:0.3:2.5 by weight. The grouting was done in stages to eliminate uplift and voids. Readings from piezometers at various levels are shown in the graph below.

The installation time, which could have been substantial using the conventional method with placement of almost 15 different layers of material (sand and bentonite seals) and that too with a great risk of failure, was drastically reduced using the fully grouted method.