SLOPE INDICATOR - -JACKOUT PRESSURE CELLS

Application

The jack-out total pressure cell measures active and passive pressures on diaphragm (slurry) walls. It can also be used in soft ground to measure radial pressures on tunnel linings. Typical applications include:

• Verifying design assumptions.

• Monitoring for safety: warning of earth pressures in excess of those the structure is designed to with-stand.

Operation

Total pressure is the intergranular pressure in the soil (effective stress) combined with the pressure of water in the voids between soil grains (pore water pressure). The total pressure cell is formed from two circular plates of stainless steel. The edges of the plates are welded together to form a sealed cavity, which is filled with a non-compressible fluid. The cell is installed with its sensitive surface in direct contact with the soil. The total pressure applied to that surface is transmitted to the fluid inside the cell and measured with a pneumatic or vibrating wire pressure transducer.

The jack-out total pressure cell is specially designed for installation in cast-in-place structures, such as diaphragm walls. Its name is derived from the use of a hydraulic jack that is activated to keep the cell in contact with the soil during concreting.

Installation Overview

The jack-out cell, together with a rigid steel support plate, a reaction plate of similar dimensions, and a double-acting hydraulic jack, is installed in the reinforcing cage. The signal cable and hydraulic hose are secured, and the cage is lowered into the slurry trench. When the cage is in position, the jack is activated and locked, forcing the cell into contact with the soil. The jack acts on the support plate, rather than directly on the cell, to prevent distortion of the cell. An equal force is applied to the reaction plate. The trench is then concreted.

In the illustration at right, the jack has been activated, and the cell is in contact with the soil. Concrete, delivered through a tremie pipe, displaces the slurry. This is a generalized description of the installation process. The project engineer should determine the actual procedures to be used.

Contact with Soil: Good contact with the soil is maintained by the hydraulic jack. This prevents concrete from entering between cell plate and soil.

Conformance: The cell is designed to have a stiffness similar to that of a typical soil. Fluid in the cell is de-aired to maximize sensitivity.

Accurate Calibration: The jack-out total pressure cell is calibrated under fluid pressure in a specially designed test fixture, eliminating errors typical of other calibration techniques.

Easy Installation: The jack-out cell is designed specifically for the job. Installation components are easy to fabricate, and the cell can withstand rough handling and concreting.

Manual or Automatic Readout: Manual readouts can be used with both the pneumatic or the vibrating wire versions. The vibrating wire version is also suitable for connecting to data loggers.