Pressure Cells

NATM Pressure Cell

Geosense® NPC-3000 Series NATM Pressure cells are designed to monitor stress of shotcrete in the construction of tunnels, particularly those using the New Austrian Tunnel Method and other underground works. Monitoring of the radial and tangential stresses within shotcrete linings is vital to its success.

The cells are constructed from two stainless steel plates welded around their periphery with the narrow gap between the plates filled with hydraulic fluid.

As the stress increases within shotcrete or concrete the fluid pressure within the cell rises as the plates are squeezed together.

A length of stainless steel tube connects the plates to a pressure transducer (VWDT 5000 or SGT 3000) that converts the pressure to an electrical signal which can be read directly with a MP12 readout or data logged. 

NATM cells installed in concrete expand with increasing temperature as the concrete or shotcrete cures. After cooling the cell will contract leaving a gap between it and the concrete so that the stresses are prevented from reaching the well. In order to continue to monitor the stresses the cell can be  re-pressurised using the attached pinch tube to re-expand the cell so that it once again comes in contact with the concrete surface. 

Total Earth Pressure Cell

TPC-4000 series Total Earth Pressure cells are designed to measure total pressure (effective stress and pore water pressure) in soils and at the interface between structures and the wall of excavation.

They are constructed from two stainless steel plates, welded around their periphery with the narrow gap between the plates filled with hydraulic fluid.

External pressure on the outside surfaces of the cell squeezes the two plates together creating an internal fluid pressure. A length of stainless steel tube connects the cell to a pressure transducer which converts the fluid pressure into an electrical signal which can be directly readout or transmitted to a data logging system.

Models 4010 and 4020 are designed to measure soil pressures on structures. It is fitted with an extra thick back plate on one side which is placed against the structure so as to avoid any warping of the cell. The other side has a thin plate welded to the back plate which provides sensitive measurement of soil pressures.

Push-in Pressure Cell

A Push-in Pressure Cell, also called Spade Cell, is designed to be pushed into the ground where it can measure total earth pressure and pore water pressure within the soil. It can be used as a site investigation tool to determine the in-situ stress state, both vertical and horizontal, depending on the direction of installation. In addition, it can be used to monitor the change in active and passive pressure around retaining structures (diaphragm walls etc) as well as in tunnelling, and other earthworks. Typical installations are in fine grained cohesive soils, including very soft to stiff clays.

Construction consists of two longitudinal stainless steel plates, welded together around their periphery. The annular space between these plates is filled with de-aired glycol. A port and filter for pore water pressure measurement are located on one of the flat sides of the support plate behind the pressure sensitive section of the cell.

The pressure cell and the port for the pore water pressure are connected via stainless steel tubes to two pressure transducers integrated in the cell, typically vibrating wire, or pneumatic, or strain gauged if dynamic measurements are to be performed. A thermistor for temperature measurement is also incorporated.

Flat Jack

Flat jacks are designed to carry out in-situ testing of masonry structures and rock.

The flat jack is constructed from two stainless steel plates welded around their periphery, with the narrow gap between the plates filled with hydraulic fluid. It is inserted into a slot cut into the structure to be monitored and gradually brought  up to pressure with a special hydraulic pump.

As stress increases within the structure or rock,the fluid pressure within the cell rises as the plates are squeezed together and it is possible to derive the stresses acting in the structure test area.

A length of stainless steel tube connects the plates to a pressure transducer (VWDT 5000 or SGT 3000) that converts the pressure to an electrical signal which can be read directly with a MP12 readout or data logged.