How is a Pressure cell used in geotechnical field?

What is a Pressure Cell?

A pressure sensor is a transducer which converts pressure into a measurable electrical output.

Main purpose to use a pressure cell is to measure total stress in various construction fields or the amount of stress being transferred.

The vibrating wire pressure transducer works on the vibrating wire technology to provide a digital output.

Pressure cells are of different type, shapes and size.

How is a Pressure cell used in geotechnical field?

The theology of soil/rock formations is a complicated function of soil/rock type, stress history, shear and normal stress levels, boundary and drainage conditions and many other environmental effects. Accurate assessment of stress in foundation soil/rock formations and changes in it caused by construction and loading is important for good engineering design. In instrumentation schemes for geotechnical or geo-structural study associated with large civil engineering structures like tall buildings, dams, underground tunnels etc., measurement of stress plays a very important part.

Stress fall into two basic categories:

  • Measurement of total stress at a point within a soil mass/foundation rock/concrete structure.
  • Measurement of contact stress against the face of a structural element.

The development of vibrating wire stress cell introduced a reliable and fast method of taking stress readings electrically. The cable is carried from the stress cell to the read out unit or data logger and is protected against any possible damage during construction to give all around reliable data.

Encardio-rite vibrating wire concrete pressure cell (or stress cell) is the electrical sensor of choice as frequency output of the vibrating wire sensor is almost immune to external noise, it is able to tolerate wet wiring conditions common in geotechnical applications and is capable of transmission of signals to long distances.

Pressure cells provided by Encardio-rite are as follows:

  • Earth Soil pressure cells

The vibrating wire earth pressure cell is designed to measure total pressure in earth fills and embankments. It basically consists of a flexible, circular flat capsule 200 mm f x 7 mm thick, constructed from two stainless steel discs welded around the periphery and connected by a stainless steel tube to a specially designed Encardio-rite pressure transducer. The whole system is fluid filled. The vibrating wire pressure transducer incorporates the latest vibrating wire technology to provide remote and digital readout.

  • Concrete pressure cells

The concrete pressure cell is used to measure total pressure in concrete. The measurement of total pressure helps in verifying design assumptions that will promote safer and more economical design and construction and also monitoring for safety and warning of pressure in excess of those, the structure is designed to withstand.

Construction: The concrete pressure cell basically consists of a flexible circular flat capsule, constructed from two stainless steel discs welded around the periphery and connected to a specially designed pressure transducer by a stainless steel tube. A pinch tube is provided to ensure positive contact between the concrete and the circular flat discs. The whole system is fluid filled.

  • Soil and rock-concrete interface pressure cells

The EPS-30V-I interface pressure cell is used to measure pressure between soil and rock or concrete interfaces. The pressure cell is very much similar to the earth soil pressure cell, except a thicker diaphragm on rock/concrete side of the pressure pad and is rigid, which minimizes point-loading effects.

  • Jackout pressure cell

The jack out pressure cell has been designed to measure the stress on base slabs, diaphragm/slurry walls etc. Construction of a jack out pressures cell is a combination of a thin, flexible, round, flat diaphragm and a thick and rigid back plate. The plates are welded around the periphery leaving a very little gap between the two. The electron beam welded vibrating wire pressure sensor has been welded concentrically with the back plate and the cavity inside the sensor is filled with de-aired fluid.

  • Shotcrete-concrete pressure cell

The Shotcrete-concrete pressure cell is used to monitor the radial and tangential stresses in the shotcrete of tunnels particularly where New Austrian Tunneling Method and underground works are being implemented. Measurement of radial and tangential stresses within shotcrete lining is a vital feature of this type of stress cell. The stress cell has a rectangular pressure pad which is constructed with two stainless steel plates, welded around the periphery. The pressure pad and the electron beam welded vibrating wire pressure sensor are connected together by a 6 mm Φ x 165 mm long stainless steel tube. The cavity inside the pressure sensor and the pressure pad is filled with de-aired fluid.

 

APPLICATIONS OF EARTH SOIL PRESSURE CELLS

It has applications in the measurement of stress in soil, concrete mass or foundation rock including:

  • Total stress on and within liners of underground excavations as input to improve design and construction practices.
  • Magnitude, orientation and distribution of principal stresses within embankments and dams as input to improve design and construction practices.
  • Total stress for studying soil/structure interaction behavior.

Most commonly used pressure cells are of two types:

  • Earth Soil pressure cells
  • Shotcrete Concrete Pressure Cells

These pressure cells are used in various applications and are presented in a nutshell:

Earth Pressure cells are commonly used for instrumenting the following types of structures:

Embankment dam

Earth pressure cell being installed in an embankment

Installation of earth pressure cells in an array provides data to determine the distribution, size and direction of total stress in the soil core.

Tailings dam

Installation of pressure cell confirms that the tailings material is consolidating at design rate. This can paln out for the increase in drainage. Also the pressure transferring on the foundation is known.

Concrete dam

In concrete dams it measure contact pressure of abutments and foundations. It also helps in conforming material competence and also the total pressure on the foundation.

Diaphragm or sheet pile wall

During construction of diaphragm or sheet pile walls, the active pressure applied on it are within the limits or not. It also measures uplift pressure on foundation

Retaining wall

In retaining walls the earth pressure cells detects the active pressures.

Buried structures

It provides estimate of total horizontal stress acting on structure. It also provide estimate of total uplift forces acting on structure

Shotcrete Concrete pressure cells are specially used in New Austrian Tunneling Method (NATM):

NATM (New Austrian Tunnelling Method)

NATM being carried out

The NATM constitutes a method where the surrounding rock or soil formations of a tunnel are integrated into an overall ring-like support structure. Thus the supporting formations will themselves be part of this supporting structure. The theory behind this method of support, particularly useful in weaker grounds, is that if the inherent strength of the ground can be preserved, it will be almost self- supporting and will require much less artificial support in the form of concrete or steel. Proper evaluation of total stress may help in:

  • The adequacy of the shotcrete lining,
  • Indicating the need for perhaps more or less shotcrete to maintain stability.

The Concrete Vibrating Wire Stress cell measures radial and tangential stresses in shotcrete, concrete and rock, usually in tunnel linings. Cells can also monitor stress changes in the rock walls of underground works. In this case, slots are first machined by diamond wheel sawing, or line drilling, and the cell is embedded in cement mortar within the slot.

Typical monitoring applications include:

  • Tunnel linings
  • Mass concrete