Earth pressure measurement

Two general types of measurements are taken

• Stresses in a soil mass

• Stresses on a boundary at soil structure interface.

Earth pressure cell design requirements

When an earth pressure cell is introduced into a soil mass the stress field in the vicinity of the cell is modified owing to redistribution of stresses.

It is this modified stress field which is recorded by the cell.

The basic problem in earth pressure cell design is to obtain a measure of the stress which would have existed in the ground had the pressure cell not been inserted.

Pe/P= B/D(N/Es-N/Ec)

1+B/D* N/Ec

Where P is the field stress existing at the plane of the cell in absence of cell

Es is stress/ strain modulas of soil

Ec is stress/strain modulas of the cell

D is diameter of cell

B is half thickness of the cell

N is a property of the soil such that N/D is analogous to coeff. of subgrade reaction.

Types of earth pressure cell.

The most popular cell has been that consisting of a diaphragm. Some Other cells rely on a rigid piston sensing element while a very flexible cell unit is also used.

Electrical earth pressure cell

The diaphragm type total earth pressure cell, the deflection of the diaphragm is also measured and by calibration the pressure can be related to the diaphragm deflection. The central deflection of the diaphragm is usually limited to less than 1/2000 times the diameter.

Methods used

Strain gauges( electrical resistance)

Vibrating wire gauges.

Strain gauges( electrical resistance)system

With this system the cell error occurs due to cross sensitivity.

Cross stress is meant a stress acting in the plane of the diaphragm eg. an earth pressure cell measuring the horizontal stress beneath a loaded foundation.

Even where the outer ring is very stiff the cell may still be sensitive to cross stresses because the cross stresses causes the diaphragm to be loaded in compression.

The error is maximum when the cross stress acts in the direction of the measurement of the gauge.

The error due to cross sensitivity can be minimized by the arrangement of the strain gauges.

Vibrating wire gauge

The cell comprises a rigid housing and a circular membrane.

The membrane has two protruding arms between which the gauge wire is stretched.

When the external pressure is applied to the membrane, the protruding arms rotates slightly and cause an increase of stress in the gauge wire.

Thus a change in frequency of the gauge wire is caused and by calibration a measure in change of the average earth pressure acting on the membrane is obtained.

For the measurement of stress in earth fill the cell is attached to a steel plate so that the membrane is flush with one side of steel plate.

Hydraulic cellsA hydraulic sensing pad is embedded within the soil.

The stresses in the soil surrounding the sensing pad applies pressure to the pad and this pressure is measured by a mechanical technique.

The sensing pad which may be circular or rectangular shaped, has large area to thickness ratio.

Thus it is a very thin disc and consequently stress distortion effects due to variation in the modulus of the sensor pad and the soil are very small.

Thus the unknown stress acting on the cell boundary or in the soil mass is measured by determining the hydraulic pressure in the connecting tubing.

A small and constant volume of oil is pumped through the cell and when the line pressure equals the pressure in the field in the cell, the pressure diaphragm in the pressure chamber deflects and thus permits oil to flow through the bypass orifice and into return line.

The pressure in the supply line cannot increase further and hence the cell pressure is equal to the line pressure at the entrance to the bypass chamber .

Because the pump may be at a very different elevation at the cell unit a gauge elevation correction equal the the difference in elevation of the cell and pressure measuring transducer times the specific gravity of the oil, must be applied.

The oil in the cell is pressurised.

Thus the resultant earth pressure is equal to the bypass gauge pressure plus gauge elevation correction minus the zero gauge reading.

eg is the Goltz cell

Main advantages

It is simple and contains no delicate partsLong term reliable measurements are possibleCorrections are simple to determine and apply gauge unit is cheapThe return line may be connected to several cells

EARTH AND CONCRETE PRESSURE CELL

Earth and concrete pressure cell is designed to measure total pressure in earth fills and embankments; as well as pressure on the surface of retaining walls, buildings, bridge abutments, tunnel linings and to measure stress in mass concrete.

Important Features: Accurate, robust and low cost Long term stability with high reliability High sensitivity and wide pressure range Low volumetric displacement Ease in data logging Thermistor included for temperature measurement

Earth Pressure Installed in fill for soil measurement in three direction

SHOTCRETE-CONCRETE STRESS CELL

The 'New Austrian Tunneling Method' (NATM), calls for the support of a tunnel by the rapid application of shotcrete to the freshly exposed ground.

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. verifying design assumptions that will promote safer and more economical design and construction.

shotcrete-concrete stress cell is designed for the measurement of radial or tangential stress in shotcrete tunnel lining.

It is suitable for measurement of stress in the rock walls of unlined caverns and pressure on & within the lining of underground excavations.

It is used to assess the adequacy of shotcrete lining, for monitoring back filling in mines and evaluating foundation bearing pressures.

The stress cell basically consists of a flat rectangular capsule comprising of two rectangular plates welded along the periphery.

A narrow gap between the plates is filled with fluid using a special process which guarantees that all the air is excluded.

Lugs are provided at the corners of the rectangular capsule to facilitate holding the cell in plane while the shotcrete is applied.

Pressure is sensed by a vibrating wire stainless steel body pressure transducer connected to the flat circular capsule by a stainless steel tube.

The pressure transducer incorporates the vibrating wire, the coil magnet assembly and the cable joint housing.

The rigidity of the cell exceeds 50,000 MPa. This ensures that it will respond immediately and accurately to the onset of increasing concrete stress.

A thermistor inside the transducer housing allows the temperature to be read.

During concrete lining, temperatures very often rise and will cause the cell to expand in the still green concrete.

On cooling, the capsule contracts, which, if allowed to remain as such, would prevent the transmission of pressure from the concrete to the cell.

A pinch tube is consequently provided to inflate the capsule after the concrete around it has fully cured and has cooled off to the ambient temperature.

One end of a 600 mm long pinch tube is welded to the pressure transducer and the other end is capped.

Increase in stress in concrete causes a corresponding rise in the fluid pressure as the rectangular steel plates are squeezed together.

This is transmitted through the fluid to an internal integral diaphragm, which forms part of the pressure transducer.

Attached to the other side of this diaphragm is a vibrating wire strain gage.

Change in pressure causes the diaphragm to deflect, thus altering the tension of the wire.

Any change in the tension of the wire, affects its resonant frequency of vibration which is proportional to the pressure acting on the cell.

Important features:

Easy installationStainless steel constructionPinch tube for re-inflation in concrete

Rugged, waterproof and highly reliableVibrating wire pressure cell assures long

term stability, quick and easy readoutThermistor included for temperature

measurement

JACKOUT PRESSURE CELL

Jack Out Earth Pressure Cells are used to measure active and passive pressures on diaphragm walls.Jack out Pressure Cells are used to measure earth pressures on slurry walls.They are designed to fit inside a plate which is pressed against the side of a slurry wall excavation using a hydraulic jack arrangement.

This method of installation ensures that the jack out cell is located with its sensitive face in contact with the adjacent soil.

Jackout Pressure Cell Hydraulic Jack Assembly

Jackout Pressure Cell Assembly Installed in Diaphragm Wall

Jack-out pressure cell is designed to measure the total stress i.e. the effective stress due to the soil together with the pore water pressure in the voids between soil grains.

It is suitable for measuring static or slowly varying stresses only.