SBP

Self-boring pressuremeter

Notes on the nulling of strain gauge bridges

(Note: This recipe is written for an HPD95 but the techniques described apply to transducers in all our instruments)

There are a number of strain gauged transducers in the probe. Occasionally the output of a transducer needs to be nulled, that is, set to a particular output for a given state. This may be because a transducer has been replaced, or because a gradual change in output over time has taken the circuit beyond an acceptable limit.

Calibrating an arm - SBP

Introduction

Strip the Instrument as described in Dismantling a Pressuremeter – SBP until the arms are exposed and are free to move. Secure the Pore Pressure Cells in the instrument using a cable tie or PVC insulation tape.

If the arms are to be calibrated after a series of tests then calibrate immediately, because it is important to know the calibrations for the tests that were made. If not, then consider whether the arms and their location slots should be cleaned and restored to pristine condition first.

Calibration Sheets

In the attachments above you will find copies of calibration sheets for arm and pressure cell calibrations for Cambridge Insitu pressuremeters.

Notes on the nulling of strain gauge bridges

Techref Number: 
CITR1007

(Note: This is written for a 95HPD but the techniques apply to all our instruments)

There are a number of strain gauged transducers in the probe. Occasionally the output of a transducer needs to be nulled, that is, set to a particular output for a given state. This may be because a transducer has been replaced, or because a gradual change in output over time has taken the circuit beyond an acceptable limit.

Using pressuremeters – worked examples

Techref Number: 
CITR1006

This technical reference document provides examples of pressuremeter tests from a range of materials with illustrations of how engineering parameters can be derived.

CASE A.
Analysis of a self bored pressuremeter test in London Clay

The most straightforward test to analyse is an undrained cavity expansion and contraction in clay, where a self boring pressuremeter has been used. The insertion disturbance is likely to be small and the undrained path means it is easy to calculate radial and circumferential stresses and strains directly from the displacement and pressure measurements made by the instrument. There are a number of analyses that can be applied; what is described here is one approach. The test itself was over water so depth is referred to bed level.

Kolkata Metro East - West Project

Twenty four self bored tests were carried out at four critical locations along the alignment of the proposed metro in Kolkata, India. The pressuremeter testing component of the site investigation had been specified by W S Atkins. For the most part the tests were conventional in material that behaved either as a clay or sand. What was different about this project was the technical and practical difficulties that had to be overcome to achieve success - see figure 1.

figure 1 self boring rig in Kolkata
Fig.1 Self boring in Kolkata (click for full-size)

Testing waste and investigating barrier walls

A view across a municipal waste site where pressuremeter testing was carried outWe were approached by Dr Neil Dixon of Loughborough University (now Professor of Engineering) about the possibility of using a pressuremeter to investigate the mechanical properties of municipal solid waste. Most of the work took place at a landfill site in Calvert, Buckinghamshire. The waste was a mixture of residential and commercial residue, not well-sorted, in various stages of degradation and depending on its age could be lightly to heavily compacted.

The primary purpose of the testing was to obtain engineering parameters that would permit the interaction between the body of the waste and the components of the protective barriers to be modelled and quantified.

The underground research facility at Mol, Belgium

Horizontal self boring in Boom Clay using air from a modified drill rigWe have at intervals over the last 15 years made visits to the SCK-CEN facility at Mol, Belgium, to carry out pressuremeter testing in the underground research facility HADES. This is a system of shafts and tunnels some 224 metres below ground level in a zone of Boom Clay in a highly plastic condition. The clay has interesting self healing properties when fractured, displays extremely low levels of permeability and offers a possible solution to the problem of the disposal of high level nuclear waste. Since 2000 the facility has been run by an expert group called EURIDICE and pressuremeter testing has been used during the construction of the facility and after to examine the engineering properties of the clay.

Using a self boring expansion pressuremeter to measure the permeability of soils

Techref Number: 
CITR1001

Background

This note describes the use of a standard self boring expansion pressuremeter of the Cambridge design to measure the coefficient of permeability of soils. The advantage of self boring a cavity is the minimal disturbance caused to the soil and the consequent likelihood that permeability parameters derived from the test will be representative of the insitu state. Testing can be carried out significantly faster than laboratory methods will allow.

The myth of the finite pressuremeter geometry correction

Techref Number: 
CITR1003

Introduction

Soil strength and stiffness properties are obtained from pressuremeter tests using analyses that depend on solutions for the expansion of an infinitely long cylindrical cavity. Real pressuremeters have length to diameter ratios between 3 and 10. Studies using finite element methods have indicated that this finite pressuremeter geometry leads to significant over-estimation of the shear strength. This paper tests the finite element results and shows that they do not predict the shear stress-strain response observed in real tests in the field. The conclusion is that end effects associated with finite pressuremeter geometry are of negligible significance for the derivation of material strength.

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