Technical Reference

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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 relationship between simple strain and true strain

Techref Number: 
CITR1002

The relationship between simple strain and true strain

Simple (or unit) strain is the change in length over the original length, so that for a pressuremeter measuring radius it can be expressed as

...[1]

$$\xi _s=\frac{r_i - r_o}{r_o}$$

where $\xi _s$ is simple strain
$r_i$ is the current radius of the cavity
$r_o$ is the original radius of the cavity

From equation [1] it follows that

...[2]

$$\frac{r_i}{r_o}=1+\xi _s$$

True (natural, or logarithmic) strain is defined as the sum of each incremental increase in radius divided by the current radius, so

...[3]

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.

Pressuremeter testing in waste

Techref Number: 
CITR1004

There is considerable interest in measuring the engineering properties of waste. Cambridge Insitu have used a self boring pressuremeter (shown in Figure 1) to make in situ tests in and around domestic waste sites.

Running a Mark 2 Permeameter Test

Techref Number: 
CITR1005

1) Calibrating the system

  1. The pressure cell and flow pump must be calibrated against a standard.

  2. The probe itself must be demonstrated to be pressure tight. This is done before field work starts by capping off the lower end of the instrument then operating the full permeameter system as if it were a real test.

  3. A successful demonstration would show that even at the lowest flow rates of which the system is capable the internal pressure continues to rise - a leakage path shows itself as an apparent steady state.

Using pressuremeters โ€“ 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.

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.

Internal tests carried out in the field at Rectory Farm

Techref Number: 
CITR1008

The test site at Rectory Farm, Little Eversden, Cambridge, England

The Cambridge Insitu/Eversden test site is located behind the Rectory Farm in the village of Little Eversden, approximately 10km south of Cambridge. The site is an unused field owned by Mr. Clive Dalton of Cambridge Insitu. Some of the early field testing of the SBP (1970's) was conducted here and the site has been used on numerous occasions for SBP demonstrations and other experimental purposes (Whittle, 1998).