SELF BORING PRESSUREMETERS - AN INTRODUCTION

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The self-boring pressuremeter test is quite unlike any other soils test. In producing large amounts of accurate data very quickly, and requiring no correction factors derived from experience, it is unique. In very soft clays and in fine running sands, a self-boring instrument is the only way to discover the full insitu properties.

The Cambridge Self-Boring Pressuremeters have been in production since 1975 and in full commercial site investigation from 1980, in all parts of the world.

THE INSTRUMENTS

There are two different Cambridge Self-Boring Pressuremeters or Camkometers. They are respectively the Expansion Pressuremeter and the Load Cell Pressuremeter. Each makes its own hole using a specially designed self-boring head. The difference between the two is that the Expansion Pressuremeter is an active device, expanding a cavity out into the soil surrounding the instrument and measuring the pressures required to produce a given change in radius. The Load Cell Pressuremeter is a passive device; once it has been placed at the correct location it senses the pressures on the outside of the instrument and so measures the total insitu stress and the pore water pressure continuously, indefinitely, and in any horizontal direction. It does no expansion or loading of the soil.

Each instrument can be used both in clays and in sand.

DRILLING

Both Pressuremeters use a specially developed and patented self-boring technique which results in the instrument entering the ground with an exceptionally small amount of disturbance to the soil. Apart from a very thin layer of sheared soil in direct contact with the instrument (estimated to be, in clays, not thicker than thirty microns), the original state of all the soil outside the instrument is maintained both as to position and to the state of insitu stress. It is this very low disturbance which accounts for the different character of the test data compared, for example, to pressuremeter tests carried out in a prebored hole.

The self-boring process works as follows. A cutter at the foot of the instrument rotates inside an internally tapered shoe. As the instrument is pressed steadily against the bottom of the hole, a plug of soil is extruded into the taper much as if it were a conical extrusion die. The top of this plug of soil is sliced off by the cutter positioned inside the shoe such that the pressure needed to drive the soil up the taper is made equal to the insitu vertical stress. The soil cuttings resulting are carried away up the inside of the instrument by a flow of flushing fluid, normally water, supplied from the surface. This water flows, in all but the most permeable soils, in a closed circuit and does not affect the properties of the soil outside the instrument.

TESTING

The differences between the two instruments become apparent at the testing stage.

The Load Cell Pressuremeter carries on its outside curved surface sensitive load cells subject to the total insitu stress from the soil on their outside and to the internal gas pressure in the instrument on their inside. By adjusting the gas pressure, the external soil stress can be nulled, directly measuring the total insitu stress at six points equi-spaced around the circumference of the probe. Additional cells measure the pore water pressure and the instrument internal gas pressure at each of the six positions.

Fig.2

The active instrument, the Expansion Pressuremeter, is surrounded over half its length by a suitably tough and protected elastic sleeve or membrane, initially the same outside diameter as the cutting shoe. The test is carried out by applying gas pressure to the inside of this sleeve and measuring the resulting changes in radius of the elastic sleeve as a cavity is formed in the soil. The pressure at which the sleeve lifts from the rigid body of the instrument gives the insitu total stress. Two pressure cells mounted through and moving with the sleeve as it expands, give continuous readings of the pore water pressure. This test is normally carried out at constant rate of strain, automatically. A holding test is also possible where a given expansion is carried out and then held constant while the changes in total and effective insitu stresses with time are recorded.

READINGS
All the measurements made by the instruments are transmitted to the surface by a protected cable passing up inside the gas supply line. At the surface the readings can be read immediately, plotted, printed out, stored on floppy disc or fed into an on-site computer, all as required by the user. We offer systems to do all or any part of this.

ANALYSIS
From the readings of the Expansion Pressuremeter the curves shown below (fig.3 and fig.4) are obtained and some of the important properties shown are marked. It should be noted that these are total, not effective stresses.

To derive the shear stress/shear strain curve from the soil several methods of analysis are available ranging from a straightforward graphical approach up to computer curve fitting techniques.

A list of what can be determined is given below:

FOR CLAYS:

• the insitu horizontal stress, both total and effective
• the direction of the major and minor horizontal stresses
• the initial shear modules
• secant and tangential shear modulus for shear strains in the range 10^-4 to 10^-2
• the undrained shear strength c_u
• the sensitivity
• the full shear stress/shear strain curve
• the pore water pressure
• a coefficient of horizontal consolidation

FOR SANDS

• the angle of Shearing Resistance
• the angle of dilatation
• secant and tangential shear modulus

The Load cell Pressuremeter gives a more restricted range of readings, but unlike the Expansion Pressuremeter, will repeat them as often as necessary, even to days and weeks if necessary. These readings are:

• The insitu horizontal stress, both total and effective
• The complete horizontal distribution of the insitu stress
• The pore water pressure

Fig.3

These plots are taken from a self boring pressuremeter test in clay at a depth of 4.9 metres. This is a typical pressure v stain graph with the corresponding shear stress/strain graph derived by the analysis due to Palmer (1972) shown in fig.4. All the other features appearing in the fully analysed set of tests in both clays and sand are discussed, illustrated and explained in a separate leaflet.

Fig.4

ADVANTAGES

For the Self-boring pressure meter these can be summarised as follows:

• the tests are performed on virtually undisturbed soil
• a large number of fundamental soil properties are obtained from a single test.
• to derive these properties, no empirical correcting factors whatever are needed
• the test is controlled by a semi-automatic system and is largely independent of operator influence
• results can be obtained quickly
• commercial operation has shown that the instruments, though more complex than conventional site investigation equipment, are reliable and have enough redundancy to permit useful readings even if a single fault appears

DISADVANTAGES

• the instrument will not penetrate gravels, claystones or the like
• operating in sands usually demands a cased borehole to a level one or two metres above the desired test locations.
• failure planes and deformation modes are not usually appropriate to those occurring in the final design
• only two stress paths can in practice be followed, undrained and fully drained
• undrained tests must usually be performed at high rates of strain so introducing errors
• the instruments and their associated equipment are complex by conventional site investigation standards
• results obtained are sometimes surprising and in several cases have challenged conventional assumptions of soil mechanics

REFERENCE

Windle D. and Wroth C.P. Insitu measurements of the properties of stiff clays with self-boring instruments. 9th Int. Conf. on S. M. F. E. Tokyo (1977)

Hughes J.M.O., Wroth C.P. and Windle D., Pressuremeter tests in sands. Geotechnique V 27 No 3 (1977)

The main leaflet contains a full list of references running now to more than 30.

EQUIPMENT

We supply any and every piece of equipment necessary for carrying out Cambridge self-boring pressuremeter tests both on land and over water shallow enough for self-boring.

Trailer mounted system - All parts necessary for drilling and testing

We can also supply complete sets of equipment mounted on a specially designed four wheel trailer or on to a pallet ready for fitting to an existing vehicle. Complete sets are usually supplied with spares covering some years’ operation.

We also advise on the conversion or adaptation of existing equipment to operate our self-boring pressuremeters.

In addition to this we also maintain a complete set of equipment which we use for demonstrations and instructional courses. It is also kept ready for commercial hiring with an Engineer and Technician.

We will supply further details of this service on request.

FURTHER DETAILS

A great deal of further information is available from us on request. This includes full and detailed specifications, photographs and advice on every detail of the operation of these instruments. This advice is freely available both before and after an instrument is purchased.

CAMBRIDGE INSITU

Set up in 1973 to manufacture and market the Cambridge self-boring pressuremeters, we have in the course of developing the instruments acquired skills and experience in a number of directions not originally expected. Some of the things we now make or do are given below. If any of them seem likely to be useful then we shall be very glad to hear about it and to discuss whether we can help.

• self-boring pressuremeter testing and analysis
• design and manufacture of electronic systems for use in harsh environments both for control and for data handling, including computer data handling
• Load Cells of numerous designs and, up to now, for soil mechanics purposes
• strain gauging of small cells and structures
• small hydraulic systems
• high pressure liquid injection moulding of polyurethane
• rubber to metal bonding

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