43 Pressuremeter tests
COMMENTARY ON CLAUSE 43
A pressuremeter can be used to give an assessment of the in-situ stress, stiffness and strength of the ground. The device is cylindrical with a flexible membrane over part of the outside curved surface, It imposes a uniform pressure on a borehole wall (see Mair and Wood, 1987 [84]), and the applied pressure and the resulting deformation are recorded.
There are two approaches to the use of Pressuremeters in ground investigations. The first is based on the methods developed by Ménard (see Baguelin, Jezequel and Shields, 1978 [85]) in which the pressuremeter is used to obtain design parameters directly. The second is to analyse pressuremeter tests to give the properties of the ground.
43.1 General
Pressuremeter tests are specialist tests and should only be carried out by specialist contractors.
43.2 Pressuremeters
COMMENTARY ON 43.2
Pressuremeters fall into three categories: those that are lowered into pre-bored holes, those that are drilled or self-bored into place and those that are pushed in. They are normally between 40 mm and 100 mm in diameter and up to 1 m long, with the expanding section being between 5 and 7 times the diameter of the instrument. Versions do exist at 142 mm diameter. The displacement capacity of the instrument is a function of the instrument design but exceeds 10% of the instrument diameter, much more in the case of a pre-bored hole pressuremeter because of the clearance necessary to fit it into the hole and much more in the case of pushed-in Pressuremeters as their expansion needs to override the disturbance caused by their insertion procedure. There are three main pressure capacities: 0 MPa to 4 MPa for soils, 0 MPa to 10 MPa for weaker rocks, and 0 MPa to 20 MPa for weak to medium strong rocks.
43.2.1 Pre-bored Pressuremeters
COMMENTARY ON 43.2.1
Pre-bored Pressuremeters are mainly single expanding cell systems in which the radial or diametrical displacement is measured directly with transducers mounted in the instrument. The membrane is inflated with gas or oil, and the pressure measured with a transducer mounted in the instrument.
Pre-bored Pressuremeters (including high pressure dilatometers) should be lowered into pockets drilled specifically for the tests. The Ménard pressuremeter (see Baguelin, Jezequel and Shields, 1978 [85]) consists of three expanding cells connected to the surface by drill rods and flexible hoses. Other types of pre-bored hole Pressuremeters (see Clarke, 1994 [86]) should be carried out in accordance with BS EN ISO 22476-5, BS EN ISO 22476-7 and BS EN ISO 22476-8, as appropriate, and BS EN 1997-2 and NA to BS EN 1997-2.
43.2.2 Self-bored Pressuremeters
COMMENTARY ON 43.2.2
Self-bored Pressuremeters are single cell instruments attached to a drilling head (see Windle and Wroth, 1977 [87] and Withers, Schaap and Dalton, 1986 [88]) so that they can be bored into the ground. The head contains a drill cutter, turned by inner rods that pass through outer rods. The outer rods connect the instrument to the surface and are used to push the probe into the ground. Mud or water is pumped down the inner rotating rods and back up the annulus between the inner and the outer rods. This removes the soil arisings from the cutter but retains the in-situ pressure on the soil. Radial displacement of the membrane is measured directly with transducers mounted within the instrument. The membrane is inflated with gas or oil under pressure, which is also measured with a transducer mounted in the instrument.
Self-bored pressuremeter testing should be specified and undertaken in accordance with BS EN ISO 22476-6, BS EN 1997-2 and NA to BS EN 1997-2.
43.2.3 Pushed-in Pressuremeters
COMMENTARY ON 42.2.3
These Pressuremeters are a single cell instrument commonly, but not always, mounted behind an electric cone penetrometer (see Withers, Schaap and Dalton, 1986 [88]). The membrane is usually inflated by oil or water under pressure, ideally measured by a transducer mounted in the instrument. Displacement of the membrane can be measured directly by transducers within the instrument, recording and showing radial displacement or can be by volume change measurements.
The push-in pressuremeter testing should be undertaken in accordance with BS EN ISO 22476-8, BS EN 1997-2 and NA to BS EN 1997-2.
43.3 Calibrations
For all types of pressuremeter calibrations should be undertaken in accordance with the relevant standard.
NOTE There are three groups of calibrations (see Clarke, 1994 [86] and Clarke and Smith, 1992 [89]):
- strain transducer or line volume calibrations;
- membrane stiffness; and
- membrane compression.
The membrane stiffness and compression calibrations are in fact corrections that are applied to the measured test data. The membrane stiffness correction accounts for the pressure required to overcome the membrane's inherent stiffness and is assessed by inflating the instrument in air. The membrane compression correction actually accounts for compliance of the whole pressuremeter and represents adjustment to the measured displacements for apparent movements due to pressurising of the pressuremeter probe. This correction is assessed by inflating the instrument in a steel or aluminium cylinder of known stiffness.
43.4 Test procedures
COMMENTARY ON 43.4
Tests can be either stress-controlled or strain-controlled. In stress-controlled tests there are increments of pressure during the loading phase, each increment being held for a specified time or alternatively a continuous loading may be applied. A strain-controlled test is also under stress control initially but a feedback system is used to ensure that the displacement of the membrane satisfies a preset strain rate (see Clarke and Smith, 1992 [89]).
The Ménard method is a standardized test procedure based on a stress-controlled (incremental loading) test (see Baguelin, Jezequel and Shields, 1978 [85]); it should be carried out in accordance with BS EN ISO 22476-4, BS EN 1997-2 and NA to BS EN 1997-2. The objective is that method-specific parameters are obtained that can be used directly in design formulae developed from observations of full scale tests.
NOTE 1 It has become practice in the UK to carry out Ménard-style tests using other types of equipment but following the Ménard testing procedure. These are referred to as emulated Ménard tests, and have been performed using, for example, high pressure dilatometers (HPD) and purpose-built smaller diameter instruments.
NOTE 2 Except in the Ménard test, it is common practice to carry out at least one unload reload cycle within a test from which values of stiffness can be obtained. In general, two or three unload reload cycles might be preferable and if included they yield data about the shear modulus and the way in which the shear modulus varies with the strain excursion.
NOTE 3 Pressuremeters can be used in most ground conditions, but not all Pressuremeters can be used in any one ground condition. The results from any individual test depend on the equipment used and the procedures adopted for installation, for testing and for interpretation. This means that results from different tests might not necessarily be compatible. Prebored hole Pressuremeters conforming to BS EN ISO 22476-4, BS EN ISO 22476-5 and BS EN ISO 22476-7 can be used in most soils and rocks with the exception of some very soft soils. The results obtained depend on the quality of the pre-drilling. Cambridge type self-boring Pressuremeters conforming to BS EN ISO 22476-6 can be used in all clays, silts and sands provided they do not contain excessive amounts of gravel size particles It might be necessary with the self-boring pressuremeter to use a separate drilling rig to clear obstructions encountered during self-boring. The weak rock self-boring pressuremeter is an upgraded version of the self-boring pressuremeter with thicker membrane, slightly oversize cutting shoe, and rock-roller or other heavy duty cutter. It can be used in dense sands, very stiff clays and weak rocks, although sometimes self-boring gives no better data than a correctly installed pre-bored hole pressuremeter. This instrument has to be used with a rotary rig. The cone pressuremeter can be used in those soils into which it is possible to push a cone.
NOTE 4 Theories of cavity expansion are well documented (see Clarke, 1994 [86]). The interpretation of a test is based on these theories but the parameters obtained might be empirical. Estimates of in-situ stress can be obtained from self-bored pressuremeter tests and in some cases prebored hole tests (see Marsland and Randolph, 1977 [90]).
NOTE 5 Average cavity stiffness can be obtained from unload-reload cycles from all pressuremeter tests (see Mair and Wood, 1987 [84]). Recent developments in interpretation show that the average cavity stiffness can be converted to a material stiffness (see Jardine, 1992 [91]). The values of stiffness vary with the strain and stress level over which they are carried out. The reliability of interpretations of deformation modulus, therefore, require the influence of strain magnitude on the modulus to be assessed and in soils where drainage occurs during a test the influence of changes in effective stress on the deformation modulus needs to be considered. Undrained shear strength can be obtained directly from self-boring pressuremeter tests and estimated from other pressuremeter tests (see Clarke, 1994 [86], Withers, Schaap and Dalton, 1986 [88] and Powell and Shields, 1995 [92]). Angles of friction and of dilation of sands can be determined from self-bored pressuremeter tests (see Hughes, Wroth and Windle, 1977 [93] and BS EN ISO 22476-6) and estimated from pushed in pressuremeter tests (see Powell and Shields, 1997 [94] and BS EN ISO 22476-8).
