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In situ Testing

m above the vane and the relationship between torque and angle of rotation was automatically drawn by an x—y recorder. The vane was rotated by an electrical motor via a gearbox which allowed variousrotation speeds. A similar arrangement was used by Merrifield (1980). These types of refinements willundoubtedly lead to improved data by removing rod friction effects and by ensuring that uniform ratesof rotation are applied.

Fig. 9.18 Diamond shear vanes.

 Pressuremeter testing

The pressuremeter was developed in France in the early 1950s (Ménard 1957). In its earliest form itwas (and remains today) a simple, robust mechanical tool, well- adapted to use in routineinvestigations. Since its development there has been a considerable growth in the number of designs of pressuremeter that are in use, as will be described below. A recent review of pressuremeter testing isgiven by Mair and Wood (1987). Higher pressure devices of this type, designed for use in hard soils orrocks, are sometimes referred to as ‘dilatometers’.

Pressuremeter tests can be carried out both in soils and in rocks. The pressuremeter probe, which is acylindrical device designed to apply uniform pressure to the ground via a flexible membrane, isnormally installed vertically, thus loading the ground horizontally (Fig. 9.19). It is connected by tubingor cabling to a control and measuring unit at the ground surface. The aim of a pressuremeter test is toobtain information on the stiffness, and in weaker materials on the strength of the ground, bymeasuring the relationship between radial applied pressure and the resulting deformation.Conventional self-boring pressuremeters cannot penetrate very hard, cemented or stoney soils, orrocks. In these materials a borehole pressuremeter is normally used.

Fig. 9.19 Basic components of the pressuremeter.

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Site Investigation

Types of pressuremeter

Three principal types of pressuremeter are in use.

1. 

The borehole pressuremeter. Originally developed by Ménard, a borehole is formed using anyconventional type of drilling rig capable of producing a smooth-sided test cavity. The pressuremeter has a slightly smaller outside diameter than the diameter of the hole, and cantherefore be lowered to the test position before being inflated.

There are two types of measuring system in use. In the original Ménard system the probecontains a measuring cell which is fluid-filled (Fig. 9.20). The radial expansion of the probewhen pressurized is inferred from measurements of volume take made at the ground surface,using the control/measuring unit. A guard cell is incorporated into each end of the probe, inorder to ensure, as far as possible, that the measuring cell expands only radially.

In more recently designed pressuremeters (for example, the Oyo ‘Elastmeter2’) the probe is pressurized using gas, and the radial displacement is sensed electronically by diametricallyopposed measuring arms. This type of pressure- meter may also incorporate a pressuretransducer in the probe, thus giving better quality (but more complex) pressure measurement.

Fig. 9.20 Diagrammatic sketch of the Ménard pressuremeter (Gibson and Anderson 1961).

2.  The self-boring pressuremeter (SBP). The self-boring pressuremeter has been developed bothin France (Baguelin et al. 1974) and in the UK (Wroth and Hughes 1973), in an attempt toreduce the almost inevitable soil disturbance caused by forming a borehole. Boreholedisturbance can have a very great effect on the soil properties determined from in situ testing,as we have already noted in the case of the SPT. A self-boring pressuremeter incorporates aninternal cutting mechanism at its base; the probe is pushed hydraulically from the surface,whilst the cutter is rotated and supplied with flush fluid (Fig. 9.21). The soil cuttings areflushed to the ground surface via the hollow centre of the probe, as the pressuremeteradvances. At least four distinct versions of the self-boring pressuremeter have been described:

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