Ground investigations - The Mechanics of Soils and Foundations - page 249

Figure 17.5 In situ loading tests.

and a value for s u can be calculated from the measured torque. If the rotation is
continued for several revolutions the strength will drop to the residual (see Sec. 9.2).
Pressuremeter tests are illustrated in Fig. 17.5(c). A flexible cylinder is inflated and
the pressures and volume changes measured. The best pressuremeters measure radial
displacements directly (instead of volume changes) and some measure pore pressures
as well. Pressuremeters may be installed in pre-drilled boreholes or self-boring devices
drill themselves into the ground with less disturbance. Results of pressuremeter tests
are used to calculate both soil strength, stiffness and the in situ horizontal stress σ h .

Methods for analysis of pressuremeter tests are described by Mair and Wood (1987).

17.6 Investigating groundwater and permeability
Whatever else you do in a ground investigation you must be sure to define the ground-
water conditions. This will include determining the current steady state pore pressures
and the final steady state pore pressures after construction. If the works involve a seep-
age flow of water, either steady state or during consolidation, you will need values of
the coefficient of permeability.
Pore pressures can be measured by observing the level of water in a standpipe (see
Sec. 14.1) in a borehole. Notice that if you drill a borehole into saturated clay with a
groundwater table, or phreatic surface, near the ground surface the hole will remain