Laboratory measurements of pore pressure are required in undrained testing where soil properties
are to be measured in terms of effective stresses and
in model tests which involve the loading or unloading of beds of clay.
Laboratory measurements of pore pressure are required in undrained testing where soil properties
are to be measured in terms of effective stresses and
in model tests which involve the loading or unloading of beds of clay.
Traditionally, pore pressures are measured in the drainage lines just outside a triaxial cell. It is essential that the pore pressure measurement system should be completely free of air and as stiff as possible so that minimal amounts of pore water flow are required in order to register the changes in pore pressure.
In research testing, miniature pore-pressure transducers may be mounted directly on a soil sample in order to speed up the expected response time.
Pore pressures can be measured in the ground with different types of piezometer. The simplest piezometer consists of a open tube or standpipe with a porous tip. The change in measured head of water requires a large flow of water into the tube, so response is slow.
A Casagrande piezometer comprises a porous tip in a filter zone at the base of a borehole, connected to a narrow tube. The cross sectional area of the tube is small by comparison with the surface area of the filter, so the flow required to register a change in pressure is smaller than in a standpipe, and the response is quicker.
Closed circuit piezometer systems are read remotely by mechanical or electrical means
and provide possibilities for de-airing the pore water circuit. The response time is dependent
on the length of connecting tubing.
Electrical transducers (using strain gauge or vibrating wire techniques) can be placed in the ground.
These are stiff devices which respond rapidly, but can be difficult to keep de-aired particularly if there is
a possibility of the surrounding soil becoming unsaturated.
Laboratory measurements of the permeability of soils can be made using a permeameter. For fine-grained soils (clays), the coefficient of permeability can be estimated directly or indirectly during one-dimensional compression tests in an oedometer.
Laboratory measurement of permeability
Constant head test
Recommended for coarse-grained soils.
Steady total head drop Dh is measured across gauge length L, as water flows through a sample of cross-section area A.
Falling head test
Recommended for fine-grained soils.
Total head h in standpipe of area a is allowed to fall; heads h1 and h2 are measured at times t1 and t2.
Hydraulic gradient Dh/L varies with time.
Laboratory measurement of permeability
Indirect measurement
Transient consolidation phenomena are controlled by the coefficient of consolidation.
With knowledge of one-dimensional compliance mv, coefficient of permeability k can be estimated from
Direct measurement
Direct measurement of permeability in oedometers is preferable.
Flow pumps can be used to maintain a constant flow rate (q) across the sample and to measure
the resultant constant head (h). The coefficient of permeability is then given by k = q.L / A.h
Field or in-situ measurement of permeability avoids the difficulties involved in obtaining and setting up undisturbed samples in a permeameter or oedometer and also provides information about bulk permeability, rather than merely the permeability of a small and possibly unrepresentative sample.
Field measurement of permeability
In a well-pumping test, the steady-state heads h1 and h2
in observation boreholes at radii r1 and r2 are monitored at flow rate q.
If the pumping causes a drawdown in an unconfined (i.e. open surface) soil stratum then
If the soil stratum is confined and of thickness t and remains saturated then
Constant head and falling head tests with in-situ piezometers can also be used.
Field measurement of permeability
Field tests equivalent to the laboratory constant head and falling head tests can be performed in which controlled heads or flows are applied to piezometer tips. In general, conditions around such piezometers are not ideally cylindrically symmetric or spherically symmetric and an intake factor F (with dimensions of length) is required for each particular geometry. Values of the intake factor may be deduced from analytical or numerical studies.
For a borehole open to its base, of diameter D, and lined to the full depth F=2.75D.
If the cased hole is through impermeable soil and the base of the casing is at the interface with a
permeable stratum F=2D.
For an intake formed by a cylindrical filter zone of diameter D and length L in an infinite isotropic stratum
for L/D > 4
Then for a steady state, constant head test in which a flow q is required to maintain a head h:
For a falling head test in which heads h1 and h2 are measured at times t1 and t2 in a borehole of area A:
Failure to control groundwater adjacent to a construction project may result in
Removal of water from the ground will cause the water level to fall.
How quickly and by how much depends on the permeability of the soil and the distance between the adjacent wells.
Drainage of clays is impractical. Silt particles may be removed along with the water causing the formation of voids in the ground and damage to pumps. The high yield in gravels may make the method impractical.
Lowering the ground water will reduce the pore water pressure and hence increase stability. Removing water will tend to cause settlement although the effect is likely to be small for sandy soils in which the technique works well.
Exclusion methods involve the installation of an impermeable barrier. This may be structural (steel sheet piles or concrete diaphragm wall) and may form a part of the permanent work.
Other methods include
Excluding water may cause a build up in pore water pressure. Heave is a particular problem where a thin layer of impermeable soil at the base of an excavation results in a high pore water pressure close to the base.