Geology Reference
In-Depth Information
ring shear boxes are used, inwhich an annulus-shaped sample is prepared
and then rotated until a constant low strength is obtained.
Young
is Modulus (E) is expressed as stress/strain (with units of stress)
and is a key parameter for predicting settlement of a structure or
deformation in a tunnel and needs to be de
'
ned at a mass scale. For
soil, samples are consolidated in oedometers andmeasurements taken of
deformation against time. The main derived parameters are m
v
,whichis
an inversion of E, i.e. strain/stress, and C
c
, which is a measure of rate of
consolidation. For normally consolidated clay that has been simply
buried by overlying sediment, there will be a gradual improvement in
strength and stiffness with depth, as illustrated for natural soils in
Figure 5.9.
Soil that has been overconsolidated because of its geological
history will exhibit relatively high stiffness up to the loading level
corresponding to its earlier pre-consolidation stress state. Once that
pressure is exceeded, the stiffness will revert to the natural consolidation
curve. At very small strains, overconsolidated clay can be much stiffer
than at higher strain levels, and this can be important for realistic
modelling of excavations (Jardine et al., 1984; Clayton, 2011).
Geophysical testing can be used to interpret stiffness parameters from
velocities of wave propagation through soil, and values are again on the
high side compared to static tests at relatively high strains (Mathews
et al., 2000). The same is true of rock masses
interpretation of
compressional or shear velocities tend to give higher stiffness values
than do static loading tests, and this probably re
-
ects the low strain
nature of loading from transient dynamic waves (Ambraseys &
Hendron, 1968). Because of the dif
culties in determining E at the
rock mass scale from
first principles or testing, it is common to rely on
empirical published data as discussed at 5.6.3.
fluid
flow through the material and strictly varies according to the
e.g. oil, water or gas. It has dimensions of area (L
2
).
In hydrogeology and geotechnical engineering, the termpermeability is
generally used interchangeably with hydraulic conductivity and is the
volume of water (m
3
) passing through a unit area (m
2
) under unit
hydraulic gradient (1m head over 1m length) in a unit of time (per
second). This reduces to m/s. For low permeability rock suitable for a
nuclear waste repository, the permeability, k, might be 10
-
11
m/s. For
an aquifer of sandstone suitable for water extraction, it might be 10
-
6
m/s and for clean gravel 10
-
1
m/s. Typical values for other soils are
given in BS 8004 (BSI, 1986).
fluid concerned
-
