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9.6. FLOW PROPERTIES
Soils exhibit flow properties that control hydraulic conductivity (k), rates of consolidation, construction
behavior, and drainage characteristics in the ground. Field measurements for soil permeability have been
discussed previously in Chapter 6 and include pumping tests with measured drawdown, slug tests, and packer
methods. Laboratory methods are presented in Chapter 7 and include falling head and constant head types in
permeameters. An indirect assessment of permeability can be made from consolidation test data. Typical
permeability values for a range of different soil types are provided in Table 9-1. Results of pressure
dissipation readings from piezocone and flat dilatometer and holding tests during pressuremeter testing can
be used to determine permeability and the coefficient of consolidation (Jamiolkowski, et al. 1985). Herein,
only the piezocone approach will be discussed.
The permeability (k) can be determined from the dissipation test data, either by use of the direct correlative
relationship presented earlier (Figure 6-7), or alternatively by the evaluation of the coefficient of consolidation
. Assuming radial flow, the horizontal permeability (k
) is obtained from:
= constrained modulus obtained from oedometer tests.
9.6.1. Monotonic Dissipation
In fine-grained soils, excess porewater pressures (
u) are generated during penetration of any probe (pile,
cone, blade). For example, in Figure 9-34, large u
readings are observed in the clay layer from 11 to 19 m
depth. If penetration is halted, the
u will decay eventually to zero (thus the porewater transducer will read
the hydrostatic value, u
). The rate of decay depends on the coefficient of (horizontal) consolidation (c
) of the medium. An example of piezocone dissipation for both type 1 and 2 filter elements
is given in Figure 6-6. These are termed monotonic porewater decays because the readings always decrease
with time and generally are associated with soft to firm clays and silts. For these cases, the strain path method
(Teh & Houlsby, 1991) may be used to determine c
from the expression:
where T* = modified time factor from consolidation theory, a = probe radius, I
= rigidity index of the
soil, and t = measured time on the dissipation record (usually taken at 50% equalization).
Several solutions have been presented for the modified time factor T* based on different theories, including
cavity expansion, strain path, and dislocation points (Burns & Mayne, 1998). For monotonic dissipation
response, the strain path solutions (Teh & Houlsby, 1991) are presented in Figure 9-40(a) and (b) for both
midface and shoulder type elements, respectively.
The determination of t
from shoulder porewater decays is illustrated by example in Figure 6-6. For the
particular case of 50% consolidation, the respective time factors are T* = 0.118 for the type 1 (midface
element) and T* = 0.245 for the type 2 (shoulder element).