CHARACTERISTICS OF SOILS - Introduction to Soft Soil Geotechnique: Content, Context and Application - page 30

fraction D 15 is a suitable measure for the average pore size cross section. The
permeability is a constitutive parameter applied in Darcy's law, which describes
fluid flow in porous media (groundwater flow).

TABLE 2.6 PERMEABILITY VALUES, CLASSIFICATION AND TESTS

10 -2 10 -4 10 -6 10 -8 10 -10

k [m/s] 1

drainage good moderate poor

class large moderate small very small relatively impervious

soil type boulders gravel clean sand silty sand clay

pumping test, constant head test falling head test, oedometer

test

monopole or dipole cone piezometer

correlation grain size distribution density clay index

The hydraulic permeability of the poorly graded sand, of which the grain size
distribution is presented in Fig 2.1, can be estimated using the empirical formula
for the intrinsic permeability, assuming the porosity n is 40% and the tortuosity T is
0.5. The kinematic viscosity of water is w = 10 -6 m 2 /s. Elaboration gives = / =

0.005 D 15 2 T n 3 /(1 n) 2 = 0.005 (0.15x10 -3 ) 2 0.5 (0.4) 3 /(1 0.4) 2 = 10 -11 m 2 . Hence,

= 10 -11 x10/10 -6 = 10 -4 m/s.
The well-graded sand of Fig 2.1 has D 15 = 0.06 mm, which for similar porosity
leads to k = 1.6x10 -5 m/s. Significantly less than the poorly graded sand, though
D 50 is higher. For the well graded sand the pores between coarse grains is filled up
by smaller grains, creating a less pervious porous structure.
The electrical conductivity of saturated soils, where the pore fluid is conducting
and the porous matrix is non-conducting, allows to determine the tortuosity of the
porous structure, by comparing the electric conductivity of an electrolyte in a
porous medium with the bulk conductivity of the electrolyte. In reservoir
engineering this ratio is referred to by the formation factor F . In literature,
empirical relations are mentioned, such as F = T m 1 n m 2 , where n is the porosity, and
m 1 and m 2 are empirical factors. For uniform equal spheres, Maxwell found F =
(3

the hydraulic permeability becomes k = w / = g /

n )/2 n . Then, a dense packing (cubic) gives n = 0.4764 and a loose packing
(rhombohedra) gives n = 0.2596, so that one finds 2.64 < F < 5.28 and for m 1 =

1, the tortuosity becomes 0.67 < T < 0.75.
Finally, the permeability is essential for contaminant and heat transport through
porous media, since it takes place by convection (pore water flow). The transport is
also affected by adsorption to/in the grains (retardation) and chemical decay, and
heat disperses by conduction (non-conservative).

0.8 and m 2 =

E STRENGTH AND STIFFNESS ASPECTS
The behaviour of soil masses is described by strength and stiffness parameters.
The most relevant are: stress-strain modulus, elasticity or Young's modulus E [Pa],
the shear modulus G [Pa], Poisson's ratio

[ o ] , soil

, the angle of internal friction

[ o ], compression
index C c , creep or secondary compression index C and the coefficient of
consolidation c v [m 2 /s]. Aspects of non-linear, anisotropic and irreversible
behaviour requires additional parameters, such as the over-consolidation ratio

cohesion c [Pa], undrained shear strength c u [Pa], dilation angle