Hydro-Mechanical Properties of an Unsaturated Frictional Material

8 CHAPTER 2. STATE OF THE ART
conductivity for clays is about ks ≈ 1 · 10 −7 to 1 · 10 −12 m/s (von Soos 2001). In this case
the flow path of the water around the clay platelets is more tortuous, the water is partially
bonded to the clay particles and thus becomes less. The oedometric stiffness modulus at
σ = 100 kPa for sand is approximately Eoed ≈ 15.000 to 70.000 kN/m 2 . The oedometric
stiffness modulus of a clay is approximately Eoed ≈ 600 to 5.000 kN/m 2 (von Soos 2001). The
incompressible grains of the sand cause the higher stiffness. Depending on the soil type, its
behavior is governed by different mechanisms. The behavior of a soil is strongly influenced
by its structure, mineralogy, fabric and the soil-water interaction.
Coarse grained materials (sand and gravel) consist of particles larger than 0.002 mm, that
are commonly minerals as silicates, oxides, carbonates and sulphates. On the other hand the
structure of fine grained materials consists of hydrated aluminium silicates and hydrous oxides
in a crystalline structure (Mitchell 1993). Fine grained soils (i.e. clay and silt) encompasses a
wide range of particle sizes varying between 10 to 1000 ˚ A. The specific surface area of silt and
clay is large and a large portion of the total molecules belong to the surface molecules. Thus
the physico-chemical interactions between the soil particles and the pore fluid are significant.
Therefor the main factors controlling fine grained soils properties are the surface area and
the cation exchange capacity. Pore water is retained by molecular bonding, electrical field
polarization caused by negative charge on the surface of the minerals, van der Waals attraction
arising from the interaction between the molecules comprising the surface of the soil particles
and the molecules comprising the pore fluid as well as the exchangeable cation hydration
(osmotic effects). In coarse grained soils, since the surface area is relatively small, surface
molecules constitute only a fraction of the total molecules. Hence surface area and the cation
exchange capacity are not affecting the properties of coarse materials. Their characteristic
behavior is mainly driven by contact forces between the grains as well as the matric suction,
which is also called capillary force or negative pore-water pressure.
The soils fabric consists of the arrangement of soil particles, groups of particles and pores
(Mitchell 1993). Gens & Alonso (1992) suggested 3 basic fabric types as shown in Fig. 2.1.
In Fig 2.1 a) the fabric consists of elementary particle arrangements of clay platelets (also
called matrix) and coarse grains, that could be sand or silt particles. The pore space between
the elementary particle arrangements are intra-matrix pores. Several elementary particle
arrangements joined together form aggregates as can be seen in Fig. 2.1 b). In the aggregates
intra-aggregate pores and between the aggregates inter-aggregate pores can be distinguish.
Coarse grains and the aggregates lead to a structure of a granular type material and to a
multi modal pore-size distribution. In both fabrics a further type of pore exists, that are the
intra-element pores (see Fig. 2.1 c). The fabric in Fig. 2.1 b) is known to be typical in natural
and compacted clays. Due to different pore sizes (i.e. multi modal pore-size distribution)
the material may become sensitive to, e.g. changes in loading condition, changes in suction
or chemical changes. In coarse grained materials the main features are grains and pores.