Part 1. Key Terminology and Engineering Parameters for Geotechnical Engineering,

Typically, soil comprises a skeleton of soil grains in frictional contact with each other forming an open-packed structure (loose/soft) or close-packed structure (dense/hard). The soil particles may be microscopic in the case of clays (which may range in hardness from soft to stiff), just visible in the case of silts, and clearly visible in the case of sands (which may range in density from loose to dense, and in particle size from fine to coarse) and the larger particle sized gravels.  The distribution of particle size is called grading. The soil skeleton, which can also be cemented, forms an interstitial system of connecting spaces or pores.  The pores in the soil will usually contain some moisture even in unsaturated soils. The flow of pore water can be restricted by the small size of the pores and degree of saturation thus giving rise to low permeability k particularly in clays. During construction, for saturated soils the change in load or total stress σ is shared between the soil structure and the pore pressure u. The time dependent flow of water in soil under applied load is referred to as consolidation (pore water flowing out of a loaded zone) or swelling (pore water flowing into an unloaded zone) and is the means whereby total stress change is transferred from pore pressure to structural loading of the soil skeleton as measured by effective stress σ΄= σ-u, the parameter that uniquely controls all deformation in soils. It is this time dependency that gives saturated clays their unique behaviour whereby they have a short term or undrained strength su that is different from the long term or drained strength sd. This is why soil supported structures (e.g. foundations) and soil structures (e.g. embankments and cuttings) have short term stability and long term stability e.g. why Victorian-era railway cuttings in England failed half a century after construction. The maximum capacity of the soil skeleton to support load is called the shear strength because soil fails in shear. This strength depends on the frictional nature of the inter-particle contact and is measured by the coefficient of friction or angle of shearing resistance ∅΄, and by the constant, cohesion c΄, with respect to effective stress as designated by the prime notation thus ΄. The deformability of the soil skeleton is measured by elastic theory deformation moduli such as Young’s modulus E, Poisson’s ratio ν and Shear modulus G. Because of formation history such as deposition by wind or water, soil insitu possesses fabric or geometric orientation of particles that gives rise to anisotropy i.e. different properties in different directions. 

Soil can be geologically loaded to a maximum past pressure or preconsolidation pressure. This pre-load constitutes a yield point. At stresses less than yield the soil behaves elastically i.e. the strains are nearly recoverable.  At stresses more than yield the soil behaves plastically i.e. the strains are not recoverable and the mathematical theory of plasticity is sometimes used to describe the post-yield soil behaviour e.g. in finding the bearing capacity of footings and piles.  Stress distributions, however, can be described generally using the mathematical theory of elasticity that is also used for the prediction of vertical movement such as settlement (downward) or heave (upwards). 

Soil properties can be studied and parameters measured in a 

variety of tests. The most common and most useful test is the triaxial test that is carried out in the laboratory so that test conditions can be carefully controlled. Other important laboratory test equipment are the resonant column apparatus that measures maximum shear modulus, the hollow cylinder apparatus that is an element test and can apply rotation of principle stresses (e.g. as pertain under moving wheel loads), and the ring shear apparatus that measures residual shear strength on an established shear surface (e.g. such as may control stability on natural slopes that have previously slipped). 

The widespread availability of commercial finite element stress analysis software has concentrated attention on measuring soil parameters, particularly ground stiffness. This has led increasingly to the use of seismic test apparatus to measure shear wave velocity. Up-hole, down-hole and crosshole methods use boreholes. The seismic cone penetration test uses a hammer at the surface to produce vibrations detected by a receiver in the cone. Using a hammer or a frequency-controlled vibrator at the ground surface generates surface waves. These include Rayleigh waves that travel parallel to the ground surface to a depth of about one wave length thus testing the soil in the mass (i.e. including the effects of fissuring and jointing) in a non-invasive way. The resulting ground vibrations are detected by an array of vertically polarised sensors or geophones. From surface wave tests, shear wave velocity is correlated with wavelength and this data can be interpreted to give stiffness-depth profiles. Shear wave velocity measurements can be used to characterise soils as well as to provide useful data for estimating sampling disturbance. 

It is important to make the distinction between a soil property and a soil parameter. A soil property is independent of test type and can be used to characterise soils (e.g. shear wave velocity). A soil parameter is dependent on test type (e.g. undrained strength) but is useful for design purposes, particularly when correlated with field performance of full scale works. 

It is therefore the structure of geomaterials (soil and rock) that controls the engineering behaviour of the ground. Take for example a landslip. In the case of soil, if the bedding of the soil follows the down slope of a potential slip, long translational slips can occur. If the soil structure is random or the material is homogeneous, failure can occur on a circular arc with a rotational slip. In the case of rock, however, slips or topples can occur only if the jointed block structure allows a feasible mechanism. 

Cohesive or fine-grained soils such as clays are highly time dependent in their behaviour. For example the recovery of pore pressures in some Victorian railway cuttings in London clay took over half a century, leading ultimately to collapse. Rocks, on the other hand, are not time-dependent in their behaviour over the life of a civil engineering structure. Rocks may change their properties due to weathering but the time scale involved will normally greatly exceed the design life of a foundation (typically 50-90 years). 

pattern and types of discontinuity present in the rock mass. The mass compressibility of the rock is a combination of the compressibility of the intact rock and the compressibility of the joints. Clearly, the greater the extent of the joints and the lower their compressibility, the more the joint properties will dominate overall mass behaviour. 

The way discontinuities can dominate stiffness of rock masses can be illustrated in Fig. 1. Here, Matthews (1993) used the formulation of Hobbs (1975) to carry out a parametric study of the effect on mass stiffness of fracture (or joint or discontinuity) frequency and fracture thickness or “aperture”. In Fig. 1, the subscripts correlation is “m” for “mass” (i.e. including joints or fractures), “j” for “joint” (i.e. joint properties only), and “i” for “intact” (i.e. intact or parent rock only). 

The case with the joints open to 1.0mm, aligned perpendicular to the direction of the applied load is shown in Fig. 1 (a). This represents a geometry often found in chalk. It will be seen from Fig. 1 (a) that as the modulus of the joint approaches that of the intact material (i.e. Ej/Ei = 0.5 in Fig. 1 (a) the number of joints has little effect on the ratio of mass modulus to intact modulus. 

Fig. 1 (a) Variation in the ratio of mass stiffness to intact stiffness with fracture frequency for fractures with different stiffnesses; (b) variation in the ratio of mass stiffness to intact stiffness with fracture frequency for fractures with different apertures (Matthews et al., 2008).  

The relationships shown in Fig. 1 (a) assume a constant joint aperture of 1.0mm. If the ratio Ej/Ei is kept constant, and the aperture varied, the set of curves shown in Fig. 1 (b) results. Not surprisingly, the greater the aperture, the greater the mass compressibility.