Friday, October 14, 2011

Geotechnical Mechanisms


Good design means not only determining the methods to be employed, but also planning well ahead which geotechnical mechanisms may be involved and which methods of calculation are needed to evaluate stability and deformation.
Some major geotechnical mechanisms are:
§         Shearing;
§         Uplift;
§         Squeezing;
§         Settlement;
§         Horizontal deformation;
§         Negative skin friction.
It should be borne in mind that the following explanatory notes on geotechnical mechanisms make no claim to be complete; there are other mechanisms and phenomena which have not been mentioned, but can occur in specific circumstance, and have a crucial effect on the geotechnical behaviour of the earth structure.

Shearing
A loss of equilibrium can occur in soil of low bearing capacity beneath the edges of fillings and excavations. In such cases, the shear resistance in the soil is insufficient to prevent the continuous movement of a particular piece of ground along a sliding surface. This mechanism is called shearing.
The origin of the shearing movement is usually the self weight of the soil.
In the case of steep slopes, because in the latter case the shear resistance of the soil can usually be mobilized over a greater length.
Sliding surface along which shearing occurs can in principle be of any kind. In practice they often seem to be curved, with the result that many methods of calculation are based on circular sliding surface; other sliding surfaces have however also been noted, in particular as a consequence of the layering of soils.
The risk of loss of stability due to shearing can be lessened by consolidation of the subsoil and also by making the slope more gradual, raising supporting berms or using a lightweight material for the slope. Stability can also be improved by lowering the water table.

Uplift
Whenever an impermeable clay or peat layer of low bearing capacity overlies a sand layer with relatively high water pressure, an unstable situation can arise in the clay or peat layer due to differences in water pressure. The water pressure in the impermeable layer may be lower than that in the sand layer after draining of the clay or peat layer or pumping out of a building excavation or trench; the difference in water pressure may also be due to the fact that the head of water in the sand layer is determined by a relatively high external water level. In such cases, the water pressure on the underside of the poorly permeable layer may cause the layer to swell or burst if its own weight is too low.
A phenomenon of this kind can be observed at floor level in excavations, for instance, or in a polder behind a dyke retaining a temporarily high external water level.

Squeezing
When a layer of low bearing capacity situated above a firm sand layer is subjected to a load from a fill whose horizontal dimensions are restricted, for instance in the case of a dyke or artificial mound, it is possible that this intermediate low bearing capacity layer may be forced horizontally outwards while the underlying layer is only slightly deformed and the top layer merely undergoes settlement. This phenomenon is known as the toothpaste effect or squeezing.

Settlement
The materials commonly found in the surface layers of the Northen and Western Netherlands, such as clay and peat, can be considerably compressed by filling, resulting in settlement of the top of the earth structure. The compression process in these water saturated and relatively incompressible materials usually goes through three phases.
  • Initial settlement, which occurs immediately and/or during the application of the load, e.g. the fill, and is relatively minor.
  • Consolidation settlement, which occurs gradually and in parallel with pore water dissipation.
Excess water pressures are set up by the application of the load, but because of the relatively low permeability of the material and the relatively long drainage route they are released only gradually. By far the greater part of the total anticipated settlement will be consolidation settlement.
  • Secondary settlement, also known as creep.
This settlement progresses more slowly with time; eventually, the rate of settlement tends asymptotically towards zero.

Horizontal deformation
At the edges of fills and excavations horizontal stresses build up in the highly compressible layers, leading to horizontal deformation. Since in layers of this kind it is only possible to mobilise a low level of passive earth pressure, these deformations can be relatively marked; they can often be observed up to ten metres in front the toe of the slope.
The result of horizontal soil deformations can be horizontal loads on any rigid foundation elements such as basement walls or foundation piles; loads of this kind, which are often unforeseen, can cause serious damage to structures.

Negative skin friction
As a result of the compression of the peat and/or clay layers, vertical shearing forces may affect foundation elements such as basement wall or foundation piles. The skin friction of filling materials is usually greater than that of the highly compressible layers. The greater the thickness of the fill, the greater the total vertical force on the foundation elements. This force is called negative skin friction; this mechanism can considerably reduce the allowable bearing capacity of a foundation. Moreover, negative skin friction can also increase the normal stress in foundation piles; in addition, extra deformation of foundation elements can trigger off settlement and differential settlement in the construction affected.

Other mechanisms
A considerable change in the shear stress level, for instance resulting from dynamic loading, may weaken loosely packed saturated sand and silt formations. Weakening of this kind is also known as liquefaction; it manifests itself as a sudden loss of stability of a mass of soil which may be of quite large extent; the ultimate result is predominantly extremely gradual slopes.
In certain circumstances, other mechanisms may also play a part. These include phenomena such as micro-instability, swelling, piping and erosion.
One possibly less obvious example is the effect of plants and animals on the stability of the structure, such as the influence of plant and tree roots or the possible consequences of holes dug by burrowing animals.
All of the phenomena listed under this heading, as well as other possible mechanisms are however excluded from consideration in the present volume.

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