Estrategias relacionadas con la satisfacción por el resultado de la venta

aND ‘LONG-tErM’ CONDItIONS

Construction on, or in, the soil normally involves stress changes at the boundary between the soil and the structure. For example, footings for an office block will normally apply an increased stress to the soil, while exca-vation to form a motorway cutting will (because soil is excavated) result in a stress decrease.

Total stress changes at the boundary of a sealed specimen of soil will result in pore pressure changes within the soil. If the soil is ‘saturated’ (i.e.

it contains no free air), the pore fluid within the soil skeleton will be very rigid compared with the soil skeleton. Of course, the individual soil par-ticles will be less compressible than the water, but since soil compression occurs as a result of expulsion of pore fluid due to a change in the arrange-ment of the soil particles, this is unimportant to the process.

We can use a spring and dashpot analogy (Figure 1.9) to describe what happens when saturated soil is loaded. With the valve closed, if a weight is placed on the piston, water cannot escape. Since the water is incompress-ible compared with the spring, and since the spring must compress if it is to carry additional load, all the weight is supported by an increase in water pressure. The piston does not move, indicating that no volume change occurs, and the load carried by the spring does not change, indicating that the effective stress and hence the strength of the soil remains unchanged.

If the valve is now opened, water will flow out of the container until the water pressure dissipates to its original value, in this case atmospheric. As water escapes through the valve, the piston will move downward, indicat-ing that the soil is changindicat-ing volume (‘consolidatindicat-ing’). Thus, compression is associated, in this case, with a water pressure decrease. Since the weight has not been removed, and the water pressure no longer supports it, its load is thrown onto the spring. Thus, a volume decrease is associated with an

effective stress increase (an increase in the spring load) and therefore an increase in strength.

In practice, soil in the ground is not normally bounded by a totally imper-meable barrier. Therefore, as soon as loads are applied, consolidation will start. Conversely, as soon as loads are removed, the soil will start to swell.

It is the rate at which swelling or consolidation occurs which is significant.

The permeability of soil to water can vary through 10 orders of magnitude, with water flowing out of the clay at perhaps 10^–10 m/s and gravel at 10^–1 m/s, for a hydraulic gradient of unity. It therefore takes a very long time for the water to be squeezed out of the clay beneath a large foundation, or to be sucked into the soil beneath a highway cutting or basement excavation.

Whereas construction of a building might take 18 months, it could take 15 years for a substantial proportion of the final settlement due to con-solidation to occur. Thus, for a clay, it is reasonable to assume that mate-rial in a zone of changing stress is unable to change volume, and remains

‘un drained’ at least in the short term until the ‘end of construction’.

Since the strength of saturated soil is a function of effective stress and moisture content, and since neither can change if volume does not change, geotechnical engineers carry out analyse for two cases:

1. ‘Short term’, or ‘end of construction’, when the maximum shear stresses are applied to the soil, but there has been little time for consolidation or swelling. The soil strength is assumed not to have changed from the original value. Tests can be carried out before the start of construction, to measure the initial strength of the soil.

Piston

Figure 1.9 Spring and dashpot analogy of consolidation of soil. (From Clayton, C. et al., Earth Pressure and Earth-Retaining Structures, Second Edition, Taylor & Francis, New York, 1993.)

2. ‘Long term’, when the shear stresses and total stresses due to con-struction have been applied, all volume changes due to consolidation or swelling have occurred, and the groundwater in the soil is assumed to have come to an equilibrium level.

As an example, consider a shallow foundation for an office block. It applies a total stress increase to the clay subsoil, and positive pore pressures are induced (Figure 1.10). In the short term, the shear strength of the soil will remain unchanged, but shear stresses will be applied to the soil by the foundation. As consolidation occurs, water will be driven out of the soil, and its strength will increase. If we were (simplistically) to define a factor of safety against failure of the soil beneath the foundation as the ratio (avail-able shear strength/applied shear stress), it can be seen that this ‘factor of safety’ decreases during construction (as more shear stress is applied) but increases after construction as consolidation occurs. The critical period, when the factor of safety is at a minimum, occurs at the end of construction (Figure 1.10), and it is not normally necessary to carry out an analysis for the long-term case.

A second group of problems exists which requires analyses in the long term, because this is the critical time for stability. Most earth pressure determinations fall into this category. Consider a form of construction such as a cutting for a motorway, where the total stress in the soil is reduced by the work carried out (Figure 1.11). As a result of a reduction in total stress, swelling will eventually occur, and the soil will lose strength in the period of pore pressure stabilisation between the end of construction and the long term. Clearly, the prudent engineer will design his structure for the long-term case, when the factor of safety is lowest.

There is, however, the problem of temporary works, such as cuttings or earth-retaining structures, which are only required to function dur-ing the construction period. Temporary works constitute a considerable part of the cost and risk associated with construction. They are normally designed by the contractor and might, for example, be cuttings or retaining walls required during excavation for a basement. It is tempting to analyse such cases in the short term, because they will not be used beyond the end of construction, but in reality, this is a risk. The simplified models in Figures 1.10 and 1.11 ignore the fact that some drainage of pore water will occur during the construction period. In cases where construction involves unloading (Figure 1.11), it is impossible to predict with certainty the rate at which pore pressures will rise, and there are very few case records to give guidance. It may be that the real situation approaches the dotted line in Figure 1.11, and the engineer should therefore be cautious, and carry out a long-term analysis.

The degree to which drainage and dissipation of excess pore water pres-sures (set up by loading or unloading) occur during the construction period

Applied load(P)

Shear stress applied

to soil (τ)

Pore water pressure generated

in soil (u)

Shear strength

of soil

Factor of safety against bearing capacity

failure

‘End of construction’ ‘Long-term’

Time Time Time Time Time

Figure 1.10 Load, pore pressure, shear strength and ‘factor of safety’ for a clay beneath an embankment. (From Clayton, C. et al., Earth Pressure and Earth-Retaining Structures, Second Edition, Taylor & Francis, New York, 1993.)

Applied load(P)

Shear stress applied

to soil (τ)

pressurePore (u)

Shear strength

of soil

Factor of safety against

slope failure

‘End of construction’ ‘Long-term’

Time Time Time Time Time

Figure 1.11 Load, pore pressure, shear strength and ‘factor of safety’ for a clay beneath a motorway cutting slope. (From Clayton, C. et al., Earth Pressure and Earth-Retaining Structures, Second Edition, Taylor & Francis, New York, 1993.)

is a function of a number of factors, such as soil particle size, fabric (i.e. fis-suring, presence of silt or sand laminations in clays), the availability of free water (either from the ground surface as a result of rainfall, or because of the existence of a high water table), and the time taken for construction to be completed (which may be significantly increased if unforeseen problems occur during construction).

It is clear, however, that granular soils (clean silts, sands and gravels) have such a high permeability that full dissipation of pore pressures will occur during the construction period, and certainly in this case, it will be unreal-istic to carry out a short-term, end-of-construction analysis which assumes that effective stresses and volumes remain unchanged.