5.2.2Plastic settlement

This is caused by underlying soft plastic cohesive materials gradually being displaced or squeezed out from under a foundation. This can happen at loading levels appreciably less than that which would cause simple shear failure of the same stratum. Also it can occur where a plastic soil is sandwiched between layers of harder material. Unless revealed in the site investi­ gation beforehand such behaviour can be puzzling to design engineers and, even if forecast, can still prove difficult to analyse accurately.

5.2.3 Settlement due to changes of conditions

Settlement can be caused by conditions which are largely or entirely independent of the new foundation loading. Instances of this phenomenon are:

•Drying and shrinkage of cohesive soils brought about by lowering the water table.

•Saturation of granular soils brought about by raising the water table. This has the effect of reducing the safe bearing capacity of the granular soils.

•Long term seepage having the ability to wash away fine materials.

•Temperature changes, such as under a boiler house, where appreciable long term temperature increase

produces shrinkage resulting from moisture loss of the stratum having contact with the foundations.

Over and above the preceding reasons, differential settlement of foundations arises from three common causes:

(a) Spacial variation of soil conditions having varying capacities to carry the uniform imposed foundation load.

(b)Variation in thickness of a compressible stratum under the foundations.

(c)Net foundation loads may vary around the struc­ ture, thereby inducing uneven ground loading. If at

all possible this should be eliminated by careful design.

Mixed types of foundations are also to be avoided if possible as they invariably increase the likelihood of differential settlement. It should be borne in mind that settlement rather than bearing capacity is the critical parameter in the design of successful foundations for most major structures.

the soil properties are known. Alternatively the load which a driven pile can carry may be determined, fairly

Foundations design and construction

reliably under favourable circumstances, from pile driving formulae when the set or penetration resulting from a known hammer blow is measured (this method is notoriously unreliable in conditions where dynamic and static loads produce dissimilar reaction in the soil). Examples of these calculations are provided in Appen­ dix A of this chapter.

However, the most accurate method of determining the capacity of a pile is to apply a test load to the head of the pile after it has been constructed and this provides the most reliable source of information on which to base a design. Full information from any test piles should be made available to companies tendering for piling contracts.

The test load is usually applied to a pile by:

•Building an independent structure loaded with kent­ ledge around the pile head and jacking from the structure down bn to the pile.

•Jacking down on to the test pile from a crossbeam; each end of the crossbeam being connected to one or more piles which carry the uplift reaction in tension. The uplift piles must be sufficiently remote from the test pile to prevent interaction.

Load testing of piles can be carried out for various reasons; on preliminary piles to derive an adequate design, as described; on working piles, to verify the adequacy of this design across a large site where conditions may vary, and again on working piles, to check that standards of workmanship are such that acceptable performance can be expected from all of the piles. The need for a number of preliminary pile tests will be dictated by the uncertainty or safety factors in the design process. The number of working pile tests will depend on the size of the contract, the variability of ground conditions, and the difficulty of construction (typically on a major piling contract one working pile in every 200 is load tested). While working piles are rarely tested to more than 150% of the working load, prelimi­ nary piles should be loaded very much higher, to 250% working load or to destruction.

The purpose of a pile test is not only to assess load carrying capacity but also to observe the settlement characteristics of the pile particularly up to its working load. The applied load is measured by either a load cell or a proving ring and the displacement of the pile by extensometer gauges mounted on a separate stationary support structure (see Fig 3.14).

The pile is loaded either in increments (some fraction of the working load), each increment only being added when settlement from the previous increment has sensibly ceased, or, in the case of preliminary piles, continuously such that the pile moves into the ground at a constant rate of penetration (CRP),

For a CRP test to be worthwhile sufficient test capacity must be provided in order that the pile can be failed. To obtain the maximum information from a pile test the cycle of loading should include monitored performance under incremental unloading since this

allows a refined estimate to be made of the elastic shortening of the pile.

Therefore, a typical working pile load test consists of two full cycles of incremental load to working load and then to 150% working load.

For preliminary pile tests it is often desirable to start with the same procedure and then add one or more incremental cycles to maxima and perhaps finish with a CRP load test.

The results of load tests should be recorded on standard sheets where ail the relevant information is

noted including full cycles of incremental load to working load and then to 150% working load.

For preliminary pile tests it is often desirable to start with the same procedure and then add one or more incremental cycles to maxima and perhaps finish with a CRP load test.

The results of load tests should be recorded on standard sheets where all the relevant information is noted including full details concerning the construction of the pile.