Tuesday, January 8, 2013

General Approach to Choice of Foundations.

Having collected the information about the site including that noted in check-list 1 and obtained from the client answers to the queries including those noted in check-list 2, the foundation selection process can start. As discussed previously the design process evolves as the effects of  the various constraints are dealt with or any problems are solved. Tables 10.1, 10.2 and 10.3 give descriptions of the basic sub-soil and site types and a general guide to suitable foundations. In broad terms these tables will assist in the selection of foundation type. The lists are by no means exhaustive nor is the selection ever as simple as these tables may suggest, however, they should prove very useful as a general guide.

Table 10.1 gives details of foundations to account for varying sub-soil types ranging from rock to peat. The table gives comments on the effects of trees and shrubs on cohesive soils and gives notes on factors to be considered when selecting foundation type.

Table 10.2 gives details of suitable foundations to account for particular site conditions covering sloping, filled or affected by mining, old foundations, groundwater problems. The table gives notes on factors to be considered when selecting the foundation type.

Table 10.3 gives details of suitable foundation types to suit varying depths and strengths of bearing strata.

While Tables 10.1–10.3 give a general guide to the foundation selection by considering the factors which can influence this choice and earlier chapters have highlighted and discussed these points, check-list 3 provides a further list of points for consideration during the foundation selection process.

Check-list 3 – points to consider when assessing sub-soil conditions

(1) The extent of site investigations.
(2) The amount of information available prior to site investigations.
(3) The possibility of errors in the information received.
(4) The variability of the ground conditions.
(5) The inaccuracy of the soil mechanics.
(6) The effects of removal of the overburden.
(7) The effects of the groundwater.
(8) The seasonal effects of the groundwater levels.
(9) The effects of frost and seasonal weather changes.
(10) The effects of trees.
(11) The effects of the water-table on the depth at which various foundations will be considered.
(12) The effects of settlement.
(13) Variations of pressure with time.
(14) Variations of loading with time.

These factors will influence the bearing/settlement capacity of the sub-soil.

When evaluating test results or information from so-called specialists these data should be very carefully interpreted since the information on which their experience is based is generally limited by their specialized activities. For example, recommendations from one expert may clash with the requirements of another. It is therefore up to the engineer to gather the data and reassess in overall terms the reliability, relevance and practicality of both the information and recommendations being made. When selecting a  foundation type it should also be appreciated that prices of materials and labour vary depending on the timing and location of the project. The size of the contract can have a significant effect on the economics of the solution. For example, the effects of fixed costs such as those for getting piling rigs on and off the site can be very small when spread over a large number of piles, on the other hand they can prove to be the major cost when a small number of piles are to be driven. It is also necessary to keep up-to-date with piling and ground improvement techniques to ensure that decisions made on cost and performance are current.

High costs can be generated by complex shuttering details to foundations. These costs can be reduced significantly  if details are simplified, for example, concrete can be cast against hand packed hardcore in raft construction. Two stage concrete pours for raft edges can use earth faces for shuttering to the first pour. Brickwork built off the raft edge can act as shuttering to the second pour (see Fig. 10.1). When adopting this form of construction it is necessary  to increase reinforcement cover against earth faces and to provide reinforcement connection between the first and second concrete pours. An appreciation of construction methods and problems is also helpful in determining which foundation type to adopt. Pouring concrete under water using tremie techniques can suggest trench fill rather than strip and masonry (where it would be necessary to pump out the water to enable the masonry to be constructed).

Fig. 10.1 Raft edge construction.

Deep excavations in waterlogged ground are best avoided and alternative foundation solutions using rafts should be considered. It is advisable to avoid deep strips, pier and beam and piled foundations where mining is a problem.

In addition to construction considerations affecting the foundation selection, basic decisions in the design process can be significant. Varying the shape, length or rigidity of the foundation can have a major influence on performance.

The introduction of joints in the substructure and super- structure can be exploited in the foundation design and selection. Adopting a composite design for the foundation can also affect the type of foundation to be selected.

The importance of the above items are dealt with in detail in other chapters but they are included here as a useful reminder of the early part of the design process and to assist in the gathering of all relevant information.

Table 10.1 Foundation selection to suit sub-soil type

Table 10.2 Foundation selection to suit varying site conditions

Table 10.3 Foundation selection to suit bearing strata strength and depth

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