Foundation types depend primarily on the depth and safe bearing pressures of the bearing stratum, also restrictions placed on differential settlement due to the type of bridge deck.
Generally in the case of simply supported bridge decks differential settlements of about 20 to 25 mm can be tolerated, whereas multi-span continuous decks 10 mm is usually considered as a maximum.
Bridge foundations generally fall into two categories:
It is possible to have a combination of both (i.e. piers being piled with abutments on strip footings).
The design of foundations comprise of the following stages :
The overall size of strip footings is determined by considering the effects of vertical and rotational loads. The combination of these two must neither exceed the safe bearing capacity of the stratum or produce uplift.
The thickness of the footings is generally about 0.8 to 1.0 m but must be capable of withstanding moments and shears produced by piers or abutments.
The critical shearing stress may be assumed to occur on a plane at a distance equal to the effective depth of the base from the face of the column.
Cover to reinforcement should never be less than values given in BS 5400: Part 4: Table 13, and crack control calculation must be carried out to ensure the crack width is less than 0.25mm (Table 1). Cover to reinforcement will need to be increased to comply with BS 8500 requirements.
The type of piles generally used for bridge foundations are :
a. to c. are known as displacement piles, and the problems of calculating the load carrying capacity and settlement require a different approach to that for bored piles.
Driven type piles can, depending on the strata, be either end bearing or friction piles; sometimes a combination of both.
Bored piles are generally end bearing and are often of large diameter. To increase their bearing capacity the bottom can be under-reamed to produce a greater bearing area. However, additional safety precautions are required with larger diameter piles.
A specialist form of pile consisting of stone aggregate consolidated by water or air using the 'Vibroflotation' technique is suitable in some granular soils.
Choice of pile type depends largely on the strata which they pass through, none of them however give the most economic and satisfactory solution under all conditions.
The art of selecting the right sort of pile lies in rejecting all those types which are obviously unsuited to the particular set of circumstances and then choosing from those which remain, the one which produces the most economical solution.
Concurrently with the choice of pile type must go the choice of the strata which will carry the main loads from the structure, because this very often influences the choice. In most all cases the rejection of conventional pad or strip foundations arises because the computed settlement is more than the structure can safely withstand and hence the main purpose of the piled foundation will be to reduce this settlement. It follows, therefore, that if more compressible strata exists within reasonable distance of the surface, it is very desirable that a high proportion of the foundation load should be carried by this more stable strata; the ideal solution is where piles support the load wholly in end bearing on hard rock where the settlement will be negligible. It follows that piles wholly embedded in the same soil that would under-lie a conventional foundation has very little effect in reducing settlement. With soft normally consolidated alluvial clays, the remoulding effect of driven piles may well increase the settlement of the soil under its own dead weight and thus increase the settlement of the foundation itself.
Aspects of design of piled foundations which influence choice of pile type
All foundations must satisfy two criteria, no shear failure in the soil and no excessive settlement; piled foundations also have to meet this criteria. There are well established methods for ensuring that the first criteria is met, but the second presents more of a problem. The working load of an individual pile is based on providing an adequate factor of safety against the soil under the toe failing in shear and the adhesion between the shaft and the soil surrounding it passing its ultimate value and the whole pile sinking further into the ground. There are basically four methods for assessing this effect :