Q.
What are the factors influencing what type of pile to use?
A.
Factors influencing choice of pile
There are many factors that can affect the choice of a piled foundation. All factors need to be considered and their relative importance taken into account before reaching a final decision.
Location and type of structure
For structures over water, such as wharves and jetties, driven piles or driven cast-in-place piles (in which the shell remains in place) are the most suitable. On land the choice is not so straight forward. Driven cast-in-place types are usually the cheapest for moderate loadings. However, it is often necessary for piles to be installed without causing any significant ground heave or vibrations because of their proximity to existing structures. In such cases, the bored cast-in-place pile is the most suitable. For heavy structures exerting large foundation loads, large-diameter bored piles are usually the most economical. Jacked piles are suitable for underpinning existing structures.
Ground conditions
Driven piles cannot be used economically in ground containing boulders, or in clays when ground heave would be detrimental. Similarly, bored piles would not be suitable in loose water-bearing sand, and under-reamed bases cannot be used in cohesionless soils since they are susceptible to collapse before the concrete can be placed.
Durability
This tends to affect the choice of material. For example, concrete piles are usually used in marine conditions since steel piles are susceptible to corrosion in such conditions and timber piles can be attacked by boring molluscs. However, on land, concrete piles are not always the best choice, especially where the soil contains sulphates or other harmful substances.
Cost
In coming to the final decision over the choice of pile, cost has considerable importance. The overall cost of installing piles includes the actual cost of the material, the times required for piling in the construction plan, test loading, the cost of the engineer to oversee installation and loading and the cost of organisation and overheads incurred between the time of initial site clearance and the time when construction of the superstructure can proceed.
Pile groups
Piles are more usually installed in groups, rather than as single piles. A pile group must be considered as a composite block of piles and soil, and not a multiple set of single piles. The capacity of each pile may be affected by the driving of subsequent piles in close proximity. Compaction of the soil between adjacent piles is likely to lead to higher contact stresses and thus higher shaft capacities for those piles. The ultimate capacity of a pile group is not always dependent on the individual capacity of each pile. When analysing the capacity of a pile group 3 modes of failure must be considered.
· Single pile failure
· Failure of rows of piles
· Block failure
The methods of insertion, ground conditions, the geometry of the pile group and how the group is capped all effect how any pile group will behave. If the group should fail as a block, full shaft friction will only be mobilised around the perimeter of the block and so any increase in shaft capacity of individual piles is irrelevant. The area of the whole base of the block must be used in calculating the end bearing capacity and not just the base areas of the individual piles in the group. Such block failure is likely to occur if piles are closely spaced or if a ground-contacting pile cap is used. Failure of rows of piles is likely to occur where pile spacing in one direction is much greater than in the perpendicular direction.
Ref:
http://environment.uwe.ac.uk/geocal/foundations/Fountype.htm#RAFT