Wednesday, February 13, 2013

Choice of Pile.

Having found a satisfactory pile and a reliable and cooperative piling contractor for a particular site and conditions, there is a temptation for a busy designer, with  inadequate time to investigate the wide choice of piles and systems, to use the same piling contractor for all future  projects. This understandable reaction does not make for cost effectiveness nor structural efficiency. A guide to the choices available is given below:

(1) The piling system must provide a safe foundation with an adequate factor of safety against failure of the foundation on supporting soil.

(2) The total settlement and differential settlement must be limited to that which the structure can tolerate.

(3) The pile should be the right type of pile for the ground conditions and structure.

(4) The driving of the piles and the load they impose on the soil must not damage neighbouring structures.

(5) The piles must be economic and durable, and where speed of construction is important, quick to place.

1 Ground conditions and structure

(1) When invited to tender for the contract the piling contractors should be provided with a soil report, the position and magnitude of structural loads and the location of the structure together with information on adjoining properties. They should also be asked to visit the site to inspect the access for piling plant movements.

(2) Driven and cast-in-place piles, where the shell is left in, are used on sites over water (jetties, piers, etc.), on sites known to contain large voids, and on sites subject to high water pressure. Driven piles should not be chosen where the ground is likely to contain large boulders but they are one of the best piles for loose-to-compact wet sands and gravels.

These types of piles are frequently the cheapest to use on building sites with light-to-moderate pile loadings
and where the charges for moving onto site are spread over a large number of piles.

(3) Bored piles are frequently the lowest cost piles when piling into firm clays or sandstone and when vibration and ground heave would cause problems to existing adjacent buildings.
(4) Jacked piles need something to jack against and tend  to be expensive. Their main use is therefore in underpinning when they can prove to be cost-effective.
(5) Steel H piles are often chosen when long length  piles with deep penetration into sands and gravels are required.

2 Durability
The ground conditions can affect the choice and method of protection of piling material. Sulfates and acids will attack poor-quality concrete, some acids will cause problems with steel piles and alternate wetting and drying can cause timbers to rot.

3 Cost
Piles are, or should be, chosen as the economic and safe alternative to strip and raft foundations but there is more to cost analysis than comparing the cost per metre run of piles; there are on-costs. In comparing piling contractors’ estimates it can be unwise to accept the lowest cost per metre run.

Examination of extra over-costs for such items as extending lengths of piles, conducting check loading tests, etc. is  prudent. The designer should examine the piling contractor’s resources available to complete the project on time, the length of notice required to start work and the contractor’s experience in piling on similar sites. The  contractor’s reputation should be investigated and proof obtained of adequate insurance to indemnify building owners for any claims or damage to adjoining buildings  or failure of piles due to design and construction faults.

To the cost of the piles must be added the cost of excavation for constructing pile caps and any necessary tie beams. This increases the cost of construction supervision and design.

Decisions must be taken early so that design, detailing, construction and planning can be completed well in advance  of starting the contract. Too often the time is restricted by delays in site investigations, change of design brief, recent changes in contractors’ prices, etc.

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