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Piles are deep foundations which transfer the load of the superstructure deep down in the ground. Piles are basically long slender columnar members which transfer the load of the superstructure to the ground through skin friction or through end bearing. A foundation is considered a pile foundation if its length is more than three times its breadth. Piles may be made of timber, concrete, or steel. In this article, we are going to discuss timber piles.
Timber pile is a trunk of a tree, trimmed of branches. A timber pile is usually designed for a maximum load of 15 to 25 tons/pile. Additional strength can be obtained by bolting fish plates to its side. These piles last for about 30 years. The breadth of these piles ranges from 12 to 16 inches.
These piles can be used as a foundation for structures with moderate load and as a foundation for temporary structures.
Advantages of timber piles
- The cost per running length of the pile is low. Hence these piles are economical.
- Timber being a resilient material, timber piles are suitable for impact absorption.
- Timber piles are easy to install.
- If necessary, it is easy to uninstall a timber pile.
Disadvantages of timber pile
- Timber piles have small bearing capacity.
- Untreated timber piles above groundwater may last more than 25 years but are not permanent.
- Timber piles are prone to damage by hard driving.
- Timber piles cannot be driven through hard stratum or boulders.
- Piles of longer length may not always be available.
A timber pile must satisfy the minimum requirements before being qualified as a foundation pile. The quality, the treatment and the constructional characteristics of timber piles are discussed below.
Quality and Classification of Timber Piles
A timber pile should be of sound quality and free of defects. It should be straight and have a uniform taper. A straight line drawn between the center of the butt to the center of the tip should be contained entirely within the pile. The quality of piles is determined by the amount or lack of defects (decay, splits, twist of wood grains, etc.), size of knots, holes, etc.
According to ASCE Manual no 17 in the section entitled “Timber piles and Construction Timber”, timber piles are divided into three classes on the basis of the quality of timber and the dimensions of the piles.
|Class A pile||The minimum diameter for this class of piles is 14 inches.|
These piles are to be used for heavy loads or large unsupported length.
|Class B pile||The minimum diameter for this class of pile ranges between 12 to 13 inches. These piles are used for medium loads.|
|Class C pile||The minimum diameter for this class of pile is 12 inches.|
These piles are used for temporary structure.
Untreated piles entirely embedded below the groundwater table are considered permanent, provided that the marine borers are not present. When projecting above water, the timber piles are subjected to decay by fungi and attacked by insects and borers. Therefore, building codes usually prohibit the use of untreated timber piles above the water table to support permanent structures.
The most effective and common method for the prevention of decay and animal and plant attacks is the treatment of piles with preservatives like creosote oil which is used universally. When a sufficient amount of creosote oil is impregnated properly in the piles, the protection against decay and attack is excellent, with the exception of few borers.
The treated piles should be handled with care. Cutting, framing, and drilling should be done before the treatment as much as possible. Cuts, abrasions, etc should be covered with coats of creosote.
Over-driving of Timber Piles
One of the most significant drawbacks of timber piles is the possibility of damage due to over-driving. Piles may be damaged at the tips or above. Providing a metal driving shoe on the tip does not materially reduce the possibility of such damages. Therefore, the design capacity of the timber piles is limited empirically to about 25 tons in order to avoid the possibility of damages due to the necessity of hard-driving. Furthermore, the behavior of the pile and the blows per foot of penetration during the pile driving operation should be carefully observed. If there is any doubt as to the possibility of such damage, the pile driving should be stopped immediately, and if necessary, one or two driven piles may be pulled out for visual examination.
Foundation Design by Wayne C. Teng