A steel pile can be a rolled section, a fabricated shape, or a piece of sheet pile. Two or more sections of sheet piles may be connected together in a box shape and driven as one pile. The most important advantage of using a steel pile is its load-carrying capacity. A steel pile can take up to 100 tons per pile. Steel piles are most commonly used for foundations of large structures with heavy loads.
Advantages of Steel Piles:
- Steel piles have a large bearing capacity.
- Steel piles can penetrate through stiff layers or boulders.
- The volume of soil displaced during the driving of steel piles is less.
- Steel piles can withstand rough handling.
Disadvantages of Steel Piles:
- Steel piles have a high possibility of damage from corrosion and electrolysis.
- Steel piles are relatively expensive unless the bearing stratum can develop large pile capacity.
- Steel piles are less effective than friction piles.
Common types of steel Piles:
H-piles and pipe piles are the most commonly used types.
H-piles are proportioned, especially, to withstand the large impact stresses during hard driving. The flanges and the web are rolled with equal thickness in order to eliminate damage on thinner parts. The flange width is made at least 85% the depth of the pile section in order to provide rigidity in the weak axis.
Pipe piles are made of seamless or welded pipes and are frequently filled with concrete. They may be driven closed-ended or open-ended. The open-end piles may be driven to the desired depth, and the soil inside the pipe is cleaned out. The closed-end piles are formed by fixing a driving point to the tip of the pile. For major columns, the pipe piles should be at least 10 inches in diameter, and the thickness should not be less than 5/16 inches.
The choice between the open-end and the closed-end types depends upon the soil conditions at the site. In some cases, an open-end pipe can be driven to a greater depth since the soil inside the pipe can be cleaned as the driving progresses. If boulders or other obstructions are encountered before reaching the desired depth, they may be removed by means of a chopping bit or in the case of large diameter pipes, by blasting. In other cases where cleaning of soil inside the pipes is difficult, closed-end pipes should be used.
Splices of steel piles:
H-piles are spliced in the same manner as steel columns. Welded or bolted (high tension bolts) splice may be used depending primarily on the preference of the engineer or the available equipment. If the splice is located above the ground surface, or if a large bending moment will act upon the pile, the splice should be designed to resist such stresses. In normal cases, even when the piles are not subjected to horizontal thrust or bending moment, there is a certain amount of bending stress in a pile. A pile is never perfectly straight and may get bent due to driving. This can cause bending moments in the piles. Hence it is customary to design the splice to resist a moment equal to one-third to one-half of the moment capacity of the H-section. Some building codes require the splice to develop the full strength of the H or pipe section. The pipe piles may be spliced by butt welding or by use of a sleeve.
Corrosion in Steel Piles:
Corrosion is a big problem with steel piles. Following are a few methods used to protect a steel pile from corrosion -
This method, employed by most codes, requires that the pile section contains a certain extra thickness (usually about 1/16 inches) in excess of the sectional area as required by strength. This may be accomplished by either deducting 1/16 inches from the actual metal thickness while computing the pile capacity or splicing another piece of steel to the pile length where corrosive action is anticipated.
Removal of corrosive soils
Organic soils, cinder fill, unburned carbon, or industrial waste near the ground surface may be removed and replaced by noncorrosive soils. This method is economical only when such soils are at shallow depths.
Near the ground surface where moisture and oxygen are abundant, the piles may be protected very effectively by encasing with concrete. Concrete encasement extending to a great depth is a costly method.
Since corrosion is a continuing process of removing electrons from the piles, a method of preventing such a removal would be effective. This is done by cathodic method whereby a slow current is introduced towards the steel piles, instead of away from them.
It should be pointed out here that painting or other types of coating applied prior to driving are of doubtful value since the coating in the embedded portion is likely to be damaged from abrasion or handling. Piles projecting into the atmosphere should be painted periodically. Piles projecting in polluted water should be protected with concrete encasement or coal tar paint. In fresh water, the section near the water surface should be protected.
Foundation Design by Wayne C. Teng