In English only

Steel Tube Pile

Hollow steel piles can overcome some of the problems encountered with the flexibility of slender H-sections. Steel tube piles can be manufactured in seamless spirally welded, or lap-welded. There is no difference between the two types of welding with respect to the allowable driving stresses. Tubes are manufactured in sizes from 0,30 to 2 m outside diameter and with wall thickness ranging from 6-12 mm for the smallest diameters, to 10-25 mm for the largest diameters. Steel tube piles can be produced with increased wall thickness than standard sections, when it is necessary to accommodate high axial loads or bending moments, or in the case of corrosion. When driving into very stiff clays, dense granular soils or rock, the pile toe can be protected from buckling by a stiffening ring or different types of proprietary cast-steel shoes. An internal stiffening ring should be used where the pile is supported mainly by skin friction. In very hard driving conditions the toe protection should consist of a thicker wall pile section about one to one-and-a--half pile diameters in length, butt-welded to the main pile.

Tubular piles are normally installed by top driving but in difficult ground conditions they can be installed by a combination of "drill-and-drive".

The pile can be driven either closed or open ended. Also open-ended piles tend to plug, especially in cohesive soils or when impact hammers are used. As hollow piles are driven into cohesive soil layers, cohesion can prevent the soil from entering the pile base, and displacement will then occur unless the plug of soil is removed. In granular soils, steel tube piles can be installed more easily by vibratory hammers, which have a lower risk of plugging, whereby the speed of installation is increased. However, it should be assumed that in most soils even open-ended tubular piles perform as displacement piles.

Whether or not the plug needs to be removed during driving depends on the soil type, the pile diameter and the installation method. The tendency of plugging is largest in long piles with small diameter, driven into cohesive soils. Removing of the plug facilitates pile penetration but may be time-consuming and costly. Open-ended steel tube piles have generally a lower bearing capacity than closed-end piles.

Ground heave can be reduced by jetting, where water, air or grout is flushed to the pile base during driving, by pre-drilling (soil loosening) or pre-coring (soil removal).

Soil displacement at depth may not necessarily be observed as heave at the ground surface, but may cause significant lateral soil movements. The depth, at which soil displacement occurs, can be difficult to predict but is usually concentrated to cohesive soils.

Hollow steel piles may be driven closed-ended through cohesive soil containing cobbles and small boulders, but heave is then more likely to occur. This pile type performs well in resisting impact and bending loads, and large-diameter sections can be used to carry considerable loads. This has led to their extensive use for marine structures, where long, unsupported pile elements are commonly used at large water depth.

Steel tube piles of high tensile steel are used for marine structures (jetties and piers), where high lateral forces from the berthing impact of ships or wave action must be resisted. Due to their circular section, tubular piles are less affected by waves and currents than box piles, or H-section piles. Also, the use of high tensile steel gives often better economy with respect to weight of material and hence reduced shipping and handling costs. The piles can be manufactured of material with different strength properties: the upper part and the lower section in mild steel, and the center section in high tensile steel. Thus the more expensive high tensile steel is used in the highly stressed zone close to the sea bed, and facilitates welding of bracing steelwork to the upper section.

Tubular piles for marine applications have usually large diameter and are therefore often driven open-ended to resist lateral and uplift loading. However, the base resistance of open-end piles can be low in loose to medium dense granular soils. Excessive penetration depth can be avoided by welding H- or T-sections to the circumference.

If driving conditions permit, it is preferable to drive box or tube piles with a closed end, since this permits inspection of the pile shaft after driving, and usually gives higher pile bearing capacity. After driving, the pile shaft is filled with concrete. This method is frequently used when corrosion may be a problem (marine structures). Alternatively, piles may he driven open-ended and can, if required, be cleaned over their full length and concreted. If long piles are driven, a convenient method is to drive the first section open-ended. The following sections can be provided with a base plate. The upper part of the pile, which may he subject to corrosion, remains empty and can be cast with concrete. The stresses along the pile cross-section can be shared between the steel casing and the concrete core, thereby increasing the capacity of the critical pile section. However, the cross-sectional area of steel in a hollow pile is likely to be governed by driving stresses rather than by the compressive stress from the working load.



Copyright ©1998-2009 GRV AB Legal disclaimer