Steel Piles.

Though most piling is carried out using some form of  concrete there are situations where steel piles should be considered. There is considerable experience of the use  of sheet piling in civil engineering which can be applied  to piles for structures. The piles are generally a standard  H section (see Table 14.1) or, for longer or more heavily loaded sections, tubular box section piles are used (see Table 14.2). In some cases they can form a structurally efficient and cost-effective solution. The advantages of steel piles compared with concrete are:

(i) They have a lighter weight for a required load-bearing capacity.
(ii) They can be used in longer lengths and extending them by butt welding is relatively simple. Similarly excess length is easily cut off.
(iii) They are more resistant to handling and driving stresses.
(iv) They can have good resistance to lateral forces, bending stresses and buckling.
(v) They are particularly useful for marine structures (piers, jetties, etc.) above water where the piles may be subjected to impact forces, ships docking, etc.

The disadvantages of steel piles are:

(i) There is a need for corrosion-protection.
(ii) The pile cost per metre run can be relatively high.
(iii) There are fewer piling sub-contractors competent to carry out the work.
(iv) There is a tendency for H sections to bend about the weak axis when being driven – the resulting curved pile has a lower bearing capacity.

The use of stones or large gravel in piling is now a well-developed technique. The authors have been involved  in extensive use of this form of piling for many years.

The method is used mainly as a geotechnical process to  consolidate and compact soils and/or to improve their drainage. The technique is discussed in detail previously and reference should be made to that chapter for further information.

Table 14.1 Steel H piles – dimensions and properties

Table 14.2 CAZ box piles – dimensions and properties

Steel Piles - Description.

Steel piles, like timber, are driven by percussion means and have a variety of suitable cross-sections. In addition to the common sheet piles, the three main types are H sections, box piles and tube piles. Typical sections are shown in Fig. 9.39.

The main use of steel piles is for temporary works, retaining walls and marine structures. The problem of corrosion of the steel can be overcome by suitable protection. However, account should be taken of the abrasion during driving on the final performance of such protection/coatings. In addition to coating, increased metal thickness and cathodic protection may be appropriate for particular locations and conditions. Detailed information on these aspects along with detailed information on steel piles and piling is outside the scope of this book due to their limited use in structural foundations.

Fig. 9.39 Typical steel pile cross-sections.

Steel Grillage Foundation - Method of Construction.

Steel grillage foundation is constructed of steel beams, structurally known as rolled steel joists(RSJ.), provided in two or more tiers. In the case of double tier grillage(which is commonly provided), the top tier of grillage beams is laid at right angles to the bottom tier. The joists or beams of each tier are held in position by 20 mm diameter spacer bars wtth 25 mm diameter pipe separators. Fig. 3.10 shows the plan and section of such a foundation. The grillage beams are embedded in concrete. Generally, a minimum clearance of 8 cm is kept betwen the grillage beams so that concrete can be easily poured and properly compacted. However, the distance between flanges should not exceed 30 cm  or 1 ½  times the flange width (whichever is small) so that the filled concrete acts monolithically with the beams. It should be noted that the concrete filling does not carry any load; it simply keeps the beams in position and prevents their corrosion. A minimum concrete cover of 10 cm is kept on the outer sides of the external beams, as weIl as upper flanges of top tier. The depth of concrete below the lower tier should atleast be 15 cm.

Method of construction. The foundation is excavated to the desired depth. Generally, the depth of foundation is shallow, just sufficient to accommodate the two tiers of grillage beams and the gusset plates etc. connecting the stanchion to the base. Howeer this depth should not be less than 90 cm in any case. After leveling the foundation base, rich concrete is poured and compacted, so that the formed thickness is not less than 15 cm. Compaction should be done properly so that the layer of concrete becomes an impervious bed.

This would protect the steel joists against ground water. After levelling the concrete bed, first layer of grillage beams of designed sizes are laid over it, at proper distances, with the help of separaters. The upper surface of all the beams should lie in one horizontal plane. Rich cement grout is then poured all around the lower flanges of the beams so that they are secured to the concrete bed. Cement concrete is then poured betwecn and around the beams of the first tier. The second tier of beams is then placed at right angles to the first tier and over the top flanges of the beams of the first tier. They are properly spaced with the help of separators. Concrete is then poured between and around the steel beams. The steel stanchion is then connected to the upper tier with the help of a base plate, side angles and gusset plate. These connecting elements are also embedded in the concrete so that joint becomes rigid..

Steel grillage foundation may also be provided for a masonry wall on soils of low bearing capacity. The grillage foundation for such a case consists of only on tier, though in some circumstances when the wall is wider and it carries heavy loads, two tiers may also be provided. Fig. 3.11 shows the details for both the cases.

FIG. 3.10 TYPICAL GRILLAGE FOUNDATION FOR STEEL STANCHION.

FIG. 3.11 STEEL GRILLAGE FOUNDATIONS FOR WALLS.