GB2315086A - Piling - Google Patents

Abstract

In the manufacture in situ of secant piles, the secondary piles 4 are formed individually by the use of cylindrical forms 14. When each form has been excavated to the desired depth, at least one longitudinal former 12 is placed in the form in parallel with its axis and close to the inner wall of the form. The cross-sectional shape of the former is chosen so that, when the form has been filled with concrete; the concrete has set, and the form has been removed axially, the resultant secondary pile has in its convex surface a longitudinal recess having at least a partially-cylindrical concave surface. When two such piles have been formed, a similar form 14 may be driven along their length by contact with the recesses, and the complementary primary pile 2 manufactured. Preferably the form for the primary pile is withdrawn at least partially while the concrete has not fully set, so that concrete from the pile is able to slump and fill the space left between the pile and each adjacent secondary pile by the withdrawal of the form.

Description

Piling This invention relates to piling, particularly of the secant or secant bored type. In such piling, the piles have their central axes spaced apart by less than their diameters, with alternating piles being in the form of primary cylindrical piles of circular cross-section, with the intermediate secondary piles being of complementary cross-section so that the rounded surface of each primary pile is received within longitudinal recesses in the surface of its contiguous secondary piles, so that all the piles are keyed together.

In the construction of secant piles, a series of cylindrical pits for the primary piles is dug by hand, and into which the concrete for the piles is poured. When the piles have set, shuttering is used to define the shape of the secondary piles, again with manual excavation, and the secondary piles are then poured.

In this specification, the term 'concrete' is to be understood as including concrete or like material which is fluid enough to be poured, and which is strong enough when set.

This method of construction is not suitable for the construction of secant piles which are located close to the sea or other large bodies of water, or where the water table ils high, because of the danger of the excavations collapsing while being dug out.

One known method of overcoming this problem is known as secant boring. In this method the pits for the secondary piles are dug (bored) by machine, with the use of hollow casings (liners). After the casings have been set in place, concrete is poured into them and allowed to set, and the casings are removed to leave a set of spacedapart cylindrical piles. A rotary cutter is then driven along an axis intermediate two piles to bore another pit. In do so doing, the cutter cuts into the convex surface of both adjacent piles lengthwise to define a right cylindrical chamber for the primary piles, into which concrete is poured later. This cutting process is time-consuming and expensive, and suffers from the additional disadvantage that the alignment of the bore with the axes of the secondary piles is difficult to control. This is because the cutter tends to follow the path of least resistance, preferring to be deflected from its intended path and cut into earth rather than into the concrete of the secondary piles.

The present invention aims at overcoming this problem by doing away with this cutting step, and accordingly provides a method of constructing secant piles which is as claimed in the appended claims.

The present invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a plan view of a series of secant piles, forming one corner of a retaining wall; Figure 2 is a plan view of three secant piles of this invention, showing on the righthand side a plan view of a form for making a secondary pile of the desired crosssection; Figure 3 is a view similar to Fig. 2, showing a second form of secant piles; Figure 4 is an axial cross-section, part in elevation, of the form and formers for forming the secondary pile of Fig. 2; Figure 5 is an isometric view of one former used for making a recess in a secondary pile; Figure 6 is a plan view of the form and formers of Fig. 2, with reinforcement mesh inserted in the form prior to concrete being poured in; Figure 7 is an axial cross-section of a partially completed secondary pile, with the form partially removed; Figure 8 is a section, on the line VIII-VIII of Fig. 7, of a secondary pile of this invention, with two formers still in place; Figure 9 is an axial cross-section of two secondary piles of this invention, with a form for the primary pile being inserted between them; Figure 10 is a view similar to Fig. 9, with the primary pile partially complete, and with the form being removed in stages, and Figure 11 is a plan view of three secant piles of this invention, as part of a retaining wall.

In Fig. 1 is shown an alternating series of primary and secondary piles of the secant type. The primary piles 2 are of circular cross-section normal to their axes, while the secondary piles 4 are of 'wasp-waisted' cross-section. The centres of the piles are spaced apart by less than the diameter of the primary piles, so that the secondary piles have to have their diameters reduced in order to fit into the space between two primary piles. It is this 'cutting' of one pile into its neighbour which gives rise to the name 'secant'.

The left-hand side of Fig. 2 shows two primary piles with a secondary one sandwiched between them. The recesses 6 in the latter pile are of lozenge-shaped cross-section, being formed by two intersecting arcs, while that embodiment shown in Fig. 3 has a recess 8 with a rectangular extension 10 to improve the lateral keying between the two types of piles.

In accordance with this invention, the recesses 8 or 10 are formed with the use of formers 12 during their construction. In order to make each secondary pile 4, a cylindrical form 14 of circular cross-section is driven into the ground to a desired depth before the interior of the form is excavated to a desired depth or completely, corresponding to the height of the retaining wall to be formed by the piles. The longitudinal formers 12 are then introduced into the interior of the form 14, being held parallel to the axis of the form as by means of temporary battens, and being kept close to, but not necessarily in contact with, its intemal surface. Fluid concrete or like high-strength setting material is then poured into the form to a depth of about two to three meters. When it has set sufficiently to have acquired a significant proportion of its eventual strength, so that the pile formed by set concrete is largely self-supporting against the force of gravity, the form and/or the battens may be withdrawn axially, leaving the formers 12 in place. This may be assisted, in known fashion, by applying a vibration hammer or casing oscillator to the casing (form) to reduce the grip which the surrounding soil and the set pile exert on it. The pile may be poured in stages, with withdrawal of the form likewise in stages, until eventually the form may be removed completely. The formers are usually longer than the form, so that their ends 16 project from the forms initially. When the forms have been removed, the ends of the formers stick up from ground level, enabling traction to be applied to their ends to remove the formers from the ground also, leaving in the ground a secondary pile of the cross-section shown in Fig. 2.

Fig. 5 is a view of the former itself. It may be made of any suitable material, such as steel plate, so that it is able to withstand the withdrawal traction applied to it, without disintegrating. In addition, its outer surfaces may be treated with a lubricant or parting material, so that the concrete does not key to it.

As shown in Fig. 6, it is possible to insert into the empty form, before or after the formers are positioned, a reinforcement structure 18 made from steel or wire welded together to give it the necessary strength. In known fashion, after the concrete has been poured in, bubbles may be removed from it by a suitable ultrasonic or other agitator, to cause the bubbles to rise to the surface. In watery environments, known methods may be employed for pouring concrete into flooded forms, from which the concrete displaces the water before the concrete sets.

The sequential construction of the pile is indicated in Fig. 7, in which the form 14 has been lifted until the surface 20 of the concrete lies just above the bottom end of the partly-withdrawn form, awaiting the next pouring of concrete. Fig. 8 shows the crosssection of the pile before the formers 12 have been removed to leave the respective recesses.

Fig. 9 shows two parallel secondary piles 2. In order to construct the intermediate primary pile, a form 14, usually of the same diameter as was used to make the secondary piles, is driven into the earth 22, being guided in its path by contact with the two diametrically-opposed recesses 6. When fully in place and excavated, then a reinforcement structure 24 may be introduced into its interior before the next batch of concrete is poured into the form.

In accordance with a preferred feature of the invention, the form 14 is withdrawn, at least partially, before the concrete at the bottom of the primary pile has fully set, so that the concrete is able to slump slightly until it comes into weight-bearing contact with the earth and the secondary piles. When it becomes fully set, this contact ensures that the primary and secondary piles are keyed together both mechanically and chemically. The resultant lowering of the surface 20 of the poured concrete is accommodated in the next pouring.

Fig. 11 shows a cross-sectional view of three secant piles formed in accordance with this invention. The reinforcement structures may be omitted; may be of the same shape, or may be of different shapes, as shown.

It will be seen that this invention provides a cheaper and quicker way of constructing structures from secant piles than by the use of cutters to form the longitudinal recesses in the secondary piles.

Claims (8)

Claims

1 A method of constructing a retaining wall in the form of a series of secant piles keyed together, the method comprising the steps of: driving into the earth a hollow cylindrical form; excavating the interior of the form down to a desired depth; inserting into the emptied length of the form at least one longitudinal cylindrical former of desired cross-sectional shape, and positioning it in parallel with the axis of the form and close to its inner surface; pouring a settable filler into the interior of the form; withdrawing the form axially from the set piles and former(s); withdrawing the former(s) to leave a secondary pile of desired cross-sectional shape; inserting between two adjacent secondary piles a hollow cylindrical form of which part enters the recess left by each removed former; excavating the form; pouring a settable filler into the interior of the form, and removing the form axially to leave the convex surface of the primary pile lying within the concave recess in the or each adjacent secondary pile, to key the piles together against relative lateral movement.

2 A construction method as claimed in claim 1, including the step of removing the form for the primary pile before the filler has completely set, whereby some of the freshly-poured filler comes into contact with the surfaces of the adjacent secondary piles to bond together the two secondary piles and the newly-set primary pile.

3 A method as claimed in claim 1 or 2, in which reinforcement structures are inserted into the interior of at least one form before the filler is poured into it.

4 A method as claimed in any preceding claim, in which the length of the former is chosen to be longer than the respective form, so that the upper end of each former projects beyond the end of the form before the filler is poured in.

5 A method as claimed in any preceding claim, in which the pouring of the filler, and the withdrawal of the form, are in stages, and in which the lower end of the form is always kept below the surface of the filler forming the partial pile until the next batch of filler has been poured.

6 A method as claimed in any preceding claim, in which extraction of the form from the ground, and from around the fresh pile, is assisted by vibrating the form longitudinally and/or angularly while axial traction is applied to it.

7 A method of constructing secant piles substantially as described herein with reference to, and as shown in, the accompanying drawings.

8 A retaining wall or like structure made from a series of keyed secant piles made by the method as claimed in any preceding claim.