The present invention relates to piles and to their production.

Piles for use in supporting structures such as buildings are commonly pre-formed , in sections, and driven into the ground from the surface. Alternatively, they may be formed by driving a head into the earth to form a hole. A pipe extending down the hole supplies concrete or other pile material to the head. The head comprises a through passage which allows the pile material to fill the hole beneath the head as the head is withdrawn from the hole. A sacrificial cap closes the lower end of the through passage while the head is being driven to form the hole, and is left at the bottom of the hole when the head is withdrawn.

A major contribution to the supporting force provided by a pile arises from friction between the surface of the pile and the surrounding earth. This friction can be increased by increasing the size of the pile, but there is a resultant increase in the volume of pile aterial required. It is an object of the present invention to obviate or mitigate this problem.

The invention provides a pile having a cross-

section which is non-circular along at least a part of the length of the pile.

Preferably the outline of the said cross-section includes at least one straight portion. The outline of the said cross-section may be a polygon. The polygon may be regular. The polygon may have six sides or less. The said non-circular cross-section is preferably a triangle.

The outline preferably includes at least one reentrant portion. The pile may incorporate at least one transverse projection (or fin) which extends substantially further in the radial direction than in the circumferential direction of the pile. The or each fin may be superimposed on the said polygon or triangular outline. The or each fin may be superimposed on an outline which is otherwise circular.

The said pile may incorporate a plurality of fins radiating from the axis of the pile to form a star-like non-circular cross-section. The star-like cross-section may consist of a circular core and a plurality of fins radiating from the centre of the core.

The invention further provides a pile-making device, comprising a head which, in use, is driven into

the earth to form a hole, means for supplying pile material to the head, and means within the head for allowing the pile material to fill the hole beneath the head as the head is withdrawn from the hole, wherein the outermost outline of the head is so shaped as to form a hole of non-circular cross-section.

The outermost outline may include at least one straight portion. The outermost outline is preferably a polygon, such as a regular polygon. The polygon may have six sides or less. The outermost outline may be triangular .

The outermost ouline may include at least one reentrant portion. The head may incorporate at least one transverse projection or fin which extends substantially further in the radial direction than in the circumferential direction.

The head may comprise one or more fins projecting from the polygon outline. The or each fin may project from an outline which is otherwise circular.

The head may comprise a plurality of fins radiating from a central axis, whereby the head forms a hole with a star-like cross-section. The head may comprise a core of

circular cross-section, and a plurality of fins radiating from the core. The said outermost ouline may be the outline of the head at only one point along its length. The head may narrow smoothly from the said one point towards that end of the head which leads when the head is forming a hole. The head may narrow at a step behind the said one point in relation to the direction of movement when forming a hole.

Embodiments of devices according to the present invention, and the piles produced by those devices will now be described in more detail, by way of example only, and with reference to the accompanying drawings in which:

Fig. 1 is a perspective view of a pile-making head according to the invention;

Fig. 2 is a cross-section of a pile formed using the head of Fig. 1 ;

Fig. 3 is an end view of an alternative head;

Figs. 4 and 5 are views of the head of Fig. 2 along the line 4-4 and in the direction of the arrow 5, respectively;

Fig. 6 is a cross-section of a pile formed using the head of Figs. 3 to 5, and

Fig. 7 is an elevation of a further alternative head.

Fig. 1 shows a pile-making device 10 comprising a head 12 which, in use, is driven into the earth to form a hole. Means 14 in the form of a pipe are provided for supplying pile material (normally concrete) to the head 12. A through passage 16 within the head allows the pile material to fill the hole beneath the head 12 as the head is ithdrawn from the hole. The device 10 may be driven into and out of a hole by any conventional technique, such as hammering, jacking or vibrating but a vibrating technique is preferred certainly for withdrawing the device from the hole.

The outermost outline of the head 12 is so shaped as to form a hole of non-circular cross-section.

In more detail, the head 12 comprises a block 18 in the form of a triangular prism whose central axis f ^' crms the axis of the head and the axis of the hole formed by the head. One end of the block 18 carries an extension 20 in the approximate form of a pyramid which causes the

head to taper from the outline of the block 18 to the mouth of the through passage 16, shown closed by a sacrificial cap 22.

The base of the pyramid is slightly larger than the cross-section of th prism, so that a small step 23 is formed where they meet. The outline of head is at its largest at this point. That is, the outermost outline of the head, when viewed along the axis of the prism, occurs only at one point along the length of the head, at the top of the step 23.

Three fins 24 project radially from the axis of the head 12 and are generally aligned in the radial planes of the edges of the block 18. It can be seen from Fig. 1 that the fins 24 project beyond the outermost outline of the block 18 and that they extend substantially further in the radial direction than in the circumferential direction. The fins may be formed, for instance, by sections of steel plate.

The head 12 is used to form a pile in the following manner. The head 12 is driven into the earth by a vibrator to form a hole to the required depth. The extension 20 and cap 22 lead as the head 12 is being driven. The pipe 14 follows the head 12 down the hole,

additional sections of pipe being added as required. Since the outermost outline of the head 12, in crosssection, is triangular, with the exception of the fins 24, the head forms a generally triangular hole 26 (Fig. 2) which is extended at each corner by slots 28 formed by the fins 24. When a hole 26 to the desired depth has been formed, the head is withdrawn by reversing the direction of force applied by the vibrator as concrete is supplied through the pipe 14 and the through passage 16 to the space left in the hole below the head 12. This space is filled with concrete to leave a pile 30 which is generally triangular in cross-section, with additional flanges 32 running in each slot 28.

The taper of the pyramid 20 and the step 23 minimise the friction resistance while driving the head 12 to form the hole 26. It may be necessary to provide some means for preventing the hole collapsing behind (above) the head 12, around the pipe 14. These means cculd be plates which form an outline slightly εrτ.ε.ller than the outline of the block 18, or the triangular prism 12 could be extended in sections back to the surface.

The frictional force between the pile 30 and the surrounding earth is proportional to the surface area of the pile 30. It will be seen from the following

explanation that the surface area of a specified volume of concrete is greater when that volume is cast in the form shown in Fig. 2 than when it is cast in the form of a conventional pile of circular cross-section. Consequently, the frictional forces and the load-bearing capacity of the pile shown in Fig. 2 are greater than those of a conventional pile formed with the same volume of concrete.

The circumference of the cross-section of a circular pile of radius R has a length (2T\R). The cross-sectional area is (7^R ² ). The friction between the pile and the earth will be proportional to the surface area of the pile, and will therefore be proportional to (2A R) per unit length of the pile. The volume of concrete in the pile is (T R ² ) per unit length.

The circumference of the cross-section of a triangular pile is 35, if the length of each side of the triangle is S. The cross-sectional area is ( ^rS ² ) . The friction created is therefore proportional to 35 per unit length, and the volume of concrete used is ( - _j T S ² ) per unit length.

If the volume of concrete per unit length is the same in each case, then

T R _ H____- ->

The following equation therefore relates the friction produced by the piles:

Friction (triangular pile) = 3 S x Friction

2 T\ R (circular pile)

= 3 4 7\ § x Friction (circular pile) 2 7\ /

= (1.28) x Friction (circular pile).

The friction produced by the same volume of concrete is therefore 28.ά higher when the concrete forms a triangular pile than hen the concrete forms a circular pile. In addition, the pile shown in Fig. 2 has the flanges 32 which provide a large surface area, and therefore a large increase in friction, with only a

relatively small increase in the volume of concrete required.

A similar analysis to the one given above shows that a square pile (without flanges) has a performance enhanced by approximately 12.ό. The performance of a hexagonal pile (without flanges) is enhanced by about 5%.

The remaining drawings show a head 40 for forming a star-shaped pile 42. The head 40 comprises a number of fins 44 which project radially from a spine formed by a length of circular pipe 46. The pipe 46 has a mouth 48 which incorporates means for mounting a sacrificial cap. The hollow of the pipe 46 provides a through passage for use in the same manner as the through passage 16 of the head 12 described above.

The fins 44 have a mid point 50 at which their outermost edge runs parallel with the axis of the pipe 46. Between the mid points 50 and the front mouth 48 of the pipe, the radially outermost edges of the fins 44 taper down from the mid points 50 to meet the outer surface of the pipe 46 near the mouth 48. The fins 44 taper similarly, but more sharply, behind the mid points 50. In addition, it can be seen from Fig. 4 that the thickness (that is, the circumferential extent) of the

fins 44 varies at different points along the length of the fin, and at different points along any radius. The thickness of the fins 44 is greatest at the base of the fin, adjacent the pipe 46, in the region of the mid points 50.

Behind the fins 44, relatively narrow flanges 51 extend back from the head 40, for reasons to be described.

The outline of the pipe is slightly enlarged in the region of the mouth 48, back to a step 49.

It can be seen from Fig. 3 that the head 40 has five fins distributed evenly around the axis of the head. The outermost outline of the head has a generally star-like form, consisting of the outlines of the fins at the mid points 50, superimposed on the outline of the enlarged front end of the pipe 46.

In order to form a pile, the head 40 is driven into the earth by a vibrator as described above in relation to Figs. 1 and 2. The fins 44 and pipe 46 together create a hole 52 of generally star-like cross-section. The fins 44 cut channels to form the points of the star. Resistance to

this cutting is reduced by the tapering front of the fins 44, and their reduced frontal thickness. The flanges 51 help to stiffen the pipe 46, and assist in preventing the collapse of the channels before concrete is cast into the hole 52.

As the head 40 is withdrawn from the hole 52, after the direction of force applied by the vibrator has been reversed, concrete is supplied through the pipe 46 to fill the space below the head 40 and leave a cast star-shape pile 42 in the hole 52. The star shape consists of fins superimposed on a core having a cross-section which is otherwise circular. It will be apparent from the shape of the cross-section that the star-shape, especially the points 56 of the star, provides a greatly increased surface area for the pile, in comparison with a circular pile using a similar volume of concrete, and therefore provide enhanced performance.

It is due to this increased surface area and consequent frictional resistance that the use of a vibrator to drive and ithdraw the head 40 is desirable. The head cannot be simply pulled from the hole it has formed by applying an upwardly directed pull as the force required would be too great for normal apparatus to produce. A vibrator can be reversed to readily provide

sufficiently great upwardly directed force.

Variations and modifications to the heads described above, and consequently to the resulting piles, can be made without departing from the spirit and scope of the present invention. The number of fins used may be chosen according to local conditions in use. An excessive number of fins could give rise to shear of the surrounding earth so that the maximum numbers of fins is six .

A most useful number of fins is three and a head 140 with three fins 144 is shown in Fig. 7. This head is similar to the head illustrated in Figs. 3 to 5 but has only three fins 144 which are substantially triangular in shape and parallel sided. The leading edge 145 of each fin is substantially perpendicular to the axis of the pipe 146. Advantageously the pipe is of the form described in our 10 - pending U.K. Patent Application 8914764.

In each of the embodiments described above at least one steel reinforcing bar is located in the pile before the cementitious mixture has set to provide reinforcement. In view of the relatively small diameter

of the circular cross-section pile core this is an important feature.