GB2113748A

GB2113748A - Piles

Info

Publication number: GB2113748A
Application number: GB08201845A
Authority: GB
Inventors: Wilfred George Kenneth Fleming; Ziemowit Jan Sliwinski
Original assignee: CEMENTATION FRANKIPILE Ltd
Current assignee: CEMENTATION FRANKIPILE Ltd
Priority date: 1982-01-22
Filing date: 1982-01-22
Publication date: 1983-08-10
Also published as: ATE16296T1; GB8300058D0; DE3361079D1; EP0084921A1; EP0084921B1; GB2113748B; GB2117432A

Abstract

The method involves driving into the ground a mandrel (1) carrying a pile shoe (2) and pumping a cementitious mixture (4) into the hole through the interior of the mandrel (1) as it is being driven into the ground. The cementitious mixture (4) leaves the interior of the mandrel (1) and enters the annular gap between the mandrel (1) and the earth surface via one or more holes (34) located at or close to its lower end and generally just above the pile shoe (2). The mixture (4) is maintained at a positive pressure relative to its surroundings, and is allowed to set after the mandrel (1) has reached the desired depth and has been withdrawn leaving the shoe (2) behind. The driving rig need not have the bulk required to exert the massive pulling forces required in conventional piling systems where a temporary casing which has been driven tightly into the soil is withdrawn prior to completion of the pile.

Description

1 GB 2 113 748 A 1

SPECIFICATION Piles

The invention relates to piles, and more particularly is concerned with piles which are of the driven cast-in-place type.

In conventional piling systems of the driven cast-in-place type, the stages of production are:

(1) a driving rig is set up at the desired pile position; (2) a casing or tube (often fitted with an 75 expandible base shoe to prevent entry of soil into the tube) is driven into the ground, the diameter of the tube being the same as that of the pile being formed; (3) a steel reinforcing cage is placed inside the 80 tube; (4) concrete is poured into the interior of the tube, usually to ground level or slightly above; and (5) the tube is then withdrawn from the ground, and the driving rig moves on to the next pile position. With current techniques, the time taken to complete these five operations for a single pile is on average around 45 to 50 minutes. The driving rig is also a robust and expensive piece of equipment, since the pulling force required to extract the tube in step (5) is often of the order of to 150 tonnes. The masts of the machines have to be designed to carry this load and some means of transferring this force to the ground must also be provided.

In one prior art piling system, generally termed the "Interpile", a tubular, perforated mandrel carrying at its lower end a detachable pile shoe is driven into the ground. The dimensions of the pile shoe exceed those of the mandrel so that there is 100 an annular gap between the outer surface of the mandrel and the earth wall(s) created by the driving of the pile show. A fluid concrete mix is poured into a hopper which surrounds the mandrel as the mandrel is driven into the ground. The 105 gravity-fed concrete flows downwards, moving with the pile shoe, and enters the interior of the mandrel via a large number of holes. Once the pile shoe and mandrel have reached the desired depth, they are left in place and the concrete is allowed 110 to set. A similar method is described in U.S. Patent No. 779,881 issued January 1 Oth, 1905. Systems of this type are not ideal for the construction of deep piles. Although in theory the hydrostatic pressure at the base of a fluid column will always 115 exceed that at higher levels, in practice this does not always apply to a column of concrete which is to form the pile. For example, if there is significant bleeding of the concrete at the top of the pile, or if a band of concrete in this area sets against the surrounding earth and thus becomes separated from the remainder of the column (known as thd "hang-up" effect), then the area in question will not contribute, or will not contribute in full, to the hydrostatic pressure acting on the concrete at the base of the pile. The normal frictional forces between concrete and the surrounding earth (which always exist when the concrete moves downwardly to follow the motion of the pile shoe) also contribute to the "hang-up" effect. The reduction of hydrostatic pressure at the base of the pile is accompanied by a proportional reduction in the concrete lateral pressure at the base of the pile, and it may thus become possible for soil to move inwards to fill any void being formed above the pile shoe. In this way the pile section is reduced to a value below that corresponding to the size of the pile shoe. In other words the pile may--- necV, and this is highly undesirable. The tendency of the pile shoe to displace earth sideways and upwards also increases the lateral concrete pressure necessary to support the pile at its base.

An object of the present invention is to provide a method of forming piles which makes it possible to reduce the time required to form a pile, to reduce the mass of the machinery involved in producing the pile, and to reduce the possibility of the pile necking.

According to the present invention, there is provided a method of forming a pile in the ground, which comprises:

(a) driving into the ground a mandrel carrying at its lower end a pile shoe the dimensions of which measured in a plane perpendicular to the direction of driving exceed the corresponding dimensions of the mandrel; (b) pumping a cementitious mixture into the hole around the mandrel during the driving of the mandrel into the ground so as to maintain the cementitious mixture at a positive pressure relative to its surroundings; and (c) when the mandrel has reached the desired depth, leaving the pile shoe in place, withdrawing the mandrel and allowing the cementitious mixture to set.

Because the mandrel is smaller in size than the pile and is surrounded by unset (i.e. fluid) cementitious material, the forces required to extract it at completion of driving a pile are small compared with the pulling forces which are required in conventional cast-in-place driven piling systems to withdraw the temporary casings which have been driven tightly into the soil.

According to a further aspect of the present invention, there is provided a method of forming a pile in the ground, which comprises:

(a) placing onto the surface of the ground at the location where the pile is to be formed a pressure box which is adapted (1) to have passed therethrough a mandrel and (2) to supply concrete under pressure into a hole around the said mandrel; (b) driving into the ground, via said pressure box, a mandrel carrying at its lower end a pile shoe the dimensions of which measured in a plane perpendicular to the direction of driving exceed the corresponding dimensions of the mandrel; (c) pumping a cementitious mixture under pressure through the pressure box and into the hole around the mandrel during the driving of the mandrel into the ground so as to maintain the cementitious mixture at a positive pressure relative to its surroundings; and 2 GB 2 113 748 A 2 (d) when the mandrel has reached the desired depth, leaving the pile shoe in place, withdrawing the mandrel and allowing the cementitious mixture to set.

The pressure box in this aspect of the invention is held against the surface of the ground with sufficient force to prevent back-pressure from the pumped cementitious mixture from causing any displacement of the pressure box. The pressure box may be provided, at its lower surface, with a tubular extension which passes downwardly into the upper part of the hole which is formed by the mandrel and pile shoe. Preferably, the cementitious mixture is maintained under pressure until after the mandrel has been withdrawn in step (d) above. The mandrel may be solid or tubular and it may be perforate or imperforate. Conveniently, the mandrel is formed of steel or other highstrength material. The mandrel may be formed by the co-operation of a plurality of unit lengths joined together, e.g. with flush joints. The pressure box preferably includes a pressure gland which in use abuts the mandrel, and an air release valve. Downward pressure may be applied to maintain the position of the pressure box through the weight of the driving rig for the mandrel or by any other convenient means. The driving of the mandrel can be effected through a driving hammer which can be diesel- or hydraulically-driven or can be gravity-acting.

According to a further aspect of the invention, there is provided a method of forming a pile in the ground, which comprises:

(a) driving into the ground a tubular mandrel carrying at its lower end a pile shoe the dimensions of which measured in a plane perpendicular to the direction of driving exceed the corresponding dimensions of the mandrel, the mandrel having at its upper end means for introducing a fluid cementitious mixture into the interior of the mandrel and having at its lower end one or more holes whereby fluid cementitious mixture can flow outwardly into the space surrounding the lower part of the tubular mandrel; (b) pumping a cementitious mixture through the 110 interior of the tubular mandrel from the top to the bottom thereof and thence into the hole around the mandrel as the mandrel is being driven into the ground so as to maintain the cementitious mixture at a positive pressure relative to its 115 surroundings; and (c) when the mandrel has reached the desired depth, leaving the pile shoe in place, withdrawing tile mandrel and allowing the cementitious mixture to set.

In this embodiment, one or more holes are provided at the base of the mandrel just above the pile shoe. A cementitious mixture under pressure is supplied to the interior of the mandrel and passes downwardly under pressure to the hole(s) 125 at the base of the mandrel and thence into the hole around the mandrel. The supply of cementitious mixture in this way ensures that the hole around the mandrel is filled with fluid cementitious mixture as the hole is being 130 generated. The supply of concrete under pressure to the base of the mandrel also means that it enters the pile section just above the shoe position, and accordingly the frictional forces which in conventional piles oppose the downward flow of concrete into the pile do not occur. Advantageously, an open ended concrete reservoir which may be constructed, for example, of steel, is provided at the ground surface and is arranged so that the mandrel passes therethrough as the pile is being formed. If desired, the concrete reservoir can be closed at its upper surface to function as a pressure box much in the way described above in connection with the first embodiment of the invention. The mandrel can be driven by a hammer which is diesel- or hydraulically-driven or which is gravity-acting.

In both of the embodiments described above, the mandrel can be of square, circular or any other desired cross-section. The pile shoe (also known as a driving shoe) may be flat or pointed at its driving end, and may be formed of steel or concrete. The driving shoe may be circular, square or any other shape as viewed in a plane perpendicular to the direction of driving.

Furthermore, an extension sleeve may be attached to the upper end of the driving shoe to prevent displacement of soil around the driving shoe into the intended pile section.

In the embodiments described above, it is possible to ensure that adequate pressures can be maintained in the cementitious mix (e.g. concrete) at all times during construction of the pile, thereby to preserve the shape of the pile and to give intimate bonding of.the pile to the surrounding ground material.

In all embodiments of the invention, the driving of the mandrel into the ground and the supply of cementitious material (e.g. concrete) to form the pile are concurrent. Also, the need for a temporary casing or tube is obviated so that the high extraction forces normally involved with driven cast-in-place piles are avoided. This means in turn that the pile driving machine or rig can be of simpler construction since its mast need not withstand the normally large forces involved in extracting the casing.

The invention will now be described with reference to the accompanying drawings, in which:

FIGURE 1 illustrates the frictional forces acting during the construction of certain prior types of pile; FIGURE 2 illustrates a first embodiment of this invention; and FIGURE 3 illustrates a second embodiment of this invention.

Referring now to Figure 1, a prior art arrangement involves a tubular mandrel 1 equipped at its lower end with with a pile shoe 2. A fluid concrete mix is supplied under gravity to a hopper 3 located at the ground surface. As the mandrel 1 is driven downwardly (by means not shown), the fluid concrete mix 4 also travels downwardly in the direction of arrow 5. This

3 GB 2 113 748 A 3 results in frictional forces indicated by arrows 6; these forces oppose the downward flow of concrete, so that the longer and narrower the pile becomes then the less will be the vertical concrete pressure towards the base of the pile. Thus the concrete lateral pressure will also reduce in proportion, and the pile may neck.

Referring now to Figure 2, a concrete pile is formed by a method in accordance with this invention using a tubular mandrel 10 of unitary construction and formed of high-strength steel.

The mandrel carries at its lower end a steel driving shoe 11 which, like the mandrel 10, is circular in cross-section. The driving shoe 11 has a flat base 12. As can be seen from the drawing, the dimensions of the driving shoe exceed those of the mandrel 10, so that an annular space is formed around the mandrel as the assembly is driven downwardly into the ground. A pressure box 13 is located at the ground surface. This includes an inlet 14 for a fluid concrete mixture under pressure, and an outlet 15 for the concrete. The outlet 15 is surrounded by a tubular extension 16 which extends downwardly from the pressure box 13 for a distance of several feet. The pressure box is provided with side lugs 17 which receive a downwards applied pressure. This is usually provided by reactions from the frame of the driving rig for the mandrel. An air release valve 18 is provided at the top of the pressure box, together with a pressure maintaining cell 19 which can be air-filled. A pressure gland 20 surrounds an aperture in the top surface of the pressure box through which, in use, the mandrel 10 is driven.

The driving rig for the mandrel (not shown in the drawing) includes a driving hammer 20 which is hydraulically driven.

In use, the mandrel 10 and driving shoe 11 are forced into the ground by driving hammer 20.

Simultaneously, fluid concrete under pressure is 105 supplied to the pressure box 13 and thence passes down tubular extension 16 into the annular gap which is formed between the earth wall 21 and the mandrel 10. The concrete is maintained at a pressure sufficient to ensure that the fluid column 110 around the mandrel is at a positive pressure relative to the surrounding earth. When the mandrel has reached the desired depth for the pile, it is withdrawn leaving the pile shoe behind.

The driving rig and pressure box are then removed, 115 and the fluid concrete column is allowed to set. After the first pile has been completed and the concrete has hardened to a reasonable degree, the continuity and general cross-sectional area of the pile can be tested; in this way the concrete pressure used in the construction of subsequent piles can be adjusted to give the desired structure of full cross-sectional area.

Referring now to Figure 3, a concrete pile is formed by a method in accordance with the present invention using a tubular mandrel 30 of circular cross-section and formed of steel. A driving shoe 11 similar to that described above with reference to Figure 2 is fitted to the base of mandrel 30. The upper end of mandrel 30 is closed by a pressure-resisting plate 3 1. Just below this plate there is an aperture 32 in the side wall of the mandrel which aperture carries a shock-absorbing pipe connection 33 which receives a pipe for the supply of a fluid concrete mix under pressure. One or more holes 34 are provided in the lower part of mandrel 30 adjacent to driving shoe 11. A reservoir 35 the walls of which are formed of steel is positioned at ground surface level. A tubular extension 36 extends downwardly from the concrete reservoir for a few feet into the hole which is formed as the mandrel 30 is driven into the ground. The driving rig for the mandrel (not shown) carries a hydraulic mechanism for driving hammer 20.

In use, concrete is supplied under pressure through pipe connection 33 and passes downwardly through mandrel 30 to aperture 34. The supply of concrete occurs simultaneously with the driving of the mandrel 30 downwards by action of driving hammer 20. The concrete is at a pressure sufficient to maintain the column of fluid concrete at a positive pressure relative to the earth wall 2 1. The reservoir 35 is of a height sufficient to support the head of concrete which is generated owing to the pressurised concrete supply. Because in this embodiment the concrete is supplied under pressure to the mandrel and enters the pile section just above the driving shoe 11, the frictional forces shown at 6 in Figure 1 which normally oppose the downward flow of concrete into a pile do not apply. In fact, the direction of motion of concrete within the pile can be the reverse of that encountered with conventional gravity feed systems: in these circumstances, the wall friction is also reversed and the pressure of concrete at the base of the pile is enhanced during the driving of the pile. Also, the "hang-up" effect is of no consequence because concrete is always supplied at the right place to fill up the void created by downward motion of the pile shoe. If very high concrete supply pressures are used, the reservoir 35 may be modified so as to resemble the pressure box 13 of Figure 3. Once the mandrel 30 has reached the desired pile depth, it is withdrawn leaving the pile shoe in place. The driving rig is then removed and the concrete mixture is left to set.

Construction of piles in accordance with the present invention is especially advantageous in soft clays, water-bearing soils and other poor ground. In these conditions, pore water pressures can be high and soil shear strength low during the driving of piles. In gravity-fed systems which do not use a temporary casing, these conditions are particularly favourable for soil to re-enter the pile section and to displace any fluid concrete which is still present. By operating in accordance with the invention, this ingress of soil is prevented and thus stronger and deeper piles can be constructed.

Claims

1. A method of forming a pile in the ground, which comprises:

(a) driving into the ground a mandrel carrying at 4 GB 2 113 748 A 4 its lower end a pile shoe the dimensions of which 55 measured in a plane perpendicular to the direction of driving exceed the corresponding dimensions of the mandrel; (b) pumping a cementitious mixture into the hole around the mandrel during the driving of the 60 mandrel into the ground so as to maintain the cementitious mixture at a positive pressure relative to its surroundings; and (c) when the mandrel has reached the desired depth, leaving the pile shoe in place, withdrawing 65 the mandrel and allowing the cementitious mixture to set.

2. A method of forming a pile in the ground, which comprises:

(a) placing onto the surface of the ground at the 70 location where the pile is to be formed a pressure box which is adapted (1) to have passed therethrough a mandrel and (2) to supply concrete under pressure into a hole around the said mandrel; (b) driving into the ground, via said pressure box, a mandrel carrying at its lower end a pile shoe the dimensions of which measured in a plane perpendicular to the direction of driving exceed the corresponding dimensions of the mandrel; (c) pumping a cementitious mixture under pressure through the pressure box and into the hole around the mandrel during the driving of the mandrel into the ground so as to maintain the cementitious mixture at a positive pressure relative to its surroundings; and (d) when the mandrel has reached the desired depth, leaving the pile shoe in place, withdrawing 8.5 the mandrel and allowing the cementitious mixture to set.

3. A method of forming a pile in the ground, which comprises:

(a) driving into the ground a tubular mandrel 90 carrying at its lower end a pile shoe the dimensions of which measured in a plane perpendicular to the direction of driving exceed the corresponding dimensions of the mandrel, the mandrel having at its upper end means for introducing a fluid cementitious mixture into the interior of the mandrel and having at its lower end one or more holes whereby fluid cementitious mixture can flow outwardly into the space surrounding the lower part of the tubular mandrel; 100 (b) pumping a cementitious mixture through the interior of the tubular mandrel from the top to the bottom thereof and thence into the hole around the mandrel as the mandrel is being driven into the ground so as to maintain the cementitious 105 mixture at a positive pressure relative to its surroundings; and (c) when the mandrel has reached the desired depth, leaving the pile shoe in place, withdrawing the mandrel and allowing the cementitious mixture to set.

4. A method of forming a pile in the ground, which comprises:

(a) driving into the ground a mandrel carrying at its lower end a pile shoe the dimensions of which measured in a plane perpendicular to the direction of driving exceed the corresponding dimensions of the mandrel; (b) pumping a cementitious mixture into the hole around the mandrel during the driving of the mandrel into the ground so as to maintain the cementitious mixture at a positive pressure relative to its surroundings; and (c) when the mandrel has reached the desired depth, leaving the pile shoe in place, withdrawing the mandrel while causing the cementitious mixture to permeate the whole volume of the pile, and allowing the cementitious mixture to set.

Printed for Her Majesty's Stationery Office by the Courier Press, Leamington Spa, 1983. Published by the Patent Office 25 Southampton Buildings, London, WC2A lAY, from which copies may be obtained.

A

4. A method according to claim 1, 2 or 3, wherein after the first pile of a series has been completed and the cementitious mixture thereof has hardened, the continuity and cross-sectional area of the pile are tested and the supply pressure of the cementitious mixture used in the construction of subsequent piles is adjusted, if necessary, in order to give the desired pile structure.

5. A method of forming a pile in the ground, substantially as hereinbefore described with reference to Figure 2 of the accompanying drawings.

6. A method of forming a pile in the ground, substantially as hereinbefore described with reference to Figure 3 of the accompanying drawings.

New claims or amendments filed on 14.1.83.

Superseded claims 3,4, 6.

New or amended claims:- 3A.

3. A method according to claim 1 or 2, wherein after the first pile of a series has been completed and the cementitious mixture thereof has hardened, the continuity and cross-sectional area of the pile are tested and the supply pressure of the cementitious mixture used in the construction of subsequent piles is adjusted, if necessary, in order to give the desired pile structure.