GB2435286A

GB2435286A - Investigating ground strength by measuring the rate of penetration of a pile

Info

Publication number: GB2435286A
Application number: GB0704338A
Authority: GB
Inventors: Simon CROOK
Original assignee: Shire Structures Ltd
Current assignee: Shire Structures Ltd
Priority date: 2004-05-11
Filing date: 2007-03-07
Publication date: 2007-08-22
Also published as: GB2414032A; GB0704338D0; GB2414032B; GB0509822D0

Abstract

A method of investigating ground strength for use in piling comprises providing a probe or a driving point 101 at a leading end of a first rod 102 of a pile, pushing the pile into the ground with the probe at the leading end, measuring the rate of penetration and using the rate of penetration to indicate the condition of the ground and thus, predict the rate of penetration and capacity of a given pile. The probe may be used as part of a piling process or as a stand-alone site investigation tool with a length and diameter to match that of a proposed pile.

Description

1 2435286

IMPROVEMENTS IN OR RELATING TO PILES

This invention concerns improvements in or relating to piles or similar load carrying structures such as ground anchors, soil nails or the like.

This invention has particular, but not exclusive application to piles for buildings and the like where the piles may be used for new build to provide a foundation on which a building can be constructed or for remedial work on an existing building to strengthen the original foundation and/or to reduce or restrict movement of the building. For convenience, the term "piles" is used hereinafter and includes all similar arrangements of load carrying structures.

Traditional piling equipment is large, heavy and expensive to establish on site. This makes it difficult/uneconomic to use on some applications such as inside buildings or where there are only a small number of piles.

A number of small diameter piling systems, commonly referred to as micro-piles, are available which can be installed using smaller equipment and can be used for applications where traditional piling equipment is

unsuitable.

These known systems still have some/a few of the following disadvantages: vibration to adjacent structures, size/cost of installation equipment, cost of pile materials, and limited load capacity.

The present invention seeks to overcome or at least to mitigate one or more of the disadvantages of known piling systems.

Thus, according to a first aspect of the present invention, there is provided a piling system wherein the interaction (friction and/or bearing) between the pile and the ground can be selectively adjusted along the length of the pile.

By adjusting the interaction between the pile and the ground along the length of the pile, the load capacity of the pile can be adapted according to requirements for a particular application. Alternatively or additionally, installation of the pile may be facilitated.

Preferably, the pile comprises a pile shaft made up of sections joined end to end to produce the required pile length. Each section may comprise a rod of small diameter, typically up to 100mm diameter. Each rod may be of solid or hollow section and may be of circular, oval or other suitable cross-section. Small diameter rods give minimal soil displacement and facilitate installation with smaller equipment, for example using impact equipment such as a post driver or a road breaker, to drive the pile into the ground.

Preferably, the rods are connected together using couplers. The couplers may be tubular sleeves and the ends of the rods to be connected are inserted into opposite ends of the sleeve.

In one arrangement, the coupling sleeves are provided with internal (female) screw threads to receive external (male) screw threads on the ends of the rods. Screwing the ends of the rods into sleeves provides a strong, robust connection better able to withstand any bending loads that may be applied to the pile in use. The screw threads on the sleeves and/or rods may be of uniform diameter. Alternatively, the screw threads on one or both may be tapered.

In another arrangement, the ends of the rods are an interference fit in the coupling sleeves and are retained by friction. One or both of the sleeves and rods may be provided with formations such as ribs to enhance the grip and/or assist insertion of the ends of the rods. For example, the sleeves may have axial ribs that bite into and grip the ends of the rods.

Alternatively or additionally, one or both of the sleeves and ends of the rods may be tapered. Connecting the rods with friction couplers does not require the formation of screw threads that are easily damaged.

The first rod preferably has a coupling sleeve at the upper end for securing one or more additional rods to extend the length of the pile shaft. The first rod may have a diameter equal to or smaller than the diameter of the coupling sleeve and the or each additional rod may have a diameter smaller than, equal to or larger than the diameter of the coupling sleeve. In this way, interaction between the pile and the surrounding soil can be adjusted along the length of the pile by varying the contact between the rods and the soil.

The interaction between the pile and surrounding soil may be adjusted by providing the rods with a smooth outer surface or a rough (textured) outer surface to control the adherence to the soil and/or with projections such as ribs or fins to increase the surface area in contact with the soil. In this way, the contact between the rods and the ground may be varied along the length of the pile such that the friction and/or bearing between the pile and the ground can be increased/decreased at selected positions along the length of the pile as desired. The projections may be of any size and/or shape to provide the pile with desired properties for a given application.

In one arrangement, the grip can be adjusted along the length of the pile by providing at least one adaptor member to vary the contact between the pile and the soil. Thus, the adaptor member can have a smooth outer surface to provide a slip' surface between the pile and the soil in applications where part of the pile length is to be kept separate from the soil. Alternatively, the adaptor member can have a rough (textured) outer surface and/or be provided with projections such as ribs or fins to provide a non-slip' surface between the pile and the soil to enhance grip between the pile and the soil.

The adaptor member may fit over all or part of the length of the pile shaft. For example, the adaptor member may comprise a separate tubular sleeve. Alternatively or additionally, the adaptor member may be a part of the pile shaft. For example, the adaptor member may comprise a modified coupler for connecting two rods.

In one arrangement, the adaptor member is a tubular sleeve that is axially movable relative to the pile shaft. For example, the sleeve may be slidable in an axial direction relative to the pile shaft. Alternatively, the sleeve may be deformable in an axial direction relative to the pile shaft.

In this way, the ground can move vertically relative to the pile shaft without placing additional compressive loads on the pile.

We may use any combination of rods and couplers with or without adaptor members to achieve the required load capacity for a given application and the invention includes all such combinations as well as a kit of parts for assembly of a pile.

According to a second aspect of the present invention, there is provided a method of providing a pile with a desired load capacity by adjusting the friction and/or bearing between the pile and the substrate into which it is driven by means of the contact between the pile and the substrate.

When a pile is driven into the ground, the load capacity is traditionally established by either tension or compression testing of the pile. Tension testing can be inaccurate in some ground conditions and compression testing is slow and relatively expensive.

According to a third aspect of the present invention, there is provided a method of establishing the load capacity of a pile based on the speed of installation (rate of penetration).

Thus, when a pile is being installed the rate of penetration of the pile is dependent on the friction at the sides of the pile together with the displacement at the pile end. The final load capacity of the pile is also dependent on the side friction and the force required to displace the soil at the end of the pile. There is therefore a relationship between speed of installation and final load capacity of the pile and the invented method uses this relationship to establish the capacity of the pile without the need for further on site testing.

We may determine the rate of penetration by recording the time taken to drive the pile a given distance into the ground, for example 100mm.

Prior to installation of a pile, it is good practice to establish the strength of the ground it is proposed to pile through. This allows the suitability of the piling system to be established and the expected length of the pile and its capacity to be estimated. The investigation of the ground is usually undertaken by processes such as auguring holes, installing boreholes, together with testing such as SPT's or the Mackintosh Probe.

According to a fourth aspect of the present invention, there is provided a method of investigating the strength of the ground by pushing into the ground a pile provided with a probe (driving point) at the leading end and measuring the rate of penetration.

The rate of penetration of the probe can be used to indicate the condition (strength) of the ground and thus, predict the rate of penetration and capacity of a given pile.

The length of the probe in contact with the soil and the diameter can be varied to match the amount of soil displacement and side friction of the probe with the displacement and side friction of the proposed pile. The probe may be used as part of the piling process or as a stand alone site investigation tool.

During installation of a pile, the friction between the pile and the soil increases as the embedded length of the pile increases. When the friction equals the force produced by the installation equipment, the pile cannot be driven any further into the ground and this limits the depth and load capacity of the pile.

With helical piles, especially small diameter (typically less than 100mm) helical piles, the load capacity primarily results from the interaction with the soil trapped within the turns of the helical section. Increasing the size of the helical section longitudinally and/or radially increases load capacity but can require a larger installation force and/or reduce the depth that can be achieved.

According to a fifth aspect of the present invention, there is provided a pile wherein at least a lower section of the pile has a helical projection adapted to reduce friction between the projection and the soil during installation.

During installation the pile is rotated to drive the helical lower section into the ground and, when installed, the pile is secured to prevent rotation. Reducing the friction facilitates rotation of the helical section, allowing the pile to be driven into the ground more easily and, when installed, has little or no effect on axial movement of the helical section.

In this way, the interaction between the soil and the helical section and thus load capacity of the installed pile is substantially unaffected by the reduced friction of the helical section. Consequently, the depth that can be achieved and thus the load capacity of the pile may be increased.

The reduced friction may be achieved by providing the helical section with a "slip" coating. For example we may apply a coating of low friction material to the helical section such as a paint finish or any other

suitable finish.

It may be desirable to minimise the friction on the sides of the upper section of the pile during installation, especially where the upper section of the pile shaft has a smooth outer surface. Thus we may provide the upper section with a "slip" coating similar to the helical section.

Alternatively or additionally, when connecting successive pile sections we may employ a coupler having a larger outer diameter than the outer diameter of the pile section above the coupler to displace the soil away from the pile section above the coupler.

According to a sixth aspect of the present invention, there is provided a pile including a lower section connected to an upper section by a coupler having a larger outer diameter than the outer diameter of the upper section.

The coupler effectively displaces the soil away from the section above the coupler and reduces the grip (friction) of the soil on this section. The coupler can be connected to the sections using either male/female threads or using a friction grip as described previously.

The coupler can be manufactured and supplied separately from the sections. Alternatively, the coupler can be manufactured separately and supplied fitted to one of the sections by screw threads, friction grip, welding, or any another suitable means.

The lower section may comprise the first section of the pile and may have a larger outer diameter than the outer diameter of the coupler. for example, the lower section may be provided with one or more helical or axial projections of larger diameter than the coupler to enhance the load capacity of the pile. The second section may have a smooth outer surface and the length of the pile may be increased to attaching one or more additional sections having a smooth outer surface. The or each additional section may be attached with a coupler having a larger outer diameter than the section above the coupler.

The interaction with the soil trapped in the turns of the helical section may be improved and thus the load capacity increased by controlling the size and/or shape of the helical projection.

According to a seventh aspect of the present invention, there is provided a pile with at least one helical projection of rectangular cross-section. We have found that load capacity is improved with a helical projection of rectangular section.

Where a helical section is employed at the leading end of the pile to improve load capacity of the pile, the soil displacement at the leading end of the pile is increased.

According to an eighth aspect of the present invention, there is provided a pile comprising a first section having a central rod provided with at least one helical projection and a second section connected to the first section by a coupler where the central rod of the first Section has a diameter smaller than the diameter of the coupler.

Reducing the diameter of the central rod may improve the rate of penetration and it may be possible to reduce the diameter without sacrificing the strength of the first section, The central rod may be of solid or tubular section.

When a helical pile supports compressive or tensile loads, the helical section may tend to cause a twist in the shaft of the pile.

According to a ninth aspect of the present invention, there is provided a piling system comprising a first helical section and a second helical section, the first section having a pitch opposite to the pitch of the second section.

By providing two helical sections, the load capacity of the pile is increased. Moreover, by arranging the sections with opposite pitches, the twist generated by each section is countered by the other section. As a result, the twist generated by the sections may be significantly reduced without adverse effect on the load capacity.

The first helical section may be provided at the lower end of a first pile shaft with the second helical section provided at the lower end of a second pile shaft. The pile shafts may be installed one within the other so that one helical section is above the other and the pile shafts secured together, for example by grouting the void between the pile shafts.

According to a tenth aspect of the present invention, there is provided a method of installing a pile to reduce twist generated by a helical section of the pile comprising providing a first pile shaft having a helical section at the lower end, providing a second pile shaft having a helical section at the lower end, installing the first and second pile shafts so that the helical section of the second pile shaft is above the helical section of the first pile shaft, and securing the first and second pile shafts together.

The pitch of the helical sections of the first and second pile shafts may be in the same direction but more preferably is in the opposite direction.

According to an eleventh aspect of the present invention, there is provided a piling system comprising an inner pile shaft and an outer pile shaft secured together.

Each pile shaft may have a helical section at the lower end with the helical section of the outer pile shaft being above the helical section of the inner pile shaft. The pitch of the helical sections of the inner and outer pile shafts may be in the same direction but more preferably is in the opposite direction.

According to a twelfth aspect of the present invention, there is provided a piling system in which the pile is installed through a hollow tubular section driven into the ground.

By driving a hollow tubular section into the ground, the pile is not exposed to friction by contact with the soil until it emerges from the lower end of the tubular section. As a result, the depth to which the pile can be driven is increased without requiring larger, more powerful equipment.

The hollow tubular section may comprise a single tube having a removable liner that can be withdrawn when the tube is in place to remove the soil that has been displaced into the bore of the tube during driving so as to leave the tube empty for installing the pile. On completion the gap between the hollow tube and pile can be filled to produce a single structural unit.

Alternatively, the hollow tubular section may comprise a series of two or more tubes of progressively smaller diameter such that the second tube can be driven into the ground through the bore of the first tube and the process repeated as required to install further tubes until a tubular section of the desired length has been obtained. After each tube is installed, the associated liner is withdrawn to remove the soil displaced into the tube before the next tube is installed. The pile can then be installed through the hollow tubular section and, on completion, the gap between the hollow tubular section and pile can be filled to produce a single structural unit.

According to a thirteenth aspect of the present invention, there is provided a method of installing a pile in a substrate by providing a hollow tubular section in the substrate and inserting the pile through the hollow tubular section.

According to a fourteenth aspect of the present invention, there is provided a method of estimating pile load capacity by measuring the force required to twist (rotate) a pile about a longitudinal axis of the pile.

The force required to twist the pile provides an indication of the interaction between the pile and the soil which, together with the pile length and diameter enables the force and hence the load capacity to be calculated.

This force can be readily determined using a torque wrench attached to the upper end or head of the pile. Torque wrenches are relatively inexpensive, are easily attached to the pile and are simple to use even when access is restricted and the available space is limited.

According to a fifteenth aspect of the present invention, there is provided a piling system comprising at least two sections connected together by a coupling sleeve having female threaded portions to receive male threaded portions on the sections.

According to a sixteenth aspect of the present invention, there is provided in or for a piling system a pile rod having an outer sleeve.

The outer sleeve may be fixed or axially movable relative to the pile rod.

According to a seventeenth aspect of the present invention, there is provided a piling system comprising a pile having adjustable coupling means adapted to raise/lower a structure supported by the pile.

According to an eighteenth aspect of the present invention, there is provided a piling system wherein the pile comprises a lead section and at least one extension section, wherein adjacent sections are joined together at the ends by a coupler.

The coupler may provide a screw fit or interference (friction) fit with the ends of the sections to be connected.

According to a nineteenth aspect of the present invention, there is provided a piling system comprising a pile having a plurality of sections joined together end to end to form a required pile length wherein each section is arranged and/or adapted to provide a desired interaction with the ground such that the adherence between the pile and the ground can be selectively adjusted along the length of the pile.

According to a twentieth aspect of the present invention, there is provided a piling system comprising any feature or combination of features of the preceding aspects of the invention as described herein.

The invention includes piles, piling systems and methods of installing and testing piles according to any of the above aspects of the invention and any combination thereof. Moreover, at least some of the methods of installation and testing may also have application to conventional piles and/or other load carrying structures such as ground anchors, soil nails or the like.

The invention in each of its aspects will now be described in more detail by way of example only with reference to the accompanying drawings in which like reference numerals indicate corresponding parts and wherein:-Figure 1 shows a piling system according to a first embodiment of the invention; Figure 2 shows components of the piling system shown in Figure 1; Figure 3 shows an alternative rod for the piling system of Figure 1; Figure 4 shows a strengthening rod for the upper section of the piling system of Figure 1; Figure 5 shows a strengthening tube for the upper section of the piling system of Figure 1; Figures 6a and 6b show two adaptor members for the piling system of Figure 1; Figures 7a and 7b show an alternative adaptor member for the pile of Figure 1 in side and plan view; Figures 8a and 8b show yet another adaptor member for the pile of Figure 1 in side the plan view; Figure 9 shows a modified pile; Figure 10 shows a smooth adaptor sleeve for the pile of Figure 9; Figure 11 shows a ribbed adaptor sleeve for the pile of Figure 9; Figures 12a to 12d show a piling system according to a second embodiment of the invention to produce either deeper piles, high load piles or piles with stronger top sections; Figures 13a to 13f show the sequence of installation of a piling system according to the second embodiment; Figure 14 shows a method of testing a piling system with a torque gauge according to a third embodiment of the invention; Figure 15 shows a driving point located in the leading end of the first rod of a pile: Figure 16 shows a pile rod provided with two integral, helical fins along the length of the rod; Figure 17 is a section on the line 17-17 of Figure 16; Figure 18 shows a pile rod provided with four integral, axial fins along the length of the rod; Figure 19 is a section on the line 19-19 of Figure 18; Figure 20 shows a coupler for connecting two pile rods; Figure 21 shows an alternative coupler for connecting two pile rods; Figure 22 is a section on the line 22-22 of Figure 21; Figure 23 is a section on the line 23-23 of Figure 21; Figure 24 is a section on the line 24-24 of Figure 21; Figure 25 shows an enlarged detail of part of Figure 24; Figures 26 to 28 show the method of joining two pile rods using the coupler of Figures 21 to 25; Figure 29 shows a cross section through a typical pile that has been in-filled and reinforced; Figure 30 shows a length of pile that has been fitted with an external tube to reduce the friction between the pile and the ground; Figure 31 shows a length of pile that has been fitted with an external sleeve to reduce the friction between the pile and the ground; Figures 32 and 33 show a method of increasing the load capacity of a pile using two helical sections with pitches running in opposite directions; and Figures 34 and 35 show a method of making piles easier to install using a low friction coating.

Referring first to Figures 1 to 11 of the accompanying drawings, a first embodiment of a piling system is shown in which the pile has a pile shaft comprising a plurality of steel rods 1,2,3,4 joined together end to end with coupling sleeves 5. In this embodiment four rods 1,2,3,4 each approximately 1 metre long are shown but it will be understood that the number and/or length of the rods can be varied according to the required depth of the pile.

The coupling sleeves 5 are provided with a female thread (not shown) at each end and the rods 1,2.3,4 are provided with a mating male thread la,2a,3a,4a at each end. The pile is installed using impact equipment such as a post driver or a road breaker (not shown). A driving show is provided over the first length of pile and it is then driven into the ground.

The second length of pile is then connected to the first length with the coupler. This is then driven and the process repeated until the pile is at the required depth.

The steel rods 1,2,3,4 have small diameters (typically 12mm to 60mm) to minimise the soil displaced during driving. In this embodiment, the lower section of the pile comprises rod 1 that is of smaller diameter than coupling sleeves 5, and the upper section comprises rods 2.3,4 that are of the same diameter as the coupling sleeves 5 to ensure friction between the upper section of the pile and the soil. The upper rods 2,3,4 could be of larger diameter than the coupling sleeves 5.

By employing rods of different diameter, the surface area in contact with the soil varies along the length of the pile. As a result, the pile/soil interaction can be altered along the length of the pile and thus the load capacity of the pile adapted to suit the ground by appropriate combination of rods of different diameter.

The rods 1,2,3,4 may be plain with a smooth surface. Alternatively, one or more of the bars may be provided with formations to increase the surface area in contact with the soil to vary the friction and/or bearing contact and thus the load capacity of the pile to suit the ground conditions. For example, the bars could have a helical thread 6 (Figure 3) or ribs. The bars are of solid section but tubular bars could be employed.

There are some circumstances (such as piles subject to lateral load) where there is a need to strengthen the top section of a pile. This can be achieved by using a larger diameter top section in place of the rod 4.

This could be a larger diameter rod 7 (Figure 4) that screws into the top coupling sleeve or a tube 8 that screws onto the outside of the top coupling sleeve (Figure 5).

Where increased soil support is required (to give increased load capacity or due to poor quality ground) adaptor members 9,10 (Figures ôa and 6b) can be fitted to the rods at appropriate points along the length of the pile.

Alternatively or additionally, the adaptor members 9,10 may comprise coupling sleeves for connecting the rods together.

The adaptor member 9 has bearing plates 9a and the adaptor member 10 has shear plates lOa that provide increased surface area in contact with the soil and thus increase the interaction between the pile and the soil and hence the load capacity of the pile. Thus number, size and shape of the plates 9a, lOa may be chosen to vary the friction and/or bearing contact along the length of the pile to provide the desired characteristics for a particular application.

The adaptor members 9,10 may be shorter than the rods 1,2,3,4 such that one or more of the adaptor members 9,10 may be fitted to a rod.

Alternatively, the adaptor members 9,10 may be continuous along the length of a rod. Figures IOa and lOb show an adaptor member 9' provided with a helical bearing plate 9a'. Figures ha and lib show an adaptor member 10' provided with four axially extending shear plates IOa'. Other constructions and arrangements of bearing plates 9a' and shear plates lOa'may be employed.

Referring now to Figure 9, there is shown a modification to the pile above-described in which the rods 2,3,4 are replaced by rods 1' of smaller diameter than the coupling sleeves 5 (typically the same diameter as the rod 1) and an adaptor sleeve 11,12,13 having a diameter equal to the coupling sleeves 5 is fitted over each of the rods 1'. The adaptor sleeves 11,12,13 can be loosely slid over the inner steel rods 1' and held in place by the coupling sleeves 5 or by bonding to the rods 1'.

The adaptor sleeves 11,12,13 are made of plastics or any other suitable material and can be plain with a smooth surface (Figure 10) to reduce grip and restrict bond of the ground onto the pile or provided with formations such as ribs 14 (Figure 9) to increase grip and enhance bond of the ground onto the pile. In this way, the friction and/or bearing contact the length of the pile may be varied by selection and fitment of the appropriate adaptor sleeve.

It will be understood that the above-described piling system may employ any combination of rods of different diameters with or without adaptor sleeves and/or adaptor members to modify the friction and/or bearing contact along the length of the pile to vary the bond between the soil and the pile according to the requirements for a given application.

As will be understood, then the pile is driven deeper the amount of friction between the pile and the ground increases. At some stage this friction force becomes equal to the driving forces being applied. At this point it is not possible to drive the pile deeper. There are some circumstances where it could be necessary to drive deeper than this to get higher load capacity or to reach stable ground.

A second embodiment of a piling system for enabling the pile to be driven deeper is shown in Figures 12a to 12d employing a steel tube 20 that is driven into the ground and the pile is then driven down inside the tube 20.

The tube 20 is hollow to minimise soil displacement by allowing the soil to flow' both around and into the tube as indicated by the arrows 21 (Figure 12a) The tube 20 is fitted with a removable inner liner 22 (Figures 12a,12b).

After driving, the liner 22 is extracted together with the soil to leave the tube 20 empty in the ground (Figure 12c).

A pile such as described above can then be driven through this empty tube 20. On completion the space 23 between the tube 20 and the pile is infilled to produce a single structural unit.

The depth to which the pile can be driven can be increased by the method of Figure 12 by employing a series of hollow tubes of progressively smaller diameter fitted with removable inners liners.

This is shown schematically in Figures 13a to 131 for two tubes 30,31 and liners 32,33 but it will be understood the process could be repeated using more tubes.

As shown, the larger tube 30 with inner liner 32 is first driven to a set depth or until it cannot be driven further (Figures 13a,13b). The soil inside the tube 30 is then extracted using the inner liner 32 (Figure 13c).

The smaller tube 31 and liner 33 is then driven down inside the larger, upper tube 30 to a set depth or until it cannot be driven further (Figure 13d). The upper tube 30 prevents friction on the lower tube 31 until it passes below the base of the upper tube 30. The soil inside the lower tube 31 is then extracted using the liner 33 (Figure 13e).

The pile 34 is then driven through the tubes 30,31 and the void 35 between the tubes 30,31 and the pile 34 is then filled to form a single structural unit. The final load capacity of the pile driven in this way is the sum of the friction of all the installed sections.

As will be appreciated from the foregoing description, the load capacity of a pile is generally achieved by friction between the pile and the soil.

Figure 14 shows how the load capacity of a pile 40 can be calculated by measuring this friction using torque wrench 41 attached to the head of the pile 40.

The torque wrench 41 is rotated and the maximum torque applied before the pile 40 starts to rotate is measured. This torque with the pile length and diameter enables the friction force and hence the load capacity to be calculated.

Referring now to Figure 15, there is shown a piling system employing a driving point 101 mounted in the leading end of the first rod 102 of a pile. In this embodiment, the rod 102 is a hollow steel tube and the driving point 101 is a solid body of steel or other suitable metal or alloy.

The point 101 may be a casting or machined to the desired shape. The point 101 has a cylindrical body 103 with an external diameter equal to the external diameter of the rod 102. The driving point 101 tapers to a sharp tip 104 at the front end of the body 103 and is provided with a spigot 105 of reduced diameter at the rear end of the body 103 for reception in the hollow end of the rod 102. The spigot 105 is an interference push fit in the end of the rod 1.02 until the rod 102 seats against a shoulder 106. Alternatively, the spigot 105 could be screwed into the end of the rod 102. The driving point 101 prevents soil entering the hollow interior of the rod 102 and reduces the force required to drive the rod 102 into the soil. Several rods can be connected end to end with suitable couplers as described previously until the desired length of pile is obtained. Where the leading pile rod has a solid section, a driving point may be formed integrally at the end of the rod.

Referring now to Figures 16 and 17, there is shown a pile rod 120 in the form of a hollow steel tube provided with two longitudinally extending helical steel fins 121,122 secured to the rod 120, for example by welding.

The fins 121,122 extend substantially the whole length of the rod 120 between externally threaded end sections 123,124 for attaching a coupler (not shown) to connect successive rods forming the pile. In a modification (not shown), the fins only extend for part of the length of the rod. The fins 121,122 increase the surface area of the rod 120 in contact with the soil. The number and or size of the helical fins 121,122 may be varied to alter the surface area of the rod 120 to Suit the soil in which the pile is to be installed without changing the diameter of the hollow tube. The interaction between the pile and the soil may be altered along the length of the pile by providing rods having a different number and/or size of fins and/or some rods may be provided with plain or rough external surfaces and/or adaptor members/sleeves to control the friction between the pile and the soil along the length of the pile.

Referring now to Figures 18 and 19, there is shown a pile rod 130 in the form of a hollow steel tube provided with four longitudinally extending axial steel fins 131,132,133,134 secured to the rod, for example by welding. The fins 131,132,133,134 extend substantially the whole length of the rod 130 between externally threaded end sections 135,136 for attaching a coupler (not shown) to connect successive rods forming the pile. In a modification (not shown), the fins only extend for part of the length of the rod. The fins 131,132,133,134 increase the surface area of the rod 130 in contact with the soil. The number and or size of the axial fins 131,132,133,134 may be varied to alter the surface area of the rod to suit the soil in which the pile is to be installed without increasing the size of the hollow tube. The interaction between the pile and the soil may be altered along the length of the pile by providing rods having a different number and/or size of fins and/or some rods may be provided with plain or rough external surfaces and/or adaptor members/sleeves to control the friction between the pile and the soil along the length of the pile.

Referring now to Figure 20, there is shown a tubular steel coupler 140 for connecting adjacent ends of two successive pile rods such as the rods 120,130 shown in Figures 16 to 19. The coupler 140 has a female screw thread 141,142 at each end to receive a male screw thread on the end of the pile rod. The female screw threads 141,142 are tapered to reduce in diameter towards the centre of the coupler 140. The male threads on the pile rods may also be tapered to match the taper of the female threads.

Alternatively, one or both of the male and female threads may be parallel.

Referring now to Figures 21 to 28, there is shown an alternative tubular steel coupler 150 for connecting adjacent ends of two successive pile rods 151,152. The coupler 150 has a socket 153,154 at each end to receive the end of a pile rod. An internal flange 155 separates the sockets 153,154 and provides stop faces 156,157 to limit insertion of the pile rods 151,152 in the associated sockets 153,154. In a modification (not shown) the internal flange may be omitted and a spring washer may optionally be employed between the ends of the rods within the coupler.

The outer surface of the pile rods 151,152 is plain (smooth) at the ends and the inner surface of the sockets 153,154 is formed with axial ribs 158. The ribs 158 are of triangular section and provide teeth 159 that bite into the outer surface of the rods 151,152 and grip the ends of the rods 151,152 inserted in the sockets 153,154. In this way, the ends of the rods 151,152 are secured and retained without having to provide the coupler 150 and rods 151,152 with male and female screw threads. The sockets and/or the ends of the rods may be of uniform diameter or tapered. Tapering may limit insertion of the rods without requiring an internal flange or the like.

The engagement of the coupler 150 is increased by the axial force used to drive the pile into the ground and is not reduced by any rotation of the pile. This is a considerable benefit compared to the use of screw threads that are costly to produce, easily damaged and can break or become loose in use, especially if the pile is rotated. For example, if the top of a rod is damaged when driving into the ground, the damaged section can be cut off on site allowing the coupler 150 to be connected without having to re-thread the end of the rod which is an expensive, difficult and time consuming operation to carry out and may not be possible on-site.

Referring now to Figure 29, there is shown a cross section through a length of pile, 160. The central core of the pile has been infilled with a grout or concrete 161, and reinforced with one or more steel or other suitable materials, 162. It is not normal practice to reinforce small diameter (less than 100 mm) piles that are driven/vibrated into position where the walls of the steel pile section are relatively thick (i.e. they can carry a significant proportion of the axial and bending loads on the pile).

Reinforcing small diameter piles such as this can be used to improve their axial load capacity and their bending resistance. Central reinforcement also prevents the pile twisting at the joints between adjacent pile lengths.

Referring now to Figures 30 and 31, these show methods of reducing the friction between the soil and the pile shaft. Piles often have to be installed through soil that can expand or contract due to factors such as long term settlement in the ground or seasonal variations in the ground moisture Content. This movement of the soil around the shaft of the pile can cause either tensile or compressive loads to be transferred into the pile.

To reduce these loads Figure 30 shows a Length of pile 163 connected by couplers, 164. A length of rigid tube, 165, has been fitted around the central pile 163. The rigid tube 165, could be constructed from steel, plastic or another suitable material. The tube is cut shorter than the distance between adjacent couplers 164. If the ground around the pile starts to move vertically downwards the ground would grip the tube 165 which would then slide over pile 163. This allows the ground to move vertically downwards without putting additional compressive loads onto the pile.

Figure 31 shows a similar arrangement where the rigid tube 165 is replaced with a thin flexible sleeve 166 made out of a material such as plastic. As the ground moves vertically downwards around the plastic sleeve, the sleeve 166 crushes slightly and this prevents the vertical loads being transferred to the pile. The effectiveness of the tube/sleeves shown in Figures 30 and 31 can be further improved by coating the pile tubes 163 with a low friction coating.

Referring now to Figures 32 and 33, these show a method of improving the load capacity of a pile and of reducing the twist generated by a helical pile under vertical loads.

If a pile containing helical fins supports either compressive or tensile loads, the shape of the fins will tend to cause a twist in the shaft of the pile. Figure 32 shows a typical arrangement of pile, consisting of helical end 167, shaft extensions 168, and couplers 169 to join the parts together.

To reduce the twist generated by the pile or to increase its load capacity, a second section of helical pile with a larger diameter than the first section can be driven around the first section of piling. In Figure 33 a second length of piling 170 of larger diameter than the shaft tube of the original pile 168 has been driven around 168 and down to the lower original helical length pile 167. Two lengths of pile (inner and outer) can then be fixed together by either grouting the void between the piles or by another suitable means. If the helical twist of the upper length of pile is in the same direction as the twist of the lower length of pile 167, this will increase the load capacity of the pile and also increase the overall twist generated. If, as shown in Figure 33, the pitch of the upper length of pile 170 is in the opposite direction to the pitch of the lower length of the pile 167, this will increase the load capacity of the pile and will either reduce or totally remove the twist that is generated by the helical sections.

Referring now to Figures 34 and 35, there is shown a method of making helical piles easier to install. It is current practice to coat the top sections of piles to reduce the friction between the pile shaft and ground when they are installed in ground that can be moving up or down. This is shown in Figure 34. The pile shown consists of a lower helical section 171, shaft extension 173 and the upper length of piles have been coated with slip coating, 174.

It is not normal practice to coat the lower section of the pile 171 (this could be helical fin or straight section) as the friction between the soil and pile normally contributes to the load bearing capacity of the pile.

With the helical pile, it would be possible to coat the lower section of the pile as once locked in place and rotational movement is prevented the load carrying capacity of the pile is derived from the soil trapped between the fins rather than the friction between the fins and soil. This is shown in Figure 35. The pile shown consists of a helical base section 175 and shaft extension 177. The upper length of the pile 177 has been coated with a low friction coating 178 and the lower base section 175 of the pile 176 has also been coated with a low friction coating 179. The coating on the helical base section assists installation of the pile without significantly reducing the load capacity of the installed pile.

It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements that can be made to the invention will be apparent to those skilled in the art and are deemed within the scope of the invention.

For example, the various embodiments described above show how the pile can be adapted for different applications and conditions and the principles outlined can be combined in any appropriate manner to provide piles having any desired load capacity using any of the rods, coupling sleeves, adaptor sleeves and adaptor members described above.

The piles described herein employ rods of small diameter that can be driven into the ground with equipment of small size and weight that is relatively easy to handle and use in a confined area where access is limited or restricted and which does not require extensive preparation of the site in order to use the equipment. The invention is therefore particularly suitable for carrying out remedial work on the foundations of existing domestic buildings and to provide a solid foundation for extending such buildings, for example when adding a porch, conservatory or similar structure.

Where the invention is employed to carry out remedial work, the upper end of the pile may be adapted to apply a jacking force to a building supported on the pile. For example, the upper end of the pile may have a screw threaded portion provided with a nut that is axially adjustable to raise/lower a bearing plate connected to the building. Such jacking force may be effective to enable localised defects to be corrected, for example to close or reduce the size of the crack in the wall.

Claims

CLAIMS

1. A method of investigating ground strength for use in piling comprising providing a probe at a leading end of a pile, pushing the pile into the ground with the probe at the leading end, and measuring the rate of penetration.

2. A method according to claim 2 wherein the rate of penetration of the probe is used to indicate the condition of the ground and thus, predict the rate of penetration and capacity of a given pile.

3. A method according to claim 1 or claim 2 wherein the length of the probe in contact with the soil and the diameter are chosen to match the amount of soil displacement and side friction of the probe with the displacement and side friction of a proposed pile.

4. A method according to any preceding claim wherein the probe is used as part of a piling process or as a stand-alone site investigation tool.

5. A method according to any preceding claim wherein the probe comprises a driving point.

6. A method of investigating ground strength for use in piling substantially as hereinbefore described.