GB2371585A

GB2371585A - Earth anchor/pile

Info

Publication number: GB2371585A
Application number: GB0130434A
Authority: GB
Inventors: William Henry Ollis
Original assignee: Individual
Current assignee: Individual
Priority date: 2000-12-21
Filing date: 2001-12-20
Publication date: 2002-07-31
Anticipated expiration: 2021-12-20
Also published as: GB0130434D0; GB2371585B

Description

EARTH ANCHOR & METHOD

The present invention relates to an earth anchor and a method of installing an earth anchor. There have been many forms of helical fastenings, fixings and anchors and there have been many varied forms of helical piles. EP0150906 shows many forms of tubular helical anchors with configurations from simple wedge shaped three projection forms to complex six ridged arrangements. Although Eh0150906 does not limit on size, fastenings are limited to proportionately thick walled sections as required to prevent buckling when top driven.

Thin walled helical anchors have been previously used in piling situations. However to prevent buckling or tubular collapse when subjected to top driven impact loads it has previously been necessary for the anchors to be pre-filled and fitted with a centrally grouted high tensile reinforcement rod. Such pre-cast pile forms are then driven via this rod. These forms by nature of their construction have to be driven as a single long length, causing both handling and flexing problems.

GB2336869B reveals a number of pile forms with wedge shaped fins running helically along and around the whole length. These piles are driven by applying axial impulse forces to the uppermost portion of the pile or jointed length. This form of pile and its method of driving are unsatisfactory for two reasons.

A pile that is helical along its entire length has no recognised capacity to relieve heave (upward movement of the ground caused by changes in moisture content). Heave zones are subject to regular cycles of expansion and desiccation Helical piles, which maintain friction and mechanical interlock characteristics within heave zones will be subjected to regular cycles of lifting and weakening forces, increasing the possibility/likelihood of adhesion and interlock failure along the entire length of the pile. Accordingly there is a requirement for the sections of these piles, which are embedded in such volatile ground layers to be sleeved. Such sleeving activities have implications in terms of time and costs.

Driving a helical pile from the near-most end results in a loss of energy along the length. Energy is dissipated both along the length of the pile itself and into the ground via the wedge shaped flanges during the driving process of such upper portions. Without a method of ensuring that the pile sections are axially aligned and their helixes precisely aligned, when subsequent helical pile sections are added, energy losses are compounded and the performance action of the helical pile and its driving speed are affected.

Accordingly there is a need for an improved thin walled tubular helical anchor with an efficient installation method.

The present invention relates to an earth anchor as defined by the claims. The earth anchor provides a number of novel attributes pertaining to the installation method and means of proportionately thin walled hollow tubular anchors.

In a preferred embodiment, the invention is concerned with substantially thin walled hollow tubes manufactured with reduced lead end diameters in which the wall thickness is increased locally to create a ballistic driving end. The tubes are mainly used as earth anchors or piles where the reduced diameter lead driving end receives a hard metal driving plug.

Preferably, the invention has not only a unique driving end configuration, but also incorporates a novel installation method using internal mandrels. The point provides a secure plug for a driving mandrel to focus, impart and concentrate the driving loads directly onto the driving point located to the active leading end. The tapered ballistic profile of the lead end and the driving point provide a means of rapid driven installation and a means of glancing aside rock debris.

The earth anchors will preferably be of a corrosion resistant material or thin wall coated or plated steel to allow the sections to be formed by normal drawing processes. The wall thickness will be less than a twelfth of the round or prie-formed round diameter. Such wall thickness also provides a lesser mass to assist in the dynamics of driving as well as material economy. The remote end driving arrangement provides the added benefit in that driving times can be optimised by not having to drive a pre-filled full-length profile and internal grout mass, which would absorb dynamic loads

To further enhance driving ease and to increase performance the portion of the thin walled tubes following the ballistic reduced lead end are preferably given an equilateral triangular cross section and are preferably helically deformed. This preferred helical profile initiates screw-like rotation and smooth passage of penetration of the anchors upon a series of axial impulse forces being applied. The characteristics of this preferred helical profile provides the maximum notional circumscribed diameter and the stiffening effect of triangulation. The cross-sectional triangular form may carry profiled fins or wedges similar to those shown in British patent GB2336869B, which in certain circumstances will provide improved driving ease with only a marginal reduction in the notional circumscribed diameter.

The earth anchor may incorporate a non-helical stem tube portion at the near end, which itself is a clearance upon the main profile core. The upper or near end ground condition is normally too suspect to provide credible performance and could cause disruptive heave action on the anchor if profiled.

Both the lead helical tube and the subsequent sections may be driven jointly whilst on the same mandrel bar, although preferably they would be driven sequentially to ease operations at height.

The leading section of helical profiled tube receives and mates with the subsequent joints and sections. Preferably a non-helical extending stem tube is used as the final and near-most component as this format has the advantage of providing a reduced diameter thereby avoiding heave stresses acting on the upper soil levels where any action of upward movement would then be minimised. The means of incorporating two section profiles within an earth anchor arrangement by mating the components also allows the anchor assembly to be telescopically fixed or driven into the final position when grouting up. This telescopic adjustment allows a degree of flexibility when varied ground conditions encountered during use require variation of final driven penetrative length.

The helical form offers numerous advantages, not least of which is that there is a positive physical helical interlock between the earth anchor surface and the ground, in effect like a slack pitch screw or skewer. In certain ground conditions this interlock can provide unique performance, however, even in softer clay type soils with high adhesion factors the form provides visibly more performance than associated with adhesion alone. The profiled tubes especially when triangular provide an equal surface adhesion area to that of the round tube from

which it is formed. With softer clay like materials the form then incorporates two complimentary securing characteristics, these being adhesion and physical helical interlock. The two characteristics acting together provide a dual action that creates a consistent and physically reliable performance.

The angular faces of the helix emit planes of stress at a tangent along the full embedment length of helical section. The mode of helical interlock failure is characterised by the ultimate failure action coring out a cylinder of soil with a proportionately larger diameter than the full notional circumscribed diameter of the triangular helical earth anchor or pile.

The helical form provides the main performance action whereby the helical troughs limit deflection movement under load whilst also retaining adhesion contact. The helical projections from the core can best be imagined as an annular disc, set at helical pitch positions along the length. This being visualised enables it to be seen how the interlock resists deflection movements under load where the soil is subjected to shear failure within the troughs under loading. Even with inconsistent soil types, incorporating soft pockets and silty layers, the helical form will provide an even distribution of grip along the full length of the helix. In these circumstances the earth anchor incorporates attributes that provide unique reliability, especially in the tension, where maximum physical interlock is imperative.

The helical interaction is provided by the active surface of the helix as loaded ('surface A'), imparting the forces into the ground to induce stresses at an oblique tangent to the direction of load. Initially the opposite surface of the helix ('surface B') would be in contact with the ground providing extra adhesive contact. However, after a relatively small amount of loaded axial deflection movement surface B would be free from contact. Prior to the surface B contact falling away, surface A would have built up more than sufficient load carrying capacity by inducing positive helical interlock stresses. By token of this interaction there is no sudden drop off in loads. Failure occurs when the ground within the notional circumscribed helical peaks is subjected to ground stress failure. When under sustained load, the notional cylinder of shear stress is considered as being much greater than the anchor's notional circumscribed diameter, as derived from calculations. The loads carried in tension would represent a stress cylinder some 120% more than the anchor itself (x 1. 2) and 200% (x2) in compression. Such figures have been derived from on site ground shear values. Such increases in performance clearly demonstrate the

interaction of helical interlock performance especially so, when you consider half the surface B contact has already fallen away from the point of view of adhesion. The build up of ground stresses and forces upon the anchor give reactions by which a cone of stress is imparted into the ground along the operative length of the anchor. This is also confirmed by basic test data in that, the loads in compression acting upon denser deeper ground are some 50% higher than the less dense upper directional tension loadings. If it were that the earth anchor's helical interlock produced no significant stress-cone effect the tensile loads would precisely replicate the compressive loads : The above concludes that the helical interlock behaviour acts in a fashion that is not definable by conventional assumptions based upon more basic geometric forms. Such other forms of interlocking earth anchors mainly consist of screw blade auger type anchors and toggle plate tendon anchors. These forms only provide a localised locking distance at the extreme end of the anchor, and do not provide an even spread of load distribution along a reasonable distance.

The auger blade type anchor provides a screw interlock, which can sometimes loosen in the ground by over twisting, whereas the helical form sets up its own self-governing rotation from first entry.

The toggle plate earth anchors can only carry tensile loads providing limited stability, and also are very localised in terms of anchorage.

The earth anchors have applications in retaining walls, embankment stabilisation, sea defences, sewer tunnels, shallow piling, raised foundation platforms and even walkways in flood planes.

The earth anchors are preferably formed from thin walled tubes with diameters of 25 to 200mm.

The preferred helical anchor will have an internal circular inscribed void being one to two thirds that of the notional circumscribed diameter. The helical twist will be between two and a half and six times the notional circumscribed diameter axially per full rotation of twist.

The preferred helical anchor will have a non-helical or a part-helical extension stem capable of providing an internal sliding fit within the helical sheath. Such an adjustable anchor or pile

construction comprising of more than one section allows the end length fine-tuning to be achieved. The various sections and profiles are locked both axially and tortionally through driven or sliding helical mating interlock or through multiple grout portholes.

The stem tubes comprising a remote helical portion to interlock with the main helical sheath tube are independent of grout and can be subjected to load testing immediately. Such internal helical interlocking connecting sections can also be used as short jointing sockets.

The grout can be pumped solely into the remote helical sheath connection by the outer end grout ports being blanked off internally by the grouting nozzle tube. These blanked off grout ports can be utilised as portholes for an engagement lug plunger as a quick means of site testing. Prior to near end grouting it will be necessary to mask off the outer bore openings with a grouting disc.

Helical anchors are also ideal for being inserted into pre-drilled holes for use as masonry or rock anchors. Helical profiled sections have been demonstrated to provide unique back-flow characteristics with grouts and resins.

There are several means of securing rods and tubes into grouted holes. However most, if not all, rely upon interference profiles such as machine threads, ribs or in some cases holes. With threads and ribs there tends to be a creation of voids around the outer extremities as the rods are pushed in, especially when into partially cured materials. Hole pressings tend to be more reliable though provide far less interlock strength over a given length.

By pushing a closed end helical tube into a part filled bore hole the spiral form presents a continuous helical path for uniform pressurised back travel of displaced grout or resin, providing absolute encapsulation and maximum interlock. Insertion of such helically deformed socket anchors provides a unique phenomenon of self-rotation on entry even in semi fluid grouts and resins.

A further aspect of the invention utilises the grouting characteristics of helical profiles by using tubular helical grouting in conjunction with expansion socks. Such socks are porous and are pressure filled with grouts where the granular filler fills the pores up causing the sock to expand in a bulbous manner. The grout is pumped through an extension sleeve to extrude grout from the

remote end of the anchor to fully inflate the expansion sock. The contained grout provides mechanical interlock between the substrate and the helical tube as well as externally with masonry voids, which fill up evenly and simultaneously to create a deformed cementitious plug.

The grouts will be of high strength, around 50N/mm, and the majority of grouts will be of the injectable cementitious type.

The connecting capping detail can incorporate a series of cross-fixed grouted in bars fitted through the stem tube tops.

Figure Listings In Brief Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figures 1, 1A and 1B show the tubular section (3) and lead end configuration (1) incorporating the driving point (2) and the stem tube extension (6) with a means of connecting anchor segments together via mechanical helical interlock (28).

Figures 2,2A and 2B show installation methods and tooling.

Figures 3, 3A and 3B show the resulting thrusting angles of forces under load and the interaction with the ground after small axial movement with the active cross section (15).

Figure 4 shows the accumulative cone of stress (16) when the earth anchor is under load.

Figure 5 shows an analytical model of how the helical profile acts in effect as intermittent disc plane interlock.

Figure 6 shows the trough (17) of the helical sheath (3) profile and how it interlocks the ground.

Figures 7 and 8 show alternative earth anchors.

Figures 9A and 9B show raised channel foundation (24) applications of the system.

Figure 10 shows a means of connecting anchor segments together and to masonry structures via grouting and portholes (7). Figure 11 shows a helical anchor pushed into a grouted bore hole in masonry (34) and the characteristics of grout back flow.

Figure 12 shows the anchor (3) being used as a grout delivery tube with and expanding sock (33) attached at the remote end.

Detailed Figure Listings Figure 1 shows an isometric view of a tubular earth anchor. The driving point (2) is seated and contained in the reduced diameter lead end (1) of the leading helical sheath (3). An interlocking stem-tube (6), which incorporates an internal helical interlock portion (28), is helically screwed into and mated with the helical sheath (3). A short length of interlock portion (28) could be used to joint together a number of sheath (3) segments within a length.

Figure lA is an end view that shows the lead end profile incorporating the drive point (2) in the reduced diameter (1) that is inscribed within the core of a triangular helical sheath (3) profile.

Figure IB shows a cross sectional view through the mechanical interlock connection (28) between the stem tube (6) and the sheath (3) on a triangular helical profile.

Figure 2 shows the installation procedure in which two component parts are driven separately and constructed as one to create an earth anchor within, in this case a limited headroom represented by the broad white are at the top left. On the left hand column of three drawings the earth anchor can be seen with its variable stem tube (6) to sheath (3) length. To the right the actual installation procedure is shown with left, a clearance hole punch (30) being driven in by a jack-hammer (4) an extended pilot punch (32) (not always required) can also be driven in by a jack-hammer (4) to increase the depth of the clearance hole yet further, within this drawing is shown the operation of initial helical sheath (3) insertion by the hand dropping of a mandrel (5) onto the driving point then finally to the right is shown the full depth insertion of the sheath (3)

and pre-interlocked stem (6) using the drive mandrel (5) and extension (31). The right hand column shows the driving and punching tooling consisting of a jack-hammer (4), a driving head (8), an extension (31), a driving mandrel (5) and pilot punches (30,32).

Figure 2A shows a driving method in operation that is being carried out using a full-length steel driving mandrel (5) that is slotted into a driving tool (8), the sheath (3) and grouting stem tube (6) being driven as one. Figure 2B is an end view of the earth anchor.

Figure 3 shows how the Helical form (3) interacts with the ground to thrust out lines of stress dispersing out at angles from the Helix shown by the arrows as the load is directed axially (13). The effect of such interlocking helical interaction results in a notional performance shear cylinder (12).

Figure 3A shows how the helical form (3) interacts with the ground when a small amount of axial deflection takes place, which produces a pulled clearance (35). In such a displaced positioning the underside of helical ridge is cleared from contact and the upper Helical faces (29) are solely those that are in full load contact.

Figure 3B shows the notional performance shear cylinder (12) which is greater than the actual anchor's notional circumscribed diameter (14) within which is seen the anchor's cross sectional area (15).

Figure 4 shows the action of stresses described in Figure 3 accumulating in a cone of force (16) when the earth anchor is loaded from either direction. The cones represent the stress plane that can be more easily expressed by the notional shear cylinder (12).

Figure 5 shows a means of visualising the Helical ground interaction and interlock in a simple model of intermittent annular ring plates (40), set at, and representing the helical pitch.

Figure 6 shows the interlock (17) of the ground upon the Helical form (3) by breaking the section half way through, this being the same at any rotational half-way view around and down the Helical sheath (3). The notional circumscribed diameter (14) of the helical sheath (3) is shown by a dotted line (41) as the minimum outer shear plane cylinder.

Figure 7 shows a tubular driven pile (19) that relies upon friction bond, it can be seen toward the top of the pile that there sometimes can be a slackening effect which creates a tapered driving void (18) which in some circumstances can have a detrimental effect on performance even at depth.

Figure 8 shows two other alternative earth anchors; on the left is a spiral bladed auger type anchor (20) with the spiral blade (21) at the extreme bottom end. For convenience, both anchors are shown in a vertical orientation where in practice most uses as tensile anchors would be closer to horizontal. The spiral bladed anchor (20) is screwed in under torque and there is occasional loosening above the spiral blade (21) seen as a white void above the blade (21) this is caused by over torqueing. The right hand drawing shows a toggle plate anchor that is purely a tension anchor, which is driven in with a shaft-like adapter that is then subsequently removed. The toggle plate (23) is swivelled back on itself by the tensioning up of a tendon (22) the tendon is also then the means of carrying the operational load.

Figure 9A shows a section through a raised foundation channel (24) where the nuts (26) clamp the channel down to the earth anchor piles through grouted in studding (25) which can extend downward to provide extra reinforcement stiffening effect into the grouting stem tube (6) which is grouted into the Helical sheath (3) in the normal manner. Figure 9B shows a similar arrangement where buffer blocks (27) have been introduced to absorb seismic shock in earthquake regions.

Figure 10 shows a grouting up operation, which is carried out by pumping grout (9) down the stem tube (6) by means of a nozzle (11) where the grout exudes through the ports (7) to lock the sheath (3) and stem tube (6) together as well as simultaneously or separately grouting up the outer masonry connection. To avoid the grout spilling out onto the masonry face and allow adequate backpressure a grouting disc (10) is inserted.

Figure 11 shows a helical anchor pushed into a grouted bore hole in masonry (34), the helical tube (3) is highly profiled and closed at its lead end creating self impelled rotation as it is pushed in, allowing the grout (9) to flow and follow an uninterrupted helical interlock passage. After

curing a piece of studding (25) can be threaded in and a nut (26) can be attached to secure the desired element to the masonry (34).

Figure 12 shows a helical sock anchor being secured into a bore hole in masonry. The helical tube (3) is used to pump, swirl and additionally mix grout (9) to the bottom of the bore hole and create a back pressure feed of grout (9) spiralling back around the helix tightly bonding the helical tube in place whilst simultaneously pumping out and expanding a sock (33) to tightly abut the masonry (34).

According to a preferred embodiments of the invention there is provided a form of tubular earth anchor, that has a reduced diameter (1) at the leading end into which a driving point (2) is incorporated to create a hard hitting surface. Its method of installation is such that the driving method utilises a rod-like mandrel (5) to focus, impart and concentrate the driving loads from a series of axial impulse blows directly onto the driving point (2) located to the active leading end.

The reduced diameter (1) creates a tapered leading drive end tapering up to the full profile diameter. The full profile diameter is in excess of 25mm and the wall thickness is less than one twelfth of the pre-formed feed tube diameter.

The hollow tubes (3), which form the anchor, have a reduced lead end diameter (1) into which a hard metal driving plug (2) is incorporated to create a ballistic driving end. The driving plug (2) acts as a hardened point for glancing past rock-like debris. The point also provides a secure plug for a driving mandrel (5) to impact upon allowing the anchor to be driven from the bottom/remote end.

The tubes are mainly used as earth anchors or piles and their helical form (3) offers a positive physical interlock between the earth anchor surface and the substrate into which they are driven or grout into which they are embedded. This interlock provides an even spread of load distribution along the length of the helix.

Claims

Claims 1. A tubular earth anchor, having a reduced diameter (1) at the leading end into which a driving point (2) is incorporated to create a hard hitting surface.
2. An earth anchor according to claim 1 where the lead section (3) incorporating the reduced diameter lead end (1) is a profiled helical tube.
3. An earth anchor according to claim 1 where the lead section (3) incorporating the reduced diameter lead end (1) is tubular and round.
4. An earth anchor according to any one of the preceding claims where the anchor consists of more than one tubular profile.
5. An earth anchor according to any one of the preceding claims where the anchor consists of at least one tubular profile and at least one solid profiled section.
6. An earth anchor according to any one of the preceding claims where the components are connected by a localised helical interlock (28) or are otherwise mechanically connected.
7. An earth anchor according to any one of the preceding claims where the components are connected by grouts or by chemical compounds.
8. An earth anchor according to any one of claims 2,4, 6 and 7, including a grout envelope formed by pumping grout through the helically profiled tubular anchor, to extrude grout from the far end and feed grout back, under pressure, around the external helix in a substantially uniform helical passage to encapsulate the tube.
9. An earth anchor according to claim 8 including an expanding grout retaining sock is fitted to the tubular anchor to create an expandable bulbous sock interlock with the surrounding rock or masonry.
10. An earth anchor according to any one of the preceding claims where the near end portion

of the earth anchor is connected to masonry by intersecting cross-ties.
11. An earth anchor according to any one of the preceding claims where a pile arrangement supports a raised foundation platform (24).
12. An earth anchor according to any one of the preceding claims where a pile arrangement supports a cast concrete beam foundation in which the upper portion of the pile is embedded.
13. An earth anchor according to any one of the preceding claims where the anchor's sections are manufactured from corrosion resistant material.
14. An earth anchor according to any one of the preceding claims where the anchor's sections have a corrosion resistant coating.
15. An earth anchor according to any one of the preceding claims, in which the reduced diameter (1) creates a tapered leading drive end tapering up to the full profile diameter.
16. An earth anchor according to any one of the preceding claims, in which the full profile diameter is in excess of 25mm and the wall thickness is less than one twelfth of the pre formed feed tube diameter.
17. A method of installing an earth anchor according to any one of the preceding claims where the driving method utilises a rod-like mandrel (5) to focus, impart and concentrate

the driving loads from a series of axial impulse blows directly onto the driving point (2) C > located to a reduced diameter (1) active leading end.
18. A method of connecting tubular helical sections according to any one of the preceding claims where the components are connected by localised helical interlock (28).