AU646902B2 - Grouted screw anchor - Google Patents

Description

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AUSTRALIA

,Patents Act 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT Name of Applicant(s): FRANCIS WILLIAM SOROHAN AND CAVLANA PTY LTD 00 0

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a a Actual Inventor: Address ,or Service: Invention Title: FRANCIS WI LLIAM SOROHAN CULLEN CO., Patent Trade Mark Attorneys, 240 Quoen Street, Brisbane, Qld. 4000, Australia.

GROUTED SCREW ANCHOR Details of Associated Provisional Application: No. PK3379 The following statement is a full description of this invention, including the best method of performing it known to us: 2 This invention is concerned with improvements in screw anchoring devices and, in particular to screw anchors for marine applications.

Screw anchors generally comprise a shaft to which is attached a continuous helical flight (similar to an auger screw) or one or more discontinuous helical flights spaced at irntervals along the shaft. In use, the screw anchor is driven into an earth mass or a sea bed by rotating the shaft whilst allowing the screw anchor to penetrate the earth mass at a rate consistent with the pitch of the helical flight.

While generally suitable for their intended purpose on land or on a sea bed, prior art screw anchors do suffer a number of limitations and disadvantages. In particular, conventional screw anchors are really only effective where the anchoring load is applied along the axis of the shaft if severe loads are to be applied.

For example, screw anchors are often used to anchor guy wires for masts or towers used for communications. For small towers where the horizontal load component is small, the

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screw anchor may comprise a relatively short shaft with a single helical flight extending about 3600 aroand the shaft at its base. Such small screw anchors are usually driven vertically into the ground surface.

For larger towers which may have a plurality of guy wires attached to the screw anchor, the horizontal component of applied load may be much grater. To reduce bending stresses on the anchor shaft, the anchor may be driven into the ground at an angle such that the anchor shaft extends colinearly with the guy wire.

In marine applications, screw anchors are particularly prone to bending stresses applied to the anchor shaft and this can ultimately lead to failure.

Where the screw anchor is used aO3 a "swinging" mooring for a vessel, the horizontal component of the applied load can be quite severe due to the combined effects of wind, waves and tidal currents. Furthermore, the horizontal load component may be applied in any direction throughout an angle a@ of 3600 from the upright axis of the anchor shaft.

*o: In "fixed" moorings where a plurality of screw anchors may be employed to anchor, say, a floating pontoon, the horizontal load component is generally unidirectional but nevertheless this component can, under appropriate weather and tidal circumstances, be quite high leading to excessive bending stresses on the anchor shaft.

Generally speaking, due to the fairly "fluid" characteristics of a sea bed comprised of mud, sand, fractured coral and the like, it is usually necessary to use a longer 9 screw anchor with two or three screw flights spaced along the e anchor shaft to meet vertical load requirements. The relatively soft sea bed medium does not however provide any real support against the horizontal component of applied load thus subjecting the anchor shaft to considerable bending stresses. In some cases, the horizontal load component can be so great as to preclude the use of a screw anchor for safety reasons. Conventional screw anchors are often precluded from use where the sea bed comprises solid coral masses. The solid 4 coral is too hard to allow penetration of a screw anchor.

It is an aim of the present invention to overcome or ameliorate the disadvantages of prior art screw anchors and to provide an improved screw anchor and anchoring system which is particularly suited to marine applications.

According to one aspect of the invention there is provided a screw anchor comprising:a hollow shaft having a tapered point forming a closure at one end and an opening at the other end of he shaft; a helical screw flight extending about the outer wall of said hollow shaft adjacent said tapered point; and, a hollow tube extending radially from said hollow shaft adjacent a rear edge of said screw flight, said hollow tube being in fluid communication with the interior of said hollow shaft at one end and having an opening at its free and.

Preferably said hollow tube is supported on said rear edge of said screw flight.

7Suitably the screw flight may comprise a circular helix extending about the hollow shaft.

Alternatively the screw flight may comprise a non circular member extending helically about the hollow shaft.

If required said screw anchor may comprise two or 0 00* more screw flights at spaced intervals along the hollow shaft.

Preferably a screw flight adjacent said tapered point comprises a non circular member having a minimum radius at the leading edge of the flight and a maximum radius at the trailing edge of the flight.

1/9 If required, said hollow shaft may comprise two or more hollow tubular members connectable at respective ends to form a single hollow shaft.

Suitably a screw flight is associated with each of said tubular members.

According to another aspect of the invention there is provided a method of installing a screw anchor in an earth mass, said method comprising the steps of:rotating a screw anchor comprising a hollow shaft and a helical screw flight adjacent one end thereof to effect 1 penetration of the earth mass by said screw flight; and, whilst rotating said screw anchor, a cementitious grout is pumped down said hollow shaft to emerge from an opening in said hollow shaft adjacent said screw flight whereby a mass of cementitious grout is mixed with earth material distir .cd by said screw flight as it penetrates the earth mass.

as In order that the invention may be more fully understood, reference is now made to various preferred embodiments of the invention illustrated in the accompanying drawings in which:-

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o FIG 1 illustrates a penetrating anchor section according to the invention.

FIG 2 illustrates an extension shaft for the screw anchor section of FIG 1.

FIG 3 shows schematically a screw anchor installation according to the invention.

FIG 4 shows an enlarged view of the digging point illustrated in FIG 1.

FIG 5 shows a plan view of a preferred configuration of a lead screw flight.

FIG 6 shows a plan view of a body screw flight.

In FIG 1 there is illustrated the penetrating or lead screw anchor section 1.

The lead section comprises a hollow square section steel tube shaft 2 with a cutting point 3 welded or bolted in one end. A lead screw flight 4 of generally helical configuration is located about shaft 2 in the region of cutting point 3.

Mounted across the trailing edge 5 of flight 4 is a hollow tube 6 which is open at' its free end and is connected to the interior of shaft 2 either directly by welding or otherwi3e via a socket (not shown) attached to shaft 2.

Optionally, a further, larger helical screw flight 7 asset: may be attached to shaft 2 at a desired distance (usually 3 x flight diameter) from lead screw flight 4.

The free end 8 of the shaft 2 is open and includes an aperture 9 extending through opposed side walls of shaft 2.

The purpose of aperture 9 will be described later.

00 FIG 2 illustrates a shaft extension section which, in combination with the lead section of FIG 1 allows deeper and more secure anchorages.

In FIG 2, the extension section 10 comprises a hollow square section steel tube 11 having the same cross sectional dimensions as the lead shaft 2 in FIG or it may be of a larger cross sectional dimension.

For cases where the lead shaft 2 and the extension shaft 11 are the same in cross-sectional dimensions, connection of shaft 2 and shaft 11 may be effected by a fabricated collar 12 secured to or attachable to shaft 11.

Collar 12 has an opening to permit the fioe snd of shaft 2 to be inserted therein and fixing is achieved by a bolt or the like inserted through aligned apertures 13 and 9 in collar 12 and shaft 2 respectively.

Optionally, extension shaft 11 may include one or more screw flights 14 having the same> or a larger diameter than screw flight 7 shown in FIG 2.

A further aperture 15 is provided near the upper end 16 of shaft 11 for attachment of yet another extension 0 0** shaft (not shown) or other fittings as will be described

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later.

FIG 3 shows an anchor system according to the invention.

Once a selected sea bed anchorage site is selected an installation frame (not shown) is located on the sea bed.

The frame typically comprises a platform and legs to elevate the platform from the sea bed. Mounted on the platform is an hydraulic drive motor powered by a source of pressurised

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hydraulic oil via hoses connected to a service vessel moored nearby.

The lead screw section 1 is positioned on the sea bed below the frame with the shaft 2 mounted in a drive collar associated with the hydraulic motor. A swivel collar (not shown) connected to a grout supply hose is then connected to the free end of lead shaft 1 by a diver.

8 Upon a command from the diver, the hydraulic motor is actuated and as soon as the lead screw flight has penetrated the surface of the sea bed, a cementitious grout is pumped from the service vessel via the grout supply hose to the interior of shaft 2. The grout flows down the shaft 2 and emerges from the free end of tube 6 as the lead screw flight 4 rotates.

As the lead screw flight 4 rotates the cementitious grout emerging from tube 6 is mixed with the loosened sea bed material eg. sand or coral rubble to form, in effect, an aggregate reinforced mass of concrete around the lead shaft 1 as it penetrates the sea bed.

Sa* The cementitious grout preferably comprises type 0.

cement powder mixed into a pumpable slurry with fresh water and any other chemical additives as may be required eg.

viscosity modifiers, curing accelerators or retardants etc.

o, The mixing action of the screw flight as it penetrates the sea bed forms a mass 20 of high strength aggregate reinforced concrete about the shaft of the screw anchor and as the cement slurry diffuses outwardly therefrom there are formed regions

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21 and 22 of medium strength and low strength concrete surrounding the high strength mass 20 within the sea bed material 23.

For greater load bearing capacity, the screw anchor may have an extension shaft 11 (with or without screw flights) attached thereto to drive the lead screw section 1 deeper into the sea bed and to form a larger mass of concrete around the shaft.

9 When the desired degree of penetration is achieved, the driver detaches the hydraulic drive assembly and disconnects the swivel coupling supplying the pressurised grout.

A mooring eye 24 or the like with a mounting collar is then attached to the free end of the screw anchor shaft by a bolt 25, thus sealing the grout filled shaft.

When cured, the cementitious mass surrounding the screw anchor provides a far more effective and stable

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anchorage both in respect of the vertical and horizontal load 0*4 components when compared with a conventional screw anchor system. In particular, the grout filled shaft and the mass of concrete about the shaft provide an effective means of reinforcing the screw anchor shaft against bending stresses resulting from the horizontal components of applied load.

FIG 4 illustrates a cutting point 30 which may be attached to the end of the lead section 1.

The point 30 typically comprises tapered steel 0 plates 31, 32 in a cruciform cross sectional configuration. A base plate 33 may be adapted for welding or bolting into the open end of the lead shaft 2.

The use of a cutting point such as that illustrated in FIG 4 facilitates the penetration of hard earth, soft rock, hardened sedimentary sea bed deposits and, in particular large masses of solid coral.

FIG 5 illustrates a preferred form of lead or penetrating screw flight 35 suitable for most penetrable earth masses.

The leading edge 36 starts with a minium radius and gradually increases to a maximum radius at its trailing edge 37. In this illustration, the grout tube 38 is secured to the trailing edge 37 for support.

FIG 6 shows an alternative configuration of screw flight 39 which is preferably positioned on the anchor screw shaft at a distance from the lead or penetrating screw. The non circular shape, which is effectively derived from a rectangle with large radiussed corners and a tapering leading 0e edge 40, has been found to substantially reduce the drive torque required while at the same time ensuring an effective mixing of the grout and disturbed particles of earth mass.

0 se The screw flight 40, being spaced form the lead screw flight of FIG 5, is usually not associated with a grout tube. However, in the event that additional grout is required, the or each flight subsequent to the ler.d screw flight may support a grout tube.

In a typical heavy duty mooring, the screw anchor shaft may comprise three sections of steel tubing, each

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measuring about 2 metres in length. The lead shaft section 0* may comprise a 75 mm square section, the next section may be 100 mm square and the uppermost section may be 150 mm square.

Typically the steel tubing may have a wall thickness of between 6 mm and 9 mm.

The lead screw helix may have a radius of about 150 mm at its trailing edge, the intermediate helix a radius of 200 mm at its trailing edge and the uppermost helix may have a trailing edge radius of 250 mm. Typically, the helices have a pitch of about 75 mm.

For a screw anchor installation of the above type in, say, coral rubble a cement slurry mixed at the rate of kg of cement to 20 litres of water may be used.

Approximately 120 kg of cement may be deposited in the region of the uppermost anchor section while about 60 kg of cement is deposited in each of the intermediate and lead sections.

The amount of cementitious grout deposited is 6O selectively controlled by controlling the rate of penetration of the lea-I screw anchor helix. Rather than just allowing the helix to penetrate at its pitch/revaluation rate, the seO ponetration rate may be restricted to allow greater deposition of grout.

A particular advantage of screw anchors according to the present invention is that due to the combined effects of flight shape and the lubricity of the cement grout, far less torque is required to drive the screw anchors into the earth mass. Accordingly, screw anchors according to the invention 5 may be employed in "dry" earth masses on land as well as a sea 5 bed.

0 Another advantage over prior art screw anchors is the ability to penetrate solid coral masses and to form a strongly bound mass of concrete comprising cement and fractured coral particles. Without the grouting capability, screw anchors inserted into a solid coral mass would have little effective holding power in the cylindrical column of fractured coral so formed.

12 It will be readily apparent to a skilled addressee that many modifications and variations may be made to the various aspects of the inventions without departing from the spirit and scope thereof.

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Claims (10)

1. An earth engaging screw anchor comprising:- a hollow shaft having a tapered point at one end and an opening at the other end of said shaft; a helical screw flight extending about ai outer wall of said shaft adjacent said tapered point; and, a hollow tube extending radially from said shaft adjacent a trailing edge of said screw flight, said hollow tube being in fluid communication at one end with the interior of said hollow shaft and having an opening at an end opposite thereto.

2. A screw archor as claimed in claim 1 wherein said hollow tube is supported on said trailing edge of said screw flight.

3. A screw anchor as claimed in claim 1 or claim 2 wherein said shaft comprises one or more axially engable shaft sections. a.

4. A screw anchor as claimed in any preceding claim oo wherein a plurality of helical screw flights are located at :2p spaced intervals along said shaft.

5. A screw anchor as claimed in any preceding claim wherein at least one screw flight comprises a non-circular member %o having a minimum radius at the leading edge of said flight and .e S. a maximum radius at the trailing edge of said flight. *a .2

6. A screw anchor as claimed in any preceding claim S wherein a further screw flighc, spaced from said helical screw flight adjacent said tapered point, has a greater surface area than said helical screw flight adjacent said tapered point. 14

7. A method of installing a screw anchor according to any one of claims 1 to 6 in an earth mass, said method comprising the steps of:- rotating said screw anchor to effect penetration of said earth mass by said screw flight; and, whilst rotating said screw anchor, pumping a cementitious grout into said hollow shaft to emerge from said hollow tube whereby a mass of cementitious grout is mixed with earth material disturbed by said screw flight as it penetrates the earth mass.

8. A screw anchor substantially as hereinbefore described with reference to the accompanying drawings.

9. A method of installing a screw anchor substantially as hereinbefore described.

10. A mooring for marine craft substantially as hereinbefore described with reference to any one of FIGS 1-3. Dated this 16th day of December 1993 FRANCIS WILLIAM SOROHAN AND CAVLANA PTY LTD By their Patent Attorneys 6' CULLEN CO 4 S. oS e oo o \o* ABSTRACT A screw anchor for land and sea bed applications comprises a hollow shaft with a cutting point at one end adjacent a helical screw flight. A hollow tube is supported along the trailing edge of the screw flight and is connected to the interior of the hollow shaft. As the screw anchor is rotated to panetrate an earth mass, a cementitious grout is pumped down the hollow shaft and emerges from the end of the hollow tube. Rotation of the screw flight in the presence of the oementitious grout forms a concrete reinforcing binder between the screw anchor and the svrrounding earth mass. at 944 086 0