GB2258680A

GB2258680A - Forming a pile

Info

Publication number: GB2258680A
Application number: GB9215410A
Authority: GB
Inventors: R A Bullivant
Original assignee: Roxbury Ltd
Current assignee: Roxbury Ltd
Priority date: 1991-08-10
Filing date: 1992-07-21
Publication date: 1993-02-17
Also published as: GB9215410D0

Abstract

A method of forming a pile in situ comprises augering a hole utilising an auger 16 of a length less than the length of the hole, rotating the auger at a speed greater than that normally employed, ie at least 150 r/min, with a rate of penetration less than that normally employed, supplying flushing material to the auger to mix with material removed by the auger, pumping off the mixture and, away from the hole separating removed material from the flushing material which may then be recycled. In another embodiment the material removed by the auger is left within the hole and a hardenable material is mixed therewith. <IMAGE>

Description

IMPROVEMENTS IN OR RELATING TO THE FORMATION OF PILES The present invention concerns improvements in or relating to the formation of piles, especially concrete piles formed in situ by a soil displacement method.

A known method of forming piles in situ comprises forming a hole in the ground at the site to be piled by removing earth from the ground with an auger, the auger being rotated by drive means mounted on the surface and having a length which is at least equal to the length of the pile to be formed. This is a relatively slow process as the auger, after advancing for a short distance, has to be retracted to allow soil removal therefrom, often by hand. Additionally, the skin friction between the external periphery of the auger and the sides of the hole it has previously drilled is high so that a large driving force is required simply to rotate the auger, in other words, only a relatively small proportion of the driving force is utilised to cut the hole.

An object of the present invention is to obviate or mitigate these and other disadvantages.

According to the present invention there is provided a method of forming a pile in situ comprising providing a hole in the ground to be piled by an auger whose length is less than the depth of the hole, rotating the auger at a speed of at least 150 r/min, applying a flushing medium at the auger to mix with material removed by the auger and removing the mixture from the hole, thereafter filling the hole with a settable material to form a pile therein.

Preferably the auger is rotated at a speed of above 300 rev/min.

Preferably the auger is advanced at a speed of .3 m/min.

Preferably the auger is mounted at the free end of a drive shaft of adjustable length. Preferably the flushing medium is supplied through said shaft.

Preferably the mixture of flushing medium and removed material is pumped from the hole.

Preferably after removal from the hole the removed material is separated from the flushing medium which is then recycled. Separation may be by means of a cyclone separator. Separated flushing medium may be recirculated to the auger by a further pump.

Preferably after an initial augering operation is carried out the auger is removed and the augered hole is lined by a steel casing.

Preferably stones and other solid debris removed by the auger are collected in a perforated container at the auger exhaust thereafter being removed mechanically from the hole.

The present invention relates also to a modified method which can be used where the ground comprises material ranging from what is defined as medium sand to material defined by the phrase sandy gravel. The modified invention realises that this material can be utilised in the foundation of the pile and need no be removed from the augered hole.According to the modified invention there is provided a method of forming a pile in situ in ground comprising material in the range medium sand to sandy gravel comprising providing a hole in the ground to be piled by an auger whose length is less than the depth of the hole, rotating the auger at a speed of at least 150 r/min and applying a cementitious containing material at the auger to mix with material removed by the auger while said material remains in the hole and after the auger has reached a predetermined depth removing it from the hole while the mixture remains therein the mixture thereafter setting to form a pile in the hole.

An embodiment of the present invention will now be described by way of example only with reference to the accompanying drawing which shows diagrammatically an assembly for forming a pile.

An initial pile forming operation is carried out by augering a hole which is substantially as deep as the length of the auger, at slow speed and high torque.

Preferably said initial operation augers a hole of up to 2 metres in depth. Preferably the auger is driven during this intial operation at a speed of between 60 and 100 rev/min.

A tracked vehicle 10 has mounted thereon a drill head 12 for imparting rotation to an extendable length of drilling tube 14 having an auger 16 fixed to its end. The auger has a hollow central shaft 18 which is mounted to and in communication with the core of the drill string 14 whereby a supply of flushing fluid, for example, bentonite can be supplied to the leading end of the auger 16 to flush material removed by the auger through the auger flights to its exhaust 20.Bentonite is supplied to the drill string 14 through the drill head 12 by a supply conduit 22 to which bentonite is pumped by a pump 24 from a separation assembly comprising a cyclone separator 26 and a settling assembly including, for example, a plurality of skips 28, the cyclone and separating assembly serving to remove separated material from bentonite pumped from the hole by a further pump 30 connected in a line 32 extending between the augered hole and the cyclone 26.

Just above the exhaust 20 from the auger there is provided a perforated basket 34 into which are exhausted stones and other relatively heavy material which cannot be removed by the flushing fluid. The basket 34 is connected to raising chains or cables 36 by which it can be periodically raised to the surface for emptying.

In a preferred embodiment the length of the auger is 2 metres and its diameter between 0.4 and 0.7 metres. The drill head has an output horse power of between 30 and 40 HP and is capable of a speed range of between 50 and 350 r/min. The drill string preferably comprises drilling tubes of 75 - 100 mm outside diameter.

In operation the drill head is positioned above the location at which a pile is to be formed. The auger 16 is attached to the drill head and a 2 metre deep hole is augered into the ground using normal augering techniques, that is, rotating the auger at a speed of between 60 and 100 r/min with a high torque input. The auger is progressed intermitently and is frequently retracted to allow the removal of soil therefrom by hand.

At the conclusion of this initial augering operation the auger is removed from the 2 m deep hole it has formed and the hole is lined by a steel casing 38. The auger is then fitted to the leading end of the drill string 14 and rotated at a much higher speed of between 200 and 300 rev/min. The auger is inserted into the casing while bentonite is supplied down the drill string by the first pump 24. The auger is then advanced at the relatively slow speed of .3 m/min. The high rotational speed of the auger coupled with the slow advance effectively planes a thin layer of material from the ground below the auger, the high rotational inertia of the auger resulting in the removal of any stones or hard material without any noticeable decrease in the auger's speed or additional power input.

The auger flights, coupled with the flushing action of the bentonite removes soil and small debris from the auger cutter, the mixture being pumped away by pumps 30.

Larger debris is thrown into the perforated basket 34 by the rotating flights and retained therein. The mixture of bentonite and small material pumped from the hole by the pump 30 is subsequently separated so that the bentonite can be recirculated by the pump 24. Periodically the stones can be removed by raising the basket 34 to the surface by means of the chains or cables 36.

When a hole of a predetermined depth has been formed the drill string 14 with the auger 16 attached thereto can be removed from the hole leaving the hole filled with bentonite. The auger 16 is then removed from the drill string 14 which is returned to the hole and separated from the drill head so that the tracked vehicle 10 can move to the next position and, reusing the equipment described above with the exception of the drill string, forms another hole. At some convenient time, concrete can be supplied to the bottom of the hole by the drill string remaining therein. The string is removed from the hole as it is filled with concrete, the concrete displacing the bentonite remaining within the hole. Before the concrete has set suitable reinforcement may be introduced.

It is important to note that the present invention utilises an auger which, during most of the hole forming operation, is mounted on the end of a drill string. The auger is rotated at a speed which is greatly in excess of that normally employed and as a result of its short length compared to the length of the hole the skin friction between the periphery of the auger and the hole is considerably reduced when compared with a normal augering technique carried out in a hole of the same depth. This means that the horse power applied to the top of the drill string is transmitted entirely to the bottom of the hole being formed, rather than all the way down the hole as experienced in earlier techniques.

Various modifications can be made without departing from the scope of the invention, for example, flushing medium other than bentonite may be employed, alternative stone removal techniques may be utilised, alternative separations means may be employed, the recirculation of flushing medium may be achieved by any suitable apparatus or method and a cutter other than an auger may be utilised.

Claims

Claims:

1. A method of forming a pile in situ comprising providing a hole in the ground to be piled by an auger whose length is less than the depth of the hole, rotating the auger at a speed of at least 150 r/min, applying a flushing medium at the auger to mix with material removed by the auger and removing the mixture from the hole, thereafter filling the hole with a settable material to form a pile therein.

2. A method as claimed in claim 1, in which the auger is rotated at a speed of above 300 r/min.

3. A method as claimed in Claim 1 or claim 2, in which the auger is advanced at a speed of .3 m/min.

4. A method as claimed in any one of claims 1 to 3, in which the auger is mounted at the free end of a drive shaft of adjustable length.

5. A method as claimed in claim 4, in which the flushing medium is supplied through said shaft.

6. A method as claimed in any one of the preceding claims, in which the mixture of flushing medium and removed material is pumped from the hole.

7. A method as claimed in any one of the preceding claims, in which after removal from the hole the removed material is separated from the flushing medium which is then recycled.

8. A method as claimed in claim 7, in which separation is by means of a cyclone separator. A further modification of the invention is applicable in specific ground conditions, that is ground where the material comprises anything within the range defined by the term medium sand to sandy gravel. The modification envisages the retention of most of the material removed by the auger as it penetrates into the ground within the hole the auger is cutting and providing at the auger, instead of the flushing medium described above, a cementitious material.

for example, cement and water mix which the auger will mix together with the material it has removed to form a settable material which remains fluent during the movement of the auger down to a predetermined depth. The auger can then be removed and after removal the material within the hole sets to form the desired pile.

9. A method as claimed in claim 7 or claim 8, in which separated flushing medium is recirculated to the auger by a further pump.

10. A method as claimed in any one of the preceding claims, in which after an initial augering operation is carried out the auger is removed and the augered hole is lined by a steel casing.

11. A method as claimed in any one of the preceding claims, in which stones and other solid debris removed by the auger are collected in a perforated container at the auger exhaust thereafter being removed mechanically from the hole.

12. A method of forming a pile in situ in ground comprising material in the range medium sand to sandy gravel comprising providing a hole in the ground to be piled by an auger whose length is less than the depth of the hole, rotating the auger at a speed of at least 150 r/min and applying a cementitious containing material at the auger to mix with material removed by the auger while said material remains in the hole and after the auger has reached a predetermined depth removing it from the hole while the mixture remains therein the mixture thereafter setting to form a pile in the hole.

13. A method as claimed in claim 12, in which the auger is rotated at a speed of above 300 r/min.

14. A method as claimed in claim 12 or claim 13, in which the auger is advanced at a speed of .3 m/min.

15. A method as claimed in any one of claims 12 to 14, in which the auger is mounted at the free end of a drive shaft of adjustable length.

16. A method as claimed in claim 15, in which the cementitious material is supplied through said shaft.

17. A method of forming a pile in situ substantially as hereinbefore described.

18. Apparatus for forming a pile in situ substantially as hereinbefore described with reference to the accompanying drawing.

19. Any novel subject matter or combination including novel subject matter disclosed in the foregoing specification or claims and/or shown in the drawings, whether or not within the scope of or relating to the same invention as any of the preceding claims.