GB2286613A

GB2286613A - Ground improvement

Info

Publication number: GB2286613A
Application number: GB9403131A
Authority: GB
Inventors: Roger Alfred Bullivant
Original assignee: Roxbury Ltd
Current assignee: Roxbury Ltd
Priority date: 1994-02-18
Filing date: 1994-02-18
Publication date: 1995-08-23
Anticipated expiration: 2014-02-18
Also published as: GB9403131D0; GB2286613B

Abstract

The method of ground improvement comprises forcing one or more mandrels into the ground to be improved by applying the vibratory force having at least a generally vertical component of movement to the mandrel 10 until it reaches the predetermined depths thereafter removing the mandrel from the hole or holes which it has formed in the ground, filling the hole or holes with particulate material and, optionally, reintroducing the mandrel into the hole to compact and outwardly displace the particulate material prior to filling the hole again with further particulate material. <IMAGE>

Description

IMPROVEMENTS IN OR RELATING TO METHODS AND APPARATUS FOR IMPROVING THE CONDITION OF GROUND.

The present invention concerns improvements in or relating to a method and apparatus for improving the condition of ground.

A known method of improving the condition of ground which is not sufficiently consolidated to support a building thereon is to provide columns in the ground formed from discrete particles of stone, that is stone which is not bound together by cement and sand to form concrete.

A method of providing such stone columns which has been proposed and used with limited success has been to suspend a hole-forming tool above the ground where the column is to be formed, the tool including a vibrating mechanism including a vibrating hole-forming member oscillating in a substantially horizontal direction. The vibrating member enters the ground under the self weight of the vibrating tool, forming a hole by effectively compacting the loosely consolidated ground to more consolidated ground surrounding the hole. An elongated hole is formed by allowing the vibrating member to descend, under its self weight, downwards into the ground.

Two methods of providing the stone column in the hole formed by the above apparatus have been suggested.

The first is simply to remove the hole forming tool and fill the hole that it has formed with stone. A further method is to allow stone to fall into the hole as it is being formed, the hole forming member, to an extent, compacting the stone into the surrounding ground. After removal of the hole forming member the remaining hole is filled with stone.

The column, in combination with other columns formed at predetermined distances therefrom, is then used as a support for the building structure.

This prior method exhibits several disadvantages which have contributed to the fact that it is used only seldom in particular grounds conditions. It is relatively time consuming and consequently expensive, suitable only in limited applications governed by the ground condition and the type of building to be supported thereon and has been shown to provide only limited support.

In the construction of roads poor ground often requires improvement and this is currently achieved by excavating the poor ground replacing it with stones and compacting the stones by vibrating and/or dead weight rollers. Clearly this method is time consuming and expensive and involves transporting the poor quality soil away from the site.

It is a further object to the present invention of obviate and mitigate the problems associated with improving poor ground in the construction of roads.

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

According to the present invention there is provided a method of ground improvement comprising forcing a mandrel into ground to be improved by applying a vibratory force having at least a generally vertical component thereto until the mandrel reaches a predetermined depth, removing the mandrel from the hole in the ground thus formed and filling the hole with a particulate material.

Preferably a downwardly directed force is applied to the mandrel as it is forced into the ground.

Preferably the hole is initially only partially filled and the mandrel is re-introduced into the hole to compact and outwardly displace the particulate material prior to filling the whole with further particulate material. Several filling and compacting operations may be carried out.

Preferably the mandrel is of inverted frusto-conical shape.

Preferably the frequency of oscillation is 1500 cycles per minute with an amplitude of 25mm and a rated impact per blow in excess of 40 tonnes.

Preferably a plurality of mandrels are operated simultaneously in close proximity of each other.

Further according to the present invention there is provided an apparatus for improving ground conditions comprising a mandrel equipped with a vibrating assembly providing oscillation in the, in use, vertical mode.

Preferably load applying means are provided to apply a downwards component of force in addition to the vibratory force to the mandrel.

Preferably the mandrel is frusto-conical and has an end tip section formed with a cone angle greater than the cone angle of the remainder of the mandrel.

Preferably the vibrating assembly comprises twin vibrators.

Preferably the mandrel has a length of 3 m, an upper diameter of 600 mm and a lower diameter of 300 mm.

Preferably the mandrel includes an extension member from its upper end, said extension member passing through the vibrating assembly and being releasably clamped thereto.

Alternatively a mandrel for a different use has a length of 0.75 m, an upper diameter of 500 mm and a lower diameter of 300 mm.

In an alternative arrangment a plate member mounts a plurality of downwardly projecting frusto-conical members, the plate member being connected to the vibrating assembly such that the plurality of members are vibrated simultaneously.

Alternatively the plate member can carry on its underside a single frusto-conical member having a length of approximately 0.6 m.

Further according to the present invention there is provided a structure for improving the ground comprising a hole preformed in the ground by a soil displacement method, particulate material in said hole and the soil surrounding said hole, said materials having been consolidated by applying a vibrating force thereto, the force having at least a vertical component of movement.

An embodiment of the present invention will now be described by way of example only with reference to the accompanying drawings, in which: Fig. 1. shows a mandrel for preforming a hole in ground to be improved; Fig. 2. shows a cross section through ground to be improved at a first and subsequent stage of a ground improvement operation; Fig. 3. shows a modified mandrel; and Figs. 4 and 5 show further modified mandrels.

Fig. 1. shows a steel mandrel 10 of an inverted frusto conical shape having an upper diameter of 600 mm a lower diameter of 300 mm and a length of approximately 3 m. The mandrel 10 has a steeply angled frusto conical tip 12 at its lower end and is attached to a twin vibrator 14 which provides, in the in-use position of the mandrel as shown in Fig. 1., a vertically oscillating driving force of 1500 cycles per minutes with an amplitude of 25 mm and the rated impact per blow in excess of 40 tonnes.

Means are provided for applying and additional downwardly directed load or crowd L to the mandrel.

These load applying means can take any convenient form, for example, dead weights or hydraulically or mechanically applied loads.

The mandrel is placed on ground, the condition of which is to be improved so that it can act as a support for a building structure,the deadweight is applied to the mandrel and the vibrator 14 is operated such that the mandrel 10 is driven into the ground, normally to its full depth but possibly to any predetermined depth less than its full depth. As it moves into the ground the mandrel displaces soil outwardly, this displacement giving a first improvement as the displaced ground consolidates itself with the surrounding ground.

When the mandrel 10 has been driven to the predetermined depth it is lifted from the hole 16 it has formed, this operation not being hindered by stiction as the vertical movement of the mandrel under the action of the vibrator 14, keeps it free of the hole sides. When the mandrel is withdrawn, stone having a suitable predetermined particle size is introduced into the hole to fill it to a third to a half of its depth. The mandrel 10 is then re-introduced into the hole 16 and the vibrator 14 set in operation. The action of the mandrel on the loosely compacted stone compacts it and furthermore forces it outwardly into the surrounding consolidated ground 18 (Fig.2) to further improve this ground.

The operation of partially filling the hole with stone and thereafter applying the vibrating mandrel 10 thereto is repeated one or more times until the stone filling reaches ground level.

The invention thus provides an improved region of ground on which a building structure can be supported.

In tests the bearing capacity of the improved ground, depending upon the characteristics of the original ground, varies from 30 to 150 Kn/m.

The application of load L, or crowd improves the consolidation operation by ensuring that ground resistance to mandrel penetration due to elastic rebound from the ground is overcome.

The method is then repeated at predetermined regions over the building site to provide sufficient regions of improved ground to support the entire building structure.

Prior to placing a structure on the improved regions plate bearing tests can be undertaken to verify the capacity of the system.

Various modifications can be made without departing from the scope of the invention, for example, the circular cross-section mandrel 10 described above could be replaced by a rectangular or any other suitable cross-section. The twin vibrators 14 could be replaced by alternative means for providing oscillation but it is important that the oscillation is applied in the ,in use, vertical direction.

In the modification shown in Fig. 3. an extension member 11 in the form of a tube is fixed to the upper end of the mandrel. The member 11 passes through the twin vibrator 14 which is fitted with clamp means 13.

The mandrel is fitted with an oversized ring or overbreaker 15 at its upper end.

The operation of this modification is similar to that described above with reference to Fig. 1. but the mandrel is forced into the ground to a depth greater than its own depth. After a first driving operation under the action of the vibrating force and crowd L the clamp 13 holding the vibrator 14 to the extension member 11 are released and the vibrator is raised up the member 11 to an elevated position where it is again clamped to the member so that vibration can recommence and the mandrel can be driven further into, the ground. This operation can be repeated several times until a hole of the desired depth is achieved after which the stone filling and consolidating operations can take place.

The overbreaker 15 ensures that the diameter of the hole behind the mandrel is greater than the maximum diameter of the mandrel thus facilitating extraction.

In a further modification a differently sized mandrel is utilised in the same manner to consolidate ground on which a road, car park, pavement or slab is to be laid. In the past, when the underlying ground is not sufficiently good to support the road, it is often removed completely by excavation and replaced by a hard core infill. Obviously this is time consuming and expensive and gives rise to the additional problem of disposal of the removed soil.

In a modification a mandrel similar to the mandrel 10 described with reference to Figs. 1 and 2 but of maximum length 6 m, top diameter 500 mm and bottom diameter 300 mm is used, the holes to be filled by stone being formed at closely spaced intervals.

In a further modification shown in Fig. 3 four similar inverted frusto-conical mandrels 20 are fixed to a plate 22 which measures 2 m square. Preferably the length of each mandrel 20 is no greater than 1 m and conveniently 0.6 m. The plate 22 is mounted on a support structure 24 which carries a vibrator pack 26 and the entire mandrel/vibrator assembly can be carried on the end of an excavator boom 28 which can apply the crowd L.

In operation the composite mandrel illustrated in Fig. 3 is operated in a manner similar to the single mandrel illustrated in Fig. 1, although in certain instances a stone layer can be spread on the ground to be improved before the first hole forming operation.

Fig. 4 shows a further modification where a single mandrel 30 is mounted on a plate 32 which has a support structure 34 and vibrator 36 similar to those described with reference to Fig. 3.

It has been calculated that the improved ground conditions for a road or the like provided by the system disclosed in Figs 3 and 4, will be considerably more economical than traditional presently-employed systems.

Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.

Claims

Claims:

1. A method of ground improvement comprising forcing a mandrel into ground to be improved by applying a vibratory force having at least a generally vertical component thereto until the mandrel reaches a predetermined depth, removing the mandrel from the hole in the ground thus formed and filling the hole with a particulate material.

2. A method as claimed in claim 1, in which a downwardly directed force is applied to the mandrel as it is forced into the ground.

3. A method as claimed in claim 1 and claim 2, in which the hole is initially only partially filled and the mandrel is re-introduced into the hole to compact and outwardly displace the particulate material prior to filling the hole with further particulate material.

4. A method as claimed in claim 3, in which several filling and compacting operations are carried out.

5. A method as claimed in any one of the preceding claims, in which the mandrel is of inverted frusto-conical shape.

6. A method as claimed in any one of the preceding claims, in which the frequency of oscillation is 1500 cycles per minute with an amplitude of 25mm and a rated impact per blow in excess of 40 tonnes.

7. A method as claimed in any one of the preceding claims, in which a plurality of mandrels are operated simultaneously in close proximity to each other.

8. Apparatus for improving ground conditions comprising a mandrel equipped with a vibrating assembly providing oscillation in the, in use, vertical mode.

9. Apparatus as claimed in claim 8, in which load applying means are provided to apply a downwards component of force in addition to the vibratory force to the mandrel.

10. Apparatus as claimed in claim 8 or claim 9, in which the mandrel is frusto-conical and has an end tip section formed with a cone angle greater than the cone angle of the remainder of the mandrel.

11. Apparatus as claimed in any one of claims 8 to 10, in which the vibrating assembly comprises twin vibrators.

12. Apparatus as claimed in any one of claims 8 to 11, in which the mandrel has a length of 3 m, an upper diamete of 600 mm and a lower diameter of 300 mm.

13. Apparatus as claimed in any one of claims 8 to 12, in which the mandrel includes an extension member from its upper end, said extension member passing through the vibrating assembly and being releasably clamped thereto.

14. Apparatus as claimed in any one of claims 8 to 12, in which the mandrel has a length of 0.75 m, an upper diameter of 500 mm and a lower diameter of 300 mm.

15. Apparatus as claimed in any one of claims 8 to 11, in which a plate member mounts a plurality of downwardly projecting frusto-conical members, the plate member being connected to the vibrating assembly such that the plurality of members are vibrated simultaneously.

16. Apparatus as claimed in any one of claims 8 to 11, in which a plate member mounts a single frusto-conical member having a length of approximately 0.6 m.

17. A Structure for improving the ground comprising a hole preformed in the ground by a soil displacement method, particulate material in said hole and the soil surrounding said hole, said materials having been consolidated by applying a vibrating force thereto, the force having at least a vertical component of movement.

18. A method of ground improvement, substantially as hereinbefore described with reference to Figs. 1 and 2 or any one of Figs. 3 to 5 of the accompanying drawings.

19. Apparatus for improving ground, substantially as hereinbefore described with reference to Figs. 1 and 2 or any one of Figs. 3 to 5 of the accompanying drawings.

20. A structure for improving the ground, substantially as hereinbefore described with reference to Fig. 2 of the accompanying drawings.

21. Any novel subject matter or combination including novel subject matter herein disclosed, whether or not within the scope of or relating to the same invention as any of the preceding claims.