GB2391035A

GB2391035A - Formation of a foundation by compaction

Info

Publication number: GB2391035A
Application number: GB0316860A
Authority: GB
Inventors: Roger Alfred Bullivant
Original assignee: Roxbury Ltd
Current assignee: Roxbury Ltd
Priority date: 2002-07-18
Filing date: 2003-07-18
Publication date: 2004-01-28
Anticipated expiration: 2023-07-18
Also published as: EP1382750A1; EP1382750B1; GB2391036A; DE60328854D1; GB2391036B; GB0316860D0; GB0316861D0; EP1382751B1; GB2391035B; ES2331238T3; ES2331239T3; ATE440183T1; EP1382751A1; ATE440182T1; DE60328853D1

Abstract

A method of forming a foundation involves driving by vibration a lance (fig 1) into the ground to form an elongate cavity (16) which is filled with material (19) such as soil and quick lime. The material (19) is compacted using the lance (fig 1) which can result in bulging (38) of the cavity (16). The process is repeated until heave (39) is detected in the ground around the position being treated.

Description

239 1 035

I Ground Improvement I The present invention relates to ground improvement, particularly, but not exclusively, prior to building.

The present invention provides a method of ground improvement, in which a lance member is repeatedly vibrated into the ground at substantially the same position, to leave on each occasion an elongate cavity as the lance member is removed, and in which additional material is introduced into the cavities so formed, the additional material being formed by mixing at least soil and a ground amelioration material, and in which the process is repeated until heave is detected in the ground around the said position.

In this specification, the temm Ground amelioration materials refers to a

material which consists of, or includes, a binder capable of increasing cohesion in material such as soil. The binder may be a gettable material such as a cementitious material. Preferably, the binder is cement. Altematively, bitumen may be used as a binder. Ground amelioration material may also incorporate additional granular material such as pulverised fuel ash, and hydrophilic material such as quick lime, to assist in controlling pore pressure of the material and of the surrounding ground. The term "ameliorated soil" is used to refer to soil, preferably derived locally, into which ground amelioration material has been mixed. The ground amelioration material may include a cementitious binder such as cement. The additional material may further include quick lime.

Preferably, after the repeating process is finished, a depression is formed in the ground, at the said position, the depression being filled with concrete to form a structural member supported by the improved ground. The depression may be formed by vibrating a tapered member into the; improved ground.

Preferably the response of the ground during the fomation of the depression is

measured to determine the load bearing capacity of the improved ground.

The lance member preferably enters the ground substantially vertically.

Preferably the lance member is at least 2m long, preferably am long. The lance may taper at an angle of between 2 and 5 .

Examples of the present invention will now be described in more detail, by way of example only, and with reference to the accompanying drawings, in which: Fig. 1 is a lance used in the ground improvement method of the invention; Figs. 2 and 3 are intermediate and final stages in the method; Fig. 4 illustrates a testing technique used in conjunction with the method; and Fig. 5 is a partial view of a cap supported by ground improved by the method. The drawings illustrate ground being prepared in accordance with the method of the present invention. The method is primarily intended for bad ground which has low natural load bearing capacity. Bad ground may, for example, include peat. It will be understood by the skilled reader that in many practical situations, no clear delineation can be made between good ground and bad ground, there being a continuous spectrum of ground quality. Indeed, the term Marginal ground" is sometimes used to refer to ground which is between good and bad ground. Consequently, the choice of technique to be used will be made in accordance with the quality of the ground in its natural state, the nature of the load imposed by the building to be constructed and other factors.

Fig. 1 shows a lance member 14, for use in the method of the invention.

The lance 14 is driven into the ground by vibration, to form an elongate cavity 16 (Fig. 3) in the ground 17. The lance 14 is then withdrawn and the cavity 16 is filled with additional material which includes soil mixed with ground amelioration material. This may include a cementitious binder matenal, such as cement and may also include a hydrophilic material, such as quick lime.

The lance 14 has an inverted frustoonical shape. In one example, the lance 14 may have a diameter of about 300mm at its upper end and about 150mm at its lower end 14A, with a length of approximately am. The lance 14 is attached to extend down, substantially vertically, from a vibrator arrangement 18 which applies vertical oscillation to the lance 14 during use. For example, the vertical oscillation may have an amplitude of about 25mm and a frequency of 1500 cycles per minute, with a rated impact per blow in excess of 40 tonnes.

The cone angle of the lance 14 may be low, such as between 2 and 5 .

In consequence of the shape of the lance 14 and the use of vertical vibration, the lance 14 will be driven into the ground substantially without impact in a horizontal direction between the surface of the lance 14 and the sides of the cavity 16. That is, the main impact with the ground will be at the bottom of the cavity 16 as the lance 14 forces itself deeper into the ground.

Contact with the sides of the cavity 16 will consist primarily of a pressing action, causing ground material to be pressed gently sideways to form the cavity 16, without harsh impacts. Avoiding impacts in this manner results in the technique minimising shocks which can result in increased pore pressure in water-laden ground and, in extreme cases, can cause liquefaction of ground. If impacts are used to form depressions, they may cause significant and sudden increases in pore pressure, with consequential disturbance to the ground at some considerable distance from the point of working.

Furthermore, the lance 14 can be driven, by vibration, to any chosen depth without causing pore pressure problems.

Once the cavity 16 has been formed, the lance 14 is withdrawn.

I Additional material 19 is then introduced into the cavity. As has been described, the additional material includes a ground amelioration material, preferably cementitious material such as cement, mixed with soil. The soil is preferably derived locally. The additional material may also include a hydrophilic material, such as quick lime. Stone or other aggregate may also be included. This injection of additional material results in the intermediate situation illustrated in Fig. 2.

The presence of quick lime, which is highly hydrophilic, causes ground water to be drawn strongly from the surrounding ground 12, causing a significant reduction in local pore pressure. Consequently, the addition of quick lime is particularly preferred in locations at which excessive pore pressure is expected to be a problem. The reduction in pore pressure helps overcome any tendency of fine-grained soils to liquefy during the procedure.

The lance 14 is then reintroduced into the ground, at the same position, and vibrated down to cause compaction of the additional material, and possible bulging of the cavity 16, as shown at 38 (Fig. 3). The degree of bulging which occurs will depend on the size of downward compacting forces created by the vibration of the lance 14, and the natural capacity of the surrounding ground 34 to provide lateral containment of the additional material. The lance 14 is repeatedly vibrated in, removed and reintroduced after additional material has been introduced, until the cavity 16 is full of material which has been adequately compacted, with resultant bulging of the cavity.

Compaction of the additional material in the cavity 16 is continued until the ground 17 is seen to heave upwardly (indicated at 39) around the position of the cavity 16. This heave indicates that no further additional material can be accommodated in the ground, and no further compaction is possible.

It is then preferred to test the condition of the ground, which will have

improved by virtue of the additional material and the compaction. This is achieved, as illustrated in Fig. 4, by driving a test member 20 down onto the top of the additional material 19. The test member 20 is an inverted frusto-conical former, having a minimum diameter approximately the same as the diameter of the cavity 16, and flaring upwardly to a wider diameter at Ks upper surface. The test member 20 is driven downwardly, as indicated by the arrow 22, by vibration, causing compaction of the additional material, until the member 20 is fully embedded to the position indicated by broken lines 24. The test member 20 is then removed, leaving a cavity which is preferably filled with concrete, or other gettable material to form a cap 25 as illustrated in Fig. 5. The concrete cap 25 forms a structural member to which downward force can be transmitted from a structure, such as a building, to be formed at the surface. The cap 25 is, in tum, supported by the improved ground which has been produced by the repeated use of the lance 14.

The use of the test member 20 provides a form of dynamic test of the ground at the position of the cavity 16. If the vibration frequency and rated impact per blow applied to the test member 20 are both known, then the time taken for the test member 20 to become embedded to a particular depth is indicative of the load bearing capacity of the ground at that position. Naturally, the final load bearing property of the ground will be greater than the capacity tested in this way, because it will increase as the cementitious material sets, but it is envisaged that with appropriate safety factors included in calculations, the final capacity can be predicted from the measurements made in this way.

In the example described above, it can be seen that the use of vertical vibration to form the cavity, results in the pore pressure being controlled in the ground, and in particular, prevents sudden pore pressure increases which can arise from the impacts or from the use of horizontally vibrating hole fonning mechanisms. The depth to which the ground can be improved is virtually unlimited, with the advantages relating to pore pressure being retained.

Many variations and modifications can be made to the arrangements described above. In particular, the dimensions and relative dimensions and choice of materials can be widely varied according to the particular condition of the ground being treated and the load to be borne by the arrangements formed.

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

1. A method of ground improvement, in which a lance member is repeatedly vibrated into the ground at substantially the same position, to leave on each occasion an elongate cavity as the lance member is removed, and in which additional material is introduced into the cavities so formed, the additional material being formed by mixing at least soil and a ground amelioration material, and in which the process is repeated until heave is detected in the ground around the said position.

2. A method according to claim 1, wherein the ground amelioration material includes a cementitious binder such as cement.

3. A method according to claim 1 or 2, wherein the additional material further includes quick lime.

4. A method according to any preceding claim, wherein after the repeating process is finished, a depression is formed in the ground, at the said position, the depression being filled with concrete to form a structural member supported by the improved ground.

5. A method according to claim 4, wherein the depression is fommed by vibrating a tapered member into the improved ground.

6. A method according to claim 4 or 5, wherein the response of the ground during the fommation of the depression is measured to determine the load bearing capacity of the improved ground.

7. A method according to any preceding claim, wherein the lance member enters the ground substantially vertically.

8. A method according to any preceding claim, wherein the lance member is

at least 2m long.

9. A method according to claim 8, wherein the lance is at least am long

10. A method according to any preceding claim, wherein the lance tapers at an angle of between 2 and 5 .

11. A method substantially as described above, with reference to the accompanying drawings.

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