Ground improvement

Description

[0001] The present invention relates to ground improvement, particularly, but not exclusively, prior to building.

[0002] Document GB-A-2286613 describes a method of ground improvement which comprises forcing one or more mandrels into the ground to be improved by applying vibratory force having at least a generally vertical component of movement to the mandrel 10 until it reaches 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.

[0003] Document GB-A-2366819 describes a pillar formed by creating a vertical hole in the ground, and filling this with particulate material such as stone. A void is created at the top of the pillar by dropping a device onto the pillar. The void is filled with concrete.

[0004] 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, characterised in that the additional material is formed by mixing at least soil and a ground amelioration material, and in that the process is repeated until heave is detected in the ground around the said position.

[0005] In this specification, the term "ground amelioration material" 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 settable material such as a cementitious material. Preferably, the binder is cement. Alternatively, 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.

[0006] The ground amelioration material may include a cementitious binder such as cement. The additional material may further include quick lime.

[0007] 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 formation of the depression is measured to determine the load bearing capacity of the improved ground.

[0008] The lance member preferably enters the ground substantially vertically. Preferably the lance member is at least 2m long, preferably 3m long. The lance may taper at an angle of between 2° and 5°.

[0009] 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.

[0010] 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.

[0011] 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 material, such as cement and may also include a hydrophilic material, such as quick lime.

[0012] The lance 14 has an inverted frusto-conical 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 3m. 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°.

[0013] 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.

[0014] 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.

[0015] Furthermore, the lance 14 can be driven, by vibration, to any chosen depth without causing pore pressure problems.

[0016] Once the cavity 16 has been formed, the lance 14 is withdrawn. 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.

[0017] 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.

[0018] 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.

[0019] 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.

[0020] 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 its 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 settable 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.

[0021] 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.

[0022] 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 forming mechanisms. The depth to which the ground can be improved is virtually unlimited, with the advantages relating to pore pressure being retained.

[0023] 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.

[0024] 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 (14) is repeatedly vibrated into the ground (17) at substantially the same position, to leave on each occasion an elongate cavity (16) as the lance member is removed, and in which additional material (19) is introduced into the cavities so formed, characterised in that the additional material is formed by mixing at least soil and a ground amelioration material, and in that the process is repeated until heave (39) is detected in the ground around the said position.

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

3. A method according to claim 1 or 2, characterised in that the additional material (19) further- includes quick lime.

4. A method according to any preceding claim, characterised in that after the repeating process is finished, a depression (24) is formed in the ground (17), at the said position, the depression being filled with concrete to form a structural member (25) supported by the improved ground.

5. A method according to claim A, characterised in that the depression (24) is formed by vibrating a tapered member (20) into the improved ground.

6. A method according to claim 4 or 5, characterised in that the response of the ground (17) during the formation of the depression (24) is measured to determine the load bearing capacity of the improved ground.

7. A method according to any preceding claim, characterised in that the lance member (14) enters the ground (17) substantially vertically.

8. A method according to any preceding claim, characterised in that the lance member (14) is at least 2m long.

9. A method according to claim 8, characterised in that the lance (14) is at least 3m long.

10. A method according to any preceding claim, characterised in that the lance (14) tapers at an angle of between 2° and 5°.

Ansprüche

1. Verfahren zur Bodenverbesserung, bei welchem ein Lanzenglied (14) im wesentlichen an der selben Stelle wiederholt in den Boden (17) vibriert wird, um jedes Mal einen länglichen Hohlraum (16) zu hinterlassen, wenn das Lanzenglied entfernt wird, und bei welchem in die so gebildeten Hohlräume zusätzliches Material (19) eingeleitet wird, dadurch gekennzeichnet, dass das zusätzliche Material gebildet wird durch Vermischen von zumindest Erde und einem Bodenverbesserungs-Material, und dass der Vorgang wiederholt wird, bis ein eine Erhebung (39) in dem Boden um die Stelle herum detektiert wird.

2. Verfahren nach Anspruch 1, dadurch gekennzeichnet, dass das Bodenverbesserungs-Material einen zementartigen Binder wie z.B. Zement enthält.

3. Verfahren nach Anspruch 1 oder 2, dadurch gekennzeichnet, dass das zusätzliche Material (19) außerdem Branntkalk enthält.

4. Verfahren nach einem der vorhergehenden Ansprüche, dadurch gekennzeichnet, dass nach dem Beenden des wiederholten Vorgangs in dem Boden (17) an der Stelle eine Vertiefung (24) gebildet wird, wobei die Vertiefung mit Beton gefüllt wird, um ein durch den verbesserten Boden abgestütztes Strukturglied (25) zu bilden.

5. Verfahren nach Anspruch 4, dadurch gekennzeichnet, dass die Vertiefung (24) durch Vibrieren eines sich verjüngenden Glieds (20) in den verbesserten Boden gebildet wird.

6. Verfahren nach Anspruch 4 oder 5, dadurch gekennzeichnet, dass die Reaktion des Bodens (17) während des Bildens der Vertiefung (24) gemessen wird, um die Tragfähigkeit des verbesserten Bodens zu bestimmen.

7. Verfahren nach einem der vorhergehenden Ansprüche, dadurch gekennzeichnet, dass das Lanzenglied (14) im wesentlichen vertikal in den Boden (17) eintritt.

8. Verfahren nach einem der vorhergehenden Ansprüche, dadurch gekennzeichnet, dass das Lanzenglied (14) mindestens 2m lang ist.

9. Verfahren nach Anspruch 8, dadurch gekennzeichnet, dass die Lanze (14) mindestens 3m lang ist.

10. Verfahren nach einem der vorhergehenden Ansprüche, dadurch gekennzeichnet, dass sich die Lanze (14) mit einem Winkel von 2° bis 5° verjüngt.

Revendications

1. Procédé d'amélioration du sol, dans lequel on fait vibrer un élément formant lance (14) de manière répétée dans le sol (17) sensiblement à la même position, pour laisser à chaque fois une cavité allongée (16) quand l'élément formant lance est retiré, et dans lequel un matériau supplémentaire (19) est introduit dans les cavités ainsi formées, caractérisé en ce que le matériau supplémentaire est formé en mélangeant au moins de la terre et un matériau d'amélioration du sol, et en ce que le processus est répété jusqu'à ce qu'une boursouflure (39) soit détectée dans le sol autour de ladite position.

2. Procédé selon la revendication 1, caractérisé en ce que le matériau d'amélioration du sol comprend un liant cimentaire tel que du ciment.

3. Procédé selon la revendication 1 ou 2, caractérisé en ce que le matériau supplémentaire (19) comprend en outre de la chaux vive.

4. Procédé selon l'une quelconque des revendications précédentes, caractérisé en ce qu'une fois le processus de répétition terminé, une dépression (24) est formée dans le sol (17), à ladite position, la dépression étant remplie de béton pour former un élément structurel (25) supporté par le sol amélioré.

5. Procédé selon la revendication 4, caractérisé en ce que la dépression (24) est formée en faisant vibrer un élément effilé (20) dans le sol amélioré.

6. Procédé selon la revendication 4 ou 5, caractérisé en ce que la réponse du sol (17) durant la formation de la dépression (24) est mesurée pour déterminer la capacité de portance du sol amélioré.

7. Procédé selon l'une quelconque des revendications précédentes, caractérisé en ce que l'élément formant lance (14) pénètre dans le sol (17) de manière sensiblement verticale.

8. Procédé selon l'une quelconque des revendications précédentes, caractérisé en ce que l'élément formant lance (14) mesure au moins 2 m de long.

9. Procédé selon la revendication 8, caractérisé en ce que la lance (14) mesure au moins 3 m de long.

10. Procédé selon l'une quelconque des revendications précédentes, caractérisé en ce que la lance (14) est effilée selon un angle compris entre 2° et 5°.

Drawing