GB2232701A - Mini-piled retaining wall and a method for its construction - Google Patents

Abstract

A retaining wall is constructed using mini-piles having a nominal diameter between 75mm and 300mm, the mini-piles being formed in two substantially parallel rows. The members of the first row are structurally linked, for example, either by being formed contiguously or by forming grouted cores in spaces between the mini-piles. The first and second rows are consolidated into a unitary structure, for example either by being formed contiguously or by forming grouted cores in spaces between the first row and the second row.

Description

MINI-PILED RETAINING WALL AND A METHOD FOR ITS CONSTRUCTION This invention relates to retaining walls of the kind formed by sinking a plurality of piles which are vertical or near vertical, and to a method of construction thereof.

In modern building construction it frequently arises that there is a need to excavate the building site to a substantial depth, for example to provide for a deep basement, or other facility below ground level. For this purpose it is often necessary to provide a retaining wall which is effective to prevent adjacent material from entering the excavation site, both as excavation progresses and after completion of construction work. This need arises particularly in cities where during redevelopment, constrwntion work must be carried out close to existing structures, and in the redevelopment of dockland sites, where there may also be a need to provide protection against the hydrostatic pressure present in the ground, where the desired excavation site may be very close to a watercourse, or static body of water.

For this type of situation, a technique has been developed for building a retaining wall by sinking a plurality of contiguous piles so as to create a continuous retaining wall. The piles are conventionally of substantial size, for example about 500 mm in diameter or more, and the resulting wall thus has sufficient thickness to resist the shear and bending loads normally imposed on it.

These conventional piles are normally created by boring i.e. a temporary steel tube of appropriate diameter is sunk progressively into the ground and the soil in its interior is progressively removed.

When the tube has been sunk to its appropriate depth, it is filled with concrete and steel reinforcement, and the tube is then withdrawn.

This technique is satisfactory in many cases, but cannot be used in ground which contains substantial hard obstructions such as old brickwork, timber or reinforced concrete because the large diameter tube used for conventional piles is not in practice easily capable of penetrating such obstructions.

Often, there is no practical technique for removing such obstructions without encroaching on an adjacent site and/or excavating up to the edge and undermining existing foundations which may be below the water table. It can therefore happen that there is no cost-effective means for excavating certain sites below ground level, and the value of a prime building site can thus be considerably diminished.

The Applicant has evelved a very successful technique using micro-piles normally of between 133 mm and 280 mm diameter, known as "Pali Radice".

According to the Pali Radice technique as normally used, the mini-piles are bored using a purely rotary drilling rig, by rotating and feeding down a tubular steel casing which has a special cutting bit at its lower end. A drilling fluid (normally water) is circulated through the drill casing which thus cools the cutting bit and returns to the surface outside the casing, while flushing away the spoil. A grout tremmie tube is inserted to fill the bore with grout, and reinforcement is placed either before or after grouting.

This technique has been applied with great success for purposes of underpinning existing structures without undue disturbance to the surroundings, and for creating new piled foundations, especially through ground containing buried obstructions and/or complex geological formations. It would not, however, in its known form, be suitable for forming a retaining wall of any substantial height, because a row of contiguous micro-piles would not provide sufficient shear and bending strength. The present invention, however, enables use of a minipiling technique and the benefit of its advantages, for the purpose of constructing a retaining wall of substantial height, for example 5 or 10 metres, or more.

Accordingly, the present invention provides a structural mini-piled retaining wall comprising a first row of substantially vertical mini-piles, which are structurally linked together, and a second row of mini-piles, substantially parallel to the first tow, the first and second rows being consolidated into a unitary structure. This structure can be both earth retaining and load bearing.

The mini-piles of the first row may be contiguous or altertanively may be structurally linked together by means of grouted cores formed in spaces between the mini-piles.

As used herein the term "mini-pile" means a pile of a diameter between 75 mm and 300 mm.

Normally the mini-piles will have a diameter between 133 mm and 280 mm.

The second row may be formed contiguous with the first.

Alternatively the structure may be consolidated by means of grouted cores in spaces between the first row and the second row.

The second row may be spaced from the first row by a distance between about 0.7 and 3 times the nominal diameter of the piles forming the first or the second row. This space is to be measured as between the centre line of the pile rows.

There may be provided a plurality of near horizontal reinforcing ties, each grouted in place within a hole bored through the first row and sloping downwardly into a mini-pile of the second row so as more effectively to provide structural interaction between pile rows.

Each mini-pile preferably contains pile-reinforcing material.

The pile-reinforcing material extending along the length of each mini-pile, will normally be in the form of a steel tube.

Preferably the first row of mini-piles is nearer than the second to an excavation site and on its side facing the site it is provided with reinforcing material, e.g. in the form of steel mesh.

The reinforcing material may be bonded to the first row by facing material, for example, sprayed concrete.

According to another aspect of the invention, there.

is provided a method for the construction of a structural mini-piled retaining wall for an excavation site comprising the steps of constructing a first row of substantially vertical mini-piles which are structurally linked together and are located along an edge of the said site, constructing a second row of mini-piles substantially parallel to the first row, and consolidating the structure forming the first and second rows.

The mini-piles of the first row may be contiguous, or alternatively may be structurally linked together by means of grouted cores formed in spaces between the mini-piles.

The structure may be consolidated by forming the second row contiguous with the first.

Alternatively the structure may be consolidated by forming the second row spaced from the first, boring a grout injection hole in spaces between the first row and the second row, and injecting grout under pressure into the said holes, so as to displace and/or consolidate material present in the spaces.

After consolidating the structure, the site adjacent thereto is preferably excavated in lifts of, for example, up to 4 metres.

After excavation of each lift and before excavation of the next, a plurality of spaced holes are preferably bored through the exposed face of the first row and each hole sloping downwardly into the second row, and a near horizontal tie (e.g. of steel rod) is grouted into place in each said hole so as more effectively to tie each member of the second row into the remaining structure.

Preferably the first row of mini-piles is nearer to the excavation site than the second row, and reinforcing material such as steel mesh is bonded to the face of the first row by means of facing material.

The facing material may be sprayed on, and may for example be sprayed concrete.

The ends of the near horizontal ties may project beyond the first row and be bent over at the face of the first row, so as further to be bonded to the structure by the facing material.

The invention will now be described by way of example only with reference to the accompanying drawings, of which Figure 1 shows a diagrammatic sectional plan view on the line I-I of Figure 2, of a mini-piled retaining wall in accordance with the invention, Figure 2 is a diagrammatic sectional elevation on the line of II-II of Figure 1, and Figures 3-7 are diagrammatic sectional plan views of various alternative structures of mini-piled retaining wall in accordance with the invention.

As shown in Figures 1 and 2, a mini-piled retaining wall in accordance with the invention comprises a first row generally indicated as 1 of contiguous mini-piles 2. The row 1 is towards the front of the wall, i.e. towards the excavation site E. Each mini-pile 2 is nominally cylindrical having a nominal diameter of 220 mm as indicated at 3. The mini-piles 2 are spaced at 250 mm centres and are substantially vertical.

The piles are rendered contiguous by virtue of the spreading of grout beyond the nominal diameter into the zone 4 as indicated in Figure 1.

The retaining wall further comprises, behind the first row, a second row generally indicated as 5 of mini-piles 6 which are also of nominally cylindrical form as indicated at 7. In this example the mini-piles 6 are also of 220 mm nominal diameter, but this need not necessarily be the same as the diameter of the mini-piles 2. The minipiles 7 are spaced at 500 mm centres, and their actual extent is beyond their nominal diameter as indicated at 8, because the grout of which they are formed has spread into the surrounding area. The distance between pile rows, known as the lever arm is established from an analysis of the applied forces.

A grout consolidation core 9 joins each member 6 of the second row to two members 2 of the first row 1 so as to form the two rows into a unitary structure.

Each mini-pile is provided with reinforcemet in the form of a tubular steel mesh 10, and a steel tie 11 is grouted into place in a bore which extends between a pair of adjacent mini-piles 2, through the core material 9, into each mini-pile 6. The ties 11 slope downwardly from the first row to the second row at a small angle up to say 150 to facilitate grouting from the front. The ends of the ties are bent horizontally in front of the first row 1 as shown at 12 in Figure 1.

Reinforcement in the form of one or two layers of steel mesh 13 is provided in front of the row 1, and a facing of sprayed concrete 14 bonds the ends 12 of the ties and mesh 13 to the front face of the row 1.

A concrete capping beam 15 is provided as shown in Figure 2, into which the tubular reinforcement 10 of each mini-pile extends, to tie the mini-piles together at their upper ends. A low retaining wall 16 of conventional construction tops the capping beam.

The retaining wall shown in Figure 1 and 2 is constructed as follows.

Alternate bores are first formed for the mini-piles 2 of the front row adjacent to the excavation site E, to a depth 17 which is substantially greater than the depth 18 of the intended excavation (Figure 2). The bores are formed by a rotary drilling rig, by rotating and feeding down a temporary steel drill casing which has a special cutting bit at its lower end. A drilling fluid (e.g. water) is circulated through the casing so that it cools the cutting bit and returns outside the casing, transporting the drilling spoil.

During boring, successive short lengths of casing are coupled together. In granular soils, the borehole is fully lined throughout its depth so that no collapse can occur. In cohesive soils, augurs or drag-bits are used, whereas to penetrate hard strata or obstructions, tricone bits or downthe-hole hammers are used. Once the borehole is formed to the required depth the bore is flushed with clean water and sand/cement grout is pumped via a tremmie tube extending to the bottom of the bore so that any water present is displaced by the rising grout. When the whole borehole is filled with good quality grout, the reinforcement 10 is placed extending the full depth of the pile.

Alternatively, the reinforcement may bo placed prior to grouting, the grout tremmie tube being placed with the reinforcement.

When the reinforcement has been correctly placed, the temporary drill casing is slowly withdrawn, the level of the grout being maintained by re-tremmieing as each section of the case is withdrawn. Because the density of the fluid grout is greater than that of surrounding strata 19 normally encountered, it exerts a positive pressure against the wall of the borehole and spreads into the zone 4, to penetrate and at least partially cement the ground in this zone. By this action, necking of mini-piles is precluded, washing out of the grout cannot take place, and a high frictional bond between the pile and the ground is obtained.

When the piles first sunk have set sufficiently, further piles 2 are sunk in the spaces between the existing piles 2, so as to complete the first row 1. These piles are contiguous and with the zones 4 included, form a continuous, substantially vertical wall.

Next, the second row 5 of piles is sunk substantially vertically at a spacing of 400 mm (centre to centre) from the first row 1. The procedure is similar to that for the first alternate set of piles in the front row. The longitudinal spacing of the second row is in an integral relationship with that of the first row, in this example the ratio is 2:1, but other integral ratios can be chosen. Each pile of the second row can be placed so that its centre is aligned with the mid-point between adjacent piles of the first row.

Grout injection holes are now drilled in the spaces between the first row and each pile of the second row. These injection holes can be of relatively small diameter, e.g. 50 mm, sufficient to inject neat cement grout which permeates and consolidates the surrounding soil so as to provide consolidation core grouting 9 which creates an integral structure with the rows 1 and 5.

Excavation now commences in lifts of about 2.5 to 3 metres. At the first lift, the retaining wall 16 is constructed and the capping beam 15 is cast in situ. As excavation proceeds, bores, e.g. of 25 mm diameter, which slope downwardly at up to about 10 are drilled between adjacent piles 2 through the olidation grouting 9 into each pile 5 in the second row. The vertical spacing of these bores is wider in the upper parts of the wall than the lower parts, for example being graded from 1000 mm spacing to 500 mm spacing. The ties 11 are inserted from the face and grouted into the bores so as to provide a positive connection across the pile rows.

Following the installation of the ties 11, the projecting reinforcing bar is bent (or may be prebent) as at 12 to provide a connection into a concrete facing wall which is dry sprayed to a thickness of 250 mm (measured from the centre line of row 1) after placing of two layers of reinforcing mesh 13.

This procedure is repeated as each lift is excavated.

In the particular example described with reference to Figures 1 and 2, a minimum ultimate moment of resistance of the order of 660KN/mm run of wall can be expected, which is at least equivalent to that for a conventional contiguous piled retaining wall or diaphragm wall.

An alternative form of construction is illustrated in Figure 3. As shown therein a mini-piled retaining wall comprises a first row 21 of contiguous mini-piles 22 and a second row 23 of mini-piles 24. In this embodiment, the members of the second row are contiguous with adjacent members of the same row 23, and also are contiguous with and horizontally aligned with adjacent members of the first row 21. The mini-piles 22 and 24 are each vertical, of 200 mm nominal diameter, and spaced at 250 mm centres, and the centrelines of the rows 21 and 23 are also 250 mm apart. The piles are rendered contiguous by the speading of grout beyond their nominal diameter so as to consolidate the whole of the piled area 25 into a single unitary structure.

Each of the mini-piles 22, 24 is provided with reinforcement in the form of steel tube 26 and steel ties (not shown) similar to the ties 11 can be used to give a stronger bond between members of the row 21 with members of the row 23.

The method of construction is similar to that described with reference to Figures 1 and 2 in that alternate piles 21 are first sunk, followed by the remaining members of the row 22. The second row 24 is then constructed in a similar way, but because of the close proximity of the pil ee bot in n the same row and in the adjacent rows, no consolidation core grouting may be required. Soil is excavated in successive lifts from the area in front of the row 21 as previously described with reference to Figures 1 and 2, and a sprayed concrete face 27 may be applied. This face 27 may include reinforcement (not shown).

In Figure 4, the construction shown is similar to that of Figure 3, and like reference numerals denote like parts. In Figure 4 however, the minipiles 24 are off-set horizontally with respect to the mini-piles 22. The spacing between the rows 21, 23 is reduced in this case to 200 mm, all other dimensions remaining unchanged. The lever arm is thus reduced, but a more integrated structure is obtained.

In Figure 5 there is shown another alternative form of construction, in which a mini-piled retaining wall comprises a first row 31 of mini-piles 32 and a second row 33 of mini-piles 34 parellel to the first. Each row comprises mini-piles 32, 34 of 220 mm nominal diameter, containing reinforcement 35, and spaced at 500 mm centres. The centrelines of the rows 31, 33 are spaced apart by 250 mm.

When the mini-piles 32, 34 have set, grout bores 36 of nominal 220 mm diameter are drilled on the centrelines of both first and second rows, in the spaces between the mini-piles of each respective row. These bores 36 are jet-grouted, which gives a wider area of influence than conventional pressure grouting, and improved structural interaction. A waterproof structure equivalent to a diaphragm wall can thereby be obtained. The grout spreads into the area 37 so as to permeate the soil in this area, and to consolidate the entire structure. The jet grouted bores 36 need not necessarily be sunk to as great a depth as the mini-piles 32, 34. For example in a case where a less cohesive or more porous material overlies London Clay, the jet grouted bores 36 may extend only as far as the London Clay horizon.If desired, reinforcement e.g. in the form of a steel tube 38 may be placed in tet grouted bores. Excavation of the site and completion of the retaining wall structure are similar to that previously described.

Figure 6 shows a construction similar to that of Figure 5, but in which a single grout bore 36 of mm mm nominal diameter, located centrally, replaces each corresponding pair shown in Figure 5.

All other components and dimensions are as for Figure 5.

Figure 7 shows a construction similar to that of Figure 5, except that the mini-piles 32 and 34 in each row are spaced at 200 mm intervals.

From the foregoing description it will be clear to those skilled in the art that a wide variety of structures are possible in accordance with the invention. The particular design and construction used in any particular case will depend upon factors which can readily be appreciated by the skilled artisan on the basis of the foregoing, for example the height and strength of the wall, the watertightness required, available space and the nature of the terrain.

As will be clear to those skilled in the art, a mini-piled retaining wall in accordance with the invention can be constructed in obstruction-laden soil, in situations where no other method of construction would be economically practical.

This might be, for example, where breaking out of pre-existing foundations or other artificial or natural obstructions would be impossible owing to ajacent buildings or water, or where the ground contains fractured rock and boulders. In effect, the present invention incorporates such obstructions into the structure of the wall, and thus makes advantageous use of their presence.

An additional advantage of the present invention is the ability to construct a retaining wall in situations where limited headroom is available.

This is mainly because the structures in accordance with the invention are formed from mini-piles which require substantially smaller and lighter machinery than the conventional piles of a considerably larger diameter (e.g. 750 mm) which would otherwise be required.

Claims (25)

1. A structural mini-piled retaining wall comprising a first row of substantially vertical mini-piles, which are structurally linked together, and a second row of mini-piles substantially parallel to the first row, the first and second rows being consolidated into a unitary structure.

2. A retaining wall according to claim 1 wherein the mini-piles of the first row are contiguous.

3. A retaining wall according to claim 1 wherein the mini-piles of the first row are structurally linked together by means of grouted cores formed in spaces between the mini-piles.

4. A retaining wall according to any one preceding claim wherein the second row is formed contiguous with the first.

5. A retaining wall according to any one of claims 1 to 3 wherein the structure is consolidated by means of grouted cores in spaces between the first row and the second row.

6. A retaining wall according to claim 5 wherein the second row is spaced from the first row by a distance between 0.7 and 3 times the nominal diameter of the piles forming the first or the second row.

7. A retaining wall according to any one preceding claim wherein there are provided a plurality of reinforcing ties each grouted in place within a hole bored through the first row and sloping downwardly into a mini-pile of the second row.

8. A retaining wall according to any one preceding claim wherein each mini-pile contains pile reinforcing material.

9. A retaining wall according to claim 6 wherein the pile-reinforcing material is in the form of a steel rod or a steel tube.

10. A retaining wall according any one preceding claim wherein the first row of mini-piles is nearer than the second to an excavation site, and on its side facing the site is provided with steel mesh reinforcement.

11. A retaining wall according to claim 10 wherein the steel mesh reinforcement is bonded to the first row by facing material.

12. A retaining wall according to claim 11 wherein the facing material is concrete.

13. A retaining wall according to claim 11 or claim 12 wherein each reinforcing tie has an end which projects beyond the first row and is bent so as to key into the facing material.

14. A retaining wall substantially as hereinbefore described with reference to any of the accompanying drawings.

15. A method for the construction of a structural mini-piled retaining wall for an excavation site comprising the steps of constructing a first row of substantially vertical mini-piles, which are structurally linked together and are located along an edge of the said site, constructing a second row of mini-piles substantially parallel to the first row, and consolidating the structure comprising the first and second rows.

16. A method according to claim 15 wherein the mini-piles of the first row are contiguous.

17. A method according to claim 15 wherein the mini-piles of the first row are structurally linked together by means of grouted cores formed in spaces between the mini-piles.

18. A method according to any one of claims 15 to 17 wherein the structure is consolidated by forming the second row contiguous with the first.

19. A method according to claim 15 wherein the structure is consolidated by forming the second row spaced from the first, boring a grout injection hole in spaces between the first row and the second row, and injecting grout under pressure into the said holes so as to displace and/or consolidate material present in the spaces.

20. A method according to any one of claims 15 to 19 wherein the site is excavated in lifts of up to 4 metres after consolidating the structure.

21. A method according to claim 20 wherein after excavation of each lift and before excavation of the next, a plurality of holes are bored through the exposed face of the first row each hole sloping downwardly into a mini-pile of the second row, and a near-horizontal tie is grouted into place in each said hole.

22. A method according to any one of claims 15 to 21 wherein the first row of mini-piles is nearer to the excavation site than the second row, and steel mesh reinforcement is bonded to the face of the first row by means of facing material.

23. A method according to claim 22 wherein the facing material is concrete and is applied by spraying.

24. A method according to claim 22 or claim 23 wherein the ends of the near-horizontal ties project beyond the first row and are bent over at the face of the first row so as to be bonded to the structure by the facing material.

25. A method of constructing a structural minipiled retaining wall substantially as hereinbefore described.