GB2233688A - Constructing an open space beneath a structure - Google Patents

Abstract

A method of constructing an open space (6) beneath a structure comprises injecting a cement grout beneath walls (1a, 1b) to a depth greater than that of the open space (6) to be constructed, so as to form pillars (8) with an axis slightly inclined from the vertical; digging between the walls so as to clear away from round the tops of the pillars; making flush the tops of the pillars (8) projecting with respect to a vertical plane passing through the internal face of the walls, embedding metal rods (11) in the ground (3) exposed between the pillars so as to reinforce it; placing a reinforcement on the vertical walls and spraying concrete so as to form a peripheral shell (13). The sequences of digging, reinforcement of the earth and spraying of concrete are repeated until the desired lower level is reached. <IMAGE>

Description

Method of constructing an open space at basement level between existing buildings and the structure obtained The present invention relates to a method for constructing an open space at basement level, between existing buildings, as well as to the structure obtained. More particularly, it relates to the construction, between existing structures, of an open space at basement level, intended to be turned afterwards into a car park, offices, shops or otherwise.

When it is necessary to build such a structure, in the absence of any existing building above the position in which the works are to be carried out, the generally known technique used is that of the diaphragm wall, or the so-called "Berlin" technique, or again the one known as the "Paris" technique. These techniques consist of constructing a wall situated in front of the existing party walls and serving to support the peripheral loads of the floors at basement level and the thrust of the ground. Tie bolts are used as the digging progresses. All these techniques require the absence of buildings at ground level, lead to a reduction in commercially usable volume and do not provide true underpinning of the adjoining buildings with, in consequence, inevitable settlement.

Where the buildings on the site are preserved entirely or even in part, the current technique consists of constructing offset shafts or narrow shuttered trenches which go down to the level of the planned foundations. Then, building up through all the shoring, a wall is constructed designed to underpin the adjoining buildings or, more usually, for reasons of economy, a wall which will pass in front of the party wall and which will then serve at the same time to take the peripheral loads of the projected building. In accordance with this method, the party walls are generally not suitably underpinned and the earth is all the more decompressed by the excavating as the time periods inherent in this type of method are longer. The consequence of this is that settlement inevitably occurs.Moreover, the offset shafts technique is dangerous for the persons carrying it out and slow, since the earth is removed in buckets pulled up by ropes, whilst the positioning of the shuttering is tricky.

Finally, the time taken is long and uncertain, and the costs high.

The present invention remedies these disadvantages. It concerns a method for constructing an open space at basement level between existing buildings delimited by party walls, of the following type in which: - a cement grout is injected beneath the party walls to a depth greater than that of the space to be constructed, so as to form pillars with an axis slightly inclined from the vertical, against which the party walls are supported after setting; - digging is carried out between the party walls so as to clear away from round the tops of the pillars; - the tops of the pillars projecting with respect to a vertical plane passing through the internal face of the party walls is made flush; - metal rods are embedded in the ground exposed between the pillars so as to reinforce it;; - reinforcement is put in place on the vertical walls and concrete sprayed dry is utilised so as to form a peripheral shell.

Thus an open space is obtained, intended to be developed as a basement, generally beneath an existing building, whilst enabling the superstructure works to be continued and especially guaranteeing the absence of subsequent settlement for the adjacent buildings.

In accordance with one particular embodiment of the invention, when the space to be constructed is situated beneath an existing building, the supports for its load-bearing walls are transferred onto piles in order to take the loads from them, before proceeding with the injection of the cement grout which will form the pillars.

The invention also relates to a basement structure, which incorporates, in an extension of the party walls, a sprayed reinforced concrete wall overlapping concrete pillars on which the said party walls rest, and metal rods for reinforcing the ground situated outside the party walls.

Other characteristics and advantages will appear during the following description and with respect to the attached drawings, given solely by way of non-limiting example, in which: - Figure 1 is a side elevation view of the existing buildings before the method of the invention is implemented; - Figures 2 to 5 are views, also in elevation, of the same buildings, during the successive stages of the method of the invention.

As shown in Figure 1, existing buildings are delimited by two party walls la, Ib which enclose a building 2 which may optionally be preserved. In this case, the building 2 incorporates stanchions or supporting walls 2a, which rest, like the walls la, lb, on a piece of ground 3, and a floor 4 at ground floor level, surmounted by one or more upper floors 5. In accordance with the invention an open space 6 is constructed at basement level, in the ground 3 situated beneath the floor 4 and inside the extension of the party walls la, lb.

To this end, the seatings of the stanchions or load-bearing walls 2a are first transferred onto shafts or piles 7 so as to take the loads from them (Figure 2) and optionally a first excavation of the ground is carried out as far as a level 9a. This first excavation aims solely at clearing the approaches to the party walls by removing any existing bricks or cellars. Then a cement grout is injected under high pressure beneath the party walls la, lb, along an axis Y-Y slightly inclined from the vertical, which forces the earth away sideways with which it mixes to form a micro-concrete; injection is carried out rapidly and continuously, starting from a depth which is lower than the floor of the space 6 which is to be constructed, and proceeding upwards to reach the foundations of the walls la, lb.This method, known in itself by the name of "jet grouting", makes it possible thus to obtain pillars 8 the bases of which are situated approximately in line with the party walls and which are at an angle because of the need to clear the nozzle of the machine during injection. The number of pillars is determined according to the state of the party wall: the more dilapidated this is the more the pillars, which have a diameter between 600 and 800 mm, must be closed up, but their distance between centres is generally between 1.5 and 3 metres. The pillars 8 provide effective transverse reinforcement of the surrounding ground 3.

The general digging can then be carried out, commencing with the periphery, beneath the lowest existing or reconstructed floor or paving 4 (Figure 3). The purpose of the digging is to reach a lower level of ground 9b and clear away from round the upper part of the pillars 8 along each party wall la, lb to a sufficient height to be able to level off the said columns approximately in the vertical plane of the inside of the party walls la, lb. Thus any projection 10 with respect to this plane is cut back.

As shown in Figure 4, the surrounding ground laid bare by the digging is reinforced between the pillars 8, on each side of theadjacent buildings, by pins 11 of a known type, and preferably consisting of shaped metal rods of variable length, whose adherence in the ground is achieved by injecting a cement grout.

The length and meshing of the pins 11 are obtained by calculation depending on the nature of the ground, the depth and the loadings of the adjacent buildings. As this operation is very rapid, the decompression of the ground 3 is limited to the greatest possible extent. A double layer of weldmesh and steel rods can then be laid along the vertical walls, forming a reinforcement 12 for a layer of concrete 13 introduced by dry spraying. Thus a peripheral shell with a minimum thickness of 20 cm is produced, generally requiring only two passes, but the number of passes may be greater if the thickness of the shell is greater. This shell serves mainly to support the thrust of the earth and to bear the vertical peripheral loads in the final phase.

The sequences of digging, reinforcement of the ground and spraying of concrete are then repeated until the desired lower level is reached. More precisely, and as shown in Figure 5, each digging operation as far as a level 9b, 9c, 9d is immediately followed by an operation of reinforcing the ground 3 by means of metal rods 11 and then by putting in place the shell 13 of concrete sprayed onto the reinforcement 12, to the height made clear by the digging, before continuing the digging to the subsequent level and so on. The depth of each digging is determined according to the state of the party walls and the nature of the ground 3. It is generally between lm and 3m.

Finally, plastering is carried out immediately after the spraying of the last layer, making it possible thus to obtain directly a facing of the same type as a smooth, and therefore finished, rendering.

In this way, a level 14 is reached, corresponding to the floor of the structure to be built, a floor situated again between the pillars 8 which are sunk to a greater depth than this lower level and which are in fact extended until they reach a soil with a suitable bearing capacity. The space 6 thus constructed is developed as required, for example by the construction of concrete floors 15, 16.

The walls of the space 6 which are at right angles to the party walls la, lb are constructed in the same manners if necessary with the injection of cement grout to form pillars and then reinforcement of the ground by pinning and dry spraying of concrete, on the reinforcement 12. The presence of the pillars is justified only if there are also transverse party walls to be supported or if the walls of the space 6 must themselves serve as a support for the wall of a new structure.

In accordance with one variant embodiment, not shown, a first pass of dry sprayed concrete is carried out as soon as the pillars 8 have been cut back and before the ground 3 has been reinforced by means of the pins 11. This- spraying of concrete, performed immediately onto the walls cleared by the digging and without reinforcement, may be necessary if the ground is of very poor quality, whilst awaiting the implementation of the pinning.

In accordance with another variant embodiment, also not shown, the pillars 8 may be constructed with a larger diameter, made necessary for example when the party walls are too dilapidated.

The projections 10 on these pillars 8 then serve as a support for a shell of concrete constructed along the party walls la, lb, and they are afterwards made flush with the internal face of this concrete shell.

The floor 4 can be the former floor of the building to be renovated or a new floor, since it is possible to construct the open space 6 whilst enabling the superstructure works to be carried on normally. It is therefore possible to work simultaneously downwards and upwards from the ground floor.

The pillars 8 provide complete underpinning of the party walls, guaranteeing the absence of any subsequent settling in the adjacent buildings. The line of these peripheral pillars forms a physical barrier which prevents practically all movement of the ground towards the excavation during the digging.

Moreover, after being excavated, the ground is very rapidly stabilised by the pins 11; the speed of execution and the presence of these pins prevents any movement of the ground towards the excavation at this stage of the operation.

The prior construction of the pillars in accordance with the "jet grouting" method provides an immediate assumption of the vertical loads of the party walls and thereby enables the wall to be uncovered over long lengths.

There is therefore no longer any problem of finding it almost impossible to achieve correct overlapping of the reinforcement, as is always the case with the shuttered wall technique.

Because the peripheral shell is constructed from sprayed concrete, the reinforcement awaiting the floors, slopes, and shells has no need to be folded before pouring, then uncovered with a pneumatic hammer, and afterwards unfolded, as is the case when formwork is used, whence great progress from the point of view of accuracy of layout, quality of joints, construction times and costs. This same technique of injection of pillars and pinning makes it possible to construct the peripheral shell using large-sized elements, whilst the other techniques necessitate construction in panels of small dimensions thus multiplying the joining areas of the reinforcement and concreting, and it is known that in these areas, apart from the additional time required for their construction, it is practically impossible to ensure good quality of performance.

The peripheral shell constructed rapidly immediately after the positioning of the pins provides the final closing off of the excavation, contains the thrust of the ground, and assumes the vertical loads of the basement floors in the final phase. Its slight thickness results in a gain in the volume which can be used and which is therefore commercially usable, important for the Client.

The technique of dry spraying is particularly well suited to the problem. In fact, apart from the fact that it avoids formwork which is difficult to handle in foundations and achieves a considerable gain in time and costs, it makes it possible to put in place rapidly a concrete of very good quality and to fill in the parts which are out of line by reinforcing the surrounding ground.

Overall, the invention brings a speed of execution at all stages, which guarantees for the adjacent structures and during the works a level of safety and quality of construction which has never been achieved up to the present day.

Claims (11)

1. Method of constructing an open space at basement level between two existing buildings delimited by party walls wherein - a cement grout is injected beneath the party walls to a depth greater than that of the open space to be constructed, so as to form pillars with an axis slightly inclined from the vertical, against which the party walls are supported after setting; - digging is carried out between the party walls so as to clear away from round the tops of the pillars; - the tops of the pillars projecting with respect to a vertical plane passing through the internal face of the party walls are made flush; - metal rods are embedded in the ground exposed between the pillars so as to reinforce it; and - reinforcement is put in place on the vertical walls and concrete sprayed dry is utilised so as to form a peripheral shell.

2. Method as claimed in claim 1, wherein, when the space to be constructed is situated beneath an existing building, the seatings of its load-bearing walls are transferred onto piles so as to take the loads from them, before proceeding to the injection of the cement grout which will form the pillars.

3. Method as claimed in claim 1, wherein the projections on the pillars serving as a support for a shell of concrete constructed along the party walls are then made flush with the internal face of this concrete shell.

4. Method as claimed in claim 1, wherein a first dry spraying of concrete is immediately carried out on the walls laid bare by the digging, as soon as the pillars have been cut back.

5. Method as claimed in any one of claims 1 to 4, wherein the operations of digging, reinforcement of the ground by the rods and spraying of concrete on the walls are repeated until the level corresponding to the floor of the structure to be built is reached.

6. Method as claimed in claim 5, wherein each operation of digging at successive levels is immediately followed by operations of reinforcement of the ground and construction of the peripheral shell.

7. Method as claimed in claim 5, wherein the concrete shell sprayed on the reinforcement is constructed over the height cleared by the digging.

8. Method as claimed in any one of claims 1 to 7, wherein the walls of the space at right angles to the party walls are constructed, after reinforcement of the ground by pinning, by dry spraying of concrete onto the reinforcement.

9. A structure at basement level built- in accordance with the method as claimed in any one of claims 1 to 8, wherein it incorporates in the extension of the party walls a reinforced sprayed concrete wall, overlapping the concrete pillars on which the said party walls rest and metal rods for reinforcing the ground situated outside the party walls.

10. Method of constructing an open space at basement level between two existing buildings delimited by party walls substantially as hereinbefore described with reference to the accompanying drawings.

11. A structure at basement level substantially as hereinbefore described with reference to the accompanying drawings.