EP0382608B1 - Method of making an excavation by the trench wall technique involving the use of releasably prestressed floor elements - Google Patents

Description

The invention relates to a method of carrying out a search using the diaphragm wall technique using releasable prestressed floor elements.

The diaphragm wall technique has become the reference method for carrying out excavations extending below the level of the water table when the environmental conditions do not allow the water table to be lowered.

Briefly carrying out a excavation by this technique consists in girding the future excavation of a wall molded in the ground carried out in a conventional manner, then in proceeding to the excavation of the excavation by putting in place, as and when the excavation progresses, anchor rods intended to retain the wall against the thrust exerted on it by the ground and the water table which are on the other side of the diaphragm wall.

However, we often encounter situations where the establishment of anchor rods is not possible or too expensive, for example when the surrounding soil lacks sufficient cohesion to be able to anchor a tie rod securely. In this case, we then replace the installation of anchor rods, the installation of floors at various levels of the excavation. This solution, as described for example in FR-A-1 450 745 is, however, not always very satisfactory because it has been observed that, in particular in the case of excavations of relatively large surface, the diaphragm wall is subjected to significant bending moments, which can reach, for example, 150-200 tonne-meters, which make it necessary to use very thick diaphragm walls, hence a significant cost. Despite this, it has been observed that the diaphragm wall could undergo significant local deformations, up to, for example, 4-5 centimeters, which are not tolerable.

The present invention aims to provide an improved method of carrying out a search by the technique of the diaphragm wall in the ground, which makes it possible to minimize the deformations and the bending moments undergone by the diaphragm wall, and consequently , reduce the thickness of the diaphragm wall and therefore the cost.

More particularly, the present invention relates to a method of carrying out a search by the technique of the wall molded into the ground, according to which a wall molded into the ground is produced which surrounds the search to be carried out, then the search is carried out by places floors at various levels of the excavation, playing the role of supports for said wall, as is known from the aforementioned patent FR-A-1450745, characterized in that one places between at least two wall walls opposite, floors made up of prestressed floor elements and in that after said installation, all or part of the prestress applied to said elements is released so that said elements expand and exert a counter-thrust on the molded wall opposite to the thrust exerted on said wall by the soil and / or the surrounding water table.

The method of the invention takes advantage of the fact that the prestressed floor elements, when all or part of their prestress is released, undergo an elongation or expansion which generates on the molded wall a thrust antagonistic to that exerted on the other side of the wall through the soil and / or surrounding water.

The counter-pressure exerted by the floor elements does not need to balance the pressure of the soil and / or the surrounding water and will usually be less than the latter.

The positioning of the prestressed floor elements will depend on the importance of the thrusts exerted by the surrounding soil on the molded wall, and therefore on the dimensions of the excavation and the nature of said soil.

In certain cases, it will be possible to set up the prestressed elements so that their prestressing directions are parallel to each other and this on the various levels of floors put in place. In other cases, it will be necessary to set up, at certain levels, prestressed elements whose direction of prestressing is perpendicular to that of the floor elements of other levels. In all cases that can be encountered in practice, the skilled person can easily determine the optimal orientation and positioning of the prestressed floors necessary to obtain the desired result, the process of the invention easily lending itself to modeling on computer.

According to a particular embodiment, the prestressed concrete floor elements used in the method of the invention comprise prestressed tension members whose ends are embedded in the concrete and whose central part is coated with a sheath with the application of a lubricating agent between said central part and said sheath, lights being provided in the concrete to allow the cutting of said central part and, thus, the relaxation of the prestressing tension.

It should be noted that in addition to the prestressing members (for example steel strands), each floor element may contain reinforcements within it. intended to reinforce it.

• Les figures 1 et 2 sont des vues schématiques en plan et en coupe transversale, respectivement, d'un élément de plancher précontraint utilisé dans l'invention;
• La figure 3 est une vue agrandie de la partie cerclée de la figure 2 ; et
• Les figures 4 et 5 sont des vues schématiques en plan et en coupe transversale selon la ligne V-V de la figure 4, respectivement, d'une fouille réalisée selon le procédé de l'invention.

The following description made with reference to the accompanying drawings will make the invention easier to understand.

• Figures 1 and 2 are schematic plan and cross-sectional views, respectively, of a prestressed floor element used in the invention;
• Figure 3 is an enlarged view of the circled portion of Figure 2; and
• Figures 4 and 5 are schematic plan views and in cross section along the line VV of Figure 4, respectively, of a search carried out according to the method of the invention.

Figures 1, 2 and 3 show a prestressed concrete floor element used in the invention. This element 1 has in plan a generally rectangular shape and, in cross section, the shape of a T. A plurality of metallic strands of prestressing 2, extending parallel to the longitudinal direction of the element over the entire length of that here, are embedded in the element, and distributed both in the wings of the T and in the foot of the T. Each strand comprises a central part 3 coated with a sheath 4 with the interposition of a lubricant (grease for example) between the strand and the sheath, and bare end portions 5 (free of sheath and lubricant). Lights 6 provided on the lower part of the element give locally access to each of the strands, on its central part, as illustrated in FIG. 2 and, in an enlarged manner, FIG. 3. Finally, the element 1 comprises reinforcing metal reinforcements 7. As an indication, each element can have a length of 15 meters, a total width of 2.50 meters (formed of two wings of a length of 1 meter each and a thickness of 0.20 meter and a central part (foot of the T) with a width of 0.50 meters and a thickness of 0.60 meters). The strands (16 in number in the embodiment shown) can be embedded in the concrete at a distance of about 5 cm from the lower surface of the wings or lateral surfaces of the foot of the T, each strand being prestressed at a rate of 10 tonnes (ie a total prestress of 160 tonnes per floor element). The stripped end portions of each strand may have, for example, a length of 0.80 meters.

The manufacture of such an element can easily be carried out by putting in place, in a mold of suitable shape, strands whose ends pass through and exceed the end faces of the mold, by applying to them the desired prestressing by means of conventional equipment. , by pouring the concrete into the mold, then after the concrete has hardened, removing the prestressing equipment and cutting off the protruding ends of the strands. The stripped ends, which are then immobilized in the concrete of the element, maintain the prestress initially applied to the central lubricated and sheathed part.

After installation of the prestressed floor elements, as described in more detail below, the prestress applied to a given strand can be easily released by locally "burning" said strand using, for example, the flame of a blowtorch, thanks to the light 6 giving access to this hole. The central part of each metal strand does not indeed adhere to the concrete of the element due to the presence of the lubricated sheath which surrounds it. You can release the prestress of all the strands or only a part of them, as desired, depending on the value of the counter-thrust that you want to apply to the diaphragm wall.

In Figures 4 and 5, there is illustrated the execution of a search in accordance with the invention. This excavation, 80 meters long, 60 meters wide and 23.75 meters deep, was carried out in soil made of clay (0 to -5 meters), sands (of -5 to -12 meters), bass (from -12 to -25.5 meters), and marl then, water (water table) being present from the coast - 1 meter to the layer of marl.

After completion, in a conventional manner, of a molded wall 10 in the ground surrounding the future excavation, having a thickness of 0.80 meters and extending to a depth of approximately 28 meters (in the marls), we put in place a plurality of posts 11 arranged in rows at intervals of 15 meters in the direction of the width of the excavation and 10 meters in the direction of the length of the excavation, then the excavation is excavated by placing, as and as the excavation progresses, floors made using prestressed elements according to the invention, at dimensions - 3.7; - 8.75; - 11.65; - 14.55; - 17.45 and - 20.35 meters, respectively.

Since the width (60 m) of the excavation greatly exceeds the practically achievable length (taking into account the production and transport constraints) of the prestressed elements, there are four elements, 15 m long, end-to-end to cover this width. As to cover the length (80 m) of the excavation, 32 elements (2.5 m wide) are required placed side by side, it would, in principle, be necessary to use 128 elements per floor level. In practice, however, it will be necessary to provide passages for circulation from one level to another and therefore, to omit a few elements per level to allow this circulation, as shown in 12. The elements of a given level are based on their ends on reinforced concrete beams 13 previously placed either on the diaphragm wall, or between two adjacent posts of two rows parallel to the width of the excavation, or between a post and the diaphragm wall, as the case may be. Once the elements are in place, the small spaces existing between the consecutive elements or between the elements and the molded wall are filled with concrete so as to form joints 14. After hardening of these joints, the prestress is released. elements in "Burning" the prestressing strands using a blowtorch, as explained above.

It was found that in the example of excavation described above, it was possible, with a molded concrete wall with a thickness of 0.8 m, to limit the maximum displacements of the wall, as a result of the thrust about 2 cm from the ground and the surrounding water, the maximum moments to which this wall was subjected being about 100 tonne-meter.

If the excavation was carried out in a conventional manner, with non-prestressed floor elements, the diaphragm wall would be subjected to much higher stresses, namely one of the maximum moments of the order of 100 tonnes-meter which would cause maximum displacements d 'about 4 cm, which is unacceptable in practice and would result in the obligation to make a thicker molded wall, of the order of 1 meter to reduce said displacement.

The method of the invention therefore makes it possible to achieve a saving of the order of 20% on the quantity of materials required for the construction of the diaphragm wall, which is equivalent to a cost saving which is also of the order of 20 %.

It goes without saying that the embodiment described is only an example and that it could be modified, in particular by substitution of technical equivalents, without departing from the scope of the invention as defined by the appended claims. .

Claims (4)

1. A method of making an excavation by the diaphragm wall technique, according to which a diaphragm wall (10) is constructed in the ground surrounding the excavation to be made, and then the excavation is made whilst putting platforms in place at various levels in the excavation, fulfilling the role of supports for the said wall, characterised in that, between at least two opposite walls, platforms (1) consisting of prestressed platform elements are put in place and in that, after the said putting in place, all or part of the prestressing applied to the said elements is released so that the said elements expand and exert on the diaphragm wall a counter-thrust opposite to the thrust exerted on the said wall by the ground and/or the surrounding ground water.

2. A method according to claim 1, characterised in that the directions of prestressing of all the platform elements are parallel to each other.

3. A method according to claim 1, characterised in that the direction of prestressing of the platform elements put in place at some levels is at right angles to the direction of prestressing of the platform elements put in place at other levels.

4. A method according to claim 1, characterised in that the platform elements comprise tension prestressing members (2), whose ends (5) are embedded in the concrete and whose middle part (3) is covered with a sheath (4) with the application of a lubricating agent between the said middle part and the said sheath, apertures (6) being provided in the concrete to allow the cutting of the said middle part and, thus, the release of the prestressing tension.

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