FR2970486A1 - Method for reinforcing foundation of electric pylon for high voltage electric lines, involves treating portion of soil located under slab by mixing soil with binder to form column portion having high cross section - Google Patents

Abstract

The method involves carrying out drilling of a borehole (20) traversing a soil volume (10) located on a slab (14) and thickness of the slab. A portion of the soil under the slab is treated by mixing the soil with a binder to form a column portion (50) having a cross section greater than that of the borehole. A filler material e.g. mortar, is injected in a part of the borehole after treating the portion of soil so as to form another column portion integrated with the former column portion. An independent claim is also included for a reinforcement structure for a foundation of a pylon.

Description

The present invention relates to a method for reinforcing the foundations of a tower, particularly but not exclusively an electrical tower for high voltage power lines, and a reinforcing structure of such a tower.

The present invention is particularly suitable for reinforcing the foundations of a pylon capable of being subjected to a compressive force. The present invention is particularly adapted to the reinforcement of existing foundations of an old pylon.

Most often, the foundations of a pylon consist of buried concrete massifs, formed of one or more horizontal slabs whose sections increase with depth and surmounted by a chimney in which are anchored fittings to which is fixed the base of the pylon.

At design, these foundations are dimensioned theoretically based on an estimate of predictable mechanical stresses to which the pylon will be subject, including the weight of cables and equipment to bear, snow, etc. In some cases, for example when changes in the operation of the foundations lead to an increase in loads (for example, an increase in the weight of the cables carried by the towers supporting power lines) or when a posteriori is found to under-dimension the foundations with respect to real loads, it is necessary to consolidate these foundations by increasing their bearing capacity with respect to compression. In this respect, a common method is to install micro-piles and to bond them with the existing foundation block using a reinforced concrete slab or beam. The patent application FR 2 881 448 gives an example of implementation of such micro-piles.

Micro-piles are generally sized to take back all the efforts alone. Indeed, their stiffness is in general much greater than that of the ground under the solid mass, so that one usually considers that they take again the totality of the load. In addition, the installation of micro-piles is a heavy intervention, the high cost of which is often disproportionate when the bearing capacity deficit of the existing foundations is not significant.

A first object of the present invention is therefore to provide a method for reinforcing the foundations of a pylon, which is simple to implement, and whose cost is not disproportionate to the lift deficit to be compensated when this deficit n is not significant.

This goal is achieved by a method of reinforcing the foundations of a pylon, the foundations having at least one slab buried in a soil and having a vertical thickness, in which a drilling is made through a volume of soil on the slab and than the thickness of the slab, then performs a treatment of a portion of soil located under the slab by mixing the soil in place with a binder reported to form a first column portion having a cross section greater than that of the drilling. The solution according to the invention makes it possible to reinforce existing foundations by associating them with a reinforcement structure intended to compensate for a lift deficit, the other efforts being taken up by said existing foundations. The reinforcing structure consists of one or more portions of columns made by one. technique well known per se under the name of "or mixing". This technique consists in reusing the soil cut in a borehole by mixing it with a binder, for example but not exclusively a grout, in particular a bentonite-cement grout, a mortar or a concrete, to obtain a rigid structure having the shape of said borehole . It is economically advantageous, since the soil in place is reused directly in the structure, without the need for extraction and pre-treatment. For this purpose, a drilling and kneading tool known elsewhere is used. This technique makes it possible to reduce the volume of materials to be transported on site, which reduces the costs related to materials and their transport. At the same time, the volume of the cuttings to be removed is also reduced, which speeds up the construction process and further reduces transport costs and constraints. The first column portion or portions thus produced in the ground constitute supports for the existing foundations, and provide two types of function. On the one hand, they improve the quality of the soil on which the existing foundations rest. On the other hand, they offer a certain resistance to the depression due to the friction with the soil which surrounds them.

Tests have shown that the mixed foundations thus produced can withstand an additional load of up to 300 kN. According to an advantageous example of implementation of the method according to the invention, the drilling is carried out so as to extend below the thickness of the slab to a depth strictly greater than the maximum depth of the slab, and the treatment of the soil portion is made at said depth, whereby the first column portion is formed away from the slab.

The layer of soil extending vertically between the lower face of the slab and the upper end of the first column, forms a "distribution" mattress that maintains a substantially homogeneous stress level on the underside of the slab ( and therefore the massif). This distribution layer or mattress makes it possible to reduce the risk of the appearance of hard spots at the intersection of the solid mass and the first portion or portions of the column. Such hard spots can appear when the soil treatment is carried out from the root of the massif (i.e. the maximum depth of the slab), and are to be avoided because they modify the intrinsic characteristics of the massif, and therefore its bearing capacity. According to another example of implementation, the drilling and the treatment of the portion of soil intended to form the first portion of the column, are carried out continuously, one after the other, with the aid of the same tool with a dual function of drilling and mixing.

In another example, the drilling is performed with a drilling tool that is removed from the borehole before the treatment of the soil portion. In another example, a casing is placed in the bore before the treatment of the portion of soil. The casing may in particular be introduced into the ground during the drilling operation, preferably simultaneously with the insertion of the drill bit. It avoids that the drilled soil, and especially the embankment located above the slabs of the massif, is weakened and loses its homogeneity during the transitional phases of drilling and treatment. Advantageously, the casing is introduced to a depth greater than the maximum depth of the slab, so that the portion of the borehole located at the level of the distribution pad defined above

(i.e. between the lower face of the slab and the upper end of the first column) is also protected from landslides. The casing may be removed from the well once the treatment of the soil portion has been completed.

According to one example, all or part of the borehole is filled with a filling material, after the treatment of the portion of soil, so as to avoid landslides and to keep unchanged the intrinsic characteristics of the soil after removal of the casing. The first column portion is then surmounted by a second column portion, extending from said first column portion, and having a cross section smaller than that of the first column portion. According to an exemplary embodiment, the filling material is sand, or earth, for example earth reserved during the drilling.

According to other exemplary embodiments, the filling material is a binder, such as a grout, in particular a bentonite-cement grout, a mortar, a concrete, or a mixture of any one of these elements with the ground in place, resulting for example from a "soli mixing" process. Preferably, the filling material comprises the same binder as that used for carrying out the treatment of the portion of soil. Thus, it is possible to perform one after the other, without changing tools, soil treatment operations and drilling infill. The present invention also relates to the use of the method defined above to strengthen the foundations of a pylon with four feet, each of the feet being integral with a slab fully buried in the ground. According to a second aspect, the invention also relates to a structure for reinforcing the foundations of a pylon, the foundations having at least one slab buried in a floor and having a vertical thickness, said structure comprising at least a first column portion disposed under the slab, away from the latter. In the present description, the first column portion is any rigid structure buried in the soil, resulting from the mixture of cuttings and a binder such as a grout, mortar or concrete. Such a

structure can result in particular from the implementation of a "soil mixing" process. According to an exemplary embodiment, the structure further comprises at least a second column portion, secured to the first column portion, having a cross section smaller than that of the first column portion, located above the first portion of the column. column and crossing the thickness of the slab. The second column portion may for example be made with a binder such as a grout, a mortar or a concrete. It can also be achieved with a mixture of any of these elements with the soil, resulting for example from a method of "soil mixing". It can also be made of coarse, non-binder material, such as earth or sand. The second column portion may also consist of several sections made with different materials selected from those mentioned above. Preferably, the first and second column portions form a continuous column. In particular, the first and second portions comprise the same binder. In another example, the first column portion, and, if applicable, the second column portion, are inclined relative to the vertical. In this way, it is possible to reach and reinforce ground areas located vertically of the pylon. Several examples of embodiments are described in this presentation. Unless otherwise specified, the features described in connection with any exemplary embodiment may be applied to another embodiment. Other features and advantages of the invention will become apparent on reading the following description of exemplary embodiments of the invention given by way of illustration and not limitation. This description refers to the accompanying drawing sheets in which: FIGS. 1A to 1F are cross-sectional views of the foundations of a pylon, illustrating various steps of the method of reinforcing these foundations according to an example of implementation of the present invention; Figure 1A shows in particular the realization of drilling and the simultaneous establishment of the casing inside the borehole;

Figure 1B illustrates the start of soil treatment under the foundation; Figure 1C illustrates a first column portion after the soil treatment is complete; - Figure ID illustrates the recovery of drilling and mixing equipment and the simultaneous extraction of the casing; FIG. 1E illustrates the filling of the borehole and thus the production of a second column portion, in the axis of the first column portion; Figure IF shows the foundations of the pylon once the reinforcement structure is completed; Figures 2 and 3 illustrate different possibilities provided by the method of the invention to enhance the compressive strength of the foundations of a quadruple pylon.

Referring first to FIGS. 1A to 1F, a first exemplary embodiment of the method for reinforcing the foundations of a tower, according to the present invention will be described. In the example, a quadrupole pylon 100 is shown as diagrammatically shown in plan view in FIGS. 2 and 3, the base of which rests on four buried concrete massifs each comprising a succession of several horizontal slabs whose sections, usually rectangular, square or circular, widening with depth, and a chimney above this succession of slabs.

In Figure 1A, there is shown the surface S of the natural ground and a foundation block 12 of a quadripode tower 100 as defined above. The foundation block 12, buried in the ground 10, here comprises the chimney 18 and two horizontal slabs 14, 16. Typically, the height H of the chimney is between 80 centimeters and 3 meters, the vertical and total thickness of the two horizontal slabs is between 20 centimeters and 2 meters and the maximum horizontal width L1 of the massif 12 (ie of the lower slab 14) is between 80 centimeters and 4 meters. As already explained, the method according to the invention consists in producing, in the ground under the horizontal slabs 14 and 16, one or

several portions of columns forming vertical or slightly oblique supports for the existing foundation foundation 12. It is therefore necessary to drill through the horizontal slabs 14 and 16 to access the soil zone located under the solid mass 12.

A first step of the method illustrated in FIG. 1A consists in making a bore 20 passing through the volume of backfill situated above the slabs 14, 16, the vertical thickness e 1 of these two slabs 14, 16 and a volume of soil located in below the lower slab 14. Preferably, the bore 20 has a diameter dl sufficiently small so that the solid 12 is not weakened, for example about 150 millimeters. The bore 20 may for example be made by means of a drilling and mixing tool 30 as diagrammatically shown in FIGS. 1A to 1E. This tool 30, which causes in its downward movement a casing 40, comprises a rotary shaft 32 terminated at its lower end by a drill bit 34 and retractable mixing elements 36 able to deploy once extracted from the casing 40. A tool of this type is described for example in the application FR 2 903 711. In the example, the bore 20 is made in a substantially vertical direction. As shown in FIG. 1B, it is stopped at a depth P2 greater than the maximum depth P1 of the solid mass 12. The introduction of a casing 40 inside the borehole 20 makes it possible to prevent the landslides from falling to the ground. interior of said borehole 20 and to keep intact the intrinsic characteristics of the soil 10 above and below the slabs 14 and 16. Once the drilling is complete, the lower end 40a of the casing 40 driven by the drill bit and 30 is at a vertical distance L2 from the lower face of the slab 14 (see Figure 1B). Typically, this distance L2 is between 20 and 50 centimeters.

The drilling and mixing tool 30 is then disengaged from the casing 40 and moved relatively to the casing so as to penetrate further into the ground. The casing 40 remains in place inside the borehole 20. When the lower end of the tool 30 begins to emerge from the lower end 40a of the casing 40, while being animated with a rotational movement around of the axis of the shaft 32, the arms 36

begin to deploy, for example under the action of springs (not shown), to achieve an extended position in which their wingspan is greater than the outer diameter of the casing 40. In a second step of the method shown in Figure 1B, a portion of soil located below the lower slab 14, in the extension of the bore 20, is treated by moving vertically the tool 30 whose mixing arms 36 are in the deployed position. This treatment is carried out by the technique known per se known as "soli mixing" in the Anglo-Saxon language, which consists in mixing the unstructured soil by the action of the rotary tool with a fluid, in particular a binder such as a grout. , a mortar or concrete, to modify the characteristics of the portion of soil treated. In the example, the treatment is carried out by rotating the shaft 32 of the drilling and mixing tool 30 and simultaneously injecting a binder into the soil that is destructured by the rotation of the arms 36. The soil mixture in place and binder forms a first column portion 50 of substantially constant diameter d2 equal to the span of the arms 36 of the drilling and mixing tool 30 in the deployed position, for example 400 millimeters.

Figure 1C illustrates the completion of the first column portion 50, which typically extends to a height of between 2 and 5 meters. As shown in FIG. 1D or 1E, the drilling and mixing tool 30 is raised before the binder is in contact with the ground. The casing 40 is also withdrawn from the borehole 20. During the ascent, binder is injected into the borehole to fill it and form a second column portion 52, integral with the first column portion 50 but having a cross section smaller than that of the first column portion. The second portion of the column is intended to prevent landslides in the borehole 20, once the intervention is completed (and the casing removed), and makes it possible to maintain a certain homogeneity of the soil and therefore of the forces applied to the massif 12. In this nonlimiting example, the borehole is filled with the same binder used for the soil treatment and the realization of the first column portion. The first and the second column

are thus performed one after the other, using the same tool, which optimizes the duration of implementation of the method. It will be noted that according to other exemplary embodiments, the drilling may be filled after removal of the drilling and mixing tool, with another material, less noble than the binder used for the treatment of soil, in particular soil or sand. Since the second column portion has no particular reinforcing function, it can indeed be made in loose materials that are not particularly intended to solidify.

It will further be noted that the second column portion is entirely optional, and only results from the need to carry out a preliminary drilling prior to the soil treatment, and the interest of resealing said drilling to prevent any landslides. Figure 1F illustrates the reinforcing structure resulting from the process according to the invention. This reinforcing structure comprises a first portion of column 50 located under the lower slab of the solid mass, extending vertically, and consisting of the hardened mixture of a binder such as a grout, a mortar or a concrete, with soil ( here the soil in situ 10).

The first column portion 50 forms an inclusion on which the foundation block 12 rests, and which has a certain bearing capacity related to the lateral friction developed at the interface with the adjacent ground. This bearing capacity is added to that, generally normalized, of the existing foundation block 12.

The first column portion 50 is surmounted by a second column portion 52, here made of cured binder, and integral with the first. The second column portion 52 extends from the upper end of the first column portion 50 to the surface S of the natural terrain 10. In the example, the first and second column portions 50, 52 form a continuous column 54. As can be seen in FIG. 1F, the reinforcing structure of a same foundation block 12 can comprise several columns. In the figure, there is shown, in addition to the column 54, a second column 54 ', inclined relative to the vertical, comprising a first column portion 50' forming a support and a second column portion 52 'surmounting this first portion of column 50 '.

Figures 2 and 3 show different examples of possible configurations for the reinforcing structure according to the invention. FIG. 2 illustrates, in plan view, four foundation blocks 12a, 12b, 12c, 12d comparable in structure to that illustrated in FIGS. 1A to 1D described above. Each of these masses 12a, 12b, 12c, 12d comprises two superimposed slabs 14, 16 with square sections and a chimney 18 on which is anchored one of the four feet of a quadripodal tower 100 with a horizontal square section shown schematically by a line continues in Figures 2 and 3.

In this example, the slabs 14, 16 of each bed 12a, 12b, 12c, 12d are arranged so that their sides extend in a direction forming an angle of 45 ° with each side face of the pylon 100. Each bed is reinforced by two columns 154a, 154b, 154c, 154d located respectively in line with the two corners of the upper slab 16 the most spaced pylon 100 (ie the two corners not at least partially covered by the pylon 100). FIG. 3 is a partial view, from above, of two foundation blocks 12e, 12f of a quadripod tower 100, each comprising two superposed slabs 14, 16 surmounted by a chimney 18, and whose sides are this time parallel. or orthogonal to each side face of the pylon. In the example illustrated on the left of Figure 3, the reinforcing structure comprises two columns 154e respectively extending to the right of the middle of the two sides of the upper slab 16 the most spaced from the pylon (ie the not covered sides at least partially by the pylon). In the example illustrated on the right of Figure 3, the reinforcing structure comprises three columns 154f respectively extending vertically from each corner of the upper slab 16 of the solid which is not covered by the pylon. Obviously, other configurations than those described above may be considered depending on the case. Typically, the reinforcing structure comprises between one and four supports per foundation, but it can understand more.

It consists of vertical or oblique columns, or a combination of vertical and oblique columns, as required. In

In particular, oblique columns may be placed in order to reach and reinforce ground zones located vertically under the base of the pylon. It is indeed essential that the base is not deteriorated by the reinforcement intervention.

Preferably, the mesh of the supports is regular, that is to say that it is substantially identical for each foundation of the same pylon. It will be noted that the method described above in connection with FIGS. 1A to IF is not limiting of the present invention.

Thus, especially when the thickness of the concrete slabs of the foundation is very important and a particular tool is necessary to cross them, drilling through the embankment and the horizontal slabs of the massif (and possibly a volume of soil under the solid) can be achieved during a pre-drilling step, by means of a tool exclusively for drilling, for example a tool of the tricone or cutting type which is more suitable for drilling concrete structure. Such pre-drilling is mainly for pre-drilling concrete slabs, to facilitate the implementation of the steps described above with reference to Figure 1A.

However, according to an exemplary embodiment, the casing can be lowered into the borehole at the pre-drilling stage. It will also be noted that, although in the examples described above, the diameters d1 and d2 of the borehole 20 and the supports 50 are substantially constant, the diameter of the first and / or second column portions may vary over part or all of their height. In addition, it will be noted that the process according to the invention can be carried out without casing, in particular when the soil is not very friable and the risks of landslide are controlled. In this case, the drilling and mixing tool is a controlled expansion tool, the span of which can vary between at least a first diameter adapted to the drilling step and at least a second diameter, larger, adapted to the treatment step. In this case in particular, filling the borehole at the end of treatment and making a second column portion may not be necessary.

Claims (16)

REVENDICATIONS1. Method for reinforcing the foundations of a pylon, the foundations comprising at least one slab (14, 16) embedded in a ground (10) and having a vertical thickness (e1), characterized in that: a drilling is carried out (20) traversing a volume of soil (10) on the slab and the thickness (el) of the slab (14, 16); then a treatment of a portion of soil under the slab (14, 16) is carried out by mixing the soil in place (10) with an attached binder so as to form a first column portion (50, 50 ') having a cross section greater than that of the borehole (20). 2. Reinforcing method according to claim 1, wherein the drilling (20) and the treatment of the soil portion are carried out continuously, one after the other, using the same tool. drilling and mixing (30). 3. Reinforcement method according to claim 1, wherein the drilling (20) is carried out with a drilling tool (30) which is removed from the borehole (20) before the treatment of the portion of soil. 25 4. Reinforcement method according to any one of claims 1 to 3, wherein the borehole (20) is made to extend below the thickness (el) of the slab (14, 16) to a depth (P2) strictly greater than the maximum depth (P1) of the slab (14, 16), and the treatment of the soil portion is carried out at said depth (P2), whereby the first portion of the column (50) , 50 ') is formed away from the slab (14, 16). The reinforcing method according to any one of claims 1 to 4, wherein a casing (40) is placed in the borehole (20) prior to performing the treatment of the soil portion. 6. reinforcement method according to claim 5, wherein the casing (40) is introduced to a depth (P2) greater than the maximum depth (P1) of the slab (14, 16). 7. A method of reinforcement according to claim 5 or 6, wherein the casing (40) is removed once the treatment of the portion of soil is completed. 8. A method of reinforcement according to any one of claims 1 to 7, wherein the borehole (20) is inclined relative to the vertical. 9. Reinforcement method according to any one of claims 1 to 8, wherein at least one filler is injected into all or part of the borehole (20) after the treatment of the portion of soil, so as to form a second column portion (52, 52 '), secured to the first column portion (50, 50'), so that the second column portion (52, 52 ') has a smaller cross-section than the first portion of the column portion; column (50, 50 '). The reinforcing method according to claim 9, wherein said filler material is the same binder as that used for treating the soil portion, and the first and second column portions (50, 50 ', 52, 52 ') are carried out continuously, one after the other. 11. Use of the method according to any one of claims 1 to 10 to strengthen the foundations of a pylon (100) having four feet, each of the feet being secured to a slab (14, 16) entirely buried in the ground ( 10) .35 12.Structure for reinforcing the foundations of a pylon (100), the foundations comprising at least one slab (14, 16) buried in a ground and having a vertical thickness (e1), characterized in that it comprises at least one first portion of column (50, 50 ') disposed under the slab (14, 16), at a distance from the latter. Reinforcing structure according to claim 12, wherein the first column portion (50, 50 ') is a rigid structure comprising a binder such as a grout, a mortar or a concrete. The reinforcing structure of claim 12 or 13, further comprising at least a second column portion (52, 521 extending from the first column portion (50, 501) having a smaller cross-section than the first portion column, located above the first column portion (50, 50 ') and through the thickness (e1) of the slab (14, 16). The reinforcing structure of claim 14, wherein the first and second column portions (50, 52, 50 ', 52') form a continuous column. Reinforcing structure according to any one of claims 12 to 15, wherein the first column portion (50, 50 ') is inclined with respect to the vertical.