Patent Publication Number: US-9428879-B2

Title: Method for manufacturing a retaining wall from a rough soil-mix wall

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the field of fabricating retaining walls in the ground, and of retaining works made up of a plurality of retaining walls. 
     More particularly, the invention relates to a method of fabricating a retaining wall with the help of a technique of digging earth and mixing it up in situ with a binder (known as “soil-mixing”). In known manner, that technique consists in in situ mixing earth with a binder in order to constitute treated soil elements, presenting mechanical and/or geotechnical characteristics that are better than those of the natural non-treated earth. To do this, appropriate tooling is used, such as that described in document WO 2007/116178 or FR 2 879 632. 
     Such machines serve to make untreated walls in the ground, and more generally continuous structures of treated soil constituted by repeatedly passing the tooling in juxtaposed excavations. 
     It is also known to excavate a volume of ground adjacent to the untreated wall in order to uncover a surface of the untreated wall, after which said surface is in the open air. 
     The material that results from mixing the soil in situ with the binder presents the advantage of requiring less natural resources in order to be performed. 
     Nevertheless, since the resulting material is produced by mixing, it presents characteristics that are very different from those of concrete: the material is much less strong, it is very non-uniform, it presents micro pores, and it provides less protection than concrete for any reinforcement that might be embedded in the untreated wall. 
     A large difference between concrete and the material that results from the mixing (referred to as soil-mix) is its porosity. Whereas concrete has porosity lying in the range 5% to 15% by volume, a soil-mix material presents porosity lying in the range 30% to 65%. It is known that the greater the porosity of a material, the greater the rate at which said material is degraded. 
     When the soil-mix material is exposed to air, the above-mentioned micro-porosity, which is four to five times greater than that of concrete, leads to phenomena of drying and of carbonation that degrade the mechanical characteristics of the material. 
     That phenomenon is extremely troublesome for retaining wall applications, since the material can become degraded very quickly. 
     In order to solve that problem, it might be envisaged to provide a web of reinforced concrete having a thickness of several tens of centimeters on the exposed surface in order to improve the mechanical characteristics of the wall. Specifically, that way it would make it possible to withstand forces applied against the untreated wall while preventing degradation of the mechanical characteristics of the material constituting the untreated wall. 
     A drawback of that technique is that it increases the thickness of a retaining work considerably, thereby leading to a loss of available space inside the work. For example, when the retaining work is in an underground car park, it can be understood that the volume available for making the car park would be significantly reduced by the large thickness of the walls, which is undesirable. 
     Document JP 2001 032235 describes a similar method in which a layer of concrete is applied to the uncovered surface in order to prevent erosion to the banks of a canal. 
     OBJECT AND SUMMARY OF THE INVENTION 
     An object of the invention is to propose a method of fabricating a retaining wall from an untreated wall obtained using a technique of digging ground and mixing it in situ with a binder, in which the retaining wall can be incorporated in a retaining work, in particular in an underground car park, without significantly increasing the thickness of the untreated wall. 
     The invention achieves its object by the fact that the method of the invention comprises:
         a step of making an untreated wall in ground, comprising a step of digging a trench in the ground and a step of simultaneously mixing the ground in situ with a binder; and   a step of excavating a volume of ground adjacent to the untreated wall so as to uncover a surface of the untreated wall; the method being characterized in that it further comprises:   a step of treating the untreated wall during which a protective treatment substance comprising a polymer substance is applied to said surface in such a manner as to obtain a retaining wall of thickness that is substantially equal to the thickness of the untreated wall.       

     Thus, by means of the invention, after performing the method, a retaining wall is obtained of thickness that differs little from the thickness of the untreated wall. In other words, unlike using a web of concrete, applying the protective treatment substance does not significantly modify the thickness of the wall. According to the invention, the protective treatment product is for treating and protecting the excavated surface (which is uncovered and exposed to air) of the untreated wall that is made using a soil-binder mixture, the binder preferably comprising a cement. That is to say, the protective treatment substance makes it possible to ensure that the soil-binder material retains its mechanical and physico-chemical properties, even when said material is exposed to air. Thus, by means of the invention, it is not necessary to have a thick web of concrete for the function specifically of mitigating any potential decrease in the mechanical properties of the untreated wall, it being understood that the protective treatment substance does not set out to withstand the mechanical forces to which the retaining wall is subjected. 
     The polymer substance is advantageously a resin, preferably an epoxy resin. 
     A program of laboratory testing has shown that the following families of substances, when used with soil-binder mixtures, present effectiveness that is much greater than expected.
         substances that cover and/or plug the soil-binder material: these substances are preferably applied as a thin layer (resin of polyamine or epoxy type).       

     It was not obvious, a priori, to use those substances since some of them combine with the lime of the cement, but soil-binder materials contain less lime than does concrete, and consequently the reactions have greater difficulty taking place, and they are less effective. Also, they are known for requiring application onto a surface that is smooth and hard in order to obtain good grip, whereas the surface of a wall made of soil-binder material is generally crumbly and a difficult to make plane. 
     By overcoming prejudices, the inventors have found with surprise that the above-mentioned substances are effective. 
     According to the invention, the thickness of the retaining wall is no greater than 105% of the thickness of the untreated wall. 
     Conventionally, the thickness of an untreated wall made using a technique of digging earth and mixing it simultaneously in situ with a binder (soil-mixing) lies in the range 400 millimeters (mm) to 1000 mm. If the untreated wall presents a thickness of about 500 mm, then the thickness of the retaining wall is no greater than 525 mm. Also preferably, the thickness of the retaining wall is preferably no greater than 103% of the thickness of the untreated wall, i.e. no greater than 515 mm. 
     In an advantageous variant, the step of making the untreated wall also includes a step of inserting at least one stiffener element into the trench, with this being done before the untreated wall solidifies. By way of example, this stiffener element is a metal bar that is inserted in the trench while the soil-binder mixture is still fresh. It enables the untreated wall to be stiffened and reinforced. Preferably, a plurality of stiffener elements are put into place, e.g. serving to fasten floors when the retaining wall constitutes the retaining work of an underground car park. 
     Once the untreated wall has solidified sufficiently, the excavation step is performed. 
     Advantageously, a step is also performed of planing the surface of the untreated wall before the treatment step. This planing step serves in particular to clean and to flatten the surface. It may also serve to uncover said at least one stiffener element locally or completely, if such an element is present. 
     The planing may be performed mechanically and/or hydraulically. It may be followed by a step of high-pressure washing. 
     Preferably, the surface is planed in its thickness, i.e. in a direction that is perpendicular to the surface, possibly in such a manner as to uncover said at least one stiffener element. Also preferably, planing is performed to a depth of about 10 mm. 
     It can be understood that the planing of the untreated wall has the effect of locally reducing its thickness. Consequently, if the treatment substance is of the penetrating type, it can be understood that the thickness of the retaining wall may be slightly less than the thickness of the untreated wall. It will nevertheless remain substantially equal to the thickness of the untreated wall. The same applies if the layer of treatment substance is thinner than the depth of the planing. In both of these situations, the thickness of the retaining wall is preferably not less than 95% of the thickness of the untreated wall. Preferably, the thickness of the retaining wall thus lies in the range 95% to 105% of the thickness of the untreated wall. 
     In a first implementation, at the end of the treatment step, an outer covering layer is obtained on the surface having a thickness that is much less than the thickness of the untreated wall, such that the thickness of the retaining wall remains substantially equal to the thickness of the untreated wall. In this implementation, the protective treatment substance is thus of the covering type. In particular, it may be a resin, e.g. of epoxy type, or an emulsion. 
     Preferably, the thickness of the covering layer is less than or equal to 10 mm. 
     For example, the protective treatment substance is a sprayable sealing diaphragm. In a second implementation of the invention, the protective treatment substance used is a substance that penetrates into the surface of the untreated wall. This penetrating substance serves to treat a shallow depth of the surface of the untreated wall without increasing its thickness. It can thus be understood that the thickness of the retaining wall remains approximately equal to the thickness of the untreated wall. 
     Advantageously, the protective treatment substance is applied by brushing or by spraying. 
     The invention also provides a retaining wall obtained by performing the method of the invention, said wall further including a stiffener element, and presenting a surface covered in a polymer substance, e.g. an epoxy resin. 
     The invention also provides a retaining work, in particular in an underground car park, and comprising a plurality of retaining walls of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be better understood on reading the following description of implementations of the invention given as non-limiting examples, and with reference to the accompanying drawings, in which: 
         FIG. 1  shows the step of making the untreated wall in the ground by a technique of simultaneously digging the ground and mixing it in situ with a binder; 
         FIG. 2  shows stiffener elements being introduced into the untreated wall while the ground-binder mixture is still fresh; 
         FIG. 3  shows the untreated wall and one of its surfaces exposed to air after the excavation step; 
         FIG. 4  shows the untreated wall of  FIG. 3  after the planing step that serves to reveal the stiffener elements; 
         FIG. 5  is a plan view of the planed wall of  FIG. 4 ; 
         FIG. 6  shows the step of treating the surface of the planed wall; 
         FIG. 7  shows the retaining wall obtained at the end of the treatment step; 
         FIG. 8  is a plan view of the  FIG. 7  retaining wall obtained by performing a first implementation of the invention, the treatment substance use being a covering substance; and 
         FIG. 9  is a plan view of the  FIG. 7  retaining wall obtained by performing a second implementation of the invention, the treatment substance use being a penetrating substance. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows the first step of the method of the invention for fabricating a retaining wall: initially making an untreated wall  10  in the ground S with the help of suitable tooling  12 . As mentioned above, this untreated wall  10  is made using a soil-mixing technique, i.e. by digging the ground and simultaneously mixing it in situ with a binder injected into the trench. This untreated wall may be made using a method that is continuous or discontinuous. By way of example, it is possible to use the tooling described in document WO 2007/116178. In this example, the binder used is a cement. It is also possible to use a slurry, these types of binder being well known from elsewhere. The untreated wall  10 , constituted by a ground-binder (ground-cement) material, may present a shape that is circular, prismatic, or any other shape. 
     As can be seen in  FIG. 1 , the untreated wall  10  presents a height H, a length L, and a thickness e 1 . In this example, the untreated wall  12  presents a thickness of about 500 mm and a height H of about 11 meters (m). At this stage, the untreated wall  10  is surrounded by ground S. 
     While the ground-binder material is still fresh, a step is performed of introducing a plurality of stiffener elements  14  into the trench. During this step, three metal bars are inserted vertically side-by-side, the length of the bars in this example corresponding approximately to the height H of the untreated wall  10 . Inserting such stiffener elements need not be necessary, even though it is preferable in the presently-described example. The bars may be arranged at the edge of the trench on its excavation side, but that is not essential. 
     Once the untreated wall  10  has solidified sufficiently, an excavation step is performed, during which a volume of ground adjacent to the untreated wall  10  is removed so as to undercover a surface  16  of the untreated wall  10 . In this example, the excavation step is performed to a vertical depth P that is less than the height of the untreated wall, e.g. over about 6 m. 
     Thereafter, a step is performed of mechanically planing the thickness of that surface so as to uncover the stiffener elements  14  and so as to remove the ground-binder material that was in contact with the surrounding terrain S. This produces a planed untreated wall of thickness e 2 , it being emphasized that the planing is performed over the entire area of the excavated surface. As can be seen in the example of  FIGS. 4 and 5 , the planing is preferably, but not necessarily performed until the metal bars  14  are reached. In this example, the surface is planed to a depth e 3  of about 10 mm. The thickness e 2  is thus equal to about 490 mm. After the planing step, it is preferable, but not essential, to perform a cleaning step, e.g. using a high-pressure jet. 
     In accordance with the invention, a step is then performed of treating the planed untreated wall  10 , during which a protective treatment substance is applied to said surface  16 . As can be seen in  FIG. 6 , the treatment step may be performed by spraying the protective treatment substance onto the surface  16 . It may also be performed by brushing, when the treatment substance is a coating. 
     According to the invention, the protective treatment substance may either provide a thin-layer covering, or else it may impregnate the surface to be treated. 
     In the first implementation of the method of the invention, shown diagrammatically in  FIGS. 7 and 8 , the protective treatment substance used is a covering substance, such that at the end of the treatment step of  FIG. 6 , an outer covering layer  18  is obtained on the surface  12  with a thickness e 4  that is much less than the thickness e 1  of the untreated wall  10 . Specifically, the thickness e 4  of the outer covering layer  18  is about 8 mm. At the end of the treatment step, a retaining wall  20  is thus obtained having thickness e 5  that is about 498 mm. In other words, the thickness e 5  of the retaining wall  20  as obtained in this way is substantially equal to the thickness e 1  of the untreated wall  10 . 
     As a protective treatment substance, use is made of a polymer substance, e.g. an epoxy resin. It is also possible to use other types of coating or other types of the resin, in particular those selected from the above-described substances. 
     In a second implementation of the method of the invention, as shown diagrammatically in  FIG. 9 , the protective treatment substance used is a penetrating substance that impregnates the surface  16  of the wall for treatment. In this example, the penetrating treatment substance is applied by brushing. Consequently, applying this penetrating treatment product increases the thickness e 2  of the planed untreated wall very little, if at all. In this example, the thickness e 2  of the retaining wall  20 ′ as obtained in this way is about 490 mm, and thus a little less than the thickness e 1  of the untreated wall  10 . It follows that the thickness e 2  of the retaining wall  20 ′ lies in the range 95% to 105% of the thickness e 1  of the untreated wall. Once again, in accordance with the invention, the thickness of the retaining wall  20 ′ remains substantially equal to the thickness e 1  of the untreated wall  10 .