The invention relates to a drilling tool comprising: two pairs of rotary drums in axial alignment on parallel axes; motor means for driving rotation of said drums; support means; and a support structure on which said drums are mounted to rotate and serving to connect said drums to the support means; said tool being characterized in that said motor means are mounted inside the drums; and in that said support structure comprises a plate forming bearings at its bottom ends for said drum; and a mounting pad for fastening to the support means, the top edge face of the plate connecting said pad to the bearing-forming means.

[0001]    This is a 371 national phase application of PCT/FR2007/051062 filed 4 Apr. 2007, claiming priority to French Patent Application No. FR 0651240 filed 6 Apr. 2006, the contents of which are incorporated herein by reference. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to a drilling tool, particularly but not exclusively for making walls in the soil as obtained by mixing the cut soil with an additional binder. 
       BACKGROUND OF THE INVENTION 
       [0003]    Soil mixing techniques whereby drilled ground is mixed in situ with a hydraulic binder are nowadays commonly used for improving substructures. The tools used generally employ special equipment resembling augers that are caused to rotate about a vertical axis. Those machines enable rectangular wall elements to be made by juxtaposing a plurality of augers, thereby requiring high-power machines to be used whenever the trench needs to reach depths greater than 10 meters (m). 
         [0004]    A new type of machine has been in existence for several years that makes it possible to make rectangular foundation elements out of soil cement, i.e. by mixing a hydraulic binder with the soil that has been dug so as to make a portion of a trench, while also mixing the mixture. This operation is referred in the present patent application by the term “digging a trench while mixing cuttings with another material”. 
         [0005]    Naturally, the mixture must be left in place in the trench that is being made so as to end up with a wall in the soil that results from the mixture of cut soil and hydraulic binder setting, which wall has its shape defined by the shape of the trench. 
         [0006]    A machine of this type is described for example in patent applications US 2005/0000123 and US 2004/0234345. 
         [0007]    That machine is constituted essentially by two pairs of cutters mounted on a support structure. Each pair of cutters is connected to a hydraulic motor. The motors are housed in a relatively bulky box located above the cutters. 
         [0008]    When the motor is mounted in a bulky box, the drawback presented by the machine consists in the box in which the motors are housed presenting a relatively large apparent area. The presence of this box of large dimensions interferes considerably with raising the tool after it has performed the mixing, since the box needs to “barge through” the mixed material constituted by soil cuttings and hydraulic binder. In some circumstances, while the machine is being raised, the presence of this box can lead to the machine becoming blocked in the panel filled with the mixture constituted by the drilling cuttings and the hydraulic binder. 
         [0009]    In the machine of that type, that is described in patent application US 2005/0229440, the two pairs of cutters are connected by a common transmission to a single motor that may be situated above the surface of the ground. The transmission is then complex and its efficiency mediocre. 
         [0010]    Furthermore, since the two pairs of cutters are driven by the same motor, all of the cutters rotate at the same speed. Unfortunately, it can sometimes be advantageous to be able to give each pair of cutters a different speed of rotation, in particular to correct departures from the vertical while digging the trench. In addition, the power from the motor is shared between the two pairs of cutters providing operation is normal. However, if one pair of cutters becomes blocked, then all of the power from the motor must be absorbed by the other pair of cutters. That requires the system to be dimensioned mechanically so as to be able to accommodate this situation. 
         [0011]    Excavator machines are also known for making trenches in the soil. Such machines are usually constituted by two pairs of rotary cutters mounted at the bottom end of a structure of large dimensions. The top end of the structure is secured to support means that are generally constituted by cables. 
         [0012]    In horizontal section, the structure of the machine is generally rectangular in shape with dimensions substantially equal to the overall dimensions of the pairs of cutters. Thus, the dimensions of the right section of the structure are substantially equal to the dimensions of the horizontal section of the portion of trench that the machine can dig as it moves downwards. 
         [0013]    Thus, the walls of the structure are substantially in contact with the walls of the portion of trench being dug, thereby ensuring that the machine is guided vertically in order to obtain a portion of trench that is likewise substantially vertical. 
         [0014]    In addition, the soil cut by the cutters is removed via a suction tube having its inlet disposed between the walls of the cutters beneath the structure. 
         [0015]    It is clear that such an excavator machine is totally incapable of mixing the cut soil with the hydraulic binder, so that the mixture is left in place in the portion of trench being dug in order to make the wall in the soil. 
         [0016]    Documents EP 0 262 050 and GB 1 430 617 describe such a machine. 
       SUMMARY OF THE INVENTION 
       [0017]    An object of the present invention is to provide a drilling tool of this type that avoids the two above-mentioned drawbacks. 
         [0018]    To achieve this object, the invention provides a drilling tool that comprises:
       two pairs of rotary drums in axial alignment on parallel axes, each drum being fitted with a cutter;   motor means for driving rotation of said drums;   support means; and   a support structure on which said drums are mounted to rotate and serving to connect said drums to the support means;       
 
         [0023]    said tool being characterized in that:
       said motor means are mounted inside the drums; and   said support structure comprises:
           a plate that is substantially orthogonal to the axes of rotation of the drums, the bottom ends of said plate forming bearings for said drums, said plate having constant thickness that is very small relative to the length of the axes of rotation of a pair of cutters; and   a mounting pad connected directly to the bottom end of said support means and fastened to the top end of the plate, the top edge face of the plate connecting said pad to the bearing-forming means having a special shape so that, in association with the small thickness of the plate, it is significantly easier to raise the tool when it is being used for digging a trench while mixing cuttings with another material.   
               
 
         [0028]    It will be understood, that since the motors driving the cutters are disposed inside the cutters, the tool does not have a box containing the motor or bulky transmission systems. Furthermore, each motor can be controlled independently to give each pair of cutters a different speed of rotation. Since there is no box above the cutters of the tool, it can be understood that raising the tool through the mixture of drilling cuttings and hydraulic binder is made considerably easier. This is made easier still by the particular shape of the support structure having only an edge that is in a position to oppose the drilling tool being raised, and this edge has dimensions that are small and a shape that is appropriate. 
         [0029]    Preferably, the motors are hydraulic motors and the tool further includes sets of pipes for powering said motors, which pipes are constituted by holes in the thickness of the plate of the support structure. Thus, these power pipes are located entirely within the plate and cannot oppose the tool being raised after the trench has been dug and the drilling cuttings mixed with the hydraulic binder. 
         [0030]    Also preferably, the top edge face of the plate of the support means is chamfered. This further facilitates raising the drilling tool through the mixture of drilling cuttings and hydraulic binder. 
         [0031]    Also preferably, the support means comprise at least one guide portion having its bottom end secured directly to the pad of the support structure. 
         [0032]    Also preferably, the dimensions of the pad, which extends horizontally, are substantially equal to those of the right section of the guide beam. 
         [0033]    Thus, while the tool is being raised through the trench filled with the mixture of cuttings and hydraulic binder, the pad lies in line with the guide portion and therefore does not oppose this upward movement. 
         [0034]    Also preferably, the thickness of the guide beam in the direction of the axes of rotation of the cutters is less that half the length of the axis of rotation of a pair of cutters, and the width of the section of the guide beam is less than one-third the overall size of the two pairs of cutters in the horizontal direction perpendicular to said axis of rotation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0035]    Other characteristics and advantages of the invention appear better on reading the following description of embodiments of the invention given by way of non-limiting example. The description refers to the accompanying figures, in which: 
           [0036]      FIG. 1  is an elevation view of a drilling installation using the drilling tool of the invention; 
           [0037]      FIG. 2  is a perspective view of the drilling tool with its guide bar; 
           [0038]      FIG. 3  is an elevation view of the drilling tool assembly; 
           [0039]      FIG. 4  is a partially phantom plan view of the drilling tool; and 
           [0040]      FIG. 5  is a perspective view of the support means for the cutters of the drilling tool. 
       
    
    
     DETAILED DESCRIPTION  
       [0041]      FIG. 1  shows a drilling machine using the drilling tool in accordance with the invention. The tool  12  is guided in the trench by a guide beam  14  of constant profile and preferably of rectangular right section. The tool  12  is fastened to the bottom end  14   a  of the beam. The guide beam  14  serves to transmit thrust forces and traction forces to the tool  12 . It also serves to protect the pipes feeding the tool with hydraulic binder, together with the pipes powering the motors that drive rotation of the cutters. The guide beam  14  is connected by guide and drive means  16 ,  18  to a vertical mast  20 . The mast is supported by a tracked vehicle  22  having installed thereon a system  24  for generating hydraulic power. 
         [0042]    It will be understood that by causing the guide beam  14  to move upwards and downwards, the tool  12  is caused to move vertically in the soil so as to make a panel of a trench by drilling the soil and mixing the drilling cuttings with the hydraulic binder. 
         [0043]      FIG. 2  shows the guide beam  14  with the drilling tool proper  12  secured to its bottom end  14   a.  The drilling tool is constituted by two pairs of cutters  26  &  28  and  30  &  32 , with the cutters in a given pair being on a common axis and with the axes of rotation of the cutters being parallel and substantially horizontal in use. As explained below, according to an essential characteristic of the invention, the motors for driving rotation of the cutters  26  to  32  are disposed inside the cutters themselves, thereby avoiding any need to provide an external motor for driving the cutters. 
         [0044]    More precisely, the pairs of cutters  26  to  32  are connected to the bottom end  14   a  of the guide bar by a support structure given overall reference  34 . In a variant, the support structure  34  may be fitted with scraper systems  36  that serve, when the soil is sticky, to remove the soil that adheres to the cutters between their teeth  38 . 
         [0045]    With reference now to  FIG. 5 , there follows a description in greater detail of the support structure  34  of the tool. The support structure  34  is constituted firstly by a plate  40  that, in the particular embodiment described, consists of two half-plates  42  and  44  interconnected by a triangular part  46  connecting the two half-plates  42  and  44  to a mounting pad  48  used for securing the support structure  34  to the bottom end  14   a  of the guide beam. The pad  48  is naturally substantially horizontal and thus orthogonal relative to the half-plates  42  and  44 . As shown in the figures, the mounting pad has substantially the same dimensions as the horizontal right-section of the guide portion  14 . The bottom ends  44   a,    42   a  of the half-plates are fitted on each of their faces with pairs of coaxial cylindrical bushings  50 ,  52  and  54 ,  56 . These bushings have axes X, X′ and Y, Y′ that are orthogonal to the two half-plates  42  and  44  and that serve firstly for mounting the hydraulic motors and secondly for guiding rotation of the drum on which the cutters proper are mounted. 
         [0046]    As is well known, the guide bar  48   a,  in horizontal right-section, is of dimensions that are very small compared with those of the drilling tool  12  and thus compared with those of the drilling performed by the tool. 
         [0047]    More precisely, the depth l′ of the pad  48  (see  FIG. 5 ) is less than half the length H of the axis of a pair of cutters  26  to  32  (see  FIG. 4 ). The width l of the pad  48  (see  FIG. 5 ) is less than one-third of the length L of the drilling tool  12  (see  FIG. 4 ), where “length” designates its maximum dimension in a horizontal plane. 
         [0048]    Preferably, the top edge face  44   b,    42   b  of each half-plate presents a first portion  44   c,    42   c  that is substantially horizontal and short in length followed by a downwardly-sloping portion  44   d,    42   d,  thereby constituting the sides of a triangle of apex that would be disposed towards the pad  48 . Also preferably, the edge faces  42   b,    44   b  of the half-plates  42  and  44  are chamfered, as can be seen more clearly in  FIG. 4 . 
         [0049]    More generally, the top edge face of the plate  40  is of a shape that makes it easier to raise the drilling tool through the mixture of cut soil and hydraulic binder that is contained in the trench. 
         [0050]    As already mentioned, the motors for driving rotation of the cutters are preferably hydraulic motors. Under such circumstances, the power fluid feed pipes are constituted by holes such as  58  and  60  made in the thickness of the half-plates  42  and  44 . The top ends of the pipes  58 ,  60  open out into orifices such as  62  that are formed in the pad  48  for connecting the pipes  58  and  60  to the power fluid feed pipes that are located in the guide bar  14 . 
         [0051]    Under some circumstances, when the soil is sticky, scraper systems  36  are fastened on either side of the central triangular part  46  of the support means  34 . These scraper systems  36  comprise scrapers such as  64  that are interleaved between the rows of teeth  38 ,  38 ′,  38 ″ of the cutters so as to remove the soil that might adhere to the cutters between these teeth. 
         [0052]    It should be observed that the scraper systems  36  present a profile that makes it easier to raise the drilling tool through the mixture of drilling cuttings and hydraulic binder. 
         [0053]      FIG. 4  shows the cutters  30  to  36  mounted on the bushings  50  to  56 . Firstly there can be seen the hydraulic motors such as  70 , which motors are fastened within the bushings  50  to  56 . The outlet shafts from the motors  70  are connected mechanically in rotation and in translation to drums such as  72  having the cutters  30  to  36  together with their teeth  38 ,  38 ′, and  38 ″ mounted thereon. The ends of the hydraulic fluid feed pipes  58  and  60  are connected by any suitable means to the system for feeding power to the hydraulic motors  70 . 
         [0054]    It will be understood that when it is desirable to raise a drilling tool that is in a trench that is filled with a mixture of drilling cuttings and hydraulic binder, the only portions of the tool that oppose this upward movement are those constituted by the support plate  40  and possibly by the scraper systems  36 . The pad  48  is located in line with the guide bar  14  and therefore does not constitute an obstacle to raising the drilling tool. 
         [0055]    The half-plates  42  and  44  are of small thickness and they have top edges  44   b,    42   b  of profile that facilitates raising the tool, as explained above. 
         [0056]    In a particular embodiment, the drilling tool presents a width H in the direction of the axes of rotation X, X′ and Y, Y′ that is equal to 800 millimeters (mm) and a length L in the direction orthogonal to these axes of 2800 mm. 
         [0057]    If consideration is now given to the support plate  40 , its long dimension is 2200 mm and its thickness e is equal to 60 mm. Furthermore, the fastener plate  48  is rectangular in shape with sides having dimensions of 600 mm and 300 mm. It will be understood that during upward movement, the fastener plate  48  does not constitute an obstacle to such movement since it is in line with the guide bar  14 . Consequently, a length of only 1600 mm of the support plate  40  needs to be taken into consideration. Thus, the area opposing upward movement is 1600 mm×60 mm=96,000 square millimeters (mm 2 ). This section should be compared with the horizontal projection of the tool assembly, which projection presents an area equal to 2800 mm×800 mm, which is more than 2 million mm 2 . The area opposing upward movement is thus less than 5% of the area of the tool. During upward movement, the cutters are caused to rotate and therefore do not oppose such movement. When a cutting tool is fitted with pairs of cutters having axes that present a width of 500 mm, this ratio is slightly less than 10%. In general, the ratio between the areas is preferably less than 10%. 
         [0058]    More generally, and preferably, the thickness e of the support plate  40  is less than 15% of the width H of the tool in the direction of the axes of rotation X, X′ and Y, Y′. More preferably, the ratio is no greater than 10%. This value for the ratio depends on the dimensions of the cutters. The larger the cutters, the smaller the ratio can be made. The means forming the plate  40  have a minimum thickness of 50 mm to 60 mm in order to ensure the plate presents sufficient strength and in order to make it possible to provide internal ducts therein for powering the motors.