Patent Publication Number: US-7585136-B2

Title: Method and equipment for making an impermeable diaphragm of secant piles

Description:
BACKGROUND OF THE INVENTION 
   The subject of the invention is a method and equipment for making an impermeable diaphragm of secant piles. 
   In the dam-building sector, there is at present very sharply felt the problem of impermeabilization both of the dam itself and of the underlying terrain, as well as of the area of transition or toe. In particular, there are recurrent problems on dams during their operation: there exist dams that after some decades require interventions of restoration. 
   SUMMARY OF THE INVENTION 
   Intervention consists in general in traversing the body of the dam with a system of contiguous excavations that are sufficiently deep to reach impermeable layers of the terrain or bedrock, and in filling said excavations with a material that consolidates, reaching a high degree of impermeability and a good adhesion to the walls of the excavation. In general, recourse is had to concrete or plastic concrete. 
   The excavations can have a rectangular cross section (diaphragm panels) or drillings with circular cross section made in arrays and partially overlapping in plan view (secant piles). The former have a limit in depth linked to the availability of suitable equipment (60-70 m in all); the latter, as the diameter decreases, can abundantly exceed 100 m. 
   The problem, especially in the latter case and to an increasing extent as the diameter decreases, is how to guarantee the overlapping between one hole and the next. In fact, the flexibility of the drill rods, the proximity to previously existing holes, and the lack of uniformity of the terrain favour deviations from the vertical of the hole that is being drilled. 
   One solution that is adopted is to use a guide structure (vertical beam with rail) inserted and anchored in a hole that passes through the entire depth, on which a slide constrained to the drilling bits of the rods that carry out the next drilling operation is made to slide. The constraint is such as to enable rotation of the rod and of the bit. 
   It may readily be understood that the depth to be reached conditions the construction of this guide structure, which will be complicated by the need for dismantling and anchorage to the hole. Hence the practicality and the convenience of use decrease as the depth increases and occasions the need for a better solution. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described with reference to the attached plate of drawings, in which: 
       FIG. 1  shows, in plan view, a series of secant excavations, along with the equipment according to the invention; and 
       FIG. 2A-2H  show, in front view and in plan view, each of the sequences of steps of excavation and of filling of a diaphragm obtained with the method and with the equipment according to the invention. 
   

     FIG. 1  shows three adjacent holes, of which the two holes designated by  1  and  1 ′ are already completed, whilst the hole designated by  2  is the one in which the excavation is being made using the drill rod  5 . 
   The reference numbers  3  and  4  designate the surface portions of the walls of the holes  2  and  1 , on which the equipment according to the invention, constituted by a guide divided into two parts  6  and  7  slides. 
   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
   According to the embodiment of  FIG. 1 , the two elements  6 ,  7  are connected to one another through a slider ( 20 ) passing through a vertical opening ( 21 ) made between the two holes ( 1 ,  2 ) and are pushed to one another through elastic means  8  that tend to bring them closer to one another, keeping them in contact against said walls of the two holes  1 ,  2  towards the cusps  9  made in a position corresponding to the opening  21 . 
   By “i” in  FIG. 1  is indicated the distance between the centres of the holes, which usually has a constant value for all the holes that are made to obtain the diaphragm. 
   The two parts  6  and  7  may be separated in order to modify the distance between centres, where required. 
   The guide element made up of the two parts  6  and  7 , as may be noted in  FIG. 2B , is connected to the drill rods  5  in the proximity of the bottom end or the bit  16 . Said guide element does not require a rail or a metal guiding structure, but slides on part of the internal surface of the excavation being made and of the adjacent or preceding one (areas  3  and  4  of the holes  1  and  2  of  FIG. 1 ). 
   In this way, the guide structure is eliminated, along with its problems of length, assembly, and fixing to the hole. The times involved and the difficulties of operation are markedly reduced with the elimination of the step of assembly, positioning, and fixing of the guide structure. 
   It is clear that the two parts  6  and  7  of the guide element may also have other configurations, provided that they are suitable for resting against the walls of the holes in which they are inserted, albeit not reproducing them perfectly. 
   Furthermore, the springs  8  may not be present if it is deemed that the adhesion of the parts  6  and  7  can in any case be guaranteed. 
   Finally, the position of the springs  8  may indifferently be either on the part of guide  6  carried by the rod  5 , which thus becomes the mobile one, or else on the part of guide  7 , as may be noted in the solution of  FIG. 1 . 
   The springs  8  can also be replaced by spacers of variable length that have the job of modifying the distance between centres of the guides. 
   The system is suited to terrains and materials of construction of the dam that are sufficiently stable and solid to guarantee a resistance and continuity of the walls of the hole throughout its depth. For example, the system is not deemed suitable for dams in soil and for soft terrains (Dutch dams). 
   The method consists in the execution of a first hole by means of a traditional drill rod shown in  FIG. 2A , with all the necessary measures taken for obtaining the best verticality (piloted holes, inclinometric controls, inverted pendulum, etc.). 
   The first hole is used as reference for drilling the second hole adjacent to the first shown in  FIG. 2B  using the guide  6 ,  7  described above. Even if the hole were not vertical, the constancy of distance between centres I with the preceding one is even so guaranteed. The first two holes  1  and  1 ′ illustrated on the left in  FIG. 1  are thus obtained. 
   In sequence, it is possible to make a first array of holes shown in  FIG. 2C  before filling them. In general, it will be necessary to avoid making arrays that are too extensive, which could undermine the stability of the dam. 
   The type of drilling to be adopted is preferably the one with removal of detritus by circulation of mud, in particular what is known as mud flush drilling. In this method, as the holes are made, they are all kept full of drilling mud for balancing the thrusts of the terrain. This drilling mud is sucked up through the rods and draws along with it the detritus excavated by the bit, then unloading it on the surface, usually in a settling tank. The desanded fluid can be re-used for the excavation. 
   Since the aspiration of the detritus is made through the bit, possible detritus present in the adjacent excavations is free to collapse towards the lowest point and is in this way sucked up by the excavating tool. 
   As an alternative to mud flush drilling, the extraction of the detritus can be performed separately with known means, such as submersed pumps or air-lift pumps. 
   The guide element  6 - 7  is of modest length if compared to the depth of excavation and is positioned above the bit  16 , at a distance such as not to be hindered in its descent by possible detritus that is found in the adjacent hole and to enable the detritus to flow away towards the suction mouth. 
   For the step of filling, the method envisages that the aggregating material is poured using the traditional “contractor” system in a certain number of holes of the array. The purpose is, on the one hand, to economize by not repeating this operation too frequently, and, on the other, not to leave an array of holes that is too extensive and for an excessively long time, factors that increase the temporary weakening of the dam. 
   In order to leave free the last hole made  12  to guide the subsequent one upon resumption of the array, the filling of the array itself will have to be limited to the next-to-last hole  10 . The method then envisages that into the next-to-last hole  10  there will be lowered a sealing member  11  shown in  FIG. 2D . 
   In the preferred, albeit not binding, option, it is envisaged to lower into the hole a pipe  11  made of plastic material sufficiently deformable under the lateral thrusts so as to rest on the walls of the hole and provide a seal. It is to be filled first with concrete as shown in  FIG. 2E  so as to ensure the conservation of its circular shape. As may be noted, as the hole  10  is filled with concrete, the drilling mud that filled the hole itself flows away or overflows into the adjacent holes according to the casting method used. 
   The last hole  12  must then be filled as shown in  FIG. 2F  with sand  13 , in order to limit the effect of possible concrete that diffuses in this direction from the adjacent hole  10 . To finish, filling of the array of preceding holes is then carried out with concrete  14 , as shown in  FIG. 2G , so that the pipe is pushed against the last pair of cusps, thus providing a sufficient seal. 
   When the drillings  15  are resumed, as shown in  FIG. 2H , the sand  13  of the last hole  12  will be sucked up along with the detritus of the hole being made. 
   As an alternative, the sealing member could be a bag made of suitable material (e.g., woven fabric) to be ballasted and lowered into the excavation provided and then be filled with incoherent materials, sand or gravel. In this case, there is envisaged final saturation of the sand or gravel with a grout-injection pipe, through which to carry out a pressurized pumping of cement grout. 
   Another variant is the provisional filling of the last excavation  12 , instead of the next-to-last one  10 , with a means that can be completely demolished and removed in the step of execution of the first hole of the next array. In this case, a bag is required, the fibres of which do not interfere with the work of excavation of the bit, or else a pipe made of relatively brittle material, such as for example PVC. The gravel or sand must be free to flow away towards the excavation mouth. 
   A further variant for carrying out casting of a first sequence of holes and starting the next sequence consists in laying at the centre of the last hole  12 , with appropriate centring means, a pipe made of material that can easily be demolished, in a way similar to what is envisaged in  FIG. 2D  in the next-to-last hole  10 . 
   In a way similar to the step shown in  FIG. 2G , the subsequent casting of concrete will fill all the holes except for the inside of the aforesaid pipe. 
   The first hole of the next series consists in a drilling operation to widen the hole, said operation being governed by the pipe made of brittle material, made without any lateral guide and with a pilot tip inserted in the aforesaid pipe. 
   Next, the holes of the new series are made with the guide device that forms the subject of the present invention. 
   Finally, it may be noted that for certain applications it may be useful to have a guide  6 ,  7  of variable length; consequently, this may also be of the telescopic type, i.e., made up of a number of tubes inserted in one another in such a way that they can be slid in and out in a controlled way by means of appropriate mechanical or electrical means.