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REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/193,007, entitled “Method for the Construction of a Cut-off Wall”, filed Oct. 22, 2008; U.S. Provisional Application No. 61/193,104, entitled “Method and Apparatus for Constructing Deep Bore Holes and Underground Walls”, filed Oct. 29, 2008; U.S. Provisional Application No. 61/193,458, entitled “Method and Apparatus for Constructing Deep Bore Holes and Underground Walls”, filed Dec. 2, 2008; and U.S. Provisional Application No. 61/193,490, entitled “Method and Apparatus for Constructing Deep Bore Holes and Underground Walls”, filed Dec. 3, 2008, which are incorporated herein by reference. 
    
    
     BACKGROUND AND BRIEF DESCRIPTION OF THE INVENTION 
     The present invention is in the field of vertical underground boreholes and vertical underground cut-off walls. 
     Methods for the construction of watertight cut-off walls in the ground are already known, for example for the construction of cut-off walls, which eliminate the necessity of expensive draining and sealing operations during excavations, especially in alluvial grounds. For example, see Veder U.S. Pat. No. 2,791,886, incorporated herewith by reference. 
     Prior art cut-off walls do not always ensure the desired water tightness. The reason for this is partly because it is difficult to obtain a perfect interlocking or interengagement of the single elements, partly because ordinary methods of drilling by means of boring tubes do not guarantee a perfect verticalness of the final wall so that supplementary injection operations are necessary in order to consolidate the wall and to fill in the empty spaces. 
     The purpose of a cut-off wall for a dam is to intercept the venues of seepage and eliminate them by constructing a permanent positive barrier. 
     The barrier should have the capacity of accommodating small movements which will occur during the raising and lowering of the reservoir level, thus it should preferably be built with plastic concrete. 
     Two major concerns must be addressed by the methodology of constructing the wall:
         stability of the excavation when seepage areas are encountered;   continuity and minimum section of the wall at the joints.       

     The first concern can be solved by utilizing a secant-wall method, which minimizes the size of the open excavation and enhances its stability by the arching effect of the round holes. 
     The second concern is harder to satisfy, but the problem is solved by using the double stage drilling system of the present invention. 
     The invention features the following: 
     A preferred embodiment of the system comprises using directional drilling techniques, commonly utilized in the oil field industry, to drill a relatively small (e.g. 12¼″) hole to full depth of the cut-off wall with a proven accuracy of ¼ degree at 10,000′. At the cut-off wall depth of 700′ for example, this translates to less than ½″. Even accounting for instrumentation and operator errors, it is safe to assume that the bottom of the hole will be within a one foot diameter of the true center. 
     The small hole is drilled with steerable mud motor/drills, using bentonite as fluid. Once the small hole is drilled and stabilized with a proper mud system, the drill string will be extracted and a casing of 12″ or less will be run to full depth of the smaller hole. 
     This will act as the guide for the drilling of a much larger (e.g. 36″-48″) hole (e.g. depending on field tests), using a doughnut shaped bit. The drilling of the larger hole be done under bentonite mud using either direct or reverse circulation. 
     It is important to note the limited size of the open hole, drilled in a ground already partially stabilized by the 12¼″ previously drilled hole. 
     When this drilling is completed, the outer drill string is withdrawn and the plastic concrete is pumped from the bottom up using the 12″ (or less) casing as the tremie pipe. 
     This wall will be constructed by alternating a series of primary piles and secondary secant ones; the diameter and spacing will be determined by field tests which will verify the accuracy of the drilling method. 
     A second preferred embodiment of the invention features a method of constructing an underground vertical hole in a selected location to a depth exceeding 100 feet comprising, 
     (a) providing a drill string assembly comprised of outer and inner drill strings with the outer drill string having a central guiding hole for telescopingly receiving the inner drill string, said inner drill string having a steerable mud motor/drill at the lower end thereof, said outer drill string having one or more drill bits at the lower end thereof, drive mechanisms for the drill outer string, 
     (b) lowering the drill string assembly in an optional starter casing and set it vertical, 
     (c) advance the inner drill string to the full depth using the steerable mud motor/drill to assure verticality of said hole, 
     (d) extracting the inner drill string and installing an inner casing to the full depth, 
     (e) advance the outer drill string to said full depth using the inner casing as a verticality guide for said outer drill string. 
     A method of constructing an underground wall along a selected path and having predetermined deep depth comprising, 
     (a) providing a drill string assembly comprised of outer and inner drill strings with the outer drill string having a central guiding borehole for telescopingly receiving the inner drill string, said inner drill string having a steerable mud motor/drill at the lower end thereof, said outer drill string having one or more drill bits at the lower end thereof, and drive a mechanism for the outer drill string, 
     (b) advance the inner drill string to the full predetermined depth of the wall using the steerable mud motor/drill to assure verticality to the full depth of said wall, 
     (c) extracting the inner drill string and installing an inner casing to the full depth, 
     (d) advance the outer drill string to the full depth of said wall using the inner casing as a verticality guide for said outer drill string, 
     (e) remove the drill string assembly and using the inner casing as a tremie pipe, fill the hole with a wall forming plastic concrete to form a first vertical wall element, 
     (f) repeat the process along the selected wall path at a distance less than the diameter of the previously constructed element, 
     (g) repeat the process in between the two previously completed elements, cutting into them to assure continuity of the wall. 
     Apparatus for excavating an underground borehole having a predetermined deep depth comprising, 
     a drill string assembly having outer and inner drill strings with the outer drill string having a central guiding borehole for telescopingly receiving the inner drill string, said inner drill string having a steerable mud motor/drill at the lower end thereof, said outer drill string having one or more drill bits at the lower end thereof, respectively, and a separate drive head for said outer drill string. 
     A method of constructing an underground vertical hole in a selected location to a depth exceeding 100 feet comprising, 
     (a) providing a drill string assembly comprised of outer and inner drill strings with the outer drill string having a central guiding borehole for telescopingly receiving the inner drill string, said inner drill string having a steerable mud motor/drill at the lower end thereof, said outer drill string having one or more drill bits at the lower end thereof, drive mechanisms for the drill outer string, 
     (b) install a vertical starter casing at the selected location (optional), 
     (c) lowering the drill string assembly in the starter casing and set it vertical, 
     (d) advance the inner drill string to the full depth using the steerable mud motor/drill to assure verticality of said hole, 
     (e) advance the outer drill string to said full depth using the inner drill string as a verticality guide for said outer drill string. 
     A method of constructing an underground wall along a selected path and having predetermined deep depth comprising, 
     (a) providing a drill string assembly comprised of outer and inner drill strings with the outer drill string having a central guiding borehole for telescopingly receiving the inner drill string, said inner drill string having a steerable mud motor/drill at the lower end thereof, said outer drill string having one or more drill bits at the lower end thereof, and drive a mechanism for the outer drill string, 
     (b) install a vertical starter casing in the path of the wall, 
     (c) lowering the drill string assembly in the starter casing and set it vertical, 
     (d) advance the inner drill string to the full depth of the wall using the steerable mud motor/drill to assure verticality to the full depth of said wall, 
     (e) advance the outer drill string to the full depth of said wall using the inner drill string as a verticality guide for said outer drill string, 
     (f) remove the drill string assembly and fill the hole with a wall forming material to form a first vertical wall element, 
     (g) repeat the process along the selected wall path at a distance less than the diameter of the previously constructed element, 
     (h) repeat the process in between the two previously completed elements, cutting into them to assure continuity of the wall. 
     A method of constructing a deep underground wall along a selected path comprising, 
     (a) providing a drill string assembly comprised of outer and inner drill strings with the outer drill string having a central guiding borehole for telescopingly receiving the inner drill string, said inner drill string having a steerable mud motor/drill at the lower end thereof, said outer drill string having one or more drill bits at the lower end thereof, and a drive mechanism for said outer drill string, 
     (b) install a vertical starter casing in the path of the wall, 
     (c) lowering the drill string assembly in the starter casing and set it vertical, 
     (d) advance the inner drill string for a predetermined distance using the steerable mud motor/drill to assure verticality to the full depth of said wall, 
     (e) remove the drill string assembly and fill the hole with a wall forming material to form a first vertical wall element, 
     (f) at a predetermined distance along the path of said wall repeating steps (b)-(g) to form a second vertical wall element, 
     (g) excavating the earth between said first and second wall elements using said first and second elements as a guide to form a panel slot there between and filling said panel slot with a wall forming material, and 
     (h) repeating steps (b)-(i) at least one further time positioned along the selected path of the wall at a distance less than the diameter of the drill holes, 
     (i) create the connection between the two panels by forming a circular element in between them. 
     A method of constructing an underground vertical hole in a selected location to a depth exceeding 100 feet comprising, 
     (a) providing a drill string assembly comprised of outer and inner drill strings with the outer drill string having a central guiding borehole for telescopingly receiving the inner drill string, each drill string having one or more drill bits at the lower end thereof, respectively, and drive mechanisms for the drill strings, 
     (b) install a vertical starter casing at the selected location, 
     (c) lowering the drill string assembly in the starter casing and set it vertical, 
     (d) advance the inner drill string for a predetermined distance using the central guiding borehole in the outer drill string as a verticality guide, 
     (e) advance the outer drill string for said predetermined distance using the inner drill string as a verticality guide, 
     (f) repeat steps (c)-(e) to final depth, 
     A method of constructing an underground wall along a selected path and having predetermined deep depth comprising, 
     (a) providing a drill string assembly comprised of outer and inner drill strings with the outer drill string having a central guiding borehole for telescopingly receiving the inner drill string, each drill string having one or more drill bits at the lower end thereof, respectively, and drive mechanisms for the drill strings, 
     (b) install a vertical starter casing in the path of the wall, 
     (c) lowering the drill string assembly in the starter casing and set it vertical, 
     (d) advance the inner drill string for a predetermined distance using the central guiding borehole in the outer drill string as a verticality guide, 
     (e) advance the outer drill string for said predetermined distance using the inner drill string as a verticality guide, 
     (f) repeat steps (c)-(e) to final depth, 
     (g) remove the drill string assembly and fill the hole with a wall forming material to form a first vertical wall element, 
     (h) repeat the process along the selected wall path at a distance less than the diameter of the previously constructed element, 
     (i) repeat the process in between the two previously completed elements, cutting into them to assure continuity of the wall. 
     A method of constructing a deep underground wall along a selected path comprising 
     (a) providing a drill string assembly comprised of outer and inner drill strings with the outer drill string having a central guiding borehole for telescopingly receiving the inner drill string, each drill string having one or more drill bits at the lower end thereof, respectively, and drive mechanisms for the drill strings, 
     (b) install a vertical starter casing in the path of the wall, 
     (c) lowering the drill string assembly in the starter casing and set it vertical, 
     (d) advance the inner drill string for a predetermined distance using the central guiding borehole in the outer drill string as a guide, 
     (e) advance the outer drill string for said predetermined distance using the inner drill string as a guide, 
     (f) repeat steps (c)-(e) to final depth, 
     (g) remove the drill string assembly and fill the hole with a wall forming material to form a first vertical wall element, 
     (h) at a predetermined distance along the path of said wall repeating steps (b)-(g) to form a second vertical wall element, 
     (i) excavating the earth between said first and second wall elements using said first and second elements as a guide to form a panel slot there between and filling said panel slot with a wall forming material, and 
     (j) repeating steps (b)-(i) at least one further time positioned along the selected path of the wall at a distance less than the diameter of the drill holes, 
     (k) create the connection between the two panels by forming a circular element in between them. 
     A wall constructed in accordance with a further feature of this invention consists of a series of substantially vertical cylindrical elements which intersect, interengage and adapt themselves to each other so as to form a single homogeneous mass. This is obtained by drilling a series of spaced circular or elongated holes, which holes are filled on the spot with cement or the like material carrying a magnetic additive. 
     Primary elements are formed by drilling one or more primary holes in spaced relation, and then filling these holes with concrete containing magnetic particles, in order to be able to drill intermediate holes intersecting or overlapping the concrete poles already present in the first mentioned primary hole or holes and the concrete then filled into the intermediate holes intimately binds itself to the concrete in the first mentioned primary holes. 
     The problem solved by this feature of the present invention is to verify a minimum section of overlap at the joint of two adjacent elements in deep cut offs. Briefly the problem can be solved as follows:
         After the completed excavation of primary elements, be it in the form of circular piles or in elongated shapes excavated by Hydromills, the element is filled with concrete or plastic concrete containing in the mix a magnetic substance (iron filings, steel fibers, magnetite sand and the like or the equivalent).   After the excavation of the secondary element, which has accomplished the task of forming a joint with the primaries by removing a portion of the adjacent primaries, a borehole probe is lowered into the excavation which will record the portion of the periphery of the hole with magnetic reading.   Since alignment is not important in deep cut-offs, but only continuity and minimum section, orienting the probe is not critical, since as long as it reads two minimum overlaps in the adjacent primaries, the purpose of the cut-off is achieved. In the absence of two minimum overlaps, remedial action may be taken, such as enlarging the borehole for the secondary wall element to achieve the desired overlap or drill a further overlapping hole and fill with concrete to correct for the misalignment and eliminate the discontinuity.   By taking the readings at preset intervals, minimum overlap for the whole depth can be verified.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other obvious advantages and features of the invention will become more apparent when considered with the attached drawings wherein: 
         FIG. 1   a  is a schematic illustration of the fundamental aspects of the invention, 
         FIG. 1   b  discloses the small diameter borehole with a casing therein and the large donut shaped drill bit being advanced to the full depth of the borehole, 
         FIG. 1   c  is a sectional view on lines  1 C- 1 C of  FIG. 1   b.    
         FIG. 2  illustrates a steerable mud motor/drill shown in  FIG. 1   a,    
         FIG. 3  is a schematic illustration of another aspect of the invention, 
         FIG. 4  is a schematic illustration of further aspects of the invention, 
         FIG. 5  illustrates in greater detail the cylindrical starter casing and the initial position of the drill string assembly, 
         FIG. 6  is an illustration of the drill string assembly with inner drill string advanced to the full depth of the borehole, 
         FIG. 7  illustrates the next phase of the process wherein the outer drill string has been advanced to the final depth of the borehole using the casing which has been placed in the small borehole as a verticality guide, 
         FIG. 8  is a sectional view of the typical drill string taken on line “X” of  FIG. 5 , 
         FIG. 9  is a plan view of a cut-off wall formed by intersecting bore piles with the spaced primary holes formed by the process disclosed herein, 
         FIG. 10  is a vertical section of the primary elements of the palisade or cut-off wall, 
         FIG. 11  is a plan view of the primary wall elements along the line of the cut-off wall, 
         FIG. 12  is a plan view of the lay out of primary elements along the line of the cut-off wall with the overlapping positions of the secondary elements, and 
         FIG. 13  is an illustration showing the magnetic borehole probe reading the magnetic properties of two adjacent primary elements. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to  FIGS. 1   a  and  1   b , optionally, an initial cylindrical casing  10  is installed as the starter casing. It must be absolutely (straight) and then after it is placed it is emptied. Drill string assembly  15  comprised of a larger outer drill string  16  and smaller inner drill string  17  telescopically fitted in a bore  19  in the outer drill string  16 . Outer drill string  16 , which may be several feet in diameter (3-6 feet) has its own donut shaped drill bit array  20 , at the lower end thereof. Inner drill string  17  is provided with a steerable mud motor/drill  21  ( FIG. 2 ) which is commonly used in oil and gas well drilling and may be several inches in diameter (about 5-13 inches). Drill string assembly  15  is lowered in the cylindrical casing  10  and set vertical. The next step is the inner drill string  17  is advanced to the full depth, being guided by the steerable mud motor/drill  21 . The inner and outer drill strings are provided with passages (see  FIG. 8 ) for the circulation of selected drilling fluids which remove the cuttings to the surface. After the inner drill string  17  and steerable mud motor/drill  21  have been advanced to the final depth, it is withdrawn and replaced by an inner casing CA. Then the outer drill string  16  is advanced by its own drive head (not shown in  FIG. 1  but see  FIGS. 5 ,  6  and  7 ) all the while being guided by the pilot inner casing CA. The outer drill string  16  is withdrawn and the mud (bentonite) filled hole is filled with a plastic concrete using the pilot inner casing as a tremie pipe. 
     Summarizing 
     Step 1 
     Install a starter casing  10  absolutely straight and empty it. 
     Step II 
     Lower the drill string assembly in the hole and again set it vertical. 
     Step III 
     Advance inner drill string to the final depth using the steerable mud motor  21  to assure verticality, and replace with casing CA,  FIG. 1   b.    
     Step IV 
     Advance the outer drill string guided by the inner drill string. 
     Drilling by the outer drill string can be done by percussion, rotary percussion or rotary method with direct or reverse circulation using air or fluids. The inner casing CA and the hollow drill string  20  are operated by a drill rig with double rotary head which allows the holding of the inner casing CA while operating the outer drill string. 
     The drilling can be done with air or fluids, direct or reverse circulation depending on soil conditions. 
     The invention can be used to make single deep vent holes for ventilation shafts in deep tunnels, water intakes and the like, deep secant pile walls for cut-offs for dams and the like, and to make deep combination walls. 
     In order to build a continuous wall, first verify the maximum deviation of the drilling system and space primary holes guarantee minimum overlap of secondary holes assuming maximum deviation at full depth. 
     Referring now to  FIG. 3 , an initial cylindrical casing  10 ′ is installed as the starter casing. It must be absolutely straight and then after it is placed it is emptied. Drill string assembly  15 ′ comprised of an outer drill string  16 ′ and an inner drill string  17 ′ telescopically fitted in a bore  19  in the outer drill string  16 ′. Outer drill string  16 ′, which may be several feet in diameter (4-6 feet) has its own drill bit array  20 ′, at the lower end thereof. Inner drill string  17 ′ is provided with a steerable mud motor/drill  21 ′ ( FIG. 2 ) which is commonly used in oil and gas well drilling and may be several inches in diameter (5-13 inches; preferably 12¼ inch). Drill string assembly  15 ′ is lowered in the cylindrical casing  10 ′ and set vertical. The next step is the inner drill string  17 ′ is advanced to the full depth, being guided by the steerable mud motor/drill  21 ′. The inner and outer drill strings are provided with passages for the circulation of selected drilling fluids which remove the cuttings to the surface. After the inner drill string  17 ′ and steerable mud motor/drill  21  have been advanced to the final depth, the outer drill string  16 ′ is advanced by its own drive head (not shown in  FIG. 3 ) all the while being guided by the pilot inner drill string  17 ′. 
     Step I 
     Install a starter casing  10 ′ absolutely straight and empty it. 
     Step II 
     Lower the drill string assembly in the hole and again set it vertical. 
     Step II 
     Advance inner drill string  10 ′- 20 ′ using the steerable mud motor  21  to assure verticality. 
     Step IV 
     In the present embodiment, advance the outer drill string guided by the inner drill string. 
     Referring now to  FIG. 4 , an initial cylindrical casing  10 ″ is installed as the starter casing. It must be absolutely straight and then after it is placed it is emptied. Drill string assembly  15 ″ comprised of an outer drill string  16 ″ and an inner drill string  17 ″ telescopically fitted in a bore  19 ″ in the outer drill string  16 ″. Each drill string  16 ″- 17 ″ has its own drill bit array  20 ″,  21 DBA, respectively, at the lower ends thereof. Drill string assembly  15 ″ is lowered in the cylindrical casing  10 ″ and set vertical. The next step in this embodiment is the inner drill string  17 ″ is advanced by a drive head (not shown in  FIG. 4 ) ten to twenty feet, being guided by the bore  19 ″ in the outer drill string  16 ″. The respective inner and outer drill strings are provided with passages for the circulation of selected drilling fluids which remove the cuttings to the surface. After the inner drill string  17 ″ has been advanced, the outer drill string  16 ″ is advanced by its own drive head all the while being guided by the pilot inner drill string  17 ″. This process is repeated with new sections of inner and outer drill strings being added, to the final depth. 
     Summarizing 
     Step I 
     Install a starter casing  10 ′ absolutely straight and empty it. 
     Step II 
     Lower the drill string assembly in the hole and again set it vertical. 
     Step III 
     Advance inner drill string for  10 ′- 20 ′, guided by the outer drill string. 
     Step IV 
     Advance the outer drill string guided by the pilot inner drill string. 
     Step V 
     Repeat process to final depth. 
     Drilling can be done by percussion, rotary percussion or rotary method with direct or reverse circulation using air or fluids. 
     Referring to  FIG. 5 , a mobile drill rig  30  (which can be used in any of the embodiments disclosed herein) having a mast  31  supported by a strut  32  for supporting drive head  33  coupled to the inner drill string  35  ( 35 - 1 ,  35 - 2 ) and drive head  34  couples to the outer drill string  36  ( 36 - 1 ). The drive heads  33  and  34  may include rotary tables and the like for rotating the respective inner and outer drill strings  35  and  36 , respectively. (Inasmuch as the embodiments shown in  FIGS. 1 and 3  incorporate steerable mud motors, rotary drive tables for the drill string are not necessary). As shown in  FIG. 5 , the drill string assembly is being lowered into the starter casing  10 ′ which has been previously installed and verticalized. It should be noted that the inner drill string  35  is double the length of the outer drill easing string  36  being constituted by two inner drill string casings joined by the flush joints  1 FJ- 1   FIG. 6 ) to be described later. The inner and outer drill strings  35 ,  36  constituting the drill string assembly are lowered into the starter casing  10 ′. Referring to  FIG. 6 , it will be noted that three additional inner drill string section ( 35 - 2 - 35 - 4 ) and one additional outer drill string casing ( 36 - 2 ) have been added. 
     In  FIG. 6 , the lower most inner drill string is illustrated as having been operated and the outer drill string  36  is set to be operated and uses inner drill string section  35  as a verticality guide. Referring to  FIG. 7 , further (nth) sections of inner and outer drill string sections have been added and the process repeated therein. The case of  FIG. 7 , shows outer drill string  36  that will serve as a verticality guide for the inner drill string  35 . It is noted that the inner and outer drill string sections have to be disconnected from their respective drill heads to allow the insertion of new sections of the outer and inner drill strings, respectively. 
     Each successive inner drill string section  35 - 1 ,  35 - 2  . . .  35 -N is joined to the preceding section by flush joints  1 FJ- 1 ,  1 FJ- 2  . . .  1 FJ-N so that inner drill string  35  smoothly telescope in the verticality guide in the outer drill string  36  sections. Likewise, each successive outer drill string sections are joined to preceding outer drill string sections by flush joints OFJ- 1 , OFJ- 2  . . . OFJ-N. 
     The drilling can be done with air or fluids, direct or reverse circulation depending on soil conditions. 
       FIG. 8  is a section of the drill string assembly taken on section line “X” of  FIG. 2 . The outer drill string casing is comprised of an outer cylindrical shell  40  joined by duct channel struts  41 - 1 ,  41 - 2 ,  41 - 3  . . .  41 -N to an inner cylindrical member  43 , the struts  41  defining ducts  44 - 1 , 44 - 2 ,  44 - 3 ,  44 - 4  for arisings or cuttings and the excavation fluid used. Inner cylindrical member  45  is spaced from member  43  and defines an air duct  47  for the outer casing and defines a guide VG for the inner drill string assembly  35 . The inner bore VG of member  45  defines the central guiding borehole for telescopingly receiving the inner drill string  35 . The inner drill string  35  comprises an outer cylindrical member  50  joined to an inner cylindrical member  51  defining a duct  52  for arisings and cuttings for the inner drill string casing. 
     The invention can be used to make single deep vent holes for ventilation shafts in deep tunnels, water intakes and the like, deep secant pile walls for cut-offs and the like, and to make deep combination walls. 
     In order to build a continuous wall as shown in  FIG. 9  using the process described above, first verify the maximum deviation of the drilling system and space primary holes to guarantee minimum overlap of secondary holes assuming maximum deviation at full depth. 
     Referring to  FIG. 10 , a plurality of holes  10 - 1 ,  10 - 2 ,  10 - 3  and  10 -N for primary wall elements PW 1 , PW 2 , PW 3  and PWN are drilled into the ground by means of the novel methods disclosed herein or by a conventional large rotary drill bit having a bore in its center through which a thick liquid slurry is introduced into the hole and caused to circulate upwards within the hole in order to carry the material dug out from the bottom of the hole up to the surface and to obtain at the same time sealing and reinforcement of the wall excavation. 
     The primary element holes  10 - 1 ,  10 - 2 ,  10 - 3  and  10 -N are filled with wall forming material. The wall forming material (preferably cementous) fills the holes to form the primary wall elements PW 1 , PW 2 , PW 3  and PWN of the cut-off wall and each contains a substance which can be sensed by a probe. Preferably the substance is a magnetic substance which is mixed with and uniformly dispersed in the wall forming material. The holes  11 - 1 ,  11 - 2 ,  11 - 2  and  11 -N for the secondary elements SE 1 , SE 2 , SE 3  are excavated forming secant joints with the primary elements by removing portions thereof. The starts of the secondary holes are positioned to maximize the overlap. After excavation of the holes for the secondary elements, a magnetic borehole probe MP ( FIG. 13 ) is lowered from a reel R into each secondary excavation before filling with wall forming material. The probe MP is connected to a recording analyzer RA which will record the portion of the periphery of the secondary holes with magnetic material from the primary wall elements PW 1 , PW 2 , PW 3  and PWN. When a dicontinuity is detected, remedial action may be taken, such as enlarging the diameter of the secondary elements or reaming the hole for the second element to eliminate the discontinuity. Another remedial method is to drill an additional overlapping hole, check with the magnetic probe and fill the hole with concrete to correct for the misalignment and eliminate the discontinuity. By continuing this process any desired number of times, a completely watertight cut-off wall is obtained. 
     The result of this is a compact palisade or cut-off wall. According to above description of the construction, the individual holes  10 - 1 ,  10 - 2 ,  10 - 3  and  10 -N of the entire series of holes may be drilled in consecutive order. It is obvious that one will not go out of the scope of the invention if the order of drilling the holes is modified; the same discontinuity detection principle is applied when the primary elements are in the form of elongated shapes excavated by a Hydromill for example as shown in Miotti U.S. Pat. No. 5,056,242.

Summary:
A method and apparatus for constructing large vertical boreholes and underground cut-off walls is disclosed. A drill rig assembly with double rotary heads drives a small diameter drill string and a much larger diameter drill string which is concentric with the smaller drill string and has a donut shaped drill bit configuration at the lower end thereof. Preferably, a steerable mud motor/drill is provided on one end of the small drill string and is guided to make as vertical a borehole as possible. In a preferred embodiment, the small inner drill is advanced to the full depth using the steerable mud motor/drill to achieve a high degree of verticality and a slight smaller casing is installed and used as a verticality guide for the much larger diameter outer drill string. In a further embodiment, the inner drill string is advanced a predetermined distance, then the outer string is advanced using the inner string as a verticality guide, these steps being repeated in alternating fashion to the final depth.