FIELD OF THE INVENTION

[ 1 ] The present invention relates to a ground drill for drilling a borehole in the ground, the ground drill having a longitudinal drill axis extending along the borehole and a drill end associated with a bore hole end having a bore hole end surface transverse to the longitudinal drill axis while drilling, wherein the drill end is configured for advancing the bore hole end surface while drilling. The ground can be the soil of a water body bed, as a sea, ocean, lake or river bed.

BACKGROUND OF THE INVENTION

[ 2 ] The worldwide growing interest in alternative energies led also to an increasing demand for the installation of on- and offshore wind turbines. Although driven piles are extensively used to fix the support structures of wind turbines, in some soil conditions the use of driven piles is not advisable nor possible also in view of preventing nuisance to the surrounding environment.

[ 3 ] In view of this, it is sometimes necessary to install pile anchoring support structures to the soil or monopiles supporting wind turbines in pre-drilled boreholes. Therefore, several drilling techniques have been developed according to the different soil profiles or characteristics.

[ 4 ] Depending on the nature of the soil and of the size of the borehole to be drilled, conventional drilling techniques involve the use of drill buckets having bullet-shaped drill teeth, cluster drills having several individual hammers combined in a single barrel, and core barrels having, at the bottom end, rotating drill bits and vertical shaft boring machines having toothed cutter heads.

[ 5 ] Conventional drills are equipped with fixed drilling teeth or with drill bits that are made to roll by the rotation of the drill itself. The rolling of the drill bits over the surface of the rock allows the creation of a borehole. During drilling operations, the borehole is filled with water to allow a suction mouth installed in the core barrel to remove the rock debris.

[ 6 ] The drilling rate of a core barrel drill on a soil of medium hardness is limited by the fact that the rock crushing is passively performed by the drill bits which rotate uniquely because of the rotation of the barrel itself. In addition, due to the often inefficient suction system, the removal of the rock debris by such devices may not be optimal so that the rock could be repeatedly crushed.

[ 7 ] Vertical shaft boring machines are used to create boreholes in soft soils with diameters ranging from 5 m to 18 m. Such machines comprise a rotatable toothed cutter head attached to a movable hydraulically operated arm. The arm moves the cutter head so as to cover the whole borehole surface. The rotation of the toothed cutter head on the soil allows the crushing of the rock and the excavation of the borehole.

[ 8 ] The major limitation of a vertical shaft boring machine is that the cutter head needs to be continuously moved to cover the entire borehole. This implies a considerably time-consuming process and, therefore, a reduction in the productivity. In addition, risks of breakdowns of hydraulically operated arms are considerable.

[ 9 ] For these reasons, a drill having at least one cutter head at its bottom end configured to cover the whole borehole surface, wherein the at least one cutter head is powered by an independent motor and is equipped with its own suction mouth, would overcome the inefficiencies of both the barrel drill and the vertical shaft boring machine.

SUMMARY OF THE INVENTION

[ 10 ] It is an objective of the invention to provide an apparatus overcoming at least some of the above-identified disadvantages of the known devices to drill a borehole in the soil.

[ 11 ] It is another or alternative objective of the invention to provide an alternative device to excavate a borehole in the soil.

[ 12 ] It is yet another or alternative objective of the invention to provide a device having a higher drill rate compared to a conventional barrel drill with bits.

[ 13 ] It is yet another or alternative objective of the invention to provide a device having a higher drill efficiency by avoiding a repeatedly crush of already cut rock.

[ 14 ] It is yet another or alternative objective of the invention to provide a device ensuring a more effective suction of the rock debris than a conventional drill.

[ 15 ] It is yet another or alternative objective of the invention to provide a device allowing a higher productivity by reducing the time needed to drill a borehole of same diameter with respect to already known drill devices.

[ 16 ] It is yet another or alternative objective of the invention to provide a device ensuring a continuous drill rate.

[ 17 ] It is yet another or alternative objective of the invention to provide a device improving time and energy efficiency of cutting operations by decoupling the force provided for cutting rocks and the force provided for rotating the drill head.

[ 18 ] At least one of the above objectives is achieved by a ground drill for drilling a borehole in the ground, the ground drill having a longitudinal drill axis extending along the borehole and a drill end associated with a bore hole end having a bore hole end surface transverse to the longitudinal drill axis while drilling, wherein the drill end is configured for advancing the bore hole end surface while drilling. The drill end comprises a cutter head drivable to rotate around a cutter axis and defining a cutter surface around the cutter axis, wherein the cutter axis is directed along a direction intersecting the longitudinal drill axis, wherein the cutter axis and the cutter surface are configured and arranged for removing by the cutter surface, while drilling, a slice area of the bore hole end surface extending along a radial direction with respect to the longitudinal drill axis, and wherein the cutter head is movable to rotate about the longitudinal drill axis for removal of a circular area of the bore hole end surface around the longitudinal drill axis by combined rotations of the cutter head around the cutter axis and the longitudinal drill axis in a single full rotation of the cutter head around the longitudinal drill axis.

[ 19 ] In an embodiment, the cutter axis and the cutter surface are configured and arranged for removing by the cutter surface, while drilling, a slice area of the bore hole end surface extending from the longitudinal drill axis in a direction of a circumference of the bore hole.

[ 20 ] In an embodiment, the cutter head is configured such that the circular area of the bore hole end surface removed by the cutter head has a diameter greater than a diameter of the ground drill.

[ 21 ] In an embodiment, a distance of the cutter surface to the cutter axis increases along the cutter axis from a first end to a second end of the cutter head.

[ 22 ] In an embodiment, the cutter surface defines a conical-like shape.

[ 23 ] In an embodiment, the first end of the cutter head is associated with the longitudinal drill axis.

[ 24 ] In an embodiment, the cutter head is driven to rotate around the longitudinal axis and the cutter axis by means of separate motors.

[ 25 ] In an embodiment, the ground drill comprises a non-rotating section configured for, in use, advancing in the borehole while not being rotated about the longitudinal drill axis and a rotating section arranged on the non-rotating section to allow rotation about the longitudinal drill axis with respect to the non-rotating section, the rotating section providing the drill end.

[ 26 ] In an embodiment, a suction arrangement comprising a suction mouth is arranged to be associated with the cutter head for removing cut material from the bore hole end surface.

[ 27 ] In an embodiment, the drill end comprises an auxiliary cutter head drivable to rotate around an auxiliary cutter axis and defining an auxiliary cutter surface around the auxiliary cutter axis, wherein the auxiliary cutter axis is directed along a direction intersecting the longitudinal drill axis, wherein the auxiliary cutter axis and the auxiliary cutter surface are configured and arranged for removing by the auxiliary cutter surface, while drilling, a slice area of the bore hole end surface extending from the circumference of the borehole in a direction of the longitudinal drill axis, and wherein the auxiliary cutter head is movable to rotate about the longitudinal drill axis for removal of an annular area of the bore hole end surface around the longitudinal drill axis by combined rotations of the auxiliary cutter head around the auxiliary cutter axis and the longitudinal drill axis in a single full rotation of the auxiliary cutter head around the longitudinal drill axis.

[ 28 ] In an embodiment, a distance of the auxiliary cutter surface to the auxiliary cutter axis increases along the auxiliary cutter axis from a first end to a second end of the auxiliary cutter head.

[ 29 ] In an embodiment, the auxiliary cutter surface defines a conical-like shape.

[ 30 ] In an embodiment, the first end of the auxiliary cutter head is associated with the longitudinal drill axis.

[ 31 ] In an embodiment, the auxiliary cutter head is driven to rotate around the longitudinal axis and the auxiliary cutter axis by means of separate motors, and the rotation of the cutter head around the cutter axis and rotation of the auxiliary cutter head around the auxiliary cutter axis are driven by separate motors.

[ 32 ] In an embodiment, the auxiliary cutter head is adjustable in a radial direction with respect to the longitudinal drill axis.

[ 33 ] In an embodiment, the auxiliary cutter axis and the auxiliary cutter surface are configured and arranged so that the annular area of the bore hole end surface removed by the auxiliary cutter head has an inner radius smaller than or equal to the outer radius of the circular area of the bore hole end surface removed by the cutter head

[ 34 ] In an embodiment, the auxiliary cutter axis and the auxiliary cutter surface are configured and arranged so that the annular area of the bore hole end surface removed by the auxiliary cutter head is greater than the diameter of the ground drill.

[ 35 ] In an embodiment, a rotational speed of each one of the cutter head and the auxiliary cutter head around a respective cutter axis is greater than a rotational speed of each one of the cutter head and the auxiliary cutter head around the longitudinal drill axis.

[ 36 ] In an embodiment, a suction mouth of a suction arrangement is arranged to be associated with a respective one of the cutter heads for removing drilled material from the drill end.

[ 37 ] In an embodiment, the suction arrangement comprises an opening or a jet for introducing water into the bore hole.

BRIEF DESCRIPTION OF THE DRAWINGS

[ 38 ] Further features and advantages of the invention will become apparent from the description of the invention by way of non-limiting and non-exclusive embodiments. These embodiments are not to be construed as limiting the scope of protection. The person skilled in the art will realize that other alternatives and equivalent embodiments of the invention can be conceived and reduced to practice without departing from the scope of the present invention. Embodiments of the invention will be described with reference to the accompanying drawings, in which like or same reference symbols denote like, same or corresponding parts, and in which

Figure 1 shows a schematic view of a ground drill operating in a borehole, according to an embodiment of the invention;

Figure 2A shows a schematic section along a plane comprising the longitudinal drill axis of a drill end of a ground drill, according to an embodiment of the invention;

Figure 2B shows a top view of a borehole during drilling operations, according to an embodiment of the invention;

Figure 3A shows a section of a ground drill along the longitudinal drill axis, according to an embodiment of the invention, wherein the ground drill comprises a non-rotating section and a rotating section and wherein the drill end comprises a cutter head or a cutter head and an auxiliary cutter head;

Figure 3B shows a top view of the bore hole during drilling operations, according to an embodiment of the invention, wherein the drill end of the ground drill comprises a cutter head or a cutter head and an auxiliary cutter head; and

Figure 3C shows a top view of the rotating section during drilling operations, according to an embodiment of the invention, wherein the drill end of the ground drill comprises a suction mouth fixed to the cutter head and/or an auxiliary suction mouth fixed to the auxiliary cutter head.

DETAILED DESCRIPTION OF EMBODIMENTS

[ 39 ] The following detailed description is made in relation to a ground drill for drilling a bore hole in the ground. The bore hole has a cylindrical shape having a symmetry axis. The symmetry axis of the bore hole generally follows the vertical direction. Alternatively, the symmetry axis may form any angle with respect to the vertical direction. In some applications, the symmetry axis of the bore hole may be substantially horizontal. The bore hole comprises a bore hole end surface transverse with respect to the symmetry axis. The embodiments will be described with reference to “soil” but generally apply to all kinds of soil, rock and ground. In addition, the term “soil” can be applied to the soil of a water body bed, as a sea, ocean, lake or river bed.

[ 40 ] During drilling operations, the ground drill is generally sustained by a surface- located device capable of holding the ground drill upright and configured to advance or retract the ground drill along the symmetry axis of the bore hole. In addition, the surface- located device may be arranged to allow the insertion of a casing down into the bore hole so as to prevent its collapse.

[ 41 ] Figure 1 schematically shows a section of a ground drill 1 , inserted in a bore hole B, along a plane containing the longitudinal drill axis A of the ground drill 1. The bore hole B is reinforced by means of a hollow cylindrical reinforcing casing RC to prevent the collapsing of the bore hole B. The reinforcing casing RC is made to slide through the bore hole B during drilling operations. The ground drill 1 is stabilized in the borehole by means of stabilizers DS protruding from the ground drill 1 in a direction perpendicular to the longitudinal axis A and at contact with the reinforcing casing RC. The ground drill has a drill end 10 associated with a bottom end BE of the bore hole B. The drill end 10 is shown in more details in Figure 2A depicting a section along the longitudinal axis A of the ground drill 1 inserted in the bore hole B. The drill end 10 can be rotated around the longitudinal drill axis A for advancing the bore hole end surface BS of the bore hole B while drilling the bore hole. The bore hole end surface BS is transverse to the longitudinal axis A. The drill end 10 comprises a cutter head 11 drivable to rotate around a cutter axis C and defining a cutter surface CS around the cutter axis C. The angle formed between the cutter axis C and the longitudinal drill axis A is such that the cutter surface CS is parallel to the bore hole end surface BS. As shown in Figure 2B, displaying a top view of the borehole end surface BS during drilling operations, the cutter head 11 is configured to remove a slice area SC of the bore hole end surface BS extending from the longitudinal drill axis A in a direction of a circumference of the bore hole B. The cutter head 11 is configured to remove a circular area CC of the bore hole end surface around the longitudinal drill axis A by combined rotations of the cutter head 11 around the cutter axis C and around the longitudinal drill axis A in a single full rotation of the cutter head 11 around the longitudinal drill axis A.

[ 42 ] To perform the excavation of the bore hole B, the ground drill 1 is lowered so as to contact the soil with its drill end 10. Alternatively, the ground drill 1 is lowered into a preexisting bore hole B to contact with its drill end 10 the bore hole end surface BS. The drill end 10 is then made to rotate around the longitudinal drill axis A and the cutter head 11 is additionally made to rotate around the cutter axis C. The rotation of the cutter head 11 allows the cutting of the soil. The combined rotations of the drill end 10 around the longitudinal drill axis A and of the cutter head 11 around the cutter axis C allow the coverage of the circular area CC corresponding to the whole bore hole end surface, i.e. allow the cutting of the soil of the whole bore hole end surface BS and the excavation of the bore hole B. The cutter head 11 can be configured, i.e. dimensioned and/or positioned, such that the circular area CC of the bore hole end surface BS removed by the cutter head 11 has a diameter greater than the diameter of the ground drill so as to allow the underreaming of the drill housing and to facilitate the insertion of the reinforcing casing RC.

[ 43 ] A distance d of the cutter surface CS to the cutter axis C increases along the cutter axis C from a first end to a second end of the cutter head 11 . The cutter head 11 has a conical-like shape. The first end of the cutter head 11 is associated with the longitudinal drill axis A, which is intended to mean that the first end of the cutter head 11 is at a proximal position with respect to the longitudinal drill axis A and the second end of the cutter head 11 is at a distal position with respect to the longitudinal drill axis A. The first end of the cutter head 11 extends slightly beyond the longitudinal drill axis A so that the projection of the first end of the cutter head 11 onto the bore hole end surface substantially corresponds to the projection of the longitudinal drill axis A onto the bore hole end surface. In this way, the slice SC of soil removed by the rotation of the cutter head 11 around the cutter head axis C starts at the center of the bore hole end surface BS, and the circular area on the bore hole end surface swept by the cutter head 11 after a complete rotation of the drill end 10 around the longitudinal axis A corresponds to the whole bore hole end surface.

[ 44 ] In the embodiment shown, the cutter head 11 is driven to rotate around the cutter axis C by a cutter head motor 14 and the drill end 10 is driven to rotate around the longitudinal axis by a drill end motor, wherein the cutter head motor 14 is different from the drill end motor, i.e. the cutter head 11 is driven to rotate around the longitudinal axis A and the cutter axis C by means of separate motors. The rotational speed of the cutter head 11 around the cutter axis C is higher that the rotational speed of the ground drill 1 around the longitudinal drill axis A. Generally, the cutter head 11 rotates around the cutter axis at a rotational speed up to 50 rpm and the drill end 10 rotates around the longitudinal drill axis A at a rotational speed up to 1 rpm.

[ 45 ] In the embodiment shown in Figure 2A, the ground drill 1 comprises a nonrotating section 12 configured for, in use, advancing in the bore hole B while not being rotated about the longitudinal drill axis A and a rotating section 13 arranged on the nonrotating section 12 to allow rotation about the longitudinal drill axis A with respect to the nonrotating section 12. The rotating section 13 provides the drill end 10. Usually, the non-rotating section 12 has a longitudinal size greater than the longitudinal size of the rotating section 13. The non-rotating section 12 can have a dimension, transversal to the longitudinal drill axis, smaller than a transversal dimension of the rotating section 13. The non-rotating section 12 may be used for the allocation of equipment useful for the functioning of the drill end 10 and for the rotation of the rotating section 12, such as a drill end motor.

[ 46 ] In an embodiment, a suction arrangement comprising a suction mouth 15 is arranged to be associated with the cutter head 11 for removing cut material from the bore hole end surface BS. During a drilling operation, the soil cut by the cutter head remains as debris on the bore hole end surface. To avoid to repeatedly crushing such debris, it is advantageous to remove it by making use of the suction mouth. The suction arrangement is arranged to be at a fixed position with respect to the cutter head 11 and may be configured to introduce water into the bore hole through an opening or a jet fixed. Additionally, the suction arrangement is arranged to be fixed to the rotating section 13. The introduction of water into the bore hole allows the suction of the cut soil on the bore hole end surface. In case water is introduced into the bore hole by means of a jet, the waterjet may be directed towards the cutter head 11 so as to allow the cleaning of the cutter head 11 from soil debris.

[ 47 ] In the embodiment shown in Figure 3A, the drill end 10 additionally comprises an auxiliary cutter head 11 a drivable to rotate around an auxiliary cutter axis Ca and defining an auxiliary cutter surface CSa around the auxiliary cutter axis Ca, directed along a direction intersecting the longitudinal drill axis A. The rotating section 13 is partly received within the lower end of the non-rotating section 12, and rotatable with respect to the non-rotating section 12 by a motor 13.1. The auxiliary cutter head 11 a is arranged to be mechanically connected to the rotating section 13 of the ground drill 1 .

[ 48 ] Figure 3B shows a top view of the borehole during drilling operations using the ground drill shown in Figure 3A. The auxiliary cutter axis Ca and the auxiliary cutter surface CSa are configured and arranged for removing by the auxiliary cutter surface CSa a slice area SCa of the bore hole end surface BS extending from the circumference of the bore hole end surface BS in a direction of the longitudinal drill axis A, i.e. the slice area SCa swept by the auxiliary cutter surface is an annular area AC of the bore hole end surface BS. The auxiliary cutter head 11 a is movable to rotate about the longitudinal drill axis A for removal of the annular area AC of the bore hole end surface around the longitudinal drill axis by combined rotations of the auxiliary cutter head 11 a around the auxiliary cutter axis Ca and the longitudinal drill axis A in a single full rotation of the auxiliary cutter head around the longitudinal drill axis.

[ 49 ] The distance of the auxiliary cutter surface CSa to the auxiliary cutter axis Ca changes along the auxiliary cutter axis Ca from a first end to a second end of the auxiliary cutter head 11 a. In one example, the distance of the auxiliary cutter surface CSa to the auxiliary cutter axis Ca linearly increases along the auxiliary cutter axis Ca from the first end to the second end of the auxiliary cutter head 11 a, thus, resulting in the auxiliary cutter surface CSa defining a conical-like shape, e.g. a conical frustum.

[ 50 ] In another example, the distance of the auxiliary cutter surface CSa to the auxiliary cutter axis Ca changes along the auxiliary cutter axis Ca from a first end to a second end of the auxiliary cutter head 11 a according to a non-linear function. In this case, the projection (profile) of the auxiliary cutter surface CSa on a plane containing the auxiliary cutter surface CSa and the auxiliary cutter axis Ca can be described by a curve being a nonlinear function of the distance between a generic point on the auxiliary cutter axis Ca and the first end of the auxiliary cutter head 11 a, In other words, the auxiliary cutter surface CSa defines a surface of a solid of revolution obtained by rotating the curve described by said non-linear function around the auxiliary cutter axis Ca. In one example, the auxiliary cutter surface CSa defines a convex shape.

[ 51 ] The first end of the auxiliary cutter head 11 a is associated with the longitudinal drill axis A, which is intended to mean that the first end of the auxiliary cutter head 11 a is at a proximal position with respect to the longitudinal drill axis A and the second end of the auxiliary cutter head 11 a is at a distal position with respect to the longitudinal drill axis A.

[ 52 ] The auxiliary cutter head 11 a is driven by an auxiliary cutter head motor 14a different from the cutter head motor 14 and different from the drill end motor, i.e. the auxiliary cutter head 11 a is driven to rotate around around the longitudinal axis A and the auxiliary cutter axis Ca. The rotational speed of the auxiliary cutter head 11 a around the auxiliary cutter axis Ca is higher that the rotational speed of the ground drill 1 around the longitudinal drill axis A. Generally, the auxiliary cutter head 11 a rotates around the auxiliary cutter axis Ca at a rotational speed up to 50 rpm and the ground drill 1 rotates around the longitudinal drill axis A at a rotational speed up to 1 rpm.

[ 53 ] In the embodiment shown, the auxiliary cutter head 11 a is adjustable in a radial direction with respect to the longitudinal drill axis A. The auxiliary cutter head 11a is movable along the radial direction with respect to the longitudinal drill axis A by means of an actuator 17, which allows setting the radius of the bore hole. In this embodiment, the auxiliary cutter head is hingedly connected to the rotating section 13 of the ground drill 1 .

[ 54 ] One end of the actuator 17 is mechanically, optionally hingedly, connected to to the rotating section 13 of the ground drill 1 while the other end of the actuator is mechanically, optionally hingedly, connected to the auxiliary cutter head 11 a. In this way, when the actuator 17 is actuated, the auxiliary cutter head 11 a is made to move along the radial (perpendicular) direction with respect to the longitudinal axis A. The actuator 17 is to be intended as any element which, when activated, can make a component, i.e. the auxiliary cutter head 11 a, to move. Non-exhaustive examples of actuators are hydraulic actuators, pneumatic actuators, electric motors, linear motors and the like.

[ 55 ] Generally, the cutter head 11 could be made adjustable in a same manner as well. A complete coverage of the bore hole end surface BS is obtained by combining the rotation of the drill end 10 around the longitudinal drill axis A, the rotation of the cutter head 11 around the cutter axis C, the rotation of the auxiliary cutter head 11 a around the auxiliary cutter axis Ca and, when the radius of the bore hole B is set, the translation of the auxiliary cutter head 11 a along the radial direction with respect to the longitudinal drill axis A.

[ 56 ] The circular and annular bore hole end surfaces areas removed by the cutter head 11 and the auxiliary cutter head 11 a are complementary so as to cover the whole bore hole end surface. The auxiliary cutter axis Ca and the auxiliary cutter surface CSa are configured and arranged so that the annular area AC of the bore hole end surface BS removed by the auxiliary cutter head 11 a has an inner radius smaller than or equal to the outer radius of the circular area CC of the bore hole end surface BS removed by the cutter head. The auxiliary cutter axis and the auxiliary cutter surface can be configured and arranged so that the annular area AC of the bore hole end surface BS removed by the auxiliary cutter head 11 a is greater than the diameter of the ground drill 1 , which can be set by appropriately driving the actuator 17, so as to allow underreaming. One of the advantage of this solution is to facilitate the advancing into or the retracting from the bore hole of the ground drill 1 . The use of the actuator 17 for moving the auxiliary cutter head 11 a along the radial (perpendicular) direction with respect to the longitudinal axis A provides the further advantage that the extent of the enlargement of the bore hole beyond the diameter of the ground drill 1 , i.e. underreaming, can be adjusted based on occurring or existing circumstances, e.g. soil condition, mud formation, bore hole instability, etc.

[ 57 ] In one alternative, the annular area AC of the bore hole end surface BS removed by the auxiliary cutter head 11a is coplanar to the circular area CC of the bore hole end surface BS removed by the cutter head. In another alternative, the annular area AC of the bore hole end surface BS removed by the auxiliary cutter head 11 a is comprised on a plane different than the plane comprising the circular area CC of the bore hole end surface BS removed by the cutter head. In still another alternative, the auxiliary cutter axis Ca, the auxiliary cutter surface CSa, the cutter axis C and the cutter surface CS are configured and arranged so that the bore hole end surface BS removed by the combined actions of the auxiliary cutter head 11 a and the cutter head 11 is the inner surface of a solid of revolution, i.e. a cone.

[ 58 ] In the embodiment shown, a suction arrangement 16 is arranged to be associated with the cutter head 11 and the auxiliary cutter head 11 a for removing drilled material from the bore hole end surface BS. The suction arrangement 16 is arranged to be at a fixed position with respect to the cutter head 11 and to the auxiliary cutter head 11 a and may comprise an opening or a jet for introducing water into the bore hole. Additionally, the suction arrangement is arranged to be fixed to the rotating section 13. The introduction of water into the bore hole allows the suction of the crushed soil on the bore hole end surface. In case water is introduced into the bore hole by means of a jet, the waterjet may be directed towards the auxiliary cutter head 11 a so as to allow the cleaning of the auxiliary cutter head 11 a from soil debris. The suction arrangement 16 optionally comprises a suction mouth 15 and/or an auxiliary suction mouth 15a, shown in Fig. 3C, configured to be at a fixed position with respect to the cutter head 11 and the auxiliary cutter head 11 a, respectively.

[ 59 ] Figure 3C shows a top view of the rotating section 13 during drilling operations on the borehole end surface BS using the ground drill shown in Figure 3A. A suction mouth 15 of the suction arrangement is arranged to be at a fixed position with respect to the cutter head 11 and an auxiliary suction mouth 15a of the suction arrangement is arranged to be at a fixed position with respect to the auxiliary cutter head 11 a. The suction mouth 15 and the auxiliary suction mouth 15a are in fluid connection with the suction arrangement 16, not shown in this figure, and allow the introduction of water into the borehole and the suction of the crushed soil on the bore hole end surface BS.

[ 60 ] It will be clear to the person skilled in the art that the invention disclosed in relation to figures 3A-3C can also be carried out by a ground drill comprising a plurality of auxiliary cutter heads each one drivable to rotate around an associated auxiliary cutter axis and defining an associated auxiliary cutter surface around the auxiliary cutter axis, directed along a direction intersecting the longitudinal drill axis A. The plurality of auxiliary cutter heads comprise an outermost auxiliary cutter head corresponding to the auxiliary cutter head farthest from the longitudinal drilling axis A.

[ 61 ] As an example, figure 4 shows a top view of the borehole during drilling operations using a ground drill comprising two auxiliary cutter heads, an inner auxiliary cutter head and an outermost auxiliary cutter head. Although in the example shown in figure 4, the ground drill comprises two auxiliary cutter heads, it will be clear to the person skilled in the art that the number of auxiliary cutter heads is not limited to two. The features and the working principles of such auxiliary cutter heads are similar to the features and the working principles of the auxiliary cutter head 11 a disclosed in relation to figures 3A-3C and to which reference is made.

[ 62 ] Similarly to the auxiliary cutter head 11 a of figures 3A-3C, each auxiliary cutter axis and each auxiliary cutter surface are configured and arranged for removing, by the associated auxiliary cutter surface, a slice area of the bore hole end surface BS extending from the circumference of the bore hole end surface BS in a direction of the longitudinal drill axis A, e.g. the slice areas SCa and SCa’ of figure 4 swept by the auxiliary cutter surfaces are annular areas AC and AC’, respectively, of the bore hole end surface BS. Wherein the slice area SCa’ and the annular area AC’ relate to the inner auxiliary cutter head and the slice area SCa and the annular area AC relate to the outermost auxiliary cutter head.

[ 63 ] Each auxiliary cutter head is driven by an auxiliary cutter head motor different from the cutter head motor, i.e. the cutter head motor 14, and different from the drill end motor. In addition, each auxiliary cutter head motor is independent from the auxiliary cutter head motor associated to the other(s) auxiliary cutter head(s).

[ 64 ] In an example, the outermost auxiliary cutter head of the plurality of cutter heads is movable in a radial direction with respect to the longitudinal drill axis A. The outermost auxiliary cutter head is movable along the radial direction with respect to the longitudinal drill axis A by means of an actuator, similarto the actuator 17 of figures 3A-3C to which reference is made, which allows setting the radius of the bore hole. A complete coverage of the bore hole end surface BS is obtained by combining the rotation of the drill end 10 around the longitudinal drill axis A, the rotation of the cutter head 11 around the cutter axis C, the rotation of each auxiliary cutter head of the plurality of cutter heads around the related auxiliary cutter axis and, when the radius of the bore hole B is set, the translation of the outermost auxiliary cutter head along the radial direction with respect to the longitudinal drill axis A.

[ 65 ] The circular and annular bore hole end surfaces areas removed by the cutter head 11 and the plurality of auxiliary cutter heads, respectively, are complementary so as to cover the whole bore hole end surface.

[ 66 ] In addition, each auxiliary cutter axis and each auxiliary cutter surface of each auxiliary cutter head is configured and arranged so that the annular area AC’ of the bore hole end surface BS removed by the inner auxiliary cutter head has an inner radius smaller than or equal to the outer radius of the circular area CC of the bore hole end surface BS removed by the cutter head. Similarly, the annular area AC of the bore hole end surface BS removed by the outermost auxiliary cutter head has an inner radius smaller than or equal to the outer radius of the annular area AC’ of the bore hole end surface BS removed by the inner auxiliary cutter head.

[ 67 ] The auxiliary cutter axis and the auxiliary cutter surface of the outermost auxiliary cutter head can be configured and arranged so that the annular area AC of the bore hole end surface BS removed by the outermost auxiliary cutter head is greater than the diameter of the ground drill 1 , which can be set by appropriately driving the actuator, so as to allow underreaming.