DRILLING TOOL AND DRILLING METHOD

A drilling tool of the invention includes an air supply pipe inserted through an inner circumference of a drilling pipe having a tip portion on which a tool main body is disposed. The air supply pipe being configured to supply compressed air. A supply channel is provided on an outer circumference of the drilling pipe. A discharge channel is formed on the tip portion of the tool main body. The discharge channel is configured to discharge cuttings generated together with the drilling fluid into a space between the drilling pipe and the air supply pipe. An exhaust gas outlet opening into the space is formed at a tip portion of the air supply pipe.

TECHNICAL FIELD

The present invention relates to a drilling tool used for a reverse circulation drilling method of taking in cuttings generated during drilling into a tool main body to discharge the cuttings through a drilling pipe, and a drilling method using this drilling tool.

Priority is claimed on Japanese Patent Application No. 2016-008874, filed Jan. 20, 2016, the content of which is incorporated herein by reference.

BACKGROUND ART

In a basic piling construction method using a casing pipe, generally, compressed air used for striking a drilling bit (tool main body) is ejected from a tip of the drilling bit, and cuttings, which are earth and sand generated by breaking a bedrock during drilling, are discharged toward a posterior end side through a space between the casing pipe and a drilling rod by this compressed air. However, if such a construction method is conducted in urban areas, there is a concern that the compressed air ejected from the tip of the drilling bit leaks out to a bedrock around a borehole to lower the strength of the surrounding bedrock and collapse of the surrounding bedrock is caused bedrock depending on the case.

In such a case, although it is effective to supply a slurry, with which bentonite is mixed, to the borehole, to prevent the compressed air from leaking out, the cuttings with which a slurry with high specific gravity is mixed have to be discharged. Thus, the pressure of the compressed air to be ejected has to be made high. Then, as a drilling tool used for such a construction method, for example, Patent Document 1 suggests a drilling tool that supplies pure water as conveyance water through a casing pipe and pumps and discharges cuttings, with which this conveyance water is mixed, using a vacuum pump.

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

Technical Problem

However, in the drilling tool described in such Patent Document 1, not only the vacuum pump is required, but also the conveyance water with which the cuttings are mixed may pass through the vacuum pump. Therefore, there is a concern that damage may occur at an early stage in the vacuum pump, and it becomes difficult to stably perform drilling over a prolonged period of time.

The invention has been made under such a background, and an object thereof is to provide a drilling tool capable of efficiently discharging water with which cuttings are mixed, without using a vacuum pump, and a drilling method using this drilling tool.

Solution to Problem

In order to solve the above problems to achieve such an object, a drilling tool of the invention includes a tool main body; a drilling pipe having a tip portion on which the tool main body is provided; and an air supply pipe configured to supply a compressed air and inserted through the drilling pipe. A supply channel is provided on an outer circumference of the drilling pipe, the supply channel being configured to supply drilling fluid to a tip portion of the tool main body. A discharge channel is formed on the tip portion of the tool main body, the discharge channel being configured to discharge cuttings generated during drilling together with the drilling fluid supplied from the supply channel into a space between the drilling pipe and the air supply pipe. An exhaust gas outlet opening into the space is formed on a tip portion of the air supply pipe.

Additionally, a drilling method of the invention is a drilling method using such a drilling tool. The method includes steps of forming a borehole using the tool main body while supplying the drilling fluid to the tip portion of the tool main body through the supply channel; discharging the cuttings generated during forming the borehole together with the drilling fluid through the discharge channel into the space between the drilling pipe and the air supply pipe; and discharging the cuttings and the drilling fluid discharged into the space toward a posterior end side of the tool main body using the compressed air discharged from the exhaust gas outlet.

In the drilling tool of the above configuration and the drilling methods using this drilling tool, the exhaust gas outlet opening into the space between the drilling pipe and the air supply pipe is formed on the tip portion of the air supply pipe. The compressed air used for striking, for example, the tool main body during drilling is supplied to the air supply pipe and is exhausted from the exhaust gas outlet. The cuttings and the drilling fluid discharged into the space between the drilling pipe and the air supply pipe is pushed out and discharged toward the posterior end side by the compressed air exhausted from the exhaust gas outlet.

Therefore, according to such drilling tool and drilling method, the compressed air supplied in order to apply the striking force to the tool main body in this way can be utilized for the discharge of the cuttings and the drilling fluid. In addition, since the drilling fluid is supplied to the tip portion of the tool main body via the supply channel during drilling, it is possible to prevent the compressed air from leaking out to a bedrock around the borehole to lower strength and causing collapse. Additionally, since the drilling fluid is supplied to the tip portion of the tool main body via the supply channel, pure water can be used as the drilling fluid, and it is also not necessary to make the pressure of the compressed air higher than needed.

Since the cuttings and the drilling fluid can be discharged toward the posterior end side of the space between the drilling pipe and the air supply pipe using compressed air in this way, no vacuum pump is required, and damage resulting from the cuttings passing through the vacuum pump does not occur, either. Additionally, if the compressed air is exhausted and the cuttings and the drilling fluid are pushed out toward the posterior end side, the space closer to the tip side than the exhaust gas outlet has a negative pressure. Thus, new cuttings and drilling fluid can be suctioned into this space from the tip of the tool main body and can be continuously discharged.

Here, the exhaust gas outlet may extend toward the outer-peripheral side of the tool main body so as to incline toward a posterior end side of the tool main body and opens into the space. Accordingly, the cuttings and the drilling fluid discharged into this space can be pushed out toward the posterior end side and can be much more reliably discharged. Moreover, in a case where the drilling tool of the configuration is applied to a basic piling construction method using a casing pipe, the supply channel may be formed between the supply pipe and the drilling pipe by inserting the drilling pipe through the inner circumference of the supply pipe, using this casing pipe as the supply pipe. Accordingly, it is possible to reliably supply the drilling fluid to the tip portion of the tool main body.

Additionally, particularly in a case where such a casing pipe is inserted through the drilling pipe as the supply channel, a plurality of grooves may be formed on an outer circumference of the tip portion of the tool main body. The grooves may extend from a tip of the tool main body toward a posterior end side of the tool main body. Some of the plurality of grooves communicate with the supply channel. The remainder of the grooves may communicate with the discharge channel. Tip portions of the grooves which are communicated with the supply channel and tip portions of the grooves which are communicated with the discharge channel may communicate with each other via a communicating groove formed on a tip surface of the tool main body. Accordingly, while the drilling fluid is supplying from the some of the plurality of grooves flows into the tip portions of the remainder of the grooves via the communicating groove, the cuttings can be efficiently recovered and can be discharged into the space between the drilling pipe and the air supply pipe from the discharge channel.

Meanwhile, a groove may be formed on an outer circumference of the tip portion of the tool main body. The groove may extend from a tip of the tool main body toward a posterior end side of the tool main body to communicate with the supply channel. A hole may be formed in the tool main body inside than the groove as the discharge channel. The hole may extend from the tip of the tool main body toward the posterior end side of the tool main body. A tip portion of the groove and a tip portion of the hole may communicate with each other via a communicating groove formed on a tip surface of the tool main body. Also by such a configuration, while the drilling fluid similarly is supplying from the groove flows into the tip portion of the hole via the communicating groove, the cuttings can be efficiently recovered. In addition, the drilling tool including the above configuration can be used for the drilling method of the invention.

Advantageous Effects of Invention

As described above, according to the invention, there is no case where the strength of a bedrock around the borehole to cause collapse is lowered and collapse is caused, it is not necessary to make the pressure of compressed air higher than needed, and no vacuum pump is not required. Thus, stable drilling can be efficiently performed at low costs.

DESCRIPTION OF EMBODIMENTS

FIGS. 1 to 4show a first embodiment of the drilling tool of the invention. In the present embodiment, a tool main body1includes a bottomed substantially multi-stepped cylindrical pilot bit2that is formed of metallic materials, such as a steel material, is configured to have a one-step larger diameter on a tip side (a left side inFIG. 1) in a direction of an axis O, and is centered on the axis O, and a ring bit3that is detachably attached to an outer circumference of a tip portion of the pilot bit2, is formed in an annular shape or a cylindrical shape, similarly, using metallic materials, such as a steel material, and is centered on the axis O. A smaller-diameter posterior end portion of the pilot bit2serves as a shank portion2A having a male thread portion formed on an outer circumference thereof, and is attached to the shank portion2A by threadedly engaging a cylindrical drilling pipe P1with the male thread portion of the shank portion2A. A rotating force around the axis O and an impelling force and a striking force toward a tip side in the direction of the axis O are applied to the pilot bit2via the drilling pipe P1. In addition, in the present specification, a direction in which the axis O extends is referred to as the direction of the axis O, a direction toward the pilot bit2from the drilling pipe P1in the direction of the axis O is referred to as the tip side (the left side ofFIG. 1), and a direction toward the drilling pipe P1from the pilot bit2is referred to as a posterior end side (a right side ofFIG. 1). Additionally, a direction, which passes through the axis O and is orthogonal to the axis O, is referred to as a diametrical direction or a radial direction. In the radial direction, a direction (radial inner side) approaching the axis O is referred to as an inner-peripheral side, and a direction (radial outer side) separating from the axis O is referred to as outer-peripheral side. Moreover, a direction going around the axis O is referred to as a circumferential direction.

In the present embodiment, the tip portion of the pilot bit2closer to the tip side than this shank portion2A is formed such that the diameter thereof is reduced approximately in three steps toward the tip side. That is, the tip portion of the pilot bit2has a larger-diameter part2B with the largest external diameter, a middle-diameter part2C with a smaller external diameter than the larger-diameter part2B, and a smaller-diameter part2D with a smaller external diameter than the middle-diameter part2C, sequentially from the posterior end side. A conical-surface-shaped pilot-bit-side abutting surface4, which is reduced in diameter gradually toward the tip side in the direction of the axis O and is centered on the axis O, is formed between an outer peripheral surface of the larger-diameter part2B located on the most posterior end side among these diameter parts and an outer peripheral surface of the middle-diameter part2C. A conical-surface-shaped pilot-bit-side contact surface5, which is reduced in diameter gradually toward the tip side in the direction of the axis O and is centered on the axis O, is formed between the outer peripheral surface of the middle-diameter part2C and an outer peripheral surface of the smaller-diameter part2D located on the most tip end side.

Here, in a section along the axis O as shown inFIG. 1, an inclined angle β that the pilot-bit-side contact surface5makes with respect to the axis O is set to be smaller than an inclined angle α that the pilot-bit-side abutting surface4makes with respect to the axis O. In the present embodiment, as shown inFIG. 1, the pilot-bit-side abutting surface4is formed such that a length A thereof in a direction parallel to the axis O is equal to or less than a radius (the reduction amount of the radius) B that is reduced by the length A when being directed toward the tip side in the direction of the axis O, that is, is formed such that the inclined angle α thereof made with respect to the axis O is equal to or more than 45°. Contrary to this, the pilot-bit-side contact surface5is formed such that a length C thereof in the direction parallel to the axis O is longer than a radius (the reduction amount of the radius) D that is reduced by the length C when being directed toward the tip side in the direction of the axis O, that is, is formed such that the inclined angle β thereof made with respect to the axis O is less than 45°.

In addition, the length C of the pilot-bit-side contact surface5is set to be sufficiently longer than the length A of the pilot-bit-side abutting surface4, and the radius D of the pilot-bit-side contact surface5is set to be slightly larger than the radius B of the pilot-bit-side abutting surface4. Additionally, the outer peripheral surfaces of the larger-diameter part2B, the middle-diameter part2C, and smaller-diameter part2D are cylindrical surfaces having constant external diameters centered on the axis O, respectively. The length, in the direction of the axis O, of the middle-diameter part2C among these, is set to be slightly longer than the larger-diameter part2B or the smaller-diameter part2D.

Moreover, a plurality of (three in the present embodiment) of grooves6, which extend toward the posterior end side from a tip surface of the pilot bit2, are formed substantially at equal intervals in the circumferential direction on the outer circumference of the tip portion of the pilot bit2. Some of the grooves (on an upper side inFIG. 1and one groove on a left side ofFIG. 2)6A among the plurality of grooves6penetrate from the tip surface of the pilot bit2to a posterior end surface of the larger-diameter part2B. The remainder of the grooves6B (on a lower side inFIG. 1and two grooves on a right side inFIG. 2) among the plurality of grooves6extend from the tip surface of the pilot bit2to a position in front of the larger-diameter part2B, and are formed in a cut-upward shape toward the outer-peripheral side. In other words, the remainder of the grooves6B extend from the tip surface of the pilot bit2to a position near a posterior end of the middle-diameter part2C. Holes, which extend toward the inner-peripheral side so as to face the posterior end side and have a circular cross-section, are formed as discharge channels7in the present embodiment from posterior end portions of the remainder of the grooves6B. One end of each of the holes opens into an inner peripheral portion (inner peripheral surface) of the bottomed cylindrical pilot bit2, and the other end thereof opens into a posterior end portion of each of the remainder of the grooves6B.

Additionally, a communicating groove8, which connects tip portions of the grooves6A and tip portions of the remainder of the grooves6B together to allow communication therebetween, is formed on the tip surface of the pilot bit2. In the present embodiment, in a plan view as shown inFIG. 2, the communicating groove8is formed in a Y-shaped such that the communicating groove8extends to a position in front of the axis O in the radial direction with respect to the axis O from the tip portions of the grooves6A, and then, reach the tip portions of the two remainder of the grooves6B while being branched into two and curved without reaching the axis O. In addition, each groove6has a substantially rectangular or substantially U-shaped cross-section, and a bottom surface of the groove that face the outer-peripheral side extends toward the posterior end side so as to slightly incline toward the outer-peripheral side with respect to the axis O as shown inFIG. 1. As shown inFIG. 4, the communicating groove8has a U-shaped cross-section and extends on a plane perpendicular to the axis O.

Moreover, a face surface on a planar central portion perpendicular to the axis O excluding the openings of the grooves6and the communicating groove8, and a gauge surface of a conical-surface-shaped outer peripheral portion that inclines so as to extend toward the outer-peripheral side as going toward the posterior end side are formed on the tip surface of the pilot bit2. Drilling tips9, which are made of cemented carbide or the like harder than the pilot bit2, are implanted on the face surface and the gauge surface perpendicularly to the face surface and the gauge surface so as to avoid the openings of the grooves6and the communicating groove8.

Moreover, a plurality of (three in the present embodiment) circular-arc plate-shaped protruded streaks2E (having outer peripheral surfaces that are arcuate surfaces centered on the axis O), which protrude toward the outer-peripheral side and are centered on the axis O, are formed at equal intervals in the circumferential direction at positions away at a slight distance from the pilot-bit-side contact surface5toward the tip side on the outer peripheral surface of the smaller-diameter part2D of the pilot bit2. In the present embodiment, each protruded streak2E extends from one end (an end portion in the counterclockwise direction in the plan view as shown inFIG. 2) of the groove6in the circumferential direction, and the drilling tips9of the gauge surface are implanted over the protruded streaks2E.

A conical-surface-shaped ring-bit-side contact surface10, which is reduced in diameter gradually toward the tip side in the direction of the axis O and is centered on the axis O is formed on an inner peripheral surface of a posterior end portion of the annular or cylindrical ring bit3attached to the outer circumference of such a pilot bit2. The ring-bit-side contact surface10makes the equal inclined angle (3with the pilot-bit-side contact surface5with respect to the axis O in the section along the axis O.

That is, the ring-bit-side contact surface10is formed such that the length C thereof in the direction parallel to the axis O is longer than the radius D that is reduced by the length C when being directed toward the tip side in the direction of the axis O, similarly to the pilot-bit-side contact surface5, that is, is formed at a inclined angle (3of less than 45° with respect to the axis O. As shown inFIG. 1, the ring bit3brings the ring-bit-side contact surface10into close contact with the pilot-bit-side contact surface5, and is attached from the tip surface of the pilot bit2to a posterior end of the pilot-bit-side contact surface5in the direction of the axis O. In other words, in the present embodiment, the length of the ring bit3in the direction of the axis O is substantially the same as the length in the direction of the axis O from the tip surface of the pilot bit2to the posterior end of the pilot-bit-side contact surface5. The ring bit3is attached to the outer circumference of the pilot bit2such that the position of the tip surface of the ring bit3is substantially the same as that of the tip surface of the pilot bit2in the direction of the axis O.

Additionally, an inner peripheral surface of a tip portion of the ring bit3has a slightly larger internal diameter than the smaller-diameter part2D of the pilot bit2. Recessed grooves3A slightly wider in the circumferential direction than the protruded streaks2E of the pilot bit2are formed by the same number as that of the protruded streaks2E at regular intervals in the circumferential direction on the inner peripheral surface of the tip portion so as to penetrate from the tip surface of the ring bit3toward the ring-bit-side contact surface10in the direction of the axis O. The depth of each recessed groove3A in the radial direction is set such that the internal diameter of the recessed groove3A is slightly larger than the external diameter of the protruded streak2E.

Moreover, an L-shaped engaging portion3B, which is equal to the recessed groove3A in depth in the radial direction, is slightly longer than the protruded streak2E in length in the direction of the axis O, and has an L-shaped section in the direction of the axis O, is formed from one end (an end portion in the same direction as an end portion, extending in the circumferential direction from the groove6, on the protruded streak2E of the pilot bit2, that is, from the end portion in the counterclockwise direction inFIG. 2) of the recessed groove3A in the circumferential direction. An inner peripheral surface of the engaging portion3B is smoothly continuous with the inner peripheral surface of the recessed groove3A in the circumferential direction on a tip side thereof. By accommodating the protruded streaks2E into the recessed grooves3A to insert the ring bit3from the tip side of the tip of the pilot bit2and rotate the pilot bit2toward one end side (in the counterclockwise direction inFIG. 2) in the circumferential direction, the protruded streaks2E are fitted to the engaging portions3B so as to be engageable therewith. Therefore, the positions of the grooves6of the pilot bit2coincide with the recessed grooves3A of the ring bit3in the circumferential direction in a state where the protruded streaks2E are fitted to the engaging portions3B in this way. In addition, the circumferential width of each engaging portion3B is set such that the positions of each groove6of the pilot bit2overlap each recessed groove3A of the ring bit3in the circumferential direction in a state where the protruded streak2E is fitted to the engaging portion3B. In the present embodiment, the circumferential width of the engaging portion3B are set so as to be approximately equal to the circumferential width of the protruded streak2E.

Moreover, the tip surface of the ring bit3also includes a face surface of an inner peripheral portion perpendicular to the axis O, and a gauge surface of an outer peripheral portion that inclines so as to extend toward the outer-peripheral side as going toward the posterior end side. Drilling tips9, which are made of cemented carbide or the like harder than the ring bit3, are implanted in the face surface and the gauge surface perpendicularly to the face surface and the gauge surface. In addition, a plurality of recessed grooves3C are formed at equal intervals in the circumferential direction between the drilling tips9implanted in the gauge surface on an outer peripheral surface of the tip portion of the ring bit3.

Meanwhile, a ring-bit-side locking groove11, which forms an oblong shape extending in the direction of the axis O in the section along the axis O and opens toward the outer-peripheral side, is formed over the entire circumference at a position with a distance in the direction of the axis O from the gauge surface of the tip surface and a posterior end surface in the ring bit3, on the outer peripheral surface of the ring bit3.

Additionally, the outer peripheral portion of the ring bit3closer to the posterior end side than the ring-bit-side locking groove11serves as an annular ring-bit-side locking portion12that protrudes toward the outer-peripheral side with respect to the ring-bit-side locking groove11. The external diameter of the ring-bit-side locking portion12is a smaller diameter than the tip portion of the ring bit3, and the length of the ring-bit-side locking portion12in the direction of the axis O is set to be shorter than the ring-bit-side locking groove11. In addition, chamfering is performed on an outer peripheral portion of a posterior end of the ring-bit-side locking portion12.

Moreover, in the present embodiment, a cylindrical casing pipe13centered on the axis O is disposed as a supply pipe P2in the present embodiment on the outer circumference of the pilot bit2to which the ring bit3is attached as described above. A supply channel F is formed between the outer circumference of the drilling pipe P1and the casing pipe13(supply pipe P2). The casing pipe13is integrated by joining a cylindrical casing top13B similarly centered on the axis O to a tip portion of a cylindrical pipe main body13A centered on the axis O by welding or the like. The pipe main body13A has a larger internal diameter than the external diameter of the larger-diameter part2B of the pilot bit2, and a plurality of the pipe main bodies13A are sequentially joined to a posterior end of the pipe main body13A by welding or the like in accordance with the depth of a borehole.

The casing top13B is formed such that the external diameter of a posterior end portion thereof is one step smaller than a tip portion thereof, and is joined such that a tip portion of the pipe main body13A at the foremost end is fitted into the stepped portion. In other words, the external diameter of a posterior end portion of the casing top13B is substantially the same as the internal diameter of the tip portion of the pipe main body13A, and the posterior end portion of the casing top13B is joined so as be fitted to the pipe main body13A. Additionally, the internal diameter of the posterior end portion of the casing top13B is set to be smaller than the external diameter of the larger-diameter part2B of the pilot bit2and to be larger than the external diameter of the middle-diameter part2C. A casing-pipe-side abutting surface14capable of abutting against the pilot-bit-side abutting surface4formed at a position closer to the posterior end side than the pilot-bit-side contact surface5of the pilot bit2is formed on an inner peripheral portion of a posterior end surface of the casing top13B.

That is, the casing-pipe-side abutting surface14is also formed in a conical surface shape that is reduced in diameter gradually toward the tip side in the direction of the axis O and is centered on the axis O. As shown inFIG. 1, in the section along the axis O, the inclined angle α made with respect to the axis O is an angle equal to the inclined angle α of the pilot-bit-side abutting surface4O and larger than the inclined angle β made between the pilot-bit-side contact surface5and the ring-bit-side contact surface10. Moreover, in the present embodiment, the length A of the casing-pipe-side abutting surface14in the direction parallel to the axis O in the section along the axis O is set to be equal to or less than the radius B that is reduced in diameter by the length A when being directed toward the tip side in the direction of the axis O, and the inclined angle α is equal to or more than 45°.

Additionally, the external diameter of the tip portion of the casing top13B is set to be equal to the external diameter of the pipe main body13A and is set to be smaller than the external diameter of the tip portion that becomes the maximum external diameter of the ring bit3. A casing-pipe-side locking groove15, which forms an oblong shape extending in the direction of the axis O in the section along the axis O and opens toward the inner-peripheral side, and a casing-pipe-side locking portion16, which protrudes toward the inner-peripheral side with respect to the casing-pipe-side locking groove15, are formed over the entire circumference in order toward the tip side on an inner peripheral portion of the tip of the casing top13B.

The lengths of the casing-pipe-side locking groove15and the casing-pipe-side locking portion16in the direction of the axis O are set to be equal to those of the ring-bit-side locking groove11and the ring-bit-side locking portion12, respectively. The internal diameter of the casing-pipe-side locking groove15is set to be slightly larger than the external diameter of the ring-bit-side locking portion12. Additionally, the internal diameter of the casing-pipe-side locking portion16is set to be slightly larger than the external diameter of the ring-bit-side locking groove11and smaller than the external diameter of the ring-bit-side locking portion12, and chamfering is performed on an inner peripheral portion of the tip of the casing-pipe-side locking portion16.

By accommodating the casing-pipe-side locking portion16in the ring-bit-side locking groove11and accommodating the ring-bit-side locking portion12in the casing-pipe-side locking groove15, the ring bit3is attached to the casing top13B in a state where the ring bit3is rotatable around the axis O and is locked toward the tip side and the posterior end side in the direction of the axis O within a range in which the ring-bit-side locking groove11and the casing-pipe-side locking groove15are formed.

In addition, in order to attach the ring bit3to the casing top13B in this way, for example, by making the chamfered portion of the outer peripheral portion of the posterior end of the ring-bit-side locking portion12coincide with (abut against) the chamfered portion of the inner peripheral portion of the tip end of the casing-pipe-side locking portion16and then pressing at least one of the casing top13B and the ring bit3toward the other thereof in the direction of the axis O, thereby elastically reducing the diameter of the posterior end portion of the ring bit3and elastically increasing the diameter of the tip portion of the casing top13B, the casing-pipe-side locking portion16is may be accommodated to be fitted into the ring-bit-side locking groove11and the ring-bit-side locking portion12may be accommodated to be fitted into the casing-pipe-side locking groove15. After the ring bit3is attached in this way, the casing top13B is joined to the pipe main body13A, and the ring bit3is disposed on the tip portion of the casing pipe13.

As described above, after the ring bit3is disposed on the casing top13B of the tip portion of the casing pipe13, the pilot bit2attached to the tip portion of the drilling pipe P1is inserted into the casing pipe13from the posterior end side. After, by rotating the pilot bit2toward the one end side in the circumferential direction after the protruded streaks2E are accommodated into the recessed grooves3A, the engaging portions3B are fitted to and engaged with the protruded streaks2E. Drilling pipes P1are sequentially joined and coupled together in accordance with the depth of a borehole. The drill pipe P1located at the most posterior end is connected to a drilling unit. In this way, the pilot bit2inserted into the casing pipe13is positioned at a location where the pilot-bit-side abutting surface4abuts against the casing-pipe-side abutting surface14of the casing top13B.

Moreover, if drilling is performed by making the tip portion of the pilot bit2and the ring bit3abut against a bedrock or the like from this state to apply a rotating force around the axis O and an impelling force and a striking force toward the tip side in the direction of the axis O from the drilling unit via the drilling pipe P1to the pilot bit2, the ring bit3is pressed toward the posterior end side due to resistance from the bedrock or the like, and the ring-bit-side contact surface10is brought into close contact with the pilot-bit-side contact surface5. In addition, the ring bit3may be pressed toward the posterior end side before drilling to bring the ring-bit-side contact surface10close contact with the pilot-bit-side contact surface5.

Here, the ring-bit-side locking portion12and the casing-pipe-side locking portion16are formed so as to be disposed at positions with a distance from both ends, in the direction of the axis O, of the casing-pipe-side locking groove15and the ring-bit-side locking groove11, respectively, as shown inFIG. 1, in a state where the pilot-bit-side abutting surface4abuts against the casing-pipe-side abutting surface14and the ring-bit-side contact surface10is brought into the pilot-bit-side contact surface5, in this way.

The air supply pipe P3is inserted through an inner circumference of the cylindrical drilling pipe P1from the posterior end side, and a tip portion of the air supply pipe P3is inserted into an inner peripheral portion of the pilot bit2in the tool main body1. Moreover, an exhaust plug17is attached to a tip portion of the air supply pipe P3, and the exhaust plug17is accommodated in the inner peripheral portion of the pilot bit2. The air supply pipe P3is formed in a cylindrical shape that has an external diameter smaller than the internal diameter of the drilling pipe P1and is centered on the axis O, and a space E having an annular section is formed between the air supply pipe P3and the drilling pipe P1. For example, the compressed air used for driving of a pneumatic hammer when the striking force is applied to the pilot bit2as described above is supplied to an inner peripheral portion of the air supply pipe P3.

The exhaust plug17is formed in a bottomed multi-stepped cylindrical shape. Specifically, the exhaust plug17includes a tip portion having a larger external diameter, a posterior end portion having a smaller external diameter, and an intermediate portion having a smaller external diameter than the tip portion and larger than the posterior end portion, and an outer peripheral surface of any of these portions has a cylindrical surface shape having a substantially constant external diameter. A male thread portion screwed into an inner circumference of the tip portion of the air supply pipe P3is formed on an outer circumference of the smaller-diameter posterior end portion of the exhaust plug17. The larger-diameter tip portion of the exhaust plug17is an external diameter that makes it possible to be fitted into the inner peripheral portion of the pilot bit2with a slight space therefrom. Additionally, a posterior end surface of the tip portion of the exhaust plug17is formed in a conical surface shape that extends toward the inner-peripheral side as going toward the posterior end side. That is, the posterior end surface of the tip portion of the exhaust plug17connecting the outer peripheral surface of the tip portion of the exhaust plug17and the outer peripheral surface of the intermediate portion of the exhaust plug17together is formed in a conical surface shape. The inclined angle of the posterior end surface of the tip portion of the exhaust plug17with respect to the axis O is set to be equal to the inclined angle of each discharge channel7of the pilot bit2, which similarly extends toward the inner-peripheral side as going the posterior end side, with respect to the axis O. Such an exhaust plug17is disposed such that the conical-surface-shaped tip portion posterior end surface is located at the tip edge of the opening of the discharge channel7on the inner peripheral portion of the pilot bit2, as shown inFIG. 1, in a state where the exhaust plug17is attached to the tip portion of the air supply pipe P3and is inserted into the inner peripheral portion of the pilot bit2. In other words, the exhaust plug17is disposed such that the position of an outer peripheral end (a posterior end of the tip portion) of the conical-surface-shaped tip portion posterior end surface thereof coincides with the position of the tip edge of the opening of the discharge channel7on the inner peripheral portion of the pilot bit2in the direction of the axis O.

An inner peripheral portion of the bottomed cylindrical exhaust plug17communicates with the inner peripheral portion of the cylindrical air supply pipe P3. A plurality of (three) the exhaust gas outlets17A, which open into the space E from the inner peripheral portion of the exhaust plug17to the outer peripheral surface closer to the tip side than the posterior end surface of the tip portion, are formed at equal intervals in the circumferential direction in the present embodiment. That is, one end of each of the exhaust gas outlets17A opens into an inner circumference of the exhaust plug17, and the other end thereof opens into a connecting position between the posterior end surface of the tip portion of the exhaust plug17and the outer peripheral surface of the intermediate portion. The exhaust gas outlet17A of the present embodiment inclines so as to extend toward the posterior end side as going toward the outer-peripheral side of the tool main body1. In addition, an exhaust gas outlet17B having a smaller diameter than the exhaust gas outlet17A is formed even from the inner peripheral portion of the exhaust plug17to a tip surface of the exhaust plug17perpendicular to the axis O. That is, one end of the exhaust gas outlet17B opens into the inner circumference of the exhaust plug17, and the other end thereof opens into the tip surface of the exhaust plug17. Additionally, the exhaust gas outlet17B inclines so as to extend toward the tip side as going toward the outer-peripheral side of the tool main body1. The exhaust gas outlet17B has a function of discharging residual earth and sand on the inner peripheral portion of the pilot bit2.

In one embodiment of a drilling method of the invention of performing drilling with such a drilling tool, the striking force and the impelling force applied from the drilling unit via the drilling pipe P1to the pilot bit2toward the tip side in the direction of the axis O are transmitted from the pilot-bit-side abutting surface4via the casing-pipe-side abutting surface14of the casing top13B to the casing pipe13, and are transmitted from the pilot-bit-side contact surface5via the ring-bit-side contact surface10to the ring bit3. Accordingly, a borehole is formed by of the drilling tips9implanted in the tip surfaces of the pilot bit2and the ring bit3, and the casing pipe13is inserted into this borehole. Additionally, the rotating force around the axis O applied to the pilot bit2is also transmitted from the pilot-bit-side contact surface5via the ring-bit-side contact surface10to the ring bit3.

Additionally, the borehole is formed in this way, and simultaneously, drilling fluid is supplied to the supply channel F between the drilling pipe P1and the casing pipe13, which is the supply pipe P2, from the posterior end side. The drilling fluid in the present embodiment is pure water, such as tap water. In the present embodiment, the drilling fluid supplied this way flows into a bottom portion of the borehole from the grooves6A of the pilot bit2opening into a tip of the supply channel F to fill this bottom portion, and reaches the remainder of the grooves6B while flowing through the communicating groove8communicating with the tip portions of the grooves6A and entraining cuttings with the rotation of the pilot bit2, flows into the space E closer to the posterior end side than the posterior end surface of the tip portion of the exhaust plug17of the inner peripheral portion of the pilot bit2through the discharge channels7communicates with the remainder of the grooves6B, and is charged up to the posterior end side from the exhaust gas outlets17A.

The drilling fluid mixed with the cuttings charged up to the posterior end sides of the exhaust gas outlets17A in this way is delivered and discharged toward the posterior end side as the compressed air supplied into the air supply pipe P3is jetted from the exhaust gas outlets17A through the inner peripheral portion of the exhaust plug17. Additionally, since the tip portion of the space E to which the drilling fluid is discharged in this way has a negative pressure, the drilling fluid remaining within the discharge channels7from the remainder of the grooves6B is sucked into the tip portion of Space E together with the cuttings. In this way, the drilling fluid and the cuttings are continuously discharged due to the jetting of the compressed air from the exhaust gas outlets17A.

In this way, according to the drilling tool and the drilling method of the above configuration, the drilling fluid with which the cuttings are mixed can be discharged using the compressed air for applying the striking force or the like to the pilot bit2and the ring bit3of the tool main body1without requiring a vacuum pump or the like. Since the drilling fluid to be discharged passes through the space E between the drilling pipe P1and the air supply pipe P3, the drilling fluid does not interfere with the discharge even if the cuttings are mixed therewith. For this reason, low-cost drilling and discharge of the cuttings can be stably and efficiently discharged over a long period of time.

Additionally, since the compressed air for applying the striking force is exhausted to the above space E toward the posterior end side and is used for the discharge of the drilling fluid with which the cuttings are mixed, the compressed air does not leak out to the periphery of a borehole. Moreover, since the drilling fluid is charged into the bottom portion of a borehole, there is no case where a surrounding bedrock collapses due to strength reduction. Moreover, since the drilling fluid is also supplied through the supply channel F between the drilling pipe P1and the casing pipe13that is the supply pipe P2, the pure water as described above, having a lower specific gravity than a slurry or the like, can be used as the drilling fluid, and a pressure more than needed is not required for the compressed air jetted from the exhaust gas outlets17A.

Additionally, in the present embodiment, since the exhaust gas outlets17A inclines so as to extend toward the posterior end side as going toward the outer-peripheral side of the tool main body1, the cuttings and the drilling fluid within the space E can be much more reliably discharged toward the posterior end side by the compressed air exhausted from the exhaust gas outlets17A. Moreover, in the present embodiment, the drilling pipe P1is inserted through the casing pipe13serving as the supply pipe P2in this way, and the supply channel F is formed between the drilling pipe P1and the casing pipe13(supply pipe P2). Thus, the present embodiment can be applied to a basic piling construction method of building the casing pipe13in a borehole while forming the borehole. In other words, the drilling fluid is supplied only to the tip side of the tool main body1via the supply channel F. For this reason, it is possible to reliably supply the drilling fluid to the bottom portion of the borehole on the tip portion of the tool main body1to discharge the cuttings, while preventing collapse of the borehole itself.

Additionally, in this way, combine with inserting the drilling pipe P1into the casing pipe13in this way to supply the drilling fluid to the supply channel F during that time. In conjunction with this, in the present embodiment, the plurality of grooves6, which extend toward the posterior end side from the tip surface of the pilot bit2, are formed at intervals in the circumferential direction on the outer circumference of the tip portion of the pilot bit2of the tool main body1attached to the tip portion of the drilling pipe P1. The grooves6A among these grooves are made to communicate with the supply channel F, and the remainder of the grooves6B are made to communicate with the space E to which the drilling fluid is discharged via the discharge channels7. Since the tip portions of the grooves6A and the remainder of the grooves6B communicate with the communicating groove8formed on the tip surface of the pilot bit2, the cuttings generated by the drilling tips9implanted on the tip surface of the pilot bit2or the ring bit3can be uniformly taken into the communicating groove8, and can be reliably discharged together with the drilling fluid.

Moreover, in the present embodiment, the pilot bit2and the ring bit3are configured so as to rotate integrally around the axis O due to the close contact between the conical-surface-shaped pilot-bit-side contact surface5and ring-bit-side contact surface10. For this reason, between the pilot bit2and the ring bit3, it is possible to prevent the drilling fluid from being supplied to the borehole from locations other than of the grooves6A or the drilling fluid including the cuttings from being discharged from locations other than the above remainder of the grooves6B. As a result, it is possible to much more reliably discharge the drilling fluid including the cuttings taken in by the communicating groove8as described above.

In addition, in the first embodiment, the tip portions of the some of the grooves6A communicating with the supply channel F among the plurality of grooves6formed on the outer circumference of the tip portion of the pilot bit2as described above are made to communicate with the tip portions of the remainder of the grooves6B communicating with the space E via the discharge channels7by the communicating groove8. However, as in the second embodiment shown inFIGS. 5 to 8, holes18may be formed on the inner-peripheral sides of the grooves6of the outer circumference the tip portion of the pilot bit2of the tool main body1as a discharge channels7, and tip portions of the holes18and the grooves6may be made to communicate with each other by communicating grooves19. In addition, in theseFIGS. 5 to 8, portions that are the same as those of the first embodiment shown inFIGS. 1 to 4will be designated by the same reference signs, and the description thereof will be omitted.

That is, in the second embodiment, the plurality of grooves6(three also in the present embodiment) formed on the outer circumference of the tip portion of the pilot bit2all open into the posterior end surface of the tip portion of the pilot bit2to communicate with the supply channel F between the drilling pipe P1and the casing pipe13(supply pipe P2), similar to some of the grooves6A of the first embodiment. Meanwhile, the holes18passing through the inner peripheral portion of the pilot bit2from the tip surface of the pilot bit2and having a circular section are formed at positions away from the axis O on the inner-peripheral sides of the respective grooves6on the tip portion of the pilot bit2. The holes18extends parallel to the axis O, tip-side end portions thereof open to the tip surface of the pilot bit2, and posterior-end-side end portions thereof open to the inner peripheral surface of the pilot bit2.

The tip portions of the holes18and the tip portions of the grooves6communicate with each other via the communicating grooves19that extends radially in the radial direction with respect to the axis O on a plane perpendicular to the axis O. Additionally, notches17C, which pass through an outer circumference of the larger-diameter tip portion in the direction of the axis O, are respectively formed between the plurality of (three) exhaust gas outlets17A in the circumferential direction on the larger-diameter tip portion of the exhaust plug17accommodated in the inner peripheral portion of the pilot bit2.

Also in such a second embodiment, the drilling fluid supplied from the supply channel F flows to the tip side of the pilot bit2of the tool main body1through the respective grooves6. Next, while the drilling fluid flows through the communicating grooves19, the drilling fluid entrain the cuttings and reaches the tips of the holes18, flows into the inner peripheral portion of the pilot bit2from the holes18, and is charged from the exhaust gas outlets17A of the exhaust plug17to positions closer to the posterior end side than the notches17C. Then, by exhausting compressed air from the exhaust gas outlets17A, the drilling fluid with which the cuttings are mixed is pushed out and discharged toward the posterior end side, and the drilling fluid with which new cuttings are mixed is sucked from the holes18.

In this way, also in the drilling tool and the drilling method using this according to the second embodiment, similar to the first embodiment, it is possible stably and efficiently perform low-cost drilling while preventing collapse of a surrounding bedrock without requiring a vacuum pump or the like and without requiring high pressure for the compressed air. Additionally, in this second embodiment, even if the number of grooves6to be formed on the tip portion of the pilot bit2is the same as that of the first embodiment, more drilling fluid can be supplied to the tip side of the tool main body1, and a distance at which the drilling fluid that has entrained cuttings flows through the communicating grooves19can be shortened. Thus, this is suitable also in a case where drilling is performed at a high speed.

INDUSTRIAL APPLICABILITY

According to the drilling tool and the drilling method of the invention, the water with which the cuttings are mixed can be efficiently discharged without using a vacuum pump. Thus, the invention is suitable for a basic piling construction method.

REFERENCE SIGNS LIST