Double bit assembly and method of using the same

A method to drill a hole in the soil by a bit assembly and a bit assembly comprising a ring-shaped bit (2) to drill the outer circle of the hole and to which a protection tube system (4), mounted in the hole while drilling, is coupled, and a cylindric inner bit (1) to drill the center portion of the hole, from which bit at least the rotative motion is transmitted to the outer bit (2). Drilling by the outer bit (2) and pulling out the protection tube (4) are disconnected upon need by reverse rotation of the inner bit, whereafter drilling is continued with the inner bit (1) only.

FIELD OF THE INVENTION 
This invention relates to a method for drilling a hole in the soil and to a 
bit assembly and a drill bit for use in drilling a hole or tunnel in the 
soil. 
BACKGROUND OF THE INVENTION 
Previously known are, for instance from patent specifications FI 933074 and 
GB 2 255 365, double bit assemblies where the inner bit that drills the 
center hole can be pulled out from the hole, while the ring bit that 
drills the outer hole circle is left in the bore. However, in these 
examples the inner bit can be pulled out from the hole after drilling and 
also reinstalled if drilling is continued. 
On drilling a hole in the soil, the most general case is that the first 
portion of the hole is made in soft soil, whereby a protection tube is 
pulled into the drilled hole. Eventually, on hitting rock, the protection 
tube is not needed anymore. Drilling can then be continued without a 
protection tube if change of the drilling procedure is easy. A problem 
arises when the drilling procedure is changed. The inner bit must be 
pulled out from the hole and replaced by a bit that drills a hole with a 
smaller diameter. During this replacement, the complete drilling equipment 
is pulled out from the hole and reinstalled furnished with a new 
rock-drilling bit. This takes time and money, especially if the first bore 
in the soil is long. 
SUMMARY OF THE INVENTION 
By means of a method and bit assembly with a proper bit according to this 
invention, a surprising improvement is achieved. 
The advantage of this invention is that a hole requiring a protection tube 
is drilled with the same bit assembly as a hole into rock. When the 
protection tube is not needed, drilling proceeds nonstop with the inner 
bit only. In case drilling is stopped in a dead end, the inner bit can be 
pulled out through the outer bit. Even the outer bit can be pulled out 
from the bore on condition that also the protection tube is also pulled 
out simultaneously. 
The bit assembly and the method can be applied to drilling by hammering but 
also to bits rotating only. The groove in the bayonet joint that opens in 
both of the axial directions is favorably arranged as a flow channel for 
flushing medium, whereas the joint is a simple construction.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 illustrates a section of the bit assembly including a cylindric 
inner bit 1 fastened to the head of hammer 7, a ring-shaped outer bit 2 
around the inner bit 1 and a protection tube 4 mounted in the hole while 
drilling and reaching the outer shell of outer bit 2. The inner bit 1 and 
outer bit 2 are interconnected by a bayonet joint, where rotation is 
transmitted to the outer bit 2 by the inner bit 1. The inner bit 1 also 
transmits the hammer impacts to outer bit 2. Outer bit 2 pulls protection 
tube 4 into the hole by means of the joint lap between them, which has 
restriction for the mutual axial motion. 
FIG. 2 shows that in the circle between bits 1 and 2 three bayonet joints 
are placed. In the inner bit 1 three axial grooves 6 are formed to run 
through the bit. Correspondingly, there are in outer bit 2 three 
projecting parts 3 with the same spacing which move in grooves 6. 
Sidewards from grooves 6 in inner bit 1 surface, as per FIG. 3, there are 
notches for projecting parts 3, into which the projecting parts 3 are 
guided due to the mutual rotation of the bits 1 and 2. Thereby, bits 1 and 
2 are in a locked state and bit 2 is rotated by bit 1, while the locking 
is retained. The locking is opened by reverse rotation. Projecting parts 3 
are square in order to receive the impacts from a larger surface area of 
the inner bit 1 and to transmit them to the outer bit 2. Groove 6 of inner 
bit 1 is also a flow channel for the flushing medium, whose direction of 
flow, in this case, is backward in the channel. 
In FIG. 4 drilling has advanced to a location, where ring bit 2 and 
protection tube 4 have been left. Drilling has proceeded only by bit 1. 
For demounting of the inner bit 1 from the outer bit 2, the rotation of 
the drill assembly has been reversed. Hammer 7 transmits the rotation to 
inner bit 1. Reverse rotation of inner bit 1 opens the bayonet joint in 
such a way that projecting part 3 turns in groove 8 and enters groove 6. 
Thereafter, drilling further by reverse rotation, projecting parts 3 glide 
out from grooves 6 to the front side of the ring bit, where the ring bit 2 
stops rotating and remains totally immobile. The inner bit 1 drills rock 
or other hard ground that does not collapse into the hole. If drilling is 
stopped at a wanted depth or length, bit 1 and the drilling equipment are 
pulled out from the hole. Bit 1 passes ring bit 2 in running through it, 
when the bit 1 is turned into a position where projecting parts 3 hit 
grooves 6 and glide through bit 2. 
FIG. 5 illustrates an example of ring bit 2 with several projecting parts 3 
for several sequentially arranged axial bayonet joints, by means of which 
construction a face to transmit impacts to ring bit 2 is produced. 
Likewise, the protection tube 4 is pulled with several sequential shoulder 
faces. On using many sequential bayonet joints and pulling shoulders of 
protection tube 4, it is possible to make ring bit 2 thinner and yet 
strong enough for drillings of longer duration. 
By means of a method and a bit assembly as per this invention it is 
possible to carry out, advantageously, s.c. anchoring drilling, where 
drilling is done by the outer bit until rock is hit and a certain distance 
by the inner bit in the rock. Since reversed flushing is used for removal 
of drill waste, whereby there is through inner bit 1 a straight channel 
tube up to the surface, an anchor wire rope can be taken through the 
channel to the bottom of the bore and, for instance, on pulling up the 
bit, concrete can be poured through the channel into the hole. When the 
hole is filled and bit 1 pulled out to the level of ring bit 2, the outer 
bit 2 and even the protection tube 4 are then pulled out and the hole is 
filled with concrete at the same time. If during drilling the ring bit 2 
and the protection tube 4 have been left in the hole at too early a stage, 
the inner bit 1 can be pulled up to the ring bit 2 and locked and drilling 
continued. In order to carry out properly the mutual rotation of bits 1 
and 2 necessary for locking, it is worth it to record on the ground the 
mutual depth position of protection tube 4 and the drill rod inside it, 
when bits 1 and 2 are locked, so that relocking could be made in the same 
position. This is of great significance, especially if there are several 
sequential bayonet lockings between the bits as shown in FIG. 5. 
The inner bit 1 according to this invention includes transverse ring 
grooves on its outer shell 8 ending in a vertical wall 9. The groove 8 
flanks function as faces transmitting impacts to ring bit 2 and the face 9 
as rotation-transmitting means. The outer shell of bit 1 lacks stepped 
impact-transmitting shoulders known from previous designs. 
By means of a method and bit assembly according to this invention both 
vertical as well as horizontal holes can be drilled. Likewise, the axial 
groove of the bayonet joint can also be in the ring bit 2 and the 
projecting part in the inner bit 1.