TUNNELING BY ROCK FRACTURING METHOD
UNDER SMALL CROSS SECTION AREA
Tatsuya Noma, Toshiro Tsuchiya, Nagayasu Mitsugochi

FUJITA CORPORATION Technology Development Division, 2025-1, Ono, Atsugi-city,
Kanagawa-pref., 243-0125, Japan

Keywords: rock fracturing method, free face, small cross section area

INTRODUCTION
Due to new, stricter fire safety standards, new evacuation tunnels are being added to Japanese highway
tunnels constructed in the period 1965-1980.
The new evacuation tunnels connect the up and down lanes of existing twin-bore road tunnels, with atleast one lane remaining open during the construction period. Therefore, blasting methods that create
large noises and vibrations can’t be used and contractors are forced to use mechanical excavation.
Machine excavation of hard rock is classified into two types: single machine excavation and rockfracturing methods. The single machine excavation has some limitations: short-distance excavations are
not economical using TBMs, and compressive strength can limit use of a partial face machine. Because
these evacuation tunnels are put into the center of existing tunnels, ground conditions of excavation areover 200MPa UCS. With tunnel, cross sections of 10-20 m2 and lengths of 50-60m, it is impossible toexcavate by single machine excavation.
So, the rock fracturing method is considered most suitable for machine excavation of hard rock in thesetunnels. For the rock fracturing method, a free face is artificially formed at working face. Then, the freeface is used to crush rocks.
In this paper, two cases of evacuation tunnel’s excavation are described, one’s cross section is 20m2, andother’s cross section is 10m2.
SIGNIFICANCE OF FREE FACE
A free face means a face that does not restrict rupture at the time of crushing. When a tunnel is excavatedby blasting, only one free face is formed at the working face. In the case of blasting method, the crushingforce is strong and excavation has become possible even with a free face as center cut blasting has beendevised.
On the other hand, the crushing capacity of the rock fracturing method is less powerful than blasting, andcrushing with one free face is impossible. In the other words, though cracks are caused at the working
face by some method, it is difficult to cause large cracks when only one free face is formed. Two or more
free faces secure rocks-moving space to make cracks larger. So, it is necessary to artificially form free1

Continuous Drill
Holes (Free Face)

Rock Bridge
Bore Hole

Figure 1 General concept of the free face formation at the tunnel.

faces at the working faces of tunnels.
Figure1 shows a conceptual diagram of free face formation in a tunnel. When a continuous hole is drilled
like a groove as shown in the Figurer the working face is divided into blocks. This brings about multiple
free faces and makes crushing rocks easy. When natural ground becomes hard, it is difficult to crack it
and many grooves should be formed. Continuity and widths of grooves formed influence rock fracturing
efficiency. If there remain rock bridges (as shown in the Figure) that interrupt continuity of grooves, alarge compressive force is necessary for cracking. Also, the wider the grooves that are formed, the larger
the moving spaces created and the easier rocks come to be cut.
However, it takes considerable labor and time to form grooves on hard rocks. So, it is very important for
excavating tunnels by the rock fracturing method to form continuous free faces efficiently andeconomically. The usual method requires a special forming device and has some problems with forming
efficiency and accuracy of continuity1). To solve these problems, they have developed the method to drill
single holes more efficiently and continuously than by the conventional method2).
After the continuous hole drilling, a large-scale hydraulic wedge was used for rock fracturing (primaryfracturing). This hydraulic wedge is mounted on a 0.4m3 backhoe, and can fracture a depth of 1m, andneeds bore hole as 100mm diameter and 1.6m depth. The hydraulic wedge has the following features (1)high crushing capacity, (2) creates wide cracks and (3) control of crack causing direction. Also, there is asmall handheld type hydraulic wedge, so-called “DARDA”, but its fracturing capacity is inferior.
After these processes, a breaker was used for the secondary crushing to complete the rock fracturing.
FREE FACE FORMING METHOD
As mentioned above, they paid special attention to the following points in developing the new free faceforming method: (1) High free face forming efficiency, (2) Sufficient maintenance of continuity accuracyand (3) Applicability to general purpose drill jumbo. The following section outlines this method andexamines the efficiency of this method. For the free face forming method, they adopted a general-purpose2

drill jumbo and the continuous drilling
method of single hole that can display
this ability to the maximum.
In the case of continuous drilling of asingle hole, the rod bit is apt to drill ahole that tends towards to a neighboring
existing hole. They developed themethod to drill a continuous hole byinserting an SAB (Spinning anti-bend)rod (φ89mm) in a neighboring existing

Fixing Nut

BracketOuter Pipe

Tip Cone

Rod T45

Center Pipe
Boring Bit

Ó 02
1

Guide Cell

hole and by making a bit touch andFigure 2 Concept of SAB rod.

strike against an SAB rod by using this property.
Figure 2 shows the schematic diagram of the SAB rod.
Contacting the bit to and striking it against the SAB rod at thetime of drilling a hole do not cause any clearance between thebit and the rod and secured continuity of a free face. This SAB
rod is structured to rotate and enables drilling a hole at highspeed when the resistance by contacting and striking at thetime of drilling a hole is reduced. The SAB rod is designed
to be fitted to the bracket on the tip of the guide cell of ageneral-purpose rock drill and is inserted and pulled out when
the guide cell is moved.
Figure 3 shows the drilling procedures. The drilling procedures
are as follows:(1) Insert the SAB rod in an existing hole.
(2) Start drilling a hole. The bit drills a hole while it touches
and strikes against the SAB rod. The SAB rod rotates as
the bit rotates, and this makes a high speed drilling
possible.
(3) Keep drilling to the specified depth.
(4) Keep inserting the SAB rods one after another anddrilling to form a continuous hole (free face). It makes
high continuity free face forming possible that the bittouches the SAB rods.

(1)

SAB Rod

Boring Bit

(2)

(3)

(4)

In addition, it reduces wear of rod and bit and makes evenFigure 3 Procedure of continuous hole drilling.
the SAB rod itself wear evenly so that the SAB rod canrotate. So, the SAB rod can be used for hours. Also, the consumable part of the SAB rod is the thick pipe
only, and the SAB rod is very economical.
As mentioned above, the general-purpose rock drill is generally used for fixing rock bolts and drilling
fracturing (or loading) holes, but mounting a SAB rod on a general-purpose rock drill makes it possible to3

drill continuous holes. So, no dedicated machine is necessary.

4,250

Traf fic L ane

Regul ation Lane

5,300

Figure 4 The arrangement of the machine for drilling rock in A tunnel.

EVACUATION TUNNELS EXCAVATIONIn this paper, the evacuation tunnel with a cross sectional area is 20m2 is called A tunnel, and anothertunnel (cross section of 10m2) is called B tunnel.
Because both tunnels’ cross sectional area is very small, machine sizes are limited. Figure 4 shows thearrangement of the machine for drilling rock in A tunnel. In order to keep one lane in service, the machinemust come in a right angle using only one regulated lane.
Table 1 shows a comparison of construction machines for tunnels of normal cross-sectional area (morethan 50m2), A tunnel, and B tunnel.
This table shows as below.
With a normal size tunnel excavation a drifter of over 170kg can be used (so-called “Rocket Boomer).
However, for evacuation tunnels a 140kg or 150kg drifter has to be used. This means that many times arerequired for drilling (both continuous holes drilling and rock fracturing holes drilling).
Also, for a normal size tunnel excavation a 2000kg breaker can be used, whereas for evacuation tunnels aless efficient 400kg or 800kg breaker has to be used.
Therefore, (1) keeping high free face forming efficiency, (2) being exploited hydraulic wedge's abilities tothe full, and (3) crushing almost bedrock by primary fracturing are very important.
Table 1 Comparing main using construction machines.

Rock Drill
Fracturing machine
Weight of breaker

Normal tunnnel
2(Cross section area over 50m )Three-boom rigs
Rock drill's weight: over 170kg
Two hyadraulic wedges
3with 0.4m backhoe

A evacuation tunnnel
2(Cross section area 20m )Two-boom rigs
Rock drill's weight: 150kg
One hyadraulic wedge
3
with 0.4m backhoe

B evacuation tunnnel
2(Cross section area 10m )One-boom rigs
Rock drill's weight: 140kg
One hyadraulic wedge
3
with 0.4m backhoe

2000kg

800kg

400kg

4

Interval ofBoreholes

Free Fac eÓ
102

Borehole102
ÓFigure 5 The drilling pattern in the A tunnel.

A evacuation tunnel
A evacuation tunnel is 58.2m long and connects theup and down lanes of existing twin-bore roadtunnels.
The geology along the route is rhyolite. Around theportal the bedrock is cracky with a uniaxial strengthof up to 100MPa, but the bedrock becomes hardwith excavation of tunnel, and the uniaxial strength
at center of tunnel is about 230MPa.
Within 8m of the portal, the large-scale hydraulicwedges can’t fit in the tunnel, “DARDA” is used for

Secondary
Fracturing
5%

Others13%Primary
Fracturing
33%

Freeface
Forming15%

Total
1638 min/m

Fracturing Holes
Drilling
34%

fracturing. Between 8m to 18m, the bed rock isFigure 6 An example of the cycle time in the A tunnel.
cracky and uniaxial strength is up to 150MPa. In thisregion an artificial free face is not necessary, only a large-scale hydraulic wedge and 800kg breaker are
needed for excavation. But after 18m, where the bedrock is intact and the uniaxial strength is over150MPa, artificial free face forming is necessary. Figure 5 shows the drilling pattern in this case. Forming
free face is one line about spring line, less than normal size tunnel’s free face as forming periphery in
Figure 1. The main reason for this is that the cross sectional area is small, and, when drilling on theperiphery, holes can’t be bored square to the tunnel face, and some angle be remains. This angle becomes
overbreak, and overbreak should be preventable.

5

Figure 6 shows an example of the cycle time. In the figure, about half time is spent by boring, and thirdpart of time is rock fracturing.

Center ofTunnel

Center ofTunnel
Evacuation Tunnel

Down LineUp Line

Figure 7 The longitudinal profile of the B tunnel.

As for the A tunnel, within 8m from the portal, thelarge-scale hydraulic wedge can’t fit into the tunnel,“DARDA” is used for fracturing. At 8m from the portal,
the entrance is enlarged for the large-scale hydraulicwedge entering.
Figure 8 shows the drilling pattern in this case. Forming
free face is one line about spring line same as A tunnel,
but at most hard rock case, free face is forming at crownabout 2m because the route is upgrade.
Figure 9 shows an example of the cycle time. In thefigure, about 60% is spent boring, but less than 20% isrock fracturing. This result is different from A tunnel.
This occurs for two reasons. One reason is number ofbooms on the rig, A tunnel uses a two boom rig, but B
6300

400

300

400

300

300

300

5 00500

300

500

500

300@5‚‚ bk

B evacuation tunnel
B evacuation tunnel is 52.5m long and connects the up anddown lanes of existing twin-bore road tunnels.
The geology of this route was expected to be sandstone andslate with a strength of about 100 MPa, but in practice was
hard sandstone with a strength of 170-230MPa..
Figure 7 shows the longitudinal profile of the tunnel. Thistunnel’s cross section area is 10m2 for man traffic, because
there is an 8m difference in elevation between up line anddown tunnels. A large quantity of water is necessary fordrilling, excavation is started from up line for drainage bygravity.

4003300

Figure 8 The drilling pattern in the B tunnel.

Secondary
Fracturing
5%

Others21%

Freeface
Forming15%Total
1370 min/mPrimary
Fracturing
17%
Fracturing Holes
Drilling
43%

Figure9 An example of the cycle timein the B tunnel.

tunnel uses a one boom rig. The second reason is the difference of drifter’s capacity. The 150kg drifter has
about 1.5 times ability in comparison with the 140kg drifter.
Thus, when making an execution plan by the rock fracturing tunneling, it is necessary to take ability andthe boom number of rock drills into account.
In tunneling such small size by rock fracturing, it is possible to excavate that making continuous free face.
Figure 10 shows the state of rock fracturing by the large-scale hydraulic wedge in the A tunnel, and figure
11 shows the state of making free face in the B tunnel.

Figure 10 The state of rock fracturing by the large-scale
hydraulic wedge in the A tunnel.

Figure 11 The state of making free facein the B tunnel.

CONCLUSION
In this paper, two cases of small size tunneling by the rock fracturing method are shown. Conclusions aredescribed as below.
• In tunneling such small size by rock fracturing, it is possible to excavate that making continuous
free face.
• The efficiency of excavating is depending on the ability of the rock drill.
REFERENCES
Hagimori, K., Furukawa, K., Nakagawa, K., Yokozeki, Y. (1991), Study of non-blasting tunneling by slot drilling method, Proc.
7th ISRM Int. Congr. on Rock Mechanics, Aachen, pp. 1001-1004.
Noma, T., Tsuchiya, T. (2003), Development of low noise and vibration tunneling methods using slots by single hole continuous
drilling, Tunneling and Underground Space Technology, vol.18, pp.263-270

7