Center-cut method for tunnel excavation utilizing large...

Ammunition and explosives – Blasting – Patterned blasting

Reexamination Certificate

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C102S313000, C102S302000, C299S013000

Reexamination Certificate

active

06431075

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to tunnel excavating methods, and more particularly to a center-cut blasting method for tunnel excavation utilizing large unloaded blast holes and a circular pre-split, which employs a pre-splitting technique that blasts loaded auxiliary holes alternately arranged along with large unloaded auxiliary holes in a circle and a sub-drilling technique of rooting away blast holes, thereby facilitating a center-cut operation by weakening the binding force of an original rock and easily achieving dual free surfaces, and shortening the period of execution and reducing execution costs.
2. Description of the Prior Art
Recently, in order to excavate tunnels and underground spaces for underground storage facilities, communication or electrical cable tunnels, waterway tunnels and traffic tunnels, blasting for tunnel excavation is frequently performed.
In general, such tunnel blasting is performed in the following three stages.
With reference to
FIG. 1
, a first stage of the tunnel blasting is the stage of drilling center-cut holes
1
, cut spreader holes
2
, floor holes
3
and roof holes
4
to predetermined depths, a second stage is the stage of loading the drilled holes
1
,
2
,
3
and
4
with detonators and explosives, and a third stage is the stage of detonating the detonators using a triggering device. Referring to
FIG. 1
, the detonators are detonated from the center-cut holes
1
to the outer holes
2
,
3
and
4
.
The center-cut holes
1
are loaded with delay detonators in such a way that the detonators are bilaterally symmetrically arranged in the center-cut holes
1
in the order of detonation in an upward direction. The cut spreader holes
2
, the floor holes
3
and the roof holes
4
are loaded with delay detonators in such a way that the detonators are arranged in the holes
2
,
3
and
4
in the progressing order from the center to the outside. In such a state, the tunnel is blasted by detonating the detonators using a triggering device. The detonations of the detonators are sequentially performed in the progressing order; the center-cut holes
1
, the cut spreader holes
2
, the floor holes
3
and the roof holes
4
.
Tunnel blasting is mostly performed using a single free surface, and generally employs center-cut blasting so as to create new free surfaces.
The center-cut blasting creates dual free surfaces by blasting the center portion of the working face, and is an important factor that governs the success of the entire tunnel blasting.
In
FIG. 1
, reference characters
1
′,
2
′,
3
′ and
4
′ designate a center-cut region, a cut spreader region, a floor region and a roof region, respectively.
The “free surface” denotes the surface of rock in contact with an alien sphere, such as air or water. The free surface considerably affects blasting. That is, a blasting effect is in proportion to the number of free surfaces and the degree of proximity of loaded positions to the free surfaces. The reason for this is that resistance is weak in a free surface side and thus blasting energy generates heavy stresses in the free surface side. A “burden” denotes the shortest distance from a free surface to the center of an explosive.
The “center-cut hole” denotes blast holes within a center-cut region of 1.5 to 2.5 m by 1.5 to 2.5 m. A “central center-cut hole” denotes a single loaded blast hole at the center of the center-cut region. “Auxiliary center-cut holes” denote the center-cut holes except for the central center-cut hole arranged around the central center-cut hole in a circle having a predetermined diameter. The auxiliary center-cut holes are comprised of unloaded and loaded auxiliary (blast) holes. “Spreader center-cut holes” denote loaded holes that are arranged in circles around the auxiliary center-cut holes.
As illustrated in
FIGS. 2
a
and
2
b
, a conventional tunnel blasting is performed, in such a way that center-cut blast is primarily performed using a cylinder-cut method to obtain dual free surfaces, and the spreader center-cut holes, the cut spreader holes
2
, the floor holes
3
and the roof holes
4
are secondly and sequentially blasted.
That is, the conventional tunnel blasting is the blasting in which the spreader center-cut holes, the cut spreader holes
2
, the floor holes
3
and the roof holes
4
are sequentially blasted after center-cut blasting is performed by blasting the loaded holes (center-cut holes) each having a diameter of 38 to 45 mm drilled around one to four unloaded holes each having a diameter of 65 to 120 mm.
In the conventional tunnel blasting employing the cylinder-cut method, when center-cut blasting is not sufficiently performed, dual free surfaces are not easily formed during the blasting of the other blast holes, thereby causing many remaining holes to exist. Accordingly, the excavation efficiency is relatively low, that is, about 90% of a drill footage (less than 80%, depending upon the quality of rock). As a result, the advance formed by a single set of blasting is relatively short, so that the number of sets of blasting should be increased.
Because of the problem, there occur problems in which the work period cannot be shortened owing to a long excavation period and a long reinforcement period, excavating costs and drilling costs per blasting set are excessively high, and divisional blasting should be performed to minimize blast vibrations.
In the meantime, conventional center-cut blasting is performed by slant hole center-cut blasting (that is, V-cut blasting) or horizontal center-cut blasting (that is, cylinder-cut blasting; the cylinder-cut blasting is an improvement from burn-cut blasting).
The V-cut blasting is applied where slant center-cut holes are short, outside holes (such as cut spreader holes, floor holes and roof holes) are long, and the drill footages and drill angles of blast holes can vary. The V-cut blasting is chiefly applied to the blasting of short holes in which its advance is less than 2 m. The cylinder-cut blasting is applied where one to four unloaded holes each having a diameter shorter than the diameter of each loaded hole are drilled in parallel with the tunnel axis to the same drill depth as that of the loaded hole. The cylinder-cut blasting is chiefly applied to the blasting of long holes in which its advance is longer than 2 m.
As illustrated in
FIGS. 3
a
and
3
b
, the V-cut blasting is performed in such a way that three or four sets of loaded central holes are drilled in the central region of a tunnel in parallel with one another with each set comprised of two opposite loaded central holes, the loaded central holes are simultaneously blasted to create a new free surface, and outer holes are blasted in the order of spreader center-cut holes, cut spreader holes, floor holes and the roof holes to expand the created free surface.
The V-cut blasting is center-cut blasting that has been employed for the longest time. In the V-cut blasting, the bottoms of the drilled center-cut holes are situated in a line with two center-cut holes of each set facing each other, and the interval between two burdens is 100 to 150 mm. Accordingly, the volume of a fractured rock portion is large and the projected area of blasting is wide due to the slant center-cut holes, large fragments are easily formed during center-cut blasting, and the center-cut holes can be drilled in various patterns.
The V-cut blasting is advantageous in that in comparison with the burn-cut and cylinder-cut blasting, the drilling of holes is easy, the drilling footages of holes are short, the flying distance of fracture is short owing to the creation of large fragments, the V-cut blasting is effectively applied to the blasting of short holes or a soft rock, the occurrence of dead pressure may be generated, and a target drill footage can be achieved regardless of inferior drilling due to a large free surface.
However, the V-cut blasting is disadvantageous in that its advance is restricted, a plastic region and extra excavatio

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