Fig. 2.3: Drilling jumbo and first steel rib at A-A
mote control. This avoids the need to erect scaffolding or use the drilling jumbo to get the nozzle operator up to the roof in large tunnels: its most significant value is in enabling an area. of unsafe tunnel arch to be treated immediately after blasting (before
mucking out) without the need for anybody to work in the danger area.
If ribs, rather than sprayed con- crete, were used it would first be necessary to scale off all dangerously loose fragments of rock, muck out rock from the previous blast and then
to install the ribs.
These processes
might occupy 12-18 hours in a large tunnel, and would all involve men working in the danger area.
The application of sprayed con- crete is usually quicker than rib in- stallation but this depends on the thickness of spray, the size of ribs and many other factors. Advantage was taken in the main tunnel of the stabilising effect of the sprayed con- crete on the rock to reduce the thick- ness of the shuttered concrete lining of the main tunnel to 10 in. This thickness was made standard for all but the very worst rock conditions where steel arches had to be used as temporary support, followed by a thicker lining.
Most of the rock encountered in the main tunne] was rhyolite which in many places was closely jointed (joints a few inches apart). The joints usually contained kaolin and so the rock had a tendency to loosen and drop out in small blocks. Scaling off the loose pieces after blasting did not eliminate this danger as the loosening soon extended into the rock behind. A thin layer (3⁄41⁄44 in. or so) of sprayed concrete was often enough to forestall this process: however, the average thickness was about 4 in. due to the greater thickness applied in areas of weak rock.
It should be borne in mind that the size of tunnel strongly influences the required strength of temporary sup- port. A particular rock would re-
56
MG REMOVED
BEFORE LINING
STEEL NO
1NVERT
SECTION A-A
BENCH
E
PILOT TUNNEL
(VOIR GROUTED.
APPLICATION OF SPRAYED CONCRETE
BOTH HO NỮ BEAM IN RENFORCED COMORE TE TO CARRY ARCH THRUST (SYSTEM REPEAT= ED AT HVERT LEVEL IF NECESSARY)
SPRAYED CONCRETE I20 PLACE OF LAGGING
TUNNEL HIVERT
GALLERY
BENCH
__SECTION__8-8
V PILOT TUNNEL
METHOD
SECTION C-C_
OF
TUNNELLING
IN
BAD
GROUND
FIG. 2.4
Far East Architect & Builder December, 1967