In order to increase the water and energy supply to the 12 million people in the metropolis of Tokyo a further storage basin, with an area of 85,3 km2 (33 miles2) and containing 1'150 million m3 (1'500 million yds3) of water, was recently constructed. An essential part of the project situated near Shiokawa - 150 km (93 miles) west of Tokyo and 900 m above sea level in the 3'000 m (9'800 ft.) high Kanto Sanchi mountains - was a cut-off wall to seal the permeable, fissured rock under the earthfill dam against underflow.
Use of conventional methods to cut the socket of the cut-off wall to a sufficient depth into the hard bedrock strata, which had compressive strengths from 150 to 230 N/mm2 (21,34 to 37,71 ksi), were unsuccessful, particularly at the almost vertical andesite slopes of a volcanic funnel on the eastern (left) side.
Several time consuming and expensive site trials were carried out in co-operation with the engineers of the main contractor, Hazama Gumi, using hydraulic grabs, down-the-hole hammers and a trench cutter armed with tungsten carbide bits. These methods came to a halt when granite and magnetic sandstone boulders with compressive strengths of more than 100 N/mm2 (14,22 ksi) were encountered. Further trials of a trench cutter laced with optimum round shank chisels were unsuccessful as it was only possible to achieve a cutting rate of 1 cm (3/8 in) per hour.
The problem was solved with a Bauer BC 30 trench cutter, equipped with Bauer´s patented new generation of cutting wheels, armed with an innovative design of roller bits. Following a short trial period to adjust the roller bits into an optimum arrangement to suit differing rock strata, a penetration rate of 10 to 20 mm (3/8 to 3/4 in) per minute was achieved. These rates allowed the socketing of the cut-off wall to be achieved particularly on the eastern side to the full satisfaction of the client and main contractor.
Excavating the single 64 cm (25 in) wide trenches in lengths of 2,60 m (8 ft. 6 in) at the above high penetration rates in such hard rock, indicates that Bauer has the ability to take on similar projects with optimism. Also when cutting with roller bit armed wheels through slightly cohesive overburden strata with a preponderance of stones, cobbles and big boulders, high performance rates, compared with normal cutting wheels armed with round shank chisels, were achieved.
Even though the hardest rock strata on the Shiokawa dam site were encountered at about 16 m (52 ft.) below ground level, Bauer´s BC 30 HRC cutters can excavate down to 55 m (180 ft.) when suspended from a standard crawler crane and to even greater depths when operated in conjunction with a customized crane.
Scope of work:
Excavation of a slurry trench, cut into hard andesite rock for a 64 cm thick, 7'500 m2 cut-off wall with a Bauer BC 30 HRC trench cutter.