LARGE HEAVY GRAB DREDGER EQUIPPED WITH ROCKBREAKER. EXPERIENCE OF ROCK EXCAVATION UNDER SWIFT TIDAL CURRENTS

摘要

This paper describes a large heavy grab dredger, which was built for rock excavation works under swift tidal currents. The dredger consists of a large heavy grab bucket (1274kN, 13m~3) together with a medium grab bucket for normal works, 3 sets of spuds to keep dredger's position, bow and stern thrusters for adjusting any shift, and a chisel type rock breaker (490kN). One peculiar feature of the dredger is that the dredging machine is driven by an electric motor, in comparison to the conventional diesel engine. The merit of DE dredger is that it can easily conduct horizontally controlled excavation, which is an important operation when a heavy grab is used to rip any bedrock. This kind of dredger took part in pier construction for the Honshu-Shikoku great bridges. The giant piers 78m in diameter and 62m in height were erected upon a fresh rock basement. Tidal currents exceeding 8 knots and a water depth extending to a depth of 57m had a significant influence on the work. Horizontally controlled techniques were introduced for the first time in order to conduct ripping work by the heavy grab bucket. From then on, the large heavy grab dredger has experienced some peculiar works different from usual dredging. This included the demolition of manmade objects partially buried in the seabed of Tokyo Bay and the preparation for a 60m deep trench for an immersed tube tunnel across the Bosphorus Strait. These experiences show the dredger's confident capability for the excavation of a variety of rocks using this type of dredger.