Controlled Shaft Sinking Using Electronic Delay Detonators and Water Ballast

摘要

The King George V (KGV) Shaft forms part of MTR Corporation Limited (MTRCL) Works Contract703 extension of the underground West Island Line on Hong Kong Island. The Shaft and the associatedadit was the sole access to excavate the running tunnels. Originally during the Detailed Design stage, itwas concluded that blasting was not feasible at the KGV Shaft due to the close proximity of sensitivereceivers in the form of residential buildings, electrical cables and hospitals. As a result, the Contractorplanned to excavate the Shaft using mechanical means with a programme time of seven months. TheContractor appointed a Sub-contractor to analyse the feasibility of excavating the Shaft by blasting. TheSub-contractor reviewed the proposal and recommended the use of an innovative blasting technique thathad never been previously employed in Hong Kong. The Sub-contractor recommended the Shaft beblasted using DetNet electronic delay detonator technology, manufactured by AEL Mining Services inSouth Africa, with an initial 1.5m depth of water ballast covering the blast face. The electronic delaydetonators provided initiation accuracy to ensure there would be no overlap between sequentiallydetonating blastholes. The water ballast makes use of the effect of energy lost through the shock frontcausing the water to break up into droplets creating a spray dome resulting in the attenuation of theenergy imparted into the surrounding rock mass. Various tests have demonstrated that both themaximum overpressure and impulse density are significantly reduced when water of a minimum depth isin direct contact with detonating explosives. This innovative blasting technique reduced the excavationprogramme by three months, saved HK$5 million in direct costs and totally mitigated the impacts on thepublic and other stakeholders nearby the KGV Shaft. Peak Particle Velocities (vibrations) rarelyexceeded 2-3mm/sec (0.08-0.12in/sec) at a distance of 30-40m (98-131ft) from the blast location and airoverpressure levels were reduced below the Hong Kong target level of 120dBL. In total, 24 blasts werefired with an average advance of 1.6m (5.25ft) with an average powder factor of 2.03kg/m~3 (0.127lb/ft~3)to 1.84 kg/m~3 (0.114lb/ft~3). The initiation pattern had 5ms between sequentially detonating blastholesand 20ms between adjoining rings.