Tunnelling and underground infrastructures are increasingly in demand around the world nowadays to improve the transport network in densely populated and urban areas. This portends that construction sites will be placed near existing metro stations, highways, buildings etc. and therefore the strict tolerance limits of environmental impacts have to be observed. Very often, we encountered hard rock regions in the geology of the proposed area which requires rock removal by blasting. The Railway Protection Zone (RPZ) code classifies blasting operations as restricted, but can be performed successfully under controlled conditions.

In the Singapore MRT Project Downtown Line (DTL) Stage 2, Asia Tunnelling & Construction Pte Ltd (ATC P/L) was engaged by main contractors in several contracts to carry out blasting of the rock profile in the access shafts and the station boxes; one of them was partially inside the RPZ. This paper discusses the rock blasting process within the RPZ and evaluates the blast performance in the restricted area which was key in managing vibration levels and eliminating/minimizing fly rock incidents. Furthermore, this paper examines the risk management strategies of the blasting process and highlights the key success factors.

1. Introduction

The Singapore's Land Transport Master Plan (LTMP) outlines the plans for upgrading and nearly doubling the existing Mass Rapid Transit (MRT) network throughout the island, mainly going underground to make space for the valuable land surface for other high valued development. In 2014, Singapore already has five MRT lines under the Land Transport Authority (LTA), while just recently the construction of a new Thomson line was launched. The new Thomson Line will have 22 stations and 6 interchange stations which will be linked to the other five existing MRT lines and of which, three of these interchange stations lay within the Bukit Timah Granite formation. Extracting rock from its natural location is a difficult task which gets even more challenging when having to do so in a rail protective zone.

From geological approach the presence of hard rock is usually gladly welcomed by geotechnical engineers and geologists but the tasks become complicated when project sites are in close proximity with existing buildings, highways or in this case within the rail protected zone.

Rock blasting is restricted in Rail Protection Zones but the limited alternative choices for removing the rock (rock hacking and splitting, breaking rock with chemical means) are usually time consuming, costly or have insufficient results. Usually, the solution would be a combination of these methods, where possible, in which the majority of the rock is extracted with the Drill & Blast (D&B) method.

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