Double decked


Tim Magub, LCAL blasting manager and Sam Jones, Leighton John Holland project manager for Contract 904 of the Hong Kong South Island Line delve into the new blasting technique developed for the project – double deck blasting.

John Holland, in partnership with Leighton Asia, is constructing the transport infrastructure project Contract 904, as part of the South Island Line (East) Rail project. The South Island Line (East) will be a medium-capacity railway covering approximately 7km from Admiralty Station to South Horizons Station on the south side of Aberdeen Harbour, with intermediate stations at Ocean Park, Wong Chuk Hang and Lei Tung. When completed, the South Island Line (East) will provide fast and reliable railway service for communities in the south of Hong Kong Island and help ease traffic congestion at critical bottlenecks like Aberdeen Tunnel and the central business district.

The project is located on the island of Ap Lei Chau at the southern side of Hong Kong island and requires extensive constructions works to be completed in close proximity to the existing residential estates of South Horizons and Lei Tung. The project also has areas of significant cultural heritage, including 18th and 19th Century temples, historical grave sites and the remains of the former Aberdeen Battery, which was destroyed by the British forces prior to the surrender of Hong Kong in 1941.

Owing to the site access constraints and location for the project, detailed planning and methodology innovations have been required to ensure the most effective and responsible means of environmental and heritage management and protection have been implemented to deliver the project works. The project team has developed and pioneered the use of 'Deck loading' in Hong Kong to minimise the impacts of blasting operations and demonstrated a strong sense of environmental duty through the project team's participation in the post typhoon clean-up of plastic pellets which spilled from a container ship and affected the marine environment.

The project was awarded in May 2011, construction commenced in July 2011 and is planned for substantial completion in July 2015. The joint venture will construct 1.1km of tunnels and two new underground stations at Lei Tung (LET) and South Horizons (SOH) as part of the contract, along with related plant and ventilation buildings. Approximately 1km will be primarily constructed by drill and blast mining techniques, with the remainder using the cut and cover method. The mined tunnel will be predominately a twin track single-bore tunnel. The tunnel splits into two single-track tunnels at the approach to the LET Station and continues as two single-track tunnels from LET to the interface with the cut and cover tunnel at Ap Lei Chau Drive. The drill and blast also includes two pedestrian access adits and a vertical shaft.

The drill and blast tunnels are predominantly beneath the mountain of Ap Lei Chau. The geology along the alignment is in the most part moderately strong to strong grade III and II rock. Indicated compressive strength is of the order of 200MPa and above, with high abrasivity.

The alignment also passes in close proximity to high voltage (132kV) cable tunnels, a water reservoir, steep slopes, retaining walls, water distribution pipes, schools and high rise housing developments. As a result, the maximum instantaneous charge (MIC) weight that can be fired while blasting for tunnel excavation has been severely limited. MIC's were reduced down to less than 0.1kg with single deck advance lengths limited to 0.5m. Here double decking could achieve 1m.

The cut and cover tunnel consists of twin, single track, reinforced concrete box structures extending from the interface with the mined tunnel to an integrated ventilation chamber structure. The shaft at South Horizons is built using cut and cover construction methods. Further there are two rock slopes that need to be cut to construct plant and ventilation buildings. The JV needs to erect a temporary traffic deck at the South Horizons Station to maintain the operation of the public transport interchange while work will continue below.

Method Management

The John Holland Tunnelling Division partnered with Leighton Asia to develop a new blasting technique for Hong Kong to counteract significant constraints on charge weights (explosive quantity per drilled hole).

The charge weight constraints imposed by the MTR were recognition that most sections of the drill and blast (D&B) tunnels were in close proximity to sensitive receivers and hence could be potentially affected by blasting operations during construction. These imposed constraints resulted in the designed the explosive weights to be low (in some cases theoretically zero), reducing the possible pull length (length of advance per blast) and as result extending the program duration. JHT was able to develop the concept of 'deck loading', effectively doubling the pull length of many originally constrained blasts.

In addition, JHT were able to develop the expertise and promote understanding within the JV for this pioneering technique, in Hong Kong, where traditionally new techniques are difficult to get accepted.

The JV was awarded the contract for the South Island Line (E) 904 Lee Wing Street Ventilation Adit in May 2011. Construction works began in July 2011 and substantial completion is expected in July 2015.

Blasting commenced in March 2012, operating under restricted maximum instantaneous charge weights (MIC's) due to the nearby presence of sensitive receivers. The decision was made to design and execute double decked tunnel blasts to maximise the metres advanced per blast.

First Implementation

In April 2012 the first double-decked blast was fired resulting in an advance of 3.2m, double that could be achieved with the traditional single explosive deck blast. Over the following 12 weeks a further 19 double decked blasts were successfully initiated culminating in July when a 5.8m advance of the 120m 2 blast face was fired resulting in 696 bank cubic metres being broken.

The successful design and execution of double decked tunnel blasts was made possible by ensuring that the pyrotechnicdelay elements of each detonator in the blast were active (burning) before the first blasthole detonated. The initiation sequence was therefore a key element in the blast design process.

Fragmentation of all double decked blasts was considered excellent, enabling mucking-out to be completed in the shortest possible time.

The Lee Wing Ventilation Adit and the running tunnels pass under numerous sensitive receivers which have specific vibration (peak particle velocity) limits expressed in mm/sec. The sensitive receivers control (usually reduce) the MIC's and therefore the advance lengths. The first 20 double decked blasts pulled a total of 77m, double the expected pull utilising a single decked (traditional) blast design using a very low average powder factor of 0.86kg/m3. The double decked tunnel blasting technique will be regularly employed to achieve the longest possible advance, shortening the excavation time required to complete the project drives.

Concluding Remarks

The technique is applicable to urban hard rock excavation and tunnelling works, there are future potential benefits to be reaped from the use of this technique, such as on deep foundation works, metro station, shaft and tunnel excavations. The deck loading method has been employed quarrying, and shaft/station works in Australia, Hong Kong and Singapore however this was the first use of the method in a tunnelling application in Hong Kong.

This can be used in tunnelling where the MIC restrictions are imposed in competent rock and production is possibly limited.

Double decking

During the tender period John Holland's tunnelling business worked closely with Leighton Asia to develop a new tunnel blasting technique that would counteract the project constraints. The JV pioneered the use in tunnels of two explosive charges (known as decks) within a single blast hole drilled to approximately twice the normal depth, a technique now called 'double decking'. The technique uses delays to initiate the two decks at different times to reduce the amount of explosive detonated at a given instant to minimise vibration and double the advance length achieved in a blast cycle.

This is a method for use when MIC's are restricted. It gives you the opportunity to effective get twice the blast length for a given MIC and increases the production rate.

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