Paul Grad for TunnelTalk

Application of an excavation method known as double deck blasting has allowed rapid and safe tunnelling work to be completed for metro extensions under the dense and sensitive urban environment that is Hong Kong. The technique was first used by the Gammon Nishimatsu JV in development association with Orica on the JV's West Island Line metro extension contract and later by the John Holland-Leighton JV on a contract for the South Island Line rail link project. Paul Grad introduces the technique and its application in meeting strict noise and vibration criteria for drill+blast excavation in Hong Kong⁽¹⁾.

Due to the sensitive urban location of most underground projects in Hong Kong, the use of drill+blast excavation through its hard bedrock deposits that provide the foundation for its dense skyline of towering high rise buildings is strictly controlled. In particular, maximum charge weights are severely limited to meet strict vibration constraints. Meeting the weight constraints under normal blasting rounds would reduce the pull/round and extend the construction period considerably. To overcome those limitations, contractors working on two recent project contracts adopted the method known as the double deck loading method.

Fig 1. 7km long South Island Line alignment

The technique is base on drilling charge holes to a depth that permits two independent explosive charges within each single hole. The two charges are separated by an inert material that is inserted to prevent desensitisation or sympathetic detonation. The technique uses delayed detonation to initiate the two separate charges, with the net effect being a reduction in the vibrations resulting from each explosion compared to what they would have been as individual charging rounds. The method doubles the advance/blast, halves the number of blasts required, and reduces disturbance to residents nearby.

After initial application on the Gammon Nishimatsu JV contract on the West Island Line extension of the Metro, the technique was adopted by the John Holland-Leighton JV for a 1.1km of drill+blast tunnelling and underground construction of two stations on the South Island Line.

Following success of the project, Richard Buckingham, Senior Project Engineer of John Holland Tunnelling, said there is now considerable interest in the technique in Hong Kong, where owners have been slow in the past to embrace new methods. Application on the South Island Line said Buckingham proved so successful, that the majority of blasting work on the whole of the 7km long project were eventually undertaken using the double deck loading method, even though the initial intention had been to use it only in key restriction zones along the alignment.

The tunnels of the John Holland-Leighton contract for the MTRC, represent one of four contracts on the South Island Line (East) Rail Project, a medium-capacity railway on Hong Kong Island between Admiralty Station and South Horizons at Aberdeen Harbour. Underground construction elements along the Line include excavation of pedestrian subways and underground stations at Admiralty, Lei Tung and South Horizons with elevated stations at Ocean Park and Wong Chuk Hang. The John Holland-Leighton JV contract comprises 1.1km of tunnel excavation and the two new underground stations at Lei Tung and South Horizons (Fig 1). Work on the new line started in May 2011 and completion is scheduled for 2015.

Analysing the result of a double deck blast
(Photos courtesy of John Holland)

The rail line passes through a highly populated area that includes culturally significant buildings. It also runs beneath two cable tunnels, a reservoir, steep slopes, retaining walls, and water distribution pipes. Great care had to be taken to minimise noise nuisance to residents, and to ensure the safety of workers and pedestrians, and to minimise disruption to local traffic.

Before blasting can start on any tunnelling project in Hong Kong a blasting assessment report (BAR) is conducted. The aim of a BAR is to assess the impact of blasting on the surrounding structures, such as buildings, bridges, manmade slopes and retaining walls, natural slopes and services. The BAR assesses all environmental impacts of blasting, including flyrock, noise, ground vibrations, dust and fumes.

The BAR focuses largely on ground vibration, which usually has the main impact on the surrounding environment. A list of all sensitive receivers along the tunnel alignment is compiled, along with the location coordinates and a vibration limit for each of those receivers. The distance of receivers along the alignment, and the corresponding vibration limits for each, creates a table of allowable maximum instantaneous charge weights as the project advances. Ground vibration levels are predicted on the basis of Hong Kong blasting records kept between 1984 and 1992. After final approval, the Hong Kong Government uses the BAR as a tool to oversee the blast designs.

Drilling charge holes at the tunnel face

While most of the underground sections are drill+blast excavations in rock, mechanical excavation methods are applied in some sensitive areas at shallow levels and cut-and-cover is used for shallow excavations in soft ground. The underground alignment is mostly a double track single-bore tunnels that split into single track tunnels at the approach to Lei Tung Station and continue in single track tunnels to the cut-and-cover tunnel at Ap Lei Chau Drive. The geology comprises mainly highly abrasive, moderately strong to strong Grade III and Grade II volcanic tuff with compressive strengths in the order of 200MPa.

Application of the technique by the John Holland-Leighton team was set up and progressed in a methodical way with careful advanced design of the blast initiation sequences and with all due consideration of potential risks. The development sequence undertaken by the team included:

• Developing expertise in the subject of deck loading for the blasting works;
• Undertaking a thorough background study;
• Developing relationships with experts and materials suppliers;
• Fostering an innovative environment;
• Working toward acceptance and trust in new ideas;
• Justifying the development process and techniques;
• Ensuring technology, equipment and materials are available to make it possible;
• Commitment from all levels of project management to pioneer the technique;
• Technical feasibility - testing of the technique prior to committing to it on site;
• Taking on an acceptable level of risk, and;
• Trial blasting.

In practice, the technique introduces new challenges to the excavation cycle. These include increased complexity of the charge hole loading process, the insertion of the inert deck to separate the two explosive charges per charge hole, and the doubling of the number of charges that have to be timed. Through the combination of a specially sequenced loading method, production of crushed aggregate stemming plugs, use of a digitally programmable electronic initiation system, and some in-field optimisation, those challenges were overcome.

Loading the charge holes with the double deck charges

In some sections of the tunnel works, maximum instantaneous charge (MIC) weights were as low as 0.2kg. Using conventional single blasting methods, such MIC weight restrictions would have restricted advance rates to about 0.5m per round. Double deck blasting allowed this rate to be doubled for every blast fired, but, crucially, without increasing the MIC.

For the South Island Line (East) rail extension, the first double deck blast was fired in April 2012, resulting in an advance of 3.2m, double that which could have been achieved with the traditional single detonation per charge hole. In July 2012, a 5.8m double deck pull of the 120m² tunnel face was recorded.

While the double deck blasting concept is simple, its execution with a conventional initiation entails several challenges. Using a conventional non-electric initiation system, it is possible to choose only predetermined delays, and limited control over sequencing and burden relief. There is also a much greater chance of error and misfire.

To overcome those limitations the project team decided to use a programmable electronic blasting system, developed in association with supplier LCAL, which allowed flexibility in design and firing time and greater accuracy. For this project, the system selected was accurate to within 0.1% of its nominated firing time and could be programmed in 1 millisecond increments from 0 to 10,000 milliseconds. This guaranteed the correct firing sequence and avoided more than one charge firing at any instant, even with the greater number of charges required.

Drilling charge holes for bench excavation

The middle stemming (inert) deck is crucial for the separation of the two charges in each hole. It must protect the second deck to ensure it functions normally after the first deck has fired. The most common material used for stemming is crushed aggregate. In a horizontal hole, however, this material presents a problem because pouring of the material, and maintaining the material in position after pouring, are not assisted by gravity. To solve this problem the project team used plastic lay-flat tubing with an outer diameter within 5mm of the hole diameter filled with crushed aggregate. After insertion, tamping forced the stemming plug to become fully coupled with the walls of the hole, locking into position.

The running tunnel blasting works from the main tunnelling site at Lee Wing adit were completed up to Lai Tung Station cavern in early February this year.

Based on an updated version of a technical paper by O. Iwata, K. Kameyama (Nishimatsu Construction Co HK), Rohan Stevens and Limning Liao (Orica Mining Services, HK) presented and published in the proceedings of the ISEE (International Society of Explosives Engineers) Conference on Explosives and Blasting Technology, February 2013, Fort Worth, Texas, USA, and on articles written and published by engineers working for the John Holland-Leighton JV

References

Fast-track drill+blast in Canada - TunnelTalk, April 2010
Record drill+blast work in Norway - TunnelTalk, January 2009
Delivering noise pollution solutions - TunnelTalk, January 2012
Keeping underground noise within limits - TunnelTalk, April 2013

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