Speeding up excavation rates in Hong Kong 14 Oct 2015

A change in the specified excavation method for the complex underground works beneath West Kowloon Station is helping with project-wide efforts to claw back time on the much delayed 26km underground section of the Guangzhou-Shenzhen-Hong Kong Express Rail mega-project.

Sandvik DQ500 rigs drilling holes for the drill+blast operation

Contractors across the entire project have been examining every way possible to hit a revised operational date of late 2017, and for the Leighton-Gammon JV undertaking Contract 810A for construction of the major underground terminus station at West Kowloon (WKT) this has meant supplementing the use of the specified mechanical drill+split with traditional drill+blast.

WKT, when complete, will be one of the world’s largest underground railway stations, a four-level structure built to a depth of 30m, with a footprint of 11 hectares, 15 platforms, customs and immigration facilities, departure lounges and retail outlets.

A review of the entire project during 2Q 2014 led to a decision to use explosives in the middle of that year, with the first blast taking place in September 2014. Projections are that the final excavations will be completed by Q1 2016.

The drilling side of the project is being undertaken by 22 drill rigs supplied and maintained by Sandvik Construction.

One especially challenging area of the WKT works is the north top-down section where the 15 tracks merge towards the two running tunnels. This requires completion of a slab at first basement level to act as a solid strut between the completed diaphragm walls that surround the site. Excavation in extremely hard granite then continues below the slab.

Express Rail Link terminus at West Kowloon, Hong Kong

Use of jumbo drilling rigs was considered by the JV at the beginning of the project, but discounted because of a lack of space. For the drill+split operations Leighton/Gammon selected Sandvik Ranger DX700 top-hammer drill rigs; for the drill+blast operations this has now been supplemented with DQ500 units, plus one Dino DC400Ri unit, which have the capacity to match the demands of drill+blast but with the maneuverability to work between the support columns and under the restricted head height.

Kenny Chen, Sales Manager for Sandvik Hong Kong, says that at peak deployment there will be a total of 22 drill rigs working underground at WKT. “These will comprise nine DQ500 units, one remote-controlled Dino DC400Ri, and a total of 12 Ranger DX700 or 800 models,” said Chen. “Leighton already owned two DQ500 units, and we are supplying 14 other rigs on a rental agreement. Local sub-contractors are supplying the other units. All 14 rental units are new, and are being delivered from the factory in Finland.” Sandvik is also supplying its own drill bits, which will be reground at its depot in the New Territories, as well as all personnel required to maintain and operate the mini-fleet of vehicles.

Drilling in hard coarse grained granite under West Kowloon Station

“We have a long working relationship with Sandvik, and as they are supplying the entire package we have all their expertise and support,” said Sean Warren, Plant Manager for the Leighton/Gammon JV. “This extends to regular data analysis that is presented to us and which details the performance of each machine. From that, we can see how effective the drilling is, how the wear on the drill bits is progressing, and how we can extract the maximum in performance.”

Francis Haden, Blasting Manager for the JV, said he has never experienced granite as hard as that which has to be broken up and removed from the WKT excavation. “The granite is ridiculously tough, very competent, has a very high UCS, minimal jointing, and is very coarse grained. Once you get through the softer upper layers and into the bedrock, it is massive, flawless. On the western side of the excavation the joint sets are 17m apart.” He added: “We are working on top of the rock, and the circumstances make this unlike any other blasting project that has been undertaken in Hong Kong.”

Drilling flexibility is a key aspect of delivering Contract 810A. “When you are blasting a tunnel, you have a generally standard cross section on the tunnel face, and you drill to a set pattern. Here, we are essentially scalping the top of the rock heads to create flat, level benches. In order to get that level surface we have to drill holes of different lengths; so we might be drilling 1.2m deep, we might be going to 2.4m or 3.3m, and the holes can be fanned, or vertical, or horizontal. Drilling flexibility and the ability of the rigs to work their way into confined areas are therefore extremely important.”

Worn drill bits cleaned with electrolysis steel depletion machine

Drill bits used for drill+blast operations

This is where the DQ500 rigs, equipped with 48mm A330 button bits, come into their own. “We hadn’t worked with the DQ500 before but Sandvik brought us a demonstration model and we could see that the articulation on the boom and the low profile were what we needed,” said Warren. “In places where the DQ500 cannot go, the Dino can be sent in on remote control. We have found this to be a very workable solution, because this is essentially for small-scale work and we can deploy the Dino quickly and with ease around what is a very congested site. In some places the steel structural support is spaced at only 6m intervals so a high degree of rotation is critical.”

DQ500 operator Tang Kin Hung explained that the rigs are drilling a rectangular pattern with 0.5m spacing, and that an average of 500 holes are drilled per shift. “We have no difficulty in meeting this target, despite the toughness of the granite. Our only real problem is space and accessibility.” The Dino DC400Ri has proved to be perfect for this tricky work. Operator Wong Ka Po said: “The Dino works where the DQ500 cannot access. It is fully remote controlled and is equipped with a 48mm bit. We can send it close up against the structural elements and the retaining wall.” The blast holes are then packed with 32mm x 200mm cartridges, with up to three being used in a single hole in a method known as triple-deck blasting. Up to 200kg of explosives are being deployed every day.

The Ranger DX700 and DX800 rigs are equipped with either 102mm RT300 button bits or 127mm RT330 button bits to drill the holes for the mechanical splitting operations. The drill bits are sent to the Sandvik depot for regrinding after each 60m of drilling. An electrolysis steel depletion machine is also being used to remove worn steel from around the carbide on the button bits before regrinding the buttons.

Remote control Sandvik Dino DC400Ri drill rig

“We need only place the bits into the machine’s salt solution and then run a low DC voltage through the solution,” explained Service Representative Chiang Kwong Chuen. “There is no need to use any hand-held grinding wheel on the bit, so that means there is no noise or dust, and it is also faster. We can also reclaim spent carbide from bits that we could no longer regrind. In this way we are sharpening each bit up to eight times, which is very cost-effective when you consider the toughness of the granite.”

The target for drill+blast operations is to break an average of 200m³ per day, but often 250m³ is being reached as the speed of excavation advantages of drill+blast begin to have an effect on project schedule. “Mechanical splitting works, but it is very plant intensive,” said Haden. “The drill+split team has the advantage that they can begin work at 7am and are permitted to work as late as 11pm. Time-wise, compared with drill+blast, they have double the time that we have.

“We don’t receive our explosives until about midday, and we generally fire at 7pm. We have a much smaller team than they have, and fewer drill rigs, and we have to drill more holes, although of smaller diameter. But even with the various constraints, the blasting team is still breaking comparable volumes of rock,” said Haden.

“We are certainly setting records and setting a new benchmark in what can be achieved with drill+blast here in Hong Kong.”