Support for tunnelling

Ground engineering company Intrafor took on a horizontal drilling job as part of the Hong Kong Link Road project, utilising Bohrtec rigs and Numa down-the-hole (DTH) hammers and bits
Support for tunnelling Support for tunnelling Support for tunnelling Support for tunnelling Support for tunnelling

The US$3.2 billion Hong Kong Link Road project (HKLR) is a massive construction initiative comprised of sea viaducts in the western waters of Hong Kong, tunnels through Scenic Hill, tunnels underneath the Airport Road and Airport Express Line, and at-grade roads on reclamation along the east coast of Airport Island.  The 7.5mi (12km) long HKLR will connect the Hong Kong -Zhuhai -Macao Bridge (HZMB) at the Hong Kong Special Administrative Region (HKSAR) boundary with the Hong Kong Boundary Crossing Facilities (HKBCF) situated at the north-eastern waters of the Hong Kong International Airport. Upon completion, it will open a direct connection link between Hong Kong, Macao and the Western Pearl River Delta.

The HKLR comprises a 6mi viaduct section going from the HKSAR boundary to Scenic Hill on the Airport Island, a 0.6mi tunnel section to the reclamation formed along the east coast of the Airport Island and a one mile long at-grade road section on the reclamation connecting to the HKBCF. The tunnel section of HKLR will pass under Scenic Hill, Airport Road and Airport Railway to minimise the environmental and visual impacts to Tung Chung residents.

Pipe canopy

Intrafor, a subsidiary of VSL International, was selected for construction of a pipe canopy required to provide a temporary support during the tunnel box excavation work. Intrafor specialises in ground engineering and foundation construction works that are technically challenging. The company's expertise is based on a long history of proven design and build capability on demanding projects. Intrafor offers its clients a complete package to meet their geotechnical requirements, from preliminary studies and concepts, through to design, construction and ongoing monitoring.

The pipe canopy was a requirement of the overall design, because it allowed for the installation of pipe jacking for two tunnels under the Airport Express Link (AEL), the railway that connects Hong Kong's International Airport to the city centre. With the railway being active, the canopy improved the stability of the ground and minimised settlement, allowing the AEL to continue operating during the construction of the tunnels. The pipes were installed just 40ft (12m) below the track bed, so accuracy on the project was critical.

The canopy design called for Intrafor to install 44 steel-casing pipes in two sections. The first section required 20 pipes and the second section had 24 pipes. The ground conditions in both sections were varied with a mixed formation profile of stiff boulders, hard granite rock, silt and clay. To efficiently drill in these conditions both sections were completed using Front Steer drilling rigs and DTH percussion hammers and bits. 

Horizontal drilling

As specified by Intrafor, two tailor-made BM800LS drilling rigs each combined with Front Steer guiding technology developed by Bohrtec, were used for the horizontal drilling project. The Front Steer system was selected because it is fitted with a sophisticated guidance system that constantly keeps and monitors the drilling alignment, ensuring high accuracy for the pipe installation. Accuracy of ±75mm vertical and ±100mm horizontal was specified due to the close proximity of the holes being drilled. Bohrtec was chosen due to their 30+ years of experience designing and manufacturing machines for horizontal directional drilling.

To drill the 44 horizontal holes, Intrafor used Numa Patriot 180 and Patriot 185 DTH hammers with Numa Impact Ring Bits. Numa's Impact Ring Bit System consists of a unique two-piece design incorporating a centre and ring bit combination. On this project, the centre bit was attached directly to the DTH hammer used to power the drilling.

The hammer and centre bit were concentrically located inside the Front Steer and the ring bit was assembled at the front end after the hammer and bit were installed. The Front Steer was attached to the casing, and the hammer, ring bit and centre bit worked in unison to install the piping while drilling. Upon completion of the hole, the centre bit and drill string were pulled back out leaving the pipe installed. The ring bit was fully recoverable in the horizontal drilling application and was reused for drilling multiple holes.

The Numa hammers and Impact bits drilled 33.86in-(860mm)-diameter holes for installation of 32in (813mm) OD pipe with 16mm wall thickness. Single pipe lengths of 39ft with fully welded joints were used to a total length of lengths between 197ft and 230ft long. The ground formation consisted of grouted granite rock fill and bedrock granite up to 250MPa. To further complicate matters, the installation points were in close proximity to existing rail lines, so impact to surrounding areas had to be kept to a minimum and accuracy was essential. Despite these challenging ground conditions, the drilling team met the requirements of drilling accuracy of vertical +/-75mm and horizontal +/-100mm while drilling 13-26ft per hour.

The use of Numa's Patriot hammers and Impact Ring Bits provided a great benefit to the drilling project. Intrafor was able to realise lower equipment costs due to the re-use of ring bits, precise accuracy of drilling and high drilling performance in the conditions encountered.

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