Transformation On Tunnel Construction

With the development of new towns, expansion of roadworks and metro networks in Hong Kong, tunnels are built by different methods under several constraint in the project. This article introduces the strategy for considering tunnel construction method and adopted recent technology development, with showcase of tunneling project delivered by Dragages in Hong Kong.

Factors affecting how the tunnel should be constructed

The tunnel design is subject to the usage and geological condition of the site which will drive the way to construct it. Each construction method has its benefit and limitation to apply in the project. Below are the examples of methodology for tunnel construction:

Tunnel Boring Machines (TBMs)

TBMs becomes one of the favorable solution due to its performance, reliability and cost-effectiveness, meanwhile create less nuisance to surrounding stakeholders and environment compared to other methods. TBMs can be designed to suit its operation need due to geological constraints. Slurry type mixed shield TBM is used in Tuen Mun – Chek Lap Kok Link (TMCLKL) Sub- Sea Tunnels in Hong Kong to construct a 5km dual line driveway tunnel 55m under sea level with a mix ground geological profile.

The cost effectiveness of TBMs is relevant to the length of tunnel built due to expensive cost on TBMs and associated support like cutter disc maintenance, slurry treatment, precast segment delivery and installation. The tunnel should be built by TBMs with at least 1km in length to compensate the first setup cost. There are two projects in Hong Kong (Heung Yuen Wai Highway – Lung Shan Tunnel, Tung Chung Line Extension – Tung Chung West Station and Tunnels Project) under Dragages HK LTD applied TBM U-turn to use 1 TBM machine boring two short distance tunnels which is one of the costs saving solution.

Drill And Blast

Drill and blast is one of the conventional methods to excavate tunnels with hard rock geological environment like quarry, mountain or underground below rockhead level. It is often mentioned as a mined tunnel due to its nature like a mined quarry.

The cost effectiveness of a tunnel constructed by drill and blast method is based on the tunnel length mined per each round of blasting, which is driven by the blast design and limited by a value called Maximum Instantaneous Charge (MIC). MIC indicate the amount of explosive (in kg) allowed in each drilled hole for blasting, the higher of MIC the longer drilled hole length hence increase the excavated length. Considering the production cycle of a mined tunnel and regulation on blasting activities (shotfiring before sun set), each excavation face is limited to one blast in a day.

Tunnel blasting will generate two major nuisances to the environment – ground vibration and air overpressure. Therefore, the surrounding sensitive receivers (existing structures, historical buildings and monuments, utilities, and service structures) should be considered and limiting the allowable MIC to apply in the blast design. Cha Kwo Ling Tunnel in Trunk Road T2 project is located in a densely populated location with surrounding historical buildings and old village, hence the mined length per each blasting is often limited to below one meter, compared to a typical mined tunnel like Central Kowloon Route – Central Tunnel (CKR-CT) with up to few meters.

Mechanical Excavation Method

Mechanical excavation is applicable to tunnel project with extremely soft ground condition or have difficulties to apply TBMs / drill and blast method. If the tunnel is under a shallow ground condition (tunnel entrance) then this method is typically adopted.

Ground treatment is often required due to the geological situation to ensure ground stability and prevent ground water ingress during excavation stage. A proper grout mix design with microfine cement / silica chemical injected by Tube a Manchette (TAM) method is commonly adopted for ground enhancement works. TAM allows the grout injected with dedicated pressure to pass through the rubber sleeve on the grout tubes with depth control by double packer which ensure the grout can reach the designated area.

Compared to TBMs / Drill and Blast method, mechanical excavation is relatively slow hence considered as a cost non-effective solution. The production cycle is limited by the excavation plant capacity and unsupported length allowed. Robust tunnel temporary support installation is expected due to weak ground condition which is time consuming and costly.

Cut and Cover Method

Similar to the mechanical excavation method, cut and cover tunnel is constructed for shallow and soft ground condition with excavation and lateral support (ELS) system installed. Tunnel entrance or approaching ramps are usually constructed by this method.

Pipe Jacking / Box Jacking Method

Similar to the TBMs but the tunnel size is relatively small usually serves as a sewer, drainage, utilities or pedestrian walkway (cross passage). Commonly used in Hong Kong urban area with densely population due to its beneficial on public nuisance and prevent open cut excavation and affecting underground utilities constructed and under service.

Plasma Blasting

Plasma Blasting is one of the alternatives for hard rock excavation with less nuisance compared with drill and blast method. This new technology is applied commonly in Korea, especially for metro network construction and several trials have been done in Hong Kong which could be one of the construction methods to be adopted in future.

The tunnel construction methodology has been evolving with technology development and consideration to the surrounding environment impacts. We are expected to see the industry is migrating from traditional labor/plant intensive method to TBMs with stakeholders’ consideration.