Dear Harbour,

Geology and Tunnel Excavation

Geological conditions in Hong Kong made the construction of the HATS deep tunnel system especially challenging. Overall, the Stage 1 tunnel excavation works took place mainly in unweathered hard rock which inherently contained interconnected fissures in the form of rock joints and fractures. At such great depths below sea level, significant amount of salty ground water under high pressure would flow into the tunnels through the rock fissures.

Such inflow, if not properly controlled, would draw down the ground water table and cause ground settlement. Depending on the quantity of inflow, water ingress would also cause practical difficulties to tunnel excavation such as rusting of construction equipment and flooding. Isolated sections of weak ground in highly weathered rock dykes and faults would also create great difficulties for tunnelling, particularly in the presence of high water ingress under great pressure.

Difficulties in Tunnelling

The Stage 1 tunnels, with lengths ranging from 3.6 to 5.5 kilometres, were excavated using five Tunnel Boring Machines (TBMs). The only exception was the shortest tunnel, at 0.78 kilometre in length, between Kwai Chung and Tsing Yi where the conventional drill and blast method was used.

 

At the time of their construction, the HATS Stage 1 tunnels were the deepest tunnels ever constructed in the world and there was hardly any experience from similar works for reference. This posed a major challenge to the tunnelling works which had to be carried out under the difficult geological conditions mentioned above.

 

The contractor for Stage 1 tunnels commenced works in early 1995. Water ingress under high pressure and the presence of weak ground at isolated locations soon created difficulties in tunnelling works. The project team tried its very best to work out feasible solutions together with the contractor to allow the works to proceed. However, the contractor eventually considered that it was “physically impossible” to complete the tunnelling works and gave up any further attempt to continue with the works.

 

Tasked with the mission to restore Your health, dear Harbour, we did not give up despite the serious difficulties encountered. We promptly took over all works from the contractor in December 1996 and relentlessly arranged for recommencement of the tunnelling works. As You will see later in this letter, it was subsequently demonstrated that the tunnelling works, though difficult, were viable. The contentions raised by the contractor were untenable and it turned out that it had to pay significant financial compensation to the Government for its unilateral decision to relinquish the tunnelling works.

Tunnelling Continued

Starting from July 1997, three new contractors were appointed to recommence the tunnelling works. Despite the serious technical difficulties, we continued the tunnelling works with unyielding and persistent efforts and actively worked out solutions to tackle the complications. We believed that grouting the ground ahead of tunnel excavation to seal rock fissures by pre-excavation grouting was the most appropriate way forward in reducing ground water ingress under high ground water pressure.

 

What followed was the difficult but successful task to modify all the existing five TBMs used for Stage 1 tunnelling works to provide them with mechanical equipment for drilling grout holes around the tunnel perimeter in front of the excavation face. In addition to pre-excavation grouting, installation of ground supports in the forms of timber laggings, reinforcement, steel support frames and concrete lining were also adopted for maintaining stability during excavation through weak ground.

 

The project team also decided that it was necessary to set a limiting figure on the amount of ground water to be allowed to enter the tunnel during excavation, but this posed another major challenge. On the one hand, allowing too much water to enter the tunnel would increase construction difficulties and cause ground water table drawdown that might lead to ground settlement. On the other hand, it would be very difficult to achieve an overly stringent allowable water ingress limit even with the use of pre-excavation grouting.

 

To tackle this, different allowable water ingress limits were worked out for different areas and a balance was carefully maintained through an observational approach to allow tunnel excavation to proceed in a carefully controlled and orderly manner. With determination and perseverance, the project team achieved a major milestone in November 2000 with the break-through of the last tunnel section from To Kwa Wan to Stonecutters Island. While settlement had occurred in some locations which required patching up to restore serviceability of the settled ground, there was no impact to public safety in any material way.

 

The many setbacks during the tunnelling process marked the most challenging aspect of Stage 1 works, not to mention community concerns and queries about HATS triggered by these issues. By 2001, all Stage 1 works including the deep tunnel system were completed. Once operational, the deep tunnel system began collecting over 1.3 million cubic metres of preliminarily treated sewage every day from the seven PTWs in Kowloon and north-eastern Hong Kong Island for chemically enhanced primary treatment at SCISTW and discharged through the outfall off Stonecutters Island. The deep tunnel system laid a solid foundation for the successful commissioning of the entire HATS Stage 1 in December 2001.

Notes:

1. Tunnel A and Tunnel B were excavated using the same tunnel boring machine (TBM).

2. “D&B” means drill and blast method.

3. “PC Segment” means precast concrete segment.