Hong Kong’s high population density and mountainous terrain have resulted in a number of notable landslides over the decades. One of the most tragic events occurred in 1972 when a catastrophic landslide on the hillside of the Po Shan area took the lives of 67 people.

This unforgettable collapse, along with other major landslide events, formed the historical background for the Slope safety efforts undertaken by the Hong Kong Special Administrative Region (SAR) Government. Detailed investigations and research studies were carried out and the results indicated that the natural hillside of the Po Shan area was affected by high groundwater levels and unfavourable geology. Subsurface drainage measures, in the form of horizontal drains, were put in place but, after some 20 years, the monitoring data showed signs that the groundwater levels in the drained area could be high during heavy rainfall. It was in this context that AECOM was appointed by the Civil Engineering and Development Department (CEDD) of the Hong Kong SAR Government to design a robust landslide preventive system to protect Po Shan residents from large-scale deep-seated failures. With this in mind, an innovative scheme involving a pair of drainage tunnels and over a hundred sub-vertical drains was adopted for the project.

Project description
The project consists of two 3m-i.d. TBM tunnels. The high tunnel cuts 310m into the face and has an overburden of 140m at its deepest point. The low tunnel cuts 219m to a depth of 80m. Some 172 sub-vertical drains were installed from inside the tunnels and extended into the soil. The drains were designed to cover the area of the entire drainage zone of the catchment, which falls largely within the Pok Fu Lam Country Park and is mainly covered by dense vegetation.

The tunnel and sub-vertical drain system provides a controllable mechanism by opening and closing of the sub-vertical drains to regulate the groundwater level and ensure that variations in groundwater levels fall within pre-defined ranges.

While the initial set-up cost for the sub-vertical drains was higher than horizontal drains, it is more practical, cost-effective and sustainable when it comes to future operation and maintenance. Also, from an environmental standpoint, the sub-vertical drains were designed to be installed inside the tunnel and did not involve any ground surface works. This means that no tree felling was required and no vegetation was adversely affected by this project.

As part of the Landslip Preventive Measures Programme managed by the Geotechnical Engineering Office (GEO) of the Civil Engineering and Development Department, the entire project was designed by Aecom and constructed by the China State-China Railway joint venture. Construction commenced in July 2006 and was completed in December 2010. The performance of the drainage system is being closely monitored for a period of two years in order to verify the effectiveness of its drainage capability and to identify the most effective operational procedures going forward.

Successful completion of the project was not without technical challenges, some of which had to be resolved by novel engineering solutions. Specifically, a number of innovative techniques and value-added features were adopted for this project, such as the use of Horizontal Directional Coring (HDC) for ground investigation, upward drilling from a confined space, a retractable TBM, and an automated wireless groundwater monitoring system in addition to collaboration with the Hong Kong Observatory to monitor seismic activities.

HDC for ground investigation
Tunnelling is traditionally acknowledged to be a high-risk construction activity as ground conditions can be challenging and difficult to predict. HDC can provide a continuous assessment and sampling of the ground conditions along the exact tunnel alignment, which is analogous to a pilot tunnel. As a result, unforeseen tunnelling conditions can be largely avoided. The continuous geological information is also valuable to assess the construction programme, grouting works and excavation methods, such as the use of TBM and/or drill-and-blast. Areas of problematic rock conditions can be located using HDC, and a specific excavation scheme can be proposed before tunneling starts. HDC also enables the measurement of groundwater inflow in different tunnel sections: it is valuable data for the grouting design as it directly impacts the programme and cost estimation of tunnel construction.

Retractable TBM
Although tunnelling by TBM has been commonly used in Hong Kong, the operation is usually carried out with both launching and receiving shafts. However, due to site constraints, no receiving shaft was available for this project. This meant that the TBM had to be retracted after completion of the first tunnel and reoriented to drive the second bore. This ‘blind boring’ technique, an approach that is rarely utilised in tunnelling, was successfully adopted for this project. Another advantage is the fact that adverse effects on the environmentally-sensitive Pok Fu Lam Country Park, a 270-hectare sylvan sanctuary, were significantly minimised with this method.

Wireless groundwater monitoring
An innovative Automatic Groundwater Monitoring System (AGMS) was established for the real-time remote monitoring of groundwater data, with the graphical output of monitoring results viewable on a secured website. Using approximately 50 sensors, this state-of-the art system uses wireless networking and the internet to deliver groundwater information directly to stakeholders in real time, without human involvement in the entire process. It currently provides groundwater pressure data for design review and serves as useful reference for future tunnelling projects, all at a very low cost. The use of advanced data acquisition, computing and communications technologies for this project has opened up new possibilities for real-time monitoring on slopes and construction sites. What’s more, the use of these techniques is not limited to groundwater monitoring and could also be used for other geotechnical applications.

Upward drilling
Sub-vertical drain drilling and installation is a special feature of this project. It is the first project in the world for long-drain drilling and installation, with a length of over 100m, which uses an “up-the-hole” hammer within a small and congested tunnel. As the first application of upward percussive drilling for tunnelling in Hong Kong, it provides an average penetration rate of over 60m per day.

A water hammer, instead of an air hammer, was used during drilling to reduce the chance of jamming. In addition, valves were installed on all drains to control the inflow of groundwater from the sub-vertical drains. While a number of difficulties were encountered during the drilling process such as deformation of casing joints, and jamming of the drill bit, these problems were finally overcome, and smooth drilling and installation ensued.

Hong Kong Observatory
The project is also notable for its deep penetration into bedrock providing an environment with stable pressure, temperature and humidity, which is also shielded from heavy traffic. The access to bedrock and to a low-noise environment makes the tunnel an ideal site for setting up a seismograph station.

The Hong Kong Observatory (the Government department responsible for monitoring earthquakes and tsunamis), GEO and Aecom collaboratively incorporated special features in the project that led to the establishment of the first broadband seismograph station in Hong Kong, meeting the stringent requirements of the Global Seismographic Network for global earthquake monitoring. In the deep rock the station is capable of detecting earthquakes thousands of kilometres away. The waveform data recorded is disseminated through the Incorporated Research Institutions of Seismology (IRIS) real-time, enabling earthquake and tsunami monitoring centres worldwide to determine the parameters of earthquakes in the vicinity of South China Sea quicker and more accurately than with existing systems.

Conclusion
Although it can be a challenge to utilize advanced techniques that have never before been applied, designers should be innovative and strive, as much as possible, to demonstrate the practicality and effectiveness of some of the more innovative methods. With full client support, contractor co-operation, quality supervision of the resident team and acceptance from the general public, implementation of these innovations in this challenging project was successfully achieved.


Figure 1, plan of tunnels and subvertical drains under Po Shan hillside Figure 2, cutaway three dimensional view of tunnels and fanned drains in the Po Shan project Figure 3, longitudinal section through drainage system and water table Figure 4, cross sections showing details of sub-vertical drains