In Hong Kong, Golder Associates needed to construct a 4.1-kilometer tunnel up to 55 meters below the ocean’s surface, overcoming complex geotechnical challenges in newly reclaimed land.
Realistic modelling of soil behaviors and tunnel excavation enabled the team to better predict ground and tunnel lining movement in response to tunneling, confirmed by the measured monitoring data. The team's advanced approach to oceanic tunnel boring won them the Year in Infrastructure 2020 Award for Going Digital: Advancements in Geotechnical Engineering.
The Problem Ahead
The awarded project was to build the Tuen Mun-Chek Lap Kok Link, meant to improve road transportation in Hong Kong. The local government planned the project to run beneath the sea and connect the northern landmass of Tuen Mun to the southern landmass of Check Lap Kok, which houses the Hong Kong International Airport.
Golder Associates's design had to work around the existing roads, coastline, and 55-kilometer Hong Kong – Zhuhai – Macao Bridge, which includes cable-stayed bridges, four human-made islands, and a subsea tunnel.
As a result, the link had to incorporate a dual two-lane subsea tunnel, with a record-breaking depth of 55 meters below the surface, a length of 4.1 kilometers, and a total development cost of USD 6.1 billion. Golder Associates Hong Kong was tasked with the geotechnical design and analysis of the dual tunnels at the southern landfall of the reclamation, requiring one of the world’s largest tunnel boring machines, with a drill of 17.6 meters in diameter, as well as two smaller tunnel boring machines.
Digging through Complex Geological Conditions
Golder Associates wanted the alignment of the tunnels at the southern landfall to go underneath seawalls supported by stone columns, under a ventilation building, into connections to ventilation shafts, and around complex geological conditions. However, they discovered many other problems once they started working on the finer details of the project.
One of the design criteria was a residual settlement of less than 500 millimeters over a design life of 120 years. Therefore, Golder Associates had to consider any residual consolidation and creep settlement in the clay layers while working on the geotechnical and structural design of the segmental tunnel linings, which run below the southern landfall.
Also, when tunneling at the southern landfall new reclamation site, the pressure of the slurry had to be maintained at a high enough amount to ensure tunnel face stability and achieve a design volume loss ratio of less than 2 percent. The actual measured volume loss ratios due to tunneling were generally less than 1 percent. Having to carefully manage these details to ensure quality, safety, and long-term use, they realized that traditional design and geological applications would not be able to complete the task.
Predicting Soil Behavior and Preventing Settlement
After considering the project complexity and scope, Golder Associates adopted Bentley applications to design the challenging tunneling project. They used PLAXIS 2D and PLAXIS 3D as the main applications to model soil-structure interaction problems.
In interpreting and managing the information on complex ground conditions at the southern landfall reclamation, Golder Associates used gINT and HoleBASE to process hundreds of boreholes, field tests and laboratory tests. This practice produced geological models in 2D and 3D, geological sections, and contour plans, which were relied on for the geotechnical design. They also used MicroStation to produce engineering drawings for construction.
Ensuring Safety and Quality while Cutting Costs
After carrying out sensitivity and parametric analyses, Golder Associates managed to reduce the number of barrettes required to control the tunnel ovalization from 158 to 106, with 46 of them shortened from 19.2 meters to 16.4 meters.
This optimization saved approximately two months of construction time, reduced carbon dioxide emissions by 1,500 tons, and saved USD 16 million. By using digital analysis and modeling with Bentley applications, Golder Associates was able to help open the road project to traffic on December 27, 2020. The new tunnel shortens travel distance between the Northwest New Territories and Hong Kong International Airport by 22 kilometers, shortening travel time from 30 minutes to 10 minutes.
Now, the Tuen Mun-Chek Lap Kok Link connects Hong Kong and shortens travel time to just 10 minutes between Tuen Mun and Hong Kong International Airport.
With PLAXIS 3D, Golder Associates reduced the number of barrettes required to control the tunnel ovalization, saving construction time, cost, and carbon dioxide emission by up to 1,500 tons.