Report on GEG Presentation “Tunnelling Experience and
Supervision of Dropshaft Construction in Hong Kong West Drainage
Tunnel Project”
By Ms FONG Hau-yin, Fiona
Mr. Eric Chan (2008 Intake GEG) gave a presentation on his site experience in
tunnelling and dropshaft construction in the Hong Kong West Drainage Tunnel
Project. He discussed the details of constructing the drainage tunnel and the
dropshaft on Lower Stubbs Road with emphasis on various measures of site control.
Flooding has been frequent during rainy seasons in the low-lying areas of Northern
Hong Kong Island due to inadequate capacity of the existing drainage system. To
alleviate this problem, a new drainage tunnel spanning from Tai Hang to Cyberport is
proposed to intercept and convey stormwater from the upper catchment at Mid-Levels
to the sea near Cyberport. Works under this Project include the construction of 34
intake structures and the 11-km long main tunnel, as well as the associated vent pipes,
dropshafts, stilling basins and adits.
To reduce construction time and minimise disturbance to the public, the major part of
the drainage tunnel was constructed with two double shield tunnel boring machines
(TBMs). Mr. Chan described the operation of TBMs in detail and highlighted the
challenges in tunnel construction. As the tunnel would run across eight fault zones,
stability problems and high groundwater inflow were anticipated. Detailed mapping
of the tunnel face and probing ahead of the TBM was thus carried out so that adverse
ground conditions could be foreseen. The flow rate of flushing water during probing
was also measured to predict the potential of groundwater inflow. When weak rocks
were encountered or infiltration rate was high, grouting would be performed at an
inclination of 5° to horizontal for strengthening the rock mass and reducing its
permeability. Upon the formation of the tunnel face by the TBM, segmental lining
was erected to support the tunnel. The advance rate of the TBM was normally 12m
per day, but it could be greatly reduced where ground conditions were poor.
On the other hand, dropshafts connecting intake structures and tunnel adits were
constructed by means of mechanical excavation. After excavation and lateral
support (ELS) works were carried out for the construction of intake structures, some
small diameter pilot drillholes were drilled down from the base of excavation to the
stilling basin. The reamer of the boring machine, which was transported to the site
through the main tunnel and adit, would then bore upwards to form the dropshaft.
This method is known as “raise boring method”. The type of temporary support to
the dropshaft was determined by the Resident Geologist using the Q-system and in
most cases, rock dowels with shotcrete were adopted. Ground movement and
groundwater levels were continuously monitored. Should the alert, action and alarm
levels be exceeded, the Contractor would be requested to review its method of
construction.
In the Question and Answer session, the audience questioned whether the tunnelling
method would change with the decomposition grade of rock mass. Mr. Chan
advised that primarily TBM was used to construct the drainage tunnel as the majority
of the sections would be excavated in hard rock. Whenever fault zone was
anticipated, more probe holes were drilled and the ground surface was mapped to
countercheck the fault zone extent. Where necessary, grouting would be performed
prior to excavation to strengthen weak rock and thicker permanent lining would be
erected.
When asked about the material used to backfill the annulus after tunnel excavation,
Mr. Chan explained that pea gravel was chosen to fill the gap between the excavated
tunnel face and tunnel lining because its round shape enabled it to be placed easily,
and it also helped relieve the groundwater pressure acting on the tunnel. However,
excessive groundwater inflow was prevented by tunnel grouting and effects on the
regional groundwater regime were considered minimal.
Furthermore, participants expressed doubts in the suitability of using the Q-system in
designing temporary support for vertical dropshafts. Mr. Chan reassured the
audience by quoting the answer from Dr. Nick Barton, the developer of the Q-system,
in a recent workshop in Hong Kong. Although this system was originally developed
for evaluating the type of support required for a horizontal tunnel, it had been applied
in similar projects in Hong Kong before and the performance was found satisfactory.
In conclusion, graduates learnt how the drainage tunnel and dropshafts were
constructed in the Hong Kong West Drainage Tunnel Project and understood the
importance of site control in tunnel construction.