SOFT GROUND TUNNELLING | TECHNICAL
Left:
Profile of geological conditions along tunnel alignment on contract G-TN-9B
To overcome this issue, MWA plans to enhance
the water production capacity in the western area by expanding the Mahasawat water treatment plant’s capacity by an additional 800,000 m3
/day, along with
the establishment of a water transmission system, including a tunnel, and valve chambers. Once this tunnel and system are completed, they will enable the water supply from the western side of the city (using raw water from the Mae Klong River) to supplement the eastern side (using raw water from the Chao Phraya River) through the construction of four additional water transmission tunnels.
WATER TRANSMISSION TUNNEL The total length of the tunnel for the project’s contract G-TN-9B is approximately 9.1km, and this is divided into three drives with two different diameters. The difference in tunnel diameter is based on the distance from the water treatment plant. Construction is being executed by a joint venture
between Italian-Thai Development (ITD) and Nawarat (NWR), under the name Italianthai-Nawarat JV. For the three drives, the JV has selected three earth
pressure balance (EPB) tunnel boring machines (TBMs) from Terratec to undertake the tunnelling work: one TBM with a diameter of 4.80m (S107 machine); and, two TBMs of 4.39m diameter (S105 & S106). The three drives are segmentally lined, and upon
completing the tunnel excavation, steel tubes will be installed as the secondary lining. Infill mortar will be applied in the space between the concrete segments and steel tubes. The space is approximately 200mm wide. The lining system will support internal water pressure within the tunnel, which will range from 5 bar to 8 bar during operation, and to enhance waterflow efficiency. For project section G-TN-9B, the inside diameter
of the steel tubes is specified in two sizes: 3800mm and 3400mm. There is 2,233m of tunnel with the final diameter of 3800mm, and 6,840m of tunnel at the smaller final diameter of 3400mm. The tunnel alignment is situated in an area of the city managed by the Department of Rural Highway
(DORH), which experiences heavy traffic during rush hour. Consequently, the launch and receiving shafts for the TBMs are positioned to avoid creating traffic obstructions, including within roadway medians and beneath flyover bridges. The alignment creates multiple curves to avoid
flyover bridge foundations, with a minimum radius of approximately 80m-90m. All TBMs have been designed with active-type articulation to accommodate the specified minimum curve radius.
GROUND CONDITIONS The geological conditions along the tunnel alignment are typical for the Bangkok region, comprising soft clay, stiff clay, silty/clayey sand, and hard clay at varying depths. For this project, the tunnel is designed to be located
at a depth of approximately -27m to -34m from the ground surface, passing through silty/clayey sand throughout the excavation.
Above: Terratec TBMs 1 & 2 (Machine Nos S106 & S105) for contract G-TN-9B of 9th Bangkok Water Supply Improvement Project
November 2024 | 19
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