My first experience with tunnelling under water was in 1976 in Bangkok when a tunnel was under construction using a Tunnel Boring Machine (TBM) in stiff clay soil.
Stiff clay is quite impermeable to water and although the tunnel was located below the groundwater table, the TBM was an open machine, which means that after excavation and before the erection of the tunnel lining, some of the ground was exposed temporarily.
All went well because the clay was impermeable, until the TBM encountered a layer of sand which was permeable, and water and sand flowed into the tunnel and flooded it. It took many months to overcome the problem before tunnelling could be resumed. The source of water was not from the river above the tunnel but from layers of water-bearing sand well below impermeable clay soil beneath the river.
Later I was working on the design for a tunnel for road and rail crossing beneath Hong Kong harbour. This tunnel is constructed as an immersed tube. Long units of the completed tunnel were cast in a temporary dockyard using reinforced concrete and were floated to the location for the tunnel. These units were lowered into a trench that had been dredged below the seabed. After placing, the units were joined together lengthwise creating a crossing beneath the harbour. After joining, the units were covered with soil and protective layers. In total, six immersed tube tunnels were constructed and are operating in Hong Kong. Three of them are for railways, two are for roads and one is combined road and railway.
Another immersed tube tunnel is included in the crossing from Hong Kong to Macau. These seven tunnels were completed without incident.
Starting in 1990, there was a scheme to construct sewer tunnels passing beneath Hong Kong harbour. Beneath the harbour there is a succession of soft soil, stiff soils, and bedrock. The tunnels, designed to be at least 30m below the top of bedrock, were located 90m to 135m below sea level. In the past, several tunnels had been excavated in the rock above sea levels where mostly minor seepages of water trapped in the fissures in the rock had been encountered, with a few notable exceptions of large inflow in faulted rock.
For Stage 1 of the system, TBMs were deployed to excavate the deep sewer tunnels assuming that generally only small seepage of rock fissure water would be encountered. Some of the tunnels encountered a substantial inflow of water beyond the capability of the contractor. Work was halted, another contractor was appointed and the tunnels were completed about six years late. The water that flowed into the tunnels during excavation did not all come directly from the sea as many would suspect. At some locations, the water drained from the bottom of overlying compressible soils causing them to consolidate, resulting in substantial settlement, more than one metre. Settlement of the seabed was not an issue, but settlement of sea walls and adjacent flat land required considerable repairs.
In deep tunnels, the water pressure can be quite high. At 135m below sea level, sewer tunnels encountered water pressure 13.5 times the atmosphere which is 202 pounds per square inch, which is about three times the air pressure inside a truck tyre. Water under this level of pressure can be kept out from open tunnels in the rock by sealing the fissures in the rock with cement or chemical grouts. If the grout is injected into fissures ahead of excavation, a tunnel can remain dry. Grouting to seal fissures in the rock ahead of tunnelling was adopted successfully for Stage 2A of the deep sewer tunnels in Hong Kong for which the excavation of the tunnels by the method of drilling and blasting the rock in free air was completed on time and within the budget.
Open TBMs as used in Bangkok and for the Stage 1 deep sewer tunnels in Hong Kong are only suitable where there is no potential for inflow of water or weak soil. Where there is potential for inflow of water, shield TBMs can be used which are designed to be completely enclosed with tight seals to prevent inflow of water and weak soil. For example, shield TBMs have been used in the last few years for the Metro in Bangkok where the open TBM was flooded many years ago. Also, two shield TBMs were used to successfully construct the Metro lines beneath the Hooghly in Kolkata. TBMs designed to pass deep beneath rivers and the sea can be quite large. For example, in Shanghai the crossing beneath the Huang Po to Pudong was competed using a 12.6m diameter TBM for two lanes of traffic and one railway line. Recently a 14.6m diameter TBM was used at Hong Kong crossing beneath the Pearl River at 14.6m diameter and enlarged to 17.2m diameter allowing two lanes plus a slip lane for traffic.
All machinery requires maintenance. For instance, replacing cutters at the face of a TBM means working in a chamber with some exposed ground where water could inflow. Such water pressure can be balanced by applying air pressure inside the working chamber.
For men working under excess air pressure there are limits to their health requiring gradual build-up of pressure and slow recovery in the same way that divers can work deep in the sea. For a drainage tunnel at Lai Chi Kok in Hong Kong at a depth of about 45m for the outfall tunnel beneath the harbour the excess air pressure was 4.5 atmospheres. This pressure is too high for workers to breathe compressed air and recover quickly.
Under medical supervision, excess oxygen was mixed with compressed air for the men to work for periods of an hour or so and covered by de-compressing safely. Under higher pressures mixed gasses including Helium are used throughout the working period and decompression. Using oxygen or mixed gasses is suitable for short durations of working under compressed air.
For working longer periods, “saturation diving” is required. For the road tunnel from Tuen Mun to Chek Lap Kok in Hong Kong located at depths of the order of 60m, teams of “divers” were deployed who worked under excess air pressure and remained in a compressed air environment in special chambers day and night for 20-day shifts, alternating between two teams of “divers”. Both tunnels, at Lai Chi Kok using enhanced oxygen, and for Tuen Mun to Chek Lap Kok using saturation diving were completed with no health issues arising for men working under compressed air.
The problems for crossing the Hooghly river were straightforward. There were zones of weak soil, sand and firm clay, that is weak and mixed ground, and the tunnel was to pass beneath the river, and flow of water into the tunnel was to be prevented.
Crossing the Hooghly was achieved successfully with two parallel tunnels and was carried out by using two shield tunnel boring machines that were built specifically for these conditions. They were purposely designed for excavating soil while keeping water-tight, and erecting a reinforced concrete robust permanent lining for the tunnel within the protection of the boring machine. The tunnel boring machines are completely enclosed with a rotating cutter head at the front and a robust steel cylindrical shield along their length which is slightly bigger than the completed concrete lining. The concrete lining is erected within the shield and remains in place behind the machine as it is advanced. Therefore, the weak and mixed ground is totally supported, firstly by the steel shield and then by the permanent concrete lining. Because the shield is slightly bigger than the lining, as the shield moves forward there is a small gap outside the lining and this is filled with cement which is pumped into the gap via pipes embedded in the steel shield that inject the cement into the gap outside the lining at the back end of the shield. This method of construction ensures that the tunnel boring machine can advance steadily and that ground movements, such as settlement, are not significant and that no nearby road or structure is affected.
The profile of the tunnel was dipped below the riverbed to provide 13m of soil above the tunnels in addition to 13m of water above the riverbed. This was necessary to prevent the tunnel from floating.
The resulting pressure on the tunnel boring machines was equivalent to more than three times the atmospheric pressure which is well within the capacity of the seals on the tunnel boring machines, and groundwater did not penetrate into the tunnel boring machines. The soil beneath the river is soft mud overlying firm clays and sand. The tunnel boring machines are well capable to excavate these types of soil.
Author is a specialist in design and construction of underground structures such as tunnels and metro stations. He is an AECOM Fellow as a world class expert with more than 50 years of experience. In 2019, at the time of subsidence at Bowbazar he was asked to come to Kolkata to help to solve the problems