Singapore pioneered the use of advanced shield tunnelling and microtunnelling technology in the Asia Pacific region in the 1980s to install sewers in built-up urban areas. The technology has been used extensively in Singapore the past two decades to install several hundred kilometres of sewers.

Through the application of advanced tunnelling technology, a considerable know-how and experience has been acquired in the use of various types of advanced shield tunnelling systems, both larger (macro shield) and smaller (remote controlled non man entry size micro shield) in a variety of ground conditions, ranging from very soft water charged clay soil strata to high strength rocky strata.

Many Asia Pacific countries have benefited from the greater use of advanced macro and micro tunnelling technology in Singapore. The lessons learned by the authorities, contractors and tunnelling equipment suppliers through the use of advanced shield tunnelling technology, particularly microtunnelling, to install sewers in Asia Pacific countries, are indeed a very valuable resource.

Introduction

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Singapore’s clean and healthy environment today is largely attributable to the comprehensive sewerage infrastructure construction programs implemented in the seventies and through eighties and nineties. Singapore has used advanced tunnelling technology, microtunnelling in particular since 1982.

Since then, many slurry shields, earth pressure balance shields and a large number of microtunnelling shields have been used to install sewers in a variety of soil strata.

In the past decade alone, more than 500 km of trenchless sewers were added to the network. In addition, Singapore has recently added 48 km of very deep sewers of diameters ranging from 3.3 m to 6 m and another 70 km of link sewers of diameters ranging from 0.75 m to 3 m to the network. All these sewers were also installed using advanced shield tunnelling technology.

Many Asia Pacific countries have also used the tunnelling technology to install sewers in a variety of ground strata from soft soils with high water table to hard rock.

Australia used laser bore microtunnelling systems and slurry shield micro tunnelling technology in the late eighties. In the mid-nineties, a slurry shield tunnelling system was used in Sydney to install sewers in strata containing silty sand and cobble stones. Slurry shield tunnelling was also used in Melbourne in the same period to install DN1500 sewers.

India used the technology in 1999 to install sewers in very complex ground strata comprising stiff silty sand with boulders and also in hard basaltic rock of compressive strengths up to 350 Mpa.

Earth Pressure Balance Shield (EPBS)

Singapore was the third country after Japan and the USA to use Earth Pressure Balance Shield (EPBS) tunnelling technology. In 1983 the EPBS was used to construct a 3 km DN3000 effluent outfall pipeline in tunnel through water logged soft marine clay.

Since tunnelling was to be carried out in very weak soft ground and in close proximity to economically sensitive and important structures, including an embankment of a water storage reservoir just 50 m away from the alignment of the tunnel, authorities imposed very stringent tunnelling requirements in the contract, including restrictions on the acceptable ground movement and disturbances to the nearby structures and installations.

To safeguard the structures and installations from any catastrophic failures due to tunnel excavation, provision was made in the contract to carefully monitor the ground response (both in terms of ground settlement and lateral ground movements) to tunnelling throughout the construction period, using inclinometers, pizeometers, tilt plates and settlement plates for the entire 3 km tunnel. To meet these stringent requirements and restrictions on ground disturbances, the contractor proposed the use of EPBS for the project.

The performance of EPBS and the soil response to the tunnelling was indeed very encouraging. The measured vertical and lateral displacements of soil during tunnel excavation, as recorded by several inclinometers and settlement points installed along the tunnel route, were found to be significantly less and the zone of disturbance was confined to a limited corridor width of 10 m on either side of the tunnel alignment.

The measured soil response in terms of final ground settlement along the tunnel alignment is plotted in Figure 1. The average ground settlement was found to be about 53 mm which is considered very small when compared to the size (3.75 m outside diameter) of the tunnel excavation.

The project demonstrated that careful planning and appropriate selection of tunnelling technology can significantly mitigate construction risks associated with tunnel excavation in poor ground conditions.

Slurry shields

A slurry shield was used in Singapore for the first time in 1982 to install medium size sewers (DN900 to DN2100) in water charged silty sand strata. Since then several dozen slurry shields have been used in Singapore to install sewers in similarly difficult grounds.

Soil response to slurry shield tunnelling was measured in the early nineties in a project where five slurry shields were used simultaneously to install 11 km of DN1200 to DN2100 sewers

A typical ground response to slurry shield excavation by a DN1500 shield through very soft marine clay strata is shown in Figure 2.

No significant ground settlement occurred along the sewer route and the average ground settlement was only 15 mm. The very low ground settlement was due to the fact that the slurry pressure at the tunnel was deliberately kept high to balance the soft marine clay.

It is not easy to balance the tunnel face in marine clay due to its fluidity. Its active pressure coefficient was estimated to be 0.9 and thus any slight reduction in slurry pressure will result in free flow of marine clay, causing the ground above to settle, while any increase in slurry pressure will result in upheaval as shown in the Figure 2. The average upheaval of the ground was only 8 mm.

Microtunnelling in Singapore

Hundreds of kilometres of sewers ranging from DN225 to DN900 have been installed by microtunnelling technology since its introduction in Singapore in 1982. Singapore’s exposure to the technology and accumulated know-how and expertise in the trenchless application has enabled many neighbouring countries including Malaysia, Brunei, Thailand China, Korea, Taiwan and Hong Kong and lately India to adopt and use the technology to install sewers in difficult and built-up urban areas.

The most commonly used microtunnelling system was the slurry shield system. However, Singapore has also extensively used the following microtunnelling systems during the initial period in the early eighties. Even today some of these machines are still being used.

• Earth Arrow method (two stage excavation process); • Iron mole method (two stage excavation process); • Horizonger method (single stage excavation process); and, • Slurry shield method (single stage excavation process).

The Singapore experience has demonstrated that trenchless sewers can be laid accurately and at a competitive cost. It shows that the method achieves faster installation at a relatively low cost. Thus the trenchless sewers are expected to lower considerably the overall of construction costs of sewerage infrastructure development and significantly shorten the construction time in developing countries where thousands of kilometres of sewers are yet to be laid in the metro cities.

Microtunnelling in Cobble Stones in Sydney, Australia

The slurry shield microtunnelling system was used in Sydney in 1995 to install a sewer in soil strata containing cobble stones. A microtunnelling system capable of boring soft soil strata and rocky strata was used to bore through cobble stones embedded in silty clay strata.

The geotechnical tests showed the river cobbles to be hard quartzite/porphyry materials of strengths of 300-400 Mpa. The rock boring shield was equipped with roller bits capable of cutting rock formation of unconfined compressive strength in excess of 200 Mpa. The shield had the capacity to exert large torque required to break up the rocks. The shield was also fitted with a more robust cone crusher capable of crushing hard rocks including cobble stones as hard as 300 Mpa.

The shield initially dipped into the ground and refused to respond to the steering jacks of the shield head due to its heavy weight in the front. It could not be brought back to the true alignment and continued to dip further. At one stage the alignment target in the shield could not be seen from the jacking pit and the shield and the jacked-in pipes had to be pulled back to the shaft.

When tunnelling resumed, the contractor restrained the shield by bolting it to the dummy pipe that followed the shield. The shield and the dummy pipe acted as a single length of pipe and restrained the shield head from dipping. The control of the shield became much easier after this modification.

Microtunnelling in siltstone in Melbourne, Australia

Microtunnelling was used in Melbourne to install a DN1500 sewer through siltstone strata in 1996. The unconfined strength of the moderately weathered siltstone was about 1.7 Mpa. About 215 m of sewer was installed in this stretch using a DN1500 slurry shield.

When the second leg of tunnelling of about 220 m commenced in the opposite direction, a much harder siltstone was encountered in contrast to the low strength siltstone that was originally envisaged. The slurry shield could not bore through the harder siltstone whose unconfined compressive strength was about 13 Mpa.

After the modification of the cutter head by welding special cutter bits supplied by the manufacturer, the shield resumed excavation but at a much slower speed. One of the main problems faced during the excavation was the rolling of the machine when high torque was applied to excavate the siltstone. Because of these constraints, the applied torque was limited to only about 50 to 60 per cent of the full torque.

About 120 m of full face tunnelling excavation was done in this harder siltstone. The next 40 m of the tunnel was excavated in siltstone and silty clay transition strata. The remaining 60 m was excavated in water charged silty clay strata. A histogram of excavation rate in siltstone (and a plot of excavation rate in soft soils) is shown in Figure 3.

This project demonstrated clearly the importance of knowing the ground conditions before commencing excavation. An appropriate (and technology specific) geotechnical investigation would have helped the contractor to choose the right machine. Adopting a machine designed to excavate soft soil to excavate rock is like trying to use a drill designed to drill holes in timber to drill holes in concrete.

Microtunnelling in basaltic rock in Mumbai, India

Trenchless Technology paved the way in Mumbai, India to install eleven missing sewer links across busy railway corridors and along bottleneck stretches of streets in the city. This World Bank-funded first microtunnelling project in India made it possible for the Municipal Corporation of Mumbai to connect to the city’s main sewage collection and disposal system to an $US25 million sewer network that was discharging sewage for decades into storm water drains. These missing sewer links could not have been installed by traditional open trench methods as it was not possible to close the railway and streets to facilitate the construction work.

It was apparent to the Municipal Corporation and World Bank that the only viable option to construct these missing links was the use of Trenchless Technology. However, they were initially reluctant to adopt this option due to the lack of trenchless expertise available in India and the risks of failure and cost escalations associated with new technologies.

Mumbai’s soil condition is complex and given the urbanised and built-up nature of the city, the project to install the sewer links by microtunnelling along congested and heavily used roads and railways turned out to be one of the most difficult and challenging tasks.

Of major significance, beside the very congested and difficult nature of the sites, was the soil conditions which varied from hard basaltic rock with compressive strengths in excess of 100 Mpa (and in some places it was more than 250 Mpa) to water charged silty and marine clay with boulder intrusions.

Three microtunnelling machines, the Herrenknecht AVN 1200, AVN1000 & AVN 800 were used in the project to install the 4.8 km of DN350 to DN1400 sewers.

The contractor initially selected two microtunnelling machines, Herrenknecht AVN 800 and AVN 1000 that met the specified performance requirement for excavating both soft and hard ground including rock as anticipated in the contract. The project specific geotechnical investigation work carried out by the Municipal Corporation enabled it to alert the contractor on the anticipated ground conditions which in turn helped the contractor to choose the right tunnelling equipment.

Another Herrenknecht AVN 1200 was mobilised by the contractor to install sewer through a much harder rock encountered in one of four sites subsequently added.

After an initial teething and familiarisation period, a record length of 2.1 km of microtunnelling was completed in a 10 month period, representing an average monthly production rate of 210 m.

This rate was achieved by working in two nine hour shifts per day. The production rate included set-up and turn around time for the machines.

The AVN 1000 machine achieved a maximum production of 15 m/day in hard rock. Its average production was 8 m/day. The AVN 800 achieved in hard rock a maximum production of 30 m/day and its average production was 10 m/day. Minimum production rate for both the machines was 3 m/day.

Excavation rate in basaltic rock

Microtunnelling excavation was carried out in basaltic rock and granite stones of strengths from 50 Mpa to 300 Mpa. The microtunnelling shields went through a very strenuous endurance test in this project as the rock encountered was not only hard to break but was also very tough. The toughness of the rock made the excavation more difficult and slow. In one sewer stretch the rock strength reached as high as 340 Mpa.

The data plotted in Figures 4 and 5 corresponds to tunnel excavation in full face basaltic rock of strength up to 100 Mpa. The table below shows the rock strength in Mpa, the maximum and average excavation rate in mm per minute and the average torque exerted by the machine in bars and the maximum jacking force applied in tonnes.

Conclusion

Mumbai’s project has created a lot of enthusiasm and confidence in Trenchless Technology in India and many Indian cities are expected to use the technology in their future sewerage infrastructure works. Mumbai Municipal Corporation has already embarked on a second microtunnelling project which is currently underway to install trenchless sewers in the city.

This project is a good example of demonstrating how a project specific geotechnical investigation and dedicated technical specification enabled both the owner and the contractor to mitigate construction problems and cost escalations.

The project in Melbourne demonstrated the importance of having accurate geotechnical information. Insufficient and inaccurate geotechnical information leads to technical difficulties, high excavation costs and time delays, as demonstrated in the project.

The successful use of mechanised shields for tunnel excavation in Singapore’s soils has demonstrated that advanced shields such as EPBS, slurry shield and microtunnelling shields offer excellent methods to install small and large diameter sewers in highly urbanised and congested city areas.

The projects have also demonstrated the importance of advance planning and instrumentation and monitoring of soil movements and disturbances to nearby structures and installations. This would indeed enable the contractor to manage the risks professionally and mitigate construction nuisances and cost escalations commonly associated with any underground excavation works.