The installation was required to further protect an important stretch of road between Sydney and Wollongong from landslides.
Unstable landslide areas are often associated with ground water levels that are too high. To fully understand the system’s drainage characteristics, the impact of rain events and to design measures to stabilise the area can be a complex task requiring detailed investigations, sophisticated technology and highly skilled practitioners.
In early 2018, Pezzimenti Tunnelbore was awarded a contract by NSW’s Roads and Maritime Services to excavate two 100 m long, DN 400 microtunnels to drain a landslip area underneath a major motorway.
However, the real story lies in the sophisticated geotechnical investigations that were carried out over a long period of time forming the basis of the eventual detail design, which specified microtunnelling. The trenchless method proved to be a simple and very effective solution to a complex geotechnical problem.
The site
The M1 Princes Motorway is the main road linking Sydney and the Illawarra. As of 2016, more than 44,000 vehicles used the motorway each day, with heavy vehicles representing up to 15 per cent of daily vehicle movements.
The motorway provides a vital link for commuters between Sydney and the Illawarra region; tourist traffic between Sydney, the Illawarra region and the NSW South Coast; commuters travelling between Sydney and the University of Wollongong; road freight traffic travelling between the Illawarra region, particularly Port Kembla, the South Coast, Sydney and the northern Illawarra collieries.
A section of the Princes Motorway is located on a historic dormant landslide, which is in turn underlain by sedimentary rocks including coal seams. The site has a long history of landsliding dating back to at least the 1950s.
Stabilisation measures were undertaken several times between the 1950s and late 1970s, including five subsurface drainage lines installed under the road in 1977 which drained into Dobinson Street.
Detailed investigations
During 2014 and 2015, Roads and Maritime’s Southern Region Asset Management decided the existing horizontal wells at the Dobinson Street site warranted replacement.
Following a careful review of all the existing information – including Department of Main Roads reports and University of Wollongong monitoring data and site inspections – it was clear a new hydrogeological investigation, combined with a review of horizontal directional drilling and microtunnelling techniques, was required to establish the scope of works and design a new solution.
The objective of the work was to replace the existing drainage system with an equivalent or improved formalised system to ensure the ongoing stability of the road. Located within a designated drainage easement, the system was designed to minimise the impact on nearby properties.
Roads and Maritime worked in collaboration with technical experts, including NSW Public Works, Trenchless Advisor, Coffey Geotechnics and the University of Wollongong to build an understanding of the subsurface conditions at the site, as well as design a suitable drainage system that would relieve the natural groundwater pressures below the surface, increasing the stability of the slope.
In early 2016, Roads and Maritime engaged NSW Public Works to manage the development of the new solution at the site and also engaged Coffey to undertake a new hydrogeological investigation of the site, supported by the university’s Dr Phil Flentje.
Coffey’s hydrogeological investigation clearly defined the subsurface profiles, hydrogeology and target permeability zones and identified that a precise laser guided microtunnelling technique would deliver the required horizontal bores of approximately 100 m.
As part of the investigation, a total of nine new sonic and cored boreholes were drilled across the site, to install inclinometers and new vibrating wire piezometers (VWP). The devices were installed to monitor ground water levels while ensuring comprehensive detail of the colluvium and subsurface bedrock profile was collected.1
Two new VWP were installed in both boreholes and connected to a University of Wollongong monitoring station by radio, enabling the monitoring of water levels in near real time. The installation of the sensors allowed the geotechnical engineers to assess the impact of the completed works by measuring the water levels in the ground before and after the microtunnelling works.
Design
Now the site was understood in geotechnical terms, it was proposed that a system of two 440 mm diameter, 100 m long horizontal drains be installed on grade under the M1 from the lower, or eastern, side of the M1 at Dobinson Street. The drains were designed to be terminated at dead ends adjacent to existing vertical wells on the western side of the motorway.
This technique would allow traffic flow to continue undisrupted and the drains to tap into the target zone of a highly permeable coal layer. Above the coal was a layer of colluvium on which the motorway was sitting.
Trenchless Advisor was engaged by NSW Public Works to investigate appropriate construction techniques. The varying geology and the potential collapse of the microtunnel, due to the overlaying colluvium, indicated a method was required with a high degree of accuracy. The method needed to be suitable for installing a casing while still being able to be retracted.
Not all off-the-shelf methods were likely to suit the site conditions and constraints, although it was assessed that some experienced microtunnelling contractors had developed systems on other sites to suit similar ground conditions and installations.
Eventually, the design methodology specified the drains be installed using microtunnelling techniques, advancing from a 4.5 m deep launch pit, located downslope at Dobinson Street. The drains were then to be connected into the local stormwater network without the use of pumps.
Microtunnelling
In early 2018, Roads and Maritime called for public tenders for the microtunnelling works and subsequently awarded the contract to Pezzimenti Tunnelbore. As this project was one of, if not, the longest dewatering project Roads and Maritime had managed to date, final details of the bore cross-section were agreed post-tender.
Excavation of the shafts was completed as specialist drilling components and casing materials were ordered and delivered. No major interruptions to drilling occurred and works were completed by the target date in late June. Clean ground water flowed continuously into the excavated shaft, then from the microtunnels during drilling and through the drainage pipes. The flow continued after the pit and connecting pipework was completed.
Assessing the success
“The horizontal drains were almost immediately successful, receiving substantial groundwater inflows,” says Roads and Maritime Geotechnical Engineer Dan Horan.
“Groundwater pressures were relieved in the vicinity of the drains, reducing the groundwater table below the M1 at the site by up to 5 m.”
Dr Flentje says early measurements of ground water flow showed about 45,000 L per day flowing from one of the collars of monitoring wells (DOB 4), an extraordinarily good result.2 He says while both flows may have settled to approximately 4,000-5,000 L per day by early August, it will be interesting to see how this develops over the next few years, especially how the flow rate varies during rainfall events.
The drop in water levels was closely monitored and logged. One of the monitoring wells (DOB 3) showed an exceptional response to the drilling operations, dropping over the three day period from 45 kPa to approximately 5 kPa and continuing to drop since to small negative values. The DOB 3 is 14 m upslope from the upper end of the one of the microtunnels and the CRT 3 borehole is 20 m south of the same microtunnel.
DOB 1 is located 10 m south of the microtunnel outlet pit and the shallow VWP is approximately 0.5 m below the base of the pit and the deep VWP is 15 m below. Both have shown 5-10 kPa reductions in pore water pressures.
The real test of the success of these works will be seen in the future during prolonged and intense wet events of the 20, 50 and even 100 year average recurrence interval magnitudes. Hopefully, these works have successfully remediated this site for the long term into the future.
In conclusion, Dr Flentje says, “The community is fortunate to have a strong proactive group of multi-disciplinary geotechnical and asset managers and staff within our industry partners who are aware of the hazardous terrain, in a hazardous climatic and hydrogeological environment who support the university-based research to develop and maintain this extensive (landslide monitoring) network.”
Acknowledgements
The majority of the technical input in this article is sourced, or are excerpts from, Dr Flentje’s paper ‘Continuous monitoring of landslides and infrastructure for asset management’, presented at the Australian Geomechanics Society’s Symposium in Sydney in November 2018.
The author would like to extend his thanks to Mr Horan of Roads and Maritime, David Gunnell of Trenchless Advisor, and especially Dr Flentje of the University of Wollongong.
References
1 Dobinson St Gravity Dewatering Project, Mt Ousley Geotechnical and Hydrogeological Investigation Report, Hunter et al, 2016
2 Continuous monitoring of landslides and infrastructure for asset management, Flentje et al, 2018
This article was featured in the March edition of Trenchless Australasia. To view the magazine on your PC, Mac, tablet, or mobile device, click here.
If you have a project you would like featured in Trenchless Australasia contact Assistant Editor Chloe Jenkins at cjenkins@gs-press.com.au