14.11.07
Dubai Metro Project – a geotechnical perspective
The Dubai Light Rail project (Dubai Metro) is the first metro project having been undertaken in the Middle East. The project currently consists of the Red and Green Lines (see figure 1), with a total length of 75.9 km, of which 13.5 km will be underground using TBM and cut & cover excavation methods. Other parts of the metro will be elevated on viaduct structures (see figure 2). There will be a total of 47 stations.
The client for the Dubai Metro is the Road and Transport Authority (RTA) Rail Agency which is responsible for planning and providing the rail network in the Emirate of Dubai.
Geotechnical issues
Since April 2006, Atkins has been engaged as lead designer for the Japan-Turkey Metro Joint Venture (JTMJV) which is part of the DURL (Dubai Rapid Rail Link) Consortium. This consortium is responsible for the design of all civil works, including all geotechnical related works from planning and management of the site investigation, geotechnical interpretation, deep excavation design, tunnel design, design of foundation systems for the viaducts and stations and management of the earthworks for three large depots. The geotechnical design is being carried out by a consortium of teams in the UK and the Middle East.
Critical geotechnical issues for the project include the:
• Absence of documented geotechnical data for the Dubai area,
• Presence of cemented sand in the deep weak rock layers,
• Potential high water ingress into excavations,
• Highly aggressive soil and groundwater conditions
• Presence of gypsum, cavities and loose marine and sabkha deposits,
• Risk of liquefaction and running sand during tunnelling,
• Reliability of pile shaft friction in high carbonate materials, and the
• Impact on adjacent structures as a result of ground movements induced by adjacent tunnelling and deep excavations.
The geotechnical investigation (see figure 3) consisted of some 2000 boreholes and cone penetration tests and associated field and laboratory testing. A jack-up rig was used for undertaking boreholes in the Dubai Creek and a geophysical resistivity survey was used to interpret the thickness of estuarine deposits beneath the creek bed. The geotechnical design has used results obtained from specialist in situ testing, including elastometer and packer tests as well as sophisticated laboratory testing.
Geotechnical models (see figure 4) and design parameters were developed for each tunnel and elevated section, station and depot. The geotechnical interpretation included assessments of liquefaction risk and free-field soil displacement under seismic loading.
Advanced geotechnical numerical modelling (see figure 5) has been carried out using the PLAXIS software for the design of the diaphragm walls in the underground stations and the cut and cover tunnels, and FLAC3D to estimate the tunnel volume loss as a result of varying the face and grouting pressures and the impact on adjacent structures.
Groundwater flow around the station boxes was modelled using the SEEP/W software. This analysis has identified the ground settlement risk resulting from dewatering and the subsequent affect on nearby buildings and utilities. In situ pumping tests have been undertaken in selected underground stations, to gather information on soil and rock permeabilities and aid design for the dewatering strategy.
The Red Line is scheduled for completion in September 2009 with the Green Line to follow in October 2010.
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