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Underground movement

Concrete

To locate huge underground voids disrupting sewer construction in Abu Dhabi, consultant Hyder turned to geophysical techniques. Bernadette Redfern reports.

One minute the Caterpillar D8 earthmover was cutting and moving earth for sewerage pipe installation and the next it had vanished. Within a space of four seconds the machine had opened up a 20m deep, 15m wide cavity and been swallowed by it. The shocked driver escaped unhurt but, fearing he would be blamed for the incident, he ran from the site and has not been heard of since.

Such a narrow escape during the preparation of 75km 2of land for commercial development forced Abu Dhabi Municipality, the government department responsible for sewerage, to stop work and carry out a more detailed site investigation.

There was simply too much potential for a huge disaster.

Smaller cavities had already been encountered by both the contractors laying the sewerage and the Abu Dhabi Roads Department which was at work levelling ground for new roads on the Abu Dhabi mainland.

'Contractors kept encountering cavities that were often full of saline water, since the site is only a few kilometres inland.

These cavities cause tunnelling machines to get stuck or diverted into open spaces, ' says Dr Ali Talib, resident engineer for Hyder.

'A 650mm micro-drilling machine was destroyed when it fell into a cavity not long ago.

As the machine moved forward the driver noticed a change in face pressure and began pumping in grout. However grout was no match for a 2m deep cavity, ' says Talib.

Apart from the danger that the cavities presented, pumping out the saline water so that the voids could be filled was becoming very time consuming and expensive. The Municipality turned to consultant Hyder to develop a means to locate and mitigate the voids. Previous site investigation was totally inadequate, consisting of a few random boreholes.

This was not helped by poor communication and information sharing between the sewerage and roads departments.

'We could have used a seismic method and measured how fast a wave travels through a material, which allows us to work out what the materials are.

We could have implemented a microgravity method which measures changes in the earth's gravitational field caused by local material changes, or a geo-radar method where electromagnetic waves reflected from subsurface materials provide information about the properties of the materials, ' says Hyder geophysicist Li Baochu After carrying out trials with a variety of geophysical companies Hyder decided on using resistivity imaging: A current is sent through electrodes into the rock and electrical potential is measured. Resistivity is consistent depending on the material: when there was an anomaly in electrical potential the team knows there is a change in material or a void and drills here to investigate.

'We know when the voids are waterlogged as salty water has a very low resistivity, ' says Talib.

The results from the US$16M survey were used to produce a colour coded risk map. Blue areas denote cavities that are greater than 1m wide, yellow areas denote cavities that are approximately 1m wide and green areas show cavities less than 1m wide. 'The client can use the map to avoid or fill in cavities where necessary, ' says Talib.

At the moment the information is being fed into a database, which will be available to a variety of government authorities such as the town planning department, water and electricity authorities and the military.

The geotechnical information management system will also include borehole maps, groundwater levels, photographs and results from chemical tests.

The system will then be used to extrude further information such as contour maps, 3D ground models and geological cross sections.

Preparing the ground

Since the British withdrawal in 1971, Sheikh Zayed Bin Sultan Al Nahan has transformed Abu Dhabi from a small town without roads, water or electricity into the bustling capital of the United Arab Emirates. Until recently development focused on Abu Dhabi Island, a 83km 2land mass situated less than 1km to the west of the mainland.

But the Island is reaching saturation point in terms of development.

High rise buildings dominate the available land and the population is still growing, as it has for the past 30 years thanks to a booming economy based on oil production. Abu Dhabi's cosmopolitan population of 700,000 is expected to double by 2013, encouraged by the government and Sheikh Zayed.

This growth requires infrastructure development in previously uninhabited areas.

Sewerage, roads and electricity networks are springing up around the Abu Dhabi mainland to support the eventual communities and businesses that will reside there.

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