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Survey maps Abu Dhabi voids

CONTRACTS

Hyder has used geophysical techniques on a £10M investigation to locate huge underground voids beneath a commercial development in Abu Dhabi.

Investigations were prompted after an earthmover cutting and moving earth for a sewerage pipe installation fell into a 20m deep, 15m wide cavity.

The Abu Dhabi Municipality, the government department responsible for sewerage, immediately stopped work to carry out detailed site investigation.

Smaller cavities had been found previously by contractors laying sewerage. 'A 650mm micro-drilling machine was destroyed when it fell into a cavity not so long ago, ' says Dr Ali Talib, resident engineer for Hyder.

Previous site investigations consisted of a few random boreholes so the authority turned to consultant Hyder to develop a means to locate and mitigate the problem of voids.

Hyder considered a number of geophysical techniques to locate the voids, including seismic, microgravity and electromagnetic methods, carrying out a number of trials with geophysical firms. In the end it opted for a resisitivity survey, locating voids by identifying anomalies in electrical potential in the ground. Where anomalies were found, drilling would be carried out. 'We also knew when the voids were waterlogged as salty water has a very low resistivity, ' adds Talib.

Survey results were used to produce a colour-coded risk map. Blue areas denoted cavities over 1m wide, yellow areas those about 1m wide and green areas those voids less than 1m wide.

Data will be available to government authorities such as the town planning department, water and electricity authorities and the military. This will include borehole maps, groundwater levels, photographs and results from chemical tests.

The system will then be used to create contour maps, 3D ground models and geological cross sections.

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