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The human factor

Technology used for computer simulations of human movement is being employed by Geotechnical Observations to measure slopes on London Underground’s Central Line.

Movement monitoring specialist Geotechnical Observations has recently employed motion capture technology used for computer simulations of human movement, to measure movement in slopes on London Underground’s Central Line.

The technology takes the form of shape accel arrays (SAAs) that are now used to monitor movement in slopes, bridges, tunnels, dams and piles.

Rigid segments

Each SAA consists of a string of rigid (300mm or 500mm) segments separated by flexible joints. SAAs require no on-site assembly and are fed into small 27mm diameter UPVC casings that have no grooves and so do not require orientating during installation.

Micro-electromechanical gravity sensors housed within each segment measure tilt in two directions. Processors within each array transform the tilt of each segment into the coordinates of each joint to represent the shape of the array.

Displacements can then be measured by recording the change in shape in a similar way to normal inclinometers.
The principal advantage that SAA has over other forms of in-place inclinometer is that displacements in millimetres can be read directly onto a computer using software that is free.

Furthermore, reading SAAs in this way takes a fraction of the time taken to read normal traversing inclinometers of similar lengths.

SAAs can be retrofitted into standard inclinometer casings where displacements have been detected using probe inclinometers. They can even be installed into casings that have been excessively distorted and can no longer be read with a probe inclinometer. SAAs can also be used in horizontal orientations to record settlement and/or heave.

London Undeground trial

Geotechnical Observations undertook the SAA trial on a slope as part of London Underground’s programme of earth structures assessments. The slope is a London clay cutting that forms part of the Hainault loop of the Central Line in east London.

Inclinometers and GeO flushable piezometers were installed to assist in assessing the stability of the slope. All of the instruments were installed using a Dando 2002 lightweight percussive rig and a safety winch.

“SAAs’ principal advantage over other forms of ­in-place inclinometer is that displacements in millimetres can be read directly onto a computer”

Three inclinometer casings were installed into independent boreholes at the same horizon close to the toe of the slope where displacements were expected.

The displacements recorded by the SAA compared favourably with the more traditional methods of recording displacements.

The trial demonstrated that SAAs are a suitable alternative to more traditional methods of measuring displacements in slopes. Traditional methods are very labour intensive and involve carrying heavy equipment to the location of each casing. Trained personnel are also needed to read data from more traditional insitu monitoring equipment.

SAAs are also claimed to be ­relatively easy to install and can be read by non-specialist ­personnel.

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