Recording structural movement in confined underground spaces requires specialist equipment.
Monitoring structural displacements in real time can be difficult, particularly when the structure being monitored is underground, for example a sewer or tunnel.
Accessing the structure at the appropriate time is difficult, so sensors must be installed inside the structure and read automatically by data loggers that are, preferably, located outside the structure.
The technology behind SAA comes from the motion capture industry, and SAA sensors are capable of working through a full circle, which is particularly useful for monitoring in tunnels and sewers
The equipment that is located inside the structure, which will almost certainly be electronic, will need to be protected to the so-called Atex standards - the name commonly given to the two European Directives for controlling explosive atmospheres.
The ShapeAccelArray (SAA), manufactured in Canada by Measurand and available in the UK, Ireland, Qatar and United Arab Emirates from Geotechnical Observations, was developed to solve this problem.
Each SAA consists of a string of rigid segments measuring between 150mm and 500mm, separated by flexible joints that do not twist. They require no on-site assembly, which significantly reduces the time required to install them, and for monitoring inside potentially explosive environments they are encapsulated inside an Atex-certified flameproof protective cover.
Micro-electromechanical gravity sensors housed within each segment measure tilt, and processors within the segments transform the tilt of each segment into the coordinates of each joint to represent the shape of the array. Displacements are measured by recording the change of shape relative to the initial shape.
The technology behind SAA comes from the motion capture industry, and SAA sensors are capable of working through a full circle, which is particularly useful for monitoring in tunnels and sewers. SAAs can also be installed into slopes, piles and diaphragm walls to monitor lateral displacements, and horizontally into concrete slabs and beneath earthworks to monitor settlement and heave.
Also available is SAAScan, an SAA that is specifically designed for rapid repeat use, rather than monitoring small displacements. It can be used to trace the locus of a borehole, and has been extensively used during injection grouting operations.
Geotechnical Observations has installed SAAs on a wide variety of projects across the UK and Ireland including Crossrail, where they are being used to monitor lateral displacements in diaphragm and piled walls at Tottenham Court Road, Plumstead, North Woolwich and Liverpool Street. They have also been used to monitor vertical settlement of concrete slabs at Royal Oak and Farringdon, and in sewers at Victoria and Bond Street.