Measuring deformation performance of geogrid reinforced structures using a terrestrial laser scanner

Geogrid Reinforced Structures (GRS) are inherently flexible and although the design for ul-timate limit state is relatively mature, GRS are often defined by their deformation performance, in the serviceability limit state (Koerner and Koerner, 2013). Currently, serviceability design protocol does not determine or prescribe deformation limits for the built wall or slope, but rather imposes limits on the theo-retical mobilised strain of geogrid reinforcement. Current understanding of the principle mechanisms for GRS deformation is weak and often the only way to assess the serviceability of structures is by the observational method. Typically this has been done with external surveying instruments such as total stations or internally using strain gauges, extensometers and inclinometers. Laser scanning has previously been used to measure the serviceability performance of conventional geotechnical structures and slopes and provided useful information (Mechelke et al., 2007) but has not yet been used on GRS. This paper assesses the potential of a Terrestrial Laser Scanner (TLS) to rapidly survey GRS. This assessment covers a range of structures including a 6.5 m high steel mesh faced retaining wall and a 3.6 m wrap faced structure. The measured behaviour obtained from this range of structures demonstrates the importance of facing stiffness on controlling deformations. Terrestrial laser scanning has potential because it is unobtrusive, only requiring lines of sight to the face and does not use targets located on the GRS. The system can be used to measure the position of the GRS face to within a noise range of ±5 mm (Kersten et al., 2008), across a large surface area from a single observation point in minutes. This paper assesses the application of using TLS to measure deformations during construction and in-service and proposes a standard scanning procedure. It also details experience gained surveying GRS constructed with a range of face systems and discusses accuracy and repeatability issues. It con-cludes with possible implications of using the TLS method for routine monitoring of GRS.