Serviceability limit state design in geogrid reinforced walls and slopes

The design of geogrid reinforced walls and slopes, although a well-established science, still contains many unknowns, particularly around long-term serviceability. Serviceability, for walls and slopes, is associated with excessive deformation or damage affecting appearance, maintenance or service life. In most designs, the serviceability limit state is not considered critical. Currently, most serviceability checks do not attempt to determine or prescribe deformation limits on the built wall or slope, but rather impose limits on the theoretical mobilised strains of geogrid reinforcement, considering the unfactored imposed loads. In many cases, these prescribed post-construction allowable strain limits are based on long-term, or accelerated creep testing, undertaken when the geogrid is not interacting with soil. In some situations, designs are grossly overconservative. This paper reviews the current state of practice, summarising some of the serviceability design issues around geogrid reinforced walls and slopes, with a particular focus on long-term post-construction deformations. The paper goes on to highlight areas of non-conformity in serviceability design, between the major national codes in Europe, assessing their strengths and weaknesses. Additionally, the paper highlights potential areas of on-going and further work that may offer a better understanding of the serviceability limit state of geogrid reinforced soil walls and slopes

Modelling deformation during the construction of wrapped geogrid reinforced structures

Although geogrids and geotextiles have been successfully used for over a quarter of a century to reinforce soil, there are currently no commonly agreed analytical methods to model their deformation behaviour. The Serviceability Limit State is becoming an ever more important design consideration, as structures are built with increasingly tighter tolerances. While there are many deformation databases and design charts available, providing information and guidance on the sensitivity to certain design variables, these are largely focused on facets such as height, shear strength and geogrid ultimate strength and do not consider construction method. Following a review of existing analytical and empirical guidance, this paper presents numerical modelling derived guidance for flexible faced Geogrid Reinforced Structures constructed using cohesionless fill that incorporates installation methods. The modelling approach is validated against measured results from three varied case studies, before analysing the changes in deformation distribution resulting from two different construction methods (layer by layer and full height construction). For the conditions analysed, including height of the structure, the lateral deformation resulting from layer by layer construction, was shown to be consistently greater, than for full height construction. In contrast, an analysis of post-construction deformation, for each of the construction methods, found full height construction to be more sensitive to post-construction loading, for the conditions considered. For low wall height structures constructed using the layer by layer method, <5 metres, this study indicates that horizontal face deformations are underestimated by current guidance

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