In this paper, an overview of optical sensor development, testing and evaluation for several geotechnical monitoring applications is presented. Additionally, sensor integration and data interpretation are addressed as key influences to the overall success of the monitoring project. They should be taken into consideration already in the design stage.Particular focus is given on strain sensor development to minimize the slippage of the fiber inside the protection. For the first time, slippage progression monitoring by high spatially resolved Brillouin measurements is presented as a new tool for sensor testing and evaluation for geotechnical projects.The main findings of the study are that in a geotechnical monitoring project, special care has to be taken by choosing the sensor slippage properties, longitudinal stiffness and robustness, as well as in the design of the sensor system itself (fixation, gauge length and bond strength). With appropriate alignment of these factors, reasonable monitoring data can be obtained, as shown in the applications proposed in this manuscript.

Horizontal strains were measured with high precision during Crossrail tunnel excavation in central London by means of distributed fibre-optic sensors embedded in shallow transverse and longitudinal surface trenches. Validation of these measurements against manual micrometer stick readings demonstrates the benefits of the fibre-optic sensing: high precision at small strains; significantly smaller spatial resolution; and the possibility for automatised measurements over larger spans. In order to further capitalise on these advantages, two additional potential applications of the fibre-optic measurements were investigated: (a) independent assessment of the vertical ground displacements using transverse strain measurements and analytical settlement trough models; and (b) early prediction of the vertical and horizontal ground displacements, using transverse and longitudinal strain measurements and analytical settlement trough models. While understanding that a single case study can only form a basis for limited conclusions, both applications do appear to be feasible.

In Geotechnical Engineering, progressive failure in soil-structure interaction is one of the least understood problems. It is difficult to study this phenomenon at laboratory scale, because of the large amount of strain gages required per unit length/area of the structure, which would interfere with the mechanical properties of both the structure and the soil. The recently developed Brillouin Echo Distributed Sensor (BEDS) technology overcomes this dilemma by distributed readings and 5cm spatial resolution. A laboratory pullout testing program has been carried out to verify applicability of BEDS for the study of progressive failure in the soil-structure interaction.

The problem of the limiting landslide pressure on an obstacle was first formulated in 1944 by Robert Haefeli of ETH Zurich, who recognised that the kinematics of the problem does not allow for classical active and passive earth pressure theories to be applied. He derived an approximate solution using a limit equilibrium approach with a number of rather arbitrary assumptions and simplifications. Since then, the Haefeli solution has been widely applied for the design and analysis of landslide retaining structures. The paper revisits this old landslide pressure problem by means of a rigorous upper- and lower-bound limit analysis and derives the exact landslide pressure solution for a planar landslide with a weak slip surface parallel to the slope. Being applicable to a wide range of natural and man-made obstacles and, unlike the classical theories, not affected by the wall friction and soil dilation, the upper-bound solution is rather robust. The landslide pressures from this solution increase with the strength of the sliding layer and are significantly higher than the active, but much lower than the passive, earth pressures. Of even higher practical importance, however, is that due to their oversimplifying assumptions, the widely used approximate solutions appear to get close to the exact solution only over a very narrow range of slope and friction angles. It appears that for mildly inclined weak slip surfaces and high strengths of the sliding layer, analysis and design of retaining structures based on well-known approximate solutions can become dramatically unsafe.

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