Abstract
Composite anchors are special passive sub-horizontal reinforcements recently developed for remediation of unstable slopes. They are composed of a hollow steel bar, installed by a self-drilling technique in the soil, coupled with tendons cemented in the inner hole to increase the global anchor tensile strength. The anchors are primarily intended to stabilise medium to deep landslides, both in soils or weathered rock masses. Among the valuable advantages of composite anchors are their low cost, ease of installation, and flexibility in execution, as testified by a rapid increase in their use in recent years. The bond strength at the soil-anchor interface is the main parameter for both the design of these reinforcements and the evaluation of their long-term effects for landslide stabilisation. After a brief description of the composite anchor technology, this paper presents a novel methodology for monitoring the strain and stress accumulated in the anchors over time when installed in an unstable slope. The new monitoring system is composed of a distributed fibre optic sensing system, exploiting the optical frequency domain reflectometry (OFDR) technique, to measure the strain exerted on the optical fibre cable embedded with the tendons inside the bar. The system permits an evaluation of the axial force distribution in the anchor and the soil-anchor interface actions with a spatial resolution of up to some millimetres. Therefore, it allows determination of the stabilising capability associated with the specific hydrogeological conditions of the site. Furthermore, upon an extensive validation, the system may become part of a standard practice to be applied in this type of intervention, aimed at evaluating the effectiveness of the anchor installation and its evolution over time.
Highlights
Many areas of the world are highly susceptible to landslides [1,2] and this type of natural hazard causes extensive damage and human losses
A few have focused on the use of optical fibre sensors to monitor strain and stress
A few have focused on the use of optical fibre sensors to monitor strain and stress along soil nails or anchors
Summary
Many areas of the world are highly susceptible to landslides [1,2] and this type of natural hazard causes extensive damage and human losses (about 4000 fatalities/year over the world in the period of 2004 to 2010). A few have focused on the use of optical fibre sensors to monitor strain and stress along soil nails or anchors The majority of these attempts were carried out in a laboratory, where the fibre can be handled more and proposed the fibre Bragg grating (FBG) sensing technique [25–. Hong et al [34] instrumented a 35 m-long GFRP bar soil nail in the field with an optical fibre in a loop configuration for the distributed measurement of strain and temperature They exploited a Brillouin optical time domain analysis (BOTDA) technique with positive results, but limited spatial resolution (0.4 m), concluding that other fully distributed sensing technologies should be used to verify the present sensor performance. This paper focuses on the presentation and discussion of the results of the first phase and only some preliminary results related to the DFOS’ installation on-site are discussed in the final part of the paper
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