ABSTRACT
A variety of robust strain sensing cables for soil with different properties was developed together with an industry partner. The cross section of the cables used in this study is shown in Figure 1. The cable type V1a is a soft metal free cable with a longitudinal stiffness nearly in the linear elastic range of EA = 2.5 kN, where E is the apparent mean elastic modulus and A the cross section area of the whole cable. The cable V2 is a better protected, longitudinally stiffer but therefore less sensitive cable (EA = 140 kN). It consists of a strain sensing tight buffered fibre and a loose fibre for temperature compensation each in a metallic tube embedded in a cable jacket. Optionally, these cables can have steel wire armouring in the cable jacket in order to
1 INTRODUCTION
Fibre optic cables embedded in concrete or attached to steel constructions are used today in research and practice for strain monitoring and measurement purposes in tunnels, piles and other geotechnical structures. In these examples, the response of a structure in soil on loadings due to soil or other sources is measured. However, in many cases, not the structural response, but a direct measurement of strains and displacements in soil is necessary. For example, in landslide monitoring a structure is rarely available for attaching the sensor and, therefore, a direct and continuous measurement of soil displacement is desirable. Distributed fibre optic strain sensing technology using BOTDA offers high accuracy and large application length and opens new horizons in the field of landslide detection and monitoring. The potential of the BOTDA technology in this field compared to traditional methods was shown by Iten et al. (2009) for an asphalt road embedded fibre optic sensor. Extending fibre optic sensor measurements of strains and displacements directly into soil environment is, however, not trivial for several reasons: vulnerable fibre optic cables tend to break during integration in the harsh soil environment and, additionally, soil can flow around the cable and thus, measured strain in the cable can be different to strain present in the soil.
