Use of fibre-optic sensors for simple assessment of ground surface displacements during tunnelling

Abstract

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.