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

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.