Monitoring displacement distribution within the rock mass during a plate load test

Abstract

A fiber optic sensor system has been developed specifically for monitoring displacements within geomaterials. Optic fiber Bragg gratings were fixed to the sides of a slender piece of plastic rod as a means for displacement measurements. An attempt was made to monitor the local displacements within an artificial soft rock sample in a laboratory model plate load test. Two 8 mm diameter fiber optic sensored Delrin rods were inserted into the rock mass at various depths below the model foundation. The amount and distribution of the displacements caused by the plate load were measured through the flexural strains occurred in the fiber optic sensors. Initial results have indicated that the technique was capable of detecting relative displacements at least for the case of a model plate load test. Reasonable relationship between moduli and strains could be derived from these displacement measurements and estimated average stresses based on Boussinesq theory. This paper describes the laboratory set up of the sensor system; present the results of the plate load test and discusses the implications in the use of fiber optic sensors for field local displacement measurements.