We Used a Unique Way of Measuring during a Highway Tunnel Construction Preparatory Works
Within the scope of works provided for Zakládání staveb company, scientists from the University Centre of Energy-Efficient Building of CTU took part on load tests of of Pohůrka tunnel that will become a part of the D3 highway project near České Budějovice. They used a method of measuring by fibre optic sensors Fibre Bragg Gratings which is unique in this context.
As soon as in several years, in the D3 highway project section “0310/I Úsilné – Hodějovice” drivers will be going under the ground. Due to the complicated geotechnical and hydrogeological conditions combined with a demanding construction, it was necessary to carry out load tests of diaphragm walls segments designed to be concurrently a sheeting, load-bearing and also sealing structure of the cut and cover tunnel.
Carrying out of the load tests as well as the design of the testing procedure were secured by our client, Zakládání staveb company. The task of CTU UCEEB employees during the load tests was to measure the actual strain and to determine values of normal stress in three tested segments of different length.
They fulfilled their task using two methods. The first of them employs the measurement using resistance strain gauges that is often used for this purpose. Each strain gauge is connected to the central measuring unit using a separate cable. Within the scope of this contract, more than 2.5 km of cables were used for connections.
The other method is measuring using fibre optic sensors Fibre Bragg Gratings (FBG); the necessary amount of measuring points is located on one optical cable and the central measuring unit is always connected to the whole measuring line. The use of FBG sensors in this context has been absolutely unique so far. Testing segments of the length of 17 and 27 meters were equipped with 52 resistance strain gauges and 52 FBG measuring points in total. In order to connect them with the central measuring points, 52 individual cables for strain gauges and 8 individual cables for FBG sensors were needed. The testing segment of the length of 40 meters was equipped with 36 resistance strain gauges; FBG sensors were not used in this case.
As compared to the commonly used strain gauge measurements, the use of fibre optic sensors for such purposes was proven to be absolutely satisfactory and, at the same time, comparably functional as well as reliable. Evaluated results of measurements were handed over to the client for further processing and assessing whether the designed construction will be satisfactory in these quite unfavorable conditions.