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Brilliouin Optical Time Domain Reflectometer (BOT-DR) is a newly developed measurement and monitoring technique, which utilizes Brilliouin spectroscopy and Optical Time Domain Reflectometer (Jiang et al., 2006;Zhang et al., 2009;Xu et al., 2011) to measure strain generated in optical fibers as distributed in the longitudinal direction. This paper introduces the principle and characters of BOTDR technique firstly, and makes an example of karst collapse monitoring at section K14 of highway from Guilin to Yangshuo. Discussion includes how to use this technique in underlying karst collapse monitoring in karst highway; environmental factors, like temperature and vehicle dynamic load; how to affect the monitoring results; and how to choose optical fiber type and paving region. At last, we compare the results between using BOTDR and geological radar, and conduct the safety diagnosis on the experimental road. The application achievements demonstrate that BOTDR is a viable technique for the karst collapse monitoring.

property damage is estimated to be at least 20 million CNY (> 3 million US dollars). This paper discusses the geologic background of the study, mechanism of sinkhole collapses, and prevention and treatment of sinkhole hazards in the study area. Geologic Setting Maohe village is located in a highly active karst area and many karst features such as sinkholes, springs, karst windows, caves, and conduit systems are widely distributed in this area. The area is a typical karst plain with isolated karst towers. The altitude of the ground surface is about 92.53-94.16 m. The unconsolidated sediments above bedrock are Quaternary alluvium and colluvium deposits with thicknesses ranging between 3.70 m and 17.10 m. The Quaternary deposits are comprised of colluvium clay, silty clay, rounded gravels, and red clay. The bedrock is thick light-gray dolostone belonging to the middle Carboniferous Dapu formation (C 2d). The dolostone is brittle with many fractures and joints. The upper layer of bedrock dips to the east with dipping angles less than 5 degree. The altitude of the top of the bedrock is 74.30-87.76 m and depth to bedrock is 4.70-38.50 m (Figure 1). The study area is a covered karst area. Bedrock is highly fractured due to faulting and tectonic movements. Karstification is highly active along joints and fault zones, which causes more relief of bedrock topography. Voids and caves are commonly encountered during drilling processes. Thirty three out of 71 (46.5 %) drilling cores showed that caves exist within bedrock. Sixty six caves are detected from drilling with heights ranging from 0.20 to 14.60 m. The number of caves decreases drastically with depth and almost all caves are distributed within the uppermost 20 m of bedrock. Most caves were filled with clay and dolostone debris, especially caves close to the top of the bedrock. Fifty two caves are filled and 14 caves are empty. The filling rate is 78.8% (Figure 2).

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