DFOS-BASED MONITORING ON QUATERNARY SEDIMENTS DEFORMATION AND LAND SUBSIDENCE IN SUZHOU, CHINA

Loose Quaternary deposits are widely distributed in Suzhou, where extracting groundwater has caused a large-scale land subsidence. The land subsidence influences the development of economy. Optical fibers are laid in a 200m borehole drilled in Shengze, Suzhou. Based on the BOTDR and FBG technologies, the Quaternary sediments deformation and land subsidence are long-term monitored. It is showed that Quaternary sediments can be divided into different aquifers(Af) and aquitards(Ad).The main compaction occurs at two thick aquitards which adjacent to the second confined aquifer. In addition, the shorter the distance between the aquitard and the pumping aquifer is,the greater the degree of compression of the aquitard is .The soil layer reaches the maximum compression in summer while a small rebound occurs in winter, showing the conformance with the pore water pressure of the pumping aquifer. We define the compression degree as the ratio of the accumulative compression and the thickness of this layer(i.e.,the unit compression).At this stage the subsidence trend can be judged by compression degree. For the aquitards, there is a tendency shows that Ad2 Ad3 Ad4 Ad1. For the aquifers, the Af2 is the main compaction layer while Af1 and Af3 remain almost constant. DFOS technique provides a very advanced monitoring method for the study of land subsidence mechanism and the evaluation of soil compression deformation potential.