Research On The Deformation Response Of Soil Structural System Under Multi-field Effect Using Optical Fiber Sensing Technology

Soil mass, with a multi-level structural system, is the gradual product of natural and historical process. In addition, soil mass, as a complex open system, will be coupled with surrounding multi-field, such as stress field, moisture field, temperature field, etc. Understanding the mechanism and influence factors of the deformation response to soil mass under the effect of multi-field is of great significance in protecting the geological environment and preventing the soil deformation disasters.The laboratory experiments and site monitoring were carried out to research the deformation response of the soil mass structural system under the multi-field effect based on the optical fiber sensing technology. A series of Fiber Bragg Grating strain sensors and strain sensing cables were developed to measure the deformation response of the soil clod, soil layer and soil mass, three structural levels of the soil structural system. A monitoring scheme was designed to measure the deformation response of the soil mass structural system. The deformation response of the soil mass caused by the stress field was studied though laboratory slope models of the loading and cut-slope experiments. Both homogeneous and heterogeneous soil masses were included in the slope model experiments. Similarly, the effect of the moisture field was studied though the soil mass dewatering and recharge laboratory model experiments. On this basis, the deformation response and change law of the field soil mass structural system coupled by the surface additional stress field and deep formation moisture field were studied though a site borehole monitoring at Shengze of Suzhou city. The results are presented as follows:(1) The 900-HY protecting jacket and ceramic tube packaging, two types of Fiber Bragg Grating strain sensors were developed to measure the deformation response of the soil clod effected by the multi-field. The Fiber Bragg Grating strain sensor with the 900-HY protecting jacket is suitable for monitoring the soil clod deformation in laboratory model test. For the Fiber Bragg Grating strain sensor with the ceramic tube packaging, it has advantages in protecting sensors from implanting into soil mass.(2) A strain sensing cable with discs was designed and improved for distributed strain measurement based on the Pulse-PrePump Brillouin Optical Time-Domain Analyzer sensing technique. The calibration tests for the strain sensing cable show that the fixed distance in centimeters and the length of gluing in millimeters almost have no effect on the strain sensing; the initial strain state and the strain cycle frequency have certain influences on the performance of the strain sensing cable. The results of the laboratory model experiments show that the strain sensing cable with discs is suitable and valid for the deformation monitoring of the soil mass structural system under the effect of surface loading and the groundwater.(3) The thesis summarized the technique points of the Fiber Bragg Grating, the Pulse-PrePump Brillouin Optical Time-Domain Analyzer and Brillouin Optical Time Domain Reflectometry sensing technology and their applications in monitoring of the deformation response of soil mass structural system.(4) The results of the laboratory slope model of loading experiments indicate that homogeneous soil mass continuously deformed under the stress field effect. The deformation characteristics depend on the distributed stress field and the structural features of soil mass. In the heterogeneous soil structural system, the deformation response of the soil mass under the stress field is mainly controlled by the weak layers of the soil mass. The results of the cut-slope experiment turn out that the redistributed stress field has a significant influence on the deformation law and the mode of soil mass disaster.(5) The results of laboratory dewatering and recharge experiments show that the soil mass consolidated and with continuous volumetric compressive deformation during the dewatering process. The volume compression rate of the clay layer soil is much slower than the sand layer because of its low permeability; and the clay layer soil has a small compressive deformation in the location of sandy lens, this is mainly due to the influence of the soil lens body. The rebound deformation of the sandy soil layer is unapparent in the recharge process; however, a significant volume expansion deformation was founded in the clay soil layer and the expansion deformation of the clay layer had a certain compression to the nearby sandy soil layer.(6) The soil structural system characteristic of the quaternary soil mass at Shengze, Suzhou city is analyzed though a site borehole with a depth of 200m. Four different types of strain sensing cable were implanted into the borehole to monitor the deformation response of the soil structural system for three years. The deformation of the deep soil layer, with a depth of 41.2-74.3m, is effected by the lower aquifer dewatering. The aquifer has a very close hydraulic connection with the upper soil layers. The moisture field of the deep soil layer changes due to their supply to the dewatering aquifer, which caused the compression of the nearby soil layers.(7) The results of laboratory experiment and the field monitoring prove the deformation response of the soil structural system, as an organic whole, under multi-field effect is interaction of the structure characteristic of soil mass, clods and soil particles. Low-level soil structure is the basis of the high-level soil structure and determines the deformation characteristics of the high-level soil structure. Meanwhile, the deformation response of the high-level soil structure has a back reaction on the low-level soil structure. Also, the deformation response of soil structural system under the multi-field effect is not a simple sum of each soil structural level, but also the interaction deformation between different structure levels.