Research On Land Subsidence Mechanism And Soil Layer Deformation Potential Evaluation Based On DFOS Technology

Excessive extraction of groundwater causes land subsidence disasters.The occurrence,development and disaster process of land subsidence is slow and difficult to detect which results in increasing difficulty and investment to control and govern subsidence.Therefore,monitoring of land subsidence is a necessary way to mitigate its disasters.However,the lack of conventional monitoring technologies hinders people's understanding of the deformation mechanism and law of land subsidence.The Distributed Fiber Optic Sensing(DFOS)technology can break through the limitations of traditional point monitoring method to realize the fine monitoring of borehole full cross section.It is of great practical significance to predict and evaluate the deformation potential of land subsidence.Based on different DFOS technologies,the relevant sensors and sensing cables for model test and in-suit land subsidence monitoring were developed and a DFOS monitoring system for soil deformation of borehole full cross section was established.Based on the coupling test of sensing cables and backfill,the feasibility and accuracy of DFOS technology applied to land subsidence monitoring were verified.It was proved that DFOS technoloy can replace the existing point-type borehole extensometer.Then,a model test was carried out to study the soil deformation response to draining-recharging conditions.Combined with Suzhou Shengze borehole,the DFOS monitoring system was estabilished.With the data that monitored nearly five years,the deformation characteristics of this area was analyzed.In addition,the PCAS software was used to reveal the deformation mechanism of the aquitard and aquifer during the land subsidence from the microscopic view.And then,the concept of land subsidence life process was put forward.Finally,the compression and rebound potential of different strata in the land subsidence life process were quantitatively evaluated by two evaluate methods based on void ratio.The main work and results of the thesis can be summarized as follows:(1)In order to reveal the relationship between land subsidence and groundwater level change and distribution,the multi-point disc sensing cable and FBG based carbon fiber heated sensing bar for model test are developed.Different kinds of sensing cables and sensors for borehole full cross section multi-fields and multi-parameters monitoring are developed.Then their monitoring performances are calibrated.(2)The key problem of coupling effect between sensing cables and borehole backfill is analyzed.The suitable backfill material is determined,and the coupling performance between the backfill material and the sensing cables under different confining pressures is tested and analyzed.The test verifies the feasibility and accuracy of implanting the sensing cables in the actual land subsidence drilling holes.Based on these,Land subsidence DFOS borehole full cross section monitoring system is set up.(3)A model test is carried out and the results show that the DFOS technology is more advantageous in the field of soil deformation monitoring than the traditional point-style extensometers.Through the simultaneous monitoring of soil strain and water content change,the deformation response law of sand layer and clay layer in repeated drainage and recharge cycles are studied.Different deformation characteristics of different soil layers are found.For clay layer,the liquid limit is a deformation cutoff point.Under the condition of repeated drainage and recharge,the soil microstructures change with decreasing porosity and permeability coefficient,the soil gradually tends toward elastic deformation.(4)Taking Suzhou Shenze,where has suffered severe land subsidence in recent years as our research target,the structural changes of the Quaternary sediments in a 200m observation borehole are analyzed in detail.Moreover,the DFOS system is applied to meticulously monitor the soil layer deformation in the borehole for the first time.The results show that the main compaction occurs at two thick aquitards which are adjacent to the pumping confined aquifer.The compression amount in the vertical distribution is not uniform which relates to the distance from the pumping aquifer and the soil properties.The compressive deformation of each layer is closely related to the change of groundwater level.The results of DFOS monitoring can be used to judge the compaction trend of each layer at this stage.(5)The microstructure changes of clay pores and sand particles in the compression process are analyzed by PCAS software,and the mechanism of land subsidence is revealed from the microscopic view.The SEM results show that the microstructure of the clay layer change from honeycomb,matrix,turbulence to laminar microstructure with increasing depth.The better the grain orientation is,the more compact the structure is,and the compaction of the soil becomes more difficult.By improving consolidation device,the change of the sand microstructure during creep process can be extracted.It is also found that the sand compression is mainly due to the deformation of the sand particle contact point,the reorganization(sliding,rolling,etc.)and the crushing of the sand particles(6)Based on the structural characteristics of Quaternary sediments,the compression and rebound potential of the soil layers are evaluated using the void ratio.The void ratio indices Ecl and Esa are put porward to rapidly evaluate the soil compression potential and the stage of the land subsidence life process for different soil layers.According to this method,the soil layer compression potential of Suzhou Shenze shows a tendency that the upper confining beds of the first and second confined aquifers>top of the upper confining bed of the third confined aquifer>bottom of the upper confining bed and the lower confining bed of the third confined aquifer.The water-bearing sand layer of the second confined aquifer has a great potential while that of the second confined aquifer has become stable.Combining the e?p curve and DFOS strain monitoring,an optical fiber monitoring e?p evaluate method is put forward.The strain of the unit water level(??)can be used to judge the deformation trend and the strain of the limit water level(?max)can be used to judge the potential of each stratum and to further estimate the limit land subsidence Smax.Combined with Shengze borehole DFOS data,the following conclusions can be drawn:?? is consistent with the DFOS monitoring tread,can be used to determine the soil compression trend in the current state;Smax will be about 1.275m of Shengze area when reaching the extreme exploitation of groundwater.(7)The relationships between soil rebound potential and consolidation pressure,consolidation time,unloading capacity and soil properties are studied.The result show that once the soil property and consolidation pressure are determined,the maximum rebound rate has a liner relationship with the consolidation pressure and can be used for the prediction of the maximum rebound rate.