Distributed Soil Moisture Monitoring Systems Design And Their Applications

Water distribution in soil is an important applied field that has significant influence on soil engineering property,however,it is very challengeable to understand water distribution and transporation in soil for geotechnical and engineering geological researchers.The quantitative index used to describe water field is called soil moisture content,which is a basic physical quantity that reflects soil engineering properties.At present,there is still lack of long-distance,large-area and in-situ soil moisture monitoring technology,as well as mini sensors with small volume,high precision and resolution.The weakness has seriously affected the study on soil engineering properties.The Bragg grating(FBG)has broken limitation of sensor size,which make it possible to gauge soil moisture in a tiny space using mini sensors.In addition,the Roman optical time domain reflectometry(ROTDR)has broken through the limitations of traditional point-based techniques and and can provide distributed real-time measurement of geotechnical temperature.In this study,a new fully-distributed moisture content system(F-DMS),and a quasi-distributed moisture content system(Q-DMS)were designed respectively,by using the temperature sensing principles of FBG and ROTDR.The basic principles and process design of F-DMS and Q-DMS are introduced as well.To verify the feasibility of F-DMS and Q-DMS,several laboratory field in-situ tests were performed.Q-DMS and F-DMS are also successfully applied in different projects to monitor soil moisture in time.The primary work carried in this study and the results achieved are as follows:(1)The main techniques of current soil moisture monitoring were reviewed and analyzed.To overcome the weakness of traditional techniques,a new fully-distributed moisture content system(F-DMS),and a quasi-distributed moisture content system(QDMS)were developed.(2)The basic principles of F-DMS and Q-DMS were are introduced,and the relationship between thermal conductivity and temperature rise was deduced.Then the factors that have influence on this relationship were analyzed.(3)A new variable,temperature characteristic value(Tt)was proposed and the method about how to determine Tt was also introduced.By averaging temperature measured by ROTDR or FBG,Tt reduces measurement precisions.The relative parameters between Tt and soil moisture were calibrated in different soils.(4)Two sensors were designed for Q-DMS,they are carbon fiber heated sensing rod(CFHS)and Alundum packed heated sensing tube(APHS).The CFHS is used to monitor soil moisture in laboratory model tests,and the APHS is used to capture field in-situ soil moisture.(5)Furthermore,two sensing cables were designed for F-DMS,they are carbon fiber heated cable(CFHC)and metal net heated cable(MEHC).The CFHC is suitable for short distance monitoring(<500 m),while the MEHC is suitable for long distance monitoring(≥ 500 m).To improve spatial resolution to surrounding soil,the CFHC was wrecked around a PVC tube to make it into a new sensor,carbon fiber heated sensing tube(CFHST)that is used for monitoring soil moisture profiles.(6)The distributed soil moisture monitoring system(DMS)was established,which consists six subsystems:heating subsystem,sensing subsystem,processing subsystem,transmitting subsystem,intelligence analysis subsystem,and service subsystem.The basic ideas,research and development process,and the integration of the DMS was also introduced.The DMS can be divided into two sections:FBG based quasi-DMS(QDMS)and ROTDR based full DMS(F-DMS).(7)The schemes for measuring soil moisture using Q-DMS in common laboratory tests and centrifuge test were designed.With guidance of the schemes,some tests were implemented to assess the performance of Q-DMS.(8)The schemes for measuring soil moisture using F-DMS in lab tests were also projected.Three tests were performed using these schemes:downwards infiltration test after a precipitation,capillary rise test and land subsidence simulation test with seepage.The results indicate that ⅰ)the hydraulic conductivity increase dramatically with impact of water movement(60 times in this study);ⅱ)the most obvious corrosion zones were observed near water table;ⅲ)there is a positive correlated relationship between soil deformation and amount of detached particles,with a relative coefficient larger than 0.95;and ⅳ)in the process of capillary rise,soil structure changes distinctly that will affect the wetting front rise rate as well.(9)The F-DMS was successfully applied in the ground source heated pump(GSHP)and therml response test(TRT).In the laboratory tests,the relationship between thermal conductivity,and soil moisture,seepage velocity,heating power and soil type were established.The methods were also put forward to analyze influence of different factors in different sites.To quantitatively evaluate the sensitivity of soil thermal conductivity to moisture content,a new parameter called relative thermal conductivity(β),was defined to describe the effect of soil moisture content on thermal conductivity.Furthermore,a field test was carried in Yixing,Jiangsu Province,China,to evaluate the influence of soil moisture on thermal conductivity using β model.(10)Taken the foundation pit dewatering and bank slope as the cases,the schemes to monitor soil moisture content in geotechnical and engineering geological fields were designed.The schemes have been used to Majiagou slope,in the Three Gorges Reservoir regions,and Changzhou metro foundation pit.The results suggested that FDMS enables to captures soil moisture information in slopes and foundations accurately.The accuracy depends on the difference between field condition and calibration tests.(11)The shortcomeings of common Green-Ampt(GA)and Modified Green-Ampt(MGA)models were evaluated,and a new model,improved MGA model(IMGA)was proposed.The accuracy of GA,MGA and IMGA were measured using F-DMS in laboratory tests.The results show that IMGA predicted the accumulated infiltration,infiltration rate and horizontal soil moisture profiles more accurately than GA and MGA.