Geological Environment Identification And Modeling-evaluation Method For Urban Underground Resources Development

Urban geological environment gives rise to a lot of restriction on the underground space development and its quality characteristic. In addition, a scientific assessment requires the support of unban geological modeling, as well as the identification of key impact factors. In order to establish the sound geological model, we should make a clear recognition of key underground stratum and understand its geotechnical engineering characteristics. However, research of this aspect is still in the initial stage at present. Therefore, based on the Suzhou Geological Survey and Sino-Swiss Cooperation Project (comparison between Sino-Swiss typical cities for sustainable underground exploration in urban areas), we need to carry out following work oriented by the subject of this research. First up, we need to take in-situ monitoring, indoor soil tests and model tests to realize multi-scale experimental study, including the study of engineering properties and inner structures of key strata. On the other hand,3-D visualizing modelling technology is also necessary to simulate the urban geological environment, conditions of strata, solution of engineering problems. Then geological and environmental evaluation for the underground space development is also indispensable. Therefore, based on the analysis of massive boreholes'data, application of multi-scale experimental tests and3-D urban geological modelling and spacial analysis functions by software ArcGIS, we can get following research results:(1) Taking into full account of the sedimentary environment and history, formation origin and soil characteristics and other factors from the geological survey in Suzhou City, engineering geological strata groups of soil mass on the eastern plains area were divided, as well as key bearing strata group and weak-sensitive strata were also indentified. Then the representative value of key bearing strata and weak-sensitive strata were predicated. In addition, horizontal and vertical spacial self-correlation distance was also calculated by using the Space Recursive Method. According to the dominant impact factors for engineering geological division, we could divide the urban planning area into two parts. It was based on the distribution characteristics of soil and rock mass and other engineering features. As for each sub-area, we have also made a detailed discussion on the site construction conditions, underground space utilization modes, potential geological hazards. And the corresponding countermeasures were also put forward. (2) A free piston thin wall sampler and frozen snapping-paste method were used to obtain undisturbed specimens for miro-structure analysis. Based on the analysis of digital images by Scanning Electron Microscope (SEM), comparative studies of two soft soil layers were carried out to analyze the natural microstructure characteristics, particles'orientation characteristics, eccentricity, orientation probability entropy and other quantitative indicators. Such a work was useful for better understanding the basic engineering properties of soft soil in Suzhou City, as well as providing important experimental data to establish the relationship between the sedimentary environment, micro-structure and macro-engineering properties. Considering the current thermal testing defects (sampling difficulty and disturbance, great errors, bad reflection of compounded formation with complex geological conditions) on surrounding rocks of underground engineering, we develop a numerical modeling technology to simulate the impact effects of soil micro-structures on its macro thermal properties. Besides, we have also designed a new sampling device and method for soil indoor model tests, in order to obtain undisturbed soil specimens for micro-structure analysis and thermal conductivity tests.(3) A lot of boreholes and stratigraphic profiles had been collected and classified in need, which could be consistent with the establishment and analysis the geological strata groups sequence. After unifying the top and bottom elevation for each stratum, ordinary kriging interpolation method and nugget-exponential model variogram were used to establish the geological model. The geological model of Suzhou Eastern Plain area can be utilized to solve the engineering problems of underground space exploration in the visual three-dimensional space. Beyond that, the spatial data could be extracted from the computed model by the geological modeling software Geolep-3D. Then the data were imported into ArcGIS, in order to convert the data to Shapefile file format. By utilizing of the function of chart drawing and statistical analysis in ArcGIS, the problems in the process of foundation pit and shield tunnelling construction were analyzed.(4) Topography conditions, types of construction sites, unfavorable geotechnical problems, hydro-geological characteristics and geological hazards were considered as the main indexes of the thematic layer for geo-environmental suitability evaluation. Furthermore, both the Analytical Hierarchy Process (AHP) and expert questionnaires survey methods were utilized to calculate the weights of evaluation indexes. At last, the mathematical evaluation model was established by massive aim linear weight function method to evaluate the exploration difficulty at the depth from0～15m (shallow depth) and15-30m (sub-shallow depth).