| Karst area is widely distributed in China, which accounts for about a third of the total land area. The karst ground surface collapse is the main engineering geological problems in karst regions. In recent years, with the rapid development of social economic, karst collapse and the corresponding disasters caused by human engineering activities are becoming increasing frequently and serious in karst regions. Limited by objective conditions, many major projects can only be arranged in the karst area. Detection accuracy in karst area is low and the ground subsidence mechanism is complex. Therefore, it is difficult to conduct the control engineering project. Study of the karst detection methods, the formation mechanism of karst ground collapse and hazard assessment, have significant value to ensure the safety of construction and operation. Besides, the study also has important academic value.Shenzhen universiade center is the main stadium of the26th world summer universiade. It was completed and put into use at the end of2010. It has a main stadium, gymnasium, swimming pool, warm-up court, underground parking, man-made lake and roads, etc. Loads of these buildings are large. They are sensitive to settlement, and have an engineering category of I. The stadium is one of the large construction projects in Shenzhen, even among the world. Engineering geological condition of this site is complex. It is the most complicated karst cave area in Shenzhen city. During the construction of a pile foundation, the actual length of the pile has a large difference from the design length. Cracks can be observed at the ground surface, some piles are crossing the karst caves. For some piles, the bearing capacity cannot meet the design requirement. After the construction of prestressed pipe piles in January2008, the surface collapse occurred in two sites. As the importance and urgency of this engineering project, study on the karst detection method, fully understand the ground surface settlement mechanism and the hazard assessment of karst area not only have an important theoretical significance, but also have an important practical significance.This research is based the research project "Shenzhen universiade center site soil cave, karst cave exploration engineering" finished by China University of Geosciences. The cave soil and the caves are the main research object. The hazard assessment of the karst surface collapse is the main line. Geology, geophysics, engineering geology, mathematical statistics, mechanics analysis and computer simulation methods are employed in this study.Based on the system analysis and study of the geological environment background, karst detection method, the regular pattern of karst development and distribution characteristics of Shenzhen universiade center, karst ground collapse mechanism and risk evaluation system are constructed and applied to the engineering practice. The research contents and main conclusions are as follows:(1) The regional geological environmental condition.The field lithology consists of limestone of Datang step stone rung subgroups (Cls). The warm-up court, stadium and the swimming pool on the east side and west side are shallow buried karst area (<15m). In the warm up court and stadium near basement, the burial depth is very shallow (<5m).The site is located in the medium or strong karst development zone in Longgang town, Shenzhen city, where the local ground foundation is unstable. Field geological structure is complicated, and is located along the GuanJing duplex anticline plunging section of northwestern wing. The east and west boundaries are Hongqiling Fault and Shatang Fault. The south and north boundary are Yougangpu Fault and Qingtang Fault. Hongqiling Fault has the biggest influence on the regional stability. Because of the gological structure of fault and fold, the rock mass group of Clc and Cls are highly fractured plays an important role to the development and evolution of karst.(2) Soil and karst caves’ survey methods and results.Based on the existing investigation data, four investigation methods such as high-density electrical method, geologic radar, CT tomography and verification of engineering geology drilling method are utilized to investigate the soil hole, location, size and material composition.84high-density electrical lines,33geological radar detection section lines,24tomographic images,50engineering geological drilling verification are conducted for this project. Among these boreholes,20are crossing the karst cave. The verification effective rate is70%. Comprehensive investigation methods make up the defects and shortcomings of a single method, which significantly improves the recognition and judgment ability of karst anomaly.(3) Distribution characteristics of karst development.The karst lava is distributed all over this site. The buried depth is more than60m, and the elevation is about50m-10m. The bedrock surface is fluctuating. The hole rate of the field is about11%-19%, with an average of14%. Empty holes accounts for3%. The karst has different characteristics at different place. The east and southeast part of stadium, west and northwest part of swimming pool and the east part of the underground parking are the severe karst area, hole rate is about50%. Soil holes are usually observed along the boundary of shale and limestone.(4) Karst evolution and karst cave and analysis of soil cave development trend.On the basis of karst survey, together with the professional knowledge such as geology and engineering geology, the development trend of the soil hole and karst cave are analyzed. The development speed of karst in ancient time is much more rapid than nowadays. Currently, because the ground water cycling is very weak, the karst development is nearly stopped. But with the influence of human activities, such as ground water pumping, engineering construction and construction load, potential of ground surface collapse still exists.(5) Mechanism of karst collapse.According to the investigation data and similar engineering projects, the statics model of subsidence is proposed. The statics mechanics analysis show that the human engineering activities such as outside force are the direct reason of this karst collapse. Based on the geological investigation and karst detection, two geological models are proposed, they are "silty clay+limestone" and "silty clay+strong weathered sandstone+limestone", respectively. Utilizing the first model and ANSYS, the influence of layer thickness under dynamic loads to karst collapse are analyzed.(6) Hazard assessment methods of karst ground collapse.Based on the MapGIS platform developed by the China University of Geosciences, mathematical models such as analytic hierarchy process (AHP) and PLS Path Model are employed to evaluate the karst surface collapse. Four first-level indicators and eight secondary level indicators are selected. The four first-level indicators of this evaluation are karst foundation condition, cover layer characteristics, building attributes and hydrogeological condition. The eight secondary level indicators are lithology, covered karst existence condition, degree of karst development, and distance from the tectonic line, coating structure, coating thickness, structure important degree and underground water level. The ground subsidence hazard grades are divided into four levels, such as high, relatively high, relatively low and low. The calculation results indicate that when the AHP method is used, the four areas of these four levels are1.3%,8.7%,23.8%and66.2%, respectively. When PLS model is used, the corresponding results are1.3%,8.7%,23.8%and8.7%, respectively. The high risk area is in the east and southeast side of the stadium, northwest side of the swimming court. Special engineering protection measures should be conducted in these areas. The relative high area is located in the east and southeast part of the stadium, east part of the underground parking, the southwest part of the swimming pool and the place near the two speculated faults. However, for the relatively low and low area, as long as the ground water level change and artificial vibration is not very significant, the ground surface will not collapse. Therefore, special engineering treatment is not necessary for this situation.In conclusion, this research follows the thought of "geological environmental conditions-karst cave, karst development law-the karst surface collapse mechanism-hazard assessment". Through this, the karst distribution of Shenzhen universiade center is figured out. The karst cave, karst development trend is discussed. These all provide the foundation and basis for the ground subsidence hazard assessment. The assessment results could reflect the reality, and engineering measures are taken for some of the caves during this research. In particular, the PLS Path Model introduced in this research enriches the karst hazard assessment model. Since the karst survey method is still a challenge problem among the world, the practicability of karst survey methods in karst area still needs further research. |
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