Mechanism And Hazard Assessment Of Covered Karst Sinkholes In Wuhan City, China

Covered karst is widely distributed in Wuhan city, China. Some karst areas are covered directly by loose sediment (i.e. the covered karst area), where easily develop sinkholes. The first terrace of Yangtze River is the main area of urban development, where covered karst is widely distributed with high risks of developing sinkholes. In recent years, many construction projects have been implemented in covered karst areas in Wuhan, such as the Baishazhou street of the total length 7.46km with 2.70 km located in the covered karst area, where occurred many sinkholes during the construction process. So, to study the mechanism of covered-karst sinkholes and to assess its risk is important for ensuring the geological safety of urban development.A karst sinkholes survey project of Wuhan city was financially supported by China Geological Survey in 2005. The project carried out a systemic survey and monitoring on karst geology, quaternary system structure and sinkholes. It showed that the sinkholes are mainly cover collapses upon open karst holes and its failure model is differ from karst collapse. With urban development, the sinkholes is affected by multi-factors from natural environment and human activities, and it has more complicated mechanism and uncertainty. Based on the 10 years’ survey data of karst sinkholes in Wuhan, we proposed the mechanism study and risk assessment of cover-karst sinkholes. The study not only has important theoretical significance for the formation of karst collapse, but also can guide engineering to prevent and mitigate disaster in hidden karst collapse area in Wuhan.Based on the data of field investigation and monitoring, the geological background of karst sinkholes in Wuhan was analyzed from these main aspects of the karst distribution, cover structure and hydrogeological conditions, and karst development history in Wuhan. According to the structure of the cover-karst system and some sinkhole cases, the geological model of karst sinkholes was established in Wuhan. Theories of saturated-unsaturated seepage, elastic-plastic mechanics and rigid limit equilibrium were employed to analyze the sinkholes evolution under the impact of natural environment and human activities, and discussed its formation mechanism.The Fuzzy Closeness Degree and Analytic Hierarchy Process were used to assess the susceptibility of the karst sinkholes in Wuhan. And, the probability of the karst sinkholes was calculated based on the sinkhole incidents of recent 10 years in Wuhan. The results in the paper are summarized as follows.1. Geological background of the karst sinkholes in WuhanThe total area of hidden karst is about 1091.51 km2, occupying 12.85% of the area of Wuhan city. The hidden karst mainly located in the axial of syncline core with NWW-SEE axial trend, and which form eight karst belts. The degree of karst development is related to formation lithology, the sort is:Permian>Trias> Carboniferous. Karst mainly formed in two periods of the late Indosinian-early Yanshanian (i.e. Late Triassic- Jurassic) and the early Himalayan. Lots of karst groove developed along the joints and fractures of these synclines’ core.In the covered karst areas, the thickness of cover is usually 10-30 m, and the maximum 60-70 m. Structure of cover could be divided into four kinds:single clay, single sand, the double structure of clay above and sand below and multi-layer structure. Single clay mainly distributed in the area where there are ridge and hillock, soil composed in the net shape the clay of pleistocene. The double structure distributes in first terrace of Yangtze river, which is composed by the upper silty clay with 5-22 m thickness and the lower silty sand, fine sand and gravel with 1-21 m thickness. Single sand and multi-layer structure are all scattered in the areas along the river.Changing the pore water in cover and the fissure-karst water in karst is the main dynamic to develop karst sinkholes. The pore water is recharged by the surface water from the Yangtze river of which groundwater level shows seasonal variation and is controlled by rainfall and the water level of Yangtze river. Pore water and fissure-karst water is usually cut off by the gravel clay layer. In some areas without the gravel clay between two aquifers, the karst water is supplied by the pore water. The discharge path of is mainly artificial pumping water. And, the water level of fissure-karst water could show large fluctuation when the foundation construction intrudes into the aquifer.2. Spatial and temporal distribution of karst sinkholes in WuhanMost karst sinkholes happen in April-November every year in Wuhan, which has abundant rainfall and frequent water level fluctuation of the Yangtze river. Since 2001, the frequency of the sinksole is higher and higher with the increase of urban construction activities.Karst sinkholes are mainly distributed in Hongshan and Wuchang districts, followly Hanyang, Hannan and Jiangxia districts. Tectonic distributed in the syncline core or near core site, and concentrated in the covered karst area with lower Triassic strata. Under the influence of human engineering activities, the sinkholes have a tendency to expand to the far city.3. Characteristics of the karst sinkholes in WuhanInfluenced by fracture, structural fissure and the location of the relative impermeable layer, the development of the covered karst has the characteristics of linear distribution and regional concentration. Drilling has revealed that the line karst rate of karst bands No.1 to No.8 are 8.47%,9.87%,13.17%,37.00%,4.95%,9.84%,5.90% and 6.77%, respectively.The cover collapses are the main failure pattern of karst sinkholes, which develop usually in the areas with the upper clay and lower sand. In 29 sinkholes incidents,25 incidents (86%) belong to the cover collapse.72% in the 25 incidents located in the area with double structure cover, and 28% is in the area with single structure.The sinkholes in the covered karst region have experienced progressive failure process to develop earth caves. Suffosion and vacuum suction erosion are the main reason of the progressive failure. With earth cave developing, gravity is the key force to induce sudden collapse. The activities of drilling and pile constructing often induce sudden collapse directly.Natural and human activities jointly trigger karst sinkholes. The former mainly includes rainfall, surface water infiltration and groundwater seasonal fluctuations. The latter includes pumping groundwater and engineering vibration, loading and drilling. The sinkholes mechanism is a comprehensive effect of suffusion, vacuum suction, vertical seepage pressure, and the gravity (weight and load).4. Geological model of karst sinkholes in Wuhan cityThe karst sinkholes models in Wuhan city can be divided into three types:(1) Corrosion-suction-gravity collapse model:taking Hanyang South Mill collapse as the representative, karst water exploitation is the main reason for vacuum suction erosion and corrosion induced collapse.(2)Suffosion-penetration pressure-gravity collapse model:taking Lujia Street collapse and Mao Tan Hong collapse as examples, long-standing pore confined water to the lower part of the karst water runoff produce the suffosion and osmotic pressure. Suffosion seepage pressure effect was strengthened when rainfall and water level of the Yangtze River changed. Rainstorm is the cause for suddencollapse.(3) Seepage pressure-earth cave failure-dynamic load, gravity induced collapse model:taking Fenghuo village project collapse and Jiangxia District Fasi Street collapse as the representative, except the natural environment of the osmotic pressure or suffosion, drilling, punching pile construction of human engineering activities are the key to collapse. It mainly shows in drilling connecting pore water and karst water Unicom, resulting in seepage pressure suddenly increasing, and vibration load and karst water fluctuation caused by punching pile construction. It increases in overburden seepage pressure, sucking erosion and load effect.5. Environmental input and evolution mechanism of cover-karst sinkholes(1) The basis of geological conditions forming karst sinkholes:cover with double structureof upper clay and lower sand, sufficient karst cavitas, linking pore water and karst water, good hydrodynamic condition.(2) The evolution of earth cave above the open karst in natural environment: propagation mechanism of openings in the top soil cave karst can be divided into two kinds of function, namely hydraulic erosion and the destruction of shear and tensile failure under stress concentration. The small scale opening of karst and the covering layer at the contact point where the hydraulic gradient is big, is easy to develop to soil erosion. With the expansion of soil cave, soil erosion action turn into the hole around where the effect of stress concentration, which tension or shear failure resulting in soil hole expansion, and gradually formed a stable equilibrium arch.When there are large open karst caves, suffosion and stress concentration lead to the soil hole extending upward. Generally it is difficult to form a stable equilibrium arch. When the groundwater level declining or rainstorm infiltration, stress concentration acting on the soil will increase, causing soil cave development or sudden collapse. The change law of concentration stress can be explained formation of karst collapse process in Lujia Street.Under natural conditions, pore water and karst water changes synchronization, which is difficult to form a vacuum suction effect. Only when the soil hole extends up to the near surface, the critical stage of collapse has vacuum suction.(3) Human activities in overburden lead to failure of the covering layers and occurrence of sinkholes:excavation of foundation pits lead to the failure of overlying clay relatively impermeable layer, shorten the rainfall infiltration access, increase the vertical seepage pressure. The foundation pit dewatering increases hydraulic gradient of layer within the local area, and accelerates the expansion of soil hole.Drilling and pile constructiing damage the internal structure of the aquifer and aquifuge, connect the pore water and karst water. The hydraulic gradient in the hole wall surrounding increase rapidly, to the critical hydraulic gradient failure stage. With vibration load of pile foundation construction and excess pore water pressure increasing, the pile strength decreased and stress increased. The result is caused to the destruction of the whole covering layer.6. Results of karst sinkhole susceptibility assessment in Wuhan cityConsidering 4 aspects of effect factors, the development degree of karst, cover characteristics, groundwater dynamic conditions and tectonic conditions, the sinkhole susceptibility assessment in hidden karst area was Carried out, four levels of susceptibility zonation was classified on as follows:(1) No karst area:it has 3596.69km2 occupying 73.58% of total assessment areas(2) Low susceptibility area:it is 486.98km2 and occupies 9.96% of total assessment areas. In this area, a thicker aquitard commonly exists between the upper confined aquifer and the lower aquifer. Karst skylights exist only in partial section, connect the upper confined aquifer and the lower aquifer, which makes karstic ground collapse possible.(3) Moderate susceptibility area:it is about 672.95km2 and occupies 13.77% of the total assessment area. Taking the covered karst as main body, underlying bedrock karst develops in medium. There is a possible for karstic ground collapse in the areas where Holocene Series loose overburden is thin.(4) High susceptibility area:it is about 131.59km2 and occupies 2.69% of the total assessment areas, which is located at the First Yangtze River’s Terrace. In these areas, the Jinkou Fracture(the Yangtze Fracture), Jiangjiadun-Qinglinghu Fracture and the Wujiashan-Huashan Fracture were developed. The shallow karst developed strongly in Triassic system-Carboniferous system limestone, and the Quaternary overburden layer has two-layer structure with sand on the top and clay on the bottom. In Third annular urban road of Wuhan, the engineering activities intensity is great, and there were 21 collapses occurred over past 20 years.7. Results of karst sinkhole hazard assessment in Wuhan cityBased on the records of karst sinkholes over past 10 years, occurrence probability of karst sinkholes in different susceptibility zonation were analyzed, the result of hazard zonation is as follows:(1) Low hazard area:it is about 224.45km2 and occupies 4.5% of the total assessment areas. It is located in Xiaxinji in Huangpi district; Jiangjiadun in Qinshan district; the south of Hanyang Yongfeng Street; Zhangwan Street-Jiangdi Street in Caidian district; the area between Zhuru Street, Zhongwancun and Fengzhengjia; Zengjialing-Guanlianhu-Zaoshuwan; Fengshuwan-Tanshuao; Daji Street-Gaohu; Zhangwan Street; Caoshuzui-Huijiazui and so on.(2) Moderate hazard area:it is about 781.17km2 and occupies 4.5% of the total assessment areas. It is mainly located in Shamao Street-Huangjiadun in Hannan district, the karst area of Jiangxia Jinshuizha- Gongjiabian- Xiangtangwan- Longsibian-Wulongquan Street; the area between Jiangxia Zhifang, Yangjiawan, Liangzi Lake and Niushan Lake; the karst area between Daji Street and the Economic Development Zone of Wuhan; the karst area between Meijia Mount-Xiahe-Dazhengcun; the karst area between Shizishan Street to Donghu new technology development district; Zhujiazui-Yanglinzhou- Jiangdi Street; Yongfeng Street; the karst area between Shenjiaji Street-Hongshengcun and so on.(3)High hazard area:it is about 131.59km2 and occupies 2.69% of the total assessment areas, which is located at the karstic area between Yongfeng Street-Wuhan Yangtze River Bridge- Hubei Provincial Government to Xindian; the area along with Yangtze river between Shizishan Street and Jiangdi Street; the area between Xiaojiawan-Shizuichang-Ye Lake-Hejia Lake; the area between Qinglingxiang-Nanhu Street and the Yangtze River; Jinyintan-Fengwan; the karst area between Xiaputan and Mengjiazui; the karst area between Jinshuizha and Hannan Shamao Street; Fasi; Yingkou; Wulongquan Street and so on.Because of the covered nature of karst sinkholes at Wuhan, there is some uncertainty in studying karst geology. Wuhan city is going on a fast track of urban development, so engineering activities are the leading factor to induce karst sinkholes. The paper provides the mechanism of cover- karst sinkholes evolution and summarizes the effect of engineering activities in sinkholes development. The study provides some clues for disaster prevention and reduction of karst sinkholes in Wuhan city.