Study on the zoning of karst development in the Jiaodong Peninsula: Take the Zhongqiao area of Yantai City as an example

Karst is developed in all provinces of China. In Shandong Province, karst is mainly developed in its southwest part, and is sporadically distributed in the Jiaodong area. Particularly, the typical development area is located in the Zhongqiao area of Yantai City—the eastern part of the Jiaodong Peninsula in Zangjiazhuang basin whose overall terrain is high at peripheral edge and low in the middle. There are four distinct seasons in the Zhongqiao area with the annual precipitation mainly concentrating in June and July. Three seasonal rivers—the Baiyang river, the Yandi river and the Shandong river—run through the whole area. With large areas of Quaternary, Cretaceous and Sinian strata exposed as well as fault structures developed, this area presents a simple geological condition, typically distributed with limestone. According to the different burial conditions, the karst area can be divided into three types: exposed karst area, covered karst area and buried karst area. There is a wide distribution of soluble rocks in this area, where karst is developed in three forms, namely, dissolved pores, dissolved cracks and karst caves. However, the irregular development of underground karst fractures leads to the relatively large difference in the abundance of karst underground water. Consequently, there have been many geological disasters of karst collapse usually occurring at a small scale after heavy rainfall in the history of the Zhongqiao area. These collapses caused certain losses to the local economy, though with no casualties.

In order to find out the distribution, degree and law of karst development in the Zhongqiao area, we have carried out a comprehensive study on the karst collapse according to geological environment survey, geophysical exploration, geological drilling, pumping testing, etc. Results of geological survey and geological drilling show that soluble rock strata in the study area are mainly made up of two limestone sections of Xiangkuang Formation of Penglai Group. The distribution of three burial types and the characteristics of karst development have also been identified. Results of geophysical exploration and geological drilling demonstrate the distribution of karst strata in the study area. Meanwhile, the distribution range, development characteristics and changes of developmental depths of dissolved pores, caves and dissolved cracks have been investigated. It is found that this area is mainly composed of the north-east fault structure. Results of geological drilling and pumping testing indicate the groundwater flow direction and the distribution characteristics of karst underground water abundance, with the maximum water inflow per well greater than 3,000 m³·d⁻¹. The degree of karst development is positively correlated with the variables such as the purity of karst strata, the water abundance of water bearing rock group in karst fissure, the distribution of anomalous points in geophysical exploration, and the level of dissolved crack ratio and cave ratio. From five evaluation factors including karst formation, water abundance, apparent resistivity, the ratio of borehole to karst and the ratio of borehole to cave, we used analytic hierarchy process to divide the karst degree in the 60 m zone and divided the degree into five grades, karst development zone, the zone with karst development at a relatively high level, the zone with karst development at a medium level, the zone with karst development at a low level and the zone with undeveloped karst. With the decline of these five grades of karst degree, both the purity of karst strata and the water abundance decrease successively, whereas the apparent resistivity, the ratio of borehole to karst and the ratio of borehole to cave all increase gradually. The distribution range of each karst degree is successively distributed without overlap. The probability of karst collapses is as follows, karst development area>the zone with karst development at a relatively high level>the zone with karst development at a medium level>the zone with karst development at a low level>the zone with undeveloped karst. Collapses mainly concentrate in the karst development area. The degree of karst development is the basic condition of karst collapse; therefore, finding out the distribution, degree and law of karst development in the Zhongqiao area may provide a geological basis for the prevention and control of geological disasters of karst collapse, and guarantee the normal life of local residents and effective operation of enterprises. The study result may also provide a basis for further research on the distribution, degree and law of karst development in the Jiaodong area.