Influence of the Xuzhou paleochannel alluvium on karst collapse and engineering construction

According to historical records, from 1128, when the Yellow River encroached the Huai river to flood the Sishui river, to 1855, when the Yellow River flowed through Xuzhou, the Yellow River flooded many times, causing several changes in the direction of the surface water system and the repeated scouring and burying of the paleochannel, which has contributed to the complexity of today's paleochannel in terms of its direction, burial depth and stratigraphic structure, over these seven hundred years.

In this paper, the characteristics of changes, erosion, identification, depth and stratigraphic structure of the Xuzhou paleochannel are analyzed through data collection, field exploration, drilling, geophysical exploration and other geological methods, and the influence of alluvium formed by the paleochannel on karst collapse and engineering construction has been studied. It is concluded that since the late Pleistocene, a total of 5 rivers have flowed through Xuzhou. Among them, there are 2 paleochannels in late Pleistocene, both originating in Shandong and entering Xuzhou City from north, and 3 in the Holocene, namely, the ancient Sishui river, the ancient Bianshui river and the ancient Yellow River. Besides, the strata of the Xuzhou paleochannel were firstly formed by the flood of the Bianshui river and the Sishui river, and then by the alluviation of the Yellow River. Therefore, the strata are characterized by "new" (The age is young, mainly formed by the flooding of the Yellow River.), "soft" (Many strata present the large compressibility with high water content), "miscellaneous" (The strata contain bricks, tiles, stones and pottery of the underground ancient city.), "changing" (The large area of cover caused by the flooding of the Yellow River not only buried the ancient city, but also changed the landform). Furthermore, the paleochannel has created favorable geological structure for the formation of karst collapse because the formation of superimposed silt and silt deposits as well as the scouring to the old clay that is steadily distributed formed a replenishment skylight of underlying karst aquifer. This is also the main reason why the collapse points are densely distributed near the paleochannel. Finally, due to the strong water abundance near the paleochannel, the construction of the subway shield is subject to the sand inrush caused by silt and mealy sand. At the same time, the special stratigraphic structure and engineering geological characteristics of the paleochannel will have a great influence on the stability of foundation pit engineering and shallow foundation buildings, and hence corresponding engineering measures should be taken during the project construction.