Influence of the Xuzhou paleochannel alluvium on karst collapse and engineering construction
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摘要:
历史上水系的变迁导致徐州古河道十分复杂。文章对徐州古河道的变迁、潜蚀特征、识别特征、深度特征、结构特征进行了分析，并分析了古河道对岩溶塌陷及工程建设的影响。结果表明：（1）自晚更新世以来，共有5条河流流经徐州，其中晚更新世古河道有2条，全新世古河道有3条；（2）徐州古河道冲积层具有“新”、“软”、“杂”、“变”等特点；（3）古河道为岩溶塌陷创造了有利的地质结构，其一形成了相互叠加的粉土、粉砂堆积物；其二对徐州市区内稳定分布的老黏土进行冲刷，形成了下伏岩溶含水层的补给天窗；（4）古河道形成的粉土、粉砂层富水性强，工程施工时易产生涌砂现象，同时古河道冲积层冲埋隐伏的构筑物对基坑工程及浅基础建筑物稳定性会产生较大影响，工程建设时应当采取相应的工程措施。
Abstract: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.
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Key words:
- paleochannel /
- karst collapse /
- engineering properties /
- influence
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表 1 古河道冲积层主要物理力学指标(平均值)
Table 1. Main physical and mechanical indexes for the paleochannel alluvium (average values)
地层时代 岩组名称 主要物理力学性质 含水率 $ w $ /%孔隙比 $ e $ 液性指数 $ {I_L} $ 压缩系数 $ {a_{1 - 2}} $ 粘聚力 $ c $ /Kpa压缩模量 $ Es $ /MPa标贯击数 $ N $ /击全新统Q4 砂土组 30.5 0.82 0.81 0.47 4.37 8 粉土组 27.2 0.81 0.80 0.42 12 3.73 9 黏性土组（新黏土） 34.2 0.98 0.97 0.61 16.3 0.80 3 更新统上段Q $_3^{3}$
更新统中段Q$_3^{2}$
更新统下段Q$_3^{1}$ 黏性土组（老黏土） 24.3 0.69 0.31 0.39 49 1.41 15 黏性土组（老黏土） 27.2 0.69 0.31 0.18 59 1.77 黏性土组（老黏土） 23.9 0.80 0.27 0.14 57 21.2 注：含水率表征土体中所含水分重量的百分比；孔隙比表征土体中孔隙体积与固体颗粒体积之比；液性指数表征土的软硬状态，表示天然含水率与界线含水率相对关系的指标；压缩系数表征土体压缩性大小的物理量；压缩模量表征物探在受三轴压缩时应力与应变的比值；粘聚力表征物质内部相邻各部分之间的相互吸引力；标贯击数表征土体的密实程度及承载能力的指标。
Note: Water ratio represents the weight percentage of water contained in a soil mass; porosity ratio represents the ratio of pore volume to solid particle volume in soil mass; liquidity index represents the hard and soft state of soil, which indicating the relative relationship between natural and boundary water content; coefficient of compressibility is a physical quantity that represents the compressibility size of soil; modulus of compressibility represents the ratio of stress to strain when geophysical exploration is under triaxial compression; cohesion represents the mutual attraction between adjacent parts of a substance; standard penetration number indicates the compactness and bearing capacity of soil mass.
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