Evolution and monitoring of karst ground collapse in the Zhifang-Miaoshan paleo-clay area of Wuhan
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摘要: 为查明武汉纸坊−庙山一带老黏土区岩溶地面塌陷成灾机理，并选取合适方法手段进行监测预警，有效减轻岩溶地面塌陷造成的危害，通过岩溶专项调查，查明了区内可溶岩分布及其地质结构，分析了岩溶地面塌陷形成演化过程及不同塌陷阶段监测方法的选取，调查分析结果表明：该区多为单层结构老黏土分布区，根据盖层黏性土、软弱土、非可溶岩（红层）的上下组合关系，将可溶岩分布区地质结构划分为3类（①、②、③）。不同的地质结构根据土体物理力学特征差异，其发生岩溶地面塌陷的形成演化机理不同：①类结构主要发生土洞型塌陷，②类结构主要发生泥流+土洞复合型塌陷，③类结构由于红层的阻隔，黏性土土洞无法形成，一般不会产生塌陷。结合塌陷的演化过程，土洞的形成演化阶段，重点是采用光纤等技术监测土洞的形成扩展情况，同时也要对地下水位、水气压力等诱发因子进行监测；土洞顶板变形塌陷阶段，因临塌前地表往往会产生少量沉降变形，主要采用精密水准测量或GPS监测等技术监测地表形变。Abstract:
This study is aimed at finding out the disaster mechanism of karst ground collapse in the Zhifang-Miaoshan paleo-clay area of Wuhan and selecting appropriate methods for the monitoring and early warning, so as to effectively reduce the harm caused by karst ground collapse. Through a special karst survey, we have identified the distribution and geological structure of soluble rocks, analyzed the formation and evolution process of karst ground collapse and selected monitoring methods for different collapse stages in the study area. The survey results show that the study area is mostly distributed by paleo-clay with single-layered structure. According to the upper and lower stacking relationship of overburden cohesive soil, soft soil and non-soluble rock (red layer), the geological structure distribution of soluble rock can be divided into three categories (①, ② and ③). In the structure of class ①, the upper part is composed of cohesive soil; the lower part is soluble rock. The cohesive soil is mostly composed of paleo-clay with the value of cohesion C mainly from 42 kPa to 70 kPa, and the φ value mostly from 10.3° to 19.1°. The free movement of the soil particles is limited, and soil holes are usually formed at the bottom of the soil layer in the soil body. In the structure of class ②, the upper part is composed of cohesive soil; the middle part is soft soil (residual red clay in the shape of soft plastic-flow plastic); the lower part is composed of soluble rock, in which the red clay—weathering residues of the limestone surface—is prone to soften in the shape of soft-fluid plastic in water. Mainly distributed in solution grooves with uneven thicknesses, the red clay usually loses with the action of gravity or suction force, and hence is developed into soil holes. In the structure of class ③, the upper part is composed of cohesive soil; the middle part is the red layer (K2E1g red sandstone); the lower part is soluble rock. In this structure, soil holes are difficult to form due to the barrier of red layer; therefore, karst ground collapse is not likely to occur under the natural condition. According to the different physical and mechanical characteristics of soil bodies, the process mechanism of karst ground collapse is various in different geological structures. Soil-hole collapse mainly occurs in the structure of class ①.The disaster evolution process can be summarized as follows: (A) In the undisturbed stage, the karst water level drops little, and the small value of the suction negative pressure leads to the failure of soil denudation caused by insufficient suction force. Therefore, there is no significant change in the soil body. (B) Affected by the repeated fluctuations of karst groundwater level or human engineering activities, the soil hole is developed initially along the surface of the open karst cave, when the corrosion absorption force is greater than the collapse resistance force of the soil itself. Then the soil of the cave roof collapsed, and the continuous collapse upward contributes to the expanding of the soil hole and the thinning of its roof. (C) The hole continues to expand upwards, and the roof of the soil hole keeps balanced under the friction force generated by the lateral pressure of soil around it. In the early stage of imminent collapse, the ground surface often presents a small amount of subsidence deformation. (D) Induced by natural or man-made factors, the surrounding friction can not balance the gravity of the roof of soil hole. Consequently, the roof may collapse quickly. In the structure of class ②, there mainly occurs compound collapse of mud flow+soil hole. The process of disaster formation and evolution can be summarized as follows: (A) Thick layers of soft soil and cohesive soil are developed on the upper soluble rock. The karst opening is in the stable stage due to the blockage of the cohesive soil. (B) Affected by fluctuations of karst groundwater level or human engineering activities, the soft soil flows and loses to the cave fissure, which forms the mudflow hole, and the hole quickly expands to the top of the soft soil. (C) The soil hole continues to expand upward in the layer of cohesive soil, and the roof of the soil hole keeps balanced under the friction force generated by the lateral pressure of soil around it. In the early stage of imminent collapse, the ground surface often presents a small amount of subsidence deformation. (D) Induced by natural or man-made factors, the surrounding friction can not balance the roof of soil hole (dead weight+loading); consequently, the roof collapsed quickly. Besides, the quick loss of red clay in shapes of soft plastic-fluid plastic accelerates karst ground collapse. In the structure of class ③, it is unlikely to occur collapse because the barrier of red layer makes it difficult to form the cohesive soil. Combined with the formation mechanism of collapse, the formation and evolution of soil hole can be monitored by optical fiber, and induced factors such as groundwater levels and water pressures should also be monitored. During the deformation and collapse stage, precise leveling or GPS monitoring can be used to monitor surface deformation because the ground surface often presents a small amount of subsidence deformation before the imminent collapse. -
图 3 研究区岩溶地质结构类型分区图
1-①型 2-②型 3-③型 4-非可溶岩区 5-分区界线及分区代号 6-剖面线及编号 7-岩溶地面塌陷及编号
Figure 3. Zoning map of karst geological structures of the study area
图 4 B-B’高密度测线综合解译成果示意图
1-第四系中更新统王家店组残坡积层老黏土 2-第四系残积层红黏土 3-二叠系中统栖霞组灰岩 4-推测地层分界 5-钻孔
Figure 4. Schematic diagram of comprehensive interpretation of B-B’ high-density survey lines
图 5 土洞型塌陷成灾演化过程示意图
1-黏性土；2-塌陷堆积物；3-溶洞及充填物；4-地下水位上下频繁波动；5-土体位移方向
Figure 5. Schematic diagram of the evolution of soil-hole-type collapse
图 6 泥流+土洞复合型塌陷成灾演化过程示意图
1-黏性土 2-塌陷堆积物 3-软弱土（软塑-流塑状红黏土） 4-溶洞及填充物 5-地下水位上下频繁波动 6-土体位移方向
Figure 6. Schematic diagram of the evolution of the compound collapse of mudflow+soil hole
表 1 研究区岩溶区地质结构类型划分
Table 1. Classification of karst geological structures of the study area
类型 模型图 结构特点 主要分布 ① 
黏性土直接覆盖于可溶岩上；可发生土洞型塌陷 工作区西北角洪山区八斗丘，多属一级阶地后缘；中部纸坊翟家村—周寺咀—魏家湾—张家湾、节义祠—黄家湾—龙塘湾、东南角梦天湖山庄—土库湾—半甲杨、西南角，多属长江二级阶地剥蚀垄岗区 ② 
黏性土和可溶岩之间夹有厚度大于1 m的软弱土（红黏土底部）；软塑−流塑红黏土先期流失形成泥流型土洞，逐渐发展破坏，地表反映为土洞型塌陷 工作区中部纸坊古堤湾—豹子山—红旗村—往南至渔牧村；多属于长江二级阶地剥蚀垄岗区 ③ 
黏性土+红层直接覆盖于可溶岩上；一般不会发生岩溶地面塌陷 工作区中部纸坊大桥镇林场—铁家湾—纺织大学；属于长江二级阶地剥蚀垄岗区 
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表 2 研究区岩溶地面塌陷类型分类表
Table 2. Classification of karst ground collapse in and around the study area
序号 发生时间 塌陷名称（地点） 塌陷规模 塌陷灾情 地质结构 诱发因素 塌陷机理 1 2014.5.2 江夏区大桥新区鹏湖湾二期工地 中型 二人死亡，一台钻机失踪。后期搜救动用10台挖掘和运输车辆，清土方8 000多方，动用大量人力，包括武警水电部队 ② 钻探、载荷 泥流+土洞
复合型2 2016.2.25 江夏区原武汉东方明浒混凝土场有限公司厂区 小型 毁坏厂房地面，造成混凝土厂暂停营业 ② 载重车辆荷载 泥流+土洞
复合型
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