Study on an in-situ dissolution experiment of gypsum boreholes
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摘要: 以往针对石膏岩溶蚀试验多局限于室内环境，难以全面反映深部石膏溶蚀的真实情况。钻孔原位溶蚀试验方法是在石膏岩场地的钻孔内不同深度放置石膏岩标准试件，模拟石膏岩在原位水动力、水化学、温度等环境下的动态溶蚀过程。目前，国内鲜有公开该方法的研究成果，本文以白垩系灌口组石膏岩为研究对象，通过试验首次获取该石膏岩原位溶蚀宏观形态、溶蚀速率、水化学变化特征，深度剖析原位环境下石膏岩的溶蚀主控因素和机理。研究结果表明：钻孔原位环境中，石膏岩溶蚀的宏观特征、溶蚀速率具有显著垂向分层效应，主要受水动力条件控制，水化学影响次之；钻孔内15 m和25 m深度处试件溶蚀速率高达0.25 mm·d−1（约90 mm·a−1），而35 m深度处试件溶蚀速率仅为上述位置的1/4；15 m和25 m处试件的溶蚀主要受水动力物理驱动作用，溶蚀过程经历吸附饱水、浅层凹坑、横向并坑、纵深解体的四个阶段；35 m处试件在近似于封闭地下水环境下，溶蚀速率缓慢，主要受控于化学溶解。通过本试验研究，构建了石膏岩钻孔原位溶蚀试验流程和评价方法，可为石膏岩场地岩溶风险评价提供重要的参考依据。Abstract:
Gypsum is characterized by high solubility and a rapid dissolution rate, which can trigger geological disasters such as subsidence and collapse, especially under the influence of inappropriate human engineering activities like groundwater extraction and drainage. However, most research on gypsum has focused on itsn occurance in the marine sedimentary environments, with relatively few studies addressing gypsum in the lacustrine sedimentary settings. The latter is generally considered to exhibit rare karst phenomena and to occur on a smaller scale. The gypsum in the Cretaceous Guankou Formation (K2g) is a typical example of lacustrine sedimentary gypsum. Its dissolution has caused abnormal settlement of buildings constructed on it. However, most research on the dissolution of the gypsum has been limited to laboratory experiments, such as the static water and flowing water dissolution tests, which do not fully capture the actual dissolution conditions of gypsum. Some in-situ dissolution tests on gypsum have been carried out in Russia, Ukraine, Spain and Italy with the use of tablet or the MEM method in boreholes and cavities. But no in-situ dissolution research on gypsum has been conducted in China. In this study, three groups of standard gypsum (K2g) tablet specimen, packed in nylon bags, were suspended at depth of 15 m, 25 m and 35 m within the borehole. Steel pipe were used to prevent the upper sandy and gravel aquifer from directly entering to the borehole. This setup simulated the dynamic dissolution process of gypsum under the in-situ hydrodynamic, hydrochemical and temperature conditions representative of the real stratigraphy environment. After 3 days, 7 days, 15 days and 30 days, the specimens were retrieved from the borehole for measurement and observation, and the chemistry of groundwater was analyzed simultaneously. The tests showed that the specimens at the depths of 15 m and 25 m exhibited intense dissolution phenomena, while the specimens at the depth of 35 m showed only slight dissolution. After 30 days of in-site testing, the average mass loss rates of the specimens at 15 m and 25 m reached 73.9% and 73.7% respectively, with average dissolution rates of 1.16×10−2 g·cm−2·d−1 and 1.13×10−2 g·cm−2·d−1, respectively. The recession rates at 15 m and 25 m were 0.246 mm·d−1 and 0.245 mm·d−1, respectively. The average mass loss rate of the specimens at 35 m was only 18.4%, and the average dissolution rate was 0.39×10−2 g·cm−2·d−1, which was only a quarter of that of the specimens at 15 m and 25 m, and was comparable to the dissolution rate of the gypsum reported by other researchers in static water environments. This dissolution rate was much greater than that of gypsum specimens in boreholes measured by Calligaris C. The dissolution rates of the specimens at 15 m and 25 m initially increased and then decreased over time,while the dissolution rate of the specimens at 35m did not change significantly with time. Groundwater samples were collected from the borehole for chemical analysis both before and during the test. It was observed that the concentrations of SO$_4^{2-}$ and Ca2+ in the groundwater at 15 m and 35 m were lower than those at 25 m. Nevertheless, neither the concentration of SO$_4^{2-}$ nor that of Ca2+ reached saturation at any point during the experiment. The dissolution rate constants (K) at the site were approximately 0.030×10−5 to 0.114×10−5 m·s−1. Influenced by hydrogeological conditions at various depths, the rapid dissolution rate of gypsum was primarily controlled by mass transport driven by the hydraulic gradient, with surface reactions driven by ion concentration playing a secondary role. The dissolution processes of the specimens at 15 m and 25 m were affected by both mechanical erosion and chemical dissolution, progressing through four stages: adsorption and saturation, shallow pit formation, lateral pit merging, and deep disintegration. In contrast, the dissolution of the specimens at 35 m was mainly controlled by chemical dissolution and only underwent the first two stages: adsorption and saturation, and shallow pit formation. This in-situ dissolution experiment and evaluation method for gypsum supplements the existing dissolution data of gypsum in China, providing an important reference for karst risk assessment in gypsum-rich areas. -
图 1 场地区域地质图
1-第四系 2-白垩系灌口组 3-白垩系 4-侏罗系 5-断层 6-隐伏断层 7-背斜 8-向斜 9-研究场地 褶皱：①龙泉山背斜 ②苏码头背斜 ③籍田铺—龙泉驿隐伏向斜 断层：（1）新津—双流—德阳隐伏断层 （2）龙泉驿断层 （3）苏码头背斜西翼断层 （4）府河隐伏断层 （5）白沙隐伏断层 （6）双桥子—包江桥隐伏断层 （7）毗河隐伏断层 （8）新都—磨盘山隐伏断层 （9）簇桥隐伏断层 （10）双流冒火山断层
Figure 1. Geological map of the study area
图 2 场地典型岩芯照片
A - 层状石膏 B - 团斑、层状石膏 C - 团斑状石膏 D-脉状石膏
Figure 2. Photos of typical cores of the study area
图 8 石膏岩溶蚀平均质量变化曲线
Figure 8. Curve of average quality variation in dissolution of gypsum
图 9 试验前不同深度地下水的化学特征
Figure 9. Chemical characteristics of groundwater at different depths before the test
图 10 15 m、25 m处石膏岩溶蚀机理概念模型
Figure 10. Schematic diagram of the dissolution mechanism of gypsum specimens at 15 m and 25 m
图 11 35 m处石膏岩溶蚀机理概念模型
Figure 11. Schematic diagram of the dissolution mechanism of gypsum specimens at 35 m
表 1 试件溶蚀平均质量变化统计表
Table 1. Statistics of average quality variation in specimen dissolution
试验
时间 t/d质量损失率 Mc/% 溶蚀速率Vd / ×10−2 g cm2·d−1 15 m 25 m 35 m 15 m 25 m 35 m 0 0.0 0.0 0.0 − − − 3 9.3 9.3 2.2 1.78 1.74 0.44 7 19.7 19.5 3.4 1.49 1.43 0.18 15 44.9 43.8 8.5 1.76 1.70 0.40 30 73.9 73.7 18.4 1.16 1.13 0.45 
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表 2 地下水化学特征统计表
Table 2. Statistics of groundwater chemistry
取样
深度/mpH ${\rm{SO}}_4^{2-}$ /mg·L−1 Ca2+/mg·L−1 总矿化度/mg·L−1 ${\rm{SO}}_4^{2-}$/Ca2+ 0 d 7 d 0 d 7 d 0 d 7 d 0 d 7 d 10 7.48 296 73 693 4.1 15 7.63 412 532 129 280 879 1098 3.2 1.9 20 8.06 655 189 1152 3.5 25 7.95 790 858 304 476 1450 1590 2.6 1.8 30 7.81 536 196 1030 2.7 35 7.32 374 428 112 306 809 1054 3.4 1.4
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表 3 石膏岩溶蚀速率常数与径流速度统计表
Table 3. Statistics of dissolution rate constants and water velocity of gypsum
试件
环境dM×10−3
/kgdt×105
/sCs-C
/kg·m−3K×10−5/m·s−1 15 m 6.22 6.048 1.67 0.114 25 m 5.99 6.048 1.21 0.075 35 m 1.02 6.048 1.81 0.030
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