Experimental study on dissolution characteristics of gypsum intercalations in red layers under still and flowing water conditions

ZHONG Zhibin; FENG Jie; LYU Lei; ZHOU Qijian; LI Sijia; XUE Changrui

doi:10.11932/karst2024y046

Abstract：Gypsum intercalations with uneven thicknesses are widely distributed within sand and mudstone in red layers of the Sichuan Basin.In the process of groundwater infiltration or erosion,these gypsum intercalations inevitably undergo dissolution reactions,resulting in the formation of numerous pores.Additionally,the unique physical and mechanical properties of red layers can significantly reduce the bearing capacity of foundations,leading to uneven settlement deformation and even damage to engineering structures.Therefore,it is necessary to carry out more in-depth research on the dissolution characteristics of gypsum intercalations in red layers.Based on an avionics hub project in Chongqing,this study took the original rock containing gypsum intercalations in the dam foundation as a research object.A self-designed apparatus for flowing water dissolution was used to perform the dissolution test.By regularly monitoring the changes in ion concentrations of Ca2+and SO42-ions within the solution,the dissolution process of gypsum in the test can be characterized,thereby allowing for an investigation into the erosion characteristics of gypsum intercalations in red layers under different flow velocities. At the end of the test,the gypsum intercalations in different flow velocities exhibited erosion grooves to varying degrees.Notably,the depth of surface grooves in the sample exposed to dissolution under still water conditions was the smallest,measuring only 2-3 mm,while the deepest groove formed by dissolution under flowing water conditions reached 12 mm.This observation indicates that the dissolution rate of gypsum significantly increased under flowing water conditions.Following the test,the concentrations of Ca2+and SO42-in the solution were measured,revealing that the contents of both ions showed an increasing trend over time.Comparing the dissolution test under flowing water conditions to that under still water conditions,the average chemical dissolution rate of the samples in still water was lower,both in terms of numerical values and the magnitude of changes.Furthermore,the slope of Ca2+content change curve for the experimental groups increased gradually with the rise in flow velocity,indicating that higher flow velocity has a promoting effect on the chemical dissolution of gypsum.Through the processing and conversion of the test data,it can be found that the average dissolution rate under different flow velocity conditions exhibited a rapid decline in a short time,followed by a gradual stabilization.At the initial time of the dissolution test,the average chemical dissolution rate of the samples indicated that the higher flow velocity corresponded to increased dissolution rates.With the experiment,the dissolution rate decreased rapidly and the decreasing trend under different flow velocity conditions was different to some extent.The greater the flow velocity was,the greater the dissolution rate became.The trend of curve for instantaneous chemical dissolution rate of each dissolution test group under flowing water conditions was generally consistent with the trend of curve for the average chemical dissolution rate.This trend exhibited a pattern of decline over time,with all curves demonstrating a sudden drop on the third day of the test,followed by a gradual decrease that eventually stabilized.According to the curve depicting the dissolution amount of each sample as a function of flow velocity,it is evident that both chemical dissolution and mechanical subsurface erosion in gypsum intercalations were enhanced with increasing flow velocity.