Coupling Internal Erosion Through Concrete Face Rockfill Dams With Damage In Face Slab Under Impounding

Concrete faced rockfill dams(CFRDs)are well-established dam type popularly used because of its ease in construction,low cost,and stability.However,much of the design in this sector is still based on empirical relations.While these relations may have been adequate for designing CFRDs in the past,as dams get larger and are faced with more complex situations a more physics-based method seems necessary.One such situation is internal erosion.While the International Committee on Large Dams,does prescribe criteria for gradations of material in the filter and cushion layers,the Gouhou Dam still failed under internal erosion even when complying with these criteria.This study,therefore,describes a way to realistically model the internal erosion after of damage in the concrete face of a concrete face rockfill dam(CFRD).Although multiple studies studying each aspect of the dam namely;settlement in rockfill,face slab deflection,seepage through the dam and internal erosion have been individually studied the study attempts to couple all these elements together.The study uses a 2D plane strain model of Tianshengqiao dam's geometry as an example,with and without consideration to the erosion of the underlying cushion and transition layers.In order to accomplish this,Mazars' concrete damage model,which is a continuum damage model,was coupled with permeability relationships on the face slab.The fluid flow through the face slab and in the rockfill was modeled using Darcy's law.The fluid velocity was used to model erosion using Sterpi's erosion model for the underlying cushion and transition layers.Mass conservation equations translate the mass lost in erosion to an increase in porosity,and the Kozeny Carman equation is used to calculate the change in permeability of rockfill due to change in porosity.Finally,a poroelastic model is used to couple the fluid pressure and velocity with the strains in the rockfill,which is otherwise assumed to be elastic.Two scenarios regarding the cushion layer and transition layer were studied: without considering erosion and considering erosion.It was found that the even if the particle size gradations selected satisfied the gradation requirements prescribed by ICOLD,it could still be internally unstable.Also,when the reservoir is linearly impounded,damage in the face slab is concentrated in the lower quarter of the slab.A stage of brittle damage was identified for the face slab,when damage grew very fast in a short span of time.Seepage velocity and internal erosion were found be highest near the dam toe.Von Mises stress,octahedral shear strain,pore pressure all exhibit a spike after brittle damage at the cushion layer underneath the damaged area.Comparing the two scenarios,the von Mises stress changed very little,changes in pore pressures were very small(maximum of 1.5%)while changes in specific discharge were very large(50%).These results allowed us to conclude that internal erosion can cause an increased loss in water tightness in a CFRD and that erosion and concrete face damage form a progressively deteriorating cycle that needs to be monitored.It must be noted that coupling related phenomena are essential in trying to realistically emulate real-world behavior in numerical models.This would also mean that even if codes defining structural safety change over time,design using physics of the processes involved,as will be described will stay relevant.