Dear reader we have learnt about piping mechanism through homogeneous dam; here we will learn about same phenomenon through foundation of zone dam. We know some dams are constructed using different materials but identical materials in same zone of several zones. Usually there have an impermeable clay core embedded by relatively permeable shells. We are providing figure: that can define zone dams.
Piping can develop inside such dam when water from seepage comes out from finer materials, remain in the core, to adjacent pervious zone (shell). The core consists of finer material and conversely shells have coarser materials.
But we have no record of such migration of erosion product (soil particles) from finer soil to zone having coarser soil (may be gravel or sand); the whole phenomenon is theoretical.
But do we have records of all failure of dam? If have, such failure is also likely to happen in old zone dams. In old dams the core having fine grained soils were left to keep contact directly with zones having quarried rock of large size.
When piping is commenced the mechanism as discussed in last post is same; the concentrated seepage force makes its way through embankment or its foundation and comes out in downstream side of dam and backward erosion does rest of the destruction by enlarging the flow channel until the dam fails completely.