SMALL SCALE PARALLEL FLOW CONTACT EROSION TEST BETWEEN SAND AND SILTY CLAY LAYERS

Abstract

Internal erosion is one of the major causes of the failure of hydraulic structures, road pavement,
and the naturally deposited ground as well. Except suffusion and piping, contact erosion is also a type of
internal erosion that can mostly occur at the foundation of embankment dams and dikes, in the road pavement
and the alluvial fan deposited valley. This phenomenon occurs when the seepage flow exists at the interface of
fine and coarse soil layers where fine particles are detached and transported through the voids of the coarse
soil layer. Between 2013-2017 numerous sinkholes have been observed in Armala area of Pokhara valley in
central Nepal. The locations of the sinkholes mainly observed at paddy fields in the alluvial fan deposit. The
sinkhole affected area was investigated twice in the year 2015, 2016, and 2017. Based on the subsurface ground
condition and the location of the hidden cavity, it assumed that an internal contact erosion occurred, and this
phenomenon leads to the formation of subsurface hidden cavity and collapse sinkholes in the Armala area.
Mechanics of contact erosion was studied in the laboratory by performing a series of small-scale parallel flow
contact erosion test. The physical model of the experiment was prepared with the soil having a similar grain
size and physical properties to the onsite granulometry. A cavity was observed at the interface when the fine
soil (DL clay) is overlying the coarse sand. Also, the amount of discharged soil decreases with the increase of
overburden pressure.