<p>Internal stability describes the capacity of the coarse fraction of a soil to retain its fine fraction during seepage flow, in order to maintain its hydraulic and mechanical properties. This article examines whether a till is internally unstable when placed against a filter sand and describes in detail the physics at the till-sand interface. The migration of fine solids is quantified with the Modal Decomposition Method (MDM), which decomposes precisely a grain size distribution (GSD) into its modes or subpopulations. Previously, the MDM has shown that the common assumption that a soil contains a coarse fraction, and a fine fraction is inaccurate. It was shown a soil can have three subpopulations of solids, or modes, the finer one being able to migrate through the void spaces of the medium and large modes. This article addresses a focused and significant challenge: accurately quantifying subtle changes in grain size distributions at the till-sand interface to achieve a precise solid mass balance. After achieving saturation in a rigid-wall permeameter (RWP), a constant-head permeability test was carried out under a constant mean gradient. The first step at a first mean gradient of 0.1 lasted two hours. It yielded variable readings in lateral piezometers, before reaching stable readings. The piezometric changes indicated an internal stability problem. Then, the test was stopped and the RWP was dismantled. Five distinct sublayers were analyzed for their GSDs: two in the till, two in the filter sand, and a 1-cm thick sublayer at the interface. The MDM quantified the migration of fine particles and found the percentages of initial materials in each sublayer while providing an accurate mass balance for all solids. The new results have quantified a slight but measurable transfer of fine solid particles from the till into the sand layer.</p>