Soap holes: Localized static liquefaction of clays on the Canadian prairies

Localized regions of liquified clays referred to as soap holes have been found across the Canadian prairies and foothills region, with similar features identified in China. The hypothesized mechanism for soap hole formation requires a fine-grained confining layer of low hydraulic conductivity, which overlies a preferential flow path that translates fluid pressure from an artesian aquifer to the base of the confining unit. The pressure below the confining layer increases while also progressively liquefying the sediments until the fluidized mud chamber reaches the surface.

Sites with multiple soap holes were studied at three locations in South-Central Alberta. Monitoring and instrumentation at all sites reveal artesian conditions and strong vertical hydraulic gradients ranging from 0.2 to nearly 3. Quaternary geologic maps indicate glaciolacustrine deposits throughout the sites that is supported by topographic maps showing the sites are in regional depressions, which is consistent with the grain size analysis that found most surface sediments (> 65%) are less than 1μm in diameter. These extremely fine-grained sediments create a thick confining unit that allows fluid pressure to build and liquefy the overlying sediments.

Sediments extracted within and surrounding the features at two sites were found to be highly plastic materials that plotted along the upper limit (U-Line) of the Atterberg plasticity chart. Samples exhibited liquid limits ranging from 28.5% to 150.3% and plasticity indexes from 6.7% to 128.1%. Smectite was the dominant clay material in all the samples, comprising on average 23% of the clay sized fraction, with Illite being second making up 7%. Due to the unusual physical and chemical properties of smectite minerals, an Atterberg Limit analyses was repeated using pore water obtained from the corresponding site in order to evaluate the difference in index properties. Test results indicated the natural waters found at each site reduced the liquid limit by 4-8% of the total water content at the liquid limit but had a minimal effect (~0.5%) of the plastic limit water content. Saline waters and high fluid pressures below the smectite clays are believed to be initiating a thixotropic response resulting in reduced viscosity and localized liquefaction of clay sediments that result in soap hole formation.