Tlakove stabilizovane pisky v Ceske republice a jejich zvlastni geomechanicke vlastnosti

Locked sand is a material with unusual geomechanical properties (Dusseault - Morgenstern 1979): relatively high unconfined compressive strength, high internal friction angle but very low tensile strength. These properties arise from the lack of permanent cement and interlocked structure of quartz grains. In the Czech Republic, the locked sands were not studied with the exception of Střeleč Locked Sand (Bruthans et al. 2014). The purpose of this paper is to introduce several locked sand localities in the Czech Republic and describe their properties, namely long-term stability of underground passages, and at the same time rapid weathering and erosion of the same material on the ground surface.
The presence of locked sand was tested at the Nevřeň and Hosín localities on kaolin sandstone, where extensive mine passages were driven, which did not collapse when flooded (Figs 1, 2), and at the Střeleč, Rudice and Běleč localities in quartzose sandstone (Table 1). Buckland/Reigate site in England was used for comparison. Locked sands were sampled by low speed dry drilling with diamond core bit 80 mm in diameter. Material characteristics was tested by flooding the cores by three different ways (Fig. 3): A) flooding the core with free sides;B) flooding the core with cylindrical side wrapped in foil (˜10 kPa compressive stress); C) flooding the core in foil in concrete sleeve (not penetrating the sample). Sandstone should be stable in all three cases. On the contrary, common sand should decay into angle of repose under all three cases. Only locked sand should decay in the first case, but it should be stable under case B and C as outer pressure avoids disintegration of interlocked structure by slaking (Fig. 3; Bruthans et al. 2014).
Based on this procedure the Střeleč, Nevřeň, Hosín and Bucklad are locked sands, Běleč is a common sand and Rudice is generally common sand with some interlocked parts (Table 2). Tensile strength was measured in the field by aluminum T profiles glued to material by epoxy, and by applying the tensile force by tensiometer (Bruthans et al. 2012) to characterize the degree of material cementation. Kaolinite sands show higher tensile strength than the other locked sands but at the same time exhibit extremely low resistance to frost weathering (Table. 2). Gravity-induced stress around mine passages was determined using geotechnical software PLAXIS 2D (Fig. 4). It shows that Hosín passages sides are parallel with principal stress component, which increases its stability. Properties of locked sands determined in the lab and tensile strength measurements are in good agreement with field observations and historical data. While loaded material in underground is able to resist flooding and forms stable passages for more than 150 years, unloaded material on ground surface is rapidly weathered and eroded by flowing water and frost weathering.