Vliv biokolonizace na erozi a odolnost ruznych typu piskovcovych povrchu v lomu a na prirozenych vychozech

Over recent years, the effect of biocolonization of sandstone surfaces has been widely discussed. A large number of studies proved deteriorating effect of organisms. On the other hand, some studies demonstrated that biologically-initiated rock crust protects sandstone surfaces against various weathering processes, and therefore decreases the erosion rate of sandstones.
In previous studies, biologically-initiated rock crust, developed on weak sandstone surfaces in the Střeleč Quarry (Bohemian Paradise, Czech Republic), was observed, and two potential ways of surface protection against weathering were indicated: tectonic hardening, and biocolonization. Recently, it was proven that the biologically-initiated rock crust in the quarry protects underlying material against rain and flowing water erosion, and significantly changes the sandstone hydraulic properties. The presented study relates to the previous investigations. Biologically-initiated rock crust in the quarry was studied from mineralogical and biological points of view. Mechanical properties and resistance to erosion of biologically-initiated rock crust at tectonic surfaces were compared with the non-tectonic surfaces. Results of the studied crust tensile strength were compared to eight sandstone outcrops in the Bohemian Paradise and the Česká Lípa region outside the quarry.
Based on sequencing and microscopy of the biologically-initiated rock crust in the quarry, the organic matter is mostly formed by various fungi and green algae. The surface zone is enriched with kaolinite and clay-to-silt sized quartz particles (Table 1). Using the drilling resistance method, relative erodibility measuring (REI) and measuring of tensile strength in situ, it was determined that there is no relevant difference in the protection by biologically- initiated rock crust between tectonic and non-tectonic surfaces (Table 1, Fig. 4). Only measuring of tensile strength in laboratory showed ˜6 times higher increase in case of biocolonized tectonic surfaces. We believe this is caused by heterogeneity of sampled material or by damage of the material during its transport. Since all the other methods found no difference, biocolonized tectonic surfaces in the quarry seem to have generally the same mechanical properties as non-tectonic biocolonized surfaces.
From comparison of tensile strength of biologically-initiated rock crust in the quarry with eight crusts in natural outcrops outside the quarry, it was demonstrated that the studied surfaces in the quarry are surprisingly resistant. Their mechanical properties are comparable to natural biologically-initiated rock crusts in sandstone rock cities (Fig. 6). Since the biologically-initiated crust in the quarry developed within the last 10-50 years, it seems that organisms are capable to harden and stabilize fresh surfaces in several years or in a few decades. Such a rate is much higher than the mineral case hardening. The stabilizing effect of organisms may provide a primary protection against weathering after rock falls, at quarry faces, road cuts, and other rapidly eroded sandstone surfaces. However, protective effect of biologically-initiated rock crust may vary according to type of material, age and degree of erosion. The stabilizing effect should be quantified as a ratio of resistance (strength) of surface and its subsurface.

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