Fast evolving conduits in clay-bonded sandstone: Characterization, erosion processes and significance for the origin of sandstone landforms
Abstrakt
In Strelec Quarry, the Czech Republic, an underground conduit network >300 m long with a volume of similar to 10(4) m(3) and a catchment of 7 km(2) developed over 5 years by groundwater flow in Cretaceous marine quartz sandstone. Similar landforms at natural exposures (conduits, slot canyons, undercuts) are stabilized by case hardening and have stopped evolving.
The quarry offers a unique opportunity to study conduit evolution in sandstone at local to regional scales, from the initial stage to maturity, and to characterize the erosion processes which may form natural landforms prior to stabilization. A new technique was developed to distinguish erodible and non-erodible sandstone surfaces.
Based on measurements of relative erodibility, drilling resistance, ambient and water-saturated tensile strength (TS) at natural and quarry exposures three distinct kinds of surfaces were found. 1) Erodible sandstone exposed at similar to 60% of surfaces in quarry. This sandstone loses as much as 99% of TS when saturated. 2) Sub-vertical fracture surfaces that are non-erodible already prior to exposure at ground surface and which keep considerable TS if saturated. 3) Case hardened surfaces that start to form after exposure.
In favorable conditions they became non-erodible and reach the full TS in just 6 years. An increase in the hydraulic gradient from similar to 0.005 to >0.02 triggered conduit evolution, based on long-term monitoring of water table in 18 wells and inflows to the quarry.
Rapidly evolving major conduits are characterized by a channel gradient of similar to 0.01, a flow velocity similar to 40 cm/s and sediment concentration similar to 10 g/l.Flow in openings with a discharge 1 ml/s and hydraulic gradient >0.05 exceeds the erosion threshold and initiates piping.In the first phase of conduit evolution, fast concentrated flow mobilizes erodible sandstone between sets of parallel fractures in the shallow phreatic zone.