Publisher © Czech Geological Survey, ISSN: 2336-5757 (online), 0514-8057 (print)
Self-organized system of underground conduits originated by groundwater flow along fractures in quartz sandstone in Střeleč quarry
AbstractIn Střelec Quarry, the Bohemian Paradise, an underground conduit network > 300 m long with a volume of ~104 m3 and a catchment of 7 km2 developed over 5 years by groundwater flow in Cretaceous marine quartz sandstone. The quarry offers a unique opportunity to study conduit evolution in sandstone from millimeter to kilometer scales, from the initial stage to maturity. An increase in the hydraulic gradient from ~ 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 ~0.01, a flow velocity ~40 cm/s and sediment concentration ~ 10g/I. Flow in openings with a discharge 1 ml/s and hydraulic gradient >0.05 exceed 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. In second phase the conduit opening mainly expands vertically upward into the vadose zone by mass wasting of undercut sandstone slabs. Mass wasting is responsible for > 90% of mobilized sandstone. Sides of the conduits are protected by non-erodible fracture surfaces.