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Fast evolving conduit network in weakly cemented sandstone by groundwater flow: Characterization, evolution and significance for origin of “open fractures” in Bohemian Cretaceous Basin

Jiri Bruthans1, Jan Soukup2, Daniel Svetlik2
1 Czech Geological Survay
2 Charles University in Prague

P 9.2 in Urban hydrogeology

In the Strelec Quarry, Czech Republic, underground conduits > 300 meters long with volume up to ten thousand m3 developed within 5 years as a result of erosion by groundwater flow (piping). Piping was caused by 20 m decrease of water table in quarry. The quarry is situated in Hruba Skala quartz sandstone bonded by kaolinite matrix (Bohemian Cretaceous Basin).

Conduits are results of self-organization of porosity and groundwater flow. Originally large number of small inflows evolved in few years into several major conduits with discharge up to 70 l/s and intense transport of sand. Continuous monitoring of water table in wells in quarry catchment (15 km2) enabled to partly reconstruct the evolution of conduit network in space and time. A new technique was used to distinguish the erodible portions of the sandstone.

Three erosion processes were observed in the quarry: piping, sandstone fluidization, and mass wasting. Piping, caused by groundwater flow 40 cm/s under a channel gradient <1%, occurred in major conduits transporting sediments. Concentrated flow exceeding 1-4 ml/s with velocities of 10-20 cm/s and channel gradients 5-6% are capable of eroding the erodible portions of sandstone. Conduits typically form by piping of erodible sandstone between sets of parallel fractures. Conduit formation was triggered by an increase of the groundwater gradient from ~ 0.005 to >0.02.

Morphologically identical conduits have evolved in Hruba Skala sandstone in the past by natural processes. Deformation bands at fracture surfaces limit erosion. As a result the conduits resemble the open fractures. The observed features and processes thus can be used to understand the character and evolution of highly permeable porosity in some areas of Bohemian Cretaceous Basin.

 

 

 

 

 

last modified 2014-05-13