SELF ORGANISED CONDUIT NETWORK IN SANDSTONE QUARRY: CHARACTERIZATION AND EVOLUTION

Self-organized conduit system created by piping was studied in Strelec Quarry, Czech Republic. Conduits are developed in marine sandstone with matrix formed by kaolinite, illite and quartz silt. Inflow into the quarry driven by groundwater pumping created within 6 years several cavities, the largest being 300 m long, several meters wide and up to 17 m high, which caused several collapses in the quarry. Some features in the quarry are morphologically identical with features observed at natural outcrops in the same sandstone. They are indicating processes, which were operating in the past under natural conditions. The objective of the study is to describe and quantify the processes responsible for conduit evolution and enlargement.

Flow velocity in conduits and sandstone matrix was studied by tracer tests, direct measurements and infiltration tests. Erodability, sandstone strength and cement were studied by water jet tests (WJT), drilling resistance and microprobe. Conduits were mapped and studied including sediment transport and evolution history. Several erosion processes were modelled in the quarry under controlled conditions.

Flow velocity in conduits is mostly 0.4 m/s, while in sandstone pore space it is below 0.0003 m/s. Stream gradient in conduits wary between 1-5%, which is 2-10 more than hydraulic gradient in wider surroundings of the quarry. In direct surroundings of conduits the hydraulic gradient in pore space may exceed 20%. In evolving conduits there is 8-16 g/L of transported sand (1 wt. %).

Conduits are evolving by side erosion of fast flowing water in fracture network. Erosion causes the undercutting of sandstone blocks limited by subvertical fractures in overburden, which provoke collapses and enlargement of the spaces dominantly upward. WJT demonstrated that vertical fracture surfaces are less erodable than inner parts of sandstone blocks. Therefore the fractures are mostly forming the conduit walls and traces of erosion are scarce. WJT was found a fast tool to quantify the relative susceptibility to erosion by flowing water and to delimit the sandstone zones, which are prone to erosion from more resistant ones.

Study was supported by grant projects: GAUK380511, IAA300130806, MSM00216220855, AV0Z30460519.