In-stream mining: supporting and undermining critical infrastructure =

Abstract

Sand and gravel mining are crucial to support massive construction and thus, countries development and economies. Best quality sand and gravel for construction are obtained from the riverbeds through in-stream mining. However, important geomorphological, hydrological, and ecological impacts of in-stream mining have been recognized. Importantly, river incision caused by sand and gravel extraction overlaps with local scour at bridge piers increasing bridges’ failure risk. Are all in-stream mining activities dangerous for bridges? Is there a threshold mining volume that causes damage to bridges? How important to scour are the number of extractions around a bridge and what is the effect of the distance between a bridge and the mining pits? Are there management options to reduce bridge failures due to in-stream mining? In this thesis, we answer these questions based on results from a statistical analysis of the relationships between 379 in-stream mining activities and 30 bridges located over 10 major Chilean rivers, 21 bridges along 950 km of the Panamerican route between the cities of Santiago and Osorno. Obtained results show that: (1) scour at bridges is explained (>80%) by the extraction volume of the largest mining activity, (2) a large (>300.000 m3 /year) and single extraction is more damaging than several smaller extractions with the same total extraction volume, and (3) the distance between extraction and bridge affects bridge scour only in cases of large extraction volumes. Policies need to be urgently improved to allow river morphology to recover from in-stream mining and to avoid bridge damage. Regulation of the mining volume and rate, minimum distance between a bridge and a mining pit, and promotion of smaller activities distributed along the river, instead of big, localized mining is recommended.