Hatch Mott MacDonald (HMM) was retained by a private coal client to assist in the mitigation and remediation of over 80,000 linear ft of streams that had been affected by longwall mining. The main objective was to restore the subject stream to its original hydrologic condition, thereby revitalizing the biotic habitat.
Work included surface and groundwater monitoring, hydrologic modeling, and subsurface investigations to determine minimum base flows for development of augmentation plans. Geologic and hydrologic conditions were then used to prepare a mitigation plan and develop a grout injection design for remediating the flow loss segments of the stream affected by mining. This grout injection design consisted of a shallow, low-pressure injection of Portland cement and bentonite mix to seal fractures and reduce water loss. Stream surveying of the remediation sites and reference streams was used to establish a stable stream geometry that would effectively carry bankfull flows. Trimble GPS units and GIS software were used to develop maps showing the flow advancement downstream as construction progressed, in addition to material utilization.
HMM also performed the construction inspection and construction management of the project. This work included full-time inspection, technical consulting, evaluation of contractor performance and work product, and approval of contractor quantities.