FLYASH GROUT TESTING IN A SIMULATED WET MINE ENVIRONMENT¹

Presented by

Steve Wald and Bruce Beechie²

The Abandoned Mine Lands (AML) Division of the North Dakota Public Service Commission (PSC) has been conducting underground mine filling projects since 1983. The intent of these projects is to protect roadways and residential areas from subsidence produced by collapsing underground lignite mines.

North Dakota’s AML Division contracted a material testing engineering firm to research the contaminating potential, physical and geochemical properties of flyash-based grouts injected into a simulated wet mine environment. The project was initiated to find a suitable replacement for Portland cement in grout mixtures due to the high cost of cement. The laboratory testing equipment was designed to simulate a worst-case scenario wet underground mine. Design of the pumped grout mixture is considered by AML staff as one of the most important aspects of subsurface reclamation projects. A grout mixture should be designed to maximize pumpability, flowability and strength. Coal combustion residual ash (flyash) has been used extensively as a cement replacement in grout being injected into dry mines in North Dakota. Until now, North Dakota’s State Health Department has not allowed the use of flyash-based grout in wet mine environments.

From 23 separate grout mixtures tested in mine emulation units, one particular mix design was chosen superior in terms of pumpability, strength, least degradation to ground water, and cost. The grout testing project was considered a success and the use of two types of flyash for injection into wet underground mines was approved by the State Health Department. The grout testing project was completed in four months at a cost of $60,000. The project generated a 150 page report delineating leach test analyses of the set-up grout cylinders, chemical analyses of water samples taken at various intervals during the project, grout strengths achieved at various intervals, conclusions and recommendations.

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¹ Presented at the 18th Annual Conference of the Association of Abandoned Mine Land Programs, Kalispell, Montana, September 15-18, 1996.

² Steve Wald is Professional Engineer, Material Testing Services, Inc., Minot, North Dakota, and Bruce Beechie is AML Project Manager, North Dakota Public Service Commission, Bismarck, North Dakota.

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