WP1 - Mixed fractions of black shale and rhomb porphyry

Abstract

When taking out potentially acid-producing black shale in construction projects, rock masses are often unintentionally mixed with masses of other rocks. The acid-producing potential of such mixed masses is uncertain, and as a result, large amounts of potentially clean rocks often end up in disposal sites. In WP1 of the Under Oslo project, the leaching from mixed masses with different fractions of rhomb porphyry with black shale has been investigated. Both dilution and additional buffering effects of the rhomb porphyry are assumed to contribute to the desired effects. To investigate this hypothesis, both small scale column experiments, as well as larger scale outdoor container experiments have been conducted over the course of two years. Different amounts of black shale have been mixed with rhomb porphyry in both experiments. Additionally, for the column experiments, the effect of grain size has been investigated. While the outdoor container experiments have been exposed to ambient conditions, with varying precipitation, humidity and temperature, the column experiments are executed under more ideal laboratory conditions, with regular watering and more stable atmospheric conditions. The columns and most containers are free drainage systems, where the system is unsaturated with water and with dry periods. Some containers are closed and water-saturated; however, these are not covered in this report. This document describes the results two years into the experiments. Additionally, one specific case related to construction of is described, where mixed fractions of syenite and black shale were analysed to evaluate an appropriate threshold for when rock masses should be deposited in appropriate landfills.