The form of iron sulfide most abundantly found in nature is pyrite. Commonly recognized by many as “fool’s gold”, pyrite has been identified in igneous, metamorphic, and sedimentary geologic formations dating from Pre-Cambrian to present-day, but is most frequently encountered in dark, carbonaceous sedimentary rocks. Where left undisturbed and unexposed, pyrite is a relatively stable mineral. However, the performance of construction activities in formations containing pyrite, or using materials excavated from such formations, can trigger reactions which may have severely adverse impacts to structures.
While sulfate attack on cementitious materials is a familiar topic in civil construction and related industries, engineers, contractors, and regulators are often less aware of the potential for foundation deflection due to oxidation of pyrite in rock or soils underlying a structure. Where natural or constructed foundation materials include a befitting combination of minerals and are expected to experience less-than-saturated moisture conditions, structures may be subjected to enough heave force to cause multiple inches of foundation heave. As foundation surfaces and resisting weight can vary across a project footprint, risk can be further increase by the introduction of differential foundation deflection. Concrete spillway integrity may be compromised by cracking which leads to displaced surfaces and concentrated seepage paths may develop along concrete monoliths joints which separate or where foundation deflection results in water stop failure.
Designers and contractors can avoid such unfavorable performance by sufficiently characterizing project sites and understanding the mechanisms which generate the subject foundation heave. Subgrades can be tailored to limit desaturation of susceptible formations and spillway structure weight can be distributed with the aim of evenly distributing heave-resisting weight. This paper describes the forms of pyrite which can be encountered in black shales, briefly explains the pyrite-related chemical processes which produce foundation heave, provides suggestions for investigating project site conditions and for accommodating foundation heave potential in design, and presents a case history where a new RCC embankment overtopping spillway design was configured to limit the potential for heave of the black organic shale on which the spillway stilling basin will be founded.
