BENTONITIC CLAYSTONE - GEOLOGIC HAZARDS, ENGINEERING PROPERTIES, AND LAND-USE ISSUES

Bentonitic claystones (i.e., claystones that contain discrete bentonite beds, typically within a mixed-layer, illite-smectite clay matrix) and their derived soils are widespread across the USA, Canada, and many other parts of the world. These rocks are most common in Cretaceous and Tertiary formations of the western USA.

The engineering properties of individual bentonite beds may vary considerably under different moisture and loading regimes. Under dry conditions, they are relatively strong materials. Upon wetting, however, they may lose their strength and cohesion and become plastic. Such claystones may expand volumetrically as well.

Expansive soil and bedrock and landslides are major hazards associated with bentonitic claystones. These hazards cause substantial damage to homes, public buildings, utility infrastructure, and roads and other transportation corridors. The hazard is increased in cases where there are significant differences between the engineering properties of bentonite beds and the surrounding bedrock. The bentonite becomes a focus for stresses that result in differential movements.

Expansive, steeply dipping bedrock is a special hazard that results from the exposure of previously uplifted and tilted strata. Found along the flanks of mountain uplifts, these bentonitic claystones have a geometry that is conducive to exposure of numerous beds in zones along the ground surface. Each zone has different engineering properties and, if initially dry, has a different swell potential when the moisture regime is changed following development. In Colorado, efforts to mitigate the "dipping bedrock problem" have recently focused on the deep (3-4 m) overexcavation and replacement of potentially troublesome bedrock. This approach is meeting with good success, provided that the replaced fill is properly engineered and quality control measures are followed.