Laboratory Characterization of Steeply Dipping Expansive Bedrock in the Rocky Mountain Front Range

Civil infrastructure in the foothills of the Rocky Mountains faces a unique geological hazard arising from underlying beds of steeply dipping expansive bedrock. Efforts to mitigate damaging differential movements that commonly occur among adjacent bedrock strata near the surface typically involve overexcavation and recompaction of the in situ materials as engineered fill. This paper examines the practice by quantifying the swelling-related properties of two dissimilar bedrock end members obtained from a site near Denver, Colo. where overexcavation is warranted. Atterberg limits, compaction, suction, and volume change (shrinkage) tests are conducted for each end member and several mixtures prepared to represent the wide range of subsurface conditions expected in the region. The objective is to examine the extent to which properties of the remolded fill may be predicted from the properties and relative amounts of the end member materials comprising it. Linear relationships are found between end member mass fraction and each of the properties measured [liquid limit, plasticity limit, plasticity index, wopt , ( ρd )max , Δ ψt ∕Δw , and Ch ], thus providing a quantitative basis for simplifying the design of future overexcavation programs. Optimum moisture content occurs at 90% saturation regardless of bentonite mass fraction. Suction compression index ( Ch ) for compacted specimens is found insensitive to molding water content.