Geologic Controls of Subdivision Damage near Denver, Colorado

This case study investigates the geologic controls on damaging ground deformations in a residential subdivision near Denver, Colo. Moderate to severe damage has occurred in certain areas where linear, parallel heave features with up to $0.3\mathrm{m}$ $\left(1\mathrm{ft}\right)$ of differential displacement have formed across roads and under houses. Other areas have small, localized depressions that have formed in the roadsides with no discernable damage to nearby houses. Still other areas show no evidence of ground movements. The bedrock beneath the subdivision consists of steeply dipping Cretaceous strata of the Benton Shale, Niobrara Formation, and Pierre Shale. Quaternary soil deposits and fill, $0–16\mathrm{m}$ $(0–53\mathrm{ft})$ thick, overlie the bedrock. The most pronounced and damaging linear-heave features are coincident with steeply dipping, silty claystone with thin layers of very highly plastic bentonite. These heave features diminish as the depth to bedrock increases, and become small to negligible where the bedrock is overlain by $3\mathrm{m}$ $\left(10\mathrm{ft}\right)$ or more of overburden soil deposits or fill. In contrast, areas having no visible damage and those having localized surface depressions are typically underlain by $1–12\mathrm{m}$ $(3–39\mathrm{ft})$ of alluvial-terrace deposits or fill. The depressions appear to have been caused by settlement over improperly filled water-and-sewer line trenches. The overall relationship between geology and ground deformations as seen in this subdivision may be useful for predicting, and thereby reducing, damage for future subdivision projects.