GEOLOGIC CONTROL OF SEVERE EXPANSIVE CLAY DAMAGE TO A SUBDIVISION IN THE PIERRE SHALE, SOUTHWEST DENVER METROPOLITAN-AREA, COLORADO

Shortly after construction of a subdivision in the southwest Denver me tropolitan area in 1986, a portion of the subdivision built directly o n steeply-dipping strata of the Pierre Shale began experiencing damagi ng differential movements, causing house foundations to fail and pavem ents to warp and crack. This formation is a Late Cretaceous marine cla y-shale composed predominantly of fluvial mixed-layer illite/smectite and quartz. During deposition of the shale, periodic and explosive vol canism generated thin beds of bentonite, consisting initially of volca nic ash and subsequently altered to nearly pure smectite. Some of thes e bentonite beds were exposed in a trench adjacent to the subdivision and perpendicular to the strike of the steeply-dipping strata. The thi ckest bentonite beds correlated well with linear heave features that t hese beds parallel the bedrock strike throughout the subdivision were mapped via severely deformed pavements. Mineralogical data show the be ntonite bed that correlates with the worst damage within the subdivisi on consists of about 62% smectite by weight with mixed-layer illite/sm ectite expandability of 92%. By comparison, a sample of the typical si lty claystone, which is fluvial mixed-layer illite/smectite mixed with detrital quartz from the adjacent strata, had about 23% smectite by w eight with 70% to 90% illite/smectite expandability. Geotechnical test s for swell potential show that samples of 2 bentonite beds swelled 39 % to 43% compared to 2% to 8% for samples of the typical silty claysto ne. It is proposed that differential swell resulting from stratigraphi cally-controlled differences in clay mineralogy and grain-size is the primary factor controlling extreme damage for this geologic setting.