Engineering Geologic Conditions for Trenchless Application in the Denver Metro Area

摘要

This paper provides insight into the challenges posed by the local geologic conditions as they affect trenchless projects.rnOn the surface, the geology of the Denver area appears pretty straightforward; young outwash deposits overlying weakrnTertiary rock. The crystalline rocks, so prominent to the west of town, are buried thousands of feet deep due to faulting.rnHowever, as with most things geological, once you look closely, at the individual project level, all sorts of challengingrnvariables begin to complicate things. Trenchless installations deal with small diameter borings and excavations, whichrnmeans that micro-variations in the geologic conditions can significantly affect the success and complexity of eachrninstallation. Alluvial deposits along the major drainage ways regularly contain cobbles and boulders. The frequencyrnand size of the clasts typically increases closer to the range front. We also often see a cobble layer near the bottom ofrnthe alluvial units. Additionally, due to paleo-meanders of the local drainage systems, the alluvium varies from fat clayrnto coarse sand and gravel with abrupt transitions both vertically and horizontally, which makes creation of accuraterngeologic profiles for trenchless projects challenging. Local bedrock in the area is really an over-consolidated clay andrnsand deposit. Typically, the material is quite weak with zones of "sandstone" even being totally uncemented. However,rnthe opposite also occurs in places with sandstone and limestone lenses up to 4,000 psi in localized areas.