INFLUENCE OF PLEISTOCENE GLACIAL OUTBURST DEPOSITS ON ENGINEERING IN THE PORTLAND METROPOLITAN AREA, OREGON-WASHINGTON, USA: WHY UNDERSTANDING THE CHARACTERISTICS OF GLACIAL OUTBURST DEPOSITS IS IMPORTANT

Pleistocene glacial outburst flooding on the Columbia River in the Pacific Northwest of USA is well known. Approximately forty large-scale Jokulhlaups from Lake Missoula in Montana have coursed down the Columbia River to the Pacific Ocean. Portland metropolitan area with a population of over one million located on the lower Columbia River is built on these Pleistocene flood deposits. The knowledge and understanding of the depositional processes and internal stratigraphy of these deposits greatly increases the confidence in the planning, designing, construction of significant infrastructure on these deposits. Examples of the impact on engineering are examined. Geologic mapping of the location of flood channels and areas of slack water provides the basic information for the understanding of the distribution of near surface materials. In the metropolitan area, coarse-grained facies provide aggregate material resource, ground water resource along rivers for public water supply, or potential upland infiltration areas for disposal (recharge) using clean storm water. The distribution and variation in the grain-size of these flood deposits greatly influence the type and bearing capacity of foundations for structures. Also, the understanding of the knowledge of the decrease in the maximum diameter of clasts with distance from flow constrictions provides a predictive tool useful in estimating maximum clast diameter which influence the risk of underground construction and excavation methods for tunnels and deep pipelines. Deposits downstream of flow constrictions contain large boulders some up to 9 m in diameter, which present construction difficulties. At greater distances from constrictions where maximum boulder diameters decrease below 1.3 m diameter, micro-tunneling techniques becomes less risky. Geographic differences less than one km can significantly alter engineering and construction risks and costs.