UNSATURATED HYDRAULIC PROPERTIES OF HILLSLOPE COLLUVIUM DERIVED FROM GLACIAL SEDIMENTS IN THE SEATTLE, WASHINGTON, AREA DETERMINED FROM OPEN-TUBE CAPILLARY RISE TESTS

Shallow landslides in colluvium on steep hillslopes near the contact between a Pleistocene age silty clay (Lawton Clay – Qlc) and an overlying Pleistocene age sand (Vashon Advance Outwash – Qva) often occur during periods of heavy rainfall in the Seattle, Washington, area. As part of a U.S. Geological Survey project to assess the hazard posed by shallow landsliding, open-tube capillary rise tests were used to determine the unsaturated hydraulic properties of hillslope colluvium derived from these glacial sediments. The effect of particle-size distribution on the soil water characteristic curve (SWCC) was examined in a controlled manner by mixing two soils in the laboratory. The Qlc silty clay was added to the Qva sand in increasing amounts, yielding soils that contained 0, 2.5, 5, and 10 percent Qlc by weight. The soils were packed in Lucite columns, 150 cm in length with an inner diameter of 7.5 cm, and were stood upright in a reservoir with a constant water table height. Water content and soil matric suction were monitored throughout the experiments at several heights in each column using Time Domain Reflectometry (TDR) and tensiometers, respectively. When the wetting front ceased to rise appreciably over a period of several days, we inferred that an equilibrium condition with the water table had been achieved. The columns were then cut into sections approximately 3 cm in height and the water content and the bulk density of the soil in each section was determined gravimetrically. The hydraulic conductivity function (HCF) was then estimated using a simple statistical technique that relates the pore-size distribution to the SWCC. Results show that the increasing Qlc content produces an increasing ultimate capillary rise, and that small amounts of fine-grained material increase moisture retention at matric suctions greater than about 30 cm of water. Saturated hydraulic conductivity was reduced from 1.3x10^-3 cm/s for the 100 percent Qva to 1.91x10^-4 cm/s with the addition of 10 percent Qlc. The data collected are being used in models to simulate the pore-pressure response to rainfall in hillslope soils. With the goal of improving landslide hazard predictions, these model results will help to better define the critical rainfall conditions that lead to shallow landslide failure in the Seattle, Washington, area.