AN EXPERIMENTAL APPARATUS FOR TESTING THE PROPENSITY OF SOIL TO FLOW

Debris flows can travel great distances from their source areas and affect communities downstream. Identification of soils susceptible to liquefying during slope failure could aid in locating potential debris-flow sources. We used a modified version of the slump test, typically used to measure the consistency of unhardened concrete, to test the proclivity of soil to flow. The method can be used to identify a threshold porosity above which the soil will flow under saturated conditions. Saturated hydraulic conductivity, initial and final bulk density, porosity, travel distance, velocity, stage, and basal pore pressure were measured for each test specimen.

The test involves placing soil loosely or packing soil using a proctor hammer into a cylindrical test mold (29.6 cm high, 29.6 cm diameter). The sample is then placed on a table and slowly saturated from below with a measured volume of water. Hydraulic conductivity is measured using the constant head method. The test is initiated by releasing a counterweight on a pulley system that rapidly lifts the mold vertically. The unconfined sample is then free to fail and spread laterally due to self-weight. Stage and basal pore pressure are monitored at 100 Hz. Video of the test is analyzed to measure velocity of the flow front. The deposit thickness is calculated by making between 50 and 150 measurements (depending on travel distance) using a laser range finder. Thickness is integrated across the entire deposit surface to determine the final deposit volume. Comparisons of initial to final volumes reveal contractive or dilative behavior of the sample during failure. A test is classified either as a slide if failure occurs along discrete failure planes or as a flow if fluid-like behavior is observed.

Soil samples collected from ten drainage basins in the Oregon Coast Range and one near Durango, CO were tested with the slump table. The samples consist of sandy and gravely soils with between 8 and 16% silt and clay. Preliminary results confirm that loose soils contract and flow upon failure while dense soils will dilate and slide. The failure mode is related to both the porosity and fine-grained fraction of the soil. These results are consistent with field observations of failure mode at the test sites. In the future, this testing apparatus could be used to identify soils susceptible to debris flows.