A Landslide Warning System for a Soil Slope on Oah

A landslide warning system for a 40-foot-high cut soil slope on Kalaniana'ole Highway was developed by performing a soil investigation, laboratory tests, installing monitoring instruments, developing and calibrating a transient hydrological flow model, using the calibrated model to predict moisture/suction response during a design storm, inputting these moisture/suction predictions into a slope stability analysis and establishing threshold values for the monitoring instruments.;The geotechnical investigation and index testing showed that the slope consists of a uniform reddish-brown, stiff to very-stiff, highly overconsolidated elastic silt (MH). Saturated consolidation tests were performed to obtain the saturated compression curve and the saturated hydraulic conductivity, which was determined to be 10-6 cm/s. A saturated multi-stage consolidated-drained triaxial shear test was performed to obtain the Hvorslev true friction angle and true cohesion, which was 27° and 18 kPa, respectively. Pressure plate and vapor sorption analyser tests were performed to obtain the unconfined soil water characteristic curve (SWCC) and hydraulic conductivity function (HCF). Three constant water content isotropic compression (CWIC) tests were performed to obtain the Basic Barcelona Model (BBM) stiffness parameters and the porosity-dependent van Genuchten parameters. Constant water content triaxial shear tests (CWTS) were performed to obtain the BBM shear strength parameters, and the critical state friction angle which was estimated to be 37°. The BBM parameters were calibrated using the modified state-surface approach (Zhang and Lytton, 2009).;A 2D finite-element transient hydrological flow model was calibrated using the field measured suctions and water contents during a two-and-a-half-year period. A good fit between measured and predicted values for all sensors was obtained by modeling hysteresis and porosity-dependency in the SWCC and HCF. The calibrated model was then used to predict the slope response due to a 500-year design storm, and to establish landslide warning thresholds for the monitoring instruments. The predicted suctions and water contents during the design storm, and the porosity at each node estimated using the calibrated BBM stiffness parameters, were inserted into a slope stability analysis. The slope was deemed stable (FSmin~1.2) during a 500-year storm.;Amber and red thresholds were developed using Nicholson et al's (1999) traffic light approach for two tensiometers and two water content sensors based on the predicted suction and water content response during a 500-year storm and the maximum water contents and minimum suctions ever measured in the field. Proposed plans of action include: 1) cover the slope with a tarp or geomembrane to prevent further infiltration if the amber threshold is triggered, and 2) close the town-bound lane of Kalaniana'ole Highway if the red threshold is triggered.;Key findings include: (1) it is important for a transient hydrological flow model to account for SWCC and HCF hysteresis and porosity-dependency in order to predict field behaviour reliably; (2) Porosity-dependent SWCCs can be derived from constant water content isotropic compression tests along with the use of the Basic Barcelona Model stiffness parameters and a modified form of the van Genuchten (1980) equation; (3) Based on 2D slope stability analyses, the slope will survive a 500-year storm with a factor of safety ≈ 1.2.