Determining the kinematics of slope movements using low-cost monitoring and cross-section balancing

For the monitoring of incremental slope displacements, electronic instruments are very sensitive to movements, but are expensive and can be disabled because of vandalism, catastrophic events, or power failures. Repeated surveys are time-consuming and detect only finite movements. Positioning of slip planes often requires boreholes or excavations. The kinematics of incremental failure can be readily and inexpensively determined using a rugged extensometer-inclinometer system consisting of ordinary building supplies. Calibrated steel cables are anchored at the tops of slopes, strung through eyebolts at the tops of deeply driven fence poles, and drawn tightly by free-hanging weights at the bases of slopes. Cable plunge angles, pole positions and heights, and pole plunge angles are recorded at regular intervals and entered into a spreadsheet that contains necessary trigonometric and sag corrections. Periodic surveys show pole position errors representing less than six percent of total displacements. Positions and plunge angles of poles are surface expressions of displacements that can be modeled at depth by construction of balanced cross-sections. Slope failure studies over a three-year period along the Lake Michigan shoreline clearly demonstrate the utility of this inexpensive monitoring system and the usefulness of numerous balanced cross-sections to produce a "motion picture" of slope displacements. Cross-section details are verified by boring, by projecting accurately the locations where slip planes eventually appeared on the surface and poles rotated above curved slip surfaces, and by reproduction of the slip surfaces using limit equilibrium digital models.