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

For the monitoring of incremental slope displacements, electronic instrumen ts are very sensitive to movements, but are expensive and can be disabled b ecause of vandalism, catastrophic events, or power failures. Repeated surve ys are time-consuming and detect only finite movements. Positioning of slip planes often requires boreholes or excavations. The kinematics of incremen tal failure can be readily and inexpensively determined using a rugged exte nsometer-inclinometer system consisting of ordinary building supplies. Cali brated steel cables are anchored at the tops of slopes, strung through eyeb olts at the tops of deeply driven fence poles, and drawn tightly by free-ha nging weights at the bases of slopes. Cable plunge angles, pole positions a nd heights, and pole plunge angles are recorded at regular intervals and en tered into a spreadsheet that contains necessary trigonometric and sag corr ections. Periodic surveys show pole position errors representing less than six percent of total displacements. Positions and plunge angles of poles ar e surface expressions of displacements that can be modeled at depth by cons truction of balanced cross-sections. Slope failure studies over a three-yea r period along the Lake Michigan shoreline clearly demonstrate the utility of this inexpensive monitoring system and the usefulness of numerous balanc ed cross-sections to produce a "motion picture" of slope displacements. Cro ss-section details are verified by boring, by projecting accurately the loc ations where slip planes eventually appeared on the surface and poles rotat ed above curved slip surfaces, and by reproduction of the slip surfaces usi ng limit equilibrium digital models.