Ferguson Rockslide Yosemite California

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The image above highlights the importance of near-real-time monitoring.  In this instance, all site personnel had left the site when the failure occurred shortly after installing the rockfall protection screen and before the construction ramp cold be removed.  No active monitoring was in place at the time of this failure.  A new repair design required drilling to locate competent rock for anchors before the drilling contractor could begin work a monitoring program to reduce risk was required.  Using near-real-time InSAR and the completion of a TARP we began a CSM monitoring program on September 25, 2017.  Daily reports and verbal communication on slope conditions ensued for the next 4-months.   

Ferguson Radar Displacement Map

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The displacement map above exhibits displacement represented different colors associated with values on the color bar to the right.  The displacement interval is from September 25 to October 15, 2017.  Notice that most of the current displacement is occurring on the slope that is still covered with rockfall protection.   

Cumulative Displacement Data

Before and after annotation of cumulative displacement time series data for the Ferguson Rockslide.  Data in the positive direction is displacement towards the radar and data in the negative direction is displacement away from the radar. 

 

The blue curve is displacement data from the area with the remaining rockfall protection screen and the red curve is the failed slope area without the rockfall protection screen.  The sinuous pattern in the blue curve matches the noisier pattern of the red curve.  The timestamps for the peaks and valleys on the data correlate to the daily high temperatures and nightly low temperatures.  The distinct pattern on the blue curve is partly a result of greater thermal conductivity properties of the steel rockfall protection but the rock failure debris described by the red curve exhibits the same but a noisier pattern.  The noise and the slight negative trend of the red curve is most likely the post debris adjustment from the recent failure.  By November to the end of monitoring the red curve exhibited a near-horizontal trend indicating the no movement was occurring except for the thermal effects. 

Overall the InSAR time series data above illustrates the accuracy of the InSAR data. The current daily average displacement rate for the September 25 to October 20 interval is 0.012 inches/day or 0.3 millimeters/day and exhibiting a slow surface displacement rate.