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<br />001038 <br /> <br />.. <br /> <br />!;t <br /> <br />3. Electronic distance measurement (EDM) techniques were decided upon <br />to provide quick and relatively inexpensive data on the rates of <br />downslope movement on the surface of slide areas. This system <br />consists of fixed reflector target posts at key locations on the <br />slide. These are read for distance periodically from strategically <br />located instrument stations on stable ground. If succeeding <br />distance measurements decrease, downslope movement is indicated. <br />Stable or constant dfstance readings indicate no significant <br />movement at a particular monitoring station. A total of 27 EDM <br />monitoring stations were established during the field work and <br />observations were begun on each soon after installation. The <br />attached table prepared by CDOH shows the monitoring lIistory of each <br />station during 1985 as computed by the Colorado Department of <br />Highways, Grand Junction. CGS plotted the weekly readings on <br />histograms showing change since previous reading. Two of the EDM <br />charts are attached, Fig. 1 shows a history of movement, Fig. 2 at <br />an apparently stable station, shows small variations (plus and <br />minus) representing normal instrument and reading variations of the <br />system. <br /> <br />The advantages of this type of monitoring include: a) it is easy and <br />fast to install and has relatively little environmental or aesthetic <br />impact, b) if carefully selected many sites can be read from a <br />single remote stable instrument station, c) cost of installation and <br />monitoring is relatively low. and d) data can be analyzed and <br />preliminarily interpreted immediately in the field when necessary. <br />Limitations of the method include a) information on downslope <br />movement does not give any indication of the depth of instability or <br />movement (this is critical because greater depth implies larger <br />volumes of material and increased potential for damage or disaster), <br />b) line of sight is necessary between the instrument station and the <br />monitored site, this can be a severe limitation in heavily wooded <br />areas or where microtopography is extreme, and c) it yields no <br />information on the composition and properties of the slide material <br />or hydrologic conditions within the mass. <br /> <br />4. Inclinometer installations were selected to provide information on <br />the location and geometry of sliding at greater depth. For tllis <br />type of instrumental monitoring a drill hole is bored at a location <br />of interest and a special casing is installed to a depth believed to <br />be somewhat deeper than the slide material. Actual measurements are <br />made by lowering a special instrument that is guided by grooves in <br />tile casing. The instrument is lowered twice - first in one set of <br />grooves then along grooves at rigllt angles to the first. From the <br />two instrument readings any deflections (bending) can be located and <br />measured and a vector (direction) of movement determined. With this <br />data and an EDM reading at the site any movement can be described in <br />three dimensions. This is very advantageous in understanding slide <br />movement, designing engineering mitigation or anticipating the <br />seriousness and consequences of new movement. <br /> <br />,..-.",", <br /> <br />i <br /> <br />'~, <br /> <br />Figure 3 is an index map showing location of the four slides and the <br />instrument arrays that were installed at Dowds Junction. <br /> <br />- 2A - <br />