2009-02-24_PERMIT FILE - M2008080

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West Gas in investigating the area and in designing and implementating miti- gating measures at individual critical locations. Strain gauges installed. Monitoring methods using vibrating-wire strain gauges have systematically been used along the lateral. Such monitorinp? methods are described elsewhere.' They are based on installations of strategically located vibrating-wire strain gauges in sets of three at select- ed locations where stability problems are suspected. Continuous monitoring of strains in the pipe provides reliable information on slope or landslide deformations and enables the line operator to insti- gate early mitigating measures to pre- vent breakage of the line. Because of the importance of the line, West Gas has been systematical- ly improving its safety. The improve- ments have included stabilization of some critical areas, relocation of cer- tain sections of the line, and special installations. Monitoring with vibrat- ing-wire strain gauges has enabled the line operator continuously to evaluate the safety of the line. During 1986, the installed monitoring system pro- vided West Gas with an early warning of landslide deformations that would be difficult to detect by any other method. The experience gained in this case, referred to as Landslide 7, is presented here. Landslide 7. During the geotechni- cal studies performed in 1984-85, a geologic feature was identified as a landslide along the alignment in the vicinity of historic Camp Hale, site of the 10th Mountain Division. The landslide did not indicate any deformations during the time of the geotechnical studies. The landslide is shown on Figs. 1 and 2. The landslide is approximately 600 ft long and 220 to 380 ft wide. Its toe extends into the Eagle River. As no deformations have been iden- tified on the opposite side of the river, it can be reasonably assumed that the landslide slip plane outcrops at ap- proximately the elevation of the river bottom. It is probable that river ero- sion contributed to the development of the landslide and possibly to the renewal of deformations at the present time. The course of the landslide slip plane has been interpreted based on topographic features and without any exploratory drilling. The intepretation is shown in Section A-A' on Fig. 2. Based on the assumed course of the slip plane, the total yardage of the landslide was estimated to be approxi- mately 200,000 cu yd. The pipeline was installed across the landslide in its slower third. In order to monitor strains in the line, Landslide Fig. 2 7 profile of A-A' F 9,400 0 so 100 200 m-? score. n c 9,300 C? O, W 9,20 m Q "' 9,100 9 ft 1. Landslide mass 2. Interpreted landslide slip plane 3. Probable location of original line installation 4. Line displaced by landslide CIGJ Fig. 3 Record of strain readings 600 O 200 O N Strain increases by landslide t -,200 Strain decreases o 600 - ^ by excavation - c -o = 1 N 1 o e? o 1 , d - K t Eo L (?' U c. -t,aoo -1,800 Excavation and -" pipe repla cement Strains keep increasing -Old pi pe cut and replaced S O N O J F M A M J J A S O N O J F M A 1985 1986 oc,, one set of vibrating-wire strain gauges was installed in the central portion of the landslide in 1985. This set of gauges comprises one part of a complex monitoring system where strain gauges were installed on the line at numerous other locations that are either landslides or areas with potential slope stability hazards. Three strain gauges were installed at this location. Strain Gauge 1 was installed at the to p.of the line, and Gauges 2 and 3 at 120° intervals. Fig. 3 shows strain increases from installation until the present. Actual strains in the pipe at the time of installation were not known. For this reason, the basic reading at the time of installation is assumed to rep- resent 0 in the plot of the strains. Steady increase. The strain plot be- tween October 1984 and April 1986 indicated a slow but steady increase of strains. While the upper gauge (Gauge 1) indicated the slowest in- crease, both the lower gauges (2 and 3) indicated a more significant strain increase. From April to May 1986, strains began to increase rapidly, and West Gas decided to inspect the area; the site inspection revealed numerous new tension cracks in the upper por- tion of the landslide. It was evident that old landslide deformations had been renewed and the safety of the line endangered. The longitudinal, relative strains in the line prior to mitigation are shown on Fig. 4. The line showed significant in- creases in longitudinal tensile strains at all three gauge locations. Maximum and minimum tensile strains are within 2° of a vertical plane. This indicates that a vertical component of landslide deformations has an important influence on the