2009-02-24_PERMIT FILE - M2008080

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! Ii; i I Strain increases in pipe before excavation o? - Olrectlon of landslide deformation _r 0 U? of Gauge 1 I 120° µc 120° ? uc 300 200 100 0 0 100 200 300 120" Gauge 3 Gauge 2 0 a S 8 S 'Strains shown are tensile. orv line. Following these findings, West Gas decided to implement mitigating measures immediately. Before implementation of these measures, the area of the landslide was surveyed so that the landslide extent could be accurately defined and the course of the landslide plane interpreted. Following this survey, the pipe alignment across the landslide and the surrounding area was sur- veyed with the pipe locator. This survey indicated clearly dis- tinct displacement of the line by ap- proximately 9 ft downhill in the direc- tion of landslide movement, and two distinct line flexures at both bound- aries of the landslide. It became evi- dent that considerable deformations of the landslide occurred in the spring of 1985 and very probably during earlier years, causing such significant dis- placement of the line. Emergency relief. An emergency re- lief of strains and stresses in the pipe was performed at the end of May 1985. Stress relief was accomplished by a trench excavated uphill from the line with a backhoe. To reduce the risk of line failure during the excava- tion, the pressure in the line was decreased from 420 psi to 50 psi. The exc:avatlon was performed, starting in the central part of the landslide, in approximately 2 days. An immediate effect of strain relief was evident when the line displaced verv rapidly uphill from its original location. This displacement reached a maxi- mum of nearly 3 ft. Fig. 5 shows the distinct flexure of the line at the is Shown on Fig. 3. Strain Gauge 1 showed the lowest stress relief of 1,430 µ-strains, while Gauges 2 and 3 showed considerably high strain relief of 2,110 and 1,314 µ•strains. respectively. With an assumption of an average strain relief of 1,721 µ-Strains, total stress relief can be calculated. Total reiief or the decrease of longi- tudinal stresses in the pipe by the excavation reached nearly 52,000 psi. Maximum decrease of stresses is even higher; it may have attained values higher than 60,000 psi. Longitudinal strain decreases moni- tored during the excavation provide information on stresses in the line induced by landslide deformations. It is evident that landslide deforma- tions, between the time of line instal- lation and that of excavation, induced longitudinal stresses in the line on the order of 52,000 psi. This is considera- biy more than the yield strength of the pipe (42,000 psi). Stability analyses. With regard to these findings, West Gas decided to replace the line section across the landslide. Stability analyses of the landslide area and analyses of possible remedial measures indicated that fairly difficult and costly methods of stabilization would be necessary to secure the long-term safety of the line if it remains buried. For this rea- son, West Gas only replaced the line section across the landslide and in- stalled it above- ground, on horizon- tal support mem- bers, to prevent line failure in case of re- peated, major land- slide deformations. It is expected that in a case of major landslide deforma- tions, the line will slide on the horizon- tal support mem- bers. The integrity of the line would prob- ably be endangered in a case of large, Sudden, catastroph- ic deformations of the landslide. Even if such a case cannot be ruled out, its probability is fairly low. The original set of Pipeline flexure caused by landslide de- iormauons (Fig. 51. boundary of the landslide after exca- vation of the stress relief trench. Strains in the line were continuously monitored during the excavation. The strain relief caused by the excavation