ASCE Seattle Section & Geotechnical Group/G-I Chapter
February Joint Meeting
Wednesday, February 15, 2023 – Online
Topic: A Revisit to the 1980 Mt St. Helens Eruption from a Civil/Geotechnical Engineer’s Experience: The Repair of the Yakima Tieton Irrigation District Feeder Canal
This talk is about one of Mr. Pita’s early projects that happened in June of 1980, when a portion of a irrigation feeder canal was washed off a steep canyon hillside caused by the St. Helens ash that blanketed the area. The runoff from a typical afternoon thunderstorm when mixed with the new 4 to 8 inches of SM type ash resulted in a slurry that carried stumps, trees & rocks into the canal and not through the culverts beneath, which were plugged by the ash. This material ‘dammed’ the gravity flowing canal & backed up the water so it went over the edge in one critical place & eroded / undermined the natural hillside soils resulting in a huge landslide of the supported material that resulted in that portion of the canal dropping nearly 100 feet. The canal water then poured out, eroding nearly all the soil down the bedrock, until shutdown at the headworks.
Frank became involved 2 days after the failure of the canal and was part of the team that got the water running again in a total of 19 days. If not for the success of this team of engineers, contractors, and a helpful County government, approximately 26,000 acres of fruit trees would have been damaged or ruined. This talk will set the stage of what was happening in 1980 related to the eruption and explain the engineering design and approach developed to speed up the solution.
Speaker Bio: Frank W Pita
Mr. Pita’s educational background in both geology and civil/geotechnical engineering, started him on a career to contribute to or solve geo-oriented problems, some emergency situations, with buildable designs. He was also allowed to implement the construction of these designs. Over the course of his 50+ year career he managed / designed / implemented the solutions to 9 emergency repair projects that required design knowledge, understanding contractors capabilities, and managing the team to come together quickly. The repairs ranged from landslide repairs and bearing capacity failures, to tunnel collapses that blocked highways & railroads and required rapid re-openings.