L2-Landslides_HazardAssessment

L2-Landslides_HazardAssessment - The U t bl E th Th...

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Unformatted text preview: The U t bl E th Th Unstable Earth: Landslides Part 2: Slope Stability & Water 1999 Rain-Triggered Landslides, Venezuela 1 of 23 Landslides: Erik Eberhardt EOSC 114 – 2009/10 Landslide Learning Goals II By the end of this lecture, you will be able to: 1) Assess the balance between the strength of the slope and the destabilizing forces acting on it (Factor of Safety). 2) Describe how groundwater affects shear strength, and how it contributes towards the increased likelihood of a landslide. 3) Outline the different factors, both natural and human, that contributed to the Vaiont landslide disaster. 2 of 23 Landslides: Erik Eberhardt EOSC 114 – 2009/10 1 Stability of Slopes Why don’t slopes fail? Frictional strength Cohesive strength 3 of 23 Landslides: Erik Eberhardt EOSC 114 – 2009/10 Slope Stability Analysis To assess the stability of a slope, a limit equilibrium analysis in typically performed, whereby force equilibrium conditions are examined for a static slope situation. The typical output from a limit equilibrium analysis is the “Factor of Safety”: FS = = resisting forces driving forces shear strength shear stress h t DRIVING force FS > 1.0 represents a stable situation RESISTING force (i.e.shear strength) FS < 1.0 denotes failure 4 of 23 Landslides: Erik Eberhardt EOSC 114 – 2009/10 2 Slope Stability, Friction & Water at Depth friction n W F 5 of 23 W cohesion n = W sin i Factor of Safety Landslides: Erik Eberhardt EOSC 114 – 2009/10 Effective Stress friction n W n F ’ W cohesion n = W sin Factor of Safety Total Normal Stress, n Pore Water Pressure 6 of 23 Effective Stress ’ ’ = n - [n – ] Landslides: Erik Eberhardt EOSC 114 – 2009/10 3 Effective Stress High pore pressures may adversely affect the stability of a slope due to a decrease in effective stresses. W n f constant [n – ] The downslope stress components remain relatively unchanged, but an increased pore pressure along any potential failure plane acts against the effective normal stress (thereby decreasing the frictional strength component along the sliding surface). 7 of 23 Landslides: Erik Eberhardt EOSC 114 – 2009/10 Case Study - The Vaiont Dam In 1956, a private hydroelectric company in northern Italy began construction on the world’s tallest concrete arch dam (265.5 m) The m ( m). construction provided employment for over 400 people and would provide power for the rapidlyexpanding northern cities of Milan and Turin. 8 of 23 Landslides: Erik Eberhardt EOSC 114 – 2009/10 4 Case Study - The Vaiont Dam Expert Opinion: During filling of the reservoir, some Opinion: local detachments of material could be expected. But these would not be of a serious magnitude. Prof. Dal Piaz Junior’s Opinion: Wait a second… if you hike to the top Opinion: of the mountain, there appears to be an indication of a prehistoric landslide on one of the reservoir slopes. Eduardo Semenza Slide Surface 9 of 23 Landslides: Erik Eberhardt EOSC 114 – 2009/10 Case Study - The Vaiont Dam The dam’s builders were under intense pressure to have the dam tested and certified. Plans were underway to nationalize the production of hydroelectric energy in Italy and a working dam would be worth significantly more than an untested one. Dam’s designer: Carlo Semenza Trouble-making Geologist son: Eduardo Semenza … the dam was completed and filling of the reservoir began. constant [n – ] 10 of 23 Landslides: Erik Eberhardt EOSC 114 – 2009/10 5 The Vaiont Dam – Initial Filling Five-months into the initial filling of the reservoir, a 2-km long tension crack opened near the top of the slope suggesting that a very large landslide had been mobilized. 11 of 23 Landslides: Erik Eberhardt EOSC 114 – 2009/10 The Vaiont Dam – 3rd Filling One year later, filling of the reservoir resumed. At first the slope appeared stable, but eventually the landslide began moving again. The water level was quickly dropped to bring the slope under control. However, unlike previous experiences where such i h h action helped halt the slope movements, this time velocities continued to accelerate. 12 of 23 Landslides: Erik Eberhardt EOSC 114 – 2009/10 6 The Vaiont Dam – Imminent Failure Wednesday, October 9th, 1963 - The situation at the dam was worsening, possibly best summed up by the letter written by the chief engineer to call back from vacation his head of construction: “In these past few days, speeds within the landslide have clearly increased. Ruptures in the roads, tilting of trees, the enlargement of the large scarp around the landslide, make me think the worst. Water level is going down, and this morning it should be at 700 m. I am sorry to send you so much bad news, and to force you to interrupt your holiday. May God help us”. Before evening, the entire slope lurched forward by 20 cm and then began to increase its velocity. The chief engineer decided to leave the dam and go back home to Venice, leaving a crew behind to monitor the situation. 13 of 23 Landslides: Erik Eberhardt EOSC 114 – 2009/10 The Vaiont Landslide At 22:39 GMT on October 9, 1963, 260 million m3 of rock collapsed suddenly. The entire mass slid 500 m northwards at 30 m/s. The mass completely blocked the gorge to a depth of 400 m and travelled 140 m up the opposite bank. 14 of 23 Landslides: Erik Eberhardt EOSC 114 – 2009/10 7 The Vaiont Landslide Half the wave, 25 million m3 of water, overtopped the dam and fell vertically 261 meters into the narrow valley below forming a 70 m high wave travelling at about 100 km/h. About 1 minute after the km/h. detachment of the landslide, a wind began to blow across Longarone and mud began to rain down Three minutes later the wave reached down. the centre of the valley and with a height of still 30 m, hit m, Longarone and a series of neighbouring towns killing 1900 people. people. 15 of 23 Landslides: Erik Eberhardt EOSC 114 – 2009/10 Vaiont – The Aftermath Two days after the disaster, the dam released a statement saying that, "the landslide was not predictable". 16 of 23 Landslides: Erik Eberhardt EOSC 114 – 2009/10 8 ...
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