International Institute for Geo-Information Science and Earth Observation (ITC) Seismic induced landslide Seismic induced landslide hazard assessment hazard assessment Cees van Westen & Mark van der Meijde Department Earth Systems Analysis, ITC Enschede, Netherlands ISL 2004
International Institute for Geo-Information Science and Earth Observation (ITC) Earthquake induced landslides Earthquake induced landslides Earthquake-induced landsliding of a hillside slope occurs • when the static plus inertia forces within the slide mass cause the factor of safety to temporarily drop below 1.0. The value of the peak ground acceleration within the slide • mass required to just cause the factor of safety to drop to 1.0 is denoted by the critical or yield acceleration a c . This value of acceleration is determined based on pseudo- • static slope stability analyses and/or empirically based on observations of slope behavior during past earthquakes. ISL 2004
International Institute for Geo-Information Science and Earth Observation (ITC) Shear strength / stress Shear strength / stress Shear stress = W sin β / A Shear strength (Mohr-Coulomb criterion) s = c + σ tan φ σ = normal stress = W cos β / A c = cohesion (KPa) φ = angle of internal friction (degrees) φ and c are geotechnical properties, which are measured in the laboratory using triaxial tests or shearbox tests. ISL 2004
International Institute for Geo-Information Science and Earth Observation (ITC) Safety Factor Safety Factor The degree of slope hazard can be expressed by the Safety Factor (F) which is the ratio of the forces that make a slope fail and those that prevent a slope from failing. F < 1 unstable slope conditions, • F = 1 slope is at the point of failure, • F > 1 stable slope conditions . • ISL 2004
International Institute for Geo-Information Science and Earth Observation (ITC) Infinite slope Infinite slope Infinite slope: Conditions at crest and toe of the slope may be ignored. • Resulting forces from left and right are equal • Weight of the block: γ = unit weight of soil (N/m 3 ). Shear component of weight: Normal component of weight: ISL 2004
International Institute for Geo-Information Science and Earth Observation (ITC) Infinite slope Infinite slope Stress = Force / area Shear component of weight: Shear stress: Normal component of weight: Normal stress: Safety factor: ISL 2004
International Institute for Geo-Information Science and Earth Observation (ITC) Infinite slope & water pressure Infinite slope & water pressure Height watertable above failure surface Weight of the water: Normal component of water weight: Pore pressure on JK: Factor of safety including pore pressure: ISL 2004
International Institute for Geo-Information Science and Earth Observation (ITC) Infinite slope & seismic acceleration Infinite slope & seismic acceleration F = safety factor c’ = effective cohesion (KPa) φ ’ = effective angle of internal friction (degrees) a A = horizontal component of seismic acceleration, PGA (m/s 2 ) γ = unit weight of soil (kN/m 3 ) γ w = unit weight of water (kN/m 3 ) ρ = bulk density of soil (kg/m 3 ) z = depth of failure surface below terrain surface (m) z w = depth of water table below terrain surface (m) β = Slope angle (degrees) ISL 2004
International Institute for Geo-Information Science and Earth Observation (ITC) FOS & critical acceleration FOS & critical acceleration FOS = Factor of Safety ϕ γ ϕ c ' tan ' m tan ' w = + − FOS γ α α γ α z sin tan tan Miles (2003), modified Legend: c' : effective cohesion [kN/m²] γ : spec. unit weight [kN/m³] γ w : spec. unit weight of water [kN/m³] z : thickness of moving layer [m] α : slope angle[ ° ] ϕ ' : effective friction angle [ ° ] m : relation of saturated to unsaturated layer within z ( ) = FOS − α a c 1 sin Legend: a c : critical acceleration [m/s²] Newmark (1965), modified α : slope angle [ ° ] ISL 2004
International Institute for Geo-Information Science and Earth Observation (ITC) 'Newmark Displacement' - - Method Method 'Newmark Displacement' ARIAS - Intensity after Wilson (1993): = − + − log I a M 2 log R ² h ² 3 , 99 'Newmark Displacement' after Jibson (1998): = − − log D 1 . 521 log I 1 . 993 log a 1 . 546 n a c This simplified formula by Jibson (1998) is used to calculate the displacement initiated by earthquake energy ISL 2004
International Institute for Geo-Information Science and Earth Observation (ITC) Methods for seismically induced landslide Methods for seismically induced landslide hazard assessment hazard assessment Pseudo-static analysis , in which the earthquake load is • simulated by an "equivalent" static horizontal acceleration acting on the mass of the landslide, in a limit-equilibrium analysis Newmark or cumulative displacement analysis , • involves the calculation of the yield acceleration, defined as the inertial force required to cause the static factor of safety to reach 1.0 Makdisi-Seed analysis. Design curves were developed to • estimate the permanent earthquake-induced deformations of embankments 100 to 200 feet high using finite element analyses. Dynamic analysis or a stress-deformation analysis. It • typically incorporates a finite-element or finite-difference mathematical model. ISL 2004
International Institute for Geo-Information Science and Earth Observation (ITC) Earthquake induced landslides Earthquake induced landslides Deformations are calculated using the approach originally • developed by Newmark (1965). Downslope deformations occur during the time periods • when the induced peak ground acceleration within the slide mass a is exceeds the critical acceleration a c . In general, the smaller the ratio (below 1.0) of a c to a is , the • greater is the number and duration of times when downslope movement occurs, and thus the greater is the total amount of downslope movement. The amount of downslope movement also depends on the • duration or number of cycles of ground shaking. Since duration and number of cycles increase with • earthquake magnitude, deformation tends to increase with increasing magnitude for given values of a c and a is . ISL 2004
International Institute for Geo-Information Science and Earth Observation (ITC) Newmark method method Newmark The procedure involves the calculation of the yield • acceleration, defined as the inertial force required to cause the static factor of safety to reach 1.0, from the traditional limit-equilibrium slope stability analysis. The procedure then uses a design earthquake strong-motion record which is numerically integrated twice for the amplitude of the acceleration above the yield acceleration to calculate the cumulative displacements. These analytical displacements are then evaluated in light of the slope material properties and the requirements of the proposed development. (Newmark, 1965; Makdisi and Seed, 1978; Hynes and • Franklin, 1984; Houston and others, 1987; Wilson and Keefer, 1983; Jibson, 1993). ISL 2004
International Institute for Geo-Information Science and Earth Observation (ITC) Newmark method method Newmark ISL 2004
International Institute for Geo-Information Science and Earth Observation (ITC) Newmark method method Newmark Step 1: The first step is to perform a limit-equilibrium stability • analysis to determine the location and shape of the critical slip surface (the slip surface with the lowest factor of safety), and the yield acceleration (Ky), defined as the acceleration required to bring the factor of safety to 1.0. Ky = ( FS - 1 )g sin a Where: g = the acceleration due to gravity a = is the angle from the horizontal that the center of mass of the landslide first moves. FS = factor of safety in static conditions ISL 2004
International Institute for Geo-Information Science and Earth Observation (ITC) Newmark method method Newmark Step 2: select an acceleration time history that represents the • expected ground motions at the project site. The selection process typically involves estimating magnitude, source-to-site distance, and peak ground acceleration seismic parameters for the project site . For Newmark analyses, Jibson (1993) recommended • using: Arias Intensity • Magnitude and source distance, • PGA and duration as criteria for selecting a suite of • strong-motion records having characteristics of interest at a project site ISL 2004
International Institute for Geo-Information Science and Earth Observation (ITC) Newmark method method Newmark Step 2: • Newmark displacement (DN) is a function of critical acceleration and Arias Intensity according to the following empirical regression equation (Jibson, 1993): log D N = 1.460 log I a - 6.642 a c + 1.546 In which: • I a = Arias Intensity in meters per second. • a c = critical acceleration, • D N = Newmark displacement • ISL 2004
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