EARTHQUAKE-INDUCED SOIL LIQUEFACTION: MOST DETRIMENTAL SEISMIC GROUND FAILURE Nien-Yin Chang (NYC), Ph.D., P.E Professor of Geotechnical Engineering and Director of Center for Geotechnical Engineering Science University of Colorado Denver Largest earthquakes since 1900 Date UTC Magnitude Location Chile 1964 Great Alaska Earthquake Off the West Coast of Northern Sumatra Near the East Coast of Honshu, Japan Kamchatka Offshore Maule, Chile Off the Coast of Ecuador Rat Islands, Alaska Northern Sumatra, Indonesia Assam - Tibet Off the west coast of northern Sumatra Andreanof Islands, Alaska Southern Sumatra, Indonesia Banda Sea, Indonesia Kamchatka Chile-Argentina Border Kuril Islands Updated 2012 April 11 Lat Long Reference 1960 05 22 9.5 1964 03 28 9.2 -38.29 -73.05 Kanamori, 1977 61.02 -147.65 Kanamori, 1977 2004 12 26 9.1 3.30 2011 03 11 9.0 38.322 142.369 PDE 1952 2010 1906 1965 2005 1950 2012 52.76 -35.846 1.0 51.21 2.08 28.5 2.311 11 02 01 02 03 08 04 04 27 31 04 28 15 11 9.0 8.8 8.8 8.7 8.6 8.6 8.6 1957 03 09 8.6 2007 09 12 8.5 1938 02 01 8.5 1923 02 03 8.5 1922 11 11 8.5 1963 10 13 8.5 95.78 160.06 -72.719 -81.5 178.50 97.01 96.5 93.063 Park et al., 2005 Kanamori, PDE Kanamori, Kanamori, PDE Kanamori, PDE 1977 1977 1977 1977 51.56 -175.39 Johnson et al., 1994 -4.438 101.367 PDE -5.05 131.62 Okal and Reymond, 2003 54.0 161.0 Kanamori, 1988 -28.55 -70.50 Kanamori, 1977 44.9 149.6 Kanamori, 1977 DEFINITION OF SOIL LIQUEFACTION In a soil mass, total stress (σ), pore water pressure (u) and effective stress (σ’) are related as in the following equation: 𝝈 = 𝝈′ + 𝒖, 𝒊 𝒆 𝝈′ = 𝝈 − 𝒖 When a loose saturated soil are subjected to dynamic load, like earthquake shaking, • Excess pore water pressure increases as the dynamic load progresses • The total stress remains constant, while the effective stress continues to decrease till pore water pressure equals total stress and the effective stress becomes zero • Since the strength of sand is a monotonically increasing function of effective stress, when the effective stress becomes zero, the soil experiences a complete loss of strength and, • Gross lateral spreading occurs The phenomenon is called soil liquefaction TERZAGHI’S EFFECTIVE STRESS PRINCIPLE Before seismic shaking Total Stress = Effective Stress + Hydrostatic Pore Water Pressure, i.e  = ’ + uo During seismic shaking σ = ’ + uo + u, i.e ’ =  - (uo + u) =  - u When ' goes to zero, the soil liquefies, its strength goes to zero, and the soil flows like a liquid FIGURE XX STRESS-STRAIN AND PORE PRESSURE RESPONSES OF SATURATED SANDS IN MONOTONIC TRIAXIAL TESTS (DEPARTMENT OF THE ARMY) Figure xx load, deformation, and pore pressures records during a cyclic triaxial test (Department of the Army, 1990) Figure xx Cyclic triaxial strength curves for Monterey No.0 Sand (USAE, 1990) EFFECTS OF LIQUEFACTION: MOST DETRIMENTAL FORM OF GROUND FAILURE Shallow Foundation Support: Because of the gradual increase of excess pore water pressure and constant overburden pressure during earthquake shaking, the effective stress continues to decrease and eventually becomes zero and the soil experiences complete loss of shear strength and its capacity to support superstructures This bearing capacity failure results in drastic dislocation and settlement of buildings on shallow foundations during the seismic shaking, as depicted in the apartment building failures in Niigata, Japan in 1964 Deep Foundation Support: Superstructures supported on deep foundations, however have a much better chance of survival, although many buildings still suffered excessive lateral dislocation and many bridges suffered sequential and progressive falls of one end of bridge deck into river due to differential side sway between neighboring bridge piers CYCLIC STRESS RATIO AND CYCLIC RESISTANCE RATIO • Cyclic Stress Ratio (CSR) = cyclic maximum shear stress / effective overburden (or confining) pressure Cyclic shear stress =0.65 x rd x max shear stress (Seed), where rd = rigidity reduction factor Max Shear stress = mass of a soil column of unit area x peak ground acceleration from MCE • Cyclic Resistance Ratio (CRR) = cyclic maximum shear stress / effective confining pressure CRR determined in laboratory cyclic triaxial (cyclic simple shear or cyclic hollow cylinder) tests CRR from adjusted BPF (N1)60 from field SPT Liquefaction failure occurs, when CRR ≤ CSR, where CRR IS OBTAINED FROM: FIELD OBSERVATION CURVES OF SEED BASED ON THE ADJUSTED BPF VALUES, (N1)60 OR LABORATORY TRIAXIAL TESTS CYCLIC SHEAR STRESS RATIO(CSR) IS OBTAINED USING THE MAXIMUM SHEAR STRESS FROM ONE-DIMENSIONAL WAVE PROPAGATION ANALYSIS USING SHAKE 2000 OR Cyclic shear stress (CSR) = 0.65 x rd x max shear stress/σvo’, where rd = rigidity reduction factor and maximum shear stress = m x ap (= peak ground acceleration) Factor of Safety against liquefaction = CRR/CSR “Cyclic-mobility” behavior, like upper curve in Idriss-Boulanger Sur / σ’v0 chart • Migration less likely if soil has any plasticity, or if liquefying layer is thin • If there is free-draining layer (or air above, excess PWP may escape • “Water films don’t normalize.” Harder • Normalized strength relies on nonuniformity of void migration, initial void ratio, strains, etc • Idriss on assuming no void migration: “Do you want to be the first one to go out on that limb?” Risk analysis is one place it might make sense to so, probabilistically For now: • For (N1)60cs < 14 or qc1Ncs < 90, check stability using both “direct” and “normalized” approaches to Sur – for a dam it’s likely to be a problem either way • For (N1)60cs > 14 or qc1Ncs > 90, consider potential for phase change and dilation causing higher strength for deformation during earthquake The jury is still out • Remember potential for void migration and water film in post-earthquake stability If it’s unstable after the earthquake, deformation during the earthquake is probably moot Questions? Sheffield Dam, 1926 Non-Liquefied Clayey Soils Bray and Sancio (2006) – Per Boulanger and Idriss (2004), expect claylike behavior if PI > ~7 – Chart above indicates no liquefaction potential if PI > 18 • Under dam, clay is likely NC or lightly OC • Assess yield and dynamic deformation using undrained strength in Newmark, FLAC, etc • Main tools: – Lab shear tests – CPT tip (peak) and sleeve (remolded) – Vane shear test (peak and remolded) • Sensitivity: Su-peak / Su-remolded – High – embankment could be unstable – Low – probably just large deformations – How much strain to get remolding? Clay in Cyclic Loading Interpretation of Clay Properties from CPT Vane Shear Test (VST) • Quick and cheap; no special rig required • Requires empirical correction factor • Peak, post-peak, remolded strengths • No stress vs strain relationship provided • Limited feasibility in stiffer clays VST Adjustment for “Static” Loading (anisotropy, strain rate, assumed shear surface ≠ actual) Slopewash, DH-09-1, CAUC, CH Tff = 3.6 tsf * (cos 35º) = 2.9 tsf Different Stress Paths in Different Areas Base Acceleration Focus on foundation under mid slope Major principal stress, σ1, at failure Organizing, Assimilating, and Presenting Data VST Peak and Remolded (Uncorrected) CAUC Post-Cyclic OCR from Lab Consol S from DSS (NC) Peak and Post-Peak Assumed Nkt: 14 “Cyclic Failure” as Defined by Boulanger and Idriss (2004) • Cyclic failure can occur with cyclic shear stress as low as 75% of monotonic peak • However, cyclic failure ≡ strain over 3%, which would generally not constitute failure in a dam embankment or foundation Strain Rate Effects in Clays • Benefit difficult to quantify, but should exist • Would apply only during dynamic deformation phase, not post-earthquake stability, where strain rate should be ZERO! Bjerrum (1972) via Mitchell (1976) ? ... PROCEDURE FOR LIQUEFACTION ANALYSIS • Determine: if the earthquake has potential to produce soil liquefaction, if the soils are susceptible to liquefaction then • Determine if liquefaction may... soils DEFINITION OF SOIL LIQUEFACTION LABORATORY TESTS FOR SOIL LIQUEFACTION Many different laboratory experiments can be used to evaluate the liquefaction resistance of soils: cyclic triaxial... the liquefaction potential of soils without actually performing cyclic tests Fig.xx Liquefaction Resistance vs Fines Content for Nl=20 FIELD FACTORS AFFECTING LIQUEFACTION RESISTANCE OF SOILS earthquake