Liquefaction phenomena, Stress strain behavior of sands, Evaluation of liquefaction susceptibility Lecture Notes by Prof. Dr. Atilla Ansal
LIQUEFACTION s ′ = s – u = 0 University of Washington, Seattle (www.ce.washington.edu/~liquefaction/)
Soil grains in a soil deposit. The length of the arrows represent Observe how small the contact the size of the contact forces forces are because of the high The height of the blue column to between individual soil grains. The water pressure. the right represents the level of contact forces are large when the porewater pressure in the soil. pore water pressure is low. BEFORE EARTHQUAKE DURING EARTHQUAKE University of Washington, Seattle (www.ce.washington.edu/~liquefaction/)
= s s = u 0 s = s CYCLIC LOADING u shear stress loose sand densification shear stress dense sand dilatation
= s s = u 0 s = s CYCLIC LOADING u shear stress loose sand densification shear stress dense sand dilatation
University of Washington, Seattle (www.ce.washington.edu/~liquefaction/)
LIQUEFACTION • Earthquake Ground Motion External factor utilizing the weakness of the soil characteristics but highly variable and in most cases difficult to predict acceleration time histories • Ground Water Table Environmental factor depending on seasonal variations • Properties of Soil Layers Depth, thickness, soil type, gradation, density, fines content, and plasticity of the fines
LIQUEFACTION “Liquefaction of soil” is a state of particle suspension resulting from release of contacts between particles. Therefore the most susceptible to liquefaction are the cohesionless and low plasticity soils, in which the resistance to deformation is mobilised mainly by friction between particles under the influence of confining pressures. When liquefaction occurs, the strength of the soil decreases and, the ability of a soil deposit to support foundations for buildings and bridges is reduced.
Niigata 1964
KOBE 1995
Adapazarı 1999
1967 ADAPAZARI DEPREMİ – SAPANCA OTELİ 1999 KOCAELİ EARTHQUAKE– HOTEL SAPANCA
Christchurch, New Zealand Earthquake 22.2.2011 M=6.2
Christchurch, New Zealand Earthquake 22.2.2011
Christchurch, New Zealand Earthquake 22.2.2011
7.1M Darfield Earthquake of Sept. 3, 2010 (New Zealand)
Wildlife site record
FACTORS AFFECTING LIQUEFACTION
FACTORS CONTROLLING LIQUEFACTION • Earthquake Characteristics External factor utilizing the weakness of the soil characteristicsbut highly variable and in most cases difficult to predict acceleration time histories • Geotechnical Site Conditions – Geologic features – Soil Stratification – Depth of bedrock – Ground water table Environmental factor with seasonal variations – Properties of soil layers Depth, thickness, soil type, gradation, density, fines content and plasticity of the fines
FACTORS AFFECTING CYCLIC STRENGTH (LIQUEFACTION) of COARSE GRAINED SOILS ➜ Relative Density, D r ➜ Overconsolidation Ratio, OCR ➜ Lateral earthpressure coefficient, K 0 ➜ Increased time under pressure ➜ Seismic history ➜ Method of sample preparation ➜ Grain characteristics (Size, shape, and distribution) ➜ Fines Content and Plasticity ➜ Saturation, depth of ground water table
Stress ratio Samples with prior shaking Samples with no prior shaking Number of cycles for initial liquefaction
0.7 Dr = 78% In-situ frozen samples 0.6 from Niigata site s ' 0 =78 kPa 0.5 Cyclic Stress Ratio Dr = 54% 0.4 0.3 Freshly Deposited 0.2 Sand Dr = 56% 0.1 0 1 10 100 Number of Cycles for Initial Liquefaction Effect of remolded versus undisturbed samples
Stress Strain Behavior of Sands Behavior under Monotonic & Cyclic Stresses
A deposit of sand is composed of an assemblage of particles in equilibrium where inter-granular forces are transmitted through points of contact. When shear stress is applied, the resulting deformation is always accompanied by a volume change. shear stress loose sand densification shear stress dense sand dilatation
Two mechanism controling behaviour in sands: 1. Slip down 2. Rollover
Critical Void Ratio The void ratio at failure when volumetric strain is zero. (Casagrande, 1936) Behavior of dense and loose sands in drained, strain-controlled triaxial tests. Loose sand exhibit contractive behavior (decreasing void ratio) and dense sand exhibit dilative behavior (increasing void ratio) during shearing.
Cyclic Stress-Strain Behaviour of Soils Dynamic shear modulus Damping Ratio
Dynamic Shear Modulus for Gravelly Soils
Undrained Cyclic Simple Shear Test on Monterey #30/0 Sand Dr=50%, σ v,i’ =85 kPa, CSR=0.22, α =0
Undrained Cyclic Simple Shear Test on Monterey #30/0 Sand ) Dr=75%, σ v,i’ =85 kPa, CSR=0.4, α=0
Undrained Cyclic Simple Shear Test on Monterey #30/0 Sand Dr=55%, σ v,i’ =85 kPa, CSR=0.33, α=0.18
LABORATORY TESTING METHODS
Different states of liquefaction: • TOTAL LIQUEFACTION • INITIAL LIQUEFACTION & CYCLIC MOBILITY
Three different types of stress-strain behavior for a very loose specimen A, a dense specimen B and specimen C at intermediate density: Specimen A: a peak undrained strength at a small shear strain and then collapse to flow rapidly to large strains at low effective confining pressure and low large-strain strength. Specimen B: initially contracted but then dilated until a relatively high constant effective confining pressure was reached. Specimen C: the exceedance of peak strength at low strain followed by a limited period of strain- softening behavior, which ended with the onset of dilatation at intermediate strains.
LABORATORY TESTS FOR LIQUEFACTION EVALUATION CYCLIC TRIAXIAL CYCLIC SIMPLE SHEAR TORSIONAL HALLOW CYLINDER RESONANT COLUMN TEST SHAKING TABLE • Some tests are designed to measure specific soil properties PROBLEMS like shearing strength or shear • Sample disturbance moduli • Effects of testing systems • Some are designed to determine • Stress Conditions soil behaviour in a simulated dynamic environment
120 EXCESS PORE PRESSURE (kPa) Dr = 62 % Specimen diameter CORRECTED 100 D = 5 cm D = 5 cm 80 D = 7 cm D = 7 cm 60 UNCORRECTED NL=95 40 s c ' = 100 kPa NL=8 NL=60 20 s d /2 s c = 0.324 0 1 10 100 NUMBER OF CYCLES FOR INITIAL LIQUEFACTION, NL Effect of membrane penetration
1.2 1.2 FC = 78% FC = 14% PLASTIC FINES 1 1 Pore Pressure Ratio Pore Pressure Ratio 0.8 FC = 65% 0.8 FC = 37% 0.6 0.6 FC =22% 0.4 0.4 0.2 0.2 NONPLASTIC FINES FC = 67% 0 0 1 10 100 1 10 100 1000 Number of Cycles Number of Cycles 6 3 FC = 14% FC = 78% 5 NONPLASTIC FINES PLASTIC FINES Shear Strain (%) Axial Strain (%) 4 2 FC = 65% FC = 37% 3 1 2 FC =22% 1 FC = 67% 0 0 0 50 100 150 200 250 300 0 20 40 60 80 100 Number of Cycles, N Number of Cycles, N
Toyouro Sand, Isotropic Consolidation, Dr=50%, s o =98 kPa, Fr=0.1 Hz 25 50 20 40 xial Stress (kPa) 15 30 Shear Stress (kPa) 10 20 5 0 10 -5 A 0 -10 Shear & -10 -15 -20 -20 Additional Axial Stress Shear Stress -25 -30 0 5 10 15 20 25 30 35 40 0 0.5 1 1.5 2 2.5 3 120 140 Pore Water Pressure, u (kPa) Pore Water Pressure, u (kPa) 120 100 100 80 80 60 60 40 40 20 20 0 0 0 10 20 30 40 0 10 20 30 40 10 15 5 Shear Strain, g (%) 10 Shear Strain, g (%) 0 5 -5 0 -10 -5 -15 -10 0 10 20 30 40 0 10 20 30 40 Number of Cycles Number of Cycles
EVALUATION of LIQUEFACTION SUSCEPTIBILITY
EMPRICAL METHODS SIMPLIFIED APPROACH Estimation of induced cyclic shear stresses Estimation of liquefaction resistance of soil layers Standard Penetration Test, SPT 1. Cone Penetration Test 2. Shear Wave Velocity 3. DETAILED ANALYSIS Site Response Analysis Cyclic Tests FACTOR of SAFETY 1. STRESS RATIO = FS r c 2. RATIO of NUMBER of CYCLES FS = N r N c
= = N N NC NC C C C C C C C C 1 1 , , 60 60 N N R R S S B B E E Evaluation of CSR C C N N s a = = av max v CSR 0 . 65 r d s s ' ' g v v C N Liao & Whitman (1986) = z 9 . 15 m r d 1 . 0 0 . 00765 z = 9 . 15 m z 23 m r d 1 . 174 0 . 0267 z Youd et al. (2001)
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