Liquefaction phenomena, Stress strain behavior of sands, Evaluation - - PowerPoint PPT Presentation
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
University of Washington, Seattle (www.ce.washington.edu/~liquefaction/)
Soil grains in a soil deposit. The height of the blue column to the right represents the level of porewater pressure in the soil. The length of the arrows represent the size of the contact forces between individual soil grains. The contact forces are large when the pore water pressure is low. Observe how small the contact forces are because of the high water pressure.
BEFORE EARTHQUAKE DURING EARTHQUAKE University of Washington, Seattle (www.ce.washington.edu/~liquefaction/)
CYCLIC LOADING
CYCLIC LOADING
University of Washington, Seattle (www.ce.washington.edu/~liquefaction/)
“Liquefaction of soil” is a state of particle suspension resulting from release of contacts between
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
Niigata 1964
KOBE 1995
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
External factor utilizing the weakness of the soil characteristicsbut highly variable and in most cases difficult to predict acceleration time histories
Environmental factor with seasonal variations
Depth, thickness, soil type, gradation, density, fines content and plasticity of the fines
Number of cycles for initial liquefaction
Samples with no prior shaking Samples with prior shaking
Stress ratio
0.1 0.2 0.3 0.4 0.5 0.6 0.7 1 10 100 Number of Cycles for Initial Liquefaction Cyclic Stress Ratio
In-situ frozen samples from Niigata site
s'0=78 kPa
Dr = 78% Dr = 54% Freshly Deposited Sand Dr = 56%
Effect of remolded versus undisturbed samples
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.
Two mechanism controling behaviour in sands: 1. Slip down 2. Rollover
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. The void ratio at failure when volumetric strain is zero. (Casagrande, 1936)
Undrained Cyclic Simple Shear Test on Monterey #30/0 Sand Dr=50%, σv,i’=85 kPa, CSR=0.22, α=0
Undrained Cyclic Simple Shear Test on Monterey #30/0 Sand ) Dr=75%, σv,i’=85 kPa, CSR=0.4, α=0
Undrained Cyclic Simple Shear Test on Monterey #30/0 Sand Dr=55%, σv,i’=85 kPa, CSR=0.33, α=0.18
Different states of 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.
PROBLEMS
CYCLIC TRIAXIAL CYCLIC SIMPLE SHEAR TORSIONAL HALLOW CYLINDER RESONANT COLUMN TEST SHAKING TABLE
measure specific soil properties like shearing strength or shear moduli
soil behaviour in a simulated dynamic environment
20 40 60 80 100 120 1 10 100
NUMBER OF CYCLES FOR INITIAL LIQUEFACTION, NL EXCESS PORE PRESSURE (kPa)
D = 5 cm D = 5 cm D = 7 cm D = 7 cm
Specimen diameter
NL=8 NL=60 NL=95 CORRECTED
Dr = 62 % sc' = 100 kPa
sd /2sc = 0.324
UNCORRECTED
1 2 3 4 5 6 20 40 60 80 100
Number of Cycles, N Axial Strain (%) FC = 78% FC = 65% FC =22% NONPLASTIC FINES
1 2 3 50 100 150 200 250 300
Number of Cycles, N Shear Strain (%) FC = 14% FC = 37% FC = 67% PLASTIC FINES
0.2 0.4 0.6 0.8 1 1.2 1 10 100 1000
Number of Cycles Pore Pressure Ratio FC = 14% FC = 37% FC = 67% PLASTIC FINES
0.2 0.4 0.6 0.8 1 1.2 1 10 100
Number of Cycles
Pore Pressure Ratio
FC = 78% FC = 65% FC =22% NONPLASTIC FINES
Toyouro Sand, Isotropic Consolidation, Dr=50%, so=98 kPa, Fr=0.1 Hz
5 10 15 10 20 30 40
Number of Cycles Shear Strain, g (%)
20 40 60 80 100 120 140 10 20 30 40
Pore Water Pressure, u (kPa)
10 20 30 40 50 0.5 1 1.5 2 2.5 3
Shear & A xial Stress (kPa)
Additional Axial Stress Shear Stress
20 40 60 80 100 120 10 20 30 40
Pore Water Pressure, u (kPa)
5 10 10 20 30 40
Number of Cycles Shear Strain, g (%)
5 10 15 20 25 5 10 15 20 25 30 35 40
Shear Stress (kPa)
1.
Standard Penetration Test, SPT
2.
Cone Penetration Test
3.
Shear Wave Velocity
FACTOR of SAFETY
c r
c r N
d v v v av
' max '
z rd 00765 . . 1 =
m z 15 . 9
z rd 0267 . 174 . 1 =
m z m 23 15 . 9
Liao & Whitman (1986)
E B S R N
C C C C NC N =
60 , 1 N
C
E B S R N
C C C C NC N =
60 , 1 N
C
N
C
Youd et al. (2001)
E B S R N
60 , 1 N = measured standard penetration resistance; CN = factor to normalise N to a common reference effective overburden stress; CE = correction for hammer energy ratio (ER); CB = correction factor for borehole diameter; CR = correction factor for rod length; CS = correction for samplers with or without liners.
Youd et al. (2001) Curve for fines content, FC<5%: “SPT
clean-sand base curve”
Valid for Magnitude 7.5 earthquakes only (N1)60 :SPT blow count normalized to an
hammer energy ratio of 60%
Influence of Fines Content and SPN - N Corrections
The following equations were developed by I. M. Idriss with the assistance of R. B. Seed for correction of (N1)60 to an equivalent clean sand value, (N1)60cs:
(N1)60cs = a + (N1)60
where a and b = coefficients determined from the following relationships: a = 0 for FC 5% a = exp[1.76 - (190/FC2)] for 5% < FC < 35% a = 5.0 for FC 35 = 1.0 for FC 5% = [0.99 + (FC1.5 /1,000)] for 5% < FC < 35% = 1.2 for FC 35
E B S R N
60 , 1
LIQUEFACTION POTENTIAL INDEX, PL
PL>15 5 ≤ PL≤15 PL<5 SEVERE LIQUEFACTION MINOR EFFECTS
Method proposed by Iwasaki et al. (1982)
= dz z w z F P
L
) ( ) (
z: depth below the ground water surface, (m) F(z): a function of the liquefaction resistance factor, FL F(z)=1- FL but if FL>1.0, F(z)=0 w(z)=10-0.5z
Iwasaki et al.(1978) developed the LPI to predict the potential of liquefaction to cause foundation damage at a site. They assumed that the severity of liquefaction should be proportional to the:
– Thickness of the liquefied layer; – Proximity of the liquefied layer to the surface; – Amount by which the factor safety (FS)
0.1 0.2 0.3 0.4 0.5 0.6 0.7 1 10 100 1000 NUMBER OF CYCLES Cyclic S hear S tress Ratio
S38, Depth=15.5-16.0m, FC=14%
0.2 0.4 0.6 0.8 1 50 100 150 200 250 300
NUMBER OF CYCLES
P ore P ressure Ratio 0.223 0.292 0.361 0.481
S42, DEPTH=7.5-8.0m, FC=29%
0.2 0.4 0.6 0.8 1 20 40 60 80 100 120 NUMBER OF CYCLES Pore Pressure Ratio 0.352 0.387
S40, DEPTH=10.5-11.0m FC= 11%
0.2 0.4 0.6 0.8 1 1.2 30 60 90 120
Number of Cycles Pore Pressure Ratio
0.209 0.284 0.479
GEMLIK BORUSAN FACTORY - 1990
0.5 1 1.5 2 2.5
SAFETY FACTOR DEPTH (m)
BORING S34 (d) BORING S8
0.00 0.50 1.00 1.50 2.00 2.50
SAFETY FACTOR DEPTH (m)
(c)
Site response analysis and laboratory tests
BORING S6
0.5 1 1.5 2 2.5
SAFETY FACTOR DEPTH (m)
(b)
0.1 0.2 0.3 0.4 0.5 0.6 0.7 1 10 100 1000
Number of Cyles for Initial Liquefaction Cyclic Shear Stress Ratio
(a)
Liquefaction resistance curve used for analysis Cyclic simple shear tests
0.01 0.1 1 4 5 6 7 8 9
Ms
Excedance Prob.
Instrumental Historical Weighted Average
7.8
Return Period (years) 100 200 500 1000 Magnitude (Ms) 6.2 6.9 7.8 8.4 Peak Horz. Acc.(g) 0.11 0.17 0.29 0.41
TOYOTOSA FABRİKA SAHASI - 1992
0.6 0.8 1 1.2 1.4 1.6 1.8 SAFETY FACTOR DEPTH (m) Seed et al. (1985) Taiping et al. (1984) Iwasaki et al. (1978) Ishihara & Perlea (1984) Yokota (1980) Yuquing et al. (1980) Atkinson et al. (1984)
21 Boring 114 SPT SAMPLES
Stress Reduction Factor, rd
Youd et al., 2001 Cetin et al., 2004
d v v r
max
v av
5 10 15 20 25 200 400 600
Shear Wave Velocity (m/sn) Depth (m )
0.1 0.2 0.3
Q10 (ST4)
FILL (CL) (GC) (CL) (SC) (SC) (CL) (CL) 5 10 15 20 25 200 400 600 800 Shear Wave Velocity (m/sn) 0.2 0.3 0.4 0.5 0.6
N9 (ST20)
FILL (CL) (GM) (SC)
5 10 15 20 25 200 400 600 Shear Wave Velocity (m/s)
0.2 0.3 0.4 0.5
G11 (SKE27)
FILL SP, SM
COMPARISON OF CSR CALCULATED BY SIMPLIFIED PROCEDURES AND SITE RESPONSE ANALYSES
5 10 15 20 25 100 200 300 400 500
Shear Wave Velocity (m/sn)
0.5 1 1.5 2
D13 (ST2)
(SC) (SM) (CL) (SM) (CH, CL) SAFETY FACTOR, FS
5 10 15 20 25 100 200 300 400
Shear Wave Velocity (m/sn)
Depth (m)
0.5 1 1.5 2
T8 (ST38)
(SC) (GC) (CH, CL) (GM) (SM) (SC) (CL) (CL) SAFETY FACTOR, FS
5 10 15 20 25 100 200 300 400
Shear Wave Velocity (m/sn)
0.5 1 1.5 2
Site response analyses Youd et el., 2001
R7 (ST51)
(GM) (CL) (SM) (CL) (CH, CL) SAFETY FACTOR, FS
VARIATION OF SAFETY FACTOR WITH DEPTH