SOIL LIQUEFACTION: PHENOMENON, HAZARDS , REMEDIATION Dr. Farhat Javed Associate Prof. Military College of Engg, Risalpur

AIM HIGLIGHT THE IMPORTANCE OF LIQUEFACTION IN ENGINEERING PRACTICE

SEQUENCE OF PRESENTATION Introduction Liquefaction phenomenon Hazards Associated with Liquefaction Evaluation of Liquefaction Potential Remediation

During an earthquake seismic waves travel vertically and rapid loading of soil occurs under undrained conditions i.e., pore water has no time to move out. In saturated soils the seismic energy causes an increase in pore water pressures and consequently the effective stresses decrease. This results in loss of shear strength of soil and soil starts to behave as a fluid. This fluid is no longer able to sustain the load of structure and the structure settles. This phenomenon is known as liquefaction.

The Phenomenon is associated with: soft young water-saturated uniformly graded fine grained sands and silts During liquefaction these soils behave as viscous fluids rather than solids . This can be better demonstrated by a video clip in which a glass container with saturated sand is resting on a vibrating table.

STRUCTURE GLASS CONTAINER SATURATED SAND

LIQUEFACTION PHENOMENON

Effective stress = (total stress - pore water pressure) The phenomenon of liquefaction can be well understood by considering shear strength of soils. Soils fail under externally applied shear forces and the shear strength of soil is governed by the effective or inter-granular stresses expressed as: Effective stress = (total stress - pore water pressure) σ’ = σ - u

τ = c + σ’tan φ Shear strength τ of soil is given as : It can be seen that a cohesionless soil such as sand will not posses any shear strength when the effective stresses approach zero and it will transform into a liquid state.

Contact forces between particles give rise to normal stresses that are responsible for shear strength. Assemblage of particles This box represents magnitude of pore water pressure

During dynamic loading there is an increase in water pressure which reduces the contact forces between the individual soil particles, thereby softening and weakening the soil deposit. Increase in pore pressure due to dynamic loading

HAZARDS ASSOCIATED WITH LIQUEFACTION PHENOMENON

Historical Evidences 1964 Nigata (Japan) 1964 Great Alaskan earthquake Seismically induced soil liquefaction produced spectacular and devastating effect in both of these events, thrusting the issue forcefully to the attention of engineers and researchers

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 …. overturned apartment complex buildings in Niigata in 1964.

Liquefied soil also exerts higher pressure on retaining walls,which can cause them to tilt or slide. This movement can cause settlement of the retained soil and destruction of structures on the ground surface Kobe 1995

Retaining wall damage and lateral spreading, Kobe 1995

 Increased water pressure can also trigger landslides and cause the collapse of dams. Lower San Fernando dam suffered an underwater slide during the San Fernando earthquake, 1971.

Sand boils and ground fissures were observed at various sites in Niigata.

Lateral spreading caused the foundations of the Showa bridge in Nigata ,Japan to move laterally so much that the simply supported spans became unseated and collapsed

Liquefaction-induced soil movements can push foundations out of place to the point where bridge spans loose support or are compressed to the point of buckling 1964 Alaskan earthquake.

The strong ground motions that led to collapse of the Hanshin Express way also caused severe liquefaction damage to port and wharf facilities as can be seen below. 1995 Kobe earthquake, Japan

Lateral spreading caused 1 Lateral spreading caused 1.2-2 meter drop of paved surface and local flooding, Kobe 1995.

Alaska earthquake, USA,1964

1957 Lake Merced slide

modest movements during liquefaction produce tension cracks such as those on the banks of the Motagua River following the 1976 Guatemala Earthquake.

Damaged quay walls and port facilities on Rokko Island Damaged quay walls and port facilities on Rokko Island. Quay walls have been pushed outward by 2 to 3 meters with 3 to 4 meters deep depressed areas called grabens forming behind the walls, Kobe 1995.

1999 Chi-Chi (Taiwan) earthquake over 2,400 people were killed, and 11,000 were injured

1999 Chi-Chi (Taiwan) earthquake

1999 Chi-Chi (Taiwan) earthquake

1999 Chi-Chi (Taiwan) earthquake

1999 Chi-Chi (Taiwan) earthquake

1999 Chi-Chi (Taiwan) earthquake

1906 sanfransisco USA earthquake

Road damaged by lateral spread, near Pajaro River, 1989 Loma Prieta earthquake                            

Liquefaction failure of shefield dam (1925, california USA)                            

Liquefaction failure of Tanks at Nigata, Japan)                            

Among the 467 foundation damage cases reported, 67 cases (14% Chi-Chi earthquake.   Among the 467 foundation damage cases reported, 67 cases (14% were caused by earthquake-induced liquefaction.                                                                                                                                 Figure 1. Foundation damage survey after the 1999 Chi-Chi earthquake (NCREE, 2000

Evaluation of Liquefaction Potential

The evaluation of liquefaction potential of soils at any site requires parameters pertaining to: cyclic loads due to an earthquake and soil properties which describe the soil resistance under those loads.

Normal Field Conditions Where σv’ = effective vertical stress K0= at-rest earth pressure coefficient K0σv’ = effective horizontal stress

During Earthquake

Two tests can be used to simulate field stress conditions Cyclic direct shear test Cyclic triaxial test

Cyclic Direct Shear Test

Cyclic Triaxial Test

Relation between cyclic direct shear and cyclic triaxial test (τh/σv) direct shear = Cr (1/2 x σd/σ3’ )triaxial where; τh = horizontal shear stress (τh/σv) = cyclic stress ratio CSR σv = vertical stress σd = deviator stress σ3’ = effective confining pressure Cr = Correction faactor obtained from figure given on next slide

If relative density in lab is different from field then the equation is modified as follows: (τavg/σv’)= Cr(1/2 x σd/σ3’)triaxial at RD1 x RD2/RD1 Where RD1 is relative density in lab and RD2 is relative density in field

Generally cyclic triaxial test is conducted at various cyclic stress ratios CSR = (1/2 x σd/σ3’) on undisturbed or remolded specimen till liquefaction occurs, and corresponding number of stress cycles is determined. A graph is plotted between CSR and number of stress cycles.

This graph can be used to read out CSR corresponding to any number of stress cycles and this value is used in following relationship to determine shear resistance that will be mobilized at any depth. (τavg/σv’)= Cr(1/2 x σd/σ3’)triaxial at RD1 x RD2/RD1

If cyclic tiaxial testing can not be conducted then this Graph can be used to determine CSR from Mean grain Size D 50

Results of Standard Penetration Test can also be used to determine CSR from this curve. Subsequently shear resistance of soil against cyclic loading can be determined by:    = CSR x σv ‘ Where, σv‘ is effective vertical stress

DETERMINATION OF SHEAR STRESSES INDUCED BY CERTAIN EARTHQUAKE IN THE FIELD BY SIMPLIFIED PROCEDURE

τ = shear stress induced during an earthquake Since soil prism is assumed to be a rigid body therefore a correction factor “rD” must be applied as soil is not rigid. τ = rD (h amax )/g Where, τ = shear stress induced during an earthquake  = unit weight of soil. amax = maximum acceleration due to earthquake g = acceleration due to gravity h = height of soil prism rD = stress reduction factor , a function of depth of point being analyzed. It can be obtained from next slide

For an actual earthquake event. Acceleration v/s time relationship For an actual earthquake event Acceleration v/s time relationship (accelerogram) looks like

During an earthquake the induced cyclic shear stresses vary with time During an earthquake the induced cyclic shear stresses vary with time. On the contrary in the laboratory shear test the specimen is subjected to a uniform cyclic shear stress. To incorporate this effect a multiplication factor of 0.65 has been suggested.

Seed et al have recommended a weighted procedure to derive the number of uniform stress cycles Neq (at an amplitude of 65% of the peak cyclic shear stresses i.e. τcyc=0.65 τmax) from recorded strong ground motion

This Table can be used to determine This Table can be used to determine equivalent number of stress cycles for an earthquake of certain magnitude.

τ = shear stress induced during an earthquake The effect of non uniform stress cycles is incorporated by determining equivalent number of stress cycles for an earthquake and shear stresses induced during an earthquake are computed by the following equation: τ = 0.65 rD (h amax )/g Where, τ = shear stress induced during an earthquake  = unit weight of soil. amax = maximum acceleration due to earthquake g = acceleration due to gravity h = height of soil prism rD = stress reduction factor , a function of depth of point being analyzed. It can be obtained from next slide

Maps like these Can be used to Determine max Ground acceleration

After determining the cyclic shear stresses induced by an earthquake and the shear resistance mobilized at the point under consideration, a graph is plotted between depth and the stresses determined above.

If induced cyclic shear stresses are more than shear resistance mobilized, liquefaction will occur.

RESEARCH ON KAMRA SAND

Soil Stratification developed after SPT and Boring

Compacted Earth Fill SAND LAYER 0.5 m SILT LAYER

Sampling being done in Test Pit

RELATIVE DENSITY DETERMINATION AT Vibrating Table for relative density CMTL WAPDA LAHORE Vibrating Table for relative density Mould for relative density Lab Relative Density =53 % Relative Density From SPT correlations =52.8 %

EVALUATION OF LIQUEFACTION

SEISMICITY OF KAMRA CITY

PHA at Kamra = 0.24 g

Sr. No Fault Name Length (km) Distance From Kamra (km) Magnitude of earthquake From equation logL=1.02M – 5.77 1 Khairabad Fault 370 3 8.2

It is concluded that an earthquake of Magnitude 7 can occur at Kamra with peak horizontal acceleration of 0.24 g

Evaluation of Liquefaction potential Standard Penetration Test (SPT) Cyclic Triaxial Test.

Hypothesis If water table rises and sand gets saturated then liquefaction will occur under magnitude 7 earthquake

τr (KN / m2 ) Evaluation Of Liquefaction On the basis of SPT Point Depth (m) Shear stress mobilized in field τ avg (KN/m2) Shear Resistance τr (KN / m2 ) Remarks A 1.50 4.17 3.24 τavg > τr (Liquefaction will occur) B 1.75 4.89 C 2.00 5.58 4.13 τ = 0.65 rD (h amax )/g  = CSR x σv ‘

ANALYSIS ON THE BASIS OF CYCLIC TRIAXIAL TEST. Analysis on the basis of triaxial was based on the method proposed by SEED AND IDRIS Shear resistance was computed from the following formula (τ(τavg/σv’)= Cr(1/2 x σd/σ3’)triaxial at RD1 x RD2/RD1 Cr(1/2 x σd / σ3’ )triaxial x RD2/RD1 τh = Cr(1/2 x σd / σ3’ ) x σv’ x RD2/RD1

0.57

0.255

Analysis By Cyclic Triaxial Test point Depth (m) Shear stress mobilized in field τ avg (KN/m2) Shear resistance by Triaxial τr (KN / m2 ) Remarks A 1.50 4.17 4.08 τavg > τr (Liquefaction will occur) B 1.75 4.89 4.46 C 2.00 5.58 5.20 (τavg/σv’)=Cr(1/2 x σd/σ3’)triaxial at RD1 x RD2/RD1 τ = 0.65 rD (h amax )/g

It is concluded on the basis of these results that the sand will liquefy under the event of an earthquake of Magnitude 7.

HOW CAN LIQUIFACTION HAZARDS BE REDUCED? REMEDIATION HOW CAN LIQUIFACTION HAZARDS BE REDUCED?

Avoid Liquefaction Susceptible Soils Build Liquefaction Resistant Structures Improve the Soil

Avoid Liquefaction Susceptible Soils

historical Criteria Soils that have liquefied in the past can liquefy again in future earthquakes. Geological Criteria Saturated soil deposits that have been created by sedimentation in rivers and lakes deposition of debris or eroded material or deposits formed by wind action can be very liquefaction susceptible. Man-made soil deposits, particularly those created by the process of hydraulic filling

Compositional Criteria D10 sizes ranging from 0.05 to 1.0 mm AND a coefficient of uniformity ranging from 2 to 10. Uniformly graded soil deposits Angularity of particles Silty soils are susceptible to liquefaction if they satisfy the criteria given below.  Fraction finer than 0.005 mm< 15% Liquid Limit, LL < 35%  Natural water content > 0.9 LL  Liquidity Index < 0.75

State Criteria Relative density, Dr Increasing confining pressure

Build Liquefaction Resistant Structures HOW CAN LIQUIFACTION HAZARDS BE REDUCED? Build Liquefaction Resistant Structures

Build Liquefaction Resistant Structures It is important that all foundation elements in a shallow foundation are tied together to make the foundation move or settle uniformly, thus decreasing the amount of shear forces induced in the structural elements resting upon the foundation.

Build Liquefaction Resistant Structures A stiff foundation mat is a good type of shallow foundation, which can transfer loads from locally liquefied zones to adjacent stronger ground.

Build Liquefaction Resistant Structures Buried utilities, such as sewage and water pipes, should have ductile connections to the structure to accommodate the large movements and settlements that can occur due to liquefaction. The pipes in the photo connected the two buildings in a straight line before the earthquake

Build Liquefaction Resistant Structures

HOW CAN LIQUIFACTION HAZARDS BE REDUCED? Improve the Soil

Vibroflotation

Vibroflotation

Improve the Soil Dynamic Compaction

Stone Columns Generally, the stone column ground improvement method is used to treat soils where fines content exceeds that acceptable for vibrocompaction

Compaction Piles

Compaction Grouting Compaction grouting is a ground treatment technique that involves injection of a thick-consistency soil-cement grout under pressure into the soil mass, consolidating, and thereby densifying surrounding soils in-place.  The injected grout mass occupies void space created by pressure-densification.  Pump pressure, as transmitted through low-mobility grout, produces compaction by displacing soil at depth until resisted by the weight of overlying soils.

Improve the Soil Drainage techniques

Improve the Soil Drainage techniques

Improve the Soil

Verification of Improvement Verification of Improvement A number of methods can be used to verify the effectiveness of soil improvement. In-situ techniques are popular because of the limitations of many laboratory techniques. Usually, in-situ test are performed to evaluate the liquefaction potential of a soil deposit before the improvement was attempted. With the knowledge of the existing ground characteristics, one can then specify a necessary level of improvement in terms of insitu test parameters.

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