SOIL LIQUEFACTION: PHENOMENON, HAZARDS , REMEDIATION

Dr. Farhat JavedAssociate 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 siltsDuring 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.

GLASS CONTAINERSATURATED SANDSTRUCTURE

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) = - u

Shear strength of soil is given as : = c + tan 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. This box represents magnitude of pore water pressureAssemblage of particles

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 Evidences1964 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.

1995 Kobe earthquake, Japan 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.

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. 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)

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 v = effective vertical stress

K0= at-rest earth pressure coefficient K0v = 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

(h/v) direct shear = Cr (1/2 x d/3 )triaxial where; h = horizontal shear stress (h/v) = cyclic stress ratio CSR v = vertical stress d = deviator stress 3 = effective confining pressureCr = 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:

(avg/v)= Cr(1/2 x d/3)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 d/3) 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.

(avg/v)= Cr(1/2 x d/3)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 v Where,v is effective vertical stress

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

Since soil prism is assumed to be a rigid body therefore a correction factor rD must be applied as soil is not rigid. = rD (h amax )/g Where, =shear stress induced during an earthquake = unit weight of soil.amax=maximum acceleration due to earthquakeg=acceleration due to gravityh=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 (accelerogram) looks like

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. cyc=0.65 max) from recorded strong ground motion

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

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: = 0.65 rD (h amax )/g Where, =shear stress induced during an earthquake = unit weight of soil.amax=maximum acceleration due to earthquakeg=acceleration due to gravityh=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 toDetermine maxGround 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 FillSAND LAYERSILT LAYER0.5 m

Sampling being done in Test Pit

Vibrating Table for relative densityMould for relative densityLab Relative Density =53 %Relative Density From SPT correlations =52.8 % RELATIVE DENSITY DETERMINATION ATCMTL WAPDA LAHORE

EVALUATION OF LIQUEFACTION

SEISMICITY OF KAMRA CITY

PHA at Kamra = 0.24 g

Sr. NoFault NameLength (km)Distance From Kamra(km)Magnitudeof earthquake From equationlogL=1.02M 5.771Khairabad Fault37038.2

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

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

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

Evaluation Of Liquefaction On the basis of SPT = 0.65 rD (h amax )/g = CSR x v

PointDepth(m)Shear stress mobilized in field avg (KN/m2)Shear Resistance r (KN / m2 )RemarksA1.504.173.24avg > r (Liquefaction will occur)B1.754.893.24avg > r (Liquefaction will occur)C2.005.584.13avg > r (Liquefaction will occur)

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((avg/v)= Cr(1/2 x d/3)triaxial at RD1 x RD2/RD1 Cr(1/2 x d / 3 )triaxial x RD2/RD1 h = Cr(1/2 x d / 3 ) x v x RD2/RD1

0.57

0.255

Analysis By Cyclic Triaxial Test = 0.65 rD (h amax )/g(avg/v)=Cr(1/2 x d/3)triaxial at RD1 x RD2/RD1

pointDepth(m)Shear stress mobilized in field avg (KN/m2)

Shear resistance by Triaxialr (KN / m2 )RemarksA1.504.174.08avg > r (Liquefaction will occur)B1.754.894.46avg > r (Liquefaction will occur)C2.005.585.20avg > r (Liquefaction will occur)

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

REMEDIATIONHOW CAN LIQUIFACTION HAZARDS BE REDUCED?

Avoid Liquefaction Susceptible Soils Build Liquefaction Resistant StructuresImprove the Soil

Avoid Liquefaction Susceptible Soils

historical CriteriaSoils 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 CriteriaD10 sizes ranging from 0.05 to 1.0 mm ANDa coefficient of uniformity ranging from 2 to 10. Uniformly graded soil depositsAngularity 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 StructuresIt 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 StructuresA 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 StructuresBuried 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

Improve the Soil

HOW CAN LIQUIFACTION HAZARDS BE REDUCED?

Vibroflotation

Vibroflotation

Improve the Soil

Dynamic Compaction

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

Compaction Piles

Compaction GroutingCompaction 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 ImprovementA 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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