LIQUEFACTION

INTRODUCTION

Liquefaction is a phenomenon in which the strength and stiffness of a soil is reduced by earthquake shaking or other rapid loading. Liquefaction and related phenomena have been responsible for tremendous amounts of damage in historical earthquakes around the world.

Soil liquefaction describes the behavior of soils that, when loaded, suddenly go from a solid state to a liquefied state, or having the consistency of a heavy liquid. Liquefaction is more likely to occur in loose to moderate saturated granular soils with poor drainage, such as silty sands or sands and gravels capped or containing seams of impermeable sediments

Liquefaction occurs in saturated soils, that is, soils in which the space between individual particles is completely filled with water. This water exerts a pressure on the soil particles that influences how tightly the particles themselves are pressed together. During loading, usually cyclic undrained loading, e.g. earthquake loading, loose sands tend to decrease in volume, which produces an increase in their pore water pressures and consequently a decrease in shear strength, i.e. reduction in effective stress.

Deposits most susceptible to liquefaction are young (Holocene-age, deposited within the last 10,000 years) sands and silt of similar grain size (well-sorted), in beds at least several feet thick, and saturated with water. Such deposits are often found along riverbeds, beaches, dunes, and areas where windblown silt (loess) and sand have accumulated. 

 Liquefaction has been observed in earthquakes for many years. In fact, written records dating back hundreds and even thousands of years describe earthquake effects that are now known to be associated with liquefaction. Nevertheless, liquefaction has been so widespread in a number of recent earthquakes that it is often associated with them. Some of those earthquakes are Alaska in USA (1964), Niigata in Japan (1964), Loma Preita in USA (1989) and Kobe in Japan (1995).

Although the effects of liquefaction have been long understood, it was more thoroughly brought to the attention of engineers and seismologists in the 1964 Niigata, Japan and Alaska earthquakes. It was also a major factor in the destruction in San Francisco's Marina District during the 1989 Loma Preita earthquake.

LIQUEFACTION

Liquefaction, as mentioned before, is a phenomenon in which the strength and stiffness of a soil is reduced by earthquake shaking or other rapid loading. Liquefaction and related phenomena have been responsible for tremendous amounts of damage in historical earthquakes around the world.

 Liquefaction occurs in saturated soils, that is, soils in which the space between individual particles is completely filled with water. This water exerts a pressure on the soil particles that influences how tightly the particles themselves are pressed together. Prior to an earthquake, the water pressure is relatively low. However, earthquake shaking can cause the water pressure to increase to the point where the soil particles can readily move with respect to each other.

Earthquake shaking often triggers this increase in water pressure, but construction related activities such as blasting can also cause an increase in water pressure. When liquefaction occurs, the strength of the soil decreases and, the ability of a soil deposit to support foundations for buildings and bridges are reduced as seen in the photo of the overturned apartment complex buildings in Niigata in 1964.

Niigata earthquake, Japan, 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. 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. Fortunately, the dam barely avoided collapse, thereby preventing a potential disaster of flooding of the heavily populated areas below the dam.

Lower San Fernando dam failure, USA, 1971

EXAMPLES OF LIQUEFACTION

Earthquake Liquefaction

Earthquake liquefaction is a major contributor to urban seismic risks. The shaking causes increased pore water pressure which reduces the effective stress, and therefore reduces the shear strength of the sand. If there is a dry soil crust or impermeable cap, the excess water will sometimes come to the surface through cracks in the confining layer, bringing liquefied sand with it, creating sand boils, colloquially called "sand volcanoes".

Quicksand

Quicksand forms when water saturates an area of loose sand and the ordinary sand is agitated. When the water trapped in the batch of sand cannot escape, it creates liquefied soil that can no longer support weight. Quicksand can be formed by standing or (upwards) flowing underground water (as from an underground spring), or by earthquakes. In the case of flowing underground water, the force of the water flow opposes the force of gravity, causing the granules of sand to be more buoyant. In the case of earthquakes, the shaking force can increase the pressure of shallow groundwater, liquefying sand and silt deposits. In both cases, the liquefied surface loses strength, causing buildings or other objects on that surface to sink or fall over.

The saturated sediment may appear quite solid until a change in pressure or shock initiates the liquefaction causing the sand to form a suspension with each grain surrounded by a thin film of water. This cushioning gives quicksand and other liquefied sediments, a spongy and fluid like texture. Objects in the liquefied sand sink to the level at which the weight of the object is equal to the weight of the displaced sand/water mix and the object floats due to its buoyancy

Quick Clay

Quick clay, also known as Leda Clay in Canada, is a unique form of highly sensitive clay, with the tendency to change from a relatively stiff condition to a liquid mass when it is disturbed. Undisturbed quick clay resembles a water-saturated gel. When a block of clay is held in the hand and struck, however, it instantly turns into flowing ooze, a process known as spontaneous liquefaction. Quick clay behaves this way because, although it is solid, it has very high water content, up to 80%. The clay retains a solid structure despite the high water content, because surface tension holds water-coated flakes of clay together in a delicate structure. When the structure is broken by a shock, it reverts to a fluid state. 

Quick clay is only found in the northern countries such as Russia, Canada, and Alaska in the U.S., Norway, Sweden and Finland. Quick clay has been the underlying cause of many deadly landslides. In Canada alone, it has been associated with more than 250 mapped landslides. Some of these are ancient, and may have been triggered by earthquakes.

Turbidity Currents

Submarine landslides are turbidity currents and consist of water saturated sediments flowing down slope. An example occurred during the 1929 Grand Banks earthquake that struck the continental slope off the coast of Newfoundland. Minutes later, transatlantic telephone cables began breaking sequentially, farther and farther down slope, away from the epicenter. Twelve cables were snapped in a total of 28 places. Exact times and locations were recorded for each break. Investigators suggested that a 60-mile-per-hour (100 km/h) submarine landslide or turbidity current of water saturated sediments swept 400 miles (600 km) down the continental slope from the earthquake's epicenter, snapping the cables as it passed. 

OCCURRENCE OF LIQUEFACTION

     The term liquefaction has actually been used to describe a number of related phenomena. Because the phenomena can have similar effects, it can be difficult to distinguish between them. The mechanisms causing them, however, are different. These phenomena can be divided into two main categories: 

1. Flow liquefaction and 
2. Cyclic mobility. 

Flow liquefaction    

Flow liquefaction is a phenomenon in which the static equilibrium is destroyed by static or dynamic loads in a soil deposit with low residual strength. Residual strength is the strength of a liquefied soil. Static loading, for example, can be applied by new buildings on a slope that exert additional forces on the soil beneath the foundations. Earthquakes, blasting, and pile driving are all example of dynamic loads that could trigger flow liquefaction. Once triggered, the strength of a soil susceptible to flow liquefaction is no longer sufficient to withstand the static stresses that were acting on the soil before the disturbance. 

An analogy can be seen in the picture above, where the static stability of a ski jumper in the starting gate is disturbed when the jumper pushes himself from the start seat. After this relatively small disturbance, the static driving force caused by gravity, being greater than the frictional resisting force between the ski and snow, causes the skier to accelerate down the ramp. The path that brings the ski jumper to an unstable state is analogous to the static or dynamic disturbance that triggers flow liquefaction - in both cases, a relatively small disturbance is followed by an instability that allows gravity to take over and produce large, rapid movements.

Failures caused by flow liquefaction are often characterized by large and rapid movements which can produce the type of disastrous effects experienced by the Kawagishi-cho apartment buildings, which suffered a remarkable bearing capacity failure during the Niigata earthquake in 1964. 

 

The Turnagain Heights landslide, Alaska earthquake in 1964 which is thought to be triggered by liquefaction of sand lenses in the 130-acre slide area provides another example of flow liquefaction.

  

Alaska earthquake, USA, 1964

Sheffield Dam suffered a flow failure triggered by the Santa Barbara Earthquake in 1925. A 300 ft section (of the 720 feet long dam) moved as much as 100 ft downstream. The dam consisted mainly of silty sands and sandy silts excavated from the reservoir and compacted by routing construction equipment over the fill.

  
                  Sheffield Dam, Santa Barbara Earthquake in 1925

As these case histories illustrate, flow failures, can involve the flow of considerable volumes of material, which undergoes very large movements that are actually driven by static stresses.

Cyclic Mobility

 Cyclic mobility is a liquefaction phenomenon, triggered by cyclic loading, occurring in soil deposits with static shear stresses lower than the soil strength. Deformations due to cyclic mobility develop incrementally because of static and dynamic stresses that exist during an earthquake. Lateral spreading, a common result of cyclic mobility, can occur on gently sloping and on flat ground close to rivers and lakes. The 1976 Guatemala earthquake caused lateral spreading along the Motagua River. 

  

Guatemala earthquake in 1976

     On level ground, the high pore water pressure caused by liquefaction can cause pore water to flow rapidly to the ground surface. This flow can occur both during and after an earthquake. If the flowing pore water rises quickly enough, it can carry sand particles through cracks up to the surface, where they are deposited in the form of sand volcanoes or sand boils. These features can often be observed at sites that have been affected by liquefaction, such as in the field along Hwy 98 during the 1979 El Centro earthquake shown below.

El Centro earthquake in 1979 

MECHANISM OF LIQUEFACTION

To understand liquefaction, it is important to recognize the conditions that exist in a soil deposit before an earthquake. A soil deposit consists of an assemblage of individual soil particles. If we look closely at these particles, we can see that each particle is in contact with a number of neighboring particles. The weight of the overlying soil particles produce contact forces between the particles - these forces hold individual particles in place and give the soil its strength.

Soil grains in a soil deposit. The height of the blue column to the right represents the level of pore water pressure in the soil. 

The length of the arrows represents the size of the contact forces between individual soil grains. The contact forces are large when the pore water pressure is low. The length of the arrows represents the size of the contact forces between individual soil grains. The contact forces are large when the pore water pressure is low.

Liquefaction occurs when the structure of loose, saturated sand breaks down due to some rapidly applied loading. As the structure breaks down, the loosely-packed individual soil particles attempt to move into a denser configuration. In an earthquake, however, there is not enough time for the water in the pores of the soil to be squeezed out. Instead, the water is "trapped" and prevents the soil particles from moving closer together. This is accompanied by an increase in water pressure which reduces the contact forces between the individual soil particles, thereby softening and weakening the soil deposit.

 The contact forces are small because of the high water pressure. In an extreme case, the pore water pressure may become so high that many of the soil particles lose contact with each other. In such cases, the soil will have very little strength, and will behave more like a liquid than a solid - hence, the name "liquefaction". 

To understand how soil liquefaction is initiated, some basic soil mechanics concepts are important. They are briefly described below. 

Critical Void Ratio      

In 1936, Dr. Arthur Casagrande performed a series of drained strain-controlled triaxial tests and discovered that initially loose and dense specimens at the same confining pressure approached the same density when sheared to large strains. The void ratio corresponding to this density was called the critical void ratio (e_c).

 

Behavior of dense and loose soils in monotonic strain controlled triaxial tests

Performing tests at various effective confining pressures, Casagrande found that the critical void ratio varied with effective confining pressure. Plotting these on a graph produced a curve which is referred to as the Critical Void Ratio (CVR) line. The CVR line constituted the boundary between dilative and contractive behavior in drained triaxial compression. A soil that plots above the CVR line exhibits contractive behavior and vice versa (figure below).

CVR-line for arithmetic and logarithmic confining pressure.

 Steady State of Deformation

 In the mid-1960s, Gonzalo Castro, a student of Casagrande, performed an important series of undrained, stress-controlled triaxial tests. Castro observed three different types of stress-strain behavior depending upon the soil state. Dense specimens initially contracted but then dilated with increasing effective confining pressure and shear stress. Very loose samples collapsed at a small shear strain level and failed rapidly with large strains. Castro called this behavior "liquefaction" - it is also commonly referred to as flow liquefaction. Medium dense soils initially showed the same behavior as the loose samples but, after initially exhibiting contractive behavior, the soil "transformed" and began exhibiting dilative behavior. Castro referred to this type of behavior as "limited liquefaction".

Static triaxial test stress paths for three specimens of different densities.

Castro plotted the relationship (figure below) between effective confining pressure and void ratio at large strains for these undrained, stress-controlled tests. Castro referred to the curved produced by this plot, which is similar to the CVR line for the drained strain controlled tests performed by Casagrande, as the Steady State Line (SSL). The difference between the CVR and SSL was attributed to the existence of what Casagrande called a "flow structure", in which the grains orient themselves so the least amount of energy is lost by frictional resistance during flow.

 

As seen above, the SSL is actually a 3-dimensional curve in e- '- space. Using the 2-D projection on the e-' plane (figure above), one can determine if a soil is susceptible to flow liquefaction. Soils in an initial state that plots below the SSL are not susceptible to flow liquefaction whereas soils plotting above the SSL are susceptible to flow liquefaction - if (and only if) the static shear stress exceeds the residual strength of the soil. Cyclic mobility, another liquefaction-related phenomenon, can occur in dense as well as loose soils.

 

 

Figure showing zones of flow liquefaction and cyclic mobility susceptibility.

 

Flow Liquefaction

      On the left below is a plot of stress paths for five undrained shear tests. Three test specimens (C, D, and E) were subjected to loads greater than their residual strengths, and experienced flow liquefaction. A straight line (shown in red in the figure) drawn through the points where flow liquefaction was initiated projects back through the origin. This line is called the Flow Liquefaction Surface (FLS). Since flow liquefaction cannot take place if the static shear stress is lower than the steady state strength, the FLS is truncated by a horizontal line through the steady state point (right figure below).The steady state strength is the strength a soil has when undergoing a steady state of deformation, i.e. continuous flow under constant shear stress and constant effective confining pressure at constant volume and constant velocity. Flow liquefaction will be initiated if the stress path crosses the FLS during undrained shear regardless of whether the loading is cyclic or monotonic loading.

Graphical explanation of Flow Liquefaction Surface.

 

Cyclic Mobility

     Cyclic mobility can occur even when the static shear stress is lower than the steady state (or residual) shears strength. The geotechnical engineering profession's understanding of cyclic mobility has advanced greatly within the past 10 years or so.
A key to this understanding came about with identification of the phase transformation line. Medium dense to dense sands subjected to monotonic loading will initially exhibit contractive behavior, but then exhibit dilative behavior as they strain toward the steady state. A plot of the stress path points at which the transformation from contractive to dilative behavior takes place reveals a Phase Transformation Line (PTL) that appears to project back through the origin.

 

A p'-q plot of the phase transformation line

 

In the contractive region, an undrained stress path will tend to move to the left as the tendency for contraction causes pore pressure to increase and p' to decrease. As the stress path approaches the PTL, the tendency for contraction reduces and the stress path becomes more vertical. When the stress path reaches the PTL, there is no tendency for contraction or dilation, hence p' is constant and the stress path is vertical. After the stress path crosses the PTL, the tendency for dilation causes the pore pressure to decrease and p' to increase, and the stress path moves to the right.

 

A stress path example.

 

Note that, because the stiffness of the soil depends on p', the stiffness decreases (while the stress path is below the PTL {indicated by the blue line}) but then increases (when the stress path moves above the PTL). This change in stiffness produces the "limited liquefaction" behavior originally noted by Castro.

Under cyclic loading conditions, the behavior becomes even more complex. Remembering that the failure envelope and PTL exist for negative shear stresses as well as positive, it is easy to see that a cyclically loaded soil can undergo the contraction/dilation transformation in two different directions. The stress-strain and stress path plots for a harmonically loaded element of soil will therefore show softening behavior in the early stages of loading (before the stress path has reached the PTL) but then show cyclic softening and hardening as the stress path moves from one side of the PTL to the other. 

 

EVALUATION OF THE LIQUEFACTION POTENTIAL

 

Evaluation of the potential for liquefaction to occur is accomplished by comparing equivalent measures of earthquake loading and liquefaction resistance. The most common approach to characterization of earthquake loading is through the use of cyclic shear stresses. By normalizing the cyclic shear stress amplitude by the initial effective vertical stress, a cyclic stress ratio (CSR) can represent the level of loading induced at different depths in a soil profile by an earthquake. There are different procedures for evaluating the cyclic shear stresses - site response analyses may be performed or a "simplified" approach may be used to estimate CSR as a function of peak ground surface acceleration amplitude.

CSR versus N or q_c

Liquefaction resistance is most commonly characterized on the basis of observed field performance. Detailed investigation of actual earthquake case histories has allowed determination of the combinations of insitu properties (usually SPT or CPT resistance) and CSR for each case history. By plotting the CSR-(N1)60 (or CSR-qc) pairs for cases in which liquefaction was and was not been observed, a curve that bounds the conditions
at which liquefaction has historically been observed can be drawn. This curve, when interpreted as the maximum CSR for which liquefaction of a soil with a given penetration resistance can resist liquefaction, can be thought of as a curve of Cyclic Resistance Ratio (CRR). Then, the potential for liquefaction can be evaluated by comparing the earthquake loading (CSR) with the liquefaction resistance (CRR) - this is usually expressed as a factor of safety against liquefaction, 

 

FS = CRR / CSR

 

A factor of safety greater than one indicates that the liquefaction resistance exceeds the earthquake loading, and therefore that liquefaction would not be expected.

FACTORS AFFECTING LIQUEFACTION

 
 

The factors affecting liquefaction are as follows:

• Grain Size
• Initial Relative Density
• Vibration characteristics
• Influence of effective stress
• Period of loading
• Previous strain history
• Drainage and Deposits
• Trapped air

1. Grain Size
   Fine and uniform sands are more prone to liquefaction than coarse grained ones.
   
    
2. Initial Relative Density
   As initial density increases, the chances of liquefaction are reduced.
   
    
3. Vibration characteristics
   Liquefaction increases rapidly with the occurrence of vibrations. 

1. Influence of effective stress
   Higher the initial confining pressure, lower will be the liquefaction potential.
   
    
2. Period of loading
   Sand deposits which are undisturbed for longer durations are resistant to liquefaction.
   
    
3. Previous strain history
   If a soil has previously been subjected to stress, the chances of liquefaction are increased.
   
    
4. Drainage and deposits
   If the drainage facility provided for the soil is more, then the chances of liquefaction are less.
   
    
5. Trapped air
   Excess pore pressure gets dissipated in the trapped void spaces in the soil, hence reducing liquefaction. 

HAZARDS DUE TO LIQUEFACTION

     

Because liquefaction only occurs in saturated soil, its effects are most commonly observed in low-lying areas near bodies of water such as rivers, lakes, bays, and oceans. The effects of liquefaction may include major sliding of soil toward the body slumping and of water, as in the case of the 1957 Lake Merced slide shown below, or more modest movements that produce tension cracks such as those on the banks of the Motagua River following the 1976 Guatemala Earthquake.

Motagua River during Guatemala earthquake

Port and wharf facilities are often located in areas susceptible to liquefaction, and many have been damaged by liquefaction in past earthquakes. Most ports and wharves have major retaining structures, or quay walls, to allow large ships to moor adjacent to flat cargo handling areas. When the soil behind and/or beneath such a wall liquefies, the pressure it exerts on the wall can increase greatly - enough to cause the wall to slide and/or tilt toward the water. As illustrated below, liquefaction caused major damage to port facilities in Kobe, Japan in the 1995 Hyogo-ken Nanbu earthquake.

Lateral displacement of a quay wall and lateral spreading of pavement surface in Port Island, Kobe 1995

Liquefaction also frequently causes damage to bridges that cross rivers and other bodies of water. Such damage can have drastic consequences, impeding emergency response and rescue operations in the short term and causing significant economic loss from business disruption in the longer term. 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 as shown in the pictures below.

Collapse of bridge in Kobe, Japan

 THE INDIAN SCENARIO

The Bhuj earthquake brought to India the disastrous effects of liquefaction. As seen in the pictures below, there was a lot of destruction caused to roads and railway embankments due to the lateral spreading of soil. 

Damage to the Malya-Bhuj highway due to lateral spreading

 

Formation of craters and damage to railway embankments

 

PREVENTION OF LIQUEFACTION

 There are basically three possibilities to reduce liquefaction hazards when designing and constructing new buildings or other structures as bridges, tunnels, and roads. 

• Avoid liquefaction susceptible soils
• Build liquefaction resistant structures
• Improvement of soil 

Avoid liquefaction susceptible soils

 The first possibility is to avoid construction on liquefaction susceptible soils. There are various criteria to determine the liquefaction susceptibility of a soil. By characterizing the soil at a particular building site according to these criteria one can decide if the site is susceptible to liquefaction and therefore unsuitable for the desired structure. 

Build liquefaction resistant structures

If it is necessary to construct on liquefaction susceptible soil because of space restrictions, favorable location, or other reasons, it may be possible to make the structure liquefaction resistant by designing the foundation elements to resist the effects of liquefaction. 

Improvement of soil

The third option involves mitigation of the liquefaction hazards by improving the strength, density, and/or drainage characteristics of the soil. This can be done using a variety of soil improvement techniques.

CONCLUSION

Liquefaction can be very dangerous if proper mitigation measures are not taken to prevent it. Fortunately, areas susceptible to liquefaction can be readily identified and the hazard can often be mitigated. Because of the relative ease of identifying hazardous areas, numerous liquefaction maps have been made by government agencies. Liquefiable sediments are young, loose; water saturated, well sorted and is either fine sands or silts. Before constructing any structure, a thorough study of the adverse affects of liquefaction is very essential.It is always advisable to evaluate the liquefaction potential of the soil deposit before undertaking any construction.And last but not the least; it is always safer to construct a liquefaction resistant structure to counteract the effects of liquefaction.

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