Liquefaction

Liquefaction

Liquefaction, the term was originally coined by Mogami

and Kubo(1953) describes the behavior of loose saturated cohesionless soils, i.e.

loose sands, which transforms from a solid state to have the consistency of a

heavy liquid, or reach a liquefied state as a consequence of increasing pore

water pressures, and thus decreasing effective stress, induced by their tendency

to decrease in volume when subjected to cyclic undrained loading like

earthquake. The generation of excess pore pressure under undrained loading

condition is a hallmark of all liquefaction phenomena or simply saying

liquefaction is more likely to occur in loose to moderate granular soils with poor

drainage, such as silty sands or sands and gravels capped or containing seams of

impermeable sediments. Some examples of liquefaction include quicksand,

quick clay, turbidity currents, and earthquake liquefaction.

Liquefaction phenomena that results from this process can be

divided into two main groups.

1) Flow liquefaction

2) Cyclic mobility

These two are very important, and any evaluation of liquefaction hazards should

be carefully consider both.

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Earthquake Engineering Seminar

http://en.wikipedia.org/wiki/Earthquake_liquefaction

http://en.wikipedia.org/wiki/Turbidity_current

http://en.wikipedia.org/wiki/Quick_clay

http://en.wikipedia.org/wiki/Quicksand

http://en.wikipedia.org/wiki/Effective_stress

Flow Liquefaction

Flow liquefaction produces the most dramatic effects of all

liquefaction related phenomena, causing tremendous instabilities known as flow

failures. It occurs when the static shear stress exceeds the shear strength of the

soil in liquefied state. Once triggered, large deformations produced by flow

liquefaction will be driven by static shear stress and the cyclic stress like

earthquakes which will push the soil to unstable state where soil looses its shear

strength. Flow liquefaction failures can be charterized by the sudden nature of

their origin, speed with which it develop and large distance over which the

liquefied material moves.

Cyclic Mobility

Cyclic Mobility is another phenomenon that also produces

large permanent deformations during earthquake shaking. Here in this case

static shear stress is less than shear strength of liquefied soil but deformations

are caused by cyclic mobility failure develop incrementally during earthquake

shaking. These deformations are termed as lateral spreading can occur on very

gentle slope or flat ground surface near to water bodies.

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Liquefaction susceptibility

All soils are not susceptible to liquefaction, thus the soil at

the construction site must be checked for liquefaction susceptibility. There are

several factors that affect the liquefaction susceptibility which are

1) Historical criteria

2) Geological criteria

3) Compositional criteria

4) State criteria

Historical criteria

From past earthquake studies and field investigations, it is

observed that liquefaction recurs at same location when soil and ground water

conditions remain unchanged. Thus liquefaction case histories can be used to

identify the specific sites or general site conditions that may susceptible to

liquefaction. Post earthquake investigation investigations also suggest that

liquefaction effects were pertain to a zone which is at a particular distance from

seismic source. Distance to which liquefaction can be expected increases

dramatically with increasing magnitude as shown graphically below.

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(Relationship between limiting epicentral

distance of site at which liquefaction occurs and moment magnitude for shallow earthquake)

Geological criteria

Soil deposits that are susceptible to liquefaction are formed

within a narrow range of geological environment. Liquefaction susceptibility is

influenced by

1) Depositional environment

2) Hydrological environmental

3) Age of soil deposit

Geological process that sorts soil into uniform particle size distribution and

deposit them in loose state, produces soil deposit with high liquefaction

susceptibility. Liquefaction is also affected by depth of ground water table. If

the ground water table is at larger depth, liquefaction susceptibility will be less.

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Compositional criteria

Since liquefaction requires the development of excess pore pressure,

liquefaction susceptibility is influenced by the compositional characteristics that

influence volume change behavior. Characteristics associated with high volume

change potential like grain size, shape, distribution will result will result in high

liquefaction susceptibility. Coarser silts with bulky particle shape that are non

plastic and cohesionless are fully susceptible to liquefaction. Fine grained soil

with following characteristics is generally susceptible to liquefaction.

Fraction finer than 0.005mm< (or equal) 15%

Liquid limit< (or equal to) 35%

Liquidity index < (or equal to) 0.75

Well graded soil are generally less susceptible to liquefaction since there void

will be less. Moreover particle shapes can also influence liquefaction. Round

shaped particular soils are generally less susceptibly against liquefaction

compared to angular grained.

State criteria

Liquefaction susceptibility also depends on the initial state of soil (i.e.

its stress and density characteristics at the time of the earthquake). This is

because the tendency to generate excess pore pressure of a particular soil is

strongly influenced by both density and initial state of stress condition.

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Critical void ratio

In 1936, Cassagrande, preformed many drained strain controlled triaxial test

on initially loose and initially dense sand specimen and following results were

obtained. All the soil samples are tested at same confining pressure and they

approach the same density when sheared at large strain, ie for loose soil, as

stress increased void ratio decrease and after reaching a particular level there

will be no increase in the void ratio for the stress increase. Similarly for dense

sand, at first void ratio decrease with stress increment and at large strain value

as stress increases void ratio will also try to increase till it reaches a particular

void ratio thereafter no increased will occur in void ratio. This void ratio

corresponding to constant density is called critical void ratio (ec). Positive

excess pore pressure will develop in loose sand while negative pore pressure

will develop in dense sand when subject stress.

Critical void ratio for a soil varies with confining

pressure, and locus of these points (ec corresponding to each confining pressure)

will give a curve called Critical void ratio (CRV) line.

These CRV line marks boundary between contractive and dilative behavior so

generally saturated soil with high initial void ratio plotted above the CRV line

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are susceptible to liquefaction, and soils below are generally not susceptible.

Steady state deformation

In 1969 Castro performed static and cyclic triaxial tests on

istropically consolidated specimen and several static tests on anisotropically

consolidated specimen. Very loose specimen (specimen A), peak strength is

attained at small shear strain and the sample collapsed suddenly which can be

referred to flow liquefaction. Dense specimen (Specimen B) initially contracts

but then after dilates until constant effective confining pressure and larege strain

is attained. An intermediate specimen (specimen C), the exceedance of a peak

strength at low starin, then limited period of strain softening behavior and end

with on a set dilation at intermediate strain. Now a graph is plotted with void

ratio against confining pressure at large strain which is roughly parallel to CRV

line obtained from drained strain controlled test, showing the development of

flow structure under stress controlled condition which is called steady state line

(SSL). The state in which the soil flowed continuously under constant effective

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confining pressure at constant volume and velocity was defined as steady state

of deformation; corresponding shear stress is called steady state strength.

Saturated soil falling above SSL will susceptible to liquefaction when static

shear stress exceeds steady state strength.

Initiation of Liquefaction

Soil susceptible to liquefaction does not simply undergo liquefaction when an

earthquake occurs. It requires a disturbance strong enough to initiate or trigger

it. Many previous studies of initiation of liquefaction have implicitly lumped

flow liquefaction and cyclic mobility together, but has to be studied separately.

Understanding the initiation of liquefaction requires identification of the state of

the soil when liquefaction is triggered.

Flow liquefaction surface

Liquefaction initiation can be effectively studied using stress path. The

effective stress condition at which initiation of flow liquefaction can be

described in a stress path space, by a 3D surface as flow liquefaction surface

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(FLS). Even it is difficult to measure, but it provides useful frame work for

understanding relationship between various liquefaction phenomena.

Monotonic loading

Initiation of flow liquefaction can be seen most easily when soil is subject to

monotonically increasing stress. As the stress increased, since the specimen is

loose saturated sand which has initial state well above the SSL, tends to show

the contractive behavior. When stress reaches the steady state strength value

(Point B), specimen tends to flow i.e. flow liquefaction takes place. Till point B

pore pressure increases at faster rate and then after the increment is slow. When

Steady strength state strength is reached, the specimen tends to flow to a steady

state of deformation i.e. point C. Here after increase in confining pressure there

will not significant variation in pore pressure.

Cyclic loading

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Liquefaction study by FLS can be applied to both cyclic and monotonic loading.

It’s said that the liquefaction to initiate, the effective stress path should cross

over FLS but in actual experiment it’s shown that stress path can go further

beyond for initiating liquefaction for both cyclic and monotonic loading. For

case of monotonic loading (under undrained condition) shearing resistance will

built up to a peak value when stress path reaches FLS (point B). There it

becomes unstable and strain to steady state of deformation (point C). Similarly

the specimen is cyclically loaded (under undrained condition) stress path moves

to point D on FLS and strains toward the steady state deformation point C. It

can be seen that for initiating liquefaction, required stress under cyclic load will

be less compared to monotonic loading even though point B and D are on FLS

Development of flow liquefaction

The flow liquefaction occurs in two stages. The first stage takes place at lower

strain level, involves generation of sufficient excess pore pressure to move the

stress path from initial position to FLS. Generation of excess pore pressure can

be either due to monotonic or cyclic loading. When effective stress path reaches

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FLS, second stage begins. The second stage involves strain softening and

additional excess pore pressure generation, which is driven by shear stress

called driving stress. Large strain developed in second stage as effective stress

path moves from the FLS to steady state deformation

Influence of excess pore pressure

The generation of excess pore pressure is the key to initiate the liquefaction.

Without it neither flow liquefaction nor cyclic mobility can occurs.

Flow liquefaction

It initiate by cyclic loading when shear stress required for static

equilibrium is greater than steady state strength. This shear stress is caused due

to gravity loading when large deformation develops. The occurrence of flow

liquefaction requires undrained disturbance strong enough to move effective

stress path from initial point to FLS. If the initial state stress condition is closer

to FLS, only small pore pressure increase is enough to trigger flow liquefaction.

Thus knowing the pore pressure ratio at a particular void ratio, it can be

assessed that at high initial stress ratio the flow liquefaction can be triggered

with very small dynamic disturbance

Cyclic mobility

It can occur when shear stress is smaller than steady state strength and

even soil is dense or loose. There are 3 initial and cyclic conditions which

generally produces cyclic mobility.

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Case 1 when ( τ static > τ cyc ) (no shear stress reversal) and (no exceedance of

steady state strength)

Stress will not cross the failure envelop and hence flow deformation will not

occur. The effective confining pressure has decreased significantly resulting in

low stiffness and significant permanent strains develop with each loading.

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Case 2 ( τ static < τ cyc ) when (no shear stress reversal and steady state

strength is exceeded momenterly)

The effective stress path, when touch FLS, momentary period of in stability

occurs resulting in permanent strain will occur.

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Case 3 when ( τ static > τ cyc ) (shear stress reversal occurs and no exceedance

of steady state strength)

In this the shear stress reversal will induces both compression and extension.

The rate of generation of pore pressure increases with increase in the stress

reversal. Each cyclic loading the stress path cross origin resulting zero effective

stress but do not cause initial liquefaction. Significant strain will accumulate but

no flow failure will occur.

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Effect of liquefaction

Liquefaction phenomena can affect building, bridges, buried pipes and other

structures. It also influences the nature of ground surface motion. Flow

liquefaction can produces massive flow slides and contribute in sinking or

tilting of heavy structures. Cyclic mobility causes slumping of slopes,

settlement of building and failure of retaining walls. The effects of liquefaction

can be listed as

1) Alteration of ground motion

The abrupt increase in the excess pore pressure causes stiffness to decrease

during an earthquake. Thus the amplitude and frequency content of the

surface motion may change considerably during earthquake. The occurrence

of liquefaction at depth beneath a flat ground surface can decouple the

liquefied soil from the surficial soil to produce large transient movement

ground oscillations. The surficial soils are often broken into block separated

by fissures that can open and close during earthquake.

2) Development of Sand Boils

Liquifaction is accompanied by the development of sand boil. During

shaking, Seismically induced excess pore pressure are dissipated

predominantly by the produces an upward force if the hydraulic gradient

exceeds a critical value the vertical effective stress will the zero, which is

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known as quick condition. During this period the water can carry sand

partical and gushes out firm cracks with high velocity called sand boil.

3) Settlement

When sands subjected to earthquake, it generally has property to get

densify. Settlement behavior of snad is also depends on condition of

saturation. Dry snad densifies very quickly compare to saturated sand.

4) Settlement of Dry Sand

The densification of dry sands subjected earthquake loading depends on

density of sand and amplitude of the cyclic shear strain induced in the

sand and number of cycles of shear strain applied during the earthquake.

Cyclic shear strain can be estimated by using realtion

γcyc = 0.65 amax γd σv

g G γcyc

Since shear modulus varies with (γcyc) and hence several iterations are

equired to calculate a value of (γcyc) which is consistent with the shear

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modulus. Knowing and SPT N value volumetric strain i.e Settlement

can be estimated.

5) Settlement of Saturated Sand

The densification of saturated sand is not a quick process. It is influenced

by the density of sand maximum shear strain induced and the amount of

excess pore pressure generated during earthquake. Large time taken for

settlement is due to slow dissipation of excess pore pressure and it

depends upon gradation and permeability. Thus the settlements of

saturated sands are post earth quake settlements.

6) Flow failures

Liquefaction induced flow failure occurs when the shear stress required to

maintain static equilibrium is greater than the shear strength of liquefied

sil. These are different mechanism for flow failure.

1. Flow liquefaction failures(N.R.C Mechanism A)

Flow liquefaction occurs when the static shear stress exceeds the shear

strength of the soil in liquefied state. Once triggered, large

deformations produced by flow liquefaction will be driven by static

shear stress and the cyclic stress like earthquakes will push the soil to

unstable state where soil looses its shear strength.

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2. Local Loosening Flow failure (N.R.C Mechanism B)

This mainly occurs when sand layer is over lain by a less permeable

material that does not permit drainage during earthquake. Thus during

earth quake liquefaction, instead of whole sand mass, sand mass at

bottom and get densifies while top layer of sand will undergo local

loosening and flow failure at top occurs.

4. Global Loosening Flow failure (N.R.C Mechanism C)

High excess pore pressure generated at depth will cause porewater to

flow toward drainages boundries during and after earthquake. Shallow

soils may be loosened by this flow to the extent that their steady state

strength drops below the shear stress required to maintain the

equilibrium. In contrast with the local loosening case, this loosening is

not compensated for by densification at a different location. Since the

steady state strength is not reduced until the water flows into the

shallower soil, failure may not occur until well after earthquake.

Cracking of the surficial soils may also contribute to the failure.

5. Interface Flow failure (N.R.C Mechanism D)

Flow type failure can also occur when the shear strength of the

interface between a liquefiable soil and a underground structure

becomes smaller than the shear stress required for equilibrium.

Plunging failure of friction piles is an example interface flow failure.

If the interface is smooth, as with steel or precast concrete interface

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flow failure does not require volume change of soil and therefore can

occur in contractive or dilative sands.

Deformation failure

Not all liquefaction related failure involves large displacements.

Cyclic mobility produces to small incremental permanent

deformations, which sufficient enough to produce large deformation.

These are deformation failure e.g. lateral spreading of ground. Lateral

spreading causes the surficial layer to break into block and

development of cracks and fissure.

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Conclusion

Liquefaction is one of the most important, interesting, complex

and conterversial topics is geotechnical engineering. All mentioned above are

just basic frame work to analyze the liquefaction phenomena.

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Liquefaction

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