Soil liquefaction

Soil liquefaction occurs when a cohesionless saturated or partially saturated

If the pressure of the water in the pores is great enough to carry all the load, it will have the effect of holding the particles apart and of producing a condition that is practically equivalent to that of quicksand... the initial movement of some part of the material might result in accumulating pressure, first on one point, and then on another, successively, as the early points of concentration were liquefied.

The phenomenon is most often observed in saturated, loose (low

Although the effects of soil liquefaction have been long understood, engineers took more notice after the 1964 Alaska earthquake and 1964 Niigata earthquake. It was a major cause of the destruction produced in San Francisco's Marina District during the 1989 Loma Prieta earthquake, and in the Port of Kobe during the 1995 Great Hanshin earthquake. More recently soil liquefaction was largely responsible for extensive damage to residential properties in the eastern suburbs and satellite townships of Christchurch during the 2010 Canterbury earthquake^[2] and more extensively again following the Christchurch earthquakes that followed in early and mid-2011.^[3] On 28 September 2018, an earthquake of 7.5 magnitude hit the Central Sulawesi province of Indonesia. Resulting soil liquefaction buried the suburb of Balaroa and Petobo village 3 metres (9.8 ft) deep in mud. The government of Indonesia is considering designating the two neighborhoods of Balaroa and Petobo, that have been totally buried under mud, as mass graves.^[4]

The building codes in many countries require engineers to consider the effects of soil liquefaction in the design of new buildings and infrastructure such as bridges, embankment dams and retaining structures.^[5][6][7]

Technical definitions

A 'flow failure' may initiate if the strength of the soil is reduced below the stresses required to maintain the equilibrium of a slope or footing of a structure. This can occur due to monotonic loading or cyclic loading and can be sudden and catastrophic. A historical example is the Aberfan disaster. Casagrande^[8] referred to this type of phenomena as 'flow liquefaction' although a state of zero effective stress is not required for this to occur.

'Cyclic liquefaction' is the state of soil when large shear strains have accumulated in response to cyclic loading. A typical reference strain for the approximate occurrence of zero effective stress is 5% double amplitude shear strain. This is a soil test-based definition, usually performed via cyclic triaxial, cyclic direct simple shear, or cyclic torsional shear type apparatus. These tests are performed to determine a soil's resistance to liquefaction by observing the number of cycles of loading at a particular shear stress amplitude required to induce 'fails'. Failure here is defined by the aforementioned shear strain criteria.

The term 'cyclic mobility' refers to the mechanism of progressive reduction of effective stress due to cyclic loading. This may occur in all soil types including dense soils. However, on reaching a state of zero effective stress such soils immediately dilate and regain strength. Thus, shear strains are significantly less than a true state of soil liquefaction.

Occurrence

Liquefaction is more likely to occur in loose to moderately saturated granular soils with poor drainage, such as silty sands or sands and gravels containing impermeable sediments.^[9][10] During wave loading, usually cyclic undrained loading, e.g. seismic 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 silts of similar grain size (well-sorted), in beds at least metres thick, and saturated with water. Such deposits are often found along stream beds, beaches, dunes, and areas where windblown silt (loess) and sand have accumulated. Examples of soil liquefaction include quicksand, quick clay, turbidity currents and earthquake-induced liquefaction.

Depending on the initial void ratio, the soil material can respond to loading either strain-softening or strain-hardening. Strain-softened soils, e.g., loose sands, can be triggered to collapse, either monotonically or cyclically, if the static shear stress is greater than the ultimate or steady-state shear strength of the soil. In this case flow liquefaction occurs, where the soil deforms at a low constant residual shear stress. If the soil strain-hardens, e.g., moderately dense to dense sand, flow liquefaction will generally not occur. However, cyclic softening can occur due to cyclic undrained loading, e.g., earthquake loading. Deformation during cyclic loading depends on the density of the soil, the magnitude and duration of the cyclic loading, and amount of shear stress reversal. If stress reversal occurs, the effective shear stress could reach zero, allowing cyclic liquefaction to take place. If stress reversal does not occur, zero effective stress cannot occur, and cyclic mobility takes place.^[11]

The resistance of the cohesionless soil to liquefaction will depend on the density of the soil, confining stresses, soil structure (fabric, age and cementation), the magnitude and duration of the cyclic loading, and the extent to which shear stress reversal occurs.^[12]

Liquefaction potential: simplified empirical analysis

Three parameters are needed to assess liquefaction potential using the simplified empirical method:

1. A measure of soil resistance to liquefaction: Standard Penetration Resistance (SPT),^[13][14] Cone Penetration Resistance (CPT),^[15] or shear wave velocity (Vs)^[16]
2. The earthquake load, measured as cyclic stress ratio ${\displaystyle CSR={\frac {\tau _{av}}{\sigma '_{v}}}=0,65{\frac {a_{max}}{g}}{\frac {\sigma _{v}}{\sigma '_{v}}}r_{d}}$ ^[17]
3. the capacity of the soil to resist liquefaction, expressed in terms of the cyclic resistance ratio (CRR)

Liquefaction potential: advanced constitutive model

The interaction between the solid skeleton and pore fluid flow has been considered by many researchers to model the material softening associated with the liquefaction phenomenon. The dynamic performance of saturated

Pressures generated during large earthquakes can force underground water and liquefied sand to the surface. This can be observed at the surface as effects known alternatively as "

Studies of liquefaction features left by prehistoric earthquakes, called paleoliquefaction or paleoseismology, can reveal information about earthquakes that occurred before records were kept or accurate measurements could be taken.^[21]

Soil liquefaction induced by earthquake shaking is a major contributor to urban seismic risk.

Effects

Over geological time, liquefaction of soil material due to earthquakes could provide a dense parent material in which the fragipan may develop through pedogenesis.^[23]

Mitigation methods

Mitigation methods have been devised by

Quicksand forms when water saturates an area of loose sand, and the sand is agitated. When the water trapped in the batch of sand cannot escape, it creates liquefied soil that can no longer resist force. 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 a 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, fluidlike 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, known as Leda Clay in Canada, is a water-saturated gel, which in its solid form resembles highly sensitive clay. This clay has a tendency to change from a relatively stiff condition to a liquid mass when it is disturbed. This gradual change in appearance from solid to liquid is a process known as spontaneous liquefaction. The clay retains a solid structure despite its high-water content (up to 80% by volume), because surface tension holds water-coated flakes of clay together. When the structure is broken by a shock or sufficient shear, it enters a fluid state.

Quick clay is found only in northern countries such as

Quick clay has been the underlying cause of many deadly

Submarine landslides are

• Atterberg limits
• Dry quicksand
• Earthflow
• Earthquake engineering
• Fluidization
• Liquefaction
• Mud volcano
• Mudflow
• Network for Earthquake Engineering Simulation#Soil liquefaction research
• Paleoseismology
• Sand boil
• Subsidence
• Thixotropy

References