Liquefaction Soil Dynamics

Liquefaction - the phenomenon Liquefaction: is the phenomenon of the loss of shear strength of a saturated soil formation as a result of shaking caused by strong seismic waves travelling through it. The shear strength loss is due to the sudden development of positive pore water pressure by a cyclic loading that reduces the effective pressure not matched by the same speed of pressure dissipation. The typical ground condition susceptible to liquefaction is a young natural or man-made deposit of loose sand with the groundwater close to the surface

Liquefaction - the phenomenon The positive pore water development is because of the tendency of the loosely placed grains to rearrange themselves and assume a denser formation. However, the loading takes place so suddenly that there is no enough time for the developed pore water pressure to dissipate and the densification to take place. This results in the loss of shear strength and the individual grains almost floating in the water. If the shaking is intense, part of the soil mass can move upwards to the surface in form of mud spouts and sand boils. Structures founded on such soils would thus fail by sliding or rotation.

Liquefaction - occurrence Liquefaction mostly occurs in low lying areas adjacent to waterbodies such as rivers, lakes and oceans, where recently deposited, loose sediments are prevalent and the groundwater is at shallow depths. Liquefaction was a major cause of loss in several earthquakes including the Nigata, Japan (1964), San Fernando, USA (1971), Kobe, Japan (1995) and Christchurch, New Zealand (2010/11) earthquakes. The pictures below give some impression about liquefaction effects.

Liquefaction: some cases Of the 310 damaged r. c. buildings during the 7.5 magnitude 1964 Nigata earthquake, Japan, 200 tilted or excessively settled as a result of liquefaction. Such failures are recognized as liquefaction induced – bearing capacity and settlement failures.

Liquefaction: some cases The upstream slope of the Lower San Fernando Dam, California, failed due to the liquefaction of a zone of a hydraulic sand fill during the 1971 6.6-magnitude San Fernando earthquake The picture is taken after lowering of the reservoir water following the failure

Liquefaction: some cases The schematic drawing shows the zone of liquefaction that induced the so- called flow slide. The flow slide caused the upper part to slump and the upstream toe to slide 46 m into the reservoir If the reservoir was not emptied immediately after the failure, thousands of lives downstream would be at steak.

Liquefaction: some cases Lateral movement of a retaining wall at a sea shore due to liquefaction during the 6.9 magnitude 1995 Kobe earthquake. This phenomenon is also called lateral spread. Lateral spread can further cause the failure of foundations of structures at distances.

Liquefaction: some cases Lateral spread could be of much larger scale like the one shown in the figure caused by the 1989 Loma Prieta earthquake. Such phenomena can break apart the foundations of any structures and utilities on top

Liquefaction: some cases Scene of asphalted roads flooded by sand boils in the 2010/11 Christchurch earthquake, New Zealand

Liquefaction: some cases Vehicles sunk into the sand boils

Liquefaction: some cases Liquefaction in Christchurch was widespread over large areas

Liquefaction: basic concepts Casagrande (1936) made the first attempt to explain the liquefaction phenomenon based on the concept of critical void ratio. While dense sand tends to dilate when sheared, loose sand tends to contract. There is a critical void ratio at which no volume change occurs.

Liquefaction: basic concepts Casagrande explained that sand deposits with void ratio larger than the critical tend to decrease in volume, and thus, if saturated, to increased pore water pressure, when subjected to shaking in a condition of impeded drainage. Based on the effective-stress principle: If σ remains constant and u keeps on increasing and ultimately equals, the total stress, the soil entirely loses its shear resistance and liquefy.

Liquefaction: basic concepts This, however, didn’t go far enough to provide adequate explanation The significance of the subject prompted extensive research since the 1960’s More pertinent concepts and knowledge were needed.

Liquefaction: basic concepts Consider a saturated loose sand specimen in a simple-shear apparatus subjected to cyclic loading. If the load is applied slow enough for drainage to take place, the void ratio reduces monotonically with increasing number of cycles as shown. The volume reduction takes place at a decreasing rate with increasing number of cycles. However, this phenomenon may not take place in natural soil deposits prone to liquefaction during earthquakes because of the speed by which cyclic straining takes place.

Liquefaction: basic concepts Thus, as shown in the figure, a point in a loosely deposited natural soil represented by Point A in a e-p curve would have the coordinates at B after a number of cycles densified the soil, if drainage was allowed as above. However, because of the development of pore water pressure associated with the speed of loading, the state would rather be represented by Point C, as there would be no volume change. Note that the effective stress reduces by the magnitude of the pore water pressure developed and would be zero if the number of cycles is big enough to produce just the same magnitude of pore pressure as the vertical total stress, in which case liquefaction takes place

Liquefaction: laboratory studies The field conditions are Before earthquake: K0-condition without shear stresses, and During earthquake: K0-condition and a cyclic shear stress with no drainage. Laboratory tests are designed to simulate these conditions. Appropriate tests include simple shear, torsional shear and cyclic triaxial tests

Liquefaction: Selected test results A summary of cyclic triaxial test results from Seed and Lee (1965) for Sacramento River sand is shown for a constant confining pressure of 100 kPa. The test on each specimen was conducted in an undrained condition until initial liquefaction and 20% axial strain takes place

Liquefaction: Selected test results The results show For a given density of the sand, the number of cycles required for liquefaction increases with decreasing magnitude of cyclic deviator stress; For a given magnitude of deviator stress, the number of cycles required increases with increasing density; For a given number of cycles, the magnitude of deviator stress required increases with soil density.

Liquefaction: Selected test results A summary of undrained results from torsional tests of hollow cylindrical samples of saturated Fuji River sand under a constant confining pressure of 98 kPa and constant-amplitude shear stress are reported by Ishihara (1985) for two different relative densities: One test is for a medium dense sand with Dr=47%; The other test is on dense sand with Dr=75%.

Liquefaction: Selected test results Results for the medium dense sand with Dr=47% in an undrained state are as shown: The constant-amplitude cyclic shear stress has an amplitude of only 23% of the confining pressure A sudden increase in shear strain occurs within a small number of cycles (<10) of the loading reaching as high as 20%. The gradual increase in pore water pressure reaches the magnitude of the confining pressure at this strain level. This implies complete loss of shear strength and hence liquefaction.

Liquefaction: Selected test results Results for the dense sand with Dr=75% in an undrained state are as shown. The constant-amplitude cyclic shear stress has an amplitude of 72 % of the confining pressure The shear strain increases gradually and does not become as dramatic at once as in the medium dense sand. The pore water pressure indeed reaches the magnitude of the confining pressure within a short time but this does not lead to excessive strain. This is because of the tendency of the dense sand to dilate and thus to gain in strength when the loading direction is reversed. The liquefaction is only momentary – a state known as cyclic mobility.

Liquefaction: influencing factors Field and lab tests in the past have identified the following factors governing liquefaction: Intensity and duration of shaking: Liquefaction potential increases with increasing ground acceleration amplitude and increasing duration; PGA < 0.1g or EQ magnitude < 5 are unlikely to cause liquefaction. Groundwater: Near surface GWL is an ideal condition for liquefaction; Unsaturated soil above the GWL does not liquefy; GWL fluctuation should be accounted for to evaluate liquefaction. Soil type: most susceptible are cohesionless soils an approximate list in order of increasing resistance to liquefaction is clean sand, non-plastic silty sands, non-plastic silt and gravel

Liquefaction: influencing factors Most cohesive soils are not prone to liquefaction. For them to be liquefiable all of the following conditions must exist: % finer than 0.005mm must be <15 %; LL < 35 %; and Wn > 0.9WL. However, cohesive soils can generally lose a significant part of their shear strength due to cyclic loading. Relative Density: Loose cohesionless soils are susceptible, whereas dense cohesionless soils are not due to tendency of the latter to dilate and thus to develop negative pore pressure Particle size gradation: Uniformly graded cohesionless soils are more susceptible than well- graded ones, because in the latter case more voids are filled with finer particles hindering tendency to be compacted during shaking.

Liquefaction: influencing factors Drainage condition: Soils prone to liquefaction become more susceptible if surrounded by less draining layers like clays. Confining pressure: The more confined soils are the less susceptible they are to liquefaction; soils located deeper than 15 m from a level ground surface are unlikely to liquefy because of the large confining pressure. Particle shape: Granular soils of rounded shape are less stable and tend to be compacted more during shaking thus becoming more susceptible to liquefaction than soil of angular-shaped particles Age and cementation: Young and less cemented granular deposits are prone to liquefaction. Superimposed load: superimposed loads from large structures induces large shearing stress making soils underneath more susceptible to liquefaction

Liquefaction Analysis: Evaluation of Susceptibility The most common method of analysis to evaluate liquefaction potential is the semi-empirical method suggested by Seed and Idriss (1971) based on SPT blow counts; it is known as the simplified procedure. The method has recently been modified by Idriss and Boulanger (2004, 2008) If a soil meets the various preliminary criteria for liquefaction discussed above, the procedure involves determining : The cyclic stress ratio (CSR): indicates stress induced, cyclic resistance ratio (CRR): indicates available resistance, and check if the ratio of CRR to CSR is larger than unity to write off liquefaction as a potential problem.

Liquefaction Analysis: cyclic stress ratio (CSR) The expression for the cyclic stress ratio (CSR) or also known as the seismic stress ratio (SSR) is established based on the sketch of soil column of unit base area above the point of investigation at depth z. The ground is assumed level and the soil column responds rigidly to the acceleration at the surface. The shear stress at the base is equal to the inertia force, F, per unit area (note that side forces cancel each other): Where

Liquefaction Analysis: cyclic stress ratio (CSR) Dividing both sides by the effective stress and introducing a depth reduction factor, rd, to account for the soil deformability, one obtains: Since the ground acceleration history is irregular and short lived, this is further reduced by a factor of 0.65 to obtain the equivalent constant-amplitude cyclic shear stress and the corresponding stress ratio, CSR, as The depth reduction factor, rd, as suggested by Seed and Idriss (1971) and later by Idriss (1999) are given on the next slide as a function of depth and earthquake magnitude. The average curves are mostly used. Alternatively, it can be obtained from the following linear relationship proposed by Kayen et al (1992):

Liquefaction Analysis: cyclic stress ratio (CSR)

Liquefaction Analysis: cyclic resistance ratio (CRR) The next step in the simplified liquefaction analysis is to determine the CRR. The CRR can be determined from Standard penetration test (SPT); Cone penetration test (CPT); or Shear wave velocity, VS

Liquefaction Analysis: CRR from SPT SPT is the most common data used to determine CRR. The larger the SPT blow count, the larger is the resistance to liquefaction. Seed et al (1985) concluded the existence of the following damage ranges for cohesionless soils. Note that 20 is the boundary between the medium dense and dense state of sand and 30 is between dense and very dense sand

Liquefaction Analysis: CRR from SPT The chart shown can be used to estimate CRR of in-situ soils by entering with the fine content and the SPT blow count The chart is applicable for silty sands and for earthquake magnitude of 7.5 The (N1)60 should be corrected for overburden pressure.

Liquefaction Analysis: CRR from SPT For other magnitudes the correction factors in the table suggested by Seed and Idriss (1985) can be used Or, the chart shown can also be used

Liquefaction Analysis: factor of safety The factor of safety, FS, is defined as FS less than unity implies liquefaction. FS > 1 does not necessarily guarantee safety against liquefaction. For example a layer that liquefied already during an earthquake can induce liquefaction of an overlying layer of marginal FS.

END