<div class='content-section' id='liquefaction-potential-assessment' title='Liquefaction Potential Assessment'><p><p><h2>Liquefaction Potential Assessment</h2>VCC technology may be used at sites with in-situ soils that may be susceptible to liquefaction during earthquakes. Saturated sands, silty sands, sandy silts, and silts are likely to be in this category. When VCCs are used for support of embankments and structures, to reduce settlements, or to increase slope stability, it is also necessary to confirm that there will not be a risk of liquefaction or other ground disturbance that could lead to loss of support and lateral spreading. The initial assessment of whether the soil at a site will liquefy in an earthquake is made in terms of whether the in-situ shear strength under cyclic loading, represented as a Cyclic Resistance Ratio (CRR), is less than the cyclic shear stress that will cause liquefaction, termed the Cyclic Stress Ratio (CSR).</p><p>Combinations of CSR and strength of the soil layer, usually determined in-situ by means of penetration tests and shear wave velocity<a href="#_ftn1" name="_ftnref1">^[1]</a> measurements, have been found that define the boundary between liquefaction and no liquefaction over a range of peak ground motion accelerations. This boundary has been determined through extensive analyses of case history data from many earthquakes. Standard Penetration Tests (SPT), Cone Penetration Tests (CPT), and Becker Penetration Tests for soils containing gravel and cobbles (BPT) are used to determine the CRR. Values of CRR are defined by the points on the boundary curve that separates liquefaction and no liquefaction zones on a plot of CSR vs. penetration resistance or shear wave velocity corresponding to the measured and corrected in-situ property. An example of such a plot for liquefaction analysis using the SPT is shown in Figure 1.</p><p>&nbsp;</p><p><a href="#_ftnref1" name="_ftn1"></a></p><p><hr /></p><p><a href="#_ftnref1" name="_ftn1">[1]</a> Owing to the lack of precision and uncertainties associated with shear wave velocity - liquefaction correlations, this method is not considered further herein.</p><p><figure id='attachment_3000' style='max-width:596px' class='caption aligncenter'><img class="wp-image-3000 size-full" src="https://www.geoinstitute.org/sites/default/files/geotech-tools-uploads/…; alt="SPT liquefaction chart showing limits of liquefaction for a magnitude 7.5 earthquake. Horizontal axis is the corrected SPT blowcount, vertical axis is the cyclic stress ratio (CSR) or cyclic resistance ratio (CRR)." width="596" height="633" /><figcaption class='caption-text'> Figure 1. SPT liquefaction chart for magnitude 7.5 earthquakes (Youd et al. 2001).</figcaption></figure></p><p>Thus, if a site underlain by saturated clean sand has a corrected blow count (N₁)₆₀ of 10 blows per foot and the anticipated cyclic stress ratio under the design earthquake is 0.25, the soil will liquefy unless the normalized penetration resistance (N₁)₆₀ is increased to greater than 22 blows per foot by densification, or the cyclic stress ratio is reduced by transferring some or all of the dynamic shear stress to reinforcing elements. Similar plots are available in terms of normalized CPT tip resistance q_c1N. In each case the penetration resistance is normalized to an effective overburden stress of 1 atmosphere.</p><p>Although straightforward in concept, the liquefaction potential analysis is complex in application, because (1) the CSR depends on the input motions within the soil layer which, in turn, depend on such factors as earthquake magnitude and intensity, distance from the epicenter, geologic setting, rock conditions, and soil profile characteristics, (2) the CRR depends on such factors as overburden stress, fines content of the soil, and static shear stress, and (3) determination of normalized values of the penetration resistance involves several corrections to the measured values, especially in the case of the SPT.</p><p>&nbsp;</p><p>Information about input ground motions can be obtained from local experience and recorded ground motions near the site, if available, or from seismicity information obtainable from the<br><ul> <li>United States Geological Survey Ground Motion Calculator (<a href="https://earthquake.usgs.gov/hazards/designmaps/&quot; target="_blank" rel="noopener">https://earthquake.usgs.gov/hazards/designmaps/</a&gt;), which can be used to obtain peak rock accelerations for the site</li> <li>USGS Earthquakes Hazards Program website (<a href="https://earthquake.usgs.gov/hazards/&quot; target="_blank" rel="noopener">https://earthquake.usgs.gov/hazards/</a&gt;), which provides design ground motions for buildings and bridges; interactive fault maps; scenarios of ground motions and effects of specific hypothetical large earthquakes; and seismic hazard maps and site-specific data which includes a Beta version of an unified hazard tool that enables determination of site-specific ground motion parameters.</li></ul>Widely used liquefaction correlation diagrams for SPT and CPT, along with discussions of how to make the necessary computations to obtain the CSR, (N₁)₆₀, q_c1N, and the CRR are given in Youd, et al. (2001) and Idriss and Boulanger (2008).</p><p>If VCCs are used in potentially liquefiable soils in a seismic area for support of embankments or structures, or for stabilization of slopes, the cyclic stresses caused by ground shaking will be shared between the columns and the untreated matrix soil. By virtue of their greater stiffness, the columns will attract a greater proportion of the cyclic shear stresses than given simply by the replacement ratio (the ratio of the treated area in plan to the total plan area). To maintain structural integrity and ensure satisfactory performance requires a design that prevents horizontal shear failure in aggregate columns or combined shear and bending failures in cemented columns and walls. Analysis of this complex soil-structure problem is usually site and project specific and requires input from someone with prior knowledge and experience.</p><p>Whether the matrix soil will liquefy with the supporting elements in place can be assessed in terms of the reduced shear stress and strain that it is subjected to after accounting for that carried by the columns. A very approximate, but conservative, means for estimating the reduced shear stress is as follows.</p><p>If the simplifying assumption is made that the shear strains in the columns are the same as the shear strains in the soil, then the ratio of the shear stress in the soil to the average stress can be expressed as:</p><p><img class="aligncenter wp-image-2800" src="https://www.geoinstitute.org/sites/default/files/geotech-tools-uploads/…; alt="Equation: The ratio of Tau sub soil over T sub average is equal to the ratio of 1 over the quantity of 1 minus a sub s plus the product of the ratio G sub c over G sub s and a sub s." width="240" height="64" />where,<br>a_s = area replacement ratio<br>G_c = shear modulus of the column<br>G_s = shear modulus of the soil</p><p>The equal strain assumption means that the stress concentration ratio, <em>n</em> = <em>t</em><em>_col</em>/<em>t</em><em>_soil</em>, will be given by <em>G_c</em>/<em>G_s</em>. Owing to various types of compliance in the system, however, the actual stress concentration ratio would be less than <em>G_c</em>/<em>G_s</em> for most situations. One source of guidance is the stress concentration that occurs in a column-soil system subjected to vertical compressive loading. For aggregate columns in soft soil this ratio is typically in the range of about 2 to 5 or 6, and the ratio can be higher for cemented columns. When isolated columns are used to resist shear deformations, the values of <em>n</em> can be smaller than for axial loading because there are more potential sources of compliance, including column rotation and bending, than for axial loading. Nevertheless, the values for axial loading can serve as a useful guide. If continuous panels are used, the stress concentration for shear deformations can be higher than for isolated columns because the rotation and bending deformation modes are inhibited.</p><p>Substituting <em>n</em> for <em>G_c</em>/<em>G_s</em> in the above expression gives:<img class="aligncenter wp-image-2801" src="https://www.geoinstitute.org/sites/default/files/geotech-tools-uploads/…; alt="Equation: The ratio of Tau sub soil over T sub average is equal to the ratio of 1 over the quantity of 1 minus a sub s plus the product of n and a sub s." width="160" height="66" /></p><p>As an example, if a safe value of <em>n</em> is assumed to be 3, then:<img class="aligncenter wp-image-2802" src="https://www.geoinstitute.org/sites/default/files/geotech-tools-uploads/…; alt="Equation: The ratio of Tau sub soil over T sub average is equal to the ratio of 1 over the quantity of 1 plus the product of 2 and a sub s." width="139" height="55" /></p><p>and if <em>a_s</em> = 0.3, then:<img class="aligncenter wp-image-2803" src="https://www.geoinstitute.org/sites/default/files/geotech-tools-uploads/…; alt="Equation: The ratio of Tau sub soil over T sub average is equal to 0.62." width="130" height="56" /></p><p>which represents a 38% reduction in the seismic shear stress applied to the soil. A reduction of this magnitude could provide a significant decrease in the liquefaction potential.</p><p>The appropriate value of <em>n </em>in any case depends on the relative stiffness of the column and soil, the slenderness ratio of the columns for unconnected column arrangements, the use of grids formed of continuous shear panels instead of isolated columns, and the overall height-to-width and height-to-length ratios of the treated zone, among other factors. As mentioned above, if the columns are arranged in a grid of overlapping continuous panels it would be expected that <em>n</em> would be higher than if isolated columns are used.</p><p>The construction methods used for installation of VCCs may provide some densification of the in-situ soil. If consistent improvement of the soil can by verified by QA testing, then there will be an increase in the CRR in addition to the decrease in the CSR caused by the load transfer to the stiffer elements, with a corresponding increase in the factor of safety against liquefaction. The increase in the CRR can be determined using the new values of penetration resistance and appropriate liquefaction charts.</p></p></div>