Liquefaction mitigation in silty soils using composite stone columns and dynamic compaction

The objective of this study is to develop an analytical methodology to evaluate the effectiveness ofvibro stone column (S. C.) and dynamic compaction (D.C.) techniques supplemented with wick drains to densify and mitigate liquethctionin saturated sands and non-plastic silty soils. It includes the following: (i) develop numerical models to simulate and analyze soil densification during S.C. installation and D.C. process, and (ii) identify parameters controlling post-improvement soil density in both cases, and (iii) develop design guidelines for densification of silty soils using the above techniques. An analytical procedure was developed and used to simulate soil response during S.C. and D.C. installations, and the results were compared with available case history data. Important construction design parameters and soil properties that affect the effectiveness of these techniques, and construction design choices suitable for sands and non-plastic silty soils were identified. The methodology is expected to advance the use of S.C. and DC. in silty soils reducing the reliance on expensive field trials as a design tool. The ultimate outcome of this research will be design charts and design guidelines for using composite stone columns and composite dynamic compaction techniques in liquefaction mitigation of saturated silty soils.     

Intergrain contact density indices for granular mixes-Ⅱ:Liquefaction resistance

Whether the presence of non-plastic silt in a granular mix soil impact its liquefaction potential and how to evaluate liquefaction resistance of sand containing different amounts of silt contents are both controversial issues. This paper presents the results of an experimental evaluation to address these issues. Two parameters, namely, equivalent intergranular void ratio (ec)eq and equivalent interfine void ratio (ef)eq, proposed in a companion paper (Thevanayagam, 2007) as indices of active grain contacts in a granular mix, are used to characterize liquefaction resistance of sands and silty sands. Results indicate that, at the same global void ratio (e), liquefaction resistance of silty sand decreases with an increase in fines content (CF) up to a threshold value (CFth). This is due to a reduction in intergrain contact density between the coarse grains. Beyond CFth, with further addition of fines, the interfine contacts become significant while the inter-coarse grain contacts diminish and coarse grains become dispersed. At the same e, the liquefaction resistance increases and the soil becomes stronger with a further increase in silt content. Beyond a limiting fines content (CFL), the liquefaction resistance is controlled by interfine contacts only. When CF＜CFth, at the same (ec)eq, the liquefaction resistance of silty sand is comparable to that of the host clean sand at a void ratio equal to (ec)eq. When CF＞CFth, at the same (ef)eq, the cyclic strength of a sandy silt is comparable to the host silt at a void ratio equal to (ef)eq.     

Intergrain contact density indices for granular mixes I： Framework

Mechanical behavior such as stress-strain response, shear strength, resistance to liquefaction, modulus, and shear wave velocity of granular mixes containing coarse and fine grains is dependent on intergrain contact density of the soil. The global void ratio e is a poor index of contact density for such soils. The contact density depends on void ratio, fine grain content （Cv）, size disparity between particles, and gradation among other factors. A simple analysis of a two-sized particle system with large size disparity is used to develop an understanding of the effects of Cv, e, and gradation of coarse and fine grained soils in the soil mix on intergrain contact density. An equivalent intergranular void ratio （ec）oq is introduced as a useful intergrain contact density for soils at fines content of less than a threshold value Crth. Beyond this value, an equivalent interfine void ratio （ef）eq is introduced as a primary intergrain contact density index. At higher values of Cv beyond a limiting value of fine grains content CVL, an interfine void ratio ef is introduced as the primary contact density index. Relevant equivalent relative density indices （Drc）eq and （Drf）eq are also presented. Experimental data show that these new indices correlate well with steady state strength, liquefaction resistance, and shear wave velocities of sands, silty sands, sandy silts, and gravelly sand mixes.     

Liquefaction in silty soils—screening and remediation issues

Current techniques for liquefaction screening, ground modification for liquefaction mitigation, and post-improvement verification rely on knowledge gained from extensive research on clean sands, field observations of liquefied ground, and judicial correlation of normalized penetration resistance [(N₁)₆₀,q_c1N] or shear wave velocity (v_s1) data with field liquefaction observations. Uncertainties prevail on the direct extrapolation of such techniques for silty soil sites. This paper examines laboratory data on liquefaction resistance, strength, and v_s1 of sands and silty soils using grain contact density as the basis. Effect of silt content on cyclic resistance, strength, m_v, and c_v is examined in this light. Rational insights on effects of silt content on the current screening techniques based on (N₁)₆₀, q_c1N, and v_s1 to silty soils are offered. Recent advances and modifications to the traditional densification, drainage, and permeation grouting techniques to make them viable for silty soils are discussed.     

Intergrain contact density indices for granular mixes——Ⅱ: Liquefaction resistance

Whether the presence of non-plastic silt in a granular mix soil impact its liquefaction potential and how to evaluate liquefaction resistance of sand containing different amounts of silt contents are both controversial issues. This paper presents the results of an experimental evaluation to address these issues. Two parameters, namely, equivalent intergranular void ratio (ec)eq and equivalent interfine void ratio (ef)eq, proposed in a companion paper (Thevanayagam, 2007) as indices of active grain contacts in a granular mix, are used to characterize liquefaction resistance of sands and silty sands. Results indicate that, at the same global void ratio (e), liquefaction resistance of silty sand decreases with an increase in fines content (CF) up to a threshold value (CFth). This is due to a reduction in intergrain contact density between the coarse grains. Beyond CFth, with further addition of fines, the interfine contacts become significant while the inter-coarse grain contacts diminish and coarse grains become dispersed. At the same e, the liquefaction resistance increases and the soil becomes stronger with a further increase in silt content. Beyond a limiting fines content (CFL), the liquefaction resistance is controlled by interfine contacts only. When CFCFth, at the same (ef)eq, the cyclic strength of a sandy silt is comparable to the host silt at a void ratio equal to (ef)eq.     

Intergrain contact density indices for granular mixes II： Liquefaction resistance

Whether the presence of non-plastic silt in a granular mix soil impact its liquefaction potential and how to evaluate liquefaction resistance of sand containing different amounts of silt contents are both controversial issues. This paper presents the results of an experimental evaluation to address these issues. Two parameters, namely, equivalent intergranular void ratio （ec）eq and equivalent interfine void ratio （ef）eq, proposed in a companion paper （Thevanayagam, 2007） as indices of active grain contacts in a granular mix, are used to characterize liquefaction resistance of sands and silty sands. Results indicate that, at the same global void ratio （e）, liquefaction resistance of silty sand decreases with an increase in fines content （Cv） up to a threshold value （Crth）. This is due to a reduction in intergrain contact density between the coarse grains. Beyond Crth, with further addition of fines, the interfine contacts become significant while the inter-coarse grain contacts diminish and coarse grains become dispersed. At the same e, the liquefaction resistance increases and the soil becomes stronger with a fttrther increase in silt content. Beyond a limiting fines content （CrL）, the liquefaction resistance is controlled by interfine contacts only. When Cr〈Crth, at the same （e）eq, the liquefaction resistance of silty sand is comparable to that of the host clean sand at a void ratio equal to （ec）eq. When CF〉CFth, at the same （ef）eq, the cyclic strength of a sandy silt is comparable to the host silt at a void ratio equal to （ef）eq.     

Intergrain contact density indices for granular mixes——Ⅰ: Framework

Mechanical behavior such as stress-strain response, shear strength, resistance to liquefaction, modulus, and shear wave velocity of granular mixes containing coarse and fine grains is dependent on intergrain contact density of the soil. The global void ratio e is a poor index of contact density for such soils. The contact density depends on void ratio, fine grain content (CF), size disparity between particles, and gradation among other factors. A simple analysis of a two-sized particle system with large size disparity is used to develop an understanding of the effects of CF , e, and gradation of coarse and fine grained soils in the soil mix on intergrain contact density. An equivalent intergranular void ratio (ec)eq is introduced as a useful intergrain contact density for soils at fines content of less than a threshold value CFth. Beyond this value, an equivalent interfine void ratio (ef)eq is introduced as a primary intergrain contact density index. At higher values of CF beyond a limiting value of fine grains content CFL, an interfine void ratio ef is introduced as the primary contact density index. Relevant equivalent relative density indices (Drc)eq and (Drf)eq are also presented. Experimental data show that these new indices correlate well with steady state strength, liquefaction resistance, and shear wave velocities of sands, silty sands, sandy silts, and gravelly sand mixes.     

Intergrain contact density indices for granular mixes-Ⅰ: Framework

Mechanical behavior such as stress-strain response, shear strength, resistance to liquefaction, modulus, and shear wave velocity of granular mixes containing coarse and fine grains is dependent on intergrain contact density of the soil. The global void ratio e is a poor index of contact density for such soils. The contact density depends on void ratio, fine grain content (CF), size disparity between particles, and gradation among other factors. A simple analysis of a two-sized particle system with large size disparity is used to develop an understanding of the effects of CF, e, and gradation of coarse and fine grained soils in the soil mix on intergrain contact density. An equivalent intergranular void ratio (ec)eq is introduced as a useful intergrain contact density for soils at fines content of less than a threshold value CFth. Beyond this value, an equivalent interfine void ratio (ef)eq is introduced as a primary intergrain contact density index. At higher values of CF beyond a limiting value of fine grains content CFL, an interfine void ratio ef is introduced as the primary contact density index. Relevant equivalent relative density indices (Drc)eq and (Drf)eq are also presented. Experimental data show that these new indices correlate well with steady state strength, liquefaction resistance, and shear wave velocities of sands, silty sands, sandy silts, and gravelly sand mixes.