S. Thevanayagam

Liquefaction in silty soils—screening and remediation issues

Abstract 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,qc1N] or shear wave velocity (vs1) 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 vs1 of sands and silty soils using grain contact density as the basis. Effect of silt content on cyclic resistance, strength, mv, and cv is examined in this light. Rational insights on effects of silt content on the current screening techniques based on (N1)60, qc1N, and vs1 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.

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

AbstractsThe objective of this study is to develop an analytical methodology to evaluate the effectiveness of vibro stone column (S.C.) and dynamic compaction (D.C.) techniques supplemented with wick drains to densify and mitigate liquefaction in saturated sands and non-plastic silty soils. It includes the following: (i) develop numerical models to simulate and analyze soil densitication 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 D.C. 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.

Mechanics of Lateral Spreading Observed in a Full-Scale Shake Test

This paper examines in detail the mechanics of lateral spreading observed in a full-scale test of a sloping saturated fine sand deposit, representative of liquefiable, young alluvial and hydraulic fill sands in the field. The test was conducted using a 6-m tall inclined laminar box shaken at the base. At the end of shaking, nearly the whole deposit was liquefied, and the ground surface displacement had reached 32 cm. The presented analysis of lateral spreading mechanics utilizes a unique set of lateral displacement results, DH, from three independent techniques. One of these techniques—motion tracking analysis of the experiment video recording—is especially useful as it produced DH time histories for all laminar box rings and a complete picture of the lateral spreading initiation with an unprecedented degree of resolution in time and space. A systematic study of the data identifies the progressive stages of initiation and accumulation of lateral spreading, lateral spread contribution of various depth ranges and sliding zones, their relation to the simultaneous pore pressure buildup, and the soil shear strength response during sliding. DOI: 10.1061/ASCEGT.1943-5606.0000409 CE Database subject headings: Soil liquefaction; Residual strength; Hydraulic fill; Full-scale tests; Lateral displacement. Author keywords: Liquefaction; Residual strength; Hydraulic fill; Full-scale tests; Lateral displacement.

Centrifuge and Large-Scale Modeling of Seismic Pore Pressures in Sands: Cyclic Strain Interpretation

AbstractCentrifuge modeling of pore pressure buildup in a sand deposit as a result of shaking is evaluated by comparison with a large-scale experiment. In large-scale Test SG-1, a 5.6-m-thick, mildly sloping deposit of hydraulic fill clean Ottawa sand of Dr=40%, was subjected to 5 s of low-intensity base shaking (<0.02g) that induced excess pore pressures short of liquefaction. Three centrifuge experiments using various soil deposits and saturation fluids were conducted and compared with the large-scale test. One of these centrifuge simulations used the same Ottawa sand and Dr=40% of the prototype, a viscous pore fluid, and dry pluviation deposition, which created a soil fabric stiffer than the prototype. The other two centrifuge simulations used silty sand saturated with water. The pore pressure buildup in one of the silty sand tests was in good agreement with the prototype, while the other two centrifuge deposits did not develop any excess pore pressure. The various responses in the four tests are expla...

Effect of non-plastic fines on undrained cyclic strength of silty sands

Whether the presence of silt adversely or beneficially affects liquefaction and the collapse potential of silty soils and how to evaluate cyclic strength behavior of a sand containing different silt contents are contentious issues. The purpose of this work is to investigate this question. Stress controlled undrained cyclic triaxial tests were conducted on specimen prepared by mixing a sand with silt in different proportions. The cyclic stress ratio (CSR=0.2) and confining pressure (100 kPa) were maintained constant. Relationship between no. of cycles required to cause liquefaction (at 5% axial strain) versus void ratio, and newly introduced equivalent void ratio indices based on intergrain contact density considerations are presented. Cyclic strength correlates well with the latter indices.

Laminar Box System for 1-g Physical Modeling of Liquefaction and Lateral Spreading

Details of a large scale modular 1-g laminar box system capable of simulating seismic induced liquefaction and lateral spreading response of level or gently sloping loose deposits of up to 6 m depth are presented. The internal dimensions of the largest module are 5 m in length and 2.75 m in width. The system includes a two dimensional laminar box made of 24 laminates stacked on top of each other supported by ball bearings, a base shaker resting on a strong floor, two computer controlled high speed actuators mounted on a strong wall, a dense array advanced instrumentation, and a novel system for laboratory hydraulic placement of loose sand deposit, which mimics underwater deposition in a narrow density range. The stacks of laminates slide on each other using a low-friction high-load capacity ball bearing system placed between each laminate. It could also be reconfigured into two smaller modules that are 2.5 m wide, 2.75 m long, and up to 3 m high. The maximum shear strain achievable in this system is 15 %. A limited set of instrumentation data is presented to highlight the capabilities of this equipment system. The reliability of the dense array sensor data is illustrated using cross comparison of accelerations and displacements measured by different types of sensors.

Micromechanical Aspects of Liquefaction-Induced Lateral Spreading

This paper reports the results of model-based simulations of 1-g shake table tests of level and sloping saturated granular soils subject to seismic excitations. The simulations utilize a transient fully coupled continuum-fluid discrete-particle model of water-saturated soils. The fluid (water) phase is idealized at a mesoscale using an averaged form of Navier-Stokes equations. The solid particles are modeled at the microscale as an assemblage of discrete spheres using the discrete element method (DEM). The interphase momentum transfer is accounted for using an established relationship. The employed model reproduced a number of response patterns observed in the 1-g experiments. In addition, the simulation results provided valuable information on the mechanics of liquefaction initiation and subsequent occurrence of lateral spreading in sloping ground. Specifically, the simulations captured sliding block failure instances at different depth locations. The DEM simulation also quantified the impact of void redistribution during shaking on the developed water pressure and lateral spreading. Near the surface, the particles dilated and produced an increase in volume, while the particles at deeper depth locations experienced a decrease in volume during shaking.

Effects of Permeability on Liquefaction Resistance and Cone Resistance

Sands and silty sands with the same liquefaction resistance can have significantly different hydraulic conductivity and coefficient of consolidation. Numerical simulations of cone penetration resistance taking into account the effects of silt content and consolidation characteristics show that the penetration resistances are significantly affected by consolidation characteristics for sands and sand-silt mixes with similar liquefaction resistances. The influence of permeability and consolidation characteristics on cone penetration resistances are presented. Relationships between liquefaction resistance, cone penetration resistance, and a normalized penetration rate are presented and compared with current field-based CPT-liquefaction screening methods.

Energy Dissipation and Liquefaction Assessment in Sands and Silty Soils

Soil liquefaction phenomenon involves progressive intergrain contact deformation, slip, reorganization of contacts, and eventual collapse of soil skeleton. During the process leading to liquefaction energy is continuously lost mainly along frictional contacts. This paper presents a theoretical framework for estimating the frictional energy loss along contacts. A pore pressure model is presented as a function of dissipated energy. Based on above developments and understanding, a numerical simulation model using finite difference method is developed to simulate energy dissipation, pore pressure generation, pore pressure dissipation, and densification in a given soil profile for a given earthquake motion. The results are compared with data from centrifuge model tests.

Numerical Simulation of Soil Densification Using Vibro-Stone Columns

Saturated loose sand and non-plastic silty sand deposits are often vulnerable to liquefaction during earthquakes. Sand deposits densified by vibro-stone column (SC) are more resistant to liquefaction, and have performed well during earthquakes. Silty sand deposits appear to perform well when improved by SC supplemented with wick drains. Wick drains aid dissipation of excess pore pressure induced during SC installation in low-permeable silty soils enhancing densification. This paper presents a numerical model to simulate, and to analyze soil densification during SC installation with and without wick drains. Design charts for SC are developed based on this work. These numerical results are compared with field test data. Design guidelines are presented based on these design charts.