Soil liquefaction is a subject of long-standing interest in earthquake geotechnical engineering. Although significant advances in liquefaction research have been achieved in the past decades, it ...remains an area of great difficulty and uncertainty, as evidenced by the extensive liquefaction-related damage in many recent earthquakes. Of particular concern is the widespread liquefaction observed in silty sand deposits, raising questions about the deficiencies of the current methods for liquefaction evaluation. This paper presents selected results of a long-term research aimed at developing a rational method for evaluating the liquefaction potential of both clean and silty sands. The method is based on a comprehensive experimental program comprising small-strain shear wave testing and large-strain undrained shear testing for a wide range of sand states and is built in the critical state framework. A remarkable feature of the method is the unified characterization of shear wave velocity for both clean and silty sands through a state parameter defined in a sound theoretical context. As shear wave velocity is a well-defined soil property and can be measured both in the field and in the laboratory, and since the state parameter has proven a useful state variable for characterizing soil behavior under both cyclic and monotonic loadings, the new method is attractive and promising in a wide range of geotechnical applications.
Kutch region has experienced widespread liquefaction and related damage during a series of past earthquakes; however, cyclic behavior of sandy soils of the region is yet to be explored. The current ...study is focused on the dynamic characteristics of natural soil deposits of the high seismicity Kutch region. In the previous studies, variations in fines content (FC) and the nature of fines (plastic, non-plastic) has been controlled and systematic and based on either non-plastic or plastic fines using mostly standard or river sands. However, the present study is different in the approach as the dynamic characteristics of natural sandy deposits with simultaneous presence, and random variations of both the plastic and non-plastic fines are explored. Results from cyclic triaxial tests indicated sandy soils of Kutch region are severe to moderately prone to liquefaction with a strong dependence on plasticity rather than FC. Pore pressure ratio (ru), mean effective pressure (p′) and cyclic stress ratio (CSR) for silty-sands signified effective stress failure, which was found to be coinciding with initial liquefaction. Clayey-sands exhibited strength degradation instead of liquefaction, and the plasticity index (PI) was found to control the magnitude of degradation. Large degradation in secant shear modulus and reduction in damping ratio with the number of cycles was observed in silty-sands whereas clayey-sands exhibited relatively lesser degradation in secant shear modulus and damping ratio.
•Sandy soils of the Kutch region exhibited different dynamic behavior to be strongly dependent on the FC and nature of fines. While silty-sand specimens attained liquefaction within 31 cycles, clayey-sand specimens did not liquefy up to 50 cycles.•Effective stress path analysis revealed that Kutch soil with non-plastic fines, irrespective of total FC, displayed effective stress failure (p¢ reaching zero, coincidental with initial liquefaction) within 31 cycles. Whereas in the presence of plastic fines, effective stress path of clayey-sand specimens ceased to reach to the stress origin indicating cyclic degradation.•Evolution of cyclic stress ratio with the number of cycles was found to be strongly influenced by the FC, and the nature of fines.
The liquefaction resistance of silty sands and the potential effect of initial static shear stress are major concerns in seismic design of dams and embankments. This paper presents a systematic ...experimental study on non-plastic silty sands to address these concerns. It is shown that the concept of threshold α (αth) proposed by Yang and Sze (2011) to characterize the impact of α (representing the initial static shear stress level) on cyclic resistance (CRR) of clean sands is applicable to silty sands as well. When α < αth, CRR increases with increasing α, otherwise it decreases with increasing α. The threshold α is affected by the initial packing density, the initial effective confining pressure and the fines content. An improved state dependency of the threshold α, which is regardless of fines content, is proposed in the framework of critical state soil mechanics by using the state parameter (ψ). An analysis platform, known as CRR-ψ platform and developed based on clean sand data, is shown to have the capability of characterizing the state dependence of CRR for sands with varying fines contents. This platform in conjunction with the unified αth-ψ correlation provides a unified and consistent framework for understanding the complicated effects of initial static shear stress on soil liquefaction and for quantifying such effects for engineering practice.
•A unified CRR-ψ platform is proposed for silty sands with different fines contents.•Linear CRR-ψ correlations exist on the platform which rotate clockwise with increasing α.•The threshold α (αth) concept is applicable to both clean and silty sands.•An αth-ψ correlation is proposed for both clean and silty sands.
In the engineering geology field increased attention has been posed in recent years to potential liquefaction mitigation interventions in susceptible sand formations. In silty sands this is a major ...challenge because, as the fines content increases, vibratory methods for densification become progressively less effective. An alternative mitigation technique can be the installation of Rammed Aggregate Pier® (RAP) columns that can increase the resistance of the soil, accounting for its lateral stress increase and for the stiffness increase from soil and RAP composite response. To investigate the influence of these factors on liquefaction resistance, full-scale blast tests were performed at a silty sand site in Bondeno (Ferrara, Italy) where liquefaction was observed after the 2012 Emilia-Romagna earthquake. A multidisciplinary team of forty researchers carried out devoted experimental activities aimed at better understanding the liquefaction process at the field scale and the effectiveness of the treatment using inter-related methods. Both natural and improved areas were investigated by in-situ tests and later subjected to controlled blasting. The blast tests were monitored with geotechnical and geophysical instrumentation, topographical surveying and geological analyses on the sand boils. Results showed the RAP effectiveness due to the improvement of soil properties within the liquefiable layer and a consequent reduction of the blast-induced liquefaction settlements, likely due to soil densification and increased lateral stress. The applied multidisciplinary approach adopted for the study allowed better understanding of the mechanism involved in the liquefaction mitigation intervention and provided a better overall evaluation of mitigation effectiveness.
•Blast tests produced liquefaction and sand boils at a silty sand site in northern Italy.•Blast tests produced volumetric strain similar to that expected in an earthquake.•Aggregate piers as a liquefaction hazard mitigation strategy improved soil performance.•Geophysical and geotechnical site investigations were carried out before and after blasting.•Field responses of treated and non-treated soils during blasting were compared.
This paper addresses the differences between two approaches of damping estimation in the resonant column testing: Steady-State Vibration (SSV) and Free Vibration Decay (FVD) method, at small ...(<0.005%) to medium (0.07%) strain range. The tests were conducted on “two types of reconstituted sands” and “two types of clayey soils” at different relative densities and confining pressures. The test results suggested to use the SSV method in small strain damping measurement and the FVD method (two or three successive cycles) in medium strain damping measurement. A systematic decrease in the damping with the increasing number of cycles was observed up to a certain strain level in the FVD method. Furthermore, the effects of relative density, confining pressures, soil types on the damping ratio derived from the two methods for the chosen soils were studied. The results showed that the damping ratio of clayey soils exhibits a little higher value than those of sandy soils.
The current research is focused on the static liquefaction and effective stress path characteristics of soils of the high seismicity Kutch region, India. In previous studies, the effects of the fines ...content and the plasticity of the fines on the undrained behavior were explored in a controlled and systematic manner with either non-plastic or plastic fines added to standard or river sands. The undrained characteristics of natural soil deposits having the simultaneous presence of both silt and clay fractions have not been explored. In the present study, the effects of a varying fines content and the nature of the fines on the static liquefaction and effective stress path characteristics of various soil deposits in the Kutch region at their in-situ densities have been studied. Various liquefaction indices were determined to quantify the static liquefaction, namely, the undrained brittleness index, the collapse potential, the liquefaction potential, and the resistance to further deformation. The variation in these parameters was analyzed in the context of the fines content, silt content, clay content, and plasticity. The effective stress paths of the Kutch soils exhibited a strain softening, limited strain softening or strain hardening response and their behavior was controlled by the fines content and its nature. Due to the presence of non-plastic fines, and irrespective of the content, the silty sands exhibited intense strain softening which was captured by the liquefaction indices.
In this study, cyclic triaxial tests and centrifuge tests using silty sands were simulated in OpenSees to investigate the effect of fines on the liquefaction potential of saturated silty sands. ...Choice of proper density index was found to be extremely important for characterizing the liquefaction potential of silty sand, as different conclusions can be derived if we use different density indices for comparing experimental results. Laboratory results reported by past researchers were found to be mutually contradictory, but such contradictions were resolved by using a proper desity index. Laboratory investigations were commonly conducted by keeping relative density or void ratio fixed to study the effect of fines on liquefaction potential. However, it was found that a unique relation between cyclic stress ratio, number of loading cycles required for triggering liquefaction, and equivalent granular void ratio (e*) can be established for silty sand with fines content less than thethreshold fines content. Furthermore, finite element method, using an advanced constitutive model with calibrated parameters, was found to be effective in simulating the dynamic responses of silty sand but it significantly underestimated the settlement of free field due to liquefaction. Moreover, parametric studies showed that the cyclic resistance of silty sand can be unified by the equivalent granular void ratio.
•Numerical modelling was used to study the liquefaction potential of silty sands in cyclic triaxial and centrifuge tests.•Equivalent granular void ratio can reflect the effect of fines content on the liquefaction potential of sands.•Cyclic resistance curve of silty sand can be derived from the cyclic resistance curve of the host sand.•Finite element simulation with a proper soil model can effectively simulate the liquefaction potential of silty sand.•Numerical finite element modelling was found to underestimate the surface settlement in free field condition.
In earthquake engineering practice, the liquefaction potential of soils is commonly evaluated through simplified procedures. These approaches are suitable for sands with very low to no fines content, ...which have been traditionally thought to be the only liquefiable materials. However, field observations and experimental research have extensively demonstrated that low plasticity silty sands can also be highly liquefiable. Thus, this paper investigates the effect of nonplastic fines contents on the liquefaction potential of soils, taking the 2010–2011 Canterbury Earthquake Sequence as a case study. The validity of standard simplified procedures for high fines content soils is critically evaluated and compared with a finite element model based on a full solid–fluid coupled formulation. The model includes a state parameter-based constitutive law within the generalised plasticity theory, which allows the fines content to be taken into account explicitly. The standard simplified procedures are shown to be less effective in the evaluation of liquefaction potential in soils with high fines content but are still indispensable tools for evaluating the performance of soils over large urban areas. As the main conclusion, it is recommended that empirical models are complemented with an advanced numerical analysis in those cases where silty sands with high fines content are identified, as its outcomes can more realistically represent the soil behaviour during a seismic event.
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