In practical engineering, earthquake-induced liquefaction can occur more than once in sandy soils. The existence of low-permeable soil layers, such as clay and silty layers in situ, may hinder the dissipation of excess pore pressure within sand (or reconsolidation) after the occurrence of liquefaction due to the mainshock and therefore weaken the reliquefaction resistance of sand under an aftershock. To gain more mesomechanical insights into the reduced reliquefaction resistance of the reconsolidated sand under aftershock, a series of discrete element simulations of undrained cyclic simple shear tests were carried out on granular specimens with different degrees of reconsolidation. During both the first (mainshock) and second (aftershock) cyclic shearing processes, the evolution of the load-bearing structure of the granular specimens was quantified through a contact-normal-based fabric tensor. The interplay between mesoscopic structure evolutions and external loadings can well explain the decrease in reliquefaction resistance during an aftershock.

Soil elements in situ are subjected to multidirectional shearing during earthquakes. Ignoring the effect of two horizontal shear components generally results in an underestimation of the liquefaction resistance of soils during earthquakes. The actual earthquake sequence generally consists of a mainshock and subsequent aftershocks. Soils may experience liquefaction during the mainshock and then reliquefy again during the subsequent aftershocks. Previous studies on multidirectional loading paths have mainly focused on single liquefaction events. This study employs 3D discrete element modeling to simulate reliquefaction behavior of sands with various multidirectional cyclic simple shear loading histories. The specimens are initially subjected to various strain histories under multidirectional loading paths and then reconsolidated to initial stress states. Subsequently, each soil specimen is subjected to unidirectional cyclic loading in two different directions in the reliquefaction tests. The influence of multidirectional cyclic loading histories on the post-liquefaction drainage compression and reliquefaction resistance are analyzed. Moreover, the evolution of soil fabrics and interaction between fabric orientation and loading direction in the reliquefaction test are investigated. The results highlight that reliquefaction behavior of soils depends on both the fabric and the interaction between the fabric orientation and the loading direction. This study aims to provide micromechanical insight for understanding the effects of multidirectional shearing histories on reliquefaction resistance of sands.

Around the world severe damages were observed due to reliquefaction during repeated earthquakes, whereas precise understanding of its mesoscopic mechanism is not much discovered. Influence of these earthquakes on reliquefaction needs to be investigated to understand its significance in contributing to inherent sand resistance. In the present study, centrifuge model experiments were performed to examine the influence of foreshocks/aftershocks and mainshock sequence on resistance to reliquefaction. Two different shaking sequences comprising six shaking events were experimented with Toyoura sand specimen with 50 % relative density. Acceleration amplitude and shaking duration of a mainshock is twice that of foreshock/aftershock. In-house developed advanced digital image processing (DIP) technology was used to estimate mesoscopic characteristics from the images captured during the experiment. The responses were recorded in the form of acceleration, excess pore pressure (EPP), subsidence, induced sand densification, cyclic stress ratio, void ratio and average coordination number. Presence of foreshocks slightly increased the resistance against EPP before it gets completely liquefied during the mainshock. Similarly, aftershocks also regained the resistance of liquefied soil due to reorientation of particles and limited generation of EPP. However, application of mainshocks triggered liquefaction and reliquefaction and thus eliminated the beneficial effects achieved from the prior foreshocks. Reliquefaction was observed to be more damaging than the first liquefaction, meanwhile the induced sand densification from repeated shakings did not contribute to increased resistance to reliquefaction. The apparent void ratio estimated from the DIP technology was in good agreement with real void ratio values. Average coordination number indicated that the sand particles moved closer to each other which resulted in increased resistance during foreshocks/aftershocks. In contrast, complete liquefaction and reliquefaction have destroyed the dense soil particle interlocking and made specimen more vulnerable to higher EPP generation. (c) 2025 Production and hosting by Elsevier B.V. on behalf of The Japanese Geotechnical Society. This is an open access article under the CC BY- NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Soil liquefaction significantly contributes to inducing catastrophic damage to the infrastructures. Different ground improvement methods were used widely to improve the seismic resistance of liquefiable deposits to mitigate liquefaction. Use of granular column technique is a popular and well-recognized improvement technique due to its drainage, shear reinforcement, and densification characteristics. However, studies relating to seismic resistance of stone column-reinforced ground against multiple shaking events were limited. Recent seismic events also have shown the possibility of liquefaction and reliquefaction due to multiple seismic events. Considering this, the performance assessment of the granular column technique in liquefiable soil under repeated shaking events is addressed in this study. The possibility of re-using construction and demolition waste concrete aggregates as an alternative to natural aggregates is also attempted to propose sustainability in ground improvement. For experimental testing, a saturated ground having 40% density was prepared and subjected to sequential incremental acceleration loading conditions, i.e., 0.1 g, 0.2 g, 0.3 g, and 0.4 g at 5 Hz loading frequency for 40 s shaking duration using a 1 g Uni-axial shake table. The efficiency of selected ground improvement was evaluated and compared with untreated ground. The experimental results showed that ground reinforced with granular columns performs better up to 0.2 g shaking events in minimizing pore water pressure and settlement. Possibility of column clogging, and inadequate area replacement ratio (5%) affects the performance of column during repeated shaking. Also, irrespective of improvement in in-situ ground density; continuous generation of pore water pressure due to absence of drainage posing reliquefaction potential in untreated ground under repeated shaking events.

The present study aims to examine the behavior of sand reliquefaction phenomenon on gently inclined sloping ground subjected to repeated seismic events. The seismic sequence represents the combinations of foreshocks and aftershocks associated with mainshock events. Free field and pile group models were experimented with in a sloping ground with 5 degrees inclination using centrifuge modelling. A 2 x 2 pile group model was inserted in Toyoura sand, and results were compared with that with free field model ground. Tapered sinusoidal waveform was inputted at a constant 1 Hz shaking frequency, whereas the acceleration amplitude and shaking duration for the mainshocks were considered twice that of foreshocks and aftershocks. Two different seismic sequences with six shaking events were imparted to the model grounds to investigate the influence of slope and presence of pile group on sand reliquefaction behavior. The time history responses were recorded in the form of acceleration, excess pore pressure (EPP), bending moment, and lateral displacement and presented. The response indicates that sloping ground was stable under the action of foreshocks, whereas it collapsed during mainshocks, primarily due to liquefaction. The mainshock has transformed the gently inclined sloping ground to levelled ground model. This transformation has resulted in increased bending moment values in the pile group. Resistance to reliquefaction was smaller compared to first liquefaction, primarily due to change in soil state and increase in magnitude of anisotropy. Presence of slope has resulted in higher EPP response and bending moment values compared to levelled ground. GeoPIV analysis and visualization show the flow of sand particles from upside to downside due to lateral spreading at shallow depths that initiated the slope failure. The foreshocks and aftershocks are not very significant in increasing the resistance to reliquefaction. Meanwhile the presence of the pile group has reduced the EPP generation during repeated shaking events.

A series of centrifuge model experiments were performed in this study to investigate the sand reliquefaction behavior in free field and pile group models subjected to repeated shakings. Toyoura sand was used to prepare the model ground with 50 % relative density and experimented at 50g centrifugal acceleration. A 2 x 2 pile group model was used to examine the response of piles and its effect on sand reliquefaction during repeated shakings. A seismic sequence comprising foreshocks-mainshock-aftershocks-mainshock pattern was imparted to both free field and pile group models. In addition, an independent mainshock was imparted to a free field model and compared with the seismic sequence to examine the effect of foreshocks in triggering future liquefaction. Acceleration time response, excess pore pressure (EPP), ground subsidence, bending moment and lateral displacement of the pile group were measured. Significant de-amplification and unsymmetrical response with presence of shear-induced dilatancy spikes were observed in both free field and pile group models. Results from this study indicate that foreshocks and aftershocks are not sufficient to induce liquefaction. However, liquefaction and subsequent reliquefaction were recorded in both models during mainshocks. Pile group recorded smaller EPP response and subsidence compared to free field model. Higher magnitude of reliquefaction was induced even at moderate depths during the second mainshock at a quicker time. This was primarily associated with the increase in magnitude of anisotropy due to repeated generation and dissipation of pore water which results in decrease in resistance to further reliquefaction. Hardening induced around the vicinity of the pile group due to sand densification has resulted in smaller bending moment values during second mainshock compared to the first.

Examining the reliquefaction resistance of sand deposits is more challenging due to the complex interplay of several factors that may increase or decrease the resistance. This resulted in severe limitations in understanding the reliquefaction mechanism of sand deposits subjected to repeated shaking events. The present study attempted to overcome this limitation by examining the reliquefaction resistance using 1-g shaking table experiments. A total of 65 shakings were performed on saturated Solani sand with varying acceleration amplitude, dynamic frequency, shaking duration, and relative density of the sand specimen. All the above factors were experimented with three different shaking patterns (incremental, uniform and decremental) and independent events. For each shaking event, generation and dissipation of excess pore pressure, soil subsidence, and relative density variations were presented. The beneficial effect of seismic preshaking were applicable in partially liquefied soils that were subjected to incremental shaking pattern. On the other hand, contrary results were reported for uniform and decremental shaking patterns, where the later found to be more damaging. The state of the soil (partially or completely liquefied) governs the reliquefaction resistance, as the beneficial effect of preshaking was applicable only in partially liquefied soils, irrespective of the shaking pattern. Whereas complete liquefaction disturbs the structure of existing sand specimens and results in reduced reliquefaction resistance for future seismic events.