Numerical analysis of earth embankments in liquefiable soil and ground improvement mitigation
Introduction
Earth embankment safety is crucial since it can endanger human life and property. The seismic performance, failure mechanism, and avoiding or minimizing liquefaction-induced deformations of embankments founded on liquefiable soils are significant research concerns regarding structural operations and earthquake countermeasures. Field investigations on seismic disasters reveal earthquake-induced liquefaction as one of the most significant causes of destruction of soil embankments. Earthquake-induced liquefaction may result in significant loss of soil strength and produce large permanent deformations and ground flow with the accumulation of EPWP. In most cases, there was extensive damage mainly due to the liquefaction of the supporting sandy soils [1,2], resulting in severe subsidence and lateral ground spreading. Typical examples of liquefaction failure include the behavior of embankments during the San Fernando earthquake of 1971, and the Hyogoken-Nanbu earthquake of 1995 [3]. Moreover, as a consequence of the 2011 off the Pacific coast of Tohoku Earthquake, many geo-structures were severely damaged due to earthquake-induced liquefaction. Thus, seismic liquefaction evaluation is necessary for embankment design and safety assessment.
Several studies have evaluated the seismic performance of embankments through laboratory tests and numerical simulations. Experiments on this subject included shaking table tests [4,5] and dynamic centrifuge experiments [[6], [7], [8]]. Consistent with the findings of field analysis, the results of the test showed that earthquake-induced liquefaction would cause destruction and collapse of the soil embankment. Although experiments require considerable manpower and material resources, it is difficult to produce universally applicable results based on a small number of experiments. In recent years, numerical simulation has become an effective tool, and several numerical investigations have been carried out on the liquefaction-related seismic behavior of earth embankments [1,[9], [10], [11], [12]]. Most of the above studies were put in the same significant category that dynamic constitutive model appeared to define only for sandy soil, and their parameters were various and difficult to determine. The selection of a constitutive model to describe the behavior of both liquefied and un-liquefied soil was a key factor in the accuracy of the numerical analysis. In this dynamic analysis, soils were modeled by the cyclic mobility model, which was originally developed to describe the features of clean sands, including its cyclic mobility and liquefaction strength, while, also describing the behavior of clayey soils. The main differences between the mechanical behavior of sand and clay were in the collapse rate of structure, and the loss rate of overconsolidation [14]. For sand, the rate of collapse of the structure was faster than the rate of loss of overconsolidation. In contrast, for clay, the rate of collapse of the soil structure is much slower than the rate of loss of overconsolidation. The characteristics of soil (sand and clay), such as stress-strain relationships, shear dilatancy, strain-hardening and softening, cyclic mobility, liquefaction behavior were difficult to be described accurately in a unified way without using this constitutive model. Meanwhile, all the parameters in the CM model could be obtained by laboratory tests. Since the focus of these studies was the earth embankment sitting on a uniform sandy foundation, the dynamic behavior of the earth embankment founded on liquefiable ground comprising a sandy layer with a low permeability layer of clay remains poorly understood. Cohesive soil (clay) had a very low permeability coefficient compared with sand; thus, more excess pore water pressure (EPWP) remained for a much longer period in the non-homogeneous soil deposits compared with uniform sandy soil deposits. The dissipation of EPWP in clayey soil layer was the dominant factor for settlement after seismic excitation, and played a key role in the extent of settlement and lateral deformations in the earth embankment. Due to the 2011 off the Pacific coast of Tohoku Earthquake, in areas such as Edo River embankment at Nishisekiyado and Eai River embankment at Fuchishiri-jyoryu [15], the most severe damage was observed in this type of soil profile, in which sandy layer existed below the embankment body bounded by the clayey layer. Moreover, there was a lack of research on the long-term consolidation characteristics of liquefied soil foundation, especially the long-term liquefaction-induced deformation after seismic loading.
Ground improvement methods, such as densification, cementation, drainage, and reinforcement, have been developed to mitigate the destruction by liquefaction. Some studies have been conducted to test the effectiveness of foundation reinforcement methods in countering liquefaction [[16], [17], [18]], among which, the anti-liquefaction measure by grouting was confirmed to be an effective approach [19]. Additional numerical studies are necessary to confirm the effects of grouting on liquefaction-related performance and liquefaction-resistant behavior for improving the stability of earth embankments in liquefiable ground.
To evaluate the liquefaction-mitigation performance using the grouting technique, a design study based strictly on numerical analysis was taken up. A kinematic hardening elastoplastic dynamic constitutive model, describing the static-dynamic properties of subsoils with different densities in a unified manner under diverse loading types (cyclic or monotonic) and drainage conditions, was applied to accurately represent the seismic performance of soils under earthquake excitations. In this numerical analysis, large liquefaction-induced settlement and lateral ground spreading that occurred during earthquake loading, and in the process of long-term post-seismic consolidation could be determined uniformly. This was different from conventional liquefaction analysis, where only the sandy layer was considered and the two different processes of seismic liquefaction and subsequent long-term consolidation were treated independently. Therefore, this study might provide a reliable evaluation of liquefaction-induced seismic performance of the earth embankment built on the sandy layer and thick clayey layer. Additionally, it numerically evaluated the mitigation effects of using grouting technology.
Section snippets
Background
The study case observed the behavior of a symmetric earth embankment 6 m high with a crest 5 m wide and a slope of 1:1.5, as illustrated in Fig. 1. This soil embankment was constructed in Aichi Prefecture, Japan. The foundation comprised a 10 m thick saturated liquefiable sandy layer (As layer) over a 20 m thick cohesive clayey layer (Ac layer) resting on hard and impermeable rock. The embankment body was a dry-compacted backfill. The backfill was plain fill, composed of gravel, sand, silty
Mechanical property of grouted soil
As illustrated in Fig. 1, the groundwater table was level with the ground surface, and the subsoil was fully saturated. Furthermore, this site was vulnerable, being in an earthquake-prone area. Under seismic activity, subsidence and lateral spreading would occur in the earth embankment. Thus, embankment made earlier may not satisfy the requirements of the anti-earthquake specifications of the new seismic design standards issued following the 2011 off the Pacific coast of Tohoku Earthquake.
Summary
Stringent seismic design requirements introduced after the 2011 off the Pacific coast of Tohoku Earthquake consider earthquake-induced liquefaction as a key factor in ensuring construction safety. This research presented a numerical investigation into the seismic capacity of an earth embankment and the mitigation of destruction through grouting for construction on liquefiable soil. The CM model was used for the seismic behavior of the foundation soil for the first time, and the finite element
Author statement
Linlin Gu: Data curation, writing-original draft preparation; Zhen Wang: Writing-Reviewing and Editing; Wenxuan Zhu: Data processing; Boan Jang: Supervision; Xianzhang Ling: Methodology; Feng Zhang: Investigation, Conceptualization. We agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.
All persons who have made substantial contributions to the work are all
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
The support of National Natural Science Foundation of China (Grant No. 51908288, No. 41627801) and National Research Foundation of Korea (Grand No. 2019R1A6A1A03033167) are greatly appreciated.
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