⚡ electrical engineering

Effect of fines content on reliquefaction resistance and consolidation–reconsolidation behavior of silty sand

This study investigates how increasing fines content (up to 35%) enhances the reliquefaction resistance of silty sand and alters its consolidation-reconsolidation behavior, revealing that resistance surpasses initial liquefaction thresholds when fines content reaches 25% and the proposed anisotropic coefficient ratio drops below 1.

Original authors: Yeming Zhong, Xueqian Ni, Sheng Zhang, Zhao Zhang, Feng Zhang, Feng Shan

Published 2026-09-03

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CC BY 4.0

Original authors: Yeming Zhong, Xueqian Ni, Sheng Zhang, Zhao Zhang, Feng Zhang, Feng Shan

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The ground beneath our feet is not always as solid as it seems. In certain conditions, when the earth shakes, water trapped between soil grains can build up so much pressure that the soil temporarily loses its strength and begins to behave like a thick liquid. This phenomenon, known as liquefaction, can cause buildings to sink and roads to crack. For decades, engineers believed that once a patch of ground had liquefied and then settled back down, it would become denser and safer, effectively hardening against future shaking. However, real-world observations after major earthquakes have challenged this comforting idea. In some places, the ground liquefied again during smaller aftershocks, causing further damage. This mystery is particularly puzzling in soils that are a mixture of sand and fine silt particles. While pure sand is well understood, these mixed soils behave in complex ways that depend heavily on how much fine silt is present, leaving engineers unsure of how to protect structures built on them.

A team of researchers at Central South University and Tongji University in China set out to solve this puzzle by looking closely at how different amounts of silt change the behavior of sand after it has already liquefied once. They prepared eight different mixtures of sand and silt, ranging from pure sand to a mixture containing thirty-five percent silt. Using a specialized machine that simulates the shaking of an earthquake, they subjected these soil samples to a first round of shaking until they liquefied. After the shaking stopped, they allowed the water pressure to dissipate and the soil to settle back into a solid state, a process called reconsolidation. Then, they shook the exact same samples again to see how they would respond to a second earthquake. The goal was to determine whether the soil became stronger or weaker after the first event and to understand how the amount of silt influenced this outcome.

The results revealed a surprising shift in behavior that depends entirely on the amount of silt in the mix. For samples with low amounts of silt, the soil behaved as expected: it became weaker after the first liquefaction and failed much faster during the second shaking. However, as the researchers increased the silt content, a turning point emerged. When the silt content reached twenty-five percent or higher, the soil actually became more resistant to the second liquefaction than it was to the first. In these high-silt mixtures, the ground could withstand more shaking cycles before failing a second time, defying the traditional assumption that liquefied soil is always more vulnerable to subsequent events. The researchers found that this improvement in resistance was not random but followed a clear pattern tied to the internal structure of the soil.

To understand why this happened, the team looked at how the soil particles arranged themselves before and after the shaking. They discovered that the presence of silt changes the way sand grains touch and pack together. In mixtures with lower silt content, the shaking creates a specific internal alignment that makes the soil unstable when shaken again. But in mixtures with high silt content, the fine particles act like a coating around the larger sand grains. This coating changes the contact points between particles and alters how the soil compresses. After the first liquefaction and the subsequent settling, this coated structure creates a new arrangement that is surprisingly stable. The researchers introduced a new way to measure this stability by comparing how much the soil squeezed down vertically versus how much it squirmed sideways during the settling process. They found that when this ratio dropped below a certain level, the soil was guaranteed to be stronger against a second quake.

The study also examined the microscopic world of the soil using powerful imaging to see the tiny details of the particle arrangement. They observed that in the high-silt mixtures, the fine particles effectively wrapped around the sand, isolating them from one another. This coating prevented the sand grains from forming the unstable, column-like structures that usually lead to rapid failure during a second shaking. Instead, the soil maintained a more uniform and robust internal fabric. This microscopic view confirmed that the silt does not just fill empty spaces but actively reshapes the soil's skeleton, changing how it responds to stress. The findings suggest that for soils with high silt content, the risk of reliquefaction might be lower than previously thought, provided the soil has had time to settle and reorganize after the initial event.

This research provides a clearer picture of how mixed soils behave under the stress of repeated earthquakes. It challenges the old rule that all liquefied ground becomes permanently weaker and shows that the specific composition of the soil, particularly the amount of silt, plays a decisive role in its recovery. By identifying the specific threshold where soil becomes more resistant to a second shaking, the study offers a new tool for engineers to assess the long-term safety of foundations in earthquake-prone areas. The work highlights that the ground is not a static object but a dynamic system that can reorganize itself, sometimes becoming stronger after a disturbance, depending on the precise balance of materials within it.

Technical Summary: Effect of Fines Content on Reliquefaction Resistance and Consolidation–Reconsolidation Behavior of Silty Sand

Problem Statement
Traditional liquefaction theory posits that soil densifies after the first liquefaction event, thereby increasing its resistance to future liquefaction. However, field observations contradict this, showing that silty sand sites can reliquefy during subsequent aftershocks. While laboratory studies on clean sand indicate that reliquefaction resistance often decreases significantly after the first event due to structural reorganization and stress-induced anisotropy, the mechanisms governing these processes in silty sand remain largely unexplored. Specifically, the influence of fines content (FC) on the consolidation/reconsolidation behavior and reliquefaction resistance of transitional sand-silt mixtures requires further elucidation.

Methodology
The study employed a series of undrained cyclic triaxial tests on eight mixtures of Fujian sand and marine-deposited silt, with fines content (FC) ranging from 0% to 35%. Specimens were prepared using the moist tamping method to control density and prevent segregation. The testing protocol involved:

Initial Consolidation: Isotropic consolidation under an effective mean stress of 100 kPa.
First Liquefaction: Application of cyclic stress-controlled loading (sinusoidal wave, 0.01 Hz) at Cyclic Stress Ratios (CSR) of 0.20 and 0.10. Liquefaction was defined as a double amplitude axial strain of 5%.
Reconsolidation: Drainage was opened, and specimens were allowed to reconsolidate under 100 kPa confining pressure.
Reliquefaction: The same specimens were subjected to identical cyclic loading to simulate a second seismic event.
Microstructural Analysis: Scanning Electron Microscopy (SEM) was used to examine meso-fabric changes (particle contact modes, pore structure, and orientation) in specimens before and after liquefaction for FC values between 20% and 35%.

Key Contributions and Results

Fines Content and Reliquefaction Resistance: The study revealed a non-monotonic relationship between FC and liquefaction resistance. During the first liquefaction, resistance generally decreased as FC increased from 0% to 15%, with a transition to flow liquefaction at 15%. However, for FC ≥ 20%, resistance began to increase. Crucially, a threshold was identified at FC = 25%. Below this threshold, reliquefaction resistance was lower than the first liquefaction resistance. Above this threshold (FC ≥ 25%), the reliquefaction resistance surpassed the first liquefaction resistance and continued to increase with higher FC.
Strain Development and Anisotropy:
- Volumetric Strain: Compressibility increased with FC. During reconsolidation, volumetric strains were consistently higher than during initial consolidation, particularly for FC > 20%.
- Anisotropy Evolution: The study analyzed the ratio of volumetric strain to axial strain ( $\epsilon_v/\epsilon_a$ ) to quantify anisotropy. In the "sand-like" region (lower FC), inherent anisotropy initially decreased then increased with FC. In the "silt-like" region (higher FC), inherent anisotropy decreased.
- Stress-Induced Anisotropy: Following liquefaction, stress-induced anisotropy increased with FC in the sand-like region but continuously weakened in the silt-like region. Clean sand exhibited higher stress-induced anisotropy than silty sand, suggesting fines weaken the expression of this anisotropy.
Proposed Index ( $R_{ac}$ ): The authors introduced an anisotropic coefficient ratio, $R_{ac}$ , defined as the ratio of the volumetric-to-axial strain ratio during reconsolidation to that during initial consolidation.
- $R_{ac}$ decreased consistently as FC increased.
- A critical condition was identified: When $R_{ac} < 1$ and $FC \ge 25\%$ , the reliquefaction resistance exceeds the first liquefaction resistance. Conversely, when $R_{ac} > 1$ (typically $FC < 25\%$ ), reliquefaction resistance is lower than the first liquefaction resistance.
Meso-Fabric Mechanisms: SEM observations provided a physical explanation for the increased resistance at high FC.
- Before Liquefaction: In sand-like soils, fines acted as fillers. In silt-like soils (FC ≥ 25%), fines replaced sand as the primary skeleton constituent, forming aggregates that encapsulated sand particles.
- After Liquefaction: The reconsolidation process created drainage pathways and densified the soil. In high-FC specimens, fines formed a coating around sand grains, transitioning contact modes from point/diffuse to intimate surface contacts. This coating disrupted the continuity of the sand skeleton, preventing the effective transmission of stress-induced anisotropy through the granular framework. Consequently, the macroscopic manifestation of anisotropy was attenuated, leading to higher reliquefaction resistance.

Significance
The paper claims that understanding the evolution of anisotropy and the specific role of fines content is essential for predicting the long-term stability of foundation soils in seismic zones. By proposing $R_{ac}$ as an indicator, the study offers a method to evaluate the potential for reliquefaction in silty sands. The findings suggest that for soils with high fines content ( $FC \ge 25\%$ ) and low $R_{ac}$ , the stress-induced anisotropy typically associated with reduced resistance may no longer be the dominant factor. This insight allows for differentiated mitigation strategies in seismically active regions, particularly for sites prone to multiple liquefaction events. The study emphasizes that the evolution of liquefaction resistance is not solely a function of density changes during consolidation but is closely tied to intrinsic mechanisms such as soil structure reorganization and particle arrangement alterations induced by the mainshock.

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