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. 2026 Jan 19;16:5769. doi: 10.1038/s41598-026-36391-8

Mechanical properties of landslide slip zone soil considering over consolidation ratio and particle grading factors

Zhongxuan Chen 1, Meng Zhao 2,✉, Shupeng Jiang 3, Qinwen Tan 4,✉, Yuangang Ma 5, Yanzhe Li 5, Xiao Yang 6
PMCID: PMC12894841  PMID: 41554831

Abstract

A precise assessment of the mechanical characteristics of the slip zone is essential to implement the stability analysis of landslides. This article examines the mechanical properties of slip zone soil using large-scale direct shear tests and ring shear tests while taking into account the over consolidation ratio (OCR) and particle grading factors. The mechanism of soil particle behavior was examined from a macro-meso perspective to account for the mechanical characteristics observed from the tests. Further, the three determined groups of mechanical characteristics were employed in the landslide numerical model, respectively. The comparability between the simulative results and field monitoring data was examined, and then the suitability of the strength parameters to the landslide was explored. Results indicate that the landslide deformation and its mechanism can only be accurately described when the OCR of samples and the original particle gradation are considered. In this way, a superior technique for determining the soil in the slip zone is established, which has great significance for landslide modeling.

Keywords: Slip zone soil, Macro-meso process, Over consolidation ratio, Particle grading

Subject terms: Engineering, Environmental sciences, Natural hazards, Solid Earth sciences

Introduction

The tremendous threat that landslides pose to the environment and human security has been acknowledged. For instance, the Vajont landslide in Italy in 1963 generated a tsunami that overtopped the dam and poured into the valley below, killing approximately 2000 people and destroying the whole town1. In 2009, Jiwei mountain landslide in China caused 74 fatalities and eight injuries2. Since 2009, a major relocation project has been implemented at the site of Badong county, China, since it was identified as being located on a giant old landslide with a profound impact on residents’ lives and the environment3. In 2015, The Lishui landslide in Zhejiang Province, China, claimed 37 lives, resulted in losses of 1.7 billion CNY, and severely harmed the biological ecosystem4.

Fundamental studies on the properties of landslides serve as the foundation for effective prevention or forecast approaches. Because a landslide has one or more slip zones, it differs from other slopes. We can only realistically assess the state of the landslide by precisely analyzing the mechanical properties of the slip zones and obtaining the genuine mechanical indices. Slip zones, particularly for massive landslides, are characterized by a high consolidation ratio due to the enormous overlying load5–9; in this situation, the condition of the slip zone soil should be fully taken into consideration for getting accurate indexes.

Existing experimental studies on landslides mostly neglected the pre-consolidation pressure of the slip zone, resulting in the deviation between the measured mechanical parameters and the true values, and cannot sufficiently characterize the state of landslides10–13. A small number of studies have attempted to resolve this issue. Chen et al.14 performed ring shear tests on specimens with various over-consolidation ratios (OCRs), and the stable residual strength over-consolidation ratio (SRSOCR) was proposed to study the strength property of slip zone soil for clearly understanding the impact of stress conditions on the shear characteristics of slip zone soil. Li et al.15 investigated the influence of OCR and shear rate on the residual strength of SM-type (silty sand) landslide soils in the Three Gorge Reservoir using ring shear tests under drained conditions. Test results showed that residual strengths of SM-type landslide soils were not affected by the over-consolidation process significantly. Even though, few investigations have explored their combined effects. This single-factor focus fails to capture the complex mechanical behavior of slip zone soil, as its strength and deformation are inherently regulated by the interaction between stress history, which is reflected by OCR, and structural composition, determined by particle grading.

On the other hand, traditional laboratory shear instruments (direct shear apparatus, triaxial apparatus, etc.) frequently have a limited specimen size. According to the criteria of geotechnical test procedures, coarse particles in the slip zone soil must be screened out during sample preparation to avoid the influence of significant particles. However, the change of particle gradation affects its strength and deformation properties16–23. Li et al.24 carried out large-scale ring shear tests in order to investigate the naturally drained shear behavior of slip zone soils of three giant landslides in the Three Gorges Reservoir area. They found that the presence of coarse particles hinders the development of a slick shear surface and that increasing shearing rate tends to positively influence residual shear stress by allowing rougher shear surfaces, but this effect is dimmed by increasing coarse fraction and density. Zou et al.25 investigated the shear mechanical property of the gravelly slip zone soil by comparing the in situ and laboratory direct shear tests, and found that the presence of coarse particles improves the shear strength of the slip zone soil.

With regard to the deformation mechanism of slip zone soil, Nian et al.26 used the improved direct shear test apparatus to study the effect of water content on the shear strength of remolded slip zone soil27. The experimental results indicate that the shear strength and stress-strain curve represents a significant “softening” type with unsaturated slip-zone clay; when the water content is greater than 20%, the curve tends to be a “hardening” shear type. Employing the ring shear test, a number of scholars have conducted a series of studies on slip zone soil. Wang et al.28 examined the effect of moisture content and shearing speed on the strength of slip zone soil using the drainage ring shear test. The results reveal that the residual strength of the soil samples decreased with increasing moisture content and shearing speed. Lian et al.29 examined the effect of moisture content on the residual strength of slip zone soil using a ring shear test; the results indicated that the residual strength generally decreased with increasing moisture content. In addition, the residual strength at any moisture content is approximately proportional to the normal stress for a given shear rate30,31. Shear stress fluctuation was discovered to be more assertive at higher shearing rates.

To address these critical gaps, this study targets the Huangtupo landslide, a major reservoir landslide in the Three Gorges Reservoir area, and systematically investigates the mechanical properties of its slip zone soil under different over-consolidation ratios. By integrating large-scale direct shear tests, which retain the original particle gradation of in-situ soil, ring shear tests, which focus on the influence of OCR and numerical simulations, this research aims to reveal the combined effects of OCR and particle grading on the shear strength and deformation mechanism of slip zone soil. The findings are expected to provide a more reliable technical basis for the precise evaluation of landslide stability and the scientific selection of strength parameters, thereby addressing the limitations of existing single-factor and small-scale test studies.

Materials

The Huangtupo landslide

The Huangtupo landslide is situated in the Three Gorges Reservoir (TGR) region of China on the south bank of the Yangtze River. It is a significant composite body of deformation made up of numerous landslides. It deforms under the influence of a specific geological environment and external dynamic factors, after evolving through a long geological history, and is characterized by multi-layer composition, complex structure, huge sliding mass thickness, etc. The Huangtupo landslide comprises the No.1 landslide, No.2 landslide, Substation landslide, and Yuanyichang landslide. The front edge elevation is 80 m, and the back edge height is 650 m, with a total area of 1.35 km2 and a volume of 6934 × 104 m3, making it one of the greatest and most impactful reservoir landslides in China’s Three Gorges Reservoir (TGR) area32.

The formation of the Huangtupo landslide occurred in four stages: the formation of the No. 1 and No. 2 landslides, the substation landslide, the Yuanyichang landslide, and the current accumulation body. The landslide mass consists primarily of gravelly soil and cataclastic rock of the Badong Formation of the Triassic system (T2b), which is grayish yellow or yellow-green in hue. The underlying bedrock of the landslide is muddy limestone, featuring a compact texture, well-developed bedding structure, and excellent mechanical stability. The enormous landslide poses a grave threat to the Yangtze River’s main channel and the lives of tens of thousands of citizens. Badong Field Test Site (BFTS) was built in the No. 1 landslide at the end of 2012, which consisted of the main tunnel (908 m long) and five branch tunnels with varying excavation lengths for monitoring and in situ testing to evaluate the landslide33. The tunnel group displays not only the spatial layout of the landslide, but also its slip zone (in No. 3 and No. 5 branch tunnels), providing optimal conditions for monitoring and testing the slip zone. The geographical location (a), plan (b), and section (c) of the Huangtupo landslide are depicted in Fig. 1.

Fig. 1.

Fig. 1

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Overview of the Huangtupo landslide: (a) Geographical location; (b) Plan of the Huangtupo landslide (the arrow indicates the No. 3 branch tunnel in the No. 1 landslide area, where slip zone soil samples were collected); (c) I-I’ sectison of the No.1 landslide.

The slip zone soil

The samples are collected from the slip zone soil exposed by BFTS’s No. 3 branch tunnel. According to the information revealed by the tunnel and boreholes, the slip zone distributes continuously along the interface between the landslide mass and the underlying bedrock, with a local cutting layer phenomenon observed in the middle section of the No. 1 landslide. The boundary between the slip zone and the overlying landslide is ambiguous and demonstrates a gradual transition state. The slip zone consists of silty clay, gravel, and fragments of rock, with a thickness of 50 to 100 cm and a soil-rock ratio of roughly 4:65. The gravels and fragments are composed of argillaceous limestone with a particle size range of 1 to 80 mm that is predominantly sub-rounded. The soil is composed primarily of silty clay and has a structure that ranges from mildly dense to dense. The slip zone soil exhibits a degree of argillization, with the argillaceous composition consisting primarily of silty clay in a hard plastic form. Table 1 presents the physical characteristics of the original, unsieved slip zone soil.

Table 1.

Physical indexes of the slip zone soil.

Water content/% Density/g cm− 3 Liquid limit/% Plastic limit/% Plasticity index
18.89 ~ 20.01 1.9 ~ 2.1 22.03 14.58 7.45
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The particle size distribution curve of the in situ slip zone soil obtained from the No. 3 branch tunnel of the Huangtupo No. 1 landslide is illustrated in Fig. 2. The curve exhibits a well-graded characteristic, with a continuous distribution spanning from fine clay particles (0.04 mm) to coarse gravels (up to 80 mm). Notably, the curve shows a bimodal trend, indicating a mixed composition of fine-grained matrix and coarse-grained inclusions. The dominant fraction lies in the gravel range (2–80 mm), accounting for approximately 60% of the total mass, while the fine fraction (< 2 mm) constitutes the remaining 40%. This grading structure suggests a composite soil-rock skeleton, which is expected to significantly influence both the shear strength and deformation mechanisms. Such a grading composition supports the use of large-scale direct shear tests to preserve the natural structure, while the sieved soil used in ring shear tests primarily reflects the behavior of the fine-grained matrix under controlled OCR conditions.

Fig. 2.

Fig. 2

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Particle size distribution of the slip zone soil.

Methods

In consideration of the great buried depth and high gravel content of the slip zone soil, the impacts of the slip zone’s composition, structure, and over consolidation ratio on its mechanical properties are compared using the ring shear test and the large-scale direct shear test.

Ring shear test on sieved slip zone soil

To investigate the strength parameters, deformation features, and mechanism of slip zone soil with varied over-compaction ratios, ring shear tests were undertaken. The ARS ring shear instrument was chosen for the test because it was acknowledged in terms of precision and automatic control. The inner and outer rings of the shear box had a diameter and effective height of 100 mm and 150 mm, respectively, and the shear area was 98.17 cm2. Due to the size constraints of the equipment, the sample utilized is reformed to pass a 2 mm sieve. The water content is set to the natural water content (20%) during sampling. Five groups of soil samples with varying over-consolidation ratios were designed, i.e., the same pre-consolidation pressure was applied to all samples, and the normal pressure in the later shear period was determined by gradient (Fig. 3).

Fig. 3.

Fig. 3

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The ARS ring shear apparatus.

During the test, the apparatus’ upper and lower shear boxes fixed the specimen in the shear box, the pressurization device provided a vertical load to the specimen, and the test began after the specimen was consolidated and stabilized. On the basis of the natural overburden load of the slip zone at the sampling location, the specimen’s preliminary consolidation stress was calculated to be 800 kPa. The pre-consolidation pressure of 800 kPa used in subsequent tests was estimated based on the in-situ overburden stress at the sampling depth. Each group’s consolidation lasted around 12 h until the vertical displacement approached zero. The axial pressure was lowered to the normal pressure intended for the shear stage (see Table 2) and stabilized for 30 min at this stage pressure. The shear test was thereafter initiated at a rate of 0.02 mm/min. All ring shear tests were conducted under drained conditions to simulate quasi-static landslide movements. Upon monitoring the shear stress displacement curve, when the shear stress reduces to a specific value and remains constant, it indicates that the sample has reached the stage of stable residual strength and the test can be terminated.

Table 2.

Applied normal stress for consolidation and shear stages.

Test process Group 1 Group 2 Group 3 Group 4 Group 5
Consolidation 800 kPa 800 kPa 800 kPa 800 kPa 800 kPa
Shear 200 kPa 300 kPa 400 kPa 500 kPa 600 kPa
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Direct shear test on original graded slip zone soil

To determine the effects of particle gradation and over-consolidation ratio on the shear strength, extensive direct shear tests were conducted on the original graded slip zone soil at various over-consolidation ratios. It was decided to use a large-scale servo direct shear instrument with a large shear box sized 500 mm×500 mm×400 mm. The instrument, which had several loading modes and was a drainage shear system, was made up of a shear module, a control module, and a power module (Fig. 4).

Fig. 4.

Fig. 4

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Direct shear apparatus and picture of the slip zone soils.

Direct shear tests were divided into two groups. Group 1 employed the over consolidation method, which involved consolidating the samples under a stress of 800 kPa, then performing the shear tests under a gradient normal stress; Group 2 employed the equipressure consolidation method, which involved setting the normal stress for the shear stage to the same value as for initial consolidation. In Table 3, a specific loading method is displayed.

Table 3.

Loading scheme for direct shear tests.

Normal stress for consolidation stage Normal stress for shear stage
Group 1 Group 2 Group 1 Group 2
200 kPa 800 kPa 200 kPa 200 kPa
400 kPa 800 kPa 400 kPa 400 kPa
600 kPa 800 kPa 600 kPa 600 kPa
800 kPa 800 kPa 800 kPa 800 kPa
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Results

Mechanical characters

Shear stress-displacement curves of slip zone soil are obtained for various consolidation ratios and particle size gradations. Under a series of normal stress, soil samples soon attain their peak strength for ring shear testing (see Fig. 5a). It is worth mentioning that as normal stress is lifted during shear, the generated shear displacement increases synchronously before reaching peak strength, and each sample of the slip zone soil exhibits obvious strain softening features in the post peak stage.

Fig. 5.

Fig. 5

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Shear stress-displacement curves of shear tests: (a) results of ring shear test; (b) comparative results of large-scale direct shear test of Group 1 and Group 2. NS denotes the normal stress.

The direct shear test results are shown in Fig. 5b. Both groups exhibit signs of strain hardening in the post-peak stage. Although the 800 kPa shearing curve is shared by both groups, the deformation processes are drastically different; specifically, Group 2 has higher peak strength for the 200 kPa, 400 kPa, and 600 kPa curves than Group 1.

The corresponding peak and residual shear strength parameters derived from all test series are summarized in Fig. 6. The figure integrates the linear strength envelopes obtained from both testing methods. Specifically, the large-scale direct shear test under isotropic consolidation (Group 1) produced a cohesion of 20.2 kPa and a friction angle of 19.1°. In contrast, the over-consolidated direct shear test (Group 2) yielded a significantly higher cohesion of 129.7 kPa with a friction angle of 12.0°. From the ring shear tests, the peak strength parameters are a cohesion of 31.5 kPa and a friction angle of 12.9°, while the residual strength parameters are 18.3 kPa and 12.7°, respectively. This consolidated presentation allows for a direct comparison of the strength characteristics across different testing methods, stress histories (OCR), and shear states (peak vs. residual). The notably high apparent cohesion from the over-consolidated direct shear test reflects the partially restored structural strength of the original graded soil.

Fig. 6.

Fig. 6

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Combined linear strength envelopes from large-scale direct shear tests (Group 1 and Group 2) and ring shear tests (peak and residual).

Further analysis reveals that the normal stress level plays a dual role in these contrasting behaviors. In the ring shear tests on sieved soil, higher normal stress promotes the reorientation of fine particles and the development of a continuous, smooth shear band, leading to more pronounced strain softening. Conversely, in the large-scale direct shear tests on original graded soil, higher normal stress enhances the interlocking and partial crushing of coarse particles, which strengthens the soil-rock skeleton and thus contributes to the observed strain hardening. This highlights how the same factor, normal stress, can exert divergent influences on the mechanical behavior, depending fundamentally on the soil’s grading and structural composition.

Therefore, based on the distinctive mechanical characteristics of slip zone soil in the ring shear tests and direct shear tests, the mechanical behavior of slip zone soil is further investigated in detail in view of particle gradation and over-consolidation ratios.

Mechanism of the post-peak displacement characters

The ring shear test exhibits a strain softening phenomenon, while the large-scale direct shear test shows a strain hardening behavior. The two tests mentioned above show different deformative properties on post-peak behaviors. Current studies usually attribute strain softening to the following four factors34–37. The first factor is the shear plane continuity and the clay particle orientation. The shear rate is the second element. Researches show the post-peak softening characteristics of over-consolidated clay are significantly influenced by the shear rate; specifically, the faster the shear rate, the wider the gap between peak strength and residual strength. Roughness is the third consideration. It was discovered that after reaching their optimum strength, smooth shear surfaces clearly weaken under strain. The last consideration is the amount of coarse particles present; strain softening is typically seen in soils with lower coarse particle contents.

Figure 7 illustrates the macro-meso variation process of the shear surface of the conducted ring shear test. Using a Scanning Electron Microscope (SEM), the micro-structure of slip zone soil before and after shearing was studied, and the enlarged pores and orientated particles were observed after shearing. As revealed by the data, water migration and local moisture changes was induced by shear expansion during the shear process, thereby decreasing the shear strength of the soil and causing the soil particles to align along the shear direction as shear displacement increased. In addition, because there are no significant particles obstructing the shear band, the shear surface continues to become smooth, accompanied by a continual fall in the cohesion and internal friction angle, and the shear strength continues to decrease.

Fig. 7.

Fig. 7

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The schematic diagram of macro and micro variation process of ring shear test. (a) structure of the apparatus; (b) picture of the shear surface after test; (c) evolving process of micro morphology of shear plane before and after shearing.

For the strain hardening behavior observed during the direct shear test, existing researches mainly attribute such phenomenon to the following aspects38. The first is that small particles in gravelly soil prevent large particles from deforming during shear, leading to the behavior of strain hardening on the shear surface. The second explanation is related to the soil’s clay content; strain hardening typically occurs in soils with low clay contents; in the absence of clay, the outcome would be the opposite. The third factor is shear rate, namely the fact that low shear rates easily bring on strain hardening. The fourth factor is the water content, where strain hardening is evident on soil with decreased water content.

For the conducted shear test in this study, as originally graded slip zone soil (depicted in Fig. 2) was utilized, the tiny particles penetrated the skeleton of gravels and filled the pores, hence increasing the shear strength of the sample. In the meantime, gravel was scattered along the shear surface (Fig. 7c) to increase shear resistance so that strain hardening phenomena occurred during the test. The sample’s moisture content is also a significant factor in strain hardening. There will always be some soil water loss prior to and during the test, as a result of, for instance, the artificial preparation of the sample and the construction or tightness of the shear box. The water loss resulted in the reduction of moisture content, which caused the weakening of water lubrication. As a result, the friction and occlusal force between particles increased, resulting in the recovery of shear strength to some degree, so strain hardening takes place.

Figure 8 depicts the particle damage and water migration of slip zone soil during shear. Large particles constitute the sample’s skeleton and small particles that fill the skeleton increase the sample’s shear strength. The displacement and crushing of soil particles along the shear plane are depicted in Fig. 8a. During shear, the particles get closer together, and some soil particles with low strength are damaged and become smaller soil particles; however, particles with high strength are not harmed and hence produce increased shear resistance on the shear plane. This should be the leading cause of strain hardening. Figure 8b depicts a cross-sectional view of the shear box. During the shear process, there was continual water loss due to the normal pressure’s compaction as well as particle damage and displacement, which decreased the sample’s water content and promoted the development of strain hardening.

Fig. 8.

Fig. 8

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Particle damage and water migration of slip zone soil during shear.

Impact of OCR on the strength of slip zone soil

Soil is an aggregate of loose particles generated by the denudation, transport, and deposit of crustal protoliths over an extended geological period of weathering under complex geological conditions. The engineering qualities of soil are strongly correlated with the soil layer’s “memory” of its formation environment and geological history changes. The pre-consolidation pressure reflects the effect of geological history and is a crucial criterion for determining the consolidation state of a soil layer. Over-consolidation ratio (OCR) is the ratio of pre-consolidation pressure to effective overburden pressure on the existing soil layer39,40.

Since the OCR is explored here utilizing laboratory testing, it is convenient to define the OCR as the ratio of the overlaying load in the consolidation stage to that in the shear stage. The peak strength of the ring shear test was determined by the OCR, which was set to 1.33, 1.66, 2, 2.67, and 4 (Fig. 9); it is essential to note that the displacement prior to reaching the peak strength decreases with the increase in OCR, which is obviously different from the cases of isobaric consolidation test41–43. In general, as the normal pressure increases, the OCR decreases, indicating that the soil mass is denser, with increased friction and occlusion between soil particles, and a higher shear strength of the sample.

Fig. 9.

Fig. 9

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Result curves of the ring shear tests with different OCR.

The pre-consolidation pressure for Group 1 is equal to the normal pressure during shear, or OCR is 1, but for Group 2, the OCR were 1, 1.33, 2, and 4, respectively, in the large-scale direct shear tests (Groups 1 and 2). The comparison curves for the two groups are shown in Fig. 10. It suggests that OCR significantly affects the shear strength of the slip zone soil. The peak strength and residual strength of slip zone soil increase with increasing OCR, which is positively connected with shear rate and normal stress.

Fig. 10.

Fig. 10

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Comparison of result curves of different OCR in the large direct shear test.

Greater OCR indicates greater pre-consolidation pressure for the same normal pressure during shear, which signifies that the soil in the slip zone will be adequately compacted. In this instance, the pores of the large particles and the spaces between them will be squeezed and filled with fine particles, strengthening the skeleton made of large particles. The force between particles and the friction bite force between particles will grow as the consolidation progresses, further boosting the specimen’s shear strength. On the other hand, if the OCR is low, these forces will be reduced, lowering the sample’s shear strength. Under specific pre-consolidation pressure, the larger OCR, the lower the sample strength, as shown by the results of the strength comparison (see Fig. 9). This is consistent with the conclusion obtained for the ring shear test.

Study on the suitability of parameters

To investigate the applicability of the strength parameters of slip zone soil obtained from the aforementioned three tests, numerical tests were conducted under each of the three situations in order to analyze the rationality of parameters and select the optimal test method. The three sets of parameters are derived from the ring shear test (different OCR), the large-scale direct shear test (OCR = 1), and the large-scale direct shear test (various OCR). Table 4 contains a list of employed parameters.

Table 4.

Simulation parameters in consideration of diverse test situations.

Structure Material Elasticity modulus/MPa Gravity/kN m− 3 Cohesion/kPa Friction angle/°
Sliding mass Gravelly soil 565 24 65 23
Slip zone Silty clay with gravels (for direct shear, OCR = 1) 28.6 23 20.2 19.1
Silty clay with gravels (for direct shear, OCR ≠ 1) 28.6 23 129.7 12
Silty clay (for ring shear, OCR ≠ 1) 23.6 19.5 32.3 12.8
Bedrock Muddy limestone 36,740 26 3800 44
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Based on the I-I’ profile of the Huangtupo No. 1 landslide (Fig. 1b and c), a finite element numerical computation model is created using the Geo-studio program, which has been proven to be effective in estimating water-soil coupling deformation. The Three Gorges Reservoir’s actual water fluctuation condition is used to determine the seepage border conditions. The Three Gorges Reservoir’s initial water impounding year, from September 2008 to September 2009, when the water level was raised to 175 m above sea level, is used to derive the variation function of the calculated water level H(t)., expressed as

graphic file with name d33e785.gif 1

Next, the SEEP/W module was used to establish a landslide seepage field during the hydrological year. On this basis, the SIGMA/W module was employed to carry out the seepage-mechanical coupling calculations.

Two GPS monitoring points (G02 and G07, marked in Fig. 1) and one borehole clinometer (JC3, marked in Fig. 1) are chosen to compare the calculated displacement with the actual displacement of the landslide (Figs. 11 and 12) in order to determine the target parameters that are most consistent with the actual situation. The Pearson correlation coefficient (R) is used to determine the degree of congruence between test results and monitoring data during quantitative characterization. The equation is as follows:

graphic file with name d33e918.gif 2

Fig. 11.

Fig. 11

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Results of simulated deformation curves of different conditions and monitoring data.

Fig. 12.

Fig. 12

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Results of simulated deformation curves of different conditions and monitoring data. RS = ring shear and LDS = largescale direct shear.

where X and Y are the two variables to be compared, and N is the sample size.

The comparability between test results and monitoring data are presented in Table 5, where higher values indicate larger correlation. It implies that, among the three groups of parameters, the deformation curves produced from the over consolidation group of the large-scale direct shear test have the best degree of congruence with the monitoring curve results. The results curves for the ring shear test (OCR ≠ 1) are closer to the actual condition than those for the large-scale direct shear test (OCR = 1). As a result, the parameters of slip zone soil strength that consider both over-consolidation ratios and natural grading conditions are the most dependable. However, there will be a significant mistake when only OCR or soil gradation is used. This is attributed to the fact that the over-consolidated LDS test best replicates the key in-situ conditions: the high pre-consolidation stress (OCR) and the preserved original grading of the slip zone soil.

Table 5.

Correlation coefficients between test results and monitoring data.

Monitoring points Conditions
Monitoring data Over consolidation LDS Equal consolidation LDS Over consolidation
RS
G02 1.000 0.629 0.565 0.604
G07 1.000 0.882 0.840 0.868
JC3 1.000 0.918 0.853 0.885
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Discussion

The observed divergence in post-peak behavior, namely the strain softening in ring shear against strain hardening in direct shear, can also be attributed, in part, to inherent differences in test principles and specimen scale. The rotational, unlimited-displacement shear mode of the ring shear apparatus promotes the development of a continuous, localized shear band and particle reorientation, conditions that favor strain softening. In contrast, the translational shear mode and the much larger specimen size of the direct shear test better preserve the interlocked structure and meso-scale heterogeneity of the original graded soil, contributing to the strain hardening response. These methodological differences are complementary; they allow the investigation of mechanical behavior from both the perspective of the fine-grained soil matrix under ideal shear and the composite response of the natural soil skeleton.

Furthermore, the evolution characteristics of simulated landslide situations employing different parameters are explored. Figure 13 depicts the distribution pattern of maximum shear strain for two groups of parameters acquired by the large-scale direct shear tests. When d = 31 (rising period of reservoir level), the strain increment at the rear edge of the landslide is basically the same for the two cases; however, the leading edge is distinct where the shear strain is significantly larger for the case OCR = 1, that is, the stability of the landslide is significantly affected by the change of reservoir water level. The alternative instance (different OCR) obtaining minor shear strain shows that the landslide is less affected by the reservoir water but is susceptible to its self-weight. Monitoring data over ten years show that the Huangtupo No.1 landslide has been deformed at a very slow speed5, despite the fact that the reservoir water level has fluctuated for many years.

Fig. 13.

Fig. 13

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Contrast of maximum shear strain distribution between over consolidation case (OCR ≠ 1) and isotropic consolidation case (OCR = 1), both strength parameters taken from the large-scale direct shear tests.

Landslide deformation is principally governed by reservoir water variations and gravitational force. According to the spatial distribution characteristics of the landslide displacement shown by the monitoring data44, even though the deformation of the leading edge of the landslide is the biggest, the deformation during a hydrological year is quite modest. In accordance with the simulation results of two sets of parameters (OCR = 1 and OCR ≠ 1), simulation with the strength parameters measured under the over consolidation condition (OCR ≠ 1) can reasonably reflect the deformation characteristics of the landslide in a hydrological year; in the isotropic consolidation case (OCR = 1), the shear strain at the leading edge of the slip zone is excessively high, which is inconsistent with the actual situations.

Therefore, after deforming within a hydrological year (d = 375), the shear strain distribution under the case OCR ≠ 1 is more in line with the actual situation than the case OCR = 1. Consequently, the parameters obtained by fully considering the in-situ stress (various OCR) can best reflect the actual deformation evolution characteristics of the landslide.

Conclusions

The strength parameters and deformation characteristics of slip zone soil with varying over-consolidation ratio (OCR) and particle grading were researched using the case of the Huangtupo landslide, and the macro-meso process was thoroughly described. Through comparison analysis, the most appropriate test conditions for determining the strength parameters of slip zone soil are explored, providing an essential reference for assessing the states of large-scale landslides. The specific conclusions are as follows:

  1. Large-scale direct tests and ring shear tests exhibit different deformative characteristics on post-peak behaviors, where strain softening phenomenon is observed in the ring shear test due to strength decrease caused by water migration and local moisture changes as well as oriented particle distribution. In contrast, the large-scale direct shear test indicates strain hardening behavior due to the soil-rock skeleton, the rock barrier on the shear plane, and the loss of water.

  2. The OCR significantly affects the shear strength of slip zone soil. When the pre-consolidation pressure is held constant, a higher OCR (implying a lower normal shear stress) results in lower shear strength. It is consistent with the direct shear test and ring shear test results. For greater pre-consolidation pressure, the minor particles are compressed and filled into the spaces between the large particles and the pores of the large particles themselves, enhancing the strength of the large particle skeleton.

  3. The deformation curves produced by the over consolidation group of the large-scale direct shear test had the highest degree of congruence with the monitoring curve results among the three groups of parameters. Therefore, the strength parameters of slip zone soil that take into account both over-consolidation ratios and natural grading circumstances are the most reliable; the result is further verified by a simulation of landslide deformation throughout an entire hydrological year.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 42377181), the Science and Technology Research and Development Project of China RAILWAY MAJOR BRIDGE Engineering Group Co., Ltd. (No. 2021-01-Major), and the Key Research and Development and Transformation Program of Qinghai Province (No. 2024-SF-150). The authors would also like to thank Jack for English language editing.

Author contributions

Zhongxuan Chen: Experiment conductMeng Zhao: Designed the overall research plan and experimental protocol.Shupeng Jiang: collected raw dataQinwen Tan: Proposed funding supportYuangang Ma: processed raw dataYanzhe Li: processed dataXiao Yang: Assisted in data collection.

Data availability

The datasets generated and/or analyzed during the current study are available from the corresponding author on reasonable request.

Declarations

Competing interests

The authors declare no competing interests.

Footnotes

Publisher’s note

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Contributor Information

Meng Zhao, Email: lovingpitt@foxmail.com.

Qinwen Tan, Email: tanqinwen@cug.edu.cn.

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Associated Data

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Data Availability Statement

The datasets generated and/or analyzed during the current study are available from the corresponding author on reasonable request.

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