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Article

Gravitational Deformation and Reactivation Mechanism of a Fault-Bounded Slope, Eastern Yanshan Mountains, China

by
Hao Sun
1,
Tiantao Li
1,*,
Xiangjun Pei
1,
Jian Guo
2,
Jingjing Tian
1,
Shoudao Wang
1,3 and
Mingfang Pu
1
1
State Key Laboratory of Geohazard Prevention and Geoenvironment Protection, Chengdu University of Technology, Chengdu 610059, China
2
Department of Civil Engineering, Panzhihua University, Panzhihua 617000, China
3
Powerchina Chengdu Engineering Corporation Limited, Chengdu 611130, China
*
Author to whom correspondence should be addressed.
Forests 2023, 14(3), 495; https://0-doi-org.brum.beds.ac.uk/10.3390/f14030495
Submission received: 31 January 2023 / Revised: 25 February 2023 / Accepted: 27 February 2023 / Published: 2 March 2023
(This article belongs to the Special Issue Landslides in Forests around the World: Causes and Mitigation—2nd Edition)
Browse Figures
Review Reports Versions Notes

Abstract

:
The Nandongzi landslide occurred in the Yanshan region of North China. From 2017, the slope of the Nandongzi landslide has been significantly deformed after several excavations. Field investigations show that the Nandongzi landslide is a special toppling deposit that does not have basic toppling conditions. The toppling deformation mechanism of the slope has become a difficult issue for engineers, attracting the attention of scientists. Joint, surface, and borehole lithology surveys revealed the surface and internal structural characteristics of the slope. The structure of the soft and hard interbedded rock and the proximity of the fault are the dominant factors of slope toppling deformation. The slope toppling failure process can be divided into four stages: initial deformation, compression and bending, toppling and overlapping, and reactivation. In the first three stages, slope toppling deformation is triggered by the downcutting of the upstream gully, gravity, and differential weathering of soft and hard rocks, which promote the dumping deformation of the slope. In the final stage, engineering excavations triggered the reactivation of residual deposits. Monitoring data indicate that slope deformation is directly related to rainfall events. Flac 3D was used to simulate the slope failure process under natural and rainfall conditions after the two excavations. The results show that multiple excavations changed the surface and runoff conditions of the slope, which led to slope failure. Rainfall promoted deformation of the back edge of the landslide, which led to shear failure from the back edge to the front edge. Our results provide new and unique understanding into the spatiotemporal evolution and deformation mechanism of similar toppling-accumulation landslides around the study area.

1. Introduction

Gravitational deformation is a common phenomenon on high-relief slopes in mountainous areas worldwide [1,2,3,4,5,6,7,8]. Toppling is the most common gravitational deformation mode. They are often formed in steep sedimentary rock formations, and a small amount also develops in block rocks [9,10,11,12,13,14,15]. This is often a long-term evolution process from toppling deformation to failure. During this process, a large displacement often accumulates. Linear gravitational morpho-structures, such as double ridges, ridge top depressions, scarps, benches, trenches, cracks, and bulges, are typical features of a toppling slope [16,17]. Generally, high and steep terrains, weak rock masses, and steep rock formations are necessary conditions for slope toppling deformation [18,19,20]. However, the relict Nandongzi landslide, occurring in Pingquan City, Hebei Province, is an exception. The topography, rock stratum occurrence, and slope aspect do not meet the general conditions of toppling deformation. This relict landslide has not been recognized because of the obliteration of the original morpho-structures affected by denudational and depositional processes along the slopes. Until 2017, excavation of the highway slope caused reactivation and exposed the structural characteristics of the slope. The formation mechanism of the slope was revealed through detailed investigation and analysis. The soft and hard interbedded rock structures and adjacent faults are the dominant factors of slope toppling deformation. Therefore, this was a reactivated landslide caused by toppling accumulation.
As relict landslides activated in past morphoclimatic conditions and have been eroded over a long period, in most cases, the formation mechanism is difficult to clarify [21,22,23]. Well-trained geological and geomorphological engineering skills are often required in these situations. Moreover, multidisciplinary approaches, based on geomorphology, geophysics, and pedology, are often applied [24,25,26,27,28]. However, the residual hazard of relict landslide reactivation remains challenging to assess [29,30]. The Nandongzi landslide was both a special and typical case of reactivated toppling deposits. The landslide is special because its toppling deformation is not only controlled by the structure of the slope but also related to adjacent faults. However, it is typical because there are many similar toppling-accumulation landslides found in the basin. Therefore, we conducted a detailed investigation and analyzed the landslide evolution mechanism.

2. Regional Geological Setting

2.1. Geology and Tectonics

The study area was located in Pingquan County, Hebei Province, China, in the eastern section of the Yanshan Mountains (N 118°48 ′ 46 ″ –N 118°48 ′ 54 ″ , E 40°42 ′ 51 ″ –E 40°43 ′ 02 ″ ). The main strata exposed in the study area are the Section II of Changzhougou Formation (Chc 2 ) of the Changchengian Period. The Changzhougou Formation (Chc 2 ) was composed mainly of quartz sandstone, siltstone and shale with a total thickness of 470.98 m and a single layer thickness of 0.5–30 cm approximately (Figure 1A) [31]. The study area experienced multiple periods of strong structural deformation superposition, and large-scale regional tectonic events can be divided into three major movement periods: the Wutai, Indosinian, and Yanshan tectonic periods. The fault system that formed during the Yanshan tectonic period controlled the tectonic path in the study area. The Yanshanian structure in the study area primarily manifests in the Carboniferous and Jurassic structural layers. Regional tectonic characteristics manifested in three aspects: (1) synsedimentary faults at the edge of the basin, (2) wide and gentle syncline structures in the basin, and (3) NNE-SSW trending fault systems. This resulted in the formation of a basin and ridge structure in the area.
The Nanlaodong Fault adjacent to the Nandongzi landslide is a branch of the synsedimentary fault at the basin margin. The fault is exposed in the western section of the study area and has a total length of 2.7 km. The fault strikes near E-W, leaning S, with a dip angle of approximately 75°. The fault is a reverse fault, and the fault zone passes through the north side of the landslide, forming its right boundary. Due to the structural compression of the Nanlaodong Fault, structural compression fracture zones formed on both sides of the fault. The fracture zones consist by fault gouge, quartz sandstone and cataclasite with total thickness of 30 m [31].

2.2. Geomorphology

The study area is a low-hill area, with elevations ranging from 606.17–806.28 m. The river valley width in the study area is approximately 80 m, and the slope on both sides of the valley is approximately 40–50°. There is a seasonal current in the river valley, which flows near NS. At the front edge of the slope, there was an obvious river redirection phenomenon. The flow direction changes from NS to N 40°–W 50° and then to N 25°–E 35° after passing through the middle line of the landslide. The Nandongzi landslide was located on a convex bank slope (Figure 1B).

3. Methods and Data

3.1. Field Investigations

Field investigations can integrate the topographic and geomorphic features of the landslide and neighboring areas, the structural features of the rock mass, and the deformation features of the landslide [32,33]. A geological and geomorphological survey was conducted on the landslide and surrounding areas, covering approximately 0.5 km 2 . Data from geological surveys and a regional geomorphological map at a 1:1000 scale were used to obtain a detailed geological and geomorphological map of the Nandongzi landslide (Figure 2). To determine the structural characteristics of the accelerated deformation of the landslide in detail, we used 231 helicopter-borne images from 4 June 2020, and created a three-dimensional (3D) surface model of the slope based on the “structure-motion” method of photogrammetry [34]. Geological and stratigraphic data were obtained from a regional geological survey conducted in 2000 [31].

3.2. Borehole Drillings

The drilling campaign was conducted in 2020 (Figure 2). It comprised drilling 11 boreholes to depths of 18.4–67.4 m, six of which were equipped with inclinometer casings (Figure 2). Twenty groups of drill cores were collected. The direct shear strength, compressive strength, and physical parameter tests were performed in the laboratory. Based on the depth of the bedrock interface, the groundwater level was intermittently measured using barometers and existing wells at different depths in the landslide body.

3.3. Monitoring

In July 2020, we designed and implemented a comprehensive system for monitoring landslide deformation and weather. This system can automatically record displacement and precipitation time histories and provide strong support for analyzing the driving factors of the landslide. In this study, we examined data collected from July 2020 to February 2022. The monitoring system contained six sets of deep inclinometers, four sets of Global navigation satellite system (GNSS) receivers, and one set of weather sensors (Figure 2).
The monitoring equipment comprised two profiles. A total of four monitors, including three inclinometer monitors and one GNSS monitor, were deployed on the A–A ′ profile. Five monitors, including three inclinometers and two GNSS monitors, were deployed on the B–B ′ profile.

3.4. Numerical Modeling

The geological prototype of the Nandongzi landslide was generalized to a numerical model. Flac 3D finite element numerical modeling software was used to analyze the displacement of the residual slope under excavation and rainfall conditions [35]. To study the impact of excavation and rainfall on landslide deformation, four cases were considered: natural conditions after the first excavation (Case I), rainfall conditions after the first excavation (Case II), natural conditions after the second excavation (Case III), and rainfall conditions after the second excavation (Case IV).
A 3D numerical model of the Nandongzi landslide was established using topography and internal structure. The grid of the numerical model is shown in Figure 3. Based on the Nandongzi landslide deformation characteristics, the slope model was simplified into four parts: siltstone interbedded with a shale landslide mass, fault zone, slide contact zone, and bedrock. The height, width, and length of the geological model were 378, 305, and 157 m, respectively. The geological model consisted of 307,453 elements and 578,855 grid points. The model was developed under the action of gravity. The fourth-side boundary was only allowed to have in-plane displacement, the bottom boundary was fixed in all three directions, with a free boundary condition for the slope surface. In numerical modeling, the contact surface between the landslide mass and bedrock was used to establish the sliding surface. All lithological groups followed the Mohr-Coulomb failure criterion. The geotechnical parameters for the numerical modeling were obtained from the laboratory test results (Table 1).

4. Landslide Characterization

4.1. Description of the Nandongzi Landslide

The Nandongzi landslide is 140 m long, 200 m wide, and 70 m high, with a planar shape proximate to a fan-shape. The main sliding direction was 275°. The average thickness of the landslide mass was 18 m, and its volume was approximately 30 × 10 4 m 3 . The landslide front edge elevation was 615 m, and the back scarp elevation was 685 m. Affected by the river valley at the front edge of the landslide, the upstream gully, and the downstream gully, the slope on which the Nandongzi landslide is located has three free face directions. Multiple excavations have changed the landslide topography, and the landslide was excavated in seven steps. The slope angles ranged from 40–45°.
The Nandongzi landslide was originally a mountain and slope, with no artificial activity (Figure 4a). Construction of the Yangsan Road in front of the slope began in June 2016 and was completed in April 2017. The slope was deformed for the first time because of the first excavation of the slope foot. The elevation of the tension crack distribution at the trailing edge ranged from 660–665 m. The width of the cracks was 5–10 cm. In March 2018, local villagers noticed that the deformation of cracks had increased from 5–10 cm to 20–30 cm (Figure 4b). The second excavation and cutoff drain engineering treatments were conducted in April 2018 and completed in March 2019. After the completion of the treatment, the aerial image showed that the slope deformed in July 2019. Multiple outfalls can be observed from the first to third step of the slope, with an obvious tension crack at the trailing edge (Figure 4c). As shown in the aerial image of February 2021, the slope deformation phenomenon was further aggravated, the height of the dislocation increased, and the cracks were clearly extended and circled. Compared to the first deformation, the slope deformation range was significantly expanded, and the trailing edge elevation increased from 665–690 m. The left boundary of the landslide expanded to the south approximately 130 m (Figure 4d).

4.2. Geological Characterization

4.2.1. Surface Investigation

Nearly 200 sets of joint attitudes were collected through compass measurement. Based on the measurement results, the author used corresponding stereographs at different positions to statistical analysis the joint attitudes and mapped the geological sketch of slope surface excavation for Nandongzi landslide (Figure 5b). The investigation results for typical joints in the Nandongzi landslide are shown in Figure 5a. The results show that the landslide rock mass was controlled by one group of rock layers (J1) and three groups of discontinuous joints (J2–J4).
The rock layer (J1) attitude is NW30°–SE77° ∠ 3°–83°. The strike of the rock layer intersects the strike of the slope direction at a large angle and the dip toward the upstream gully. Under the influence of the Nanlaodong Fault, the dip angle of the rock layer in the study area is steep. Undeformed rock layers with steep dip angles are distributed on the foot wall of the fault. It can be seen in Figure 5f–h, located at the second step of the slope, that the dip angle of the rock layer (J1) decreases significantly from north to south. From the north to the middle of the slope, the dip angle of J1 decreases from 65° (Figure 5f) to 37° (Figure 5g). Finally, the dip angle is nearly horizontal on the south of the slope (Figure 5h). This phenomenon indicates that the rock layer has obvious toppling deformation (Figure 5b).
The attitude of the J2 joints is SE49°–NW82° ∠ 30°–76°, perpendicular to the rock layers and parallel to the upstream gully strike. It is widely distributed on slopes and cuts through multiple layers of rock. This shows that the downcutting of the upstream gully played a crucial role in the formation of J2. The attitudes of joints J3 and J4 are SE28°–SW16° ∠ 54°–73° and SE42°–SE6° ∠ 75°–83°, respectively, mostly parallel to the landslide strike. J3 joints were found in the siltstone interbedded with shale layers, and J4 joints were found in the quartz sandstone layers. The number of J3 and J4 joints was relatively small (Figure 5c–h).
Based on this, the distribution characteristics of the Nandongzi landslide were statistically analyzed. When the dip angle of the rock layers was less than 27°, the dip direction ranged from NW30° to SE77°. When the dip angle was higher than 66°, the dip direction was distributed from NW15° to NE7°, and the distribution was relatively concentrated. The results show that steep rock layers without large fluctuations in the dip direction are considered undeformed or weakly toppling rock layers. The strongly toppling deformed rock mass with gentle dip angle.

4.2.2. Fault

The regional geological investigation shows that the Nanlaodong Fault developed a wide damage zone and pinched out in the study area. According to the field survey, the Nanlaodong Fault formed the right boundary of the landslide. Further detailed investigation of this fault is necessary. Figure 6a shows the distribution of the Nanlaodong Fault and the location of the outcrops around the fault. The Nanlaodong Fault is exposed in the excavated slope and part of the gully, and the rest of the area is covered by Quaternary deposits. As shown in Figure 6b, the fault extended from west to east to the fifth step of the landslide. No fault was exposed from the back of the landslide to the slope watershed. Figure 6c shows the strong tectonic extrusion phenomena exposed in the upstream gully, which strongly proves that the Nandongzi landslide is located near the fault zone.
The author excavated a trial trench at the third step of the fault zone. The internal structure and composition of the fault zone were investigated and measured. In the trial trench, it can be seen that the rock structure of the fault zone was broken (Figure 6d). The rock mass consisted of massive-fractured gray-white quartz sandstone with sharp angularity and grain sizes ranging from 5–30 cm. The soil comprised gray-white clay minerals with a high degree of weathering (Figure 6e). Figure 6f shows quartz sandstone on the left bank of the river valley opposite the Nandongzi landslide. The rock structure was relatively intact with a low degree of weathering. At different locations on the same fault, the rock quality of the slope was obviously different (Figure 6d,f).

4.2.3. Lithology

Through surface investigation, the lithology was shown to contain primarily quartz sandstone and siltstone interbedded with shale. To obtain more information regarding the lithology, eleven boreholes were established in the landslide area to form two profiles (Figure 2) and observe the lithology inside the slope body.
The photos of the core exposed by drilling are shown in Figure 7, and the drilling depth was between 18.4 (B06) and 67.4 m (B09). The fault zone was exposed by boreholes B01 to B04 with thicknesses ranging from 10.4–16.0 m (Figure 7A). The lithology of the landslide consisted of a fault gouge, quartz sandstone, and cataclasite with broken and scattered rock structures. Landslide mass was distributed in all boreholes except B01 and B06, with landslide mass thicknesses ranging from 12.5 to 21.3 m (Figure 7B). The sliding zone was a weak layer within the silty clay interior of the slope, formed by long-term eluviation, with a thickness ranging from 1.0–1.6 m (Figure 7C). All boreholes revealed the siltstone interbedded with shale, ranging in thickness from 4.5–48.3 m. The degree of weathering ranged from fully to moderately weathered. Fully to strongly weathered siltstone interbedded with shale was exposed from B06 to B11. Thickness ranged from 6.8–11.4 m (Figure 7D,E). Medium-weathered siltstone interbedded with shale had a thickness between 3.9 and 39.9 m. The core structure was relatively intact with a short column (Figure 7F).

4.2.4. Slope Structure and Rock Strength

Based on the lithology of the slope, we studied the toppling deformation feature of the slope rock mass using the variations in the attitude and structure of the rock layers in the core.
The author used a compass to measure the dip angle of the rock layers in the core. In borehole B09 0–13.8m, the rock layers were flat with a dip angle of less than 23°. The rock structure was broken and loose, owing to strong unloading and weathering. A set of joints (J2) perpendicular to the rock layers were clearly visible, filled with mud, and formed a crack zone (Figure 8A). As the depth increased, the rock dip angle further increased. Constrained by lithology, siltstone interbedded with shale exhibited poor weathering resistance, and the core was loosely broken under weathering. The dip angle was only visible in a few cores, and the rock dip angle was approximately 30°. A 1.5 m thick sliding layer comprised of rock debris and silty clay developed in the slope (Figure 8B). The exposed cores were short columnar, and the rock dip angle was further increased to 49°. Several shales were muddied and weathered, forming interlayer slip zones. Interlayer sliding occurred only between rock layers (Figure 8C). When the depth exceeded 30 m, the weathering and unloading degree of the rock mass was further weakened. The cores were short to long columnar, and structurally intact. The dip angle of the rock layer was steeper at approximately 67°. The core dip angle was similar to that of the undeformed rock mass found on the slope surface (Figure 8D).
Based on quantitative indices, such as the deformation characteristics, dip angle, unloading degree, and weathering degree, the rock mass exhibited different degrees of toppling deformation [36,37]. Therefore, the slope toppling deformation can be divided into down-slope overlapping, highly toppled, weakly toppled, and original stratigraphic zones (Table 2).
Based on the topography of the slope and core exposed by drilling, two profiles of the Nandongzi landslide were drawn. As shown in (Figure 9), slope toppling was caused by the toppling and bending of rock layers and down-slope overlapping of the rock mass. The Nandongzi landslide was primarily distributed in the down-slope overlapping and highly toppled zones. The down-slope overlapping zone shown in Figure 9b comprised highly weathered and unloaded quartz sandstone that breaks brittle under gravity, landing on siltstone interbedded with shale. The lithology controlled the highly toppled zone and plastic bending occurred under gravity. The Nandongzi landslide did not slide along the bottom boundary of the toppling deformation, but slid along the bottom boundary of the highly toppled zone where fissures developed.
Finally, twenty groups of drill cores were sampled, and data from direct shear strength, compressive strength, and physical parameter tests were obtained from the laboratory (Table 1). Under fully saturated conditions, the cohesion and friction angle of the sliding surface were 77.8% and 69.2% of the field-condition strength, respectively. The above data indicate that water had an obvious influence on the mechanical strength of the slipping zone.

4.3. Landslide Deformation Characteristics

By studying the deformation of the slope, it was found that the main crack had completely penetrated the landslide. The landslide was divided into deformation zones I and II based on crack distribution, lithology, and degree of deformation. The distribution of each deformation zone is shown in (Figure 10a).
Deformation zone I was distributed in the downstream fault zone of the landslide and composed of gouge, quartz sandstone, and cataclasite. Signs of deformation in the zone were obvious in this area, and there was a significant bulge at the landslide shear outlet (Figure 6b).
Deformation zone II can be further divided into deformation zones II-1 and II-2 based on the deformation signs. Deformation zone II-1 was distributed in the middle of the landslide and composed of siltstone interbedded with shale and quartz sandstones. There were clear signs of deformation in this zone. The back edge had a large deep tension crack 52 m in length, 0.5 m wide, and 3 m deep (Figure 10d). The front edge of the deformation zone exhibited an evident deformation bulge (Figure 10e). Deformation zone II-2 was the remaining part of the landslide, and its stratigraphic lithology was the same as that of II-1. The steep scarp was approximately 2 m high and extended along the landslide back edge (Figure 10b). The left deformation zone boundary had fresh striae, and the height of the dislocation was approximately 10 cm (Figure 10f). Compared to the left boundary, the right boundary deformation was more obvious. The sixth step of the landslide developed with an obvious steep scarp, and the height of the dislocation was approximately 1 m (Figure 10c).

5. Deformation Monitoring

Landslide deformation monitoring started on 20 July 2020, ended on 10 February 2022, and lasted 19 months. The composition of the landslide deformation monitoring system is presented in Figure 11A and Table 3. The landslide deformation monitoring data results are shown in Figure 11B–D.

5.1. Monitoring of Surface Deformations

Figure 11B shows the results of the Nandongzi landslide surface displacement monitoring, including cumulative horizontal, settlement, and total displacement. In the horizontal direction, the deformation outside the slope was positive and the deformation inside the slope was negative. In the vertical direction, the upward deformation was positive and subsidence deformation was negative. From the surface monitoring data, all monitoring equipment recorded obvious deformations. There was a clear trend of gradual increase. Every year from July to October, the cumulative displacement curves exhibited a sharp upward trend. During other periods, the deformation curves remained stable. By combining the surface displacement and daily rainfall data, we found that the time of landslide deformation coincided with that of intensive rainfall (see 1, 2 in Figure 11B). Therefore, rainfall controls the landslide deformation.
By analyzing the surface displacement monitoring curves, the G02 monitoring equipment in deformation zone I was shown to be dominated by horizontal displacement, and the vertical displacement was also significant. The main sliding direction was 278°, parallel to the fault strike. This indicates that deformation zone I was mainly controlled by faults. The G03 monitoring equipment in deformation zone II-1 exhibited the largest displacement of 1878.6 mm. The deformation was mainly horizontal, and the vertical displacement was small. The main sliding direction of the deformation zone was 268°, which was consistent with the direction of the free face. This indicates that deformation zone II-1 was controlled by gravity. Based on the deformation monitoring curves of the G01 and G04 monitoring equipment, the deformation of II-2 was large at the back edge and small at the front edge. The deformation mode of the landslide changed. The horizontal displacement was dominant, and the vertical displacement was strong at the back of the landslide. The horizontal displacement at the front of the landslide was also dominant with a small amount of vertical displacement. The main sliding direction was approximately 260°.
The author investigated the cracks on 16 May 2020 and 26 August 2020. In the field investigation, the author found that it is difficult to accurately measure the horizontal deformation of cracks. Therefore, typical cracks were selected to measure the vertical deformation increments. The measurement results were compared with the vertical deformation increments from GNSS monitors at the same period (Table 4). The location and ID of typical cracks are shown in Figure 10. The results of the above monitoring data analysis were consistent with the results of the surface crack investigation of the Nandongzi landslide.
Moreover, Figure 11A indicates that the deformation degree of the front edge of deformation zone II-2 is relatively weak. However, cracks at the back edge of the landslide were highly developed, and the misplacing phenomenon was obvious. A possible explanation for these phenomena is that storm rain penetrated along the cracks to the back of the rock mass, resulting in a reduction in the shear strength and the occurrence of creep. In the middle of the first step, there were several rock masses with a layered structure and low toppling deformation degree. This formed a locked patch at the front edge of the landslide, which prevented its deformation. This is the key factor that causes deflection in the sliding direction of deformation zone II-2.

5.2. Monitoring of Deep Deformations

The monitoring curves shown in Figure 11C,D were obtained from the deep inclinometers on profiles A–A′ (B02, B04, and B05) and B–B′ (B07, B09, and B10). The B04 deep inclinometer borehole had four sensors at depths of 6, 13, 18, and 25 m. The installation of the inclination-monitoring equipment is presented in Table 3 and Figure 11A. The sliding surface position was first determined by drilling the cores and then calibrated based on the relative displacement between the sensors monitored by the inclinometer.
Based on the inclinometer monitoring curves of section A–A′, the increasing trend of the deep landslide displacement is consistent with the surface displacement. The multi-year average precipitation in the study area is 529.1 mm, and June to August is the most concentrated period of rainfall each year. From 3–6 in Figure 11B,C, it is obvious that the monitoring curves of deep deforamtion increase steeply in July and August each year, and the monitoring curves are flat in the remaining time. Monitoring data also showed that landslide deformation was related to rainfall. From a spatial perspective, the displacement of sensor B05-1 at the back of the landslide was the largest. The displacements of the B04 and B02 equipment were smaller than that of B05. From a time perspective, the B05 equipment first deformed on 13 July 2021. Approximately six days later, B04 began to deform. The deformation of B02 began 14 d later. This indicates that the deformation time and amount at the back of the landslide were earlier or larger than those at the front.
The deformation pattern of profile B–B′ Figure 11D is generally similar to that of profile A–A′. The difference is that sensor B09-3 exhibited a significant anomaly with the largest cumulative displacement. The reason for this phenomenon is that B09 is located at the back of deformation zone II-1, which pulls the monitoring equipment with obvious deformations.

6. Discussion

6.1. Fault Tip Effect

Previous studies have shown that fault zones are primarily classified into fault cores and damage zones [38,39,40]. In the fault formation process, the fault core has the most concentrated shear stress with the largest displacement. The rock mass in the fault core was broken into blocks. Its lithology was mostly gouge, cataclasite, and breccia. The damage zone is a deformed rock mass located around the fault core, which is related to the occurrence, extension, and evolution of the fault. Damage zones can be further divided into tip, wall, and linking [41,42,43,44,45].
According to the field investigation results and regional geological survey data [31], the Nanlaodong Fault extends from west to east to the fifth step of the Nandongzi landslide. The fault tip pinches out at the fifth step. During the initiation and propagation of the fault, the rock mass deformed at the fault tip owing to the concentration of the tip stress, which is called the tip damage zone [38,39,46]. The Nandongzi landslide was located in the tip damage zone. The fault is characterized by large displacements in the middle and small displacements on both sides. The displacement at the fault tip tends to zero. Far from the fault tip, stress inside the fault can be transferred to both sides of the fault through deformation. The closer it is to the fault tip, the less deformation allowed in the rock mass [47]. Therefore, there is a significant stress concentration at the tip Figure 12. According to the theory of rock fracture, when the concentrated stress exceeds the yield strength of the rock, the rock is more likely to fracture. Owing to the continuous concentration of stress, the degree of rock damage at the fault tip is stronger than that in other areas of the fault, and fault tip rock fragmentation with significant strength reduction occurs.

6.2. Toppling Failure Process

The main factors affecting slope toppling deformation include topography, lithology, rock attitude, rock thickness, slope height, and slope gradient [48,49,50,51,52,53,54,55]. The Nandongzi landslide had a gentle slope with a low slope height, and the rock dip direction intersected the slope direction at a large angle. In general, the Nandongzi landslide did not have toppling deformation conditions. However, according to our investigation, the Nandongzi landslide experienced strong toppling deformation in the upstream gully. From north to south, the rock dip angle gradually changed from to 60–70° to nearly horizontal. The gravity drive alone cannot adequately explain the toppling deformation of the Nandongzi landslide.
Downcutting of the river valley in front of the landslide is an important trigger for toppling deformation. At the end of the Mesozoic, the Yanshan Mountains, which formed during the Yanshan tectonic period, were influenced by regional planation. The planation surface was formed in the Beitai period. During the Himalayan period, the distribution of the Yanshan and rift basins in neighboring regions was controlled by synsedimentary faults. Simultaneously, the surrounding mountains were rapidly uplifted. The elevation of the Beitai period planation surface increased to more than 1000–1500 m. During this process, the valley in front of the landslide was rapidly downcut, forming a valley topography with three free surface directions. This provided free conditions for the occurrence of slope toppling deformation.
Soft and hard interbedded rock mass structures are dominant factors in toppling deformation. The landslide can be divided into two lithologies from top to bottom. The overlying quartz sandstone has a layer thickness of approximately 30–50 cm, and the underlying siltstone is interbedded with shale with a layer thickness of approximately 8–12 mm. Because the shale between the siltstone layers is soft, it was the first to be eroded by weathering and river erosion. Siltstone creeps under gravity. The rock layers appear hollow, fractured, and compressed on the slope. Affected by siltstone sandwich shale deformation, the quartz sandstone shows brittle fracture and dislocation between the rock layers under self-weight.
The proximity of the fault is another dominant factor in the toppling deformation. As shown in Figure 13C, the Nanlaodong Fault cuts the wedge-shaped quartz sandstone rock mass on the fault hanging wall. The continuity of the rock layers was disrupted, which was conducive to toppling deformation. Thus, the rock structure at the fault tip was further damaged by the fault tip effect. The overlying quartz sandstone was more susceptible to toppling and overlapping after compression and bending of the underlying rock mass.
Based on this study, the evolution of toppling failure was divided into four stages: initial deformation, compression and bending, toppling and overlapping, and reactivation. The fault-bounded slope toppling deformation evolution process is illustrated as follows (Figure 13).
(1)
During the Himalayan period, regional planation surfaces and rapid uplift occurred in the Yanshan area. The bottom and lateral erosions of the river were strong. Gullies on both sides of the slope were formed. Slope unloading was evident during river undercutting. The rock mass exhibited different response mechanisms because of its particular slope structure. The deformation of the overlying quartz sandstone was dominated by interlayer shear dislocations with local tensile damage on the slope surface. The underlying siltstone is interbedded with shale unloading relaxation and shear dislocation along the steeply dipping rock layers Figure 13D.
(2)
With further downcutting of the river, the thickness of the exposed siltstone sandwich shale increased significantly. Compared with siltstone, shale has a lower strength and poor weathering resistance. Under the same environment, shale is more easily weathered and denuded with significant cavities in the rock mass. The siltstone was compressed and plastic bending occurred owing to the gravity of the overlying rock. During toppling deformation, the dislocations between the rock layers continuously increased. Owing to the high mechanical strength of quartz sandstone, the tensile stress within the rock layer continuously increased. Tensile cracks perpendicular to the rock layer developed within the rock mass, and the rock structure became loose and fractured Figure 13E.
(3)
With further development of toppling deformation, the dip angle of the underlying rock mass slowed and became horizontal. The depth of toppling deformation increased. Free conditions were provided for quartz sandstone toppling and overlapping. Simultaneously, under the influence of the fault, the hanging wall formed wedge-shaped quartz sandstone separated from the parent rock. Owing to the improvement in free conditions, the wedge-shaped rock mass was deformed. The degree of toppling deformation was further aggravated. Under the combination of gravity and faulting, the wedge-shaped rock mass sheared along the existing joints and cracks. Deformation rock mass toppling and overlapping occurred on siltstone interbedded with shale. Loose rock masses accumulated in the upstream gully (Figure 13F,G). Before the excavation, the Nandongzi landslide was at this stage.
(4)
With the development of the valley, its present pattern was formed. Slope toppling deformation entered a relatively stable stage. In 2017, under the influence of multiple excavations and rainfall, toppling-deformation rock masses were reactivated. Finally, the Nandongzi landslide formed. The reactivation mechanism of the relict landslide is discussed further in the next section.

6.3. Reactivation Mechanism of Rrelict Landslide

The Nandongzi landslide has deformed twice since 2017. Through historical images, field investigations, and deformation monitoring, the deformation of the relict slope was shown related to excavation and rainfall. Therefore, the reactivation mechanism of the landslide after multiple excavations and rainfall events was analyzed using numerical modeling.
Figure 14 shows the displacement contours of the first excavation under different conditions. Part of the rock mass within the fault zone was deformed. In Case I, which was affected by the fault tip effect, the rock mass of the fault zone had a low mechanical strength with a wide crack distribution. Excavation at the front edge of the slope changed the free condition. Owing to the increase in the free condition, the slope stability decreased. This resulted in rock mass deformation in the fault zone (Figure 14A,B). Simultaneously, the brittle fracture of the rock mass in the fault zone produced a large amount of debris. This debris was transformed into clay minerals cemented between breccias by chemical processes and weathering [56,57,58]. In Case II, under rainfall conditions, a large amount of water seeped into the landslide. Clay minerals absorbed the water and expanded, and the strength of the fault zone decreased significantly. The deformation range of the rock mass extended to the middle of the slope, with a maximum displacement of 14.16 cm. The relict landslide was reactivated for the first time (Figure 14C,D).
Figure 15 shows the displacement contour of the secondary excavation under different conditions. After the large-scale excavation, the slope stress-state was redistributed, resulting in stress relaxation and local stress concentration on the surface. In Case III, compared with the first excavation, the slope deformation range further increased. Deformation was concentrated at the slope foot Figure 15A–C. In Case IV, Figure 15D–F show that there was a significant difference in the deformation response between the fault zone and the siltstone interbedded with the shale landslide mass. The rock mass with strong deformation was widely distributed in the fault zone. The displacement of the back edge was the largest (1.94 m). The displacement contours of profiles A–A ′ and B–B ′ show that the back edge of landslide deformation is larger than the front. The damage mode of the landslide was the progressive development from the back to the front. Numerical modeling results are consistent with field survey and deformation monitoring.
During long-term slope toppling deformation, a large amount of debris and earth material formed on the slope under the combined action of gravitational deformation and weathering. Through long-term eluviation, fine-grained materials were brought into the interior of the slope by water. Eventually, these soils stayed at the bottom boundary of the highly toppled zone, where the rock structure was intact with poor permeability. Field investigations and historical aerial images showed obvious water outlets in the first and second steps of the landslide (Figure 4c). The existence of this weak layer was confirmed by the drill core (Figure 7C). After the second excavation, a large amount of the rock mass was exposed. This aggravated the degree of weathering and changed the runoff conditions on the slope surface. The rainfall infiltration recharge on the slope increased and induced landslide deformation. Water seeped into the weak layer along the exposed rock at the back edge of the landslide. Owing to poor permeability, the water remained within the weak layer. The back of the weak layer gradually became saturated.
During long-term toppling deformation, the mechanical strength of the relict-slope rock mass continuously decreased. Slope deformation changed from being controlled by the rock layer to being controlled by gravity. The deformation mode transformed from toppling deformation toward the upstream gully to shear failure toward the maximum free direction. Water significantly weakened the sliding surface strength. The back of the landslide was first deformed by gravity. The internal arrangement of the sliding-zone soils changed under the influence of the shear force generated by deformation. The pore water pressure of the sliding-zone soils increased, and the effective stress decreased. Subsequently, the sliding force at the back of the landslide increased. Shear damage occurred along the weak layer from the back edge to the front edge of the slope. Finally, a sliding surface developed at the front edge of the landslide, with an obvious bulging deformation. The layered rock mass that was not completely disintegrated in the slope suffered from shear damage. The bedding of the rock mass was completely destroyed.

7. Conclusions

This study provides novel and important insights into the toppling failure process and reactivation mechanism of relict landslides based on field investigations, borehole data, deformation monitoring, and numerical modeling. The main conclusions are as follows.
(1)
The soft and hard interbedded rock structure and proximity of the fault are the dominant factors of the Nandongzi landslide toppling deformation. The proximity of the fault broke the continuity of the rock layer. The tip effect of the fault further damaged the rock structure. Significant differences were observed in the response of soft and hard interbedded rocks to river erosion and weathering. Shale weathering-denuding, siltstone bending deformation, and quartz sandstone brittle fractures occurred. Gravitational deformation of the slope was caused by rock layer compression, bending, toppling, and overlapping.
(2)
The toppling process of the slope can be divided into four stages: initial deformation, compression and bending, toppling and overlapping, and reactivation. With the erosion of the upstream gully, the soft and hard interbedded rocks deformed from the bottom to the top due to gravity. The overlying hard rock gravity downslope overlapped and accumulated at the top of the slope.
(3)
Multiple excavations and rainfall events accelerated the transformation of the slope toppling deformation from the toppling and overlapping stages to the reactivation stage. Multiple excavations changed the slope air and surface runoff conditions, which induced landslide damage. During the rainy season, the surface infiltration recharge increased. The weak layer formed by long-term eluviation became saturated, and the shear strength decreased. The balance of relict landslide stability was broken. Owing to gravity, shear failure occurred along the weak layer in the maximum free face direction. The Nandongzi landslide was finally formed.

Author Contributions

Data curation, writing—original draft preparation, H.S.; writing—review and editing, formal analysis, T.L.; conceptualization, supervision, project administration, X.P.; visualization, validation, J.G.; software, J.T.; investigation, S.W.; methodology, M.P. All authors have read and agreed to the published version of the manuscript.

Funding

This research was jointly supported by the National Natural Science Foundation of China (grant numbers 41907238 and 41931296), National Key R&D Program of China (grant number 2018YFC1508804), Sichuan Science and Technology Program (grant numbers 2019YJ0534 and 2021YFSY0036), and State Key Laboratory of Geohazard Prevention and Geo-environment Protection Independent Research Project (SKLGP2021Z008).

Data Availability Statement

Not applicable.

Conflicts of 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.

References

  1. Agliardi, F.; Crosta, G.B.; Frattini, P. 18 Slow rock-slope deformation. In Landslides: Types, Mechanisms and Modeling; Cambridge University Press: Cambridge, UK, 2012; p. 207. [Google Scholar]
  2. Hippolyte, J.C.; Bourlès, D.; Léanni, L.; Braucher, R.; Chauvet, F.; Lebatard, A.E. 10Be ages reveal >12 ka of gravitational movement in a major sackung of the Western Alps (France). Geomorphology 2012, 171–172, 139–153. [Google Scholar] [CrossRef]
  3. Crosta, G.; Frattini, P.; Agliardi, F. Deep seated gravitational slope deformations in the European Alps. Tectonophysics 2013, 605, 13–33. [Google Scholar] [CrossRef]
  4. Jaboyedoff, M.; Penna, I.; Pedrazzini, A.; Baroň, I.; Crosta, G.B. An introductory review on gravitational-deformation induced structures, fabrics and modeling. Tectonophysics 2013, 605, 1–12. [Google Scholar] [CrossRef]
  5. Pánek, T.; Klimeš, J. Temporal behavior of deep-seated gravitational slope deformations: A review. Earth-Sci. Rev. 2016, 156, 14–38. [Google Scholar] [CrossRef]
  6. Della Seta, M.; Esposito, C.; Marmoni, G.; Martino, S.; Mugnozza, G.S.; Troiani, F. Morpho-structural evolution of the valley-slope systems and related implications on slope-scale gravitational processes: New results from the Mt. Genzana case history (Central Apennines, Italy). Geomorphology 2017, 289, 60–77. [Google Scholar] [CrossRef]
  7. Tu, X.; Dai, F.; Lu, X.; Zhong, H. Toppling and stabilization of the intake slope for the Fengtan Hydropower Station enlargement project, Mid-South China. Eng. Geol. 2007, 91, 152–167. [Google Scholar] [CrossRef]
  8. Glueer, F.; Loew, S.; Manconi, A.; Aaron, J. From toppling to sliding: Progressive evolution of the Moosfluh Landslide, Switzerland. J. Geophys. Res. Earth Surf. 2019, 124, 2899–2919. [Google Scholar] [CrossRef]
  9. Nichol, S.L.; Hungr, O.; Evans, S. Large-scale brittle and ductile toppling of rock slopes. Can. Geotech. J. 2002, 39, 773–788. [Google Scholar] [CrossRef]
  10. Tatone, B.S.; Grasselli, G. ROCKTOPPLE: A spreadsheet-based program for probabilistic block-toppling analysis. Comput. Geosci. 2010, 36, 98–114. [Google Scholar] [CrossRef]
  11. Brideau, M.A.; Stead, D. Controls on block toppling using a three-dimensional distinct element approach. Rock Mech. Rock Eng. 2010, 43, 241–260. [Google Scholar] [CrossRef]
  12. Zhang, J.; Chen, Z.; Wang, X. Centrifuge modeling of rock slopes susceptible to block toppling. Rock Mech. Rock Eng. 2007, 40, 363–382. [Google Scholar] [CrossRef]
  13. Tu, G.; Deng, H.; Shang, Q.; Zhang, Y.; Luo, X. Deep-seated large-scale toppling failure: A case study of the Lancang slope in Southwest China. Rock Mech. Rock Eng. 2020, 53, 3417–3432. [Google Scholar] [CrossRef]
  14. Ning, Y.; Tang, H.; Wang, F.; Zhang, G. Sensitivity analysis of toppling deformation for interbedded anti-inclined rock slopes based on the Grey relation method. Bull. Eng. Geol. Environ. 2019, 78, 6017–6032. [Google Scholar] [CrossRef]
  15. Ning, Y.; Tang, H.; Smith, J.V.; Zhang, B.; Shen, P.; Zhang, G. Study of the in situ stress field in a deep valley and its influence on rock slope stability in Southwest China. Bull. Eng. Geol. Environ. 2021, 80, 3331–3350. [Google Scholar] [CrossRef]
  16. Agliardi, F.; Crosta, G.; Zanchi, A. Structural constraints on deep-seated slope deformation kinematics. Eng. Geol. 2001, 59, 83–102. [Google Scholar] [CrossRef]
  17. Ambrosi, C.; Crosta, G.B. Large sackung along major tectonic features in the Central Italian Alps. Eng. Geol. 2006, 83, 183–200. [Google Scholar] [CrossRef]
  18. Amini, M.; Majdi, A.; Veshadi, M.A. Stability analysis of rock slopes against block-flexure toppling failure. Rock Mech. Rock Eng. 2012, 45, 519–532. [Google Scholar] [CrossRef]
  19. Regmi, A.D.; Yoshida, K.; Nagata, H.; Pradhan, B. Rock toppling assessment at Mugling–Narayanghat road section: ‘A case study from Mauri Khola landslide’, Nepal. Catena 2014, 114, 67–77. [Google Scholar] [CrossRef]
  20. Huang, R. Understanding the mechanism of large-scale landslides. In Engineering Geology for Society and Territory-Volume 2: Landslide Processes; Springer: Cham, Switzerland, 2015; pp. 13–32. [Google Scholar]
  21. Mather, A.E.; Griffiths, J.S.; Stokes, M. Anatomy of a ‘fossil’landslide from the Pleistocene of SE Spain. Geomorphology 2003, 50, 135–149. [Google Scholar] [CrossRef]
  22. Discenza, M.; Esposito, C.; Martino, S.; Petitta, M.; Prestininzi, A.; Scarascia Mugnozza, G. The gravitational slope deformation of Mt. Rocchetta ridge (central Apennines, Italy): Geological-evolutionary model and numerical analysis. Bull. Eng. Geol. Environ. 2011, 70, 559–575. [Google Scholar] [CrossRef]
  23. Cevasco, A.; Termini, F.; Valentino, R.; Meisina, C.; Bonì, R.; Bordoni, M.; Chella, G.; De Vita, P. Residual mechanisms and kinematics of the relict Lemeglio coastal landslide (Liguria, northwestern Italy). Geomorphology 2018, 320, 64–81. [Google Scholar] [CrossRef]
  24. Lopez Saez, J.; Corona, C.; Stoffel, M.; Astrade, L.; Berger, F.; Malet, J.P. Dendrogeomorphic reconstruction of past landslide reactivation with seasonal precision: The Bois Noir landslide, southeast French Alps. Landslides 2012, 9, 189–203. [Google Scholar] [CrossRef]
  25. Migoń, P.; Kacprzak, A.; Malik, I.; Kasprzak, M.; Owczarek, P.; Wistuba, M.; Pánek, T. Geomorphological, pedological and dendrochronological signatures of a relict landslide terrain, Mt Garbatka (Kamienne Mts), SW Poland. Geomorphology 2014, 219, 213–231. [Google Scholar] [CrossRef]
  26. Amini, M.; Ardestani, A.; Khosravi, M.H. Stability analysis of slide-toe-toppling failure. Eng. Geol. 2017, 228, 82–96. [Google Scholar] [CrossRef]
  27. Zheng, Y.; Chen, C.; Liu, T.; Zhang, H.; Sun, C. Theoretical and numerical study on the block-flexure toppling failure of rock slopes. Eng. Geol. 2019, 263, 105309. [Google Scholar] [CrossRef]
  28. Weng, M.C.; Chang, C.Y.; Jeng, F.S.; Li, H.H. Evaluating the stability of anti-dip slate slope using an innovative failure criterion for foliation. Eng. Geol. 2020, 275, 105737. [Google Scholar] [CrossRef]
  29. Brunsden, D. Geomorphological roulette for engineers and planners: Some insights into an old game. Q. J. Eng. Geol. Hydrogeol. 2002, 35, 101–142. [Google Scholar] [CrossRef]
  30. Cardinali, M.; Reichenbach, P.; Guzzetti, F.; Ardizzone, F.; Antonini, G.; Galli, M.; Cacciano, M.; Castellani, M.; Salvati, P. A geomorphological approach to the estimation of landslide hazards and risks in Umbria, Central Italy. Nat. Hazards Earth Syst. Sci. 2002, 2, 57–72. [Google Scholar] [CrossRef] [Green Version]
  31. Liu, P.j. Guo Zhangzi Sheet K50E020020 1/50,000 Regional Geological Survey Report; Jilin University: Changcun, China, 2005. [Google Scholar]
  32. De Vita, P.; Cevasco, A.; Cavallo, C. Detailed rock failure susceptibility mapping in steep rocky coasts by means of non-contact geostructural surveys: The case study of the Tigullio Gulf (Eastern Liguria, Northern Italy). Nat. Hazards Earth Syst. Sci. 2012, 12, 867–880. [Google Scholar] [CrossRef] [Green Version]
  33. Cevasco, A.; Vita, P.D. Engineering Geological Analyses for Hazard Assessment of the Urbanized, Relict Lemeglio Coastal Landslide (Eastern Liguria, Italy). In Engineering Geology for Society and Territory—Volume 5; Springer: Cham, Switzerland, 2015. [Google Scholar]
  34. Westoby, M.J.; Brasington, J.; Glasser, N.F.; Hambrey, M.J.; Reynolds, J.M. ‘Structure-from-Motion’photogrammetry: A low-cost, effective tool for geoscience applications. Geomorphology 2012, 179, 300–314. [Google Scholar] [CrossRef] [Green Version]
  35. Billaux, D.; Detournay, C.; Hart, R.; Rachez, X. FLAC and Numerical Modeling in Geomechanics-2001; CRC Press: London, UK, 2020. [Google Scholar]
  36. Liu, M.; Liu, F.z.; Huang, R.q.; Pei, X.j. Deep-seated large-scale toppling failure in metamorphic rocks: A case study of the Erguxi slope in southwest China. J. Mt. Sci. 2016, 13, 2094–2110. [Google Scholar] [CrossRef]
  37. Runqiu, H.; Yusheng, L.; Ming, Y. The implication and evaluation of toppling failure in engineering geology practice. J. Eng. Geol. 2017, 25, 1165–1181. [Google Scholar]
  38. Kim, Y.S.; Peacock, D.C.P.; Sanderson, D.J. Mesoscale strike-slip faults and damage zones at Marsalforn, Gozo Island, Malta. J. Struct. Geol. 2003, 25, 793–812. [Google Scholar] [CrossRef]
  39. Kim, Y.S.; Peacock, D.C.; Sanderson, D.J. Fault damage zones. J. Struct. Geol. 2004, 26, 503–517. [Google Scholar] [CrossRef]
  40. Choi, J.H.; Edwards, P.; Ko, K.; Kim, Y.S. Definition and classification of fault damage zones: A review and a new methodological approach. Earth-Sci. Rev. 2016, 152, 70–87. [Google Scholar] [CrossRef]
  41. Segall, P.; Pollard, D. Mechanics of discontinuous faults. J. Geophys. Res. Solid Earth 1980, 85, 4337–4350. [Google Scholar] [CrossRef]
  42. Cowie, P.A.; Shipton, Z.K. Fault tip displacement gradients and process zone dimensions. J. Struct. Geol. 1998, 20, 983–997. [Google Scholar] [CrossRef]
  43. Pachell, M.A.; Evans, J.P. Growth, linkage, and termination processes of a 10-km-long strike-slip fault in jointed granite: The Gemini fault zone, Sierra Nevada, California. J. Struct. Geol. 2002, 24, 1903–1924. [Google Scholar] [CrossRef]
  44. Fossen, H. Structural Geology; Cambridge University Press: Cambridge, UK, 2016. [Google Scholar]
  45. Gudmundsson, A.; Simmenes, T.H.; Larsen, B.; Philipp, S.L. Effects of internal structure and local stresses on fracture propagation, deflection, and arrest in fault zones. J. Struct. Geol. 2010, 32, 1643–1655. [Google Scholar] [CrossRef]
  46. Peacock, D.; Dimmen, V.; Rotevatn, A.; Sanderson, D. A broader classification of damage zones. J. Struct. Geol. 2017, 102, 179–192. [Google Scholar] [CrossRef]
  47. Qiu, D.; Yun, J.; Liu, Q.; Zhou, Y.; Song, H. The stress concentration effect at fault end under various boundary conditions and its implications for fracture development. Oil Gas Geol. 2019, 40, 205–214. [Google Scholar]
  48. De Freitas, M.; Watters, R. Some field examples of toppling failure. Geotechnique 1973, 23, 495–513. [Google Scholar] [CrossRef]
  49. Goodman, R.E.; Bray, J.W. Toppling of rock slopes. In Proceedings of the Specialty Conference on Rock Engineering for Foundations and Slopes, Boulder, CO, USA, 15–18 August 1976. [Google Scholar]
  50. Wyllie, D.C. Toppling rock slope failures examples of analysis and stabilization. In Proceedings of the 21st US Symposium on Rock Mechanics (USRMS), Rolla, MO, USA, 27–30 May 1980. [Google Scholar]
  51. Piteau, D.; Stewart, A.; Martin, D. Design examples of open pit slopes susceptible to toppling. In Proceedings of the Third International Conference of Stability in Surface Mining, Vancouver, BC, Canada, 1–3 June 1981; pp. 679–712. [Google Scholar]
  52. Woodward, R. The investigation of toppling slope failures in welded ash flow tuff at Glennies Creek Dam, New South Wales. Q. J. Eng. Geol. Hydrogeol. 1988, 21, 289–298. [Google Scholar] [CrossRef]
  53. Adachi, T.; Ohnishi, Z.; Arai, K. Investigation of toppling slope failure at Route 305 in Japan. In Proceedings of the 7th ISRM Congress, Aachen, Germany, 16–20 September 1991. [Google Scholar]
  54. Cruden, D.; Hu, X.Q.; Lu, Z. Rock topples in the highway cut west of Clairvaux Creek, Jasper, Alberta. Can. Geotech. J. 1993, 30, 1016–1023. [Google Scholar] [CrossRef]
  55. Barnett, W.P. Geological control on slope failure mechanisms in the open pit at the Venetia Mine. S. Afr. J. Geol. 2003, 106, 149–164. [Google Scholar] [CrossRef]
  56. Holdsworth, R.E. Weak faults–rotten cores. Science 2004, 303, 181–182. [Google Scholar] [CrossRef]
  57. Moore, D.E.; Rymer, M.J. Talc-bearing serpentinite and the creeping section of the San Andreas fault. Nature 2007, 448, 795–797. [Google Scholar] [CrossRef]
  58. Collettini, C.; Niemeijer, A.; Viti, C.; Marone, C. Fault zone fabric and fault weakness. Nature 2009, 462, 907–910. [Google Scholar] [CrossRef]
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Figure 1. (A) Geological setting of study area [31]. (B) Digital terrain model of the study area with location of the Nandongzi landslide. Key: (1) tuff and andesite (Jurassic); (2) siltstone and mudstone (Jurassic); (3) dolomite (Jixian period); (4) limestone (Jixian period); (5) limestone (Changcheng period); (6) limestone and sandstone (Changcheng period); (7) limestone, siltstone and dolomite (Changcheng period); (8) quartz sandstone and siltstone interbedded with shale (Changcheng period); (9) quartz sandstone and siltstone (Changcheng period); (10) quartz syenite porphyry; (11) plagioclase gneiss (supracrustal rock); (12) monzonitic granite gneiss; (13) migmatitic gneiss; (14) biotite monzonitic granite gneiss; (15) granite porphyry; (16) diabase; (17) lamprophyre; (18) monzonite porphyry; (19) reverse fault; (20) normal fault; (21) translational normal fault; (22) translational reverse fault; (23) Ductile brittle detachment zone; (24) Nandongzi landslide; (25) study area; (26) Fault names: FLC-F: Fengjiadian-Laomaojia-Chagou fault; NLD-F: Nanlaodong fault; DL-F: Dongling fault; HY-F: Huangzhangzi-Yangjiagou fault; LTG-F: Longtangou fault; YD-F: Yaogoumen-Dongmenzhangzi fault; and LH-F: Lijiawopu-Heshangling fault and (27) Rivers.
Figure 1. (A) Geological setting of study area [31]. (B) Digital terrain model of the study area with location of the Nandongzi landslide. Key: (1) tuff and andesite (Jurassic); (2) siltstone and mudstone (Jurassic); (3) dolomite (Jixian period); (4) limestone (Jixian period); (5) limestone (Changcheng period); (6) limestone and sandstone (Changcheng period); (7) limestone, siltstone and dolomite (Changcheng period); (8) quartz sandstone and siltstone interbedded with shale (Changcheng period); (9) quartz sandstone and siltstone (Changcheng period); (10) quartz syenite porphyry; (11) plagioclase gneiss (supracrustal rock); (12) monzonitic granite gneiss; (13) migmatitic gneiss; (14) biotite monzonitic granite gneiss; (15) granite porphyry; (16) diabase; (17) lamprophyre; (18) monzonite porphyry; (19) reverse fault; (20) normal fault; (21) translational normal fault; (22) translational reverse fault; (23) Ductile brittle detachment zone; (24) Nandongzi landslide; (25) study area; (26) Fault names: FLC-F: Fengjiadian-Laomaojia-Chagou fault; NLD-F: Nanlaodong fault; DL-F: Dongling fault; HY-F: Huangzhangzi-Yangjiagou fault; LTG-F: Longtangou fault; YD-F: Yaogoumen-Dongmenzhangzi fault; and LH-F: Lijiawopu-Heshangling fault and (27) Rivers.
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Figure 2. Geomorphological map of Nandongzi landslides (1:1000 scale). Legend: (1) landslide scarp; (2) the boundary of toppling deformation rock mass; (3) the boundary of landslide mass; (4) deep large tension crack; (5) deformation zones; (6) Nandongzi fault; (7) boreholes; (8) deep inclinometers; (9) Global navigation satellite system (GNSS) monitors; (10) weather sensor; (11) track of geological cross sections; (12) slide direction; (13) the attitude of the rock layers and (14) seasonal current.
Figure 2. Geomorphological map of Nandongzi landslides (1:1000 scale). Legend: (1) landslide scarp; (2) the boundary of toppling deformation rock mass; (3) the boundary of landslide mass; (4) deep large tension crack; (5) deformation zones; (6) Nandongzi fault; (7) boreholes; (8) deep inclinometers; (9) Global navigation satellite system (GNSS) monitors; (10) weather sensor; (11) track of geological cross sections; (12) slide direction; (13) the attitude of the rock layers and (14) seasonal current.
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Figure 3. Landslide 3D numerical model. (A) The lithology groups. (B) The excavation groups.
Figure 3. Landslide 3D numerical model. (A) The lithology groups. (B) The excavation groups.
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Figure 4. Historical aerial images from the development of the Nandongzi landslide (Image from Google Earth). (a) Original stable slope before the first excavation. (b) The collapse in the slope of the highway after the first excavation. (c) The displacement and several water outlets on the slope surface of the highway after the second excavation. (d) The entirety of the Nandongzi landslide.
Figure 4. Historical aerial images from the development of the Nandongzi landslide (Image from Google Earth). (a) Original stable slope before the first excavation. (b) The collapse in the slope of the highway after the first excavation. (c) The displacement and several water outlets on the slope surface of the highway after the second excavation. (d) The entirety of the Nandongzi landslide.
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Figure 5. Rock outcrops and corresponding stereographs at different positions (equal area, upper hemisphere). (a) Overall view of landslide and outcrop locations of a typical joint. (b) Geological sketch of slope surface excavation for Nandongzi landslide. (c) Highly toppled rock masses at the back scarp. (d) Horizontal rock masses exposed by the wide tension crack in the middle of the slope. (e) Highly toppled rock masses at the landslide surface. (f) Steeply dipping rock layers in the downstream slope. (g) Moderately toppled rock layers at the landslide surface. (h) Highly toppled rock layers at the landslide surface.
Figure 5. Rock outcrops and corresponding stereographs at different positions (equal area, upper hemisphere). (a) Overall view of landslide and outcrop locations of a typical joint. (b) Geological sketch of slope surface excavation for Nandongzi landslide. (c) Highly toppled rock masses at the back scarp. (d) Horizontal rock masses exposed by the wide tension crack in the middle of the slope. (e) Highly toppled rock masses at the landslide surface. (f) Steeply dipping rock layers in the downstream slope. (g) Moderately toppled rock layers at the landslide surface. (h) Highly toppled rock layers at the landslide surface.
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Figure 6. Outcrop locations of the Nanlaodong Fault. (a) Historical aerial image of the Nanlaodong Fault. (b) Distribution of fault zone in Nandongzi landslide. (c) Phenomenon of tectonic extrusion in the downstream gully. Rock structures in the (d,e) tip and (f) middle of the Nanlaodong Fault.
Figure 6. Outcrop locations of the Nanlaodong Fault. (a) Historical aerial image of the Nanlaodong Fault. (b) Distribution of fault zone in Nandongzi landslide. (c) Phenomenon of tectonic extrusion in the downstream gully. Rock structures in the (d,e) tip and (f) middle of the Nanlaodong Fault.
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Figure 7. Stratigraphy from the geotechnical boreholes inside the landslide body and photos of the principal rock types detected (A–F). Legend: 1 fault zone (photo (A)); 2 landslide mass (photo (B)); 3 sliding surface (photo (C)); 4 fully-weathered to strongly-weathered siltstone interbedded with shale (photos (D,E)); 5 medium-weathering siltstone interbedded with shale (photo (F)); and 6 toppling deformation bottom boundary.
Figure 7. Stratigraphy from the geotechnical boreholes inside the landslide body and photos of the principal rock types detected (A–F). Legend: 1 fault zone (photo (A)); 2 landslide mass (photo (B)); 3 sliding surface (photo (C)); 4 fully-weathered to strongly-weathered siltstone interbedded with shale (photos (D,E)); 5 medium-weathering siltstone interbedded with shale (photo (F)); and 6 toppling deformation bottom boundary.
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Figure 8. Core photographs of B09 borehole. (A) Down-slope overlapping zone. (B) Highly toppled zone. (C) Weakly toppled zone. (D) Original stratigraphic zone. (E) Stratigraphy from the B09 geotechnical borehole carried out inside the landslide body.
Figure 8. Core photographs of B09 borehole. (A) Down-slope overlapping zone. (B) Highly toppled zone. (C) Weakly toppled zone. (D) Original stratigraphic zone. (E) Stratigraphy from the B09 geotechnical borehole carried out inside the landslide body.
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Figure 9. The slope structure of Nandongzi landslide: geological cross section along profiles. (a) A–A′ and (b) B–B′ shown in Figure 3. Legend: (1) Quaternary alluvial-pluvial deposit; (2) down-slope overlapping zone; (3) highly toppled zone; (4) weakly toppled zone; (5) undeformed siltstone interbedded with shale; (6) undeformed quartz sandstone; (7) sliding surface; (8) down-slope overlapping zone boundary; (9) highly toppled zone boundary; (10) weakly toppled zone boundary; (11) weakly weathered boundary; (12) borehole and borehole ID; (13) Nanlaodong Fault; and (14) inclinometer and inclinometer ID.
Figure 9. The slope structure of Nandongzi landslide: geological cross section along profiles. (a) A–A′ and (b) B–B′ shown in Figure 3. Legend: (1) Quaternary alluvial-pluvial deposit; (2) down-slope overlapping zone; (3) highly toppled zone; (4) weakly toppled zone; (5) undeformed siltstone interbedded with shale; (6) undeformed quartz sandstone; (7) sliding surface; (8) down-slope overlapping zone boundary; (9) highly toppled zone boundary; (10) weakly toppled zone boundary; (11) weakly weathered boundary; (12) borehole and borehole ID; (13) Nanlaodong Fault; and (14) inclinometer and inclinometer ID.
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Figure 10. Surface deformations of Nandongzi landslide at different positions. (a) Nandongzi landslide panoramic photos. (b) Back scarp of the landslide. (c) Right boundary of the sixth step of Nandongzi landslide. (d) Back deep-large tension crack and (e) front bulge of deformation zone II-1. (f) Fresh striae on the left boundary of Nandongzi landslide.
Figure 10. Surface deformations of Nandongzi landslide at different positions. (a) Nandongzi landslide panoramic photos. (b) Back scarp of the landslide. (c) Right boundary of the sixth step of Nandongzi landslide. (d) Back deep-large tension crack and (e) front bulge of deformation zone II-1. (f) Fresh striae on the left boundary of Nandongzi landslide.
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Figure 11. Monitoring data representing the temporal evolution of Nandongzi landslide from 20 July 2020 to 10 February 2022. (A) Location of monitors on ortho-image. (B) Vertical displacements, horizontal displacements, and cumulative displacements of GNSS monitors with daily rainfall precipitation of Nandongzi landslide. The two boxes in the figure show the displacement sharply increased in the rainfall intensive period. (C,D) Cumulative displacement of inclinometer monitors with daily rainfall precipitation along profile A–A′ and B–B′, respectively.
Figure 11. Monitoring data representing the temporal evolution of Nandongzi landslide from 20 July 2020 to 10 February 2022. (A) Location of monitors on ortho-image. (B) Vertical displacements, horizontal displacements, and cumulative displacements of GNSS monitors with daily rainfall precipitation of Nandongzi landslide. The two boxes in the figure show the displacement sharply increased in the rainfall intensive period. (C,D) Cumulative displacement of inclinometer monitors with daily rainfall precipitation along profile A–A′ and B–B′, respectively.
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Figure 12. Schematic model of the Nanlaodong Fault tip.
Figure 12. Schematic model of the Nanlaodong Fault tip.
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Figure 13. Toppling failure process of Nandongzi landslide. (A) Location of the profile. (B) Slope structure before the formation of the Nanlaodong Fault. (C) Initial topography of the slope before the formation of the river valley. (D) Initial deformation stage. (E) compression and bending stage. (F,G) Toppling and overlapping stage.
Figure 13. Toppling failure process of Nandongzi landslide. (A) Location of the profile. (B) Slope structure before the formation of the Nanlaodong Fault. (C) Initial topography of the slope before the formation of the river valley. (D) Initial deformation stage. (E) compression and bending stage. (F,G) Toppling and overlapping stage.
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Figure 14. The displacement contour of the first excavation under different conditions. (A,C) 3D-model displacement contour under natural working and rainfall conditions. (B–D) A–A ′ profile displacement contour under natural working and rainfall conditions.
Figure 14. The displacement contour of the first excavation under different conditions. (A,C) 3D-model displacement contour under natural working and rainfall conditions. (B–D) A–A ′ profile displacement contour under natural working and rainfall conditions.
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Figure 15. Displacement contour of the second excavation under different conditions. (A,D) 3D-model displacement contour under natural working and rainfall conditions. Profile (B,E) A–A ′ and (C,F) B–B ′ displacement contours under natural working and rainfall conditions.
Figure 15. Displacement contour of the second excavation under different conditions. (A,D) 3D-model displacement contour under natural working and rainfall conditions. Profile (B,E) A–A ′ and (C,F) B–B ′ displacement contours under natural working and rainfall conditions.
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Table
Table 1. Physical and mechanical parameters of rock and soil mass.
Table 1. Physical and mechanical parameters of rock and soil mass.
MaterialDensity (Kg/m 3 )Elastic Modulus (MPa)Poisson’s Ratio ( μ )Cohesion (KPa)Friction Angle (°)
Fault zone (field condition)2260400.3532.126.6
Fault zone (fully saturated condition)2350340.4025.721.1
Fault zone (fully saturated condition)2350340.4025.721.1
landslide mass (field condition)2170500.3536.227.8
landslide mass (fully saturated condition)2250420.4032.825.3
slide contact zone (field condition)2000360.3430.825.2
slide contact zone (fully saturated condition)1970300.3721.319.6
bedrock260090000.2240045
Table
Table 2. Toppling intensity grading system of the Nandongzi landslide.
Table 2. Toppling intensity grading system of the Nandongzi landslide.
Zone NameDown-Slope Overlapping ZoneHighly Toppled ZoneWeakly Toppled ZoneOriginal Stratigraphic Zone
Deformation degreeRock layers are strongly toppled and overlapped. Rock mass structure is cataclastic and loose, and local rock mass is broken and overhead.The rock stratum is strongly toppled and flexible bending can be seen. The fracture of rock mass is well developed with a good opening.The rock mass is compact, the discontinuity is locally tensioned and cracked. Interlayer slip zone is developed.The rock mass is compact, the discontinuity is locally tensioned and cracked, and there is no toppling deformation in the zone.
Dip angle (º)≤2525–4040–60≥60
Unloading degreeHighly unloadedHighly unloadedHighly to slightly unloadedSlightly unloaded
Weathering degreeHighly weatheredHighly weatheredSlightly weathered and locally highly weatheredSlightly weathered
Table
Table 3. Borehole characteristics and related types of installed monitors.
Table 3. Borehole characteristics and related types of installed monitors.
IDDepth (m)Types of MonitoringGeological Cross-Section
B02-18InclinometerA–A′
B02-213InclinometerA–A′
B02-320InclinometerA–A′
B04-113InclinometerA–A′
B04-218InclinometerA–A′
B04-325InclinometerA–A′
B05-18InclinometerA–A′
B05-216InclinometerA–A′
B05-320InclinometerA–A′
B07-16InclinometerB–B′
B07-212InclinometerB–B′
B07-318InclinometerB–B′
B09-16InclinometerB–B′
B09-212InclinometerB–B′
B09-318InclinometerB–B′
B09-425InclinometerB–B′
B10-18InclinometerB–B′
B10-218InclinometerB–B′
B10-324InclinometerB–B′
G01SurfaceGNSSA–A′
G02SurfaceGNSSB–B′
G03SurfaceGNSSB–B′
G04SurfaceGNSS-
W01Surfaceweather sensor-
Table
Table 4. The comparison of vertical deformation increment of manually measured cracks and GNSS monitors on the Nandongzi landslide.
Table 4. The comparison of vertical deformation increment of manually measured cracks and GNSS monitors on the Nandongzi landslide.
Typical Crack IDMeasured Vertical Deformation Increment (cm)GNSS Monitor IDGNSS Vertical Deformation Increment (cm)
LF0122–25G0128.7
LF0228–34G0224.4
LF0314–20G0311.6
LF046–10G044.4
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Sun, H.; Li, T.; Pei, X.; Guo, J.; Tian, J.; Wang, S.; Pu, M. Gravitational Deformation and Reactivation Mechanism of a Fault-Bounded Slope, Eastern Yanshan Mountains, China. Forests 2023, 14, 495. https://0-doi-org.brum.beds.ac.uk/10.3390/f14030495

AMA Style

Sun H, Li T, Pei X, Guo J, Tian J, Wang S, Pu M. Gravitational Deformation and Reactivation Mechanism of a Fault-Bounded Slope, Eastern Yanshan Mountains, China. Forests. 2023; 14(3):495. https://0-doi-org.brum.beds.ac.uk/10.3390/f14030495

Chicago/Turabian Style

Sun, Hao, Tiantao Li, Xiangjun Pei, Jian Guo, Jingjing Tian, Shoudao Wang, and Mingfang Pu. 2023. "Gravitational Deformation and Reactivation Mechanism of a Fault-Bounded Slope, Eastern Yanshan Mountains, China" Forests 14, no. 3: 495. https://0-doi-org.brum.beds.ac.uk/10.3390/f14030495

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