Failure mechanism of gently inclined bedding slope under hydraulic driving: The case of the Jiangyuan landslide along Heda Highway

Abstract

The case of the Jiangyuan landslide along Heda Highway is discussed on the basis of the engineering geological conditions of slopes and the effect of hydraulic action on slope stability. From the analyzed characteristics of hydraulic pressure on slopes, groundwater pressure is found to include three aspects: hydrostatic pressure in the posterior border cracks caused by tension, uplift force on potential sliding surface, and dynamic water pressure. Then, the mechanical model of the slope under hydraulic driving and the formula to calculate critical water height for tension fractures are developed. Subsequently, the failure mechanism of the Jiangyuan landslide along Heda Highway is analyzed. Results show that the potential sliding surface of the landslide is gently inclined and interfaced between the overlying conglomerate and underlying weathered mudstone. The sliding tendency of the slope is aggravated by the reduced anti-sliding force of tension fractures in the posterior border, the gently inclined rock interfaces connected by rainfall-induced hydraulic driving, and the excavation of the constructed highway.

Keywords

Hydraulic driving gently inclined bedding mechanical model groundwater failure mechanism

1. Introduction

Infrastructure construction is vigorously promoted. However, slope stability issues are common in the construction of traffic, hydropower, mining, and other infrastructures. By scientifically studying slope problems, the failure mechanisms of rock slopes and corresponding solutions are highlighted.

In recent years, new studies on the failure mechanism of slopes have been conducted by scholars around the world. The American scholar Terzaghi [1] pioneered the research on landslide mechanisms; as early as the 1950s, he explained landslides in the context of changes in pore water pressure in sliding zone soils. Goomdna and Kieffer [2] classified the deformation and failure model of rock slopes in six forms. Bhasin [3] proposed that rainfall and human activities were the main inducing factors of the Sikkim landslide. On the basis of the discontinuous deformation analysis of the Vaiont landslide in Italy, Sitar [4] studied the effects of intermittent structures on landslide deformations and their failure modes. According to Bonzanigo [5], a stability coefficient could not clearly explain the deformation process of landslides; thus, creep theory was proposed to explain the movement of landslides (i.e., creep – fast moving – unstable failure). The study also found that water played a key role in landslide deformation and its failure process. Severin [6] investigated the 3D deformation characteristics of open-pit mine slopes by using synthetic aperture radar. On the basis of laboratory experiment, back analysis, and Flac3D reports, Hatanaka [7] analyzed the Songjiang Rokushima landslide in Japan and reported snowmelt as its main inducing factor. Hungr [8] and Khositashvili [9] studied the classification of landslides and the development of these classifications. By using InSAR, dGNSS, and dGPS (a deformation monitoring equipment), Booth [10] investigated the deformation regularity of the Sognefjord Osmundneset landslide in Norway and discussed its damage mechanism.

In China, following the works of Gu Dezhen, Sun [11] proposed a theory on rock mass structure control. Meanwhile, by focusing on real cases, Feng [12] studied the causes and mechanisms of gently inclined beddings of rock landslides and discovered that sliding and tensile deformation failure developed along composite surfaces between weak landslide and steep fracture layers. Jian [13] found that a rainstorm induced the Minguochang landslide in Wanzhou District. Liu [14] analyzed the mechanisms of sliding failure under the hydraulic driving of rock slopes. By using the Tiantai Township landslide in Sichuan Province as an example, Fan [15] analyzed the formation of translational landslides. Huang [16] studied several landslides related to the Wenchuan earthquake to determine the deformation failure characteristics and mechanisms of landslides. Wu [17] used the method of unsaturated soil mechanics to study the sliding mechanisms of accumulation landslides under rainfall conditions. Lu [18] analyzed the engineering geological conditions of the Shuping landslide in the Three Gorges Reservoir and its unstable failure mechanism. Zhang [19] selected red-layer landslides as a research object to determine the effects of pore water pressure and shear strength, as well as the mechanisms involved in rainfall-induced landslides. Xia [20] developed a formula to calculate critical water height for tension fractures and related failure mechanisms and discussed the sliding failure of gently inclined bedding of compound rock mass slopes on the said basis.

The above-mentioned studies have offered outstanding contributions to slope failure mechanism and slope stability literature. In several reports, landslides failures are induced by gently inclined bedding slopes, which develop gradually over time. Furthermore, slope failures are triggered by external hydrodynamic forces. Usually, under the action of soft construction plane, circulation channels are formed along weak structural planes in slope bodies after rainfall infiltration. Subsequently, slope instability occurs because of hydrostatic pressure in the tension fissures along trailing edges and uplift pressure in potential sliding surfaces. This study highlights the case of the Jiangyuan landslide along Heda Highway to establish a mechanical model for gently inclined bedding slopes under hydraulic driving. The formula to calculate critical water height for tension fractures is developed, and the failure mechanism is analyzed alongside the engineering geological conditions of the slopes.

2. Sliding failure mechanism of gently inclined bedding slopes under hydraulic driving

2.1 Mechanical model of gently inclined bedding slope under hydraulic driving

In gently inclined bedding slopes, rainfall and groundwater interaction induces mechanical effects in four aspects: (1) physicochemical action of rock and soil on slopes triggered by groundwater effects (e.g., reduced shear strength of the sliding surface); (2) hydrostatic pressure in the tension fissures of posterior borders; (3) uplift pressure at the bottom part of sliding surfaces; and (4) seepage pressure on sliding surfaces (e.g., surface force on the sliding surface in the direction of the seepage, also known as drag force).

The failure mode of the landslide depends on the morphological characteristics of the sliding surface. If the sliding surface is arc-shaped, the groundwater uplift pressure will be distributed in the arc of the vertical arc-shaped sliding surface, and the water pressure in the trailing edge crack will still be hydrostatic pressure. At this time, the pressure exerted on the bottom of the sliding surface reduces the normal stress on the sliding surface, and reduces the sliding resistance of the sliding surface. At the same time, the sliding surface is more likely to be unstable and destroyed under the hydrostatic pressure in the trailing edge crack. Under this condition, the shear strength of the slip surface should follow the Mohr-Coulomb strength criterion, and the stability of the slider body should be calculated by the Sweden slice method.

Jiangyuan landslide has the following characteristics: (1) the potential sliding surface is linear; (2) the angle of the potential sliding surface is less than that of the slope; (3) friction is non-existent at both ends of the slope when landslide failure is developed; and (4) the shape of the tension fissures at the posterior border is vertical. The distribution of hydrostatic pressure in the tension fissures at the posterior border and uplift pressure of the sliding surface is triangular. The extreme value is set to $\gamma_{wh}$ , and drag force $t$ is evenly distributed on the sliding surface.

The hydrodynamic model of the gently inclined bedding slope under hydraulic driving is developed with the above-mentioned assumptions (Fig. 1).

Figure 1.

Hydrodynamic model of the slope.

Following the typical hydraulic model of rock slopes of E. Hoek and J.W. Bray, the corresponding seepage pressure model for the slope when groundwater flows between rock layers is established (Fig. 2).

Figure 2.

Calculation model of seepage pressure for the slope body.

A single wide micro unit from the landslide and a unit for the water sample are obtained as research objects. The following parameters are selected: micro body length (dl), height (b), section area (A), and porosity (n). The groundwater forces on the element along the flow direction are described by the following.

2.1.1 Pore water pressure

The pore water pressure at the two ends of the micro body is $P$ and $P+dP$ , respectively. Their difference is $dP=(dh-dz)\gamma_{w}dA$ , where $dz=$ head difference (m) of the two heads and $dh=$ height difference (m) of the two heads.

2.1.2 Buoyancy of rock and soil skeleton

The component force of buoyancy along the seepage direction along the micro unit is based on the following equation:

$\displaystyle dM=\gamma_{w}(1-n)\textit{dAdS}\frac{dz}{dS}$

where $n=$ porosity ratio of rock and soil slope, $dA=$ section area of micro element (m2), and $dS=$ length of the infinitesimal body (m).

2.1.3 Dynamic water pressure

The reaction force of the skeleton on micro body resistance corresponds to the dynamic water pressure of water flow to the framework. In this study, $f$ denotes the unit resistance of the infinitesimal body water. The flow resistance caused by the framework on the flow of the element is based on the following equation:

$\displaystyle dF=-\textit{fdAdS}$

Without considering the inertial force of groundwater flow, the static equilibrium relation of micro body flow can be expressed as

$\displaystyle dP+dT+dM+dF=0.$

Thus,

$\displaystyle(dh-dz)\gamma_{w}dA+n\gamma_{w}\textit{dAdS}\frac{dz}{dS}+\gamma_% {w}(1-n)\textit{dAdS}\frac{dz}{dS}--\textit{fdAdS}=0$

The unit resistance of the skeleton is calculated as follows:

$\displaystyle f=\gamma_{w}\frac{dh}{dS}$

The hydraulic pressure of water flow to the skeleton is expressed as

$\displaystyle f^{\prime}=f=\gamma_{w}\frac{dh}{dS}=\gamma_{w}i$

where $i$ is the hydraulic gradient along the seepage direction.

2.2 Slope failure mode under hydraulic driving

For the engineering excavation or artificial cutting of a slope in the gently inclined bedding, the rock mass near the slope toe is considered a free boundary without constraint, which hinders the natural stress state of the slope, thereby causing the stress in the slope to redistribute. At the same time, tensile cracks with nearly vertical appearances form at the trailing edge of the slope. When the cracks are transfixed to the weak structure surface in the gently inclined bedding slope or rock interface, the requirements of bedding sliding are satisfied. Subsequently, tensile fractures and weak structural planes also form in groundwater circulation channels. In the case of hydraulic-driven landslides, the dip angle of a rock is usually less than the angle of internal friction; therefore, the slope does not meet the sliding requirements under natural conditions. However, when driven by rainfall and groundwater, the surface water easily flows into the tension fracture at the trailing edge of the slope and form a water head at the transfixion plane of the weak structure surface. If water discharge is impossible, then uplift pressure is formed. Then, a drag force along the potential sliding surface causes wedge fractures, and this drag effect on the slope reduces the shear strength of the sliding surface. At the same time, the drag force increases the joint opening of the weak structure surface. Apart from hydrostatic pressure along the direction of the sliding surface, dynamic water pressure also adds glide force to the slope body. The resultant uplift pressure reduces the normal stress of the sliding surface and the anti-sliding force of slope. The above-mentioned analyses show that, when the tension fractures at the trailing edge of the slope are deep, the hydrostatic pressure at the trailing edge and the uplift pressure at the sliding surface are high, and the adverse effects of groundwater are strong. When rainfall or groundwater pressure reaches a certain level, the confined water in the slope forms a sufficient driving force, thereby causing instability and destruction of the slope.

3. Criterion of slope instability under hydraulic driving

The mechanical model of the gently inclined bedding slope under hydraulic driving is shown in Fig. 3. The external forces acting on the landslide slope include the following: weight force (G), uplift pressure (U) on the sliding surface, hydrostatic pressure (V) on the tensile fracture, and drag force (D) on the sliding surface. The slope angle is $\beta$ , the dip angle of the sliding surface is $\alpha$ , the internal friction angle of the sliding surface is $\varphi$ , the length of the sliding surface is $L$ , and the water height of the tension fracture is $z_{w}$ .

Figure 3.

Mechanical model of the slope under hydraulic driving.

Under the action of groundwater, the slope stability coefficient can be expressed as

$\displaystyle Fs=\frac{({G\cos\alpha-U-V\sin\alpha})\tan\varphi+c\cdot L}{G% \sin\alpha+V\cos\alpha+D}$

where $V=\frac{1}{2}\gamma_{w}z_{w}^{2}$ , $U=\frac{1}{2}\gamma_{w}z_{w}L$ , and $D=\frac{1}{2}\gamma_{w}bL$ , with $b$ as the height of the rectangle equal to the slider area.

When the slope body is in the state of limit equilibrium, $V$ , $U$ , and $D$ are taken in their upper forms. The critical water height expression for the trailing edge fracture is

$\displaystyle z_{w}=\frac{1}{2({\cos\alpha+\sin\alpha\tan\varphi})}\left\{% \left[\left({b+L\tan\varphi}\right)^{2}+\frac{8}{\gamma_{w}}({\cos\alpha+\sin% \alpha\tan\varphi})\phantom{{\frac{8}{\gamma_{w}}}^{\frac{1}{2}}}\right.\right% .\!\!\left.\left.\phantom{\frac{8}{\gamma_{w}}}({G\cos\alpha\tan\varphi+cL-G% \sin\alpha})\right]^{\frac{1}{2}}-({b+L\tan\varphi})\right\}$

4. Engineering geological conditions of the landslide

4.1 Engineering situation

The engineering-related landslide is located at the northeastern part of Jiangyuan District (Baishan City in Jilin Province) and to the right of the cutting slope at the construction mileage section of Heda Highway (from K288 $+$ 400 to K289 $+$ 030).

The left-side length of the landslide is 530 m from K288 $+$ 400 to K289 $+$ 030. Meanwhile, the maximum depth of 30.69 m is located at K288 $+$ 900. The slope is designed as a step-shaped five-grade slope with a height of 6.0 m at each level, with 1:1.5 and 1:1.25 slope rates in the upper two parts and lower three parts of the slope, respectively. During construction, earth is excavated at the lower part of the cutting, after which a down-sliding slope develops gradually. The front part of the slope appears with seepage while the rest of the slope and the trailing edge appear with tension fractures.

4.2 Natural geographical conditions of the landslide

4.2.1 Geographic and geomorphic conditions

The slope is located on a low mountain. The highest point elevation in the site is 900 m, the lowest point is 660 m, and the relative height difference is 240 m. The ridge is northeastern oriented, and the mountain slope is 10 ${}^{\circ}$ –20 ${}^{\circ}$ . The site is characterized by a tectonic denudation landform. The landslide is V-shaped along the mountain valley with the slope facing the southern direction at 15 ${}^{\circ}$ .

4.2.2 Meteorological and hydrological conditions

The area of the landslide is located in Jiangyuan District (Baishan City in Jinlin Province), which belongs to the cold temperate zone with continental monsoon climate. Annual average temperature is 4 ${}^{\circ}$ C with highest temperature of 37 ${}^{\circ}$ C and lowest temperature of $-$ 38 ${}^{\circ}$ C. The area has been frost-free for 120 days. Average annual precipitation is approximately 850 mm, annual evaporation volume is approximately 1030.0 mm, and annual average relative humidity is 72%. The rainy season is concentrated in July and August, with precipitation (mainly July) accounting for 51% of the whole year. Freezing and thawing occurs in late October and late April, respectively. The maximum freezing depth is 1.40 m.

No surface water is observed in the field and surrounding gully, the north-side V-shaped valley is developed, and water flows are controlled by seasons. These conditions do not influence the survey site.

4.3 Engineering geological conditions of the landslide

4.3.1 Formation lithology

The slope body is covered with loose strata of quaternary Holocene series (Q4) and the lower part is composed of Jurassic sedimentary rocks (J3y). Dikes with extruding basalt are developed locally. The characteristics and distribution of the rocks and soils are as follows.

(1) Silty clay with gravel

The soil is grayish, slightly wet, and loose, and it is composed of gravel mixed with cohesive soil (i.e., gravel is approximately 50% with particle sizes of 40–80 mm and maximum size of 20 cm) and crushed stone blocks (intermediary weathered basalt). Silty clay content is approximately 30%; the rest is gravel. This layer is 3–16.9 m thick. At the top layer, the thickness of the thin humus soil is 0.5–1.5 m.

(2) Intermediary weathered basalt

The grayish black layer is composed of a cryptocrystalline structure, massive structure, joint fissures and development, mineral composition pyroxene, phenocryst olivine, other hard rocks, and core fragments. The local rock distribution penetrates the Jurassic strata in the form of dikes.

(3) Completely weathered and intermediary weathered conglomerate

The grayish yellow layer is composed of a clastic structure, thick-bedded structure, shale cementation, gravel (diameter of 0.3–10 cm), and locally visible boulders (20–70 cm). Gravel composition is complicated and can be described as follows: presence of basalt, quartz sandstone, and quartzite; intermediary weathered; good gravel rounding (mild round); presence of argillaceous cement weathered clay; and layer thickness of 1.2–19.7 m, including a thick layer of sandstone and mudstone interlayer.

(4) Completely weathered and intermediary weathered clay shale

The yellow green and grayish green layer appears with a pelitic texture, bedded structure, upper completely weathered mudstone mineral weathering alteration, rock mass weathering clay in wet hard-plastic state. In the lower part of the weathering, the joint fissures are undeveloped. Lamellation is developed locally (i.e., rust and weak rocks are observed at the surface). This layer comprises the main component of the lower sliding belt, which presents a thickness of 0.5–1.8 m. The layer is affected by ancient landslide, which forms a smooth sliding surface.

(5) Completely weathered and strong-weathered carbonaceous shale

The dark gray layer is composed of a peaty carbonaceous structure, bedded structure, relatively strong weathered layer, some undeveloped joint fissures, lamellation and local development, core columnar (3–4 cm), and rock pillars with air-drying dehydration cracking in the air. The strength of this layer decreases significantly. This type is distributed from K288 $+$ 580 to K289 $+$ 100 in the conglomerate layer and appears with a thin breccia layer.

The engineering geological planar graph of the landslide is shown in Fig. 4.

Figure 4.

Engineering geological plane of the landslide.

4.3.2 Geological structure

The geodetic tectonic unit in the area of the landslide is located at the edge of Taizi River in the Liaodong upwarp and the concave breaking site of the Hunjiang depression in the platform between China and North Korea. From the newly compiled data (area: 1/250,000), the edge fracture zone of the northern China platform and the Liaoning – Jilin Rift spreads from Tonghua City (northeastern orientation) to Banshi, Sanchazi, Huayuankou, and Fusong (southeastern direction). The site also covers a basalt area in Jingyu. Structural features can be easily observed. Arc distribution is composed of several faults, cross-sections (northwestern direction), local development (southeastern direction), and dip angles of 50 ${}^{\circ}$ –60 ${}^{\circ}$ . The fracture passes through the east side of the working area. The depression edge of the Jurassic fault basin is a multi-stage inheritance composite structure that forms as normal or reverse fault structures at high angles.

Although fractures exist in the surrounding area, fold fractures are not developed. Active faults are also not discovered in the site.

4.3.3 Hydro-geological conditions

According to the investigation, groundwater occurs in conglomerate pores. Groundwater is characterized as a pore water of clastic rocks, and the water level is deeply buried. The aquifer appears with weak water-richness but good permeability. The permeability of shale and carbonaceous shale in between layers is poor and causes a relative water-barrier effect. From the geological survey, groundwater seepage is found to occur in the contact area of shale and conglomerates along the sliding surface of the two leading edges of the landslide (see engineering geological plan). The field measurement results show that the flow rates for the K288 $+$ 580–K289 $+$ 050 section with two water points are 0.039 and 0.027 L/S and those for the K290 $+$ 270–K290 $+$ 500 section with two water points are both around 0.014 L/S.

The water levels of all boreholes are measured during drilling. No groundwater level is found, which indicates that the groundwater from the survey site is locally situated and unevenly distributed. The groundwater in the site can be described as stagnant water interlayers between the slippery surface and upper part of the shale. When drilling penetrates the lower aqueduct, the stagnant water leakage in the upper layer flows along with the drilling.

The hydrochemical composition of the groundwater is HCO3-Ca, PH = 6.5–7.5, and salinity is 0.17 g/l according to the results of analysis. Upon verification with the code for geotechnical engineering investigation (GB 50021-2010), the groundwater is slightly corrosive to concrete and slightly corroded to the reinforcement in concrete.

5. Analysis of the current situation of the landslide

The original natural slope is 15 ${}^{\circ}$ –20 ${}^{\circ}$ . The gently inclined bedding at the top of the cutting slope is less than 10 ${}^{\circ}$ corresponding to the maximum digging depth of 30.69 m. During landslide excavation, the rock mass and soil down-slide slowly and the trailing edge forms a tension fracture. The upper part of the sliding body also forms a series of parallel tension cracks, and the front part of the sliding body slowly extrudes shear exports toward the free surface of the excavation. These observations imply a seepage phenomenon.

Intercepting gutters are designed for the trailing edge of the landslide (i.e., 10–50 m apart from the intercepting gutters) at the periphery. The formation of tension fractures is not continuous, and its overall direction is in line with the slope. The north-to-south length of the fracture is approximately 530 m. The shear cracks in the back part of the sliding body are locally formed and appear as feather formations. The spacing between shear gaps is 1–4 m. At the trailing edge of the landslide and tension cracks, the fracture width is approximately 10–50 cm and vertical downward depth is approximately 20–150 cm (Figs 5 and 6). The front part of the sliding bodies extrudes forward along the sliding surface of shale. The rock mass and soil in the upper cutting collapse toward the free surface. Groundwater seepage on the local sliding surface is shown in planar graph 3-3 (section around). The flow rate is 0.014–0.039 L/S (Figs 7 and 8).

Figure 5.

Tailing edge of the slope.

Figure 6.

Tension fractures in the landslide.

Figure 7.

Water seepage of the sliding edge at the front.

Figure 8.

Water seepage of the sliding edge at the front.

At the K288 $+$ 430–K288 $+$ 600 section (i.e., near the 6-6 profile of the planar graph), the area is pushed by the left-side sliding block, the left edge of the embankment is bulged, and vertical drum tension cracks are formed. In the middle of the embankment and at the right side of the road shoulder, several cracks with lengths of 50–100 m and widths of 1–3 cm are formed. The earth-retaining wall of the embankment is cracked, and the reserved expansion joints are dehisced (Figs 9 and 10).

The anterior sliding belt of the landslide is 1.0–2.5 m thick. The 0.5–1.5 m-thick upper part of the black carbonaceous shale is crumpled and eroded by the sliding action of rock structures and eventually takes the shape of a clay. The lower part is yellow brown, the argillaceous shale is maroon, and the thickness of the layer is 0.5–1.0 m. Owing to the sliding force, the rock mass is broken into clay. The sliding surface and other sliding marks are visible. A slip band is visible on both sides of the excavated cutting road. The left side of the landslide on the cutting road is slowly moved to the northwestern direction along this sliding surface (Figs 11 and 12). The gently inclined bedding surface on the right side of the cutting slides to the reverse side of the slope, which indicates that the road is subject to a landslide before construction; thus, the road is temporarily stable.

Figure 9.

Crack status of the retaining wall.

Figure 10.

Surface cracks of the subgrade.

Figure 11.

Sliding band and sliding surface.

Figure 12.

Sliding surface of the excavated cutting road.

The plane surface of the landslide is somewhat rectangular. The length from north to south is approximately 530 m while the width from east to west is nearly 70 m. The thickness of the sliding body is 5–15 m with average thickness of 10 m. Sliding mass is approximately 400,000 m ${}^{3}$ , which can be classified as a large bedding landslide for mid-thickness layers. The sliding body from southeast to northwest in the northern direction down-slides slowly due to a type of pushing. Owing to drillings (i.e., for geophysical and excavation prospecting and geological mapping), the upper landslide cut sections appear as a completely weathered conglomerate while the lower bedding slide sections along the relatively weak shale appear as a completely weathered to strongly weathered conglomerate.

The landslide in 3-3 and 6-6 sections are shown in Figs 13 and 14.

Figure 13.

Cross-sectional view of the 3-3 section.

Figure 14.

Cross-sectional view of the 6-6 section.

6. Sliding mechanism analysis of the sliding slope

6.1 Physical and mechanical properties of the landslide rock mass and soil

According to the engineering geological profile, the upper part of the landslide is a completely weathered to strongly weathered conglomerate, the lower part is a strongly weathered shale, and the sliding bed is a strongly weathered conglomerate and strongly to intermediary weathered shale. The middle upper part of the sliding surface cuts the conglomerate layer. Furthermore, the lower part of the weathering shale slides along the bedding. The physical and mechanical indicators used in the site experiment and laboratory geotechnical tests are shown in Table 1.

Table 1
Results of physical and mechanical analysis of landslide rock and soil

Stratigraphic name	Water	Bulk	Internal	Cohesion	Deform-ation	Compre-ssive	Elastic	Poisson
	content	density	friction	(kPa)	modulus	strength	modulus	ratio
	(%)	(kN/m ${}^{3}$ )	angle ( ${}^{\circ}$ )		(MPa)	(MPa)	(MPa)
⟂ ${}_{1}$ completely weathered conglomerate		25.7	34.4		21.0
⟂ ${}_{2}$ strongly-weathered conglomerate		25.7	38.4		33.2
⟃ ${}_{1}$ completely weathered clay	25.9	18.7	15.1	27.8
shale			${}^{*}$ 11.6	${}^{*}$ 14.7
⟃ ${}_{2}$ strongly-weathered clay shale		20.4	23.9	37.8		0.24	7.75	0.41
⟃ ${}_{3}$ intermediary-weathered clay shale		25.0	35.0	47.0		12.0	1300	0.22
⟄ ${}_{1}$ completely weathered	38.0	17.9	16.3	26.2
carbonaceous shale			${}^{*}$ 12.7	${}^{*}$ 13.7
⟄ ${}_{2}$ strongly-weathered carbonaceous shale		19.3	21.4	30.6		0.11	7.56	0.46

Note: Data of band ${}^{*}$ are considered the remodeling parameter.

6.2 Analysis of the causes and sliding mechanism of landslide

6.2.1 Rock and soil structure

From the investigation, the sliding belt zone is found to be mainly located in a relatively weak position between the clay shale and the carbonaceous shale in the conglomerate strata. The shear strength of shale is controlled by soil consistency (i.e., the shale is weathered into the shape of clay). Owing to weak water seepage, the blocked groundwater cannot discharge on time, thereby resulting in increased water content and softened rock mass. As the consistency of the structure is increased, shear strength is decreased and anti-sliding force is greatly reduced.

6.2.2 Topographic and geo-morphological structure

The natural slope of the mountain inclines to the northwestern direction at approximately 15 ${}^{\circ}$ with the top part at less than 10 ${}^{\circ}$ . The landslide and slide bed are considered a set of thickness-layer formed by sedimentation at 330 ${}^{\circ}$ –345 ${}^{\circ}$ and 9 ${}^{\circ}$ –15 ${}^{\circ}$ . The inclination of natural slope is consistent with that of the landslide. The slope of bedding is formed by cutting at the ratio of 1:1.5 (approximately 39 ${}^{\circ}$ ). The bedding thrust of the soil of slope caused by gravity is disadvantageous to slope stability.

6.2.3 Groundwater effect

Atmospheric precipitation infiltrates into the slope. However, precipitation is blocked by the completely weathered shale in the lower part of the slope. Thus, water cannot discharged on time. As a result, the water level of the slope rises. Owing to groundwater immersion, the relative bulk density of the soil in the upper part of the slope increases, the rock and soil in the lower part of the sliding zone soften, and shear strength decreases, thereby resulting in a landslide. In addition, when groundwater flows in the sliding surface, dynamic water pressure is formed, thereby resulting in mechanical and chemical erosion action in the upper part of the soil. The phenomenon reduces soil strength at the bottom of sliding face, thereby decreasing slope stability.

6.2.4 Ancient landslide and engineering factors

According to geological survey and prospecting, the cutting excavation is located in the anti-slip section of the leading edge of the ancient landslide. The anti-slip earthwork can be removed and the load can be reduced to minimize anti-slip resistance. However, owing to the overlap between the potential sliding surface of the existing sliding slope and the sliding surface of the ancient landslide, the ancient landslide may resurrect under hydraulic action.

During highway construction, the original foundation slope at the front part of the embankment fill (from K288 $+$ 430 to K288 $+$ 600) does not undergo longitudinal and lateral blind drains. Drilling reveals that the original flour clay on the ground is not clean. Moreover, the retaining walls on the right side intercept groundwater. Subsequently, water invades and softens the silty clay layer, thereby reducing the compressive and shear strengths of the site. During cutting, a subgrade transits from the weakly weathered rock (left side) to the relatively loose settlement and relatively large gravel soil layer (right part). As the embankment fill increases (the highest is 19.5 m on the right side), embankment gravity causes an uneven settlement of subgrade mainly with vertical subsidence. Subsequently, the left side of the landslide pushes the fill subgrade. In addition, longitudinal tensile cracks on the central embankment push the retaining wall sliding outward. Therefore, the failure phenomenon of the road bed is caused by a combination of landslide sliding (left side) and uneven subsidence of the foundation.

Geotechnical structure, strata structure and groundwater, roadway excavation, and existing ancient landslides are considered the main factors of the cutting landslide. The landslide described above can be classified as a broaching landslide.

7. Conclusions

From the results, the following conclusions are drawn:

(1)

The bedding slope destruction under hydraulic driving occurs frequently in the free surfaces after slope foot excavation. After the tension fissures at the posterior border are transfixed to the potential sliding surface in the slope, the surface water flows in and forms a section of confined water, thereby resulting in hydraulic driving and the occurrence of landslides.

(2)

For the gently inclined bedding slope, when atmospheric precipitation and groundwater flow onto tension fissures at the posterior border and the potential sliding surface, hydraulic actions on the slope occur. Hydraulic actions appear in three forms: hydrostatic pressure in the cracks of the posterior border caused by tension; uplift force on the potential sliding surface; and dynamic water pressure.

(3)

The mechanical model of gently inclined bedding slope under hydraulic driving and the criterion for slope instability in the hydraulic driving type are established. Consequently, the formula to calculate critical water height for tension fractures is developed.

(4)

The engineering geological conditions and sliding mechanism of the landslide in Jiangyuan along Heda Highway are analyzed. Consequently, the composite failure mechanisms of the rock and soil structure, slope topography, and groundwater and engineering factors are established.

Footnotes

Acknowledgments

This work was supported by the National Natural Science Foundation of China (No. 41502270), Open Fund of State Key Laboratory of Frozen Soil Engineering (Grant No. SKLFSE201506).

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Failure mechanism of gently inclined bedding slope under hydraulic driving: The case of the Jiangyuan landslide along Heda Highway

Abstract

Keywords

1. Introduction

2. Sliding failure mechanism of gently inclined bedding slopes under hydraulic driving

2.1 Mechanical model of gently inclined bedding slope under hydraulic driving

2.1.2 Buoyancy of rock and soil skeleton

2.1.3 Dynamic water pressure

2.2 Slope failure mode under hydraulic driving

3. Criterion of slope instability under hydraulic driving

4.1 Engineering situation

4.2 Natural geographical conditions of the landslide

4.2.1 Geographic and geomorphic conditions

4.2.2 Meteorological and hydrological conditions

4.3 Engineering geological conditions of the landslide

4.3.1 Formation lithology

(1) Silty clay with gravel

(2) Intermediary weathered basalt

(3) Completely weathered and intermediary weathered conglomerate

(4) Completely weathered and intermediary weathered clay shale

(5) Completely weathered and strong-weathered carbonaceous shale

4.3.3 Hydro-geological conditions

5. Analysis of the current situation of the landslide

6.1 Physical and mechanical properties of the landslide rock mass and soil

Table 1 Results of physical and mechanical analysis of landslide rock and soil

6.2.1 Rock and soil structure

6.2.2 Topographic and geo-morphological structure

6.2.3 Groundwater effect

6.2.4 Ancient landslide and engineering factors

7. Conclusions

Footnotes

Acknowledgments

References

Table 1
Results of physical and mechanical analysis of landslide rock and soil