geosciences

Article Classification of Red-Bed Rock Mass Structures and Slope Failure Modes in South China

Cuiying Zhou, Xu Yang, Yanhao Liang, Zichun Du, Zhen Liu * , Wei Huang and Weihua Ming

Guangdong Engineering Research Centre for Major Infrastructure Safety, Sun Yat-sen University, Guangzhou 510275, China; [email protected] (C.Z.); [email protected] (X.Y.); [email protected] (Y.L.); [email protected] (Z.D.); [email protected] (W.H.); [email protected] (W.M.) * Correspondence: [email protected]



Received: 30 January 2019; Accepted: 17 June 2019; Published: 21 June 2019


Abstract: Red beds are Meso–Cenozoic continental sedimentary strata that are mainly composed of gravel stone, sandstone, siltstone, mudstone, and shale and occasionally have interlayers of limestone, halite, and gypsum. As a typical rock mass, red beds are widely distributed throughout South China. In a typical tropical and subtropical continental environment, red beds are the product of multiple sedimentary cycles, which have resulted in complicated rock mass structures that play an important role in rock mass stability. It is thus of great significance to investigate the influence of different rock mass structures on the stability of red-bed slopes. In this paper, the geological formation history of red beds in South China is described. The main features of red-bed rock mass slopes in South China are discussed. The main combinations of inner geomechanical structures comprise: (1) mega-thick soft rock structures; (2) mega-thick hard rock structures; (3) thick hard rock structures with weak intercalation; and (4) soft–hard interbedded structures. In addition, the features of slope failure are analyzed, and four common failure modes are identified from the statistical data: (a) weathering spalling and scouring; (b) rock falls; (c) landslides; and (d) tensile dumping.

Keywords: red beds in South China; structural plane; structural morphology; rock mass structure types; slope failure model; forming feature of hazards

1. Introduction The term red beds refers to red clastic rocks weathered in laterite from the Mesozoic–Cenozoic era. They include lacustrine deposits, fluvial facies deposits, interbedded lacustrine–fluvial deposits, and piedmont diluvial deposits. Red beds primarily consist of conglomerate, sandstone, mudstone, siltstone, and shale, as well as a small amount of limestone, rock salt, and gypsum [1–5]. These beds are formed under unique hot and dry tropical–subtropical environmental conditions, and they exhibit a series of unique engineering properties, such as high porosity. We can investigate the diagenetic evolution of red beds through a petrophysical, fluid evolution, and thermal history analysis [6]. No distinct boundary exists to distinguish red-bed rock mass from conventional soil mass. In particular, the water-induced instability problem is significant for red beds. According to a previous model, the instability of red-bed soft rock slopes is mainly due to the softening that is induced by water infiltration [7,8]. Previous research has shown that red beds are widely distributed all over the world [1,9–13], including Asia, Europe, and America. In China, the exposed area of red beds is approximately 0.826 million km2, which is second only to the carbonatic area, which is an estimated 0.910 million km2 [1,2]. The statistics show that 80% of the red-bed area in China is distributed in a humid region that spreads from the east of the Qinghai–Tibet Plateau to the south of the Qinling and Huaihe River, and the involved red-bed area

Geosciences 2019, 9, 273; doi:10.3390/geosciences9060273 www.mdpi.com/journal/geosciences Geosciences 2018, 8, x FOR PEER REVIEW 2 of 21 the involved red-bed area accounts for approximately 60% of the total area of the humid regions. Figure 1 shows the distribution of red-bed basins in China [1]. Red beds have undergone long-term geological formation and transformation [14–23], which have resulted in a typical sedimentary soft–hard interbedded structure of mudstone and sandy conglomerate. A weak bedding layer with a low coalescence degree but good extensibility is usually formed at the interface between the hard and soft rocks and it is one of the main structural planes observed in red beds. The strata of red-bed soft rock generally contain one or more weak interlayers. For example, sandstone contains a mudstone interlayer, which softens easily and fails when it meets water. The mechanical properties of red-bed soft rock containing weak interlayers can be studied by laboratory and field analyses of the mineralogical composition of the weak interlayer [24,25]. Structural features such as faults, joints, interlayer slippage zones, and pelletization dissectionGeosciences are usually2019, 9, 273 well developed and are considered to be other types of main structure2 of 20plane in red beds, such as the interlayer slippage zone (a structurally related feature) of red beds located in southwesternaccounts forChina approximately [26] and 60%faults of theor totalfractures area of at the the humid depth regions. of red Figure beds1 shows in the the distributionSouth Georgia of Rift (SGR) red-bedbasin [27]. basins in China [1].

FigureFigure 1. 1.DistributionDistribution of red bedsbeds in in China China [1]. [1].

Red beds have undergone long-term geological formation and transformation [14–23], which have It resultedis well inknown a typical that sedimentary rock mass soft–hard structures interbedded play a structure dominant of mudstone role in andthe sandystability conglomerate. of a rock mass [11,16,28].A weak The bedding characteristics layer with of a lowslope coalescence failures in degree red beds but good are extensibilitysummarized is usuallyfrom field formed investigations at the and caseinterface studies between as follows: the hard and soft rocks and it is one of the main structural planes observed in red beds. (1)The A strataprogressive of red-bed failure soft rock is generallyprone to contain occur one on or a moreslope weak with interlayers. structures For controlled example, sandstone by steep dip joints andcontains fissures. a mudstone Once the interlayer, initial whichdamage softens occurs, easily sustained and fails whendestruction it meets will water. occur The mechanicalprogressively. (2)properties The slope of of red-bed rock mass soft rock structur containinges that weak are interlayerscontrolled can by be weak studied interlayers by laboratory is prone and fieldto causing analyses of the mineralogical composition of the weak interlayer [24,25]. Structural features such as hazards. Alternatively, a rock slope with thick layers of hard rock mass may also tend to suffer from faults, joints, interlayer slippage zones, and pelletization dissection are usually well developed and are considerableconsidered serious to be damage other types if ofunderlying main structure weak plane layers in red exist. beds, such as the interlayer slippage zone (3)(a A structurally slope with related structures feature) controlled of red beds locatedby the in structural southwestern plane China of [a26 composite] and faults rock or fractures mass atis likely to fail, theeven depth if it of is red a rock beds slope in the Southwith nearly Georgia horizontal Rift (SGR) basin layers. [27 ]. The classificationIt is well known of that rock rock mass mass st structuresructures play is useful a dominant for rolethe instability the stability analysis of a rock of mass slopes, [11,16 tunnels,,28]. or foundationThe characteristics pits in red-bed of slope areas. failures In in addition, red beds are th summarizede examination from field of the investigations structures and of case a red-bed studies rock mass isas essential follows: for investigating the characteristics of structural planes, such as internal bedding (1) A progressive failure is prone to occur on a slope with structures controlled by steep dip joints planes, joints, cracks, and weak interlayers. In addition, it is important to investigate the concurrent and fissures. Once the initial damage occurs, sustained destruction will occur progressively. (2) The slope of rock mass structures that are controlled by weak interlayers is prone to causing hazards. Alternatively, a rock slope with thick layers of hard rock mass may also tend to suffer from considerable serious damage if underlying weak layers exist. (3) A slope with structures controlled by the structural plane of a composite rock mass is likely to fail, even if it is a rock slope with nearly horizontal layers. Geosciences 2019, 9, 273 3 of 20

The classification of rock mass structures is useful for the stability analysis of slopes, tunnels, or foundation pits in red-bed areas. In addition, the examination of the structures of a red-bed rock mass is essential for investigating the characteristics of structural planes, such as internal bedding planes, joints, cracks, and weak interlayers. In addition, it is important to investigate the concurrent action of structural planes and structural morphologies. Many researchers have been studying the classification of rock mass structures in red-bed slopes by concentrating on factors such as the distribution characteristics (fractal character, crack width, intensive degree, etc.) of the structural plane, the intersection angle of the structural plane relative to the slope, and the integrality and strength of the rock mass [19,29–31]. The strength, stiffness, and dip angle of the rock mass structural plane directly influence failure modes [32,33]. The key structural plane can effectively reveal the nature of rock slope stability and provide a dimensional discriminant approach for studying the stability of the rock mass slope [34]. However, the current classification of red-bed rock structures has mainly been based on the red beds in Sichuan and Hunan provinces in China. To the best of our knowledge, no extensive studies have been carried out with respect to the red beds in South China regions, such as Guangdong and Guangxi provinces. The red beds in these regions exhibit special regional characteristics due to abundant rainfall and intense engineering activities, which might cause more disastrous slope failures. Thus, there is high value in carrying out an extensive investigation to classify red-bed rock mass structures in South China. In this study, a classification of the red-bed rock structures in South China was conducted, and the main failure modes of the red-bed slopes were identified on the basis of an extensive analysis of red beds in the north of Guangdong Province. The results were obtained by analyzing more than 10 highway projects using on-site measurement, remote sensing, drilling, and geological survey. This study can provide reference values for intended large-scale projects in the red-bed distribution area of south China.

2. Geological Background of Red-Bed Formation in South China Because of the unique hot and dry tropical–subtropical formation environment, the red-bed rock layer is rich in Fe2O3, which gives the rock mass its red color [1]. From the perspective of the formation and evolution history of the geological conditions in South China, the red beds in this area possess the following four major characteristics: (1) The red-bed distribution in South China is affected by the main fault zones. The fault zones (mainly oriented northeast (NE) and north-northeast (NNE) in South China) are deformations along the Cathaysian–New Cathaysian tectonic system [15,35]. The red beds in South China are scattered along these fault zones. Figure2 shows the distribution of red-bed basins and major fault zones in Guangdong Province in South China. (2) Four topographic features are present from the edge to the center of the red-bed basin in South China [1]:

I. A basin surrounded by mountains, which specifically refers to a red-bed basin surrounded by mountainous areas and faults that bound the basin. II. Danxia landform, in which several vertical structural planes are well developed, cutting the rock bed into columnar or platelike structures. III. Red-bed hill zone, which refers to hills with moderate slopes that are made of rock beds consisting of sandstone or shale. IV. Center plain zone [1], which refers to the central part of a red-bed basin. This zone is generally formed by the sedimentation of fine-grained clastic lacustrine facies and alluvial flat facies. The lamellar-structured red beds were formed with small dip angles and were slightly influenced by tectonic movement. Geosciences 2018, 8, x FOR PEER REVIEW 3 of 21 action of structural planes and structural morphologies. Many researchers have been studying the classification of rock mass structures in red-bed slopes by concentrating on factors such as the distribution characteristics (fractal character, crack width, intensive degree, etc.) of the structural plane, the intersection angle of the structural plane relative to the slope, and the integrality and strength of the rock mass [19,29–31]. The strength, stiffness, and dip angle of the rock mass structural plane directly influence failure modes [32,33]. The key structural plane can effectively reveal the nature of rock slope stability and provide a dimensional discriminant approach for studying the stability of the rock mass slope [34]. However, the current classification of red-bed rock structures has mainly been based on the red beds in Sichuan and Hunan provinces in China. To the best of our knowledge, no extensive studies have been carried out with respect to the red beds in South China regions, such as Guangdong and Guangxi provinces. The red beds in these regions exhibit special regional characteristics due to abundant rainfall and intense engineering activities, which might cause more disastrous slope failures. Thus, there is high value in carrying out an extensive investigation to classify red-bed rock mass structures in South China. In this study, a classification of the red-bed rock structures in South China was conducted, and the main failure modes of the red-bed slopes were identified on the basis of an extensive analysis of red beds in the north of Guangdong Province. The results were obtained by analyzing more than 10 highway projects using on-site measurement, remote sensing, drilling, and geological survey. This study can provide reference values for intended large-scale projects in the red-bed distribution area of south China.

2. Geological Background of Red-Bed Formation in South China Because of the unique hot and dry tropical–subtropical formation environment, the red-bed rock layer is rich in Fe2O3, which gives the rock mass its red color [1]. From the perspective of the formation and evolution history of the geological conditions in South China, the red beds in this area possess the following four major characteristics: (1) The red-bed distribution in South China is affected by the main fault zones. The fault zones (mainly oriented northeast (NE) and north-northeast (NNE) in South China) are deformations along the Cathaysian–New Cathaysian tectonic system [15,35]. The red beds in South China are scattered alongGeosciences these2019 fault, 9, 273 zones. Figure 2 shows the distribution of red-bed basins and major fault zones4 of in 20 Guangdong Province in South China.

Geosciences 2018, 8, x FOR PEER REVIEW 4 of 21

(2) Four topographic features are present from the edge to the center of the red-bed basin in South China [1]: I. A basin surrounded by mountains, which specifically refers to a red-bed basin surrounded by mountainous areas and faults that bound the basin. II. Danxia landform, in which several vertical structural planes are well developed, cutting the rock bed into columnar or platelike structures. III. Red-bed hill zone, which refers to hills with moderate slopes that are made of rock beds consisting of sandstone or shale. IV. Center plain zone [1], which refers to the central part of a red-bed basin. This zone is generally formed by the sedimentation of fine-grained clastic lacustrine facies and alluvial flat facies. The Figurelamellar-structured 2. DistributionDistribution red of of red-bed red-bedbeds were basins formed and principa principalwith smlall fault dip zones angles in and Guangdong were slightly Province influenced [[1].1]. by tectonic movement. (3) The(3) particleThe particle size size distribution distribution and and the the attitudeattitude of of the the stratum stratum are are closely closely related related to the to the sedimentationsedimentation process. process. The red-bedThe red-bed sedimentation sedimentatio materialsn materials mainly mainly originate originate from from the the ancient ancient basin and thebasin effl andorescence the efflorescence of rocks, soof thickrocks, gluteniteso thick gl emergesutenite emerges at the edgeat the ofedge the of red-bed the red-bed basin basin with the with the sedimentation of large particles, the size distribution of which is consistent with that in sedimentation of large particles, the size distribution of which is consistent with that in the fracture the fracture zone at the edge of the basin. Fine particles are transported to the center of the basin zone at the edge of the basin. Fine particles are transported to the center of the basin during the during the sedimentation process. From the edge to the inner zone of the basin, the size of sedimentationsedimentary process. particles From gradually the edge decreases, to the inner and the zone rock of bed the becomes basin, the thinner, size of as sedimentary shown in Figure particles gradually3. In decreases,addition, as and a result the rockof the bed influence becomes of local thinner, sedimentation, as shown inrock Figure beds3 .in In South addition, China as usually a result of the influencetend to be of localinterbedded sedimentation, without uniform rock beds thickness. in South The China adhesion usually force tend between to be neighboring interbedded rock without uniformbeds thickness. is weak, and The the adhesion axial planes force of between gentle undulating neighboring folds rock usually beds have is weak, dip angles and the of more axial than planes of gentle60° undulating [15] (see Figure folds 4). usually have dip angles of more than 60◦ [15] (see Figure4).

FigureFigure 3. 3.Typical Typical red-bed red-bed sedimentarysedimentary model model in inSouth South China. China. Geosciences 2019, 9, 273 5 of 20 Geosciences 2018, 8, x FOR PEER REVIEW 5 of 21

40

30

20 Proportion/% 10

0 0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 80-90 Inclination angle/º

FigureFigure 4. 4. StatisticsStatistics of of the the red-bed red-bed inclination inclination an anglegle of of bedding bedding attitude attitude in in South South China. China.

(4)(4) TheThe quaternaryquaternary cover, cover, consisting consisting mainly mainly of the of red-bed the red- weatheringbed weathering products andproducts quaternary- and quaternary-impactedimpacted proluvium with proluvium high laterization, with high can laterization, be easily found can inbe red-bed easily areasfound in Southin red-bed China. areas The soilin Southin the China. cover tends The soil to have in the poor cover engineering tends to have properties. poor engineering properties.

3.3. Inner Inner Structures Structures of of Red Red Beds Beds in in South South China China TheThe survey areaarea isis the the key key distribution distribution area area of redof bedsred beds in Guangdong in Guangdong province, province, and the and Danxia the Danxialandform landform has been has declared been declared a world naturala world heritage. natural Atheritage. the same At time,the same the survey time, areathe survey is also crossedarea is alsoby many crossed expressways. by many Geologicalexpressways. exploration Geological is mainly exploration carried outis mainly by means carried of remote out sensing,by means in situof remotemonitoring, sensing, and in referencing situ monitoring, geological and surveys.referencin Theg geological structural surveys. planes andThe theirstructural features planes in South and theirChina features were classified in South byChina investigating were classified the exploration by investigating data (including the exploration over 10 data expressways) (including of over red 10beds expressways) in the north of of red Guangdong beds in the Province. north of Guangdong Province. (1)(1) Primary Primary stratification stratification TheThe red red beds beds in in South South China China demonstrate demonstrate a a distinct distinct bedding bedding structure structure with with a a low low degree degree of of coalescence,coalescence, as shownshown inin Figure Figure5a. 5a. This This bedding bedding can can be strippedbe stripped o ff naturallyoff naturally after after weathering. weathering. Most Mostfresh fresh bedding bedding has a monoclinichas a monoclinic attitude attitude and dip and angles dip angles in the rangein the of range 0–30 ◦of, as 0–30°, shown as inshown Figure in4. FigureThe dip 4. anglesThe dip of someangles local of some rock masseslocal rock may ma varysses considerablymay vary considerably because of tectonicbecause movements,of tectonic movements,but they are but generally they are still generally below 50 still◦. below 50°. (2)(2) Secondary Secondary stratification stratification ComplicatedComplicated faults faults develop develop as as a a result result of of strong strong geological geological structure structure activity. activity. There There are are more more thanthan 1000 1000 large-scale fracturesfractures inin Guangdong Guangdong province, province, among among which which 26 26 are are deep deep (most (most are are in sial in sial and andsima) sima) and largeand large (over (over hundreds hundreds of kilometers of kilometers in length) in length) in the NE in andthe NNENE and directions NNE directions [1]. Structural [1]. Structuralactivities causeactivities the fracturescause the to fractures slide along to slide the soft–hard along the interface soft–hard and interface primary and weak primary interlayer weak in interlayerthe red beds, in the contributing red beds, tocontributing the formation to the of beddingformation fault of bedding zones and fault even zones shattered and even zones, shattered namely, zones,the weak namely, interlayer the weak reworked interlayer by tectonism. reworked by tectonism. AsAs a a result result of of the the long-term long-term influence influence of of underground underground water water and and continuous continuous wet–dry wet–dry cycling, cycling, pelletizationpelletization phenomena can bebe observed,observed, andand muddedmudded intercalation intercalation is is thus thus formed. formed. The The thickness thickness of ofmost most weak weak interlayers interlayers ranges ranges from from 2 2 to to 9 9 cm, cm, and and some some ofof themthem cancan bebe 10–1810–18 cm.cm. The distribution distribution densitydensity isis 1–41–4 layers layers per per meter meter and canand locallycan loca reachlly 7–10reach layers 7–10 per layers meter. per The meter. primary The stratification primary stratificationnear the faulted near and the cataclastic faulted and zones cataclastic usually has zone a largers usually density has (15–20a larger layers density per meter).(15–20 Inlayers addition, per meter).the joints In ofaddition, the red beds the injoints South of Chinathe red tend beds to have in South a high China distribution tend density,to have anda high their distribution intersection density,angles are and mainly their vertical, intersection which angles decreases are themainly continuity vertical, and which strength decreases while increasing the continuity the degree and of strengthanisotropy while of the increasing rock mass. theWeathering degree of anisotropy fissures are of usually the rock developed mass. Weathering on the surface fissures with are dip usually angles developedthat range betweenon the surface 70 and with 90◦. dip Both angles weathering that range fissures between and main 70 and bedding 90°. Both distribution weathering are shown fissures in andFigure main6. bedding distribution are shown in Figure 6. Geosciences 2018, 8, x FOR PEER REVIEW 6 of 20

GeosciencesGeosciences2019 2018, 9,, 2738, x FOR PEER REVIEW 6 of6 21 of 20

(a) Primary stratification (substance differentiation stratification) (a) Primary stratification (substance differentiation stratification)

(b) Secondary stratification made by clay intercalations (b) Secondary stratification made by clay intercalations Figure 5. Two typical structural stratifications of red beds in South China. FigureFigure 5. 5.Two Two typical typical structuralstructural stratificationsstratifications of of red red beds beds in in South South China. China.

Schmidt Concentrations (% of total per 1.0% area)

No Bias Correction Max. Conc.=4.0219% Lower Hemisphere 3829 Poles 3829 Entries

(a) (Wa)eathering Weathering fissures fissures

(b) The main bedding (b) The main bedding

Figure 6. Average distribution of red-bed superficial weathering fissures and main bedding in northern Guangdong province in South China. Geosciences 2018, 8, x FOR PEER REVIEW 7 of 21

Figure 6. Average distribution of red-bed superficial weathering fissures and main bedding in Geosciencesnorthern2019, 9Guangdong, 273 province in South China. 7 of 20

In addition to interbedded stratifications, steep-inclined stratifications also develop from the top Into additionthe bottom to interbedded of many red stratifications, beds’ slopes steep-inclined in South China. stratifications According also to develop statistical from analysis, the top tobetween the bottom two ofand many four red groups beds’ of slopes bedding in South planes China. with According a spacing toof statistical 70–90 cm analysis, develop between in most twored andbeds. four The groups lengths of of bedding these bedding planes with planes a spacing are not of uniform 70–90 cm and develop mainly in range most from red beds. 10 to The 50 cm. lengths The ofproportion these bedding of bedding planes planes are not with uniform spacings and mainly of less range than 90 from cm 10 is toapproximately 50 cm. The proportion 90%, as indicated of bedding in planesFigure with7. This spacings type of of bedding less than plane 90 cm is iscommonly approximately observed 90%, in as the indicated hard rock in Figure mass 7at. Thisthe fringe type of of beddingred-bed planebasins, is which commonly can extend observed from in the several hard rockmete massrs to atdozens the fringe of meters, of red-bed cutting basins, the whichrock mass can extendinto columnar from several or platelike meters topieces. dozens of meters, cutting the rock mass into columnar or platelike pieces.

100 f

89.99% 80

60

40

20

0 Cumulative distribution rate/% 0 20 40 60 80 100 120 140

Spacing/cm FigureFigure 7.7. CumulativeCumulative distribution curve of thethe steepsteep structuralstructural faceface spacingspacing ofof red-beds’red-beds’ slopesslopes inin SouthSouth China.China.

3.1.3.1. Red-BedRed-Bed Litho-StructuralLitho-Structural FaciesFacies inin SouthSouth ChinaChina AccordingAccording to the the field field investigation investigation and and data data analysis, analysis, three three rock rock mass mass facies facies of red of beds red bedswere wereidentified. identified. (1)(1) Mega-thickMega-thick stratumstratum Mega-thickMega-thick stratumstratum includesincludes mega-thickmega-thick ((>150>150 cm) hard rockrock andand mega-thickmega-thick softsoft rock.rock. InIn thethe formerformer gluteniteglutenite predominates, predominates, while while the the latter latter is mainlyis mainly composed composed of mudstone, of mudstone, silty silty mudstone, mudstone, and argillaceousand argillaceous siltstone, siltstone, as well asas intensivelywell as intensively and fully weathered and fully glutenite weathered layers. glutenite This combination layers. This of lithologiescombination can of be lithologies easily found can at be the easily edge offound the red-bedat the edge basins. of Inthe this red-bed type of basins. lithology, In this weathering type of andlithology, erosion weathering occur along and vertical erosion joints occur in along the multiple vertical mega-thick joints in the stratums. multiple The mega-thick Danxia landformstratums. formedThe Danxia gradually landform and formed it tends gradually to collapse and in theit tend red-beds to collapse distribution in the areas. red-bed Danxia distribution landforms areas. are sedimentaryDanxia landforms red clastic are rockssedimentary deposited red in clastic intermountain rocks deposited basins in in Neogene, intermountain which formedbasins in through Neogene, the dynamicwhich formed action through of tectonic the movement dynamic [action36,37]. of The tecton rockic mass movement with this [36,37]. structural The condition rock mass is assumedwith this tostructural be affected condition by water is andassumed have weakto be engineeringaffected by propertieswater and andhave usually weak suengineeringffers from slidingproperties failures. and usually(2) Soft–hardsuffers from interbedding sliding failures. of sandstones and mudstones These(2) Soft–hard facies are interbedding commonly observedof sandstones in the and center mudstones of red-bed basins. Particles constituting sandy stonesThese are usually facies coarse.are commonly Sandstones observed have high in strengththe center (compressive of red-bed strengthbasins. ofParticles about 30constituting MPa) and good permeability (permeability coefficient of about 1 10 2 cm/s), whereas mud shale is generally sandy stones are usually coarse. Sandstones have high× strength− (compressive strength of about 30 fineMPa) and and very good weak permeabi (compressivelity (permeability strength < 20coefficient MPa) with of about low permeability 1 × 10−2 cm/s), (permeability whereas mud coe shalefficient is of about 1 10 4 cm/s). Hence, this type of litho-structural facies tends to soften in water. Slopes generally fine× and− very weak (compressive strength <20 MPa) with low permeability (permeability containingcoefficient thisof about type of1 × lithology 10−4 cm/s). are Hence, likely to this undergo type of sliding litho-struct failuresural along facies the tends weak to mud soften shale in layers.water. InSlopes particular, containing slopes this made type of of soft–hard lithology interbedding are likely to ofundergo sandstone sliding and failures mudstone along can the cause weak sliding mud failuresshale layers. [38,39 In]. particular, slopes made of soft–hard interbedding of sandstone and mudstone can cause(3) sliding Mega-thick failures hard [38,39]. rock with soft–hard interlayers below or thick hard rocks at the top and bottom with soft–weak intercalated layers in between. These facies are observed at the transitional zone between the edge and the center of red-bed basins. Soft–weak interbedded layers are influenced by underground water and rainfall and experience Geosciences 2018, 8, x FOR PEER REVIEW 8 of 21

(3) Mega-thick hard rock with soft–hard interlayers below or thick hard rocks at the top and bottom with soft–weak intercalated layers in between. These facies are observed at the transitional zone between the edge and the center of red-bed basins. Soft–weak interbedded layers are influenced by underground water and rainfall and Geosciences 2019, 9, 273 8 of 20 experience wet–dry cycling. Thus, there also exists a notable pelletization phenomenon, and mudded intercalation has weak mechanical properties. wet–dry cycling. Thus, there also exists a notable pelletization phenomenon, and mudded intercalation 3.2.has Structural weak mechanical Types of Rock properties. Mass in Red Beds in South China As analyzed in the above sections, dip angles of rock mass in red beds in South China range 3.2. Structural Types of Rock Mass in Red Beds in South China mainly from 0 to 30°. According to the intersecting angle formed between the rock strata and the slope Assurface, analyzed the inrock the mass above structures sections, dipcanangles be divided of rock into mass nearly in red horizontal/slightly beds in South China inclined range structuresmainly from (angles 0 to 30 from◦. According −15 to 15°) to the [30,40] intersecting and inclined angle formed structures between (angles the rock from strata 15 andto 50°) the slope[19]. Inclinedsurface, rock the rock mass mass structures structures can canbe further be divided divided into nearlyinto consequent horizontal layered/slightly inclined inclined structures structures and(angles anti-dip from layered15 to 15 inclined◦)[30,40 ]structures. and inclined Steep structures or vertical (angles slope from structures 15 to 50◦ )[are19 ].seldom Inclined seen rock in massred − bedsstructures in South can China. be further Considering divided intothe different consequent litho- layeredfacies inclinedand anisotropies structures (e.g., and joints, anti-dip fractures, layered andinclined soft–weak structures. interlayers), Steep or the vertical rock mass slope structures arein South seldom China seen inare red further beds incategorized South China. as shownConsidering in Table the 1. di fferent litho-facies and anisotropies (e.g., joints, fractures, and soft–weak interlayers), the rock mass structures in South China are further categorized as shown in Table1. 4. Results and Discussion 4. Results and Discussion 4.1. Disastrous Characteristics and Failure Mode of Red-Bed Slopes in South China 4.1. Disastrous Characteristics and Failure Mode of Red-Bed Slopes in South China The red beds in South China are the result of a long geological process. Most of them possess The red beds in South China are the result of a long geological process. Most of them possess soft–hard interbedded internal structures due to the alternate sedimentation of mudstone. The folds soft–hard interbedded internal structures due to the alternate sedimentation of mudstone. The folds and faults zones are well developed. Meanwhile, the original weak bedded planes and interlayers, and faults zones are well developed. Meanwhile, the original weak bedded planes and interlayers, which significantly influence the stability of slopes, experienced wet–dry cycling and substantial which significantly influence the stability of slopes, experienced wet–dry cycling and substantial pelletization induced by rainfall and underground water. Because of rainfall and human activities, pelletization induced by rainfall and underground water. Because of rainfall and human activities, geological hazards (e.g., weathering, slope scouring, slumping, and landslides) have become geological hazards (e.g., weathering, slope scouring, slumping, and landslides) have become notable. notable. According to the statistics [41], the occurrence probability of weathering in the red-bed According to the statistics [41], the occurrence probability of weathering in the red-bed slopes in South slopes in South China is approximately 100%. In addition, slumping, landslides, and tensile cleft China is approximately 100%. In addition, slumping, landslides, and tensile cleft dumping, as shown dumping, as shown in Figure 8, account for 68.37%, 23.26%, and 8.37%, respectively, of all in Figure8, account for 68.37%, 23.26%, and 8.37%, respectively, of all geological hazards in the red geological hazards in the red beds [41]. beds [41].

Figure 8. Proportion of geological hazards in red-bed slopes in South China. Figure 8. Proportion of geological hazards in red-bed slopes in South China. Geosciences 2018, 8, x FOR PEER REVIEW 9 of 21 Geosciences 2018, 8, x FOR PEER REVIEW 9 of 21 Geosciences 2019Geosciences, 9, 273 2018, 8, x FOR PEER REVIEW 9 of 2021 9 of 20 Geosciences 2018, 8, x FOR PEER REVIEW Table 1. Rock mass structure of red-bed slopes in South China. 9 of 21 Table 1. Rock mass structure of red-bed slopes in South China. The Type and Class of Rock Mass Table 1. Rock mass structure of red-bed slopes in South China. Rock Mass Structure StructuralTable 1. FeatureRock mass structure of red-bed slopes in Engineering South China. Geological Evaluation The Type andStructure Class of Rock Mass Table 1. Rock mass structure of red-bed slopes in South China. Rock MassDiagram Structure The Type and Class of Rock Mass Structural Feature Engineering Geological Evaluation Rock Mass Structure The Type andStructure Class of Rock Mass Structural Feature Engineering Geological Evaluation Rock MassDiagram Structure Structure Structural Feature Engineering Geological Evaluation Diagram The Type and ClassStructure of Rock Mass Structure Structural Feature Engineering Geological Evaluation Rock MassDiagram Structure Diagram Complete rock mass consisting of mega-thick glutenite with a Hard and locally uniform structure with primary closure Global-layered Completelayer spacing rock ofmass more consisting than 150 of cm; mega-thick no more thanglutenite one setwith of a jointsHard in and majority; locally mega-blockuniform structure is the structural with primary morphology closure Complete rock mass consisting of mega-thick glutenite with a Hard and locally uniform structure with primary closure Global-layered Completelayerorthogonal Completespacing rock steepmassof more rock dipconsisting thanmass joints 150 consistingdeveloped of cm; mega-thick no more ofin the mega-thick thanglutenite inner one zone setwith glutenite of a jointsHardwith in andgoodHard majority; locally stability and mega-blockuniform locally but prone uniformstructure is to the local structural structurewith collapse primary morphology withand closure slide Global-layered Completelayer spacing rock massof more consisting than 150 of cm; mega-thick no more thanglutenite one setwith of a jointsHard in and majority; locally mega-blockuniform structure is the structural with primary morphology closure Thick-layere Global-layered layerorthogonal spacingwith a steepof layer more dip spacing than joints 150 developed ofcm; more no more thanin the than 150 inner one cm; zone set no of more jointswithprimary in good majority; stability closure mega-block but joints prone in is to majority;the local structural collapse mega-block morphology and slide Global-layered layerorthogonal spacing steepof more dip than joints 150 developed cm; no more in the than inner one zone set of jointswith in good majority; stability mega-block but prone is to the local structural collapse morphology and slide Thick-layered hard rock Global-layered orthogonal steep dip joints developed in the inner zone with good stability but prone to local collapse and slide Thick-layere than one set of orthogonal steep dip joints developed is the structural morphology with good Thick-layered hardmass rock orthogonal steep dip joints developed in the inner zone with good stability but prone to local collapse and slide d hard rock in the inner zone stability but prone to local collapse and slide Thick-layered structurehardmass rock mass Relatively complete rock mass consisting of thick-layered Relatively hard structure with a block or cylinder as the main structuremass dstructure hard rock gluteniteRelatively with complete a layer rock spacing mass of consisting between of100 thick-layered and 150 cm; Relativelystructural hard morphology; structure with relatively a block orstable cylinder with ascracks the main Thick-layeredstructure hard rock Block-layered Relatively complete rock mass consisting of thick-layered Relatively hard structure with a block or cylinder as the main mass structuralRelativelyglutenite plane with Relativelycomplete adeveloped layer rock spacing complete mass with of consistinga betweenspacing rock mass ofof100 betweenthick-layered consistingand 150 80 cm; andof RelativelydevelopedstructuralRelatively hard along morphology; structure the hard structural with structure relatively a blockplane; with orstableprone cylinder a block withto collapse ascracks or the main and mass structure Block-layered glutenite with a layer spacing of between 100 and 150 cm; structural morphology; relatively stable with cracks structure Block-layered structuralglutenite plane thick-layeredwith adeveloped layer spacing glutenite100 with cm of a betweenspacing with aof100 layerbetween and spacing150 80 cm; and of developedstructuralcylinder along morphology; as the the structural main sliderelatively structural plane; stableprone morphology; withto collapse cracks and Block-layeredBlock-layered structuralRelatively plane complete developed rock masswith consistinga spacing of of between thick-layered 80 and Relativelydeveloped hard along structure the structural with a plane;block or prone cylinder to collapse as the mainand structuralbetween plane developed 100 and150 100with cm cm; a spacing structural of between plane 80 developed and developedrelatively along stable the structural withslide cracks plane; developed prone to collapse along and glutenite with a layer spacing100 cm of between 100 and 150 cm; structural morphology; sliderelatively stable with cracks Block-layered with a spacing100 ofcm between 80 and 100 cm the structural plane; proneslide to collapse and slide structural plane developed with a spacing of between 80 and developed along the structural plane; prone to collapse and

100 cm slide Composed of thick-layered soft–weak rock mass deposits or Thick-layered soft–weak rock Relatively weak slope; developed structural plane; rock ComposedconsistsComposed of thick-layered of thickof layers thick-layered soft of –weakintensively rock soft–weak weatheredmass deposits rock massor Thick-layeredmass structure soft–weak rock Composed of thick-layered soft–weak rock mass deposits or RelativelythicknessRelatively weak of 0.5 weak slope; to 15 slope; develom; prone developedped to structural collapse structural andplane; slide rock Thick-layered soft–weak rock Composedconsistsconglomeratesdeposits of thick-layered of thick or consistslayers(more soft ofthan –weak ofintensively 20 thick m rock in layers thickness) massweatheredof deposits intensively or Relatively weak slope; developed structural plane; rock Thick-layeredThick-layered soft–weakmass structure soft–weak rock mass rock structure Composedconsists of thick-layered of thick layers soft of –weakintensively rock weatheredmass deposits or Relativelythicknessplane; weak rock of 0.5 thicknessslope; to 15 develom; prone ofped 0.5 to tostructural collapse 15 m;prone andplane; slide torock Thick-layeredmass structure soft–weak rock consistsconglomeratesweathered of thick layers(more conglomerates ofthan intensively 20 m in (more thickness) weathered than 20 m in Relativelythickness weak of 0.5 slope; to 15 develom; proneped to structural collapse andplane; slide rock mass structure consistsconglomerates of thick layers(more ofthan intensively 20 m in thickness) weathered thickness of 0.5 tocollapse 15 m; prone and slideto collapse and slide mass structure conglomerates (more thanthickness) 20 m in thickness) thickness of 0.5 to 15 m; prone to collapse and slide conglomerates (more than 20 m in thickness)

With soft–weak Hard rock mass, such as thick-layered glutenite, with a Withinterlayer soft–weak thin-layeredHardHard rock rockweakmass, mass,clayey such suchas rock thick-layered asmass, thick-layered which glutenite, includes glutenite, with a clayey a with The a upper hard rock mass can easily slide horizontally along WithWith soft–weak soft–weak Hard rock mass, such as thick-layered glutenite, with a The upper hard rock mass can easily slide Thick-layere (includingWithinterlayer soft–weak clayey thin-layeredlayerHard ofthin-layered soft–weakrock weak mass, clayeyrock such weak mass as rock clayeythick-layered formed mass, rock which by the mass,glutenite, includes wet–dry which with a cycling clayey includes a theThe a plane upper of hard the soft–weakrock mass rockcan ea masssily orslide collapse horizontally to a free along face Thick-layere interlayerinterlayer (including thin-layered weak clayey rock mass, which includes a clayey The upperhorizontally hard rock along mass thecan ea planesily slide of the horizontally soft–weak along Thick-layered hard rock (includinginterlayerlayer) clayey thin-layeredlayer of clayeysoft–weak weak layer clayeyrock ofand mass soft–weakrock weathering formed mass, which rockby the mass includes wet–dry formed a cycling clayey by thetheThe plane upper of hard the soft–weakrock mass rockcan ea masssily slideor collapse horizontally to a free along face Thick-layere (includingclayey clayey layer) layer of soft–weak rock mass formed by the wet–dry cycling the plane ofrock the soft–weak mass or collapserock mass to or a collapse free face to a free face Thick-layered hardmass rock (includinglayer) clayey layer of soft–weakwet–dry rockand mass weathering cycling formed andby the weathering wet–dry cycling the plane of the soft–weak rock mass or collapse to a free face d hard rock layer) and weathering d structurehardmass rock layer) and weathering Thick-layeredmass hard rock layer) and weathering withstructuremass soft– mass structurestructure with soft–weakweakwith rockstructure soft– rock mass weakwith soft– rock WithWith soft–hard soft–hard Mega-thick-layeredMega-thick-layered or thick-la oryered thick-layered hard rock mass hard with rock a mass The upper hard rock mass can easily slide weakwithmass soft– rock With soft–hard Mega-thick-layered or thick-layered hard rock mass with a The upper hard rock mass can easily slide horizontally along weak rock interbeddedinterbedded rock rocksoft–hardwith interbedded a soft–hard rock interbedded mass below, rock such mass as sand below, mud such asThe upperhorizontally hard rock along mass thecan ea planesily slide of the horizontally soft–weak along weakmass rock interbeddedWith soft–hard rock Mega-thick-layeredsoft–hard interbedded or thick-larock massyered below, hard such rock as mass sand with mud a the plane of the soft–weak rock mass or collapse to a free face mass interbeddedWith masssoft–hardmass rock Mega-thick-layeredsoft–hard interbedded orsandand thick-larock sand mudmassyered shale andbelow, hard sand such rock shale as mass sand with mud a theThe plane upper of hardrock the soft–weakrock mass mass or collapserockcan ea masssily to orslide a collapse free horizontally face to a free along face Geosciencesmass 2018, 8interbedded, x FORmass PEER rock REVIEW soft–hard interbeddedand rock sand mass shale below, such as sand mud theThe plane upper of hard the soft–weakrock mass rockcan ea masssily slideor collapse horizontally to a free along face 9 of 21 interbeddedmass rock soft–hard interbeddedand rock sand mass shale below, such as sand mud the plane of the soft–weak rock mass or collapse to a free face mass and sand shale the plane of the soft–weak rock mass or collapse to a free face mass and sand shale

Soft–hardSoft–hardSoft–hard interbeddedinterbedded interbedded rockrock massmass withwith rock differentdifferent mass with thicknessesthicknesses different

withwith thicknesses aa layerlayer spacingspacing with rangingranging a layer fromfrom spacing 1010 toto 5050 ranging cm;cm; developeddeveloped from 10 to 50 The density of the structural planes near the Soft–hardSoft–hard interbeddedinterbedded rockrock massmass TheThe densitydensity ofof thethe structuralstructural plplanesanes nearnear thethe faultfault fracturefracture Soft–hard interbedded rock mass structurestructuralstructuralcm; planesplanes developed withwith thicknessesthicknesses structural rangingranging planes fromfrom with 22 toto thicknesses 99 cmcm andand fault fracture zone is the highest; easily prone structurestructure zonezone isis thethe highest;highest; easilyeasily proneprone toto aa cuttingcutting landslidelandslide aa densitydensityranging ofof 2–72–7 from perper 2metermeter to 9 (15–20 cm(15–20 and perper a meter densitymeter inin some ofsome 2–7 locallocal per meter to a cutting landslide (15–20 perparts)parts) meter in some local parts)

Geosciences 2018, 8, x FOR PEER REVIEW 21 of 20

4.1.1. Weathering and Slope Scouring Weathering and scouring are the most common predisposing factors of slope failure in the red Geosciencesbeds in2019 South, 9, 273 China. As a result of abundant and frequent rainfalls, the rocks in red-bed slopes,10 of 20 especially clayey rocks, undergo wet–dry cycles; the rocks gradually soften and eventually become loose rock blocks or laterite. Additionally, this process may easily lead to the occurrence of 4.1.1.disintegration Weathering and and spalling, Slope Scouring as depicted in Figure 9. The weathering and scouring of red-bed slopes at a small scale are usually not easily observed. Weathering and scouring are the most common predisposing factors of slope failure in the red beds As time goes on, red-bed slopes may suffer from serious erosion, and a large number of weathering in South China. As a result of abundant and frequent rainfalls, the rocks in red-bed slopes, especially fractures may appear, which could potentially lead to large-scale (over millions of cubic meters clayey rocks, undergo wet–dry cycles; the rocks gradually soften and eventually become loose rock with an exposed slope of 1 × 106 m2) slope collapses [42–45]. As suggested by statistics, the volume blocks or laterite. Additionally, this process may easily lead to the occurrence of disintegration and of rock blocks as a result of weathering and scouring can reach 7 million m3 with an exposed slope spalling,of 1 × 10 as6 m depicted2 [46]. The in rocks Figure in9 .an exposed red-bed slope are expected to transform into laterite [47].

Geosciences 2018, 8, x FOR PEER REVIEW 21 of 20

4.1.1. Weathering and Slope Scouring WeatheringFigureFigure and 9. 9.scouringWeathering Weathering are and andthe surfacesurfacemost common erosion in predisposing a a red red-bed-bed rock rock factors slope slope in of in South Southslope China China.failure. in the red beds in South China. As a result of abundant and frequent rainfalls, the rocks in red-bed slopes, especiallyThe weathering clayey rocks, and undergo scouring wet of– red-beddry cycles; slopes the rocks at a smallgradually scale soften are usually and eventually not easily be observed.come Asloose time rock goes blocks on, red-bed or laterite. slopes Additionally, may suffer from this serious process erosion, may easily and a lead large to number the occurrence of weathering of fracturesdisintegration may appear, and spalling, which as could depicted potentially in Figure lead 9. to large-scale (over millions of cubic meters with an exposedThe slope weathering of 1 10 and6 m 2scouring) slope collapses of red-bed [42 slopes–45]. As at suggesteda small scale by are statistics, usually the not volume easily ofobserved. rock blocks × asAs a resulttime goes of weathering on, red-bed and slopes scouring may suffer can reach from 7serious million erosion, m3 with and an a exposed large number slope ofof 1weathering106 m2 [ 46]. × Thefractures rocks in may an exposed appear, which red-bed could slope potentially are expected lead to totransform large-scale into (over laterite millions [47]. of cubic meters with(2) an Failure exposed mode slope of 1 × 106 m2) slope collapses [42–45]. As suggested by statistics, the volume of rockPrevious blocks studies as a result have of shown weathering that under and scouring certain climatic can reach and 7 geologicalmillion m3conditions, with an exposed the weathering slope 6 2 ofof slopes 1 × 10 in m red-bed[46]. The areas rocks in Southin an exposed China is red mainly-bed inducedslope are by expected temperature to transform and moisture into laterite [47],especially [47]. during hot summers, during(a) Site which location heavy rainfalls frequently occur(b in) Slope South scouring China. erosion Fractures induced by thermal stress continue to grow and aggregate on the outer surface of red-bed slopes, causing the Figure 10. Slope scouring erosion of a slope in the expressway A, Guangdong, China. clastic spalling of the slope surface being affected by weathering and rain wash and contributing to the loss of soil(2) Failure and water. mode Typical slope scouring erosion along expressway A in Guangdong province is shownPrevious in Figure studies10. With have the loss shown of rock that and under soil mass certain from climatic the outer and surface geological of the conditions, slope, the the inner partweathering of the slope of slopes is gradually in red- exposedbed areas to in repeated South China weathering, is mainly which induced weakens by temperature the slope stability. and Notably,moisture weathered [47], especially and washed during loose hot deposits summers, with during a high which degree heavy of laterization rainfalls freq mayuently finally occur become in theSouth source China. materials Fractures for debris induced flows by [ thermal48,49]. As stress shown continue in Table to 1grow, mudstone, and aggregate shale, silty on the mudstone, outer andsurface other clay of red rocks-bed on slopes, the slope causing of a soft–hard the clastic interbedded spalling of structure the slope are surface easily scouredbeing affected and soften by in theweathering presence of and water. rain wash and contributing to the loss of soil and water. Typical slope scouring Figure 9. Weathering and surface erosion in a red-bed rock slope in South China. erosion along expressway A in Guangdong province is shown in Figure 10. With the loss of rock and soil mass from the outer surface of the slope, the inner part of the slope is gradually exposed to repeated weathering, which weakens the slope stability. Notably, weathered and washed loose deposits with a high degree of laterization may finally become the source materials for debris flows [48,49]. As shown in Table 1, mudstone, shale, silty mudstone, and other clay rocks on the slope of a soft–hard interbedded structure are easily scoured and soften in the presence of water.

(a) Site location (b) Slope scouring erosion

FigureFigure 10.10. SlopeSlope scouring erosion erosion of of a aslope slope in in the the expressway expressway A, A,Guangdong, Guangdong, China China..

(2) Failure mode Previous studies have shown that under certain climatic and geological conditions, the weathering of slopes in red-bed areas in South China is mainly induced by temperature and moisture [47], especially during hot summers, during which heavy rainfalls frequently occur in South China. Fractures induced by thermal stress continue to grow and aggregate on the outer surface of red-bed slopes, causing the clastic spalling of the slope surface being affected by weathering and rain wash and contributing to the loss of soil and water. Typical slope scouring erosion along expressway A in Guangdong province is shown in Figure 10. With the loss of rock and soil mass from the outer surface of the slope, the inner part of the slope is gradually exposed to repeated weathering, which weakens the slope stability. Notably, weathered and washed loose deposits with a high degree of laterization may finally become the source materials for debris flows [48,49]. As shown in Table 1, mudstone, shale, silty mudstone, and other clay rocks on the slope of a soft–hard interbedded structure are easily scoured and soften in the presence of water. Geosciences 20192018, 98, 273x FOR PEER REVIEW 2111 of of 20

4.1.2. Rock Falls 4.1.2. Rock Falls The occurrence of rock falls in a typical red-bed rock slope in South China is shown in Figure The occurrence of rock falls in a typical red-bed rock slope in South China is shown in Figure 11. 11.

Figure 11.11. Collapse and falling in a red-bedred-bed rock slope in South China.China

Rock fallsfalls are are the the most most common common geological geological hazards hazards in the in red the beds red in beds South in China. South These China. hazards These hazardsusually take usually place take in an place area in of an a slopearea of with a slope a thick with layer a thick of glutenite, layer of in glutenite, which joint in which fissures joint are fissuresdensely distributedare densely (shown distributed in Table (shown1). Rock in Table falls also 1). occurRock onfalls slopes also containingoccur on slopes soft–hard containing intercalated soft– hardstructures intercalated and thick structures weak rock and blocks. thick Slopes weak with rock soft–hard blocks. S intercalatedlopes with structures soft–hard can intercalated collapse structuresbecause of thecan loss collapse of support because from of hard the rocks, loss of which support arises from from hard the pelletization rocks, which eff ect arises aswell from as the pelletizationserious loss of effect soil and as well water as under the serious wet–dry loss cycling of soil conditions. and water The under collapse wet of–dry slopes cycling with conditions. thick weak Therock c blocksollapse is of induced slopes with by the thick high weak connectivity rock blocks of weathered is induced fractures, by the high which connectivity leads to tension of weathered cracks. fractures,According which toleads statistics, to tension failures cracks. of red-bed slopes in the form of rock falls occur mostly at a small-to-medium-sizedAccording to statistics, level. failures Small-scale of red collapses-bed slopes account in the for form over 70%, of rock with falls a volume occur ofmostly collapsed at a smallmaterials-to-medium of 10–500-sized m3, while level. medium-sized Small-scale collapses collapses accountaccount for for approximately over 70%, with 30%, with a volume a volume of collapsedof collapsed materials materials of of 10 800–5000–500 m³, mwhile3 [50 ]. medium-sized collapses account for approximately 30%, with (2)a volume Failure of mode collapsed materials of 800–5000 m³ [50]. (2)Two Failure sets of mode conjugate fractures usually develop in slopes with hard rock blocks (e.g., thick sandy conglomerates)Two sets of in conjugate South China. fracture Withs a usually fracture develop that is parallel in slopes to the with slope hard surface, rock blocks the rock (e.g., mass thick on sandytop of theconglomerates) slip surface canin South eventually China. slip With under a fracture gravity, that causing is parallel tension to failure the slope of other surface, fractures the rock and masscontributing on top toof thethe collapseslip surface of rock can blocks. eventually slip under gravity, causing tension failure of other fractureThes and collapse contributing scale is mainlyto the collapse controlled of rock by theblocks. cutting density and connectivity degree of the structuralThe collapse planes. scale Medium-sized is mainly controlled collapsing by occurs the incutting large density rock blocks and ofconnectivity low cutting degree density of andthe structuralgood connectivity, planes. Medium while in-sized rocks collapsing of high cutting occurs density,in large small-sizedrock blocks collapsingof low cutting occurs density with a thend goodgeneration connectivity, of small while broken in rocks. rocks of According high cutting to thick-layered density, small soft–weak-sized collapsing rock mass occurs structure with and the generationsoft–hard interbedded of small broken rock mass rocks. structure According described to thick in- Tablelayered1, the soft collapse–weak of rock thick mass weak structure rock slopes and softoccurs–hard because interbedded the rock rock mass mass in the structure superficial described zone becomes in Table loose 1, from the collapse the effect of of thick weathering weak rock and slopesrain wash occurs and thus because collapses the rock under mass gravity. in theThe superficialclayey rock zonemass in becomes slopes with loose soft–hard from the intercalated effect of weatheringstructures (e.g., and mudstone,rain wash and shale, thus and collapses powder under sandy gravity. mudstone) The tendsclayey to rock undergo mass geomechanicalin slopes with softdegradation–hard intercalated and is rapidly structure laterizeds (e.g., [20 mudstone,–22]. Long-term shale, rain and wash powder and sandy serious mudstone) soil erosion tends due to undergorainfall can geomechanical give rise to the degradation loss of support and foris rapidly the hard laterized rock mass [20 (sandstone–22]. Long- andterm conglomerate) rain wash and at seriousthe top ofsoil rock erosion slopes, due which to rainfall will then can lead give to arise small-to-medium-scale to the loss of support collapse for the (from hard hundreds rock mass to (sandstonethousands of and cubic conglomerate) meters). at the top of rock slopes, which will then lead to a small-to-medium-scale collapse (from hundreds to thousands of cubic meters). 4.1.3. Landslide 4.1.3.Landslides Landslide account for only 23.76% of the hazards that take place in the red beds in South China and theyLandslides are mostly account deep for (about only 5–20 23.76% m) sliding of the failures hazards [51 that]. The take induced place indamage the red is generally beds in South more Chinaserious and than they that arecaused mostly by shallow deep (about weathering 5–20 m) and sliding spalling, failures scouring, [51] and. The collapsing. induced damage Figures 12 is generallyand 13 describe more a serious landslide than occurring that caused on expressways by shallow B and weathering C in Guangdong and spalling Province., scouring, and Geosciences 2018, 8, x FOR PEER REVIEW 21 of 20 Geosciences 2018, 8, x FOR PEER REVIEW 21 of 20 colGeoscienceslapsing.2019 Figure, 9, 273 12 and 13 describe a landslide occurring on expressways B and C in Guangdong12 of 20 Province.

Figure 12.12. Landslide failure of a slope onon expressway B,B, Guangdong, China.China

Figure 13. LandslideLandslide failure failure of a slope on expressway C C,, Guangdong, China China..

Landslides in in the the red red beds beds in South in South China China are closely are closely related related to rainfall to rainfall[52]. In general,[52]. In glutenite general, glutenitehas a high has permeability, a high permeability, while the while permeability the permeability of shale is of relatively shale is relatively low in glutenite–mudstone low in glutenite– mudstoneinterbedded interbedded layered structures layered (thick-layeredstructures (thick hard-layered rock mass hard structure rock mass with structure soft–weak with rock soft mass–weak in rockTable 1 mass). In in rainy Table weather, 1). In perched rainy weather, water is perched mainly generated water is mainly at the sand–mud generated interface, at the sand causing–mud interface,an increase causing in pore-water an increase pressure. in pore In-water addition, pressure. periodic In wet–dryaddition, cycling periodic leads wet to–dry pelletization cycling leads in the to weakpelletization intercalated in the layers weak and intercal interlayerated fracture layers and zones. interlayer Sliding failure fracture occurs zones. along Sliding the weak failure structural occurs alongplane inthe the weak bedding structural slopes plane as well in the as thebedding reverse slopes pour as slopes. well as the reverse pour slopes. We investigated investigated the the red-bed red-bed and and slope slope engineering engineering in northern in northern Guangdong, Guangdong, which which has the has typical the typicalred-bed red distribution-bed distribution area of area South of China. South China. The specific The specific research research area is shownarea is shown in Figure in 14Figurea, and 14a, its andcontour its contour map is shownmap is inshown Figure in 14Figureb. 14b. Geosciences 2019, 9, 273 13 of 20 Geosciences 2018, 8, x FOR PEER REVIEW 21 of 20

(a) Research area

(b) Contour map (precision is 5 meters)

FigureFigure 14. 14.Major Major researchresearch areasareas in n northernorthern G Guangdonguangdong p province.rovince.

TheThe daily daily rainfall rainfall data data of Guangdong of Guangdong Meteorological Meteorological Bureau Bureau were were used used to determine to determine daily daily rainfall dischargerainfall in discharge Heyuan, inMeizhou, Heyuan, Qingyuan, Meizhou, and Qingyuan, Shaoguan and where Shaoguan the red-bed where soft the rock red mainly-bed soft distributes. rock Inmainly Figure 15 distributes., it can be Inseen Figure that the 15, rainfall it can be in seenthese t areashat the is abundantrainfall in and these mainly areas concentrated is abundant and in the periodmainly between concentrated February in andthe September,period between of which February February–March and September, is the of plum which rain February season,–March April–June is is thethe monsoonplum rain rainyseason, season, April– andJune July–September is the monsoon israiny the season, typhoon and rainy July season.–September Long-term is the typhoon abundant rainfallrainy isseason. the main Long cause-term of abundant slope weathering rainfall is andthe main landslides cause inof theslope research weathering area. and landslides in the research area. GeosciencesGeosciences 20182019, 8,,9 x, 273FOR PEER REVIEW 21 14of of20 20

(a) Daily rainfall distribution map of Qingyuan-Heyuan

(b) Daily rainfall distribution map of Shaoguan-Heyaun

FigureFigure 15. 15. RainfallRainfall in intypical typical red red-bed-bed regions regions of of Guangdong Guangdong province province..

TheThe statistics statistics show show that thethe landslidelandslide pattern pattern in in the the red red beds beds of Southof South China China is governed is governed by factors by factorssuch as such the rock as the mass rock structure, mass structure, the slope height,the slope and height, the height and di thefference height between difference the landslide between nichethe landslideand foot niche [53]. According and foot [ to53 Cruden]. According and Varnes’ to Cruden landslide and Varnes’ classification, landslide landslides classification, fall into l twoandslides groups: falltranslational into two landslide groups: translational and rotational landslide landslide and [54]. rotational Global translational landslide landslides[54]. Global are translational characterized landslidesby the failure are mode characterized of translational by the slides, failure and mode tractive of translational landslides follow slides, the and mode tractive of rotational landslides slides. follow the mode of rotational slides. Geosciences 20192018, 98, 273x FOR PEER REVIEW 2115 of of 20 Geosciences 2018, 8, x FOR PEER REVIEW 21 of 20 (2) Failure mode (2)A(2) globalFailure Failure translational mode landslide occurs in nearly horizontal/slightly inclined layered slopes, especiallyA global in slopes translational containing landslide landslide thick occurshard occurs rock in in nearly nearly mass with horizontal horizontal a thin// slightlyslightly weak rock inclined inclined mass layered layered interlayer slopes, slopes, (see especiallyFigureespecially 16 and in in slopes slopes soft– containinghard containing interbedded thick thick hard rock hard rock mass rock mass structure mass with witha thin in a weakTable thin rock1 weak). When mass rock interlayera massslope interlayeris (see subjected Figure (see 16to Figuretoeand erosion soft–hard 16 and and soft interbedded artificial–hard cutting,interbedded rock massthe support rock structure mass in the instructure Table front1 edge). in When Table is weakened, a 1 slope). When is subjectedand a slope failure is to occursubjected toe erosions along to toeweakand erosion artificial layers and and cutting, artificial structural the cutting, support planes. the in With thesupport front the development edgein the is front weakened, edge of weathered is andweakened, failure fractures, occursand failure the along shear occur weak strengths layersalong weakisand expected structural layers to and decrease planes. structural With because planes. the developmentof rainfallWith the infiltration. development of weathered The of fractures,thick weathered rock the mass fractures, shear layer strength will the slideshear is expected along strength the to isweakdecrease expected structural because to decrease plane of rainfall to because the infiltration. free of face rainfall of The the infiltration. thickslope. rock During massThe thickthe layer sliding rock will mass slideprocess, alonglayer the will the relative weakslide along structuralpositions the weakbetweenplane structural to the different free plane face rockto of the blocks free slope. face remain During of the almost slope. the slidingunchanged, During process, the and sliding thethe relative slidingprocess, surface positions the relative is straight. between positions The the betweenvolumedifferent ofthe rock the different blocks sliding remain rock part blocks isalmost approximately remain unchanged, almost 3800 and unchanged,–7000 the sliding m³, and surface the this sliding typeis straight. of surface slope The is failure volumestraight. usually of The the volumeleadssliding to part catastrop of the is approximately slidinghic landslides part is 3800–7000 approximately [55]. m3, and 3800 this–7000 type ofm³, slope and failure this type usually of slope leads failure to catastrophic usually leadslandslides to catastrop [55]. hic landslides [55]. Rainfall infiltration Unloading fissure Rainfall infiltration Unloading fissure

Argillaceous siltstone Argillaceous siltstone Silty mudstone Silty mudstone Cutting slope Cutting slope Figure 16. Schematic description of a global translational landslide in a red-bed slope in South FigureChina. 16. 16. SchematicSchematic description description of of a global a global translational translational landslide landslide in a red-bedin a red slope-bed inslope South in China. South China. The tractive landslide accounts for over 50% 50% of of landslide landslide event eventss in in the the red red beds beds in in South South China, China, with The the tractive average landslide sliding volumeaccounts ranging for over from 50% of 600 landslide to 3500 event mm³.3. Theses in the landslides red beds in are South considered China, withsmallsmall-to-medium-scale -theto- mediumaverage- scale sliding landslides. landslides. volume Tractive ranging Tractive landslides from landslides 600 are to are initiated 3500 initiate m³. by Thesed a reductionby landslides a reduction in the are shear in considered the strength shear smallstrengthof rock-to mass- mediumof rock because mass-scale ofbecause landslides.the water of the wash Tractive water from wash landslides rainfalls from in rainfallsare the red-bedinitiate in thed zone, byred a- leadingb ed reduction zone, to lossleading in of the support to shear loss strengthoffrom support the of front fromrock edge massthe front [56 because]. Inedge this of [56 context,the]. Inwater this the washcontext, sliding from the force rainfalls sliding cannot forcein the be cannot counterbalancedred-bed be zone, counterbalanced leading by the to shear loss by oftheresistance support shear resistance fromfrom thethe rockfromfront mass theedge rock itself. [56 ]mass. In As this shownitself. context, As in shown Figure the sliding 17in ,Figure the force rock 17 cannot, mass the rock slides be counterbalancedmass step slides by step step from byby thestepfront shear fromto back. resistance front The to surface from back. ofthe T rupturehe rock surface mass is curved itself. of rupture concavelyAs shown is curved upward, in Figure concavely and 17, the the slide rock upward, movement mass andslides is the roughlystep slide by stepmovementrotational. from The front is roughly height to back. di ff rotational.erence The betweensurface The heightof the rupture front difference and is back curved between edges concavely is approximately the front upward, and 10–55 backand m, the edges and slide the is movementapproximatelylargest thickness is roughly 10 varies–55 m, fromrotational. and 10 the to largest 22.5 The m. height thickness In addition, difference varies tension frombetween cracks 10 to the usually 22.5 front m. develop In and addition, back at the edges tension back ofis approximatelycracksthe slope. usu Withally 10increasing develop–55 m, at and excavation the the back largest depth, of the thickness the slope. sliding Withvaries interface increasing from develops 10 to excavation 22.5 from m. the In depth, superficial addition, the tension zonesliding to cracksinterfacethe inner usu develops zone.ally develop from the at thesuperficial back of zone the to slope. the inner With zone. increasing excavation depth, the sliding interface develops from the superficial zone to the inner zone. STEP n …… STEP n …… STEP 3 STEP 3 STEP 2 STEP 2 STEP 1 STEP 1 Potential Sliding Surface Potential Sliding Surface

Figure 17. 17. SchematicSchematic description description of the of theprogress progress of rotational of rotational landslides landslides on red on-bed red-bed slopes slopesin South in FigureChinaSouth. China.17. Schematic description of the progress of rotational landslides on red-bed slopes in South China. Geosciences 2019, 9, 273 16 of 20 GeosciencesGeosciences 2018 2018, 8, ,x 8 FOR, x FOR PEER PEER REVIEW REVIEW 21 of21 of20 20

UsingUsing rainfall rainfall rainfall data data data for for for the the investigated investigated investigated area, area, area, model model model tests tests (Figuretests (Figure (Figure 18) were 18) 18) were carried were carried out carried to out simulate out to to simulatethesimulate landslides the the landslides landslides of three of slopes ofthree three slopes in slopes this in rainfall inthis this rainfall mode, rainfall mode, and mode, the and results and the the results were results in were good were in agreement ingood good agreement agreement with the withactualwith the the situation.actual actual situation. situation. In Figure In In Figure19 Figurea, a slope19a, 19a, a with slopea slope a thick-layeredwith with a thicka thick-layered soft–weak-layered soft soft rock–weak–weak mass rock rock structure mass mass structure collapsesstructure collapsesnearcollapses the near footnear the of the foot the foot slope,of ofthe the andslope, slope, the and upper and the the partupper upper of part the part freeof ofthe surfacethe free free surface loses surface its loses support,loses its itssupport, support, which which leads which to leadsaleads tractive to toa tractivea landslide.tractive landslide. landslide. A slope A withAslope slope a with thick-layered with a thicka thick-layered hard-layered rock hard hard mass rock rock structure mass mass structure withstructure soft–weak with with soft soft rock–weak–weak mass rockisrock shown mass mass isin is Figureshown shown 19 in b. in Figure The Figure soft–weak 19b. 19b. The The rock soft s massoft–weak–weak succumbs rock rock mass tomass the succumbs esuccumbsffects of to rain to the softening the effects effects ofand of rain loses rain softeningitssoftening strength, and and loses and loses anits empty itsstrength, strength, surface and and appearsan an empty empty above surface surface the appears weak appears interbedded above above the the weak rock weak mass. interbedded interbedded The hard rock rock mass.massmass. The aboveThe hard hard the rock weakrock mass mass interbedded above above the the rockweak weak mass interbedded interbedded loses its support,rock rock mass mass which loses loses leads its itssupport, to support, a landslide. which which leads A leads soft–hard to toa a landslide.interbeddedlandslide. A A soft rock soft–hard– masshard interbedded structure interbedded is shown rock rock mass in mass Figure structure structure 19c. There is isshown shown are many in in Figure structural Figure 19c. 19c. planes There There of are weakare many many rock structuralmassstructural in the planes planes upper of partof weak weak of the rock rock slope, mass mass and in inthe the the sliding upper upper range part part isof concentrated of the the slope, slope, and near and the the the sliding structural sliding range range plane is of is concentratedtheconcentrated upper weak near near interlayer.the the structural structural The upperplane plane weakof ofthe the interlayer upper upper weak isweak located interlayer. interlayer. between The The the upper upper first andweak weak second interlayer interlayer grades is ofis locatedthelocated slope. between between the the first first and and second second grades grades of ofthe the slope. slope.

(a) (a)Diagrammatic Diagrammatic sketch sketch (b)(b) Physical Physical photograph photograph

FigureFigure 18. 18. Sl opeSlSlopeope model model test test monitoring monitoring system system (A . (A.( ATransparent. Transparent observation observationobservation box box.box. B. BTest B.. Test slope slope.slope. C.. C C.. HighHigh-resolutionHigh-resolution-resolution camera camera.camera. D.. D D.Rainfall. Rainfall device device device.. E.. WaterE E.. Water storage storage and and groundwater groundwater level level control control device device.device. F.. F.F. RainfallRainfall measuring measuring device device.device. G.. G G.V.alve V Valve.).alve.). .).

(a) (a) (b)( b) (c) (c)

FigureFigure 19. 19. Simulated Simulated slope slope failure failure failure modes modes modes:: (a:) ( (slopeaa)) slope slope with with with thick thick thick-layered-layered-layered soft soft soft-weak-weak-weak rock rock rock mass mass mass structure structure structure.. . (b)( bT)hick Thick-layeredThick-layered-layered hard hard hard rock rock rock mass mass mass structure structure structure with with soft-weak soft soft-weak-weak rock rock rock mass. mass mass. ( c ()c. Soft-hard) (cSoft) Soft-hard-hard inter-bedded inter inter-bedded-bedded rock rockmassrock mass mass structure. structure structure. .

4.1.4.4.1.4. Toppling Toppling TopplingToppling is theis the phenomenon phenomenon in inwhich which a hard a hard rock rock mass mass topples topples to tothe the free free face face about about a certain a certain pointpoint at atthe the bottom bottom in inresponse response to tounloading unloadingunloading and and rebounding rebounding or orwhen when subjected subjected to tothe the actions actions of of gravitygravity and andand forces forcesforces exerted exerted exerted by by by adjacent adjacent adjacent units units oror byorby fluids.byfluids. fluids. If If the theIf dumpingthe dumping dumping amplitude amp amplitudelitude is adequatelyis adequatelyis adequately large, large,thenlarge, then it then may it mayit collapse may collapse collapse under under theunder moment the the moment moment of gravity, of ofgravity, gravity, and this and and might this this might even might lead even even to lead a lead deep to toa sliding deep a deep sliding failure sliding at failurethefailure base at atthe [ 2the ,base45 base,48 [].2, [452,,4548,]48. ] . TopplingToppling in inthe the red red beds beds in inSouth South China China usually usually occurs occurs in inthick thick hard hard rock rock slopes slopes or orin inslopes slopes withwith weak weak rock rock mass mass interlayers interlayers (see (see Table Table 1) , 1) which, which mostly mostly consist consist of offractured fractured stratum, stratum, weak weak Geosciences 2019, 9, 273 17 of 20

GeosciencesToppling 2018, 8 in, x theFOR red PEER beds REVIEW in South China usually occurs in thick hard rock slopes or in slopes 21 withof 20 weak rock mass interlayers (see Table1), which mostly consist of fractured stratum, weak clayey rock, mudclayey intercalation, rock, mud intercalation, and so on. Steep and so wedge-shaped on. Steep wedge tension-shaped cracks tension develop cracks at the develop top of at a hardthe top rock of slope.a hard Asrock shown slope. in As Figure shown 20, in these Figure cracks 20, arethese wide cracks at the are top wide but at narrow the top at but the narrow bottom. at According the bottom. to theAccording statistics, to thethe width statistics, of such the cracks,width of which such arecracks, filled which with looseare filled soil, with generally loose varies soil, generally from 6 to varies 12 cm andfrom can 6 to reach 12 cm a maximum and can valuereach of a maximum about 16 cm value [41]. of With about the 16 tension cm [41 cracks]. With developing the tension vertically, cracks thedeveloping rock mass vertically, is gradually the cutrock into mass strips is gradually or blocks, cut toppling into strips around or ablocks, certain toppling point at thearound bottom a certain of the rockpoint mass at the (thick bottom hard of rockthe rock mass mass with (thick weak hard rock rock interlayers mass with topples weak at rock the contact interlayers between topples hard at and the softcontact rocks) between under hard the e ffandect soft of gravity. rocks) under the effect of gravity.

FigureFigure 20. 20. SchematicSchematic description description of of the the tension tension crack crack destruction destruction in in red red-bed-bed slope slopess in in South South China China..

(2)(2) FailureFailure modemode Slopes composedcomposed ofof a a sandy sandy conglomerate conglomerate at at the the edge edge of of the the red-bed red-bed basin basin in Southin South China China are cutare bycut orthogonal by orthogonal joints, joints, along along which which weathering weathering and erosion and erosion develop, develop, making making slopes deformslopes ondeform the side on ofthe the side free of face.the free It isface. generally It is generally observed observed that one that group one ofgroup steep of structural steep structural planes planes is dragged is dragged apart, causingapart, causing wedge-shaped wedge-shaped tension tension cracks. cracks. With increasingWith increasing depth depth and width and width of the of tension the tension cracks, cracks, rock massrock mass units units rotate rotate forward forward about about some some pivotal pivotal point, point, which which is low is or low below or below the unit. the unit. For thick hardhard rockrock massmass slopesslopes containingcontaining weakweak rockrock interlayersinterlayers (like thethe thick-layeredthick-layered hard rock mass structure with soft–weak rock mass in Table1), di fferential deformation between hard and rock mass structure with soft–weak rock mass in Table 1), differential deformation between hard weakand weak rock masses rock masses causes cause tensions tension cracks cracksin the upper in the sandy upper conglomerate sandy conglomerate to develop to until develop toppling until deformationtoppling deformation appears. Furthermore, appears. Furthermore, the weak strata the are weak extruded strata through are extruded plastic flow through under plastic the weight flow ofunder the upper the weight hard rock of the mass upper and the hard actions rock of mass gravity and and the forces actions exerted of gravity by adjacent and forces units or exerted by fluids by inadjacent the cracks. units Recessed or by fluids cavities in the are cracks. correspondingly Recessed cavities created are in the correspondingly weak strata. Hence, created the in support the weak of thestrata. upper Hence, hard the rock support mass becomesof the upper ineff ective,hard rock causing mass tension becomes cracks ineffective, to develop causing and thetension slope cracks to be toppled.to develop This and destruction the slope to tends be toppled. to cause This a large-scale destruction landslide tends atto thecause base. a lar Thege- volumescale landslide of a landslide at the canbase. be The over volume one million of a landslide cubic meters. can be over one million cubic meters. 5. Conclusions 5. Conclusions This paper discusses the geological background of the formation of the red beds in South China This paper discusses the geological background of the formation of the red beds in South and analyzes as well as summarizes the features of the structural planes and the morphologies of the China and analyzes as well as summarizes the features of the structural planes and the red-beds in the north of Guangdong Province. The structures of the rock mass in the red-bed areas in morphologies of the red-beds in the north of Guangdong Province. The structures of the rock mass South China were classified on the basis of soft rock slope engineering features of the red beds along in the red-bed areas in South China were classified on the basis of soft rock slope engineering more than 10 highway projects in South China. The features of the geological hazards in the red-bed features of the red beds along more than 10 highway projects in South China. The features of the slopes possessing different structures were investigated, and typical failure modes in the red beds in geological hazards in the red-bed slopes possessing different structures were investigated, and South China were identified. The main findings are summarized as follows: typical failure modes in the red beds in South China were identified. The main findings are (1) The red beds in South China formed mainly through deposition under high-temperature and summarized as follows: dry tropical–subtropical conditions. The sedimentation layers are cross-cut by fractures in the NE, (1) The red beds in South China formed mainly through deposition under high-temperature NNE, and east–west (EW) directions. The red-bed basins formed along faults are generally randomly and dry tropical–subtropical conditions. The sedimentation layers are cross-cut by fractures in the scattered in a small-to-medium-sized (hundreds of square kilometers on average) elongated area. NE, NNE, and east–west (EW) directions. The red-bed basins formed along faults are generally Because of the influence of the sedimentation environment, the red beds in South China are mainly randomly scattered in a small-to-medium-sized (hundreds of square kilometers on average) elongated area. Because of the influence of the sedimentation environment, the red beds in South China are mainly interbedded in unequal thick and dip angles below 30°. In particular, the quaternary overburden layer usually exists in the red beds in South China. Geosciences 2019, 9, 273 18 of 20

interbedded in unequal thick and dip angles below 30◦. In particular, the quaternary overburden layer usually exists in the red beds in South China. (2) The structural planes of rock mass in the red beds in South China include primary weak stratification, a weak intercalation layer generated during sedimentation, and a tectonic weak cross-cutting plane created during tectonic movement. These planes also include a mudded intercalation layer created by the wet–dry cycling of the primary and tectonic weak interlayers. According to the characteristics of the structural planes and their morphologies, the rock mass in the red beds in South China are categorized into four types: (a) thick hard rock mass structures, (b) thick hard rock mass structure with weak intercalations, (c) thick soft rock mass structures, and (d) hard–soft interbedded rock mass structures. (3) On the basis of the different rock mass structures in the red-bed soft rock areas in South China, four slope failure modes were identified: (a) weathering spalling and scouring, (b) rock falls, (c) landslides, and (d) tensile dumping. Weathering spalling and scouring occur frequently, and the other main types of geological hazards include rock falls, landslides, and tensile dumping.

Author Contributions: Conceptualization, C.Z., X.Y. and Z.L.; methodology, C.Z., X.Y. and Z.L.; software, Z.Y. and Y.L.; validation, C.Z., X.Y., Y.L. and Z.L.; formal analysis, C.Z., X.Y. and Z.L.; investigation, X.Y. and Y.L.; resources, C.Z. and Z.L.; data curation, X.Y. and Y.L.; writing—original draft preparation, X.Y., Y.L., Z.D., W.H. and W.M.; writing—review and editing, C.Z., X.Y., Y.L., Z.D., Z.L., W.H. and W.M.; visualization, C.Z., X.Y. and Z.L.; supervision, C.Z. and Z.L.; project administration, C.Z. and Z.L.; funding acquisition, C.Z. and Z.L. Funding: The research was supported by the National Natural Science Foundation of China (NSFC) (Grant No. 41530638), the Special Fund Key Project of Applied Science and Technology Research and Development in Guangdong (No. 2015B090925016), the Special Support Plan for High-Level Talents of Guangdong Province (No. 2015TQ01Z344), the Science and Technology Program of Guangzhou, China (No. 201803030005). Conflicts of Interest: The authors declare no conflict of interest.

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