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Identification of a locking segment in a high-locality landslide in Shidaguan, Southwest China

Identification of a locking segment is crucial to assess the potential runout risk and the overall stability of a high-locality landslide. However, it is difficult to identify a locking segment through field surveys before a landslide failure due to their concealment. Taking the Tizicao landslide—a high-locality landslide in Southwest China—as a field example, this study aims to identify and analyze a locking segment of the landslide. Based on geomorphology and spatial–temporal deformation of the landslide, the characteristics for identifying the locking segment of the Tizicao landslide are analyzed, and the locations and area of the locking segment are determined based on the analytical results. Furthermore, the identification results are interpreted through fieldwork including the displacement monitoring of ground surface and deep parts, borehole drilling, and oblique photography using unmanned aerial vehicles. The results reveal that the locking segment of the Tizicao landslide lies at the southern slope toe and covers an area of approximately 4.69 × 104 m2, accounting for 15.2% of the total area of the landslide. Significantly different characteristics are observed in respect of surface displacement, deep displacement, surface cracks, and sliding zone soil between locking and non-locking segment.


Introduction
Locking segments, also called rock bridges (Einstein et al. 1983;Frayssines and Hantz 2006) or locked sections (Huang 2012;Chen et al. 2018) in rockslides, are a critical factor in the occurrence of a high-locality landslide. The overall stability of landslides depends on the strength and deformation of locking segments (Jennings 1970;Jaeger 1971;Pariseau et al. 2008;Chen et al. 2018). When the locking segments of a high-locality landslide detach from their parent rocks, the fragmental materials formed in the landslide tend to result in catastrophic effects and severe casualties because of their huge potential energy. Catastrophic landslides with locking segments have been investigated in several previous studies. For example, the Vajont slide resulted in both loss of life and economic cost in Italy in 1963. This landslide caused a reservoir to overflow and sent a wave over the crest of the dam, destroying five villages and killing approximately 2000 lives (Muller 1968). On June 5, 2009, a huge catastrophic rockslide with a rock mass barrier in the front occurred at the crest of the Jiweishan Mountain in Wulong County, Chongqing, China, with a long runout distance of 1500 m. A landslide body with a volume of 7.0 × 10 6 m 3 moved toward the valley; as a result, approximately 0.47 km 2 of the residential area was buried, an iron ore mine was destroyed, and 74 people were killed (Xu et al. 2010;Yin et al. 2011). On August 27, 2014, a similar large-scale landslide with one locking segment at its front toe occurred in Fuquan County, Guizhou Province, China, which killed 23 people and destroyed 77 houses (Lin et al. 2018). On June 24, 2017, a catastrophic landslide with locking mass in the front was triggered above the Xinmo Village, Maoxian County, Sichuan Province, China; the whole Xinmo Village was buried (10 deaths, 73 people missing) and the sliding mass blocked the Songping River over a distance of 1300 m (Kang et al. 2019;Wang et al. 2019). A disastrous landslide occurred in the Pettimudi Village, Kerala State, India, on August 6, 2020 due to a heavy downpour. This disaster caused 66 fatalities of residents, among which four are still missing, making it the most catastrophic landslide event reported in the state (Achu et al. 2021). Therefore, landslides with locking segments pose a huge threat to people's lives and properties. Given this, it is essential to identify the location, geometry, and area of the locking segments before landslide failure.
Landslides with locking mass usually show some noticeable characteristics. For example, nearly horizontal Cretaceous strata with a thickness of 350 m are observed in the antisliding section in the front of the Vajont landslide (Broili 1967). Moreover, fresh rock surface is observed in the locking section in the front of the Jiweishan collapse reported by Yin et al. (2011). The Dongmiaojia landslide close to the Xiaolangdi Hydropower Station on the Yellow River, China, is a typical bedding rocky landslide. The accumulation body in the front is the anti-sliding section of the whole landslide, on which great numbers of bulging-induced and compressive cracks have developed (Xu et al. 2014). The Xiaoba landslide took place in Guizhou Province, China, which has a locking section of dolomite in the front. Meanwhile, this landslide shows obvious stress concentration and plastic deformation on the locking mass in the front, as discovered by Lin et al. (2018). Based on the spatial-temporal deformation analysis of the Xinmo landslide, Kang et al. (2019) found the bulge reduction on the sliding surface and inferred that the locking mass should lie in the front of the landslide. Two landslides occurred on the right bank of the Jinsha River in Baige Village, China, which blocked the river (Deng et al. 2019). As discovered in field surveys, fresh shear surfaces were distinctly visible in the anti-sliding areas in the front of the Baige landslide. According to the above-mentioned cases, a direct evidence of the presence of locking mass is a fresh rock surface after a landslide occurs. However, it is difficult to acquire accurate information on the fresh surface prior to sliding due to the concealment of the locking mass. Despite this, information (e.g., location and area) of locking mass can still be analyzed using some identifying characteristics, such as horizontal anti-sliding strata, bulging-induced and compressive cracks, obvious stress concentration, plastic deformation in the front of a landslide, the deformation intensity of the surface and deep parts of the failed slope caused by a landslide, and the quality of rock mass.
Taking the Tizicao landslide-a large-scale and high-locality landslide that was active from 2013 to 2017-as an example, this study aims to recognize a locking segment of the landslide. The information regarding the locking segment in the Tizicao landslide was analyzed based on geomorphological characteristics and spatial-temporal deformation. In addition, the validity of the identification results was interpretated using field survey methods, including the monitoring of surface and deep displacement, geological borehole drilling, and oblique photography using UAVs.

Location
The Tizicao landslide is located in Maoxian County, Sichuan Province, Southwest China ( Fig. 1), with geographical coordinates of 31°53′14.89″N and 103°40′51.12″E. This landslide lies on the right bank of the Minjiang River valley and faces Shidaguan Town on the left bank of the river, with National Highway G213 passing by its foot.

Geological context
The Tizicao landslide is located in an area with medium-height mountains and river valleys subject to tectonic erosion. The majority of this area is a part of the Minshan Mountain of the Qionglai Mountains, with the southeast border belonging to the final segment of the Longmen Mountains. This area shows steep and dangerous valleys and slopes, narrow river valleys, and deeply downcutting rivers. The Minjiang River flows through the area in a nearly N-E direction, with no terrace developing.
The site area of the Tizicao landslide is in the shape of a round-backed armchair (Fig. 2), high in the west and low in the east. The grade of the failed slope is 44°-78° and its mean gradient is 30°-40°. The portion from the lower riverside of the Minjiang River to the upper part at an elevation of 1950 m is a straight steep slope with a grade of approximately 70°, where bedrock is exposed mostly. The failed slope is mainly distributed from an elevation of 1950 m to the rear of the landslide (at an elevation of 2415 m), and the mean slope is approximately 25°. The alternate steep and gentle steps and the development of free-face conditions provide favorable topographic conditions for the development of the Tizicao landslide.
The Tizicao landslide is approximately 680 m long and 570 m wide ( Fig. 2b), with an average thickness of approximately 45.1 m and a volume of 1388.2 × 10 4 m 3 . The elevations of the rear and toe are 2415 m and 1950 m (Fig. 2a), respectively; thus the relative elevation difference of the landslide reaches 465 m. The relative elevation difference between the toe of the landslide and the lower riverside of the Minjiang River is 220 m. The Tizicao landslide features a high-elevation shear outlet, development of free-face conditions, and high potential energy, which make it a typical high-locality landslide (Weidinger 2006;Yin et al. 2017).
The Tizicao landslide was active during 2013−2017. Owing to the anti-sliding of a local locking segment in the front, this landslide had gradually become stable by the end of 2017 after multistage deformation and partial collapse. Once large-scale sliding occurs, this landslide would directly threaten the lives of 30 people on the slope body, even seriously threaten the lives of 113 people in Shidaguan Town below the landslide, and would directly strike more than 30 buildings and the National Highway G213 at a distance of 2 km. Meanwhile, the Minjiang River might be blocked to form barrier lakes and induce secondary floods, which would endanger the towns, villages, and infrastructures along the Minjiang River and directly threaten the properties of approximately CNY 86 million.
The Tizicao landslide body mainly consists of silty clay on the surface and broken phyllite (Q 4 del ) below, while the sliding bed mainly comprises carbonaceous phyllite of the Devonian Weiguan Group (Dwg 2 ). The phyllite is grayish black and grayish yellow and is locally interbedded with a small number of quartz veins. The phyllite layers show an attitude of 197°∠71°. The carbonaceous phyllite is weak and broken, indicating that it has poor physical and mechanical properties and poses risks of failure, slide, and deformation during the rainy seasons.

Hydrological frame
In the area of the Tizicao landslide, there are small quantities of precipitation in winter and spring, but large quantities of precipitation in summer and autumn. The average monthly rainfall is approximately 73 mm (2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018), with a maximum of 400 mm. Rainfall in the area mainly occurs from June to October, during which the rainfall accounts for 91.8% of the annual rainfall. As shown in Fig. 3a, the historical monthly rainfall and deformation data of the area show that the deformation mainly occurred from June to September in 2008-2018. Moreover, many sliding events occurred after peak rainfall and continuous intensive rainfall. On September 6, 2013, the middle-front part of the Tizicao landslide began to deform after three months of continuous rainfall (from May to July). As a result, the houses on the slope body started to crack and dislocate downwards at the same time. In September 2014, the middle part of the landslide front gradually collapsed after the peak rainfall of 252 mm (June) followed by continuous precipitation for three months. Consequently, a sliding area with a width of approximately 60 m and a height of 200 m (Fig. 2a) formed on the northern side of the landslide. The accumulation body fell into the Minjiang River and formed a landslide dam. At the same time, the rear of the landslide began to crack. From August to September in 2015, the landslide deformed more obviously and severely after the peak rainfall of 400 mm in July, resulting in additional wider, longer cracks. After continuous deformation in rainy seasons during 2016-2017, the rear and front of the northern landslide dislocated downwards for more than 10 m locally by July 2017, leading to continuous collapse and rockfall near the front toe. Figure 3b shows the daily rainfall statistics and the curve of typical surface displacement of the area monitored from June 1 to October 30 in 2017. Violent sliding events occurred after 20 days of heavy rain and 10 days of short-term rain, with the maximum displacement reaching 1210 mm. Therefore, sliding events lag behind rain. Thus, before the sliding events happened, the peak rainfall was 143-400 mm and the average intensive rainfall that happened 2-3 months was 114-184 mm. The minimum values of both the peak rainfall and the average intensive rainfall can be used as the thresholds for the critical stability of the landslide.

Seismotectonic conditions
The massive quantity of broken carbonaceous phyllite developing in the slope body is closely related to active structures. Shidaguan Town lies in the middle part of the NStrending seismic belt (Fig. 4), where historically intense earthquakes developed. Since the beginning of the twentieth century, Shidaguan Town has successively experienced the Diexi earthquake in 1933 (Ms 7.5), the Songpan-Pingwu earthquake in 1976 (Ms 7.2), the Wenchuan earthquake in 2008 (Mw 7.9), and the Jiuzhaigou earthquake on August 8, 2017 (Ms 7.0; Fig. 4). Among them, the Diexi earthquake, with a hypocenter depth of 6.1 km and an epicenter of merely 1.3 km away from the landslide center, caused a variety of disasters, such as massive rockfalls, landslides, and debris flows, and devastated the former Diexi Town . The Diexi earthquake shattered numerous mountains due to its epicentral intensity up to 10 degrees in the macroseismic intensity (MMI), thus resulting in broken rock masses on the slope surface. When seismic waves are transmitted to the site, the peak amplification of motion will occur at the crest of a slope (Ashford et al. 1997). This will lead to the cracking of the mountain at the rear of the Tizicao landslide and damage to rock masses. In addition, the topographic effect is most apparent for slopes steeper than 60° (Boore et al. 1972). The slope of the Tizicao landslide area has a grade of 44°−78°, thus, the rock masses on the slope part with a higher grade are highly liable to undergo damage and fracture. Moreover, the Songpan-Pingwu, Wenchuan, and Jiuzhaigou earthquakes occurring in recent years have further fractured the rock masses inside the landslide.
In addition, a great number of secondary folds ( Fig. 5a) have developed in the bedrock of the site area and clear corrugations are visible on the steep wall below the front landslide toe. Meanwhile, fractured rock masses, secondary folds, hollow structures, and argillaceous fillings (Fig. 5b) can also be observed at the slope foot. Many holes or empty spaces have been reserved in the fractured rock masses after earthquakes. The hollow structures are formed, some of which were filled with argillaceous materials. Therefore, earthquakes and tectonic movements are the critical factors leading to the formation of the broken phyllite masses in the landslide body. These raw data of the surface displacement were processed using the measurement adjustment software DDM to obtain their deformation amount and deformation rate.
To find out the deformation characteristics of the landslide, eleven major surface cracks and bulging-induced cracks were first identified and then plotted in the topographic plan

Geomorphological identification
As shown in Fig. 2a and c, a typical S-shaped river landform is developed below the Tizicao landslide, which is a concave bank under the northern side of the slope and a convex bank beneath the southern side. From a geomorphological point of view, landslides rarely occur on the convex banks, but occur more frequently on concave banks (Lulseged and Hiromitsu 2004). From a topographical point of view, the convex slope is more stable than the concave slope under the same conditions (Cala 2007). In the concave bank part, the slope foot is approximately 220 m higher than the water level of the Minjiang River and has an average grade of approximately 65°. Broken phyllite   Fig. 2e) is visible on the slope surface in this part due to the strong downcutting of the Minjiang River. On July 3, 2017, a local collapse occurred in the concave bank (Fig. 2b) in the front of the northern landslide body. As a result, materials in the landslide body ran through the flow area and fell into the Minjiang River to produce a fan-shaped landslide dam (Fig. 2c) with a maximum thickness of up to a 75 m, an average thickness of approximately 25 m, an area of 2,400 m 2 , and a volume of 6 × 10 4 m 3 . The materials from the landslide body moved to the farthest place that is just 5 m from the residential houses in Shidaguan Town after crossing the Minjiang River. Meanwhile, the landslide dam blocked the Minjiang River for several hours. A convex bank part is located under the southern side of the Tizicao landslide, with a gentle slope of approximately 35°, where vegetation is evenly distributed and relatively intact phyllite is exposed on the slope surface (Fig. 2d). Noticeably, the concave and convex bank parts of the S-shaped valley under the Tizicao landslide are greatly different in slope and lithology. Therefore, the rock masses on the southern side of the landslide above the convex bank part reserve their integrity and constitute the potential locking segment of the landslide (Fig. 2a−b).

Identification of spatial-temporal deformations
The deformation history of the Tizicao landslide indicates that the landslide body has undergone intensive sliding on the northern side, with a sliding distance of more than 10 m. A steep wall is observed at the rear of the landslide's northern side (Fig. 9e), and the collapse occurred in the front of the landslide, with a volume of approximately 6 × 10 4 m 3 . As indicated by the expansion and penetration of the cracks, the rear of the landslide is completely penetrated by cracks L07 and L08 (Fig. 8). Crack L11 (Fig. 8) on the northern part of the landslide body penetrates the landslide from the rear to the slope foot. However, crack L08 (Fig. 8) only penetrates 1/3 of the landslide's southern side. The Tizicao landslide tends to deform northeastwards without distinct deformation in the front of the landslide's southern side. Therefore, the rock masses in the front of the landslide's southern side serve as potential locking mass of the Tizicao landslide.
According to the identification analysis of geomorphology and spatial-temporal deformations of the Tizicao landslide, the locking segment of the landslide lies at the south toe of the slope (Fig. 2a−b; Fig. 7 c−d). It covers an area of approximately 4.69 × 10 4 m 2 , accounting for 15.2% of the total landslide area.

Surface deformation
The isoline map of the surface displacement (Fig. 10) was plotted with the data acquired from the 24 monitoring points (Fig. 8). As shown in Fig. 10, a sliding event occurred in a general northeast direction (closer to the north) from August 13, 2017 to January 25, 2018. The maximum surface displacement (1210 mm) occurred at the northern toe, which coincides with the location where the front collapsed (Fig. 2b). The surface displacement in the rear and middle parts was similar, which ranged from 150 to 300 mm, indicating that the rear and the middle parts slid as a whole. The minimum surface displacement occurred in the southern area of the slope toe throughout the whole landslide, which ranged between 30 and 150 mm. Therefore, the southern area serves as the anti-sliding area of the entire landslide. The deformation vs. time curves of five representative monitoring points are shown in Fig. 11. Among these five monitoring points, T23 lies in the front of the northern side, T17 and T21 lie on the southern side of the landslide front, and T10 and T11 lie on the north and southern sides of the rear, respectively. According to Fig. 11, the displacement at T23 was noticeably higher than that at T17 and T21, which was higher than the displacement at T10 and T11. These results coincide with the results from Fig. 10. Hence, the area (i.e., the front of the southern side) with the minimum surface displacement can be used to characterize the locking segment of this landslide.

Deep displacement
The curves of the monitored cumulative deep displacement are shown in Fig. 12. Significant deep displacement is found in boreholes D01, D02, D03, and D04, which shows a trend of first increasing and then decreasing with increasing depth. The maximum displacement S max is in the order of S max (D01) > S max (D04) > S max (D02) > S max (D03), and the depth corresponding to S max coincides with the depth of the sliding surface. The maximum value of S max (5 mm) was acquired at a depth of 45 m at D01. This borehole was deployed behind the front collapse area of the landslide, where the maximum surface displacement was also obtained. The results indicate the consistency between the deep displacement and the slope surface displacement.
No displacement was monitored in boreholes D05, D06, and D07, as shown in Fig. 12. Based on the monitored surface displacement (Fig. 10), deep rock masses produced no displacement but the slope surface deformed to a certain extent in boreholes D05, D06, and D07. As shown in Figs. 6 and 8, D05 is adjacent to the locking segment, and D06, D07, and the locking segment are all located in profile D-D'. Therefore, the locking mass in the front can well prevent the rear and adjacent slope bodies from sliding.

Surface cracks
Surface cracks are an important phenomenon associated with the formation and development of landslides (Wang et al. 2020). The felid survey of cracks allows for objective assessment and prediction of the mechanical properties and development trends of landslides, such as the distribution scope, mechanical mechanisms, and developmental stages of landslides, as well as the thickness of the sliding masses.
The development and distribution map of the cracks is shown in Fig. 8. According to this figure, numerous tension cracks and shear cracks have mainly developed at the landslide rear, in the middle part, and on the northern side of the landslide front, while bulginginduced cracks and a small number of compressive cracks primarily occur in the middle and southern parts of the landslide front. Although it is very common for bulging-induced cracks and compressive cracks to develop in the front of thrust-induced landslides, the properties of the rock masses bearing these two kinds of cracks differ greatly. As shown in Fig. 13, compressive cracks form in the rock masses with the locking mass in the front under the pushing force of landslides. For intact rock masses, only a small number of compressive cracks (e.g., compressive cracks C1, C2, L09, and L10) are produced due to the high strength of the rock masses. As the pushing force of landslides increases, these cracks gradually expand in the direction of the pushing force until they completely penetrate the rock masses. In contrast, for broken rock masses, large numbers of compressive or bulging-induced cracks (Fig. 9m) are formed along the push direction of landslides due to their low strength. These rock masses undergo large compressive deformation, which causes the original cracks to gradually expand along two directions that are perpendicular to the pushing force of landslides. Such crack expansion has been observed in many uniaxial compression tests and numerical simulations of rock masses with massive joints (Huang et al. 2019;Gao et al. 2021;Guo et al. 2021). This crack expansion is an important factor in the formation of numerous bulging-induced cracks (Fig. 9m) inside the landslide body. The two cases are shown in Fig. 13a and b, respectively, that can produce different effects on landslides. For the former, cracks in the rock masses gradually expand as the pushing force of the landslide increases. In this case, slope failures can only occur laterally along C1 or C2. In comparison, for the latter, sliding masses cause general damage along the newly produced crack C3 as the bulging-induced cracks extend, expand, and penetrate in the direction perpendicular to the pushing force of landslides.
As shown in Fig. 2, the rock masses with a width of approximately 50 m in front of the bulging-induced cracks (Fig. 9m) are the locking masses. However, the rock masses bearing bulging-induced cracks are not part of the locking mass since they have been completely crushed. In contrast, the rock masses containing compressive cracks ( Fig. 9k and l) and the rock masses in front of the compressive cracks are all part of the locking mass. They are approximately 300 m wide, and the width of the locking segment increases from the middle part to the southern side in the front of the slope. The bulging-induced and compressive cracks, especially separate compressive cracks, serve as important indicators for identifying and positioning the locking mass of landslides and determining the geometric morphology of the locking mass.

Borehole cores
The Tizicao landslide body mainly consists of a heavily weathered mass on the surface and fractured rock masses below. The fractured rock masses are distributed throughout the landslide, with a thickness variation from 10 to 50 m. The protoliths are grayish black and black carbonaceous phyllite. The sliding zone soil is 1.2−3.0 m thick and has a dense structure, which is mainly gray or tawny and consists of silty clay interbedded with breccia Fig. 13 Sketches showing the formation processes of compressive and bulging-induced cracks in landslides a crack propagation in a relatively intact rock mass b crack propagation in a relatively fractured rock mass; C1 and C2 are compressive cracks, and C3 is a bulging-induced crack and broken stones (Fig. 14a). The silty clay mainly appears to be hard plastic, dampishhumid, and dense (Fig. 14b). The breccia and broken stones mainly consist of phyllite. They are angular and subangular in shape, with a general diameter of 2−20 mm. Compared with the fractured rock masses in the landslide body, the rock masses in the sliding bed are generally less weathered and consist of moderately weakly weathered rock masses. Their borehole cores are generally intact and in the shape of long columns (Fig. 14a).
The cores from boreholes zk15, zk19, and zk20 ( Fig. 14d) are more intact than those from other boreholes, which is consistent with the relatively intact phyllite revealed in exploratory well TJ04 (Fig. 14c). They are moderately weakly weathered and are roughly the same as the rock masses in the sliding bed. In contrast, cores from other boreholes are more fractured (e.g., rock masses above the sliding zone in Fig. 14a) and strongly weathered. In addition, sliding zone soil with the mirror surface and scratch (Fig. 14b) can be observed in these borehole cores, while no sliding zone soil is visible in the cores of zk15, zk19, and zk20. Therefore, the rock masses in boreholes zk15, zk19, and zk20 are stable and have not slid; thus there is a locking segment at these positions.
Rock quality designation (RQD) was introduced by Don Deere (1964) as a means of using diamond cores to classify rock quality. The RQD is defined as the ratio (unit: percentage) of the total length of sound core pieces [0.1 m (4 inches) or longer] to the length of the core run. The distribution of RQD values of all borehole cores is plotted as shown in Fig. 15. According to this figure, the RQD values of cores from boreholes in profiles A-A' (zk01, zk02, zk03, zk04, and zk05) and B-B' (zk06, zk07, zk08, zk09, and zk10) are all less than 20%, with a joint spacing of 2-5 cm, and the RQD values of cores from boreholes in profiles C-C' (zk11, zk12, zk13, zk14, and zk15) and D-D' (zk16, zk17, zk18, zk19, and zk20) are between 25 and 80%, with a joint spacing of 8-115 cm. According to the strength and deformability criteria of jointed rock masses (Hoek and Bray 1981), the rock masses of the cores from boreholes in profiles A-A' and B-B' are very poorly jointed (broken), and the rock masses of cores from borehole in profiles C-C' and D-D' are poorly heavily jointed (intact). The RQD values of zk15, zk19, and zk20 that lie in the middle part and on the southern side of the landslide front (Figs. 6, 15) are greater than 68% (joint spacing: 15-115 cm). Therefore, the rock masses revealed by these boreholes are relatively intact, which coincides with the position of the locking mass (Fig. 2b).

Discussion
The Tizicao landslide is a typical landslide with a locking segment, and the existence of locking mass has prevented this landslide from complete failure. However, identifications and characteristics of a locking segment are yet to be analyzed in a systematical and refined manner, and only the field characteristics of the locking mass of a few landslides have been analyzed. Taking the Tizicao landslide as an example, this paper proposed a method combining geomorphology with spatial-temporal deformation analysis to identify the locking mass. In addition, this study acquired the characteristics of the locking mass through refined exploration. A locking mass of the Tizicao landslide has developed on the convex bank, while the nonlocking mass has developed on the concave bank, indicating that the locking mass is directly related to that of the S-shaped river valley. Based on the surveys of the landslides in the upper reaches of the Minjiang River, Qiao (1994) found that more than 83% of landslides occurred on the concave bank parts, 13% in the straight river valley, and only 4% on the convex bank parts. Therefore, the convex bank of the S-shaped valley provides geomorphological conditions that are favorable for the formation of the locking segment in the landslide, which is one of the main geomorphological characteristics of the locking segment.
The temporal-spatial analysis of landslide formations is an important means to predict the deformation trends of landslides and explore the mechanical mechanisms of landslides. Numerous landslide cases indicate that the existence of areas with few deformation signs in the landslide body is considered an anti-sliding section of landslides and that other areas with massive deformation signs or sliding on a large scale are considered major sliding sections of landslides. Owing to the anti-sliding effects of the locking mass in Tizicao landslide, deformation signs are nearly invisible on the southern side of the landslide front. Similar phenomena were also observed in the locking segment of the Xiaoba landslide (Lin et al., 2018).
As indicated by the monitoring data of surface and deep deformation of the Tizicao landslide, the deformation of locking segment is noticeably smaller than that of non-locking segment, and no deep formation was monitored in the locking segment. According to the simulations of slopes with rock bridges, Romer and Ferentinou (2019) concluded that the surface and deep displacement of the slopes is noticeably higher than that in the parts without rock bridges, and distinct stress concentration occurs in the slope with rock bridges. Therefore, the location of the locking segment can be determined by analyzing the monitoring data of surface and deep deformation before the overall sliding of landslides.
In the deformation process of the Tizicao landslide, cracks formed from different mechanical mechanisms can be observed on the landslide surface, including tensile cracks at the rear, lateral shear cracks, and bulging-induced and compressive cracks in the front. Owing to the anti-sliding effects of the locking segment, massive bulging-induced cracks and a small number of compressive cracks formed at the rear of the locking mass in the Tizicao landslide. Such a phenomenon has also been observed in the Dongmiaojia landslide (Xu et al. 2014).
The broken degree of rock masses changes during the deformation of landslides. Kvapil and Clews (1979) divided a rocky slope into three zones from the rear to front of a landslide, namely the active zone, transition zone, and passive zone. Among them, the transition zone is the part where rock masses are initially damaged, crushed, and penetrated by cracks. Therefore, the rock masses in this zone are far more broken than those in the active and passive zones. For the Tizicao landslide, the locking segment lies in the passive zone, and thus the rock masses in the locking segment maintain intact and their cores have higher RQD values (Fig. 15), which are also one of the characteristics of the rock masses in the locking segment.

Conclusions
To identify the locking segment of the Tizicao landslide, the location and area of the locking segment are analyzed through geomorphological identification and spatial-temporal deformation identification. The identification results are interpreted through field investigation methods, including the monitoring of surface and deep displacement, geological borehole drilling, and oblique photography using drones. Furthermore, the different developmental characteristics of the surface displacement, deep displacement, surface cracks, and sliding zone soil between locking and non-locking segments were analyzed based on the field investigation results. The following conclusions were drawn: (1) The convex bank of the S-shaped valley provides ideal geomorphological conditions for the formation of the locking segment in the landslide, and it is one of the main geomorphological characteristics of the locking segment. The area with few deformation signs in the landslide body is another characteristic of the locking segment in the Tizicao landslide.
(2) The locking segment of the Tizicao landslide lies at the southern slope toe and covers an area of approximately 4.69 × 10 4 m 2 , which accounts for 15.2% of the total area of the landslide. (3) The surface displacement in the locking segment is the minimum throughout the whole landslide, and it is much less than the surface displacement in the non-locking segment. The deep displacement in the locking segment is zero on the sliding surface, indicating that the deep rock masses in the locking segment have not slid. Separate compressive cracks develop in the locking segment, and bulging-induced cracks usually develop at the rear of the locking segment. The bulging-induced and compressive cracks (especially separate compressive cracks) serve as essential indicators for identifying and positioning the locking mass and determining the geometrical morphology of the locking mass. No sliding zone soil develops in borehole cores of the locking segment, while scratches and mirror surfaces of sliding zone soil can be observed in borehole cores of non-locking segments. In the locking segment, rock masses remain relatively intact and the RQD values of the borehole cores are much higher compared to non-locking segments.