Based on the field investigation data of 40 large landslides in Longquan Mountain in the hilly area of central Sichuan, this paper makes a preliminary discussion on the main characteristics and prevention measures of rock landslides in the basin during this rainstorm.
1 rainstorm characteristics and geological survey in the survey area
1981during the three months from July to September, the rainstorm in the study area has the following characteristics.
(1) The rainstorm occurred in a concentrated time with high intensity, and most of the rainfall in three months fell within a few days. The first rainstorm occurred on July 13, with the maximum intensity of 262.7 mm/d; The second rainstorm occurred on August 15, and its intensity was lower than the previous one. The third rainstorm occurred around September 2, with an intensity of 278.9 mm/d.
(2) The regional differences of rainstorm distribution are significant. There were two rainstorms in Longquanshan area of Chengdu, and the torrential rain (> 200 mm/d) occurred on July 13. There were three heavy rains in Suining, but there was no heavy rain. The maximum rainfall intensity reaches199.4 mm/d; /July d 13. There were two rainstorms in Santai and Shehong, and a heavy rainstorm occurred on September 2. Even in the small area of Longquan Mountain, there are obvious differences. Heavy rain occurred in jiepai commune in July 13, and in Ping 'an commune in July 12 and June 13 for two consecutive days.
The above characteristics of rainstorm are of great significance in the discussion later in this paper.
The survey area is the distribution area of Jurassic and Zichaer continental red clastic rocks. Longquan Mountain is located on the west side of the survey area, and the mountain range is a box anticline structure. The watershed of the mountains roughly coincides with the axis of the anticline. In the core of anticline, purple sandstone of Shaximiao Formation of Middle Jurassic mixed with mudstone, purple mudstone of Suining Formation mixed with siltstone, and purple sandstone and mudstone of Penglaizhen Formation of Upper Jurassic interbedded with Cretaceous brick red sandstone and mudstone are exposed. The rest of the survey area is hilly area, where Suining Formation, Penglai Town Formation and Cretaceous strata are exposed respectively. The occurrence of strata is gentle, and the dip angle is generally less than 3. Except for the large reverse fault connecting the two wings of Longquanshan anticline and the gentle structure in hilly area, no large fault has been found in other parts, but the signs of interlayer dislocation are very common, especially at the interface between sandstone and mudstone. There are four groups of X-plane fractures nearly perpendicular to the bedding plane, and their occurrences are n10 w, N80°E, N60°W and N40°E respectively.
The slope shape in this area is obviously controlled by geological structure. The occurrence of rock strata in hilly areas and anticline axis is gentle, mostly stepped slopes. Sandstone or siltstone forms a platform, and mudstone forms a platform. In the area where the dip angle of strata on the two wings of anticline is greater than 20, a unilateral mountain slope is formed along the dip, and the topographic slope angle is almost the same as the dip angle of strata; In the opposite direction, a stepped steep slope is formed, and the steps are often composed of mudstone.
2 landslide formation mechanism and stability analysis
During the rainstorm, almost all slopes with different geological structures and shapes will have landslides, but due to different geological structures, their characteristics are also different, which can be summarized into six categories, namely, sliding-crack landslide, sliding-pressure-induced landslide, plastic-flow landslide (soil creep), sliding-crack landslide, sliding-bend landslide and landslide. Due to the limitation of space, this paper mainly discusses three kinds of landslides which are harmful and widely distributed, namely, sliding crack type, sliding pressure crack type and sliding bending type.
2. 1 push-slip landslide
2. Formation mechanism and conditions of1.1
The basic feature of this kind of landslide is that the sliding surface is nearly horizontal, and the landslide residue can be roughly divided into four parts along the sliding direction, namely, slider, subsidence zone, leading edge uplift zone and trailing edge landslide zone (Figure 1). The slide block still maintains the original structure of the rock mass, and can develop longitudinal and transverse tensile cracks, but the occurrence of the rock mass has no obvious change. Sometimes bumps and raised areas can be seen at the toe of the slope at the leading edge of the slider. If the slope is blocked by the overburden, uplift, fold or overthrust fault will occur in the overburden. The back of the slider will slide backward or bend backward along the steep crack surface, or even fall off. Compared with the position before sliding, the elevation of the top surface of the slider has no obvious change, and the slope of the connecting line between the two is close to the apparent dip angle of the strata along the sliding direction, indicating that the slider slides outward along the strata without obvious rotation. The back side of the slide block in the subsidence area is caused by the collapse of the rock mass when the slide block slides out, and some tunnels formed by tensile cracking are still preserved to a depth of more than ten meters. The settlement area may be filled with materials that slide or dump backwards and fall from the trailing edge.
The above characteristics show that the landslide originated from the sliding body sliding along the weak surface (or belt) in a very gentle or even slightly inclined slope to the outside of the slope. This sudden sliding is mainly caused by the joint action of the hydraulic pressure in the gap between the trailing edge cracks and the hydraulic pressure in the sliding surface, and its evolution process can be roughly divided into three stages (Figure 2).
2. 1. 1. 1 deformation stage (① ② in Figure 2)
The investigation data show that all the slopes with this kind of landslide have obvious deformation before sliding, and the plastic flow-tensile deformation is the main one. That is to say, the soft rock near the toe of the slope will produce plastic flow outside the slope under the action of the self-weight of the overlying rock mass, which will cause cracks on the slope along the weak zone. This tension crack will expand upward from the weak zone and gradually connect with the tension crack on the slope, creating conditions for slope water to penetrate into the slope. In general rainfall, the water level is difficult to rise rapidly due to fissure drainage, so it can not cause strong interstitial water pressure to make the sliding body start to slide out, and can only promote the development of deformation.
2. 1. 1.2 slider starting and braking stage (③ in Figure 2)
When the rainstorm is enough to make the water filling height in the trailing edge tensile crack reach the critical height hcr, the slider will start. When the formation is horizontal, hcr can be obtained according to the following formula:
Surface crust and ergonomics
Where: w is the weight of the slider, l is the length of the sliding surface, and φ is the internal friction angle of the sliding surface.
Figure 1 Horizontal Push Landslide Profile
Once the slider is started, the water-filled water column will also decrease rapidly after the rear edge crack is opened. At the same time, the clearance water thrust and jacking force at the sliding surface will also decrease sharply, and the slider will brake itself because of losing the clearance water pressure. Therefore, the time from the beginning of the landslide to the braking is very short, usually only a few seconds to more than ten seconds.
Because the jacking force of the soil layer at the toe of the landslide and its front edge is much lower than that at the back side. Therefore, this area actually plays an anti-sliding role, so it is often pushed into uplift, expansion, fold bending and even thrust faults, and buildings in this area are often pulled apart, collapsed or buried (such as Honghua landslide).
2. 1. 1.3 Settlement zone filling and sliding body compaction stage (④ in Figure 2)
After the slider stops sliding, the rear side of the slider slides backward and topples, and the rear edge collapses and slides, so that the subsidence area is filled, and the slider gradually compacts and tends to be stable.
This kind of landslide is suitable for gently occurring layered slopes. Usually, the sliding body is composed of sandstone or siltstone and slides along the underlying mudstone interface. However, it is worth noting that there is the opposite situation in Suining Formation, that is, the sliding block is composed of mudstone and the sliding layer is siltstone, which is probably related to the development of primary cracks and structural cracks in thick massive mudstone rich in calcium and gypsum after unloading, which is more water-rich than siltstone interlayer.
Geomorphologically, the most favorable places for this kind of landslide are ridges, solitary bags and mountain mouths, where the rock mass is relaxed and unloading cracks are developed, which often undergo a long period of deformation, and three sides are more conducive to sliding out.
2. 1.2 stability evaluation
Because the occurrence of this landslide is mainly due to the action of interstitial water pressure, the trailing edge cracks and collapses after sliding. Not only the water permeability is obviously enhanced, but also the middle part of the subsidence area is high and the two ends are low, which is beneficial to drainage. Therefore, in the later rainstorm, it is difficult to make the height of pore water pressure reach the critical value again, so this kind of landslide is stable as a whole. The Laizilong ancient landslide in Zhongjiang County (Figure 3) and the Hengshan ancient landslide in Suining County (occurred in June, 1976) showed no signs of overall sliding during this rainstorm, which is strong evidence.
Fig. 2 landslide evolution process
The following problems should be paid attention to in this kind of landslide distribution area:
(1) With the increase of the off-slope angle of the slip surface, its overall stability will gradually decrease and its response to rainfall will become more sensitive.
(2) When the settlement zone is filled with silt, once the fill is saturated and softened during the rainstorm, plastic flow will occur, and under the action of interstitial water pressure, the slider can be pushed to slide again. The Dengjiaci landslide in Zhongjiang County is a typical example. During the rainstorm on September 2, the slide block was pushed by the residual soil climbing on the upward slope and slowly slid to one side for 2.2m, resulting in the collapse of the hidden arch of the channel (Figure 3).
Fig. 3 Longitudinal section of Dengjiaci landslide
(3) Local variation of sliding residual. For example, in the Laiwulong landslide, the residual body may still collapse locally in the Laiwulong area, and the dumping of the rear edge of the residual body may also cause local collapse. The sliding body absorbs water at its front edge, which softens the soil in the groundwater overflow area and leads to soil creep.
2.2 Tensile landslide induced by sliding pressure
2.2. 1 Formation conditions and mechanism
The formation conditions of this kind of landslide are similar to the former, and generally occur in layered slopes with gentle strata. The appearance of the landslide presents a number of anti-slope steps (Figure 4A), and the occurrence of strata in the sliding body is obviously reversed, indicating that it rotates during the sliding process and disintegrates into a number of secondary sliding bodies, and the sliding speed of the sliding bodies is generally fast, and tends to be stable in a short time (several minutes to several hours).
Each divided slider in the sliding body has a steep circular sliding surface at its rear edge (Figure 4B), so the gravity component of the sliding body still plays an important role in the downward force of the sliding body. The softening of the sliding surface and the interstitial water pressure caused by heavy rain are the inducing factors of the landslide. Circular sliding surface is developed by the progressive failure of tensile cracks caused by sliding pressure, and its evolution process can be roughly divided into three stages.
Fig. 4 Cross-sectional view of tensile landslide induced by sliding pressure
Fig. 5 Slip evolution diagram of tensile crack caused by slip pressure
2.2. 1. 1 unloading rebound sliding stage (figure 5a)
In the process of slope formation, the slope bounces and slides towards the airport, resulting in tensile cracks perpendicular to the sliding surface.
2.2. 1.2 Tensile fracture propagation stage caused by compression (Figures 5b and c)
Under the stress of the slope, with the development of deformation, the compression-induced tensile crack surface expands from bottom to top, forming a steep step surface, and the slope rotates slightly, but the whole slope is still in a stable fracture stage.
2.2. 1.3 Step plane infiltration stage (Figure 5d)
The step surface becomes the stress concentration area, and the chimera at the corner of steep slope and gentle slope is cut, rolled and expanded one by one, and the slope begins to rotate obviously, which leads to the slope uplift. When the tensile crack at the trailing edge is closed, the deformation enters the progressive failure stage. Once the chimera is completely cut, landslides will inevitably occur along the through surface with the participation of the rapidly rising interstitial water pressure during the rainstorm.
According to the above analysis, it can be considered that the slopes where this kind of landslide may occur during the rainstorm should be those slopes where the tensile crack deformation induced by sliding pressure has progressed to a considerable extent (the rear edge step surface has been penetrated). A crack with a width of 20cm and a length of 30m was found in the Shanshinian landslide in Longquan 10 years ago, which proved that the slope had undergone obvious deformation and transformation before the landslide occurred.
Fig. 6 Vertical profile of Wudong landslide
Stability evaluation
After the landslide began, with the dispersion of groundwater, the decrease of pore water pressure and the decrease of slip potential energy, the landslide gradually stabilized. At this time, the average slope of the sliding body has become very gentle, but due to the steep arc of the sliding surface, there is still the possibility of local or even whole sliding in the late rainstorm. For example, the stability coefficient F= 1.48, regardless of pore water pressure, the sliding body is stable; If the jacking and horizontal pushing of interstitial water pressure are considered, its F=0.87, that is, it will still slide during the rainstorm. The resurrection of Wudong landslide in Shehong county (Figure 6) and Simaoya landslide in Santai county during this rainstorm is strong evidence. Therefore, buildings and residential areas should be evacuated within the possible impact of this landslide.
2.3 Slip-bend landslide
2.3. 1 Formation conditions and mechanism
This kind of landslide is mainly developed on a single-sided hillside with a dip angle of more than 20 in the wing of Longquanshan anticline.
Landslides can also be generally divided into sliding blocks, trailing rift subsidence zones and pre-uplift fold zones (Figure 7), and their characteristics are very close to those of sliding rift landslides. However, the front uplift and fold belt is wide and the fold is strong, and there are often pits on the back side of the uplift mound. After the landslide begins, the sliding speed is slow and lasts for a long time, usually more than half a day. For example, the landslide of Shunhe No.4 Team lasted for 24 hours, and the sliding distance was only about 8 meters.
When the sliding surface is straight and the toe of the slope is not empty, the leading edge uplift zone mostly occurs near the toe of the slope. It can also occur in the part where the half slope is washed by gully and the sliding surface is shallow; When the sliding surface is spoon-shaped, the sliding surface can be empty at the foot of the slope, and a fold belt appears where the sliding surface changes from steep to slow.
Mechanical mechanism of rock fold on slope. Euler theory can be used for analysis (Figure 8).
Let φ be the internal friction angle of the sliding surface, and the critical load of rock formation folding is:
Surface crust and ergonomics
or
Surface crust and ergonomics
According to Euler's theory:
Surface crust and ergonomics
Substituting (2) into (1) can give l:
Surface crust and ergonomics
Where: γ is the bulk density of rock stratum, e is the elastic modulus of rock stratum, and other symbols are shown in Figure 8.
Assuming that the elastic modulus of mudstone is 5000 kg/cm2 and the bulk density is 2.5, the critical stress of rock instability of arch back landslide is 50.77kg/cm2. The landslide rate of Heibai forest is 48.89 kg/cm2.
Under the action of interstitial water pressure, the expression of sliding thrust when the sliding body starts is:
Surface crust and ergonomics
Based on this, it is calculated that σ = 5. 15 kg/cm2 at the beginning of the arch back landslide and σ = 6.27 kg/cm2 at the beginning of the Heibailin landslide. They are 6 ~ 10 times smaller than the critical stress required for rock buckling.
Fig. 7 Longitudinal section of slip-curved landslide
According to formula (3), the critical length of rock instability under this condition is calculated, which is 260m at the arch back and 294m at Heibailin, while the actual lengths of the two landslides are 90m and 1 10m respectively, which are about 2.5 ~ 3 times smaller than the calculated values.
From the above analysis, it can be seen that the sliding thrust during sliding is not enough to make the rock stratum buckle, so it can be considered that the rock stratum has undergone sliding-bending deformation before the landslide (the signs of this deformation have been found in the front of the Heibailin landslide), and it developed into a landslide during the rainstorm due to the softening of the sliding surface, the promotion and jacking of interstitial water pressure and other factors. Its evolution process can be roughly divided into three stages (Figure 9).
Fig. 8 mechanical mechanism analysis of slope rock fold
2.3. 1. 1 creep-slight bending stage (Figure 9a)
The data show that this kind of landslide mostly occurs in the layered slope outside the inclined slope, and the inclination angle of the weak surface is greater than the residual friction angle of this surface. Under the long-term action of slope stress, rock creep occurs, and uplift and bending occur near the foot of slope.
2.3. 1.2 sliding-strong bending stage (fig. 9b)
Under the action of strong pore water pressure and other triggering factors, the strata slide along the weak surface and the rear edge cracks; Strong bending uplift occurred in the front edge, and X-shaped cross-section dislocation appeared. Among them, the gradual development of the gentle dip angle is a sliding section. Due to the strong expansion of the curved part, the slope surface is obviously raised, and the sliding of rock mass is intensified, and local caving or sliding often occurs. This "load reduction" near the toe of the slope further promotes the development of deep deformation.
Fig. 9 Evolution Diagram of Slip-bend Landslide
2.3. 1.3 continuous deformation-sliding stage (fig. 9c)
Due to the continuous development of deformation, the sliding surface runs through and develops into a landslide.
The situation is different for spoon-shaped or armchair-shaped sliding surfaces. The strong bending part occurs at the turning point of the sliding surface and slides along the original weak surface without forming a tangent plane.
Stability evaluation
Once this kind of landslide starts, with the aggravation of the front fold, the rock stratum is crushed, the sliding resistance is also reduced, and the stability of the whole slope deteriorates sharply. But at the same time, due to the further development of cracks, groundwater is rapidly dispersed, pore water pressure is rapidly reduced, and sliding thrust is also sharply reduced. Therefore, some sliding bodies have just started to have local sections, and some even have sharp front folds without sections, and then gradually stabilize. Some sliding bodies with spoon-shaped sliding surfaces have not been penetrated, and sliding stops when the leading edge of the sliding body can't see obvious displacement. These characteristics show that local deformation or even overall sliding may still occur in the later period of rainstorm. The Moziwan landslide in Zhongjiang County on September 3 is a typical example of this kind of landslide revival, which destroyed nearly 600 meters of rivers and more than 300 houses. Therefore, it is suggested to evacuate the buildings located in this landslide and its front edge.
3 Some laws of landslide occurrence and development and slope stability prediction in rainstorm period
3. 1 Some basic laws
3. 1. 1 Relationship between landslide and rainstorm characteristics
(1) According to its degree, the influence of rainstorm on landslide can be divided into two situations: ① Start-up type-the sliding of landslide body is mainly caused by rainstorm, such as slip-pull type landslide; ② Induced type —— The interstitial water pressure caused by rainstorm only induces the softening of the sliding surface, and the sliding component of the sliding body itself still plays an important role in the sliding process, such as sliding pressure-induced cracking, sliding and bending landslide, etc.
(2) From the sensitivity of landslides to rainstorm, these three types of landslides are all sensitive and respond quickly to rainfall, and they will slide as long as they reach the critical rainstorm intensity required by landslides.
(3) About the critical rainstorm intensity. In Longquanshan area of Chengdu, most of the landslides occurred on July 13, when the rainstorm intensity was 262.7 mm/d. Although there was a rainstorm with the intensity of 145.2mm/d/d in some areas on July 12, no landslides occurred. Landslides in Santai County mainly occurred on September 2, with the rainstorm intensity reaching 278.9 mm/d, and on July 13, the rainstorm intensity in this area reached 160mm/d/d, without a large number of landslides. 13 On July 3, the rainstorm intensity in Suining County reached 199.4mm/d/d, and basically no bedrock landslide occurred. On July 3, the rainstorm intensity in Rongchang County was 253 mm/d, and 33 landslides occurred. According to the above situation and referring to relevant information at home and abroad, the critical rainstorm intensity of rock mass landslide can be preliminarily set at 250 mm/d.
3. 1.2 Relationship between landslide formation and geological environment
(1) The weak structural plane that develops into sliding surface is mainly the contact surface between mudstone and sandstone or siltstone in red bed. The fissures in the overlying strata are well developed and have strong water permeability, which is beneficial to the formation of higher pore water pressure under rainfall infiltration.
(2) The types of landslides are obviously controlled by the occurrence of rock strata. In the axial part of Longquanshan anticline and the central hilly area, the dip angle of rock stratum is less than 10, and the main types of landslides are slip cracking and slip pressure cracking. In the two wings of Longquan Mountain anticline, where the dip angle of strata is greater than 20, slip-bend landslides mainly develop.
(3) The relationship between the distribution of different types of landslides and landforms. Sliding-crack landslides and sliding-pressure-induced landslides generally develop in mountain mouths or isolated packages along ridges or watersheds; Slip-bending landslide with spoon-shaped sliding surface often occurs near the valley, which is related to the open air of weak structural surface caused by the undercut of the valley. For a slippery slope with smooth surface, the trailing edge can start from the ridge and the exit is close to the bottom.
3. 1.3 Relationship between landslide formation and human factors
(1) Most of the diversion channels along the mountain are excavated in weathered rocks with developed cracks, and some of them are not treated with seepage control. During the rainstorm, slope water quickly gathered in the ditch, which provided abundant water for groundwater, and some even filled the ditch with a certain head, causing landslides. For example, the landslides in Baihe Temple, Laohuzui, Lion Mountain and Chaiwanya in Shehong County are all along the bottom of the ditch.
(2) In some places, stone is mined, leaving deep quarries in the hillside rocks, where a large number of landscapes gather during the rainstorm, which promotes the occurrence of landslides, such as the landslide in Santai Chinese Medicine School.
3.2 Slope stability prediction
After 198 1 year rainstorm, most of the obvious deformation bodies on the slope have developed into landslides, and hidden dangers have been exposed. Therefore, it can be considered that in the next few years, if there is no more intense rainstorm, such widespread and massive landslides will not happen again. However, we should pay attention to the following aspects.
(1) Due to the regional differences in rainstorm distribution, a large number of landslides may still occur in those places where the annual rainstorm intensity exceeds 250 mm/d198/kloc-0.
(2) In the area hit by rainstorm in 198 1 year, the deformed parts that have not developed into landslides (such as slip-bend deformed bodies) may continue to develop into landslides in the future rainstorm period, and should be monitored.
(3) The stability of the above-mentioned different types of landslides will be different in the future rainstorm period, and the treatment and prevention work should focus on the tension cracks and slip-bend landslides induced by slip pressure.
3.3 Prevention and control measures
In order to improve the stability of landslide, we should fill up the rift zone at the rear edge, repair the drainage system, eliminate the water pit on the landslide, and take effective seepage control measures for the diversion channel to prevent surface water from infiltrating. For slip-bend landslide, in addition to the above necessary measures, large excavation in strong fold zone should be avoided. The revival of Moziwan landslide during the rainstorm on September 2 is related to the excavation of ditches along its fold zone. If it passes through the platform in front of the fold belt in the form of a light aqueduct, its stability conditions will not be destroyed.
In addition, we should strengthen group monitoring, group prevention and meteorological forecast (especially the rainstorm forecast with intensity greater than 250 mm/d) to nip in the bud and reduce the possible losses caused by landslide disasters.