Failure mechanism and stability analysis of bank slope deformation under the synergistic effect of heavy rainfall and blasting vibration

Underwater blasting technique is often adopted to clear away the obstructed reef and dredge-cut in the mountainous waterway dredging engineering. However, the vibration of the underwater blasting and excavation of the riverbed has a certain influence on the stability of bank slopes. Combined with the partial bank collapse phenomenon in the waterway regulation project between two dams in the Three Gorges reservoir area, the change of pore water pressure of unsaturated soil under heavy rainfall and the increase of permanent displacement of soil under blasting vibration in the reservoir area are discussed. This paper explains the weakening effect of rainfall infiltration and blasting vibration on the stability of the bank slope. By studying the law of slope failure deformation under the separate action and coupling action, it is found that rainfall infiltration promotes the increase of slope permanent displacement and the blasting vibration enlarges the infiltration area of the soil, which demonstrated the accelerated failure mechanism of the rainfall-blasting slope.


Introduction
With the vigorous development of inland waterway construction in China, underwater reef blasting engineering presents a trend of large volume, large dosage and long period. However, underwater drilling and blasting will have adverse effects on the surrounding environment. When there is a potential landslide in the reef blasting area and heavy rainfall occurs within the construction period, underwater drilling and blasting can easily induce a landslide which poses a great threat to the stability of the reservoir bank.
Scholars around the word have done a lot of work on the evaluation of slope stability in the Three Gorges area . Many common reservoir landslides are caused by factors such as seasonal rainfall (Li et al. 2010;Wu et al. 2010) and changing reservoir water levels (Yin et al. 2016). The frequent microseismic events after the Three Gorges Dam impoundment are also one of the key factors leading to the dynamic instability of the bank slope (Li et al. 2016;Yang et al. 2020). Hence, when studying the bank slope stability of the reservoir, the coupling effect of multiple factors should be established simultaneously to avoid the analysis from a single aspect.
In traditional analysis of mine slope induced by the blasting vibration, the studies of dynamic response mainly involved in different conditions, but not enough in the impact of rainfall infiltration and underwater blasting (Wu et al. 2020;Jiang et al. 2018;Peng et al. 2019;Liu et al. 2018a, b). In analysis of the slope stability induced by the rainfall, the rainfall characteristics (quantity, intensity and distribution) (Tang et al. 2015a, b;Liu et al. 2018a, b) and reservoir water level fluctuation Hu et al. 2012;Yang et al. 2017;Zhao et al. 2017.) are considered more, while the disturbing factors of underwater blasting excavation is considered less Miao et al. 2018;Tang et al. 2015a, b;Li et al. 2018). With high intensification of human activities, landslides in mining and bank region have occurred frequently in recent years (Xu and Huang 2013;Yin 2013;Ma and Hu 2016). Blasting induced seismic waves as well as heavy rainfall have a damage effect on high-steep slopes or excavation surfaces, especially in joint fissure development and an important effect on rock slope stability (Li et al. 2006;Mikos et al. 2004;Panek et al. 2011). Both rainfall infiltration and blasting vibration degradation will weaken the strength of rock and soil mass (Ma et al. 2018;Song et al. 2016). However, there are still few studies on the effects of rainfall and blasting. So, it's an urgent problem in the geotechnical field to study the catastrophic model and stability analysis of bank slope under the synergistic effect of heavy rainfall and blasting in waterway dredging engineering.
Based on the investigation and numerical simulation of local bank collapse points in the dredged area of the waterway, the stability of bank slope and failure modes of bank collapse were analyzed in this paper, which can provide a theoretical basis for the protection designing of bank collapse and control technology of underwater drilling and blasting in the heavy rainfall period.
2 Slope geological environmental conditions

Engineering geological conditions
The study area is located at 14.8 km downstream of the Three Gorges dam site, as shown in Fig. 1, which is cut by the Yangtze River. The topography along the bank of Shaijingping is changeable. Owing to the Three Gorges special highway and provincial S334 road construction, the phenomenon of the slope artificially reconstructed is obvious. The soil of the bank slope is mixed with a lot of artificial masonries retaining wall roadbed. The bank slope presents the ''finger-like'' landform between ridges and valleys. The outcrops in this region are Sinian crystalline granite and quaternary artificial deposits. There are no large fault zones. Small faults are not well developed, too. The seismic intensity is VI degree. The seasonal gullies on the ground surface are well developed in the region. The groundwater level changes obviously with rainfall and drains to the low-lying belt such as the Yangtze River nearby or supplies the weathering fissure water of the crystalline rock below. According to the investigation in 2017, the slope of this section was completely cut off due to bank collapse, which hindered the traffic and waterway operation.

Features of local bank collapse points
Affected by the water soaking, wave erosion, rainfall, and the effects of dredging and blasting works, in the middle part of the reef blast point LT7, the steep soil bank slope collapsed and deformed. The collapsed point is 40 * 50 m away from the S334 and S58 Expressway as the crow files and its plane morphology is lingual. The main collapse direction is 205°. The vertical length is more than 15 m. The bank caving width is 10 * 15 m and the area is about 45 m 2 . The thickness is 3.0 * 5.0 m and the volume is about 180 m 3 . The upper layer of the collapsed section has a loose structure of artificial backfill rock and sand. The mechanic strength of the underlying fully-strongly weathered granite is low and its resistance to erosion and denudation is poor. The water sensitivity of upper and lower soil mass is strong. The inhomogeneous binary structure of the granite under the special surface deep overburden provides the material and space conditions for the bank collapse.
3 Simulation analysis of slope stability

Model and material properties
According to the survey data and geological engineering profile of the bank collapse area (Fig. 2), a typical section B-B' was selected, as shown in Fig. 3. The slope generalization simulation model was established by Geo-studio software and the geometric dimensions are shown in Fig. 4.
The main rock and soil mechanical parameters shown in Table 1; Fig. 4 are synthetically determined on the premise of engineering geomechanical analysis (Kiran et al. 2016.) and by comparing the rock and soil mass test parameters with the same geological conditions in adjacent areas (Zheng et al. 2018;Liu et al. 2005). In the table, c is the unit weight, E is elasticity modulus, v is poisson ratio, Sr is saturability, c is cohension, u is internal friction angle, K sat is saturation permeability coefficient.

Rainfall and blasting boundaries
According to Yichang weather station's more than 20 years of statistics, the average annual rainfall was 1192.70 mm and 70 % of the annual rainfall was concentrated in June * August. Figure 5 shows the histogram of rainfall in Yichang from 8 June to 8 July, which was the heaviest rainfall season in history. At the same time, on 27 Jun, the 24-hour rainfall of 235 mm was the largest daily precipitation in the past 30 years. Therefore, the precipitation data measured from 27 Jun to 2 July in this area were selected for analysis. Based on the monitoring data, the designed rainfall intensity is 60 mm/d, and last for six days in the SEEP/ W module. The left side and bottom of the slope are set as impervious boundaries. The top and slope are rainfall infiltration boundaries. The water head boundary at the front edge of the slopes is determined according to the reservoir water level, and the stable groundwater at the stable groundwater at the back edge is the constant water head boundary.
During the rainfall period, the water depth of the drilling blasting construction area is 20 m, and the average blasting rock layer is 15 m. The 2# rock emulsion explosive of U70 mm is selected, and the maximum single-stage charge is 40 kg. A large number of seismic damages shows that horizontal seismic action is the main cause of slope failure. Hence, the field blasting vibration data were collected. Figure 6 shows the time-history curve of blasting seismic waves in the horizontal direction and was applied for the QUAKE/W module. In the Quake/w module, a fixed constraint is applied around the bedrock, and the time-history analysis method is adopted after the horizontal blasting vibration accelerate curve is input. So that, the dynamic response of each point of the slope under blasting vibration can be obtained.

Working condition and evaluation
According to the location conditions of the reservoir bank, the maximum variation of water level in this area is within 10 m, and the most likely factors leading to deformation and failure of the bank slope are rainfall and construction vibration. Infiltration of rainfall can change the saturation of the soil and the pore water pressure. The pore water pressure at different positions on the slope has been monitored and found to gradually increased as the rainfall continues. As seen in Fig. 7, the pore water pressure between 80 m * 90 m elevation increases sharply at the beginning of rainfall. After the rainfall began 1 h, it increased sharply from -195 kPa to -125 kPa. With the continuous rainfall, the growth rate of pore water pressure gradually slows down and finally reached -50 kPa. In the range of 70 m * 80 m elevation, the pore water pressure is not significant with the continuous change of rainfall.
Therefore, the effect of rainfall infiltration decreases gradually from the surface of the slope to the depth of the slope with continuous rainfall. Figure 8 shows the variation trend of pore water pressure at the bottom of the slope in 60 m * 70 m elevation. The pore water pressure at the bottom of the slope has an approximately linear relationship.
The pore water pressure of the 65 m original characteristic water level finally reached 50 kPa at the end of rainfall, which suggested that the water level rose and the soil gradually changed from unsaturated state to saturated state.
In the process of rainfall infiltration, the increased pore water pressure of soil leads to the decrease of matric suction of soil mass, which is finally reflected in the weakening soil strength and safety factor. It can be seen from Fig. 9 that the slope safety factor decreases continuously with the continuous rainfall, from 1.136 to 1.016. When the rainfall ended, the safety factor will continue to fall to some extent, but not by much, about 6 %. That means the minimum safety factor is not the end of the rainfall period, there is a certain lag phenomenon. According to the Table 2, it can be inferred that the bank slope is in an understable state.

Slope stability under blasting action
As shown in Fig. 10, under the blast load, the instantaneous instability of the slope does not necessarily lead to direct failure of the slope. The safety factor of slope fluctuates with different vibration time. The safety factor of the slope at the beginning of blasting is 1.136. After the blasting, the safety factor is reduced to 1.076. The horizontal displacement of the slope top monitor point M1 which was marked in Fig. 3, was analyzed. At 0.602 s, the slope safety factor reaches the minimum value of 0.614, but the slope displacement value at the corresponding time does not reach the maximum value. Therefore, the Newmark method is needed to further calculate the permanent displacement and evaluate the bank slope stability under blasting vibration ). From Fig. 11, it can be found that the change of permanent displacement in the early stage of blasting vibration is not obvious. After the 0.4 s, the permanent displacement surges to 0.12 cm with the rapid change  of acceleration, which is a medium-dangerous landslide.

Safety and stability of slope under the combined action of rainfall and blasting
Rainfall typically lasts for hours or even days, while an explosion typically occurs within seconds. In order to explore the stability of slope under the action of blasting vibration and heavy rainfall, blasting seismic waves were applied at different rainfall duration points (1 h, 1 d, 3 d, 6 d) to observe the slope changes as shown in Fig. 12.
When there is no rainfall in the early stage, the permanent displacement of the slope under blasting vibration is only 0.12 cm. After 1 h of rainfall, the blasting vibration load was applied and the permanent displacement of the slope reached 1.24 cm. It reflects that the rainfall infiltration intensifies the blasting vibration to the failure of the slope. When the blasting load is applied after the duration of rainfall 1 d, 3 d and 6 d, the permanent displacement of the slope is respectively 2.7 cm, 3.56 and 12.9 cm. That is to say, the longer the duration of rainfall in the early stage, the greater the permanent displacement of slope caused by blasting vibration. In short, the weakening effect of rainfall infiltration and blasting vibration on slope stability is synergistic promotion rather than simple superposition. 5 failure mechanism of bank slope deformation under rainfall-blasting action

Rainfall infiltration stage
Rainfall infiltration first changes the water content of soil on the slope. Under the action of heavy rainfall, the soil is rapidly reached the saturated moisture Fig. 5 Daily and cumulative rainfall data of 2020 meiyu season in Yichang city content and formed a transient saturation region that penetrates to the foot of the slope under the action of hydraulic gradient and gravity. With the continuous rainfall, the transient saturation area of the slope foot and slope surface gradually expands to the interior of slope. During rainfall infiltration, the moisture content and negative pore water pressure of unsaturated soil increase gradually, which leads to a marked reduction in stromal suction. Especially in the position of the transient saturation zone, the matric suction drops to zero, and the adsorption capacity between soil particles is greatly reduced, which easily leads to slope deformation and failure. In addition, rainfall also reduces the shear strength of the soil and increases the self-weight of the slope. According to a large number of experimental studies (He et al. 2019), the internal friction and cohesion will decrease with the increase of water content. The increase of the self-weight of the slope will also promote the force of the slide. The above reasons are not conducive to the stability of the slope.

Blasting vibration stage
Under the influence of underwater and drilling blasting, the rock mass fissure in the blasting area develops gradually and expands to the interior. It is easy to form a sliding surface when the internal cracks of the slope are accumulated and connected. However, the soil material and dynamic response of the sliding surface   are often different. The physical and mechanical properties of soil decreased due to blasting vibration and repeated rubbing. When the vibration inertia force of blasting is consistent with the direction of the sliding surface, the thrust formed will promote the slope to accelerate sliding. In addition, the underwater steep slope formed by blasting excavation can also be regarded as the cutting process of the bank slope. All  The slope is in an understable state under the action of rainfall alone, and in a basically stable state under the condition of blasting vibration alone. But after coupling action, the slope is in an extremely unstable state, as shown in Fig. 13. The coupling effect is not only the superposition of the two kinds of actions but also the promotion effect of this process. Under the action of rainfall infiltration, the moisture content of unsaturated soil in slope increases greatly. Once subjected to blasting vibration, the pore water pressure of soil will increase rapidly and even produce excess pore water pressure. The excess pore water pressure superimposed with the pore water pressure accelerates the reduction of matrix suction and shear strength of slope. At the same time, the inertial force generated by the blasting seismic wave will increase with the increase of the slope weight under the effect of rainfall, which will weaken the stability of the slope from all aspects. In another aspect, the surface soil becomes more loosely under the blasting vibration. The soil compactness decreases and the pores between soil particles are enlarged. The blasting vibration promotes the infiltration of rainwater deeper and expands the scope of rainfall infiltration. Eventually, the bank slope accelerated to collapse and recede.

Conclusions
Using the Shaijingping landslide as an example, we analyzed the failure mechanism and stability of bank slop in adjacent dredging engineering under the (1) The process of the variation of pore water pressure under the continuous heavy rainfall infiltration is simulated based on the measured rainfall data. It reveals that the pore water pressure increases with the infiltration of rainwater and the matric suction drops as the soil water content increases. The effect of the rainfall infiltration, in general, decreases from the surface to the deep part of the slope and improves the water content of soil, which can weaken the shear strength parameters of soil and reduced the safety factor of slope.
(2) Underwater blasting excavation plays a key role in the bank collapse and failure. Under the effect of elevation amplification, the seismic wave generated by drilling blasting in deep water propagates to the top of the slope through bedrock, which promotes the looseness and dispersion of the shallow slope soil, increases the density between soil particles, and expands the scope of rainfall infiltration. Also, blasting vibration changes the structure form of the underwater bank slope, making the slope more vulnerable to instability and failure. (3) The permanent displacement of the slope under the synergetic action of heavy rainfall and blasting vibration are compared with the single action. It reveals the accelerated failure and instability mechanism of the rainfall-blasting bank slope, which is of guiding significance to the safety construction and risk control of blasting engineering near the bank slope of the reservoir during the heavy rainfall.