2.1. Engineering Situation and Disaster Overview
The Yonglian Tunnel, situated in Jiangxi Province, China (
Figure 1), is a split tunnel spanning 2500 m in length. It traverses a mountainous and hilly region characterized by rolling topography. The tunnel area is known for its subtropical monsoon climate, resulting in groundwater with an average depth of approximately 50 m. Notably, the tunnel intersects a significant fault known as the F2 fault, which spans 520 m with an inclination of 84 degrees, from K91 + 350 to K91 + 368.
The F2 fault, however, has been significantly impacted by long-term weathering and tectonic activity, resulting in extensive fragmentation and weathering. It contains a substantial amount of mylonite, breccia, and fault mud, with a rock quality designation (RQD) of less than 5%. These geological characteristics indicate the poor quality of the rock mass in the F2 fault zone, posing formidable challenges for tunnel construction in this area.
During the preliminary geological investigation, there was a severe lack of accurate information regarding the F2 fault, and the geological report indicated that the intersection length between the F2 fault and the tunnel was only 5 m, far less than the actual length of 18 m. Additionally, the strength of the surrounding rock did not match the actual excavation conditions. As a result, more cautious excavation and support methods were not adopted during construction, nor were more precise disaster control methods employed.
When the tunnel excavation neared the F2 fault using a three-step excavation method, a large number of geological hazards occurred, resulting in the suspension of tunnel construction for a period of 2 years and causing substantial economic losses. Initially, there were four small-scale collapses that occurred intermittently, resulting in deformations of the surrounding rock and cracks in the lining. In response, treatment measures such as pipe roof grouting and small pipe grouting were implemented to mitigate the gushing water and reinforce the broken surrounding rocks. Subsequently, tunnel construction resumed.
However, just seven days later, the first mud inrush disaster occurred suddenly, with substantial amounts of mud–water mixtures inundating the tunnel. Although construction workers had detected significant deformation of the tunnel surrounding rocks in advance and identified the risk of instability, the mud inrush disaster occurred too violently and quickly for them to take measures to prevent it. This was followed by a total of 15 enormous mud inrush disasters that occurred successively within a span of four months (as shown in
Table 1). It was estimated that approximately 53,000 m
3 of mud–water mixture gushed into the tunnel in total, resulting in nearly 30 m of the constructed tunnel being inundated. These disasters triggered a series of engineering accidents, including equipment damages, large-scale collapses, and construction interruptions [
19].
Furthermore, a collapse pit with an area of about 2000 m
2 emerged at the mountain peak after the continuous disasters (as shown in
Figure 2), resulting in severe vegetation destruction and soil erosion. The engineering geological schematic diagram is shown in
Figure 3 [
3].
2.2. Disaster Causes
The mud inrush disasters were a dynamic destruction hazard caused by excavation disturbances. Based on the results of the engineering geological investigation, the presence of ample groundwater and broken surrounding rocks in the F2 fault created favorable conditions for these disasters.
First and foremost, the characteristics of the F2 fault in the mountain surface were large depressions whose terrain was obviously lower than that of those in other places. Hence, a great deal of rainwater or groundwater would gather in the area. Subsequently, there were a large number of fissures among the loose and broken surrounding rocks in the F2 fault, which provided the groundwater with convenient migration pathways and plentiful storage space. As a consequence, the accumulated water could flow down the migration pathways into the mountain interior easily and was stored in the fissures. It is noteworthy that the stored groundwater would develop into confined water gradually as time went on, and a great deal of potential energy was accumulated in the groundwater and nearby surrounding rocks. Furthermore, when the groundwater began to flow, the weakened and unstable surrounding rocks in the F2 fault were unable to withstand the groundwater erosion, resulting in easy erosion and disintegration. Last but not least, the fault mud typically contained various hydrophilic clay minerals such as montmorillonite and illite, meaning that the surrounding rocks in the F2 fault would be very easy to swell and disintegrate, turning into weakened and deformable rocks with worse physical and mechanical properties under the effects of groundwater.
Although all varieties of natural conditions had been available, mud inrush disasters rarely occurred before excavations, as the majority of groundwater remained stable under the balance of water pressure, rock stress, and tectonic stress. However, during tunnel excavation, the previously stable equilibrium state between water and rock was disrupted. This led to an increasing number of fissures extending towards the tunnel face due to stress distribution, resulting in active and accelerated groundwater seepage. Additionally, the broken surrounding rocks in the F2 fault were continuously flushed by the active groundwater, causing the migration pathways to gradually enlarge.
However, due to inaccurate geological surveys in the early stages, a correct understanding of the danger level of the F2 fault was absent. During the construction phase, conventional three-step excavation methods and simple stability monitoring measures were employed, without using methods such as grouting to reinforce the surrounding rock. Subsequently, the expanded migration pathways led to persistent increases in seepage velocity and pressure, intensifying the loss of surrounding rocks. As the pathway expansions and particle migrations aggravated, the tunnel face eventually connected to groundwater. Unable to resist the energy released by the groundwater and the surrounding rocks, the frail tunnel face gave way, leading to mud–water mixtures instantly pouring into the tunnel. Furthermore, due to the sustained interrelationship between the seepage field and stress field, the mud–water mixtures continued to flow from the F2 fault without stopping, resulting in repeated mud inrush disasters.
2.3. Disaster-Causing Factors of Mud Inrush Disasters
Taking into account the underlying causes of mud inrush disasters, it can be concluded that groundwater serves as a significant driving force, while the fractured surrounding rocks provide migration pathways for the groundwater. Additionally, the weakened rocks in the fault fracture zone act as material sources for these disasters. Moreover, excavation disturbances are unequivocally identified as direct triggers of such disasters. Therefore, the factors that contribute to the occurrence of mud inrush disasters are closely intertwined with groundwater dynamics, characteristics of the surrounding rocks, and tunnel construction activities. Based on the foregoing analyses, the disaster-causing factors were categorized as follows.
2.3.1. Geological Factors
The low-lying areas, such as gullies and hollows, provide convenient sites for the aggregation of rainwater, which serves as the primary recharge source for groundwater. As a result, there exists a significant linkage between the terrain characteristics and the occurrence of mud inrush disasters.
- 2.
Physical and mechanical properties of fault-surrounding rocks
The fissures and voids among the fault-surrounding rocks play a crucial role in providing migration pathways and storage space for groundwater, and their characteristics such as porosity, density, and pore structure are closely related to the stability of the fault zone. It can be inferred that fault-surrounding rocks with high porosity and loose structure are conducive to the migration and storage of groundwater, increasing the likelihood of mud inrush disasters. Furthermore, the strength characteristics of fault-surrounding rocks, including compression strength, shear strength, and elastic modulus, are critical in determining their stability under dynamic groundwater conditions. High-strength rocks are less likely to lose stability easily and are more resistant to erosion by flowing groundwater due to their passable strength and stability. These rocks can effectively withstand the potential energy released by groundwater, reducing the risk of mud inrush disasters. In contrast, fault-surrounding rocks with poor strength characteristics are highly prone to causing disasters. Predictably, the occurrence of mud inrush disasters is closely correlated with the strength and stability of fault-surrounding rocks, as well as their porosity and structure.
- 3.
Fault scale
The fault mud, gravel soil, and tiny breccia that originate from the fault zone serve as the main source of the gushing mud during mud inrush disasters. The severity of these disasters is closely related to the scale of the fault. It can be deduced that larger faults would provide more fillers, thus exacerbating the disasters. Furthermore, larger faults are likely to have more migration pathways and storage space, which can further contribute to the occurrence and severity of mud inrush disasters.
2.3.2. Hydrologic Factors
As mentioned previously, the groundwater that drives the mud inrush disasters is primarily supplied by rainfall, which is closely linked to climate conditions. Therefore, the occurrence of these disasters is inherently connected to the climate. For instance, during rainy seasons, continuous heavy rainfall can cause the groundwater level and hydrostatic pressure to rise continuously. As a result, a significant amount of energy can accumulate in the fault zone, potentially leading to catastrophic disasters. Conversely, if the tunnel construction takes place during dry seasons when evaporation exceeds recharge, there may not be enough groundwater to trigger the disasters.
- 2.
Groundwater pressure
The groundwater pressure, encompassing both hydrostatic pressure and dynamic water pressure, serves as the driving force for mud inrush disasters. The hydrostatic pressure gradually compresses the surrounding rocks, resulting in the accumulation of stress energy over time. Additionally, the fissures and pores within the rocks may expand under the compressive stress, facilitating groundwater seepage.
Moreover, the dynamic water pressure exerts both impact force and drag force on the surrounding rocks. The impact force weakens the strength of the rocks, making them more susceptible to deformation and failure. The drag force, on the other hand, enables the transfer of rock particles along with the flowing groundwater. In essence, the groundwater pressure contributes to the accumulation of energy, expansion of channels, and migration of particles, all of which have a profound effect on the occurrence of mud inrush disasters.
- 3.
Chemical properties of groundwater
The chemical properties of groundwater, including iron exchange, dissolution, hydration, and hydrolysis, can significantly impact the mineral composition of surrounding rocks. Such chemical actions can result in altered mineral compositions, leading to changes in the structure of the rocks, which in turn affect their strength and stability. As a result, there exists a close relationship between the chemical properties of groundwater and the occurrence of mud inrush disasters.
2.3.3. Human Factors
Due to limitations in survey technologies or instruments, it is possible that some crucial engineering geological information may not be accurately acquired during the actual engineering process. This can result in potentially unreasonable tunnel designs. In such cases, there is a high likelihood of mud inrush disasters occurring if the tunnel is constructed without proper consideration of the accurate engineering geological information. The lack of detailed geological survey during the early stages was an important factor in the disaster’s occurrence.
- 2.
Construction method
During the tunnel construction process, excavation disturbances can cause deformations in the surrounding rocks, leading to potential instability and triggering geological hazards. However, the use of appropriate excavation methods can partially reduce these disturbances, and suitable support methods can resist large deformations, thereby enhancing the stability and bearing capacity of the surrounding rocks. As a result, the probability of disasters can be effectively reduced by adopting proper construction methods that take into consideration the actual geological characteristics.