Stability Analysis and Control Measures of Tunnel Face in Water-Rich Sandy Dolomite Stratum

: Revealing the conditions for the occurrence of sand gushing accidents in tunnels with water-rich sandy dolomite strata 7 can help the tunnels to be constructed safely. In this paper, we found that sandy dolomite and the water environment are the key 8 factors causing sand surges through geophysical prospecting at the tunnel face where 12,000 m 3 sand surge occurred. Through the 9 flow-solid coupling model, the extrusion deformation at the tunnel face is the main deformation form in the tunnel of water-rich 10 sandy dolomite strata. The influence of different factors on the deformation value of the tunnel face is from strong to weak: 11 sandification degree, head height, and tunnel depth. Combined with the study of pressure arch characteristics of the model, the 12 limit equilibrium theory considering seepage effects and pressure arch characteristics is proposed. We get that sand surges will 13 occur in the tunnel in the intensively sandy dolomite strata where the head exceeds 80m. For the intensively sandy dolomite strata 14 with head below 80m, the stability of the tunnel face is controlled by the degree of seepage damage. According to the reason of 15 sand gushing, the comprehensive control measures consisting of risk identification by over-detection, double-layer close-packed 16 pipe shed, grouting sealing technology and dewatering technology, and Milling and blasting combined construction method are 17 proposed, and the effective control of tunnel working face is realized in practical application.


24
As a sedimentary carbonate rock, dolomite is widely distributed in Yunnan-Guizhou-Sichuan mountain area of China. 25 Sandification is a weathering karst phenomenon. The dolomite rock mass without sandification is intact and has good engineering 26 properties. After sandification, dolomite is characterized by argillaceous composition, unclear bedding and loose fragmentation of 27 surrounding rock. The sandy dolomite is a mixture of sand and powder after excavation disturbance. In addition, sandy dolomite 28 becomes saturated sand state when soaked in water. Under the action of groundwater osmotic pressure, it is prone to osmotic 29 deformation, resulting in water inrush and sand inrush, which poses extremely high construction safety risks (Schneider and 30 Lavdas, 2013; Wu et al., 2017;Cui et al., 2015). 31 There have been relevant studies on the disaster mechanism of water-rich karst tunnels. Based on statistical analysis of 221 32 water-inrush and mud-inrush disaster cases, Li et al. (2018) divided the disaster-induced structures into three categories: karst, 33 fault and other causes. Four typical hazard modes of water and mud inrush in tunnels were proposed, including the directly 34 revealed type of water/mud inrush, the progressive failure type of water/mud inrush, the seepage instability type of water/mud 35 inrush and the intermittent failure type of water/mud inrush. Many scholars have carried out a lot of research on different types of 36 water and mud inrush. Through theoretical derivation and laboratory tests, the disaster-causing mechanism of karst was analyzed, 37 and the risk evaluation system and treatment measures were proposed ( found that the causes of instability of tunnel face are poor stability of fully weathered granite, high groundwater level, high 42 groundwater flow rate, poor pre-reinforcement of surrounding rock and disturbance caused by tunnel construction. 43 The stability control of the tunnel face is related to the safety of tunnel construction. The limit equilibrium analysis theory is 44 one of the common methods to analyze the stability of tunnel face. Horn (1961) introduced a 3D wedge model that assumed a 45 sliding wedge loaded by a soil silo. This model was extended to layered soils by Broere (2001). Based on the limit equilibrium 46 analysis theory, Anagnostou, G (2012) used the slice method to obtain the supporting force required for the face balance. Murayama et al. (1966) proposed a two-dimensional logarithmic spiral failure model based on the actual tunnel face instability In this paper, we take the sand gushing accident of water-rich sandified dolomite stratum at mileage D4K307+375 in Jixin 73 Tunnel as the research object, and the volume of gushing sand reaches 12,000 square meters (Fig. 2 ). Geological radar and 74 transient electromagnetic are used to investigate the surrounding rock condition and water-rich situation of the tunnel. Geological 75 radar transmits high-frequency electromagnetic waves to the detected body through the transmitting antenna, and the reflected 76 electromagnetic waves are received by the receiving antenna of geological radar, thus forming a radar image. Large amount of 77 distorted signals in radar images of fractured rock masses. Transient electromagnetism works by generating a primary magnetic 78 field propagating in the direction normal to the receipt loop line from the sending loop line. Under the excitation of the primary 79 magnetic field, the geological body will generate eddy currents whose magnitude depends on the degree of electrical conductivity 80 of the geological body. For the water-rich fractured rock, the detection results will show low resistance characteristics, and for the 81 water-free cavity, it will show high resistance characteristics.  water; the area with the apparent resistivity value more than 550Ω-m is judged as the physical exploration IV anomaly area, which 87 corresponds to the weak fragmentation rock body. It can be seen that the stratum that the tunnel traverses is a Type V anomaly 88 zone, indicating that the stratum is extremely broken and water-rich. 89 face due to sand surge (high resistance area in Fig. 4), and the front of the tunnel face is water-rich broken sandy dolomite in the 93 detection range (low resistance area in Fig. 4). 94 Geological radar was used to detect the stratigraphy around the tunnel (Fig. 5). The sandy dolomite stratum was 97 severely weathered and broken, and the distortion range of geo-radar detection was large. The detection results reveal that the sand surging is due to the loose and broken characteristics of the sandy dolomite stratum 101 and the joint action of the water environment. Through the statistics of the geological situation revealed on the tunnel face, the 102 sandy dolomite stratum can be divided into three types of sandification degrees: lightly sandy, moderately sandy, and intensively 103 sandy dolomites (Fig. 6). The different degrees of sandification and the water environment make it difficult to predict the stability 104 of the tunnel surface. Later in this paper, the analysis focuses on the catastrophic occurrence conditions of sanded dolomite. 105    Table 1. According to the experimental research of 122 related scholars (Chen, 2009), the elastic modulus of the surrounding rock is improved by more than 30%, the cohesion and 123 friction angle are increased by 20%~30%, and the permeability coefficient is reduced to 0.1%~0.5% of the original one after the 124 reinforcement of the surrounding rock by grouting. The water pressure value at the tunnel surface is adjusted by setting different 125 water level heights of the model. Adjustment of burial depth and initial ground stress by applying stress to the upper surface of the 126 model. 127 The steel frame is equivalently replaced by beam elements with equal section stiffness. The initial support uses linear elastic 129 elements to simulate C25 shotcrete, and the support structure parameters are shown in Table 2. 130

Deformation characteristics of tunnel face in sandy dolomite stratum under seepage action 132
As show in Fig. 8 and Fig. 9, the sandification degree of the dolomite strata, the depth of tunnel burial and the water level 133 have a negative impact on the extrusion deformation of the tunnel tunnel face and the settlement of the vault. Among them, the 134 sandification degree of the dolomite stratum has a dominant position, and the increase of sandification degree will significantly 135 increase the sensitivity of tunnel deformation to burial depth and water level. In addition the effect of water level variation is 136 greater than the effect of burial depth variation. Comparing the extrusion deformation values with the vault settlement values (Fig.  137 well to the accident of sand gushing at the tunnel face that is prone to occur when building tunnels in sandy dolomite strata. 139 The most notable feature of the pressure arch is that the streamline of the maximum principal stress vector can form a 148 complete ring. In the pressure arch, the direction of the maximum principal stress in the arch is the horizontal direction, and the 149 maximum principal stress outside the arch will be restored to the vertical direction. Therefore, the boundary of the pressure arch is

Analysis of pressure arch characteristics 155
Using 300m burial depth and 100m water level with intensively sandy dolomite as the base condition, the effect of burial 156 depth, water level and sandification degree on the form of pressure arch after tunnel excavation was investigated under the effect 157 of seepage. The cross section at the tunnel face was selected as the study object. As we all know, before the tunnel strata are 158 disturbed by excavation, the maximum principal stresses around the tunnel are in the vertical direction. After the tunnel excavation, 159 the strata were in force imbalance, and in order to reach a new equilibrium, the direction of the principal stress in the rock around 160 the tunnel was significantly deflected, and the direction of the streamline distribution of the maximum principal stress showed a 161 ring shape (Fig.11). As shown in Fig. 11, at a certain height directly above the top of the tunnel, the direction of the maximum 162 principal stress is deflected to the horizontal direction (rotated by nearly 90°). Moreover, the stress deflection point appears at the 163 top of the shaded part, forming a stress deflection zone.The outer contour of the shaded part is the outer boundary of the pressure 164 arch. 165 The load on the wedge comes from the prism. Therefore, it is necessary to analyze the stress distribution of the prism first. As 208 shown in Fig. 16, using the slice method, the vertical earth pressure in the prism can be calculated according to Terzaghi theory. 209 Since the stress direction in the pressure arch area is deflected (Fig. 11), the influence of the pressure arch effect is generally 210 reflected by the correction of the prism side pressure coefficient in the calculation. The value can be obtained by referring to the 211 literature (Handy,1998 As shown in Fig. 16 , the load 0 where the coefficient R is the ratio of the area to the circumference of a horizontal cross-section of the prism;   is the buoyant where t h is the water head at the tunnel face; h  is the model water head; a and b are parameters obtained by fitting the 230 water head distribution of the model, as shown in Table 3. 231

Stress analysis of the wedge 244
The wedge is sliced for analysis. As shown in Fig. 17 The parameters in the Eq. (10) are shown in Appendix A. The differential equation (11) The vertical force at the wedge-prism interface reads as follows: 259   Fig. 18. It can be seen that the virtual supporting force of the tunnel face in the water-rich sandy dolomite stratum has a 285 strong correlation with the water level. Due to the better bearing capacity of the moderately and lightly sandy dolomite stratum, 286 the tunnel face can maintain its own stability. However, for intensively sandy dolomite stratum, when the water level reaches 80m 287 or more, the tunnel face will lose stability, which is consistent with the phenomenon of water and sand inrush induced by 288 excavation in high-head area. However, according to the site construction situation in low-head area, the tunnel face will also lose 289 stability over time. The phenomenon that the fine particles in the loose rock body will be transported and lost under the action of seepage, thus leading to changes in the pore structure of the filling medium, cannot be ignored (Fig. 19). This seepage damage can lead to an 294 increase in water surges and eventually induce sudden water and mud disasters (Ma et al., 2017;Zhang et al., 2016). In order 295 to explore the degree of weakening of the loss of fine particles on the bearing capacity of surrounding rock (Fig. 20) (1) when water head>80m, the tunnel face will burst under the action of water (Fig. 22(a)); (2) when water 311 head<80m, the tunnel will experience seepage loss first, then the load-bearing capacity of the tunnel face decreased, followed by a 312 surge disaster (Fig. 22(b)). 313 314 Fig. 22 The failure modes of the water-rich and sandy dolomite stratum 315

316
Combined with the stability analysis of the tunnel face in the sandy dolomite tunnel, in order to prevent disasters, it is 317 necessary to timely reveal the sandy dolomite stratum, take reliable advance reinforcement measures, and take effective control 318 measures against water seepage. As shown in Fig. 23, a comprehensive control technology for tunnel construction in water-rich 319 and intensively sandy dolomite stratum is constructed based on the identification technology of high-risk areas, advanced control 320 measures in high-risk areas and low-disturbance construction methods. 321

323
As shown in Fig. 23(a), the geological radar and transient electromagnetic technology were used to discover the broken 324 surrounding rock and water-rich section in time before construction, and the geological radar signal distortion section and 325 transient electromagnetic low-resistance section were initially determined as high-risk sections. For high-risk sections, 326 pre-construction geological drilling was carried out to further explore the degree of sandification and water content of the 327 surrounding rock. If the drilled rock sample is complete, it is lightly sandy dolomite, if the rock sample is blocky with fissures 328 developed, it is moderately sandy dolomite, if the rock sample is gravelly, it is intensively sandy dolomite. For the area where 329 water emerged from the borehole and the drilled rock sample is strongly sanded dolomite can be finalized as high risk area. 330 For high-risk areas, grouting sealing technology and dewatering technology are used to release water and pressure, and 331 double-layer close-packed pipe shed (as shown in Fig. 23(b),) are used to control the stability of the tunnel face. Among them, 332 grouting sealing technology can control seepage damage and water inrushing at the tunnel face, dewatering technology can 333 effectively reduce the water pressure at the tunnel face, and the double-layer close-packed pipe shed can effectively share the load 334 of the overlying rock mass of the wedge and improve the bearing capacity of the tunnel. 335 The borehole-blasting method is easy to cause deterioration of the surrounding rock around the excavation area and increase self-stability, ordinary borehole-blasting method construction is easy to induce catastrophe. Therefore, combination method of 338 milling and blasting (Fig. 23(c)) is proposed to replace the original three-step method, thereby reducing temporary measures, 339 controling over-under-excavation, reducing surrounding rock disturbance, and preventing surrounding rock from peeling and 340 collapsing. The key to the combination method of milling and blasting is that the reserved thickness in the excavation contour of 341 the upper and middle steps of the tunnel is not less than 1m. The reserved thickness is used to reduce the impact of blasting, and 342 this part is chiseled off with a milling machine. In addition, weak blasting and short footage (1.2m) is adopted to reduce the 343 disturbance of surrounding rock. 344 Because of the poor mechanical properties of sandy dolomite, If there is no control measures in the water-rich conditions, it is 351 very easy to gush sand, which will form a huge collapse cavity, for the later construction of the formation of engineering hidden 352 trouble at the same time to improve the management of sand gushing difficulty. 353

Conclusion
(2) Under the combined action of seepage force and ground stress, the deformation of the tunnel face in the water-rich sandy 354 dolomite stratum is dominated by extrusion deformation. The influence of different factors on the deformation value of the tunnel 355 face is from strong to weak: sandification degree, head height, and tunnel depth. Excavation of tunnel in sandy dolomite stratum 356 results in significant pressure arches, and the order of the influence of each factor on the thickness of pressure arches is: 357 sandification degree, head height, and tunnel depth. 358 (3) Combined with the force characteristics of the pressure arch, an analytical solution to the limit equilibrium of the tunnel 359 face considering the seepage flow is constructed. The analysis shows that for tunnels with intensively sandy stratum with a water than 80m, the stability of the tunnel face is related to the reveal time of the tunnel face. With the occurrence of seepage, the 362 bearing capacity of the tunnel face area decreases, leading to catastrophe. Therefore, there is a catastrophic risk in the water-rich, 363 deep-buried and intensively sandy stratum. 364 (4) For the water-rich and intensively sandy dolomite stratum, risk control must first start with risk identification, and the 365 risk identification in front of the tunnel face is completed by means of advanced forecasting. Based on the stability analysis of the 366 tunnel face, grouting sealing technology and dewatering technology, as well as the construction of double-layer close-packed pipe 367 shed, can effectively realize the control of the slip section of the tunnel face. The construction method combining milling and 368 blasting is put forward innovatively to reduce the disturbance of surrounding rock and prevent the surrounding rock from peeling 369 and collapsing. These methods have been proved to be effective in realizing stable excavation of the tunnel face in actual 370