Study on early-warning System of Karst Tunnel Collapse

During the construction of karst Tunnel, tunnel collapse often occurs. The establishment of early-warning system of tunnel collapse can effectively ensure the construction safety, but the research is insufficient. Based on the combination of theoretical analysis and engineering practice, this paper puts forward to early-warning system for karst tunnel collapse. (1) The evaluation index system of karst tunnel collapse is composed of anchor axial force, displacement, anchor pull-out force and steel arch stress. (2) The relationship between evaluation index and warning level is determined, and the early-warning system of tunnel collapse is established. (3) The model applied to Chong'anjiang tunnel collapse warning, and achieved good results.


Introduction
Tunnel engineering has the characteristics of many risk factors, complex construction technology and huge investment. Compared with other tunnels, the risk of collapse in karst tunnel during construction is higher. In Southwest China, more than 1/3 of the carbonate areas are concentrated, which are easily to form karst areas. Tunnel collapse seriously endangers the safety of construction in Southwest China.
The establishment and application of karst tunnel collapse early-warning system can effectively ensure the safety of tunnel construction and reduce losses. However, the research on the establishment of early-warning system of tunnel collapse in karst tunnel construction is still insufficient.
Wang Tianyu [1] established a landslide early-warning system based on tunnel characteristics and climate factors. And Zhou Xiaoyong [ 2 ] established a landslide early-warning model based on the mechanical properties of surrounding rock. Ma Shiwei [ 3 , 4 ] has studied the establishment method of collapse early-warning system for tunnels with weak surrounding rock and loess. Wu Ke [5] , put forward the establishment mode of landslide early-warning from monitoring measurement and from geological advance prediction. Xue Yiguo [ 6 ] has established an early-warning system for karst tunnels by combining advanced prediction with monitoring measurement.
In this paper, based on the Chong'anjiang tunnel in Guizhou Province, fully considering the characteristics of karst tunnel collapse, the tunnel collapse early-warning system is established.
2 Establishment of karst tunnel collapse early-warning system

Determination of warning degree
Five color early-warning is established to judge the degree of harm, degree of emergency and development trend of collapse during the construction process. The warning degree is indicated by red, orange, yellow, blue and white, as shown in Table 1.

Evaluation index and grade
There are many evaluation indexes that affect the stability of karst tunnel. Based on the relevant research and comprehensive analysis, the indexes of bolt axial force, displacement, bolt pull-out force and steel arch stress are selected to form the evaluation index system of tunnel stability. In order to achieve the purpose of dynamic early-warning, the geological and topographical factors of the tunnel or the tunnel itself are no longer considered here. The evaluation indexes are divided into two levels according to the different positions of anchor and different displacement monitoring positions, as shown in Table 2 Table 3. (2) Tunnel convergence displacement The convergence displacement of tunnel includes vault settlement and peripheral displacement, as shown in Table 4. (3) Pull out strength of anchor rod The tensile pull-out test of anchor rod refers to the maximum tensile force that the bolt can bear. It is a comprehensive reflection of the quality of bolt material, processing and construction and installation. Whether the pull-out force of the bolt meets the requirements directly affects the effect of the bolt. When the pull-out force of the bolt is insufficient, it can not play the role of anchoring the surrounding rock, so the detection of the tensile pull-out force of the bolt is very important.
According to (JTJ 042-94), at least three anchors shall be selected randomly for each installation of 300 anchors, and the three bolts shall be taken as a group for pull-out test in the same section, and another group shall be conducted when the surrounding rock changes.

Table 5 Relationship between pullout resistance of bolt and warning level Evaluating indicator
Warning level Ⅴ Ⅳ Ⅲ Ⅱ Ⅰ Qualified rate of anchor rod pullout force 90%-100% 80%-90% 65%-80% 50%-65% Less than 50% (4)Stress of steel arch The stress of steel arch frame can directly reflect the stability of tunnel support system. The stability of support system can be judged by monitoring the stress of steel arch frame in the section.

Determination of alarm degree and limit
The determination of the alarm degree is generally based on the data and change of the monitoring indicators. If the index value falls in a certain alarm limit area, it is determined as the corresponding alarm degree.
The division of alarm degree and alarm limit is shown in Table 7. Table 7 The division of alarm degree and alarm limit Five color warning degree

Red Giant warning
The warning level of any index is grade I, or two or more indicators are grade II, and the change trend is increasing Orange Severe warning No warning level is higher than grade I, and there is one index whose warning level is grade II Yellow Moderate warning No warning level is higher than grade II, and there is one index whose warning level is grade III Blue Mild warning No warning level is higher than grade III, and there is one index whose warning level is grade IV White No warning The warning level of all indicators is grade IV.

Early-warning and treatment
Based on the summary of experts' opinions and referring to the existing research results, the construction treatment of each early-warning level are determined, as shown in Table 8. Taking the tunnel application in a certain mileage section as an example, the accuracy of the model is verified.

Early-warning results of tunnel collapse
According to Table 2 to Table 6, the early-warning level of tunnel collapse in certain mileage section is determined, as shown in Table 9.  Table 9, the early-warning level of tunnel collapse in certain mileage section is Ⅰ, Warning degree is red.
In view of the red warning issued in this mileage section, timely measures were taken during the construction process, including: stopping the excavation and closing with C20 shotcrete; adding I22 steel arch protection with the same spacing as permanent steel frame; adding I22 temporary inverted arch, large lock foot and soil nailing according to the monitoring data; radial grouting with Φ 42 self-propelled anchor rod with a length of 4.5 m, spacing of 1.5m× 1.5 m, injection of double liquid slurry.
After the reinforcement measures are adopted, the initial support of each section has a large growth rate in 5-8 days, which is in a slow growth period in 8-20 days, and tends to converge after 20 days.