Influence of Blasting on Anchoring Force of Long Anchor Cable Passing through the Karst Cave

In order to study the effect of blasting on the anchoring force of long anchor cable through the karst cave, numerical simulation method is used, relying on a subway station pit support project, respectively, to explore the location of the karst cave, the diameter of the karst cave and the modulus of elasticity of the karst cave filler on the anchoring force of the long anchor cable through the karst cave under the effect of blasting. The results show that when the anchor cable anchoring section through the length of the karst cave the longer or the smaller the modulus of elasticity of the karst cave filler, the smaller the loss of anchor cable anchoring force before and after blasting; when the core of the karst cave and the distance from the pile is 10m, the loss of anchor cable anchoring force is the smallest; when the diameter of the karst cave is 4.46m or the modulus of elasticity of the karst cave filler is 13.3MPa, the void effect of the cavity absorbs the blasting energy, and the anchor cable anchoring force will not be lost after the blast. not be lost.


Introduction
With the continuous development of China's subway construction, there are many examples of subway stations built on karst strata, and the blasting action will inevitably have an impact on the anchorage force of long anchor cables through the karst cavern.
Wang et al. [1] and others studied the mechanical response of anchor rods under the joint action of prestressing and blasting dynamic loads, and analysed the blasting factors that affect the mechanical properties of anchor rods.Kang Tianhe et al. [2] used numerical simulation method to get the change rule of axial force when underground anchor rods are subjected to dynamic perturbation, and put forward the anchoring parameter selection method of roadway anchor rods.Qingfeng Li et al. [3] analysed the factors that have influence on the dynamic characteristics of anchor rods by establishing a numerical model of anchor dynamics.Wang Guangyong et al. [4] analysed the mechanical problems and technical methods to strengthen the anti-explosion performance of anchored cavities through mechanical model tests.Yang Ziyou et al. [5] analysed the dynamic properties of anchor rods and the reinforcing effect on the surrounding rock through ANSYS calculation software, and analysed the impact of the force on the surrounding rock reinforced with anchor rods.Xue Yadong et al. [6] studied the effect of ground vibration load on the axial force of anchor rods by numerical simulation, and concluded that the end anchors can provide better support effect to the supported tunnel under the influence of dynamic load.Wang Aiwen et al. [7] analysed the coupling effect and damage mechanism of anchor rods and surrounding rock under impact dynamic loads by establishing a numerical model of anchor dynamics.Tahmasebinia et al. [8] investigated the force performance of anchor rods under static and dynamic loads, and proposed a numerical simulation method for the force performance of anchor rods under static and dynamic loads.The numerical and analytical models were verified with the experimental results reported in the literature, which proved the reliability of the models.Shan Renliang et al. [9] analysed the mechanical response law of end anchor rods under the action of blast load, and the deformation of anchor rods caused by blasting action is mainly bending deformation.Wu Qiuhong et al. [10] developed a set of test device to study the dynamic response of anchor rods based on SHPB test platform by themselves, and carried out the study of the mechanical response characteristics of full-length bonded anchor rods under dynamic perturbation, and the results showed that the damage of anchorage interface under dynamic load started from the outer end of the anchorage rods, and the failure of anchor rods was related to the damage of anchorage interface.
From previous studies, it can be seen that most of the previous research focused on the tunnel or tunnel power under the anchor stress, strain research, for the long anchor cable under the action of blasting through the karst cavern anchorage force how to change and the factors affecting the research is less.Therefore, this paper is based on a subway station pit support project, the use of FLAC3D software for numerical simulation, in order to explore the long anchor cable through the karst cavern anchorage force under the action of blasting load change rule as well as the influence factors, with a view to providing guidance for the construction control of similar projects.

Project Profile
A subway station center mileage for the right K11+450.682, station starting mileage right K11+394.961, end mileage for the right K11+522.582, the station length of 167.6m, the main standard section of the pit depth of excavation is about 23.5m, the pit width of 24.9m, the end of the pit depth of the pit is about 25.3m, the pit width of about 29.2m. the paper's Numerical analysis will rely on the section K11+470.000 of the right mileage of the station pit which has a karst cave.The top plate of the station is covered with soil of about 3m, and the planned site level elevation on the station is 33.50 m.The soil layer consists of artificial miscellaneous fill, clay soil, yellowish-brown ~ brownish-yellow, late-Pleistocene, and hard-plasticized clay layer of the Quaternary Holocene and Late-Pleistocene, and the subducted bedrock is dominated by grey rock.In the proposed site, Ordovician gray rock and Cambrian gray rock are underneath the bedrock, all of which contain karst and are more developed.According to the boreholes, the hole rate is 81.5%, the karst rate is 15.11%, and the height of the largest hole is about 5.0 m.The holes are mainly of filling type, and the filler is hard ~ plastic brownish-yellow clay, sandwiched with chert and marl fragments.

Geometric Modeling of Deep Foundation Pit Engineering
According to the actual situation of the right mileage K11+470.000section of the subway station pit, the numerical model shown in figure 1 below is established.In the model, the soil model is 84m×40m×72m, and the inside of the pit takes half of the width of the pit, which is 12m; the length of the enclosing piles is 16m, and the spacing between the piles is 2.6m; the free section of the first anchor cable is 6m long, and the anchoring section is 9m long, and the anchor head end is 6m away from the ground surface of the pit; the free section of the second anchor cable is 5m long, and the anchoring section is 9m long, and the anchor head end is 10m away from the ground surface of the pit, and the inclination angle of the two anchor cables are both 20°; The distance between the center of the karst cave and the enclosing pile is 7m, and the diameter of the karst cave is 4m.
Foundation pit ground below 10m range for the soil layer, the construction of ordinary excavation; in the foundation pit 10m below the medium-weathering limestone, the construction of blasting construction.Rock blasting construction usually use layered blasting, so in the simulation of blasting load application surface in the pit, selected 1m high rock as a layer, that is, the blasting load application surface from the pit ground below 10m~11m.analysis of the second anchor cable closer to the blasting point of the anchor cable anchoring force for analysis.

The Composition of the Calculation Unit
The whole process of foundation pit excavation is simulated in FLAC3D calculation software, for the soil body, it is simulated by solid unit (zone); for the excavated soil body, it is simulated by null unit (null); for the simulation of the supporting structure, structural units can be called for simulation in FLAC3D, i.e., the pile (pile) unit is used to simulate the perimeter piles, the anchor (cable) unit is used to simulate the anchor cable is simulated by the pile unit, and the waist beam is simulated by the beam unit.
To simulate the pile-anchor enclosure structure, the enclosure piles are firstly applied, then the waist beam is used to connect the enclosure piles, and then the anchor cable is applied, and the end of the anchor cable is fixed on the waist beam.The pile unit and the anchor cable unit are connected through the "medium" of the gird beam, and a "node-node" connection is used between the pile and the beam, and a "node-node" connection is also used between the beam and the cable.The same "node-node" connection is used between beam and cable.The specific form of the "node-node" connection is to make the reconnected nodes rigid and coordinate the forces together.

Soil Parameters and Principal Structure.
According to the ground investigation data, the soil body of the proposed excavation pit is divided into three layers from top to bottom, which are miscellaneous fill, clayey soil and medium-weathering gray rock layer, and the depths of the first two layers are 2m and 8m, respectively, and the physico-mechanics of each soil layer is shown in table 1.The Mohr-Coulomb elastic-plastic model is used in the model for the intrinsic relationship.

Structural Unit Parameters.
The main enclosure structure of the pit of subway station adopts Φ1000@1300 drilled piles, and the diameter of the anchor hole of the anchor cable is 150mm, as shown in figure 2. The relevant parameters of the enclosing piles and anchor cables are shown in tables 2 & table 3 respectively.In this paper, Rayleigh damping is used for simulation.Rayleigh damping requires two parameters to be set: the minimum critical damping ratio ξmin and the minimum center frequency ωmin.It is found that: the critical damping ratio of geotechnical materials is more appropriate to take 2% to 5%, which is taken as 5% in this paper, and the center frequency is taken as the superposition of the input frequency and the self-oscillation frequency of the system [11].

Blasting Dynamic Parameters.
The blasting load in the tunnel during the blasting process can be simplified as a triangular linear load with a rising phase and a falling phase, as shown in figure 3, the blasting load action time t2 is assumed to be 7 ms, of which the blasting load pressure rise time t1 is 2 ms, the fall time is 5 ms, and the total computation time is taken to be 600 ms [12,13].The maximum value of the stress of the blasting load is solved according to the following empirical formula [14,15] ,is the proportional distance, where, R for the distance from the eye to the loading surface, Q for the explosive charge, when the flush blasting to take the total charge, segment detonation to take the largest segment charge.Table 4 for the peak pressure of the blasting load calculation of the relevant parameters.

Modelling of Numerical Calculations
Through the above constructed modeling of the effect of blasting load on the anchoring effect of long anchor cables, numerical simulation analysis can be carried out.The numerical simulation calculation results of the effect of blasting load on the anchoring force of long anchor cables under the conditions of karst-free strata and karst strata are compared with the changes in the anchoring force actually monitored on site to verify the reasonableness of the calculation model.In order to determine the change of anchor cable anchorage force before and after blasting, YT-1200 anchor gauge was used for anchor cable anchorage force monitoring, and the schematic diagram of anchor cable anchorage force testing device is shown in figure 4.After the excavation simulation of the soil layer 10m below the pit ground, FLAC3D power calculation module is used to establish the blasting power model, the blasting charge is 8kg, the blasting initiation point is at the interface between the soil layer and the rock layer (10m away from the pit ground), and the horizontal distance from the anchor head of the anchor cable is 3 m.The blasting charge is converted into the value of the blasting stress by the equation (1) to carry out the power calculation.
In the numerical model, the diameter of the karst cavern is 4m, and the basic mechanical parameters of the karst cavern fill are shown in table 5 below.The time course plot of the variation of the anchoring force of the second anchor cable obtained from the dynamic calculation is shown in figure 5 below.Anchor gauge was used to measure the second anchor cable directly opposite the detonation point, the value of the anchorage force before and after blasting, after three consecutive detonations were carried out, the changes in the anchorage force of the anchor cable are shown in figure 6 below.The comparison of the change values of the anchoring force of the second anchor cable before and after the numerical simulation of blasting and the change values of the actual three monitoring in the field is shown in figure 7. Accordingly, it can be concluded that the established model of the influence of blasting load on the anchoring effect of long anchor cables under karst stratum is applicable to the blasting simulation of the pit.

Influence of the Relative Positions of the Karst Cavern and the Anchor Cable on the Anchoring Force of the Anchor Cable under Blasting Action
In order to analyze the size of the anchoring force of the anchor cable by the relative position of the karst cavern and the anchor cable, respectively, consider the center of the karst cavern from the perimeter pile for 4m, 7m, 10m, 13m, 17m and so on five kinds of calculation conditions.In the model, a single blast explosive amount of 8kg, the detonation point from the anchor anchor head distance of 3m, the diameter of the hole is 4m, and for the full-fill type, the mechanical parameters of the karst cave filling is shown in table 5. Five models will be five before and after blasting the second anchor cable anchorage force difference with the distance from the center of the karst cave hole change rule is shown in figure 8.As can be seen from figure 8, the distance between the center of the karst cave and the pile in the range of 7m to 13m, the anchor cable is through the karst cave, the range of the anchor cable before and after blasting the anchor force change value is relatively small compared to other ranges.This is because when the anchor cable passes through the karst cave, the incompactness of the karst cave around the anchor cable will produce a certain cavity effect, the existence of the cavity will absorb the blast energy to a certain extent, the shorter the length of the anchor cable anchoring section through the karst cave, the greater the impact on the rock mass near the anchor head after the blast energy is absorbed by the cavity, and the loss of anchoring force will also increase accordingly.
Setting the difference between the post-blast and pre-blast anchoring force as j F , and the distance from the center of the karst cavern hole to the enclosing pile as j r , the scatter points were fitted to give the fitting equation as: -0.0148 0.3099 -1.9574 0.848 From equation ( 2), it can be seen that the loss of anchoring force of the anchor cable is minimized when the distance between the center of the karst cavern hole and the pile is 10m.

Influence of Karst Cave Dimensions on the Anchoring Force of Anchor Cables under Blasting Action
In order to analyze the size of the anchor cable anchoring force by the size of the karst cavern, respectively, considering the karst cavern diameter of 1m, 2m, 3m, 4m and other four calculation conditions.In the model, a single blast explosive amount of 8kg, the detonation point from the anchor anchor head distance of 3m, the center of the karst cavern and the distance between the pile is 10 m.The change rule of the difference in anchoring force of the second anchor cable with the size of the karst cavern before and after blasting the four models is shown in figure 9. From figure 9, it can be seen that the larger the diameter of the karst cavern, the loss of anchoring force will be reduced accordingly, which is due to the longer the length of the anchoring section of the anchor cable through the karst cavern, the smaller the impact of the blasting energy absorbed by the cavity on the rock mass near the anchor head.
Given that the difference between the post-blast and pre-blast anchoring force is d F and the diameter of the karst cavern is d r , the scatter points are fitted to give the fitting equation: 0.0475 0.0635 -1.2325 0.9999 From equation ( 3), when the diameter of the cavity is 4.46m, the cavity effect of the cavity absorbs the blasting energy, and there will be no loss of anchoring force after blasting.

Influence of the Modulus of Elasticity of Karst Cavern Fillings on the Anchoring Force of Anchor Cables under Blasting Action
To analyze the effect of the modulus of elasticity of the karst cavern filling on the anchoring force of the anchor cable under the action blasting, the modulus of elasticity of the karst cavern filling was considered to be 40MPa, 80MPa, 160MPa, 320MPa, and other four types of calculation conditions.In the model, a single blast explosive amount of 8kg, the detonation point from the anchor anchor head distance of 3m, the diameter of the karst cave is 4m, the core of the karst cave and the distance from the pile is 10m.The variation rule of the difference in anchoring force of the second anchor cable before and after blasting the four models with the elastic modulus of the karst cavern with the filler is shown in figure 10.As shown in figure 10, the larger the modulus of elasticity of the cavity filler, the loss of anchorage force will increase accordingly, which is due to the larger the modulus of elasticity of the filler, the closer its mechanical properties to the rock body, the smaller the energy absorption effect of its cavity, the larger the impact of the energy of the blast on the anchor cable anchor head, the larger the loss of anchorage force.
Set the difference between the anchoring force after blasting and before blasting is t F , the diameter of the cavity is t r , the scatter is fitted, resulting in the fitting formula is: ( ) 0.00002 -0.0104 0.135 0.9919 From equation ( 4), when the modulus of elasticity of the karst cavern filling is 13.3 MPa, the cavity effect of the karst cavern absorbs the blasting energy, and there will be no loss of anchoring force after blasting.

Conclusion
(1) Based on the actual situation of the right mileage K11+470.000section of a pit, a model of the influence of blasting load on the anchoring effect of a long anchor cable through a karst cavern was established, and the reasonableness of the model was verified by comparing it with the actual monitoring data in the field.
(2) The relative positions of the karst cave and the anchor cable, the size of the karst cave and the elastic modulus of the karst cave filler on the anchoring force of the long anchor cable through the karst cave under the blasting load were analyzed under the condition of karst stratum.When the anchor cable through the karst cavern, the longer the length of the anchoring section through the karst cavern, the lower the loss of anchoring force before and after blasting; the greater the modulus of elasticity of the karst cavern filling, before and after the blasting, the greater the loss of anchoring force through the anchor cable therein.
(3) For a pit right mileage K11 + 470.000 section, when the center of the karst cavern and the distance between the pile is 10m, the loss of anchoring force of the anchor cable is minimized.When the diameter of the hole is 4.46m or when the hole filler modulus of elasticity is 13.3MPa, the cavity effect of the hole to absorb the blasting energy, the anchoring force will not be lost after blasting.

Figure 2 .
Figure 2. Diagram of relative position between anchor cable and pile.

Figure 4 .
Figure 4. Schematic diagram of anchor cable anchoring force test device.

Figure 5 .
Figure 5.Time history diagram of blasting anchoring force change at right mileage K11+470.000.

Figure 6 .
Figure 6.Anchoring force change map of right mile K11+470.000section before and after blasting.

Figure 7 .
Figure 7.Comparison of the change values of the anchoring force of the second anchor cable before and after blasting with the change values of the actual three monitoring in the field.

Figure 8 .
Figure 8. Difference diagram of anchoring force at different relative positions of karst cave and anchor cable.

Figure 9 .
Figure 9. Variation diagram of anchoring force difference with different karst cave diameter.

Figure 10 .
Figure 10.The difference figure of anchoring force with different elastic modulus of karst cave filling.

Table 1 .
Calculation parameters of each soil layer.

Table 2 .
Calculation parameters of pile.

Table 3 .
Calculation parameters of anchor cable.

Table 4 .
Calculation parameters of each soil layer.

Table 5 .
Mechanical parameters of karst cave filling.