Blasting vibration monitoring scheme and its application

2.1. Engineering background of BET

As a technical research facility for the geological disposal of high-level radioactive waste in China, BET facility has carried out various construction skills related to the excavation engineering, such as blasting test, rock deformation monitoring, EDZ monitoring, advanced detection, grouting test and so on.

The main difference between BET project and the other underground projects is that it demands the EDZ value as small as possible. As the underground repository facilities need to be safe for thousands of years. It means that the nuclide must be ensured not migrate from the surrounding rock to the nature. In view of the actual conditions of the BET, the blasting parameters of the drilling and blasting test are designed, and the monitoring of blast vibration are carried out. The blasting effect, the speed of blasting vibration is analyzed and discussed.

BET facility is located in Gobi, Gansu Province, about 80km northeast of YuMen City. The main project of BET facilities includes: tunnel door, inclined shaft, alley, water storehouse, test chamber, shelter, ventilation hole, as well as the water supply, power supply and ventilation system. The surrounding rock of the project is mainly granite, and its static compressive strength is about 150 MPa, tensile strength is about 13 MPa [17].

2.2. Blasting scheme

According to the geological conditions, blasting plans are designed, as shown in Fig. 1 and Table 1. It covers the space arrangement of all boreholes, the charge of each borehole and the detonator stages. It needs to be explained that the cutting way used here is the straight parallel cutting [17].

The emulsion explosive is used in the surrounding blasting holes to be cut into several segments on average, using air spacing with uncoupled charge. The weight of each volume of explosive is 300 g. And the blasting vibration monitoring designed has been located in front of the drilling and blasting face. In order to study the damage degree and blasting vibration effect of different blasting parameters on rock mass, each cycle footage should be kept the same, which means 2 m.

A millisecond delay with non-electric detonator is used to detonate, and the cutting holes adopt continuous coupling charge, the auxiliary holes and the bottom holes adopt continuous non-coupling charge, the surrounding holes adopt the air interval with non-coupling charge to bind the interval of the explosive to the detonator and then attach the bamboo sheet to the bottom of the hole. The blockage length of the surrounding hole and auxiliary hole is not less than 20 cm, while the cutting hole is not less than 40 cm.

Fig. 1Blasting network diagram

3.1. Monitoring equipment

As the test and the monitoring has been carried out in the tunnel, its special test environment requires that the test instrument should have the following functions.

(1) as the construction environment is relative bad, water flow is much more and the dust is heavy. The instrument must have the functions of moisture proof and dust proof.

(2) because of limited space, blasting impact is strong. The instrument should be lightweight, portable, strong enclosure and impact resistance.

(3) the vibrometer requires batteries with long power supply.

(4) vibrometer should match corresponding analysis software to facilitate data processing in later stage.

According to the above requirements, TC-4850 blasting vibration recorder produced by Chengdu Zhongke Measurement and Control Co., Ltd. is selected, as shown in Fig. 2.

Fig. 2Schematic of the blasting vibration monitoring system TC-4850

The main index of the recorder system is shown in Table 2.

3.2. Principle of Blasting vibration monitor

When the blasting occurs, the vibration wave will propagate outward along the rock medium. If the vibration wave is received by the vibrator, the voltage will be produced. When the voltage signal is greater than the trigger level, the vibration signal will be automatically recorded by the blasting vibration recorder. The signal collector will adjust the input signal according to the setting of the instrument. Then it is stored in each channel after A/D conversion. When the data stored exceeds the maximum capacity of the memory, it will overlay the original data. The computer can be connected through USB interface for data analysis and copy.

The instrument consists of an independent acquisition module and an internal computer system. It means that each module contains a time-based controller and four acquisition channels. The modules are synchronized by clock to ensure simultaneous triggering and recording of all channels. The acquisition channel stores the collected data into their respective memory, and CPU accesses the specified channel data through a unified system, and controls the parameters of each acquisition module. As each channel has 16 bits A/D and memory, the phase difference between channels can be neglected in parallel acquisition. The instrument and testing principle are shown in Fig. 3.

After the in site monitoring is finished, testing data is connected to the computer through the USB interface of the instrument. The data is analyzed and processed by the professional software. Before the blasting vibration acquisition device is used, it should be firstly to estimate the basic characteristics of the collected signal, such as frequency range, amplitude range etc. Then, the acquisition parameters such as sampling rate and range could be set.

Fig. 3Block diagram of blasting vibration monitoring system

3.3. Blasting vibration test scheme

Usually, the blasting vibration velocity is subject to the vibration monitoring scheme. How to place the blasting vibration sensors and where to arrange them are significant. It could be performed as follows.

Firstly, to estimate the distribution of vibration intensity on the roadway excavation section, and the most powerful location of blasting vibration could be found out.

Secondly, according to the previous data analysis, it is considered that the blasting vibration speed at the top of the roadway is the largest. It means that the top vibration resistance of the roadway is the worst. Therefore, there are three blasting vibration velocity sensors are arranged on the top of the roadway, and the sensor is arranged at the side wall of the roadway at the same position. In which, a three direction sensors are placed at each measuring point, corresponding to the radial, vertical, and tangent directions. The base is fixed on the rock surface by lime powder coupling. As it is shown in Fig. 4 and Fig. 5.

Fig. 4Installation of blasting vibration meter for measuring the top point

Fig. 5Installation of blasting vibration meter for measuring the side point

Fig. 6Layout diagram of blasting vibration observation point of roadway roof and side wall

Thirdly, the first measuring sensor point is arranged 10 m away from the blasting face, and the other sensors are arranged at intervals of 5 m. That is, three sets of sensors and six measuring points are linearly arranged along the top and side of the roadway from the blasting face. As it is shown in Fig. 6.

The damage of blasting vibration to surrounding rock is different due to the difference of each charging parameter, cutting way and geological condition.

The maximum value of blasting vibration velocity generally appears at the position of maximum tensile stress, while the position of maximum tensile stress of excavation roadway is generally at the top of roadway. Therefore, one of the vibration velocity sensors has been installed at the top of roadway.

Therefore, the measuring points of blasting vibration are closely followed after each blasting cycle, and the distance between the sensors and the blasting face and the sensors of each measuring point are consistent with the former blasting cycle. Only this, the same objective condition of test data could be ensured and each blasting vibration value is more comparable.

Meanwhile, the instrument needs to be pretested carefully before blasting vibration test, and the blasting vibration test procedure is referred to the blasting vibration detection instruction.

4.1. Blasting vibration test process

The key to complete the blasting vibration monitoring depends on whether the monitoring system selected is reliable and whether it can fully meet the requirements of blasting vibration or not. Therefore, the frequency range and amplitude range of the monitored signal should be estimated and grasped in advance. And then the suitable blasting vibration recorder can be chosen.

The TC-4850 blasting vibration recorder produced by Chengdu Zhongke Measurement & Control Co., Ltd. is used in this test. The radial, vertical and tangential direction data can be obtained through the data acquisition equipment, which can reflect the characteristics of blasting vibration signals more comprehensively. The sensors’ installation position on the rock surface must be kept smooth. The harmonic binder is generally bonded with lime paste, and the vibration measuring sensors are fixed at the corresponding test positions to ensure that the sensors are fixed and not easy to fall off.

It should be observed that there is no gap between the vibration measuring sensors and the rock wall, so that the blasting vibration data can be directly transmitted to the vibration measuring system through the vibration of the rock wall induced by the blasting.

The connecting wire is drawn out from the vibration measurement sensor, and the blasting vibration monitoring equipment is installed. The corresponding protective measures are given to the monitoring equipment to prevent the blasting shock wave and the blasting flying stone damage. Turn on the monitoring equipment, set the frequency range and the amplitude range of the monitoring, and then wait for the blasting to start.

The blasting vibration data are recorded automatically. The monitoring equipment and the vibration measurement sensors are taken out after the blasting operation. The blasting vibration data is derived and data analysis and processing are carried out with the corresponding data software on the computer.

4.2. Blasting vibration data

Three vertical values of particle vibration should be simultaneously measured during the blasting vibration monitoring.

The direct analysis method of blasting vibration signals has been applied to analyse the measured blasting waveform, so the blasting vibration characteristic quantity is determined from the waveform diagram.

The vibration waveform of each component for one monitoring sensor are shown in Fig. 7.

Fig. 7Vibration waveforms of component for one monitoring sensor

a) Vibration waveform of radial component

b) Vibration waveform of vertical component

c) Vibrational waveform of tangential component

For each blasting cycle, the vibration instrument test parameters should be kept the same. And the acquisition parameters are used to maintain a uniform vibration test condition.

The maximum particle vibration velocity at the distance 10 m away from blasting source can be obtained by changing the designed blasting parameters. For the straight hole cutting method adopted in this experiment, the maximum vibration velocity is 25 cm/s.

It can be seen from Fig. 7 that under the condition of different detonators delay, the trend of the blasting vibration velocity changes with time is clear, and the peak value of vibration velocity is in a fine agreement with the detonator section. At the same time, the blasting vibration velocity is controlled within a reasonable range, which should be within 30 cm/s. It illuminates that the blasting vibration of the BET structure is controllable.

Moreover, the blasting parameters design is also reasonable, especially the explosive charge and detonator section are representative.

4.3. Blasting vibration attenuation law

According to the monitoring results of blasting vibration, the accurate influence coefficient $K$ and attenuation index $\alpha$ can be obtained by the least square fitting with the Sodev’s empirical formula. The concrete regression fitting process is as follows:

In which, $V$ is the blasting vibration velocity, $Q$ is the maximum explosive quantity, $R$ is the distance between the blasting source and the monitoring site.

Logarithm the left and right sides of Eq. (1), then it is obtained:

Order $\frac{\sqrt[3]{Q}}{R}=\rho$, then there is:

Furthermore, setting $Y=\mathrm{l}\mathrm{n}V$, $b=\mathrm{l}\mathrm{n}K$, $X=\mathrm{l}\mathrm{n}\rho$, the original formula transfers as a linear equation, it follows as:

For each group of monitoring data, $R$, $Q$ and $V$ are determined, and the estimated values of $\widehat{a}$ and $\widehat{b}$ based on the $a$ and $b$ can be obtained by the least square method:

In which: $\stackrel{-}{x}=\frac{1}{n}\sum _{i=1}^n{x}_i$, $\stackrel{-}{y}=\frac{1}{n}\sum _{i=1}^n{y}_i$.

During the blasting cycle, the fitting curves at each measuring point on the tunnel vault using the Sodev’s empirical formula are shown in Fig. 8.

Fig. 8Velocity fit on the tunnel vault (correlation coefficient = 0.93498)

The blasting vibration velocity data arranged at the side wall were simultaneously monitored. The fitting curve is shown in Fig. 9.

According to the fitting formula, the $K$ and $\alpha$ values in Sodev’s empirical formula can be calculated as 125 and 1.32 respectively, in which it shows that the rock mass in the tunnel belongs to a hard rock, and it also has proven that the above blasting scheme is feasible and rational.

Fig. 9Velocity fit on the tunnel side wall (correlation coefficient = 0.9326)

4.4. Vibration damping method

Based on the above understanding of blasting vibration characteristics, the vibration reduction and damping method in this project is put forward.

(1) millisecond blasting. A large blasting source is transformed into several small blasting sources. In view of the disturbance and separation of blasting vibration signals, the blasting vibration intensity is greatly reduced and the main vibration frequency is correspondingly increased under the condition that the total explosive charge is invariable. Thus, the damage effect of blasting vibration is reduced. It can be controlled by the delay of detonators.

(2) pre-splitting blasting. Because the propagation of blasting vibration wave will stop at the interface of air interval, a certain depth of groove or pre-splitting surface could be excavated between the blasting source and the protective object. Although the diffraction effect of blasting vibration wave cannot be prevented by the pre-splitting surface, the Rayleigh wave on the ground surface will be greatly weakened, thus greatly reducing the blasting vibration effect.

(3) avoid slag blasting. Slag blasting can effectively improve the quality of blasting and crushing. However, when the blasting vibration safety is considered, as there is a large amount of slag before the blasted body, which will strengthen the restraint of it, so that there would be more vibration energy can be transferred into the rock mass, which will have a negative vibration impact on the protective object. Therefore, in order to reduce the vibration hazards, the bottom ridge should be cleaned up, so that the blasted rock could be smoothly pushed along the direction of the minimum resistance line.

(4) control the main vibration frequency. Under the same blasting vibration intensity, the lower the main vibration frequency, the closer to the natural frequency of the protective object, and the greater the possibility of causing damage to the object. The main vibration frequency can be effectively increased by reducing the maximum explosive charge and shortening the delay time, so as to improve the blasting vibration safety.

(5) vibration reduction using interference. Since the vibration wave arriving at the protective object can reduce the vibration intensity when the peak and valley intersect, the vibration can be reduced by setting the delay interval in half period. However, it is difficult to determine the appropriate interval time because there is more than one protective object in each blasting cycle, the distance is also different, and the main vibration frequency varies uncertainly. In addition, even if the appropriate interval is found, the delay accuracy of the ordinary detonator is at least 10ms, which the desired precise delay control cannot be achieved, so that the vibration reduction is basically in a theoretical level. With the development of electronic detonation technology, high-precision time delay control has been realized.

(6) vibration reduction by buffering blasting. In the late 1990s, it was put forward that a section of air was reserved at the bottom of the hole to avoid the direct action of explosive wave on the bottom. While the rock mass at the bottom would still be destroyed by the detonation gas. So, the vibration energy was weakened.