Smooth blasting is a controlled blasting technology. First, the rock mass at the main excavation site is blasted, then the peripheral hole charges arranged on the design contour line are blasted, and then the smooth blasting layer is blasted to form a flat excavation surface. By correctly selecting blasting parameters and reasonable construction methods, the wall after blasting is smooth and regular, and the contour line meets the design requirements. Full-face smooth blasting of tunnel is a construction method of excavating full-face tunnel at one time by using smooth blasting technology. Compared with the traditional blasting method, the most obvious advantage of this method is that it can effectively control the blasting effect of the surrounding hole explosives, thus reducing the disturbance to the surrounding rock, maintaining the stability of the surrounding rock and ensuring the construction safety. At the same time, it can reduce over-excavation and under-excavation, and improve the quality and progress of the project.

I. Principle of Smooth Blasting Action

The rock breaking mechanism of smooth blasting is a very complicated problem, which is still being explored. Although it is not mature in theory, it has knowledge of qualitative analysis. It is generally believed that there are two kinds of effects on rock mass when explosives are detonated; The second is the role of explosive gas expansion in doing work. Smooth blasting is the simultaneous initiation of peripheral holes, and the shock wave of each hole propagates radially around it. When the impacts of adjacent holes meet, the stress waves are superimposed, resulting in tangential tension. The maximum tension occurs at the midpoint of the center line of adjacent holes. When the ultimate tensile strength of rock mass is less than this tensile force, the rock mass is pulled apart and cracks are formed on the center line of the hole. Subsequently, the expansion and crack of the explosive gas further expanded, forming a flat burst surface.

Second, the technical points of smooth blasting

To make smooth blasting achieve good results, it is generally necessary to master the following technical points:

1, according to the characteristics of surrounding rock, reasonably select the spacing between surrounding holes and the minimum resistance line, and try our best to improve the drilling quality.

2. Strictly control the peripheral eye charge, and distribute the charge evenly along the eye length as far as possible.

3. Small-diameter cartridge and low-speed and low-detonation explosives should be used for peripheral eyes. In order to meet the requirements of charge structure, air-spaced charge can be realized by detonating cord.

4, millisecond millisecond delay is adopted for orderly initiation. It is necessary to arrange the excavation procedure so that smooth blasting has a good free surface.

5. The diameter of the side hole is less than or equal to 50 mm. ..

Three, presplitting blasting and smooth blasting

In order to ensure that the reserved rock mass is shaped according to the designed contour surface and prevent the surrounding rock from being destroyed, contour controlled blasting technology must be adopted. Commonly used contour controlled blasting techniques include presplitting blasting and smooth blasting. The so-called presplitting blasting means that the presplitting blasting holes arranged on the design contour line are first detonated to form cracks running through the design contour line, and then the main excavation part is blasted under the shielding of artificial cracks to ensure that the reserved rock mass is not damaged; Smooth blasting means blasting the rock mass at the main excavation site first, and then detonating the peripheral holes arranged on the design contour line, thus blasting the smooth blasting layer and forming a flat excavation surface.

Presplitting blasting and smooth blasting have been widely used in the excavation of dam foundation, slope and underground cavern.

(A) the mechanism of joint formation

Both presplitting blasting and smooth blasting require regular blasting-induced crack surfaces along the design contour line, and the mechanism of joint formation is basically the same. Now, taking pre-cracks as an example, their crack formation mechanism is discussed.

Pre-splitting blasting adopts uncoupled charge structure, which is characterized by an annular air separation layer between the charge and the hole wall. The existence of this air separation layer reduces the peak explosion pressure acting on the hole wall. Because the dynamic compressive strength of rock is much greater than the tensile strength, it can be controlled that the reduced blasting pressure will not cause obvious compression damage to the hole wall, but the tangential tensile stress will cause radial cracks around the hole. In addition, the accumulation of energy between holes leads to the stress concentration of the connecting line between holes, and the initial cracks on the connecting line between holes develop further, while the quasi-static action of lagging high-pressure gas leads to the splitting of the gas blade along the seam, so that all the cracks on the connecting line between the surrounding holes run through to form a seam.

(2) Quality control standards

1) The integrity of the excavated surrounding rock is measured by the hole trace rate on the rock wall. The hole trace rate, also called the half hole rate, refers to the percentage of the total length of the hole trace on the excavation wall to the total length of the hole. In the hydropower sector, for rock mass with extremely developed joints and fissures, the tracer rate of blastholes should generally reach10% ~ 50%; Moderately developed joints and cracks should reach 50% ~ 80%; More than 80% of joints and cracks are undeveloped. There should be no obvious explosion cracks on the surrounding rock wall.

2) The allowable value of surrounding rock roughness (also called fluctuation difference) is 15cm.

3) On the airport surface, the width of pre-crack should generally not be less than 1cm. Practice shows that for soft rocks (such as siltstone of Gezhouba project), the presplitting width can reach more than 2cm, and only when it reaches more than 2cm can it play an effective isolation role; But for hard rock, the width of pre-crack is difficult to reach 1cm. The pre-splitting width of granite in Dongjiang project is only 6m·m, which can still play an effective isolation role. The pre-crack width of underground engineering is much smaller than that of open-pit engineering, generally only reaching 0.3 ~ 0.5 cm. Therefore, the width standard of pre-crack is related to lithology and engineering site, and should be finally determined through field test.

Besides blasting parameters, the factors affecting the quality of contour blasting mainly depend on geological conditions and drilling accuracy. This is because blasting cracks are easy to develop along the primary cracks and joints of rock mass, and drilling accuracy is the premise to ensure the quality of surrounding blasting control.

(3) parameter design

The parameter design of presplitting blasting and smooth blasting generally adopts engineering analogy method, which is finally determined through field tests.

(1) presplitting blasting parameters

1) Open-cut engineering with the aperture of 70 ~ 165 mm; Tunnel excavation is 40 ~ 90 mm; Large underground workshop is 50 ~ 1 10 mm.

2) The hole spacing is related to the rock characteristics, explosive properties, charging conditions, requirements for the flatness of the excavation wall and the hole size. Generally, the hole spacing is 7 ~ 12 times of the hole diameter. Those with high blasting quality requirements, weak rocks and developed cracks take the small value.

3) The decoupling coefficient of charge refers to the ratio of the radius of the blast hole to the radius of the cartridge, which is generally 2 ~ 5 to prevent the damage of the blast hole wall.

4) The linear charge is the average charge per unit length of the blast hole. The factors affecting the parameters of presplitting blasting are complex, so it is difficult to deduce a strict calculation formula in theory, and the empirical formula is the main one. At present, the basic forms of commonly used formulas in China are as follows

Where qx is the linear charge density of presplitting blasting, kg/m;

σ c-ultimate compressive strength of rock, MPa；;

A- hole spacing, m;

D- borehole diameter, mm;

K, α, β and γ-empirical coefficients.

The linear charge density of presplitting blasting with different lithology is generally 200 ~ 500 g/m. In order to overcome the pinch of rock at the bottom of the hole, the linear charge density of the hole should be increased to 2 ~ 5 times.

(2) Smooth blasting parameters

1) The thickness of smooth blasting layer, that is, the minimum resistance line, is generally 10 ~ 20 times of the hole diameter. For those with weak rocks and developed cracks, take a small value.

2) Generally, the hole spacing is 0.75 ~ 0.90 times of the thickness of smooth blasting layer. For those with weak rocks and developed cracks, take a small value.

3) The borehole diameter and charge decoupling coefficient should be selected according to presplitting blasting.

4) The line charge Qx is generally determined according to the calculation formula of loose blasting charge.

Where q is the unit consumption of loose blasting, kg/m;

A -- smooth blasting hole spacing, m;

W—— thickness of smooth blasting layer, m

(4) Charge structure and initiation

(1) cost structure

1) The function of the plugging section is to prolong the action time of explosive gas and ensure that only cracks are produced in the orifice section without blasting funnel. For deep hole blasting, the length of this section is generally 0.5 ~1.5m. ..

2) The length of the hole bottom reinforcement section is approximately equal to that of the plugging section. Because the bottom of the hole is sandwiched by rocks, it is necessary to use a larger linear charge density.

3) Uniform charge section This section is generally axially spaced and uncoupled charge, which requires uniform distribution along the hole axis. Axial interval charges must be rolled up and detonated in series with detonating cord. In order to ensure that the hole wall is not crushed, the cartridge should be placed in the center of the hole as much as possible. In foreign countries, the core locator is generally used to locate the cartridge, while in China, the cartridge and detonating cord are usually tied to bamboo sheets to locate the cartridge.

(2) Start

In order to ensure simultaneous initiation, presplitting blasting and smooth blasting generally use detonating cord to initiate, and usually adopt subsection parallel method.

Because the smooth blasting hole is the last initiation, the detonating cord may be damaged in advance. In order to ensure the quasi-explosion of the surrounding holes, the double initiation method of high-stage delay detonator and detonating cord can be used for smooth blasting holes. If the pre-splitting hole and the hole in the main blasting area form the same network for initiation, the pre-splitting hole should be initiated 75 ~ 100 ms before the first row of main blasting holes. v