Long-Distance Impact Detonation of Energy-concentrating Interval Charges and Its Application in Tunnel Blasting

Deep inside the earth, where tunnels are carved through solid rock, the work of excavation relies on a delicate balance of force and precision. To break rock without shattering the tunnel's walls or wasting energy, engineers often use a technique called air-deck charging. Instead of filling a drill hole completely with explosives, they leave small gaps of air between sections of the charge. This arrangement allows the blast energy to spread more evenly, giving the rock a better chance to fracture cleanly rather than crumble. For decades, the standard way to set off these separated charges has been a thin, flexible cord filled with high explosives, running the length of the hole. While effective, this method is complex to install and carries significant safety risks. In recent years, engineers have sought a safer, more reliable alternative that can trigger multiple explosive sections from a single point without the need for long, fragile cords.

A team of researchers from Shandong University of Science and Technology and several railway construction groups has explored a new approach that replaces the traditional cord with a focused burst of metal. Their work focuses on a device that, when triggered, creates a high-speed jet of metal that travels down the tunnel hole to strike the next section of explosive. The challenge lies in the distance; as the metal jet travels, it slows down and spreads out, much like a stream of water losing pressure as it moves away from a hose. The researchers wanted to know exactly how far this metal jet could travel before it became too weak to trigger the next explosion, and whether it could reliably set off a chain of explosives in a real tunnel environment.

To answer these questions, the team combined advanced computer modeling with physical field tests. They began by simulating the behavior of the metal jet as it formed and traveled through a steel pipe, which mimics the conditions of a drilled blast hole. Using powerful software, they tracked the speed of the jet and the pressure it exerted on the explosive material it hit. The simulations revealed that the metal jet moves faster than the shockwave from the initial explosion, reaching the next charge first. When the jet strikes the explosive, it creates a tiny, intense spot of heat and pressure, known as a "hot spot," which ignites the material. The computer models showed that this process works reliably over distances of up to 2.49 meters in theory, though the researchers noted that real-world conditions would likely reduce this slightly.

The team then moved from the computer to the ground to test these findings in reality. They constructed a setup using seamless steel pipes to simulate the tight constraints of a tunnel blast hole. They placed sections of emulsion explosive, a common type used in mining, at specific intervals. One section was equipped with a special cone-shaped liner designed to focus the explosion into a metal jet, while the subsequent sections waited to be triggered. In the first set of tests, they placed the explosives 2.35 meters apart. The result was a success: the metal jet traveled the full distance, struck the second charge, and caused it to detonate without fail. In a second set of tests, they arranged four sections of explosive with 1-meter gaps between them. The metal jet successfully triggered the first three sections, and even when the jet began to break apart, the shockwave from the exploding sections was strong enough to set off the final charge.

These experiments confirmed that the metal jet could reliably initiate explosives over long distances, far exceeding the capabilities of previous short-range tests. The researchers found that as the distance increased, the speed of the jet decreased, which in turn increased the time it took for the explosion to start. However, even at the maximum tested distance of 2.35 meters, the jet retained enough energy to trigger the explosion. This discovery suggests that the method is robust enough to replace the traditional detonating cord in many tunneling scenarios.

The practical implications of this finding were demonstrated in a real tunnel project. The construction team replaced the standard detonating cords with these energy-concentrating devices for ten separate blasting cycles. The results were immediate and measurable. The new method improved the stability of the blast, meaning the holes were used more effectively to break the rock. Specifically, the rate of "half-holes"—the clean, visible marks left on the tunnel wall that indicate a precise blast—increased by nearly 5 percent compared to the traditional method. Furthermore, the new technique reduced the cost of consumables by approximately 3,000 yuan per cycle, primarily by eliminating the need for expensive detonating cords and reducing the time required for installation. The study concludes that this long-distance impact detonation method offers a safer, more efficient, and more economical way to blast tunnels, potentially changing how engineers approach deep excavation in the future.