What Rock Drilling Tool Is Used in Tunnel Construction

Tunnel construction is one of the most demanding jobs in civil engineering, and the tools used to bore through rock determine how fast, how safe, and how economical the whole project runs. Whether a crew is driving a highway tunnel through granite, excavating a metro line under a city, or cutting a drainage adit through mixed ground, the rock drilling tools on site have to match both the geology and the excavation method. This article walks through the main rock drilling tools used in tunnel construction, what each one does, and how to pick the right set for a project.

Two ways to excavate a tunnel

Broadly speaking, tunnels are excavated in two ways. The first is drill-and-blast, where crews drill a pattern of holes into the tunnel face, load them with explosives, and blast the rock loose. The second is mechanical excavation, where a tunnel boring machine (TBM) or roadheader cuts the rock directly. Drill-and-blast is still the most widely used method worldwide, especially for hard rock and for tunnels of irregular shape or variable length. Mechanical excavation is preferred for long, uniform tunnels and in soft ground where blasting is not allowed.

Each method relies on different rock drilling tools, and the sections below look at them in turn.

Thread button bits: the workhorse of drill-and-blast

In drill-and-blast tunneling, the holes in the face are drilled by tunneling jumbos — multi-boom rigs that position hydraulic rock drills against the rock. The actual cutting edge is the thread button bit threaded onto the drill rod. Thread button bits are available in the standard thread sizes used across the industry — R25, R32, T38, T45 and T51 — and in diameters from around 45 mm up to 127 mm, so they can be matched to the blast hole size the tunnel design calls for.

Button bits come in different face shapes. Flat face bits are the general-purpose choice for most ground. ｄｒｏｐ center bits have a recessed center that helps keep the hole straight in broken or fissured rock. Retrac bits, which have a slightly reduced gauge, are used where the ground is soft or where the bit has to be pulled back easily through the hole. The buttons themselves — the tungsten carbide inserts that do the crushing — come in ballistic, spherical and conical profiles, and the right profile depends on the rock. Spherical buttons resist impact and abrasion well in hard rock, while ballistic buttons bite faster in softer formations.

For a tunnel crew, the practical point is that the bit, the rod and the rock drill have to work as one system. A bit that is too small for the rod wastes energy, and a bit with the wrong button profile wears out early in hard ground and slows down the round cycle.

DTH drilling tools: for deep and large-diameter holes

When a tunnel project needs larger-diameter holes — for example, for pilot holes, drainage holes, or probe drilling ahead of the face — down-the-hole (DTH) drilling is often the better choice. In DTH drilling, the hammer sits directly behind the bit at the bottom of the hole, so the impact energy is delivered straight to the rock instead of being lost through a long string of rods. That makes DTH drilling tools efficient in hard, abrasive rock and at depth.

DTH bits are matched to the hammer by their connection (such as DHD, CIR, QL or BR series) and come in diameters from about 76 mm up to 254 mm and beyond. High-pressure hammers are used in hard rock for maximum penetration, while low-pressure hammers are a more economical option in softer ground. DTH drilling tools are also the standard choice for the water wells and boreholes that often accompany tunnel projects.

Drill rods and accessories: the link between drill and bit

Between the rock drill and the bit, the drill rod carries both the rotation and the impact energy, so rod quality has a direct effect on drilling speed and hole straightness. Tunnel crews commonly use threaded rods in the same R and T series as their bits — R32, T38, T45, T51 — plus tapered rods for smaller hand-held drilling and shank adapters that let one rod fit different rock drills. Keeping rods and bits in the same thread series avoids mismatches and premature thread wear.

Mechanical excavation: TBM cutters

On TBM-driven tunnels, the rock is not blasted but cut by disc cutters mounted on the cutterhead. Disc cutters press into the rock face under high load and roll across it, fracturing the rock along the cutting path. In hard rock, single disc cutters with wear-resistant rings are the norm; in mixed ground, double disc cutters spread the load. TBM cutters are a specialist product in their own right, and most tunneling contractors source them separately from the drill-and-blast tooling described above.

Core drilling: knowing the ground before you blast

Before a single blast hole is drilled, the ground ahead of the tunnel has to be understood. Geotechnical investigation holes are drilled with diamond core bits — electroplated, impregnated, TSP or surface set — to recover rock cores for testing. Impregnated core bits, which release fresh diamond as the matrix wears, are the usual choice in hard, abrasive rock, while surface set bits suit softer formations. The same core drilling equipment is used to probe ahead of the face during construction to check for faults, water, or weak zones.

Choosing the right rock drilling tool for the job

There is no single "best" rock drilling tool for tunnel construction — the right choice depends on the formation, the hole size and depth, and the drilling equipment on site. As a rule of thumb:

• Hard, abrasive rock with a drill-and-blast cycle calls for thread button bits with spherical buttons in the R32–T51 range, matched to the jumbo's rock drills.
• Large-diameter or deep holes call for DTH hammers and bits sized to the hammer connection.
• Soft or broken ground calls for retrac button bits that can be pulled back cleanly.
• Geotechnical and probe holes call for diamond core bits matched to the rock.

For contractors and project teams, working with a supplier that carries the full range — button bits, DTH tools, drill rods, core bits and the accessories in between — makes it easier to keep the whole system consistent. Xi'an Heaven Abundant Mining Equipment Co., Ltd. (TY Drill Bits), a manufacturer based in Xi'an, China, supplies exactly this kind of one-stop range of rock drilling tools, from thread button bits and DTH drilling tools to diamond core bits and drill rods, with OEM support and ISO9001-certified production. If you are planning a tunnel project and need to match your rock drilling tooling to the ground conditions, it is worth discussing the specifications with a supplier before the first hole is drilled.