How to select the appropriate drill bit for a rotary drilling rig?

During the construction process, the drill bit of a rotary drilling rig experiences the most rapid wear and frequently requires replacement.

How do you select the appropriate drill bit for a rotary drilling rig? The following guide provides an overview categorized by drill bit type and geological formation conditions.

Numerous factors influence the selection of rotary drilling bits: ground conditions, drilling rig capabilities, and specific requirements such as hole depth, hole diameter, sediment thickness, and hole-wall protection measures.

Classification of Rotary Drilling Bits

Common rotary drilling tools include augers, rotary buckets, core barrels, underreamers, impact tools, grab augers, and hydraulic grabs.

• Spiral Drill Bit

Conical Type:Double-head, double-helix bits are suitable for hard bedrock. Double-head, single-helix bits are suitable for weathered bedrock, pebbles, frozen soil, and similar strata. The aforementioned drill bits can be fitted with various cutting teeth; furthermore, by varying the tooth profile, pitch, and helix angle, numerous specialized bit types can be derived.

• Straight Spiral Auger

Bucket Teeth Straight Flight Type:

Double-head double-flight, suitable for sandy soil and small-diameter gravel layers with poor cementation;Double-head single-flight, suitable for sandy soil and soil layers;Single-head single-flight, suitable for large-diameter pebbles with poor cementation, cohesive soil and hard gumbo clay.

Cutter Bit Augers:

Available in double-flight, triple-flight, and quadruple-flight configurations; suitable for hard bedrock or cobble-and-gravel formations.

Rotary Drilling Buckets

Based on the type of teeth installed, they can be classified into cutting-tooth buckets and bucket-tooth buckets.

Based on the number of bottom plates, they can be classified into double-bottom buckets and single-bottom buckets.

Based on the number of discharge gates, they can be classified into double-gate buckets and single-gate buckets.

Based on the taper of the barrel, they can be classified into tapered-barrel buckets and straight-barrel buckets.

Based on the shape of the bottom plate, they can be classified into hemispherical-bottom buckets and flat-bottom buckets.

By combining the structural configurations described above—and incorporating variations such as the presence of vent holes or different gate-opening mechanisms—dozens of distinct types of rotary drilling buckets can be produced. Generally speaking, double-bottom buckets are suitable for a wide range of geological strata, whereas single-bottom buckets are applicable only to highly cohesive soil layers. Similarly, double-gate buckets are suitable for a broad spectrum of ground conditions, while single-gate buckets are best suited for drilling through large-diameter pebbles and hard clay.

•    Core Drill Bit

Currently, there are two common types:

Cutter Bit Core Barrel (suitable for medium-hard bedrock and cobble-gravel).

Roller Bit Core Barrel (suitable for hard bedrock and large boulders).

Among the two main categories of core bits—those equipped with a coring mechanism and those without—the choice primarily depends on the difficulty of the coring operation. Since roller-cone core bits are predominantly utilized for drilling through hard rock and cover a large annular surface area, it is also possible—where conditions permit—to enhance drilling efficiency by integrating a reverse-circulation system into the bit assembly.

• Underreamer

To enhance the bearing capacity of a single pile—without increasing its diameter or depth—design departments frequently opt for underreamed piles. When constructing underreamed sections using a rotary drilling rig, no structural modifications to the machine itself are required; one simply needs to select the appropriate underreaming bit. Mechanical-type underreaming bits are the most commonly used variety; they are relatively simple to operate and maintain, and are available in both upward-opening and downward-opening configurations. Their expansion mechanisms typically utilize a four-bar linkage system, making them suitable for use in a wide range of geological conditions—including soil layers, strongly and moderately weathered strata, and even hard bedrock. Since rotary drilling is a non-circulatory process, debris removal is straightforward: once the underreaming operation is complete, a muck bucket is simply used to clear the excavated material.

•  Percussion Drill Bit, Percussion Grab Cone Bit

When drilling through large-diameter pebbles, massive boulders, or hard bedrock, the combined use of impact bits and grab-cone bits—integrated with rotary drilling operations—proves particularly effective. These specialized bits are deployed via the auxiliary hoist of the rotary drilling rig; given the requirement for an impactful striking action, the auxiliary hoist must be equipped with a free-fall function to ensure optimal performance.

•     Hydraulic Grab

Currently, the construction of diaphragm walls and impermeable cutoff walls is becoming increasingly common. With only minor modifications, a rotary drilling rig can be adapted to perform this work. The opening and closing of the hydraulic grab are hydraulically actuated; since the grab features only a single hydraulic cylinder, its operation requires merely two hydraulic lines—one for inflow and one for outflow—and a single control valve.

Select drill bits based on geological conditions.

The primary function of a rotary drilling rig is to excavate boreholes and trenches in the ground; its operational target is rock. Since the piles being constructed are typically shallow, the rock—having been subjected to tectonic movements as well as natural mechanical and chemical processes—exhibits complex variations in its structure, grain size, porosity, cementation, attitude, and compressive strength. Furthermore, given the vast geographical expanse of my country, the rock formations encountered by rotary drilling rigs are exceptionally complex. Broadly speaking, they can be categorized into the following types.

Based on lithology (common types), rocks are classified as mudstone/shale, sandstone, limestone, granite, etc.; based on origin, they are classified as igneous, sedimentary, or metamorphic rocks; and based on mechanical properties, they are categorized as hard, plastic, or loose. So, how does one select the appropriate drill bit based on specific formation conditions? The following sections provide a categorized overview.

• Clay: Select a rotary drilling bucket with a single-layer bottom; if the diameter is relatively small, a two-petal bucket or a bucket equipped with a soil-ejection plate may be used.
• For silt, weakly cohesive soil layers, sand, and poorly cemented pebble layers with small grain sizes, a double-bottom drilling bucket may be utilized.
• Hard Clay: Select a rotary drilling bucket with a single soil inlet (either single-bottom or double-bottom configurations are suitable), or one equipped with straight-spiral bucket teeth.
• Permafrost: For layers with low ice content, straight-tooth auger buckets and rotary drilling buckets may be used; for layers with high ice content, conical spiral auger bits are recommended. It should be noted that spiral auger bits are effective in all soil strata (with the exception of silt); however, they must be used in the absence of groundwater to prevent suction effects that could cause the tool to become jammed.
• Well-cemented cobbles and gravel, and highly weathered rock: Requires the use of a conical auger and a rotary drilling bucket with a double-layer bottom (single-opening type for larger particle sizes; double-opening type for smaller particle sizes).
• Bedrock Drilling Configuration: Equipped with a roller-bit core barrel—a conical auger—and a double-bottom rotary drilling bucket; alternatively, a cutter-bit straight auger paired with a double-bottom rotary drilling bucket.
• Slightly weathered bedrock: Utilize a combination of roller-cone core bits, tapered augers, and double-bottom rotary drilling buckets; if the diameter is excessively large, a staged drilling process must also be employed.

• Select the drill bit based on the functions of the rotary drilling rig.

If the rotary drilling rig is equipped with continuous operation capabilities, it can be fitted with a hydraulic grab or a double-wheel cutter to perform continuous construction. If the auxiliary winch features an automatic rope-release function, percussion bits or percussion grabs can be selected to drill through large-diameter pebbles or boulders. If equipped with an air chamber and a water swivel, it can perform air-fluid drilling. Furthermore, if fitted with a dual power head or a casing oscillator, it can execute casing drilling—an effective method for addressing issues involving flowing sand, fractured pebble and gravel layers, and silt strata.

• Select the drill bit based on other factors.

If strict requirements exist regarding the thickness of sediment accumulation, or when performing underreaming operations, a specialized muck bucket must be utilized. When drilling inside casing, the clearance between the drill bit and the casing is minimal; to prevent the bit from snagging against the casing and to mitigate suction effects, the selected bit should be equipped with vent holes. Furthermore, the bit should feature a guiding mechanism, and its opening-and-closing mechanism should be positioned as close as possible to the bit’s center—ideally, integrated directly into the bit assembly. Alternatively, this specific challenge can be addressed by opting for a modular or composite-style drill bit.

Whether you are looking for tricone bits, DTH bits, PDC bits, or special application drilling tools, our experienced engineering team can provide tailored solutions based on your specific working conditions, such as rock hardness, drilling depth, and equipment compatibility.