Rotary drilling construction is an advanced pile construction method that has developed rapidly in China in recent years. Rotary drilling rig construction features high efficiency, low noise, environmental friendliness, high hole-forming quality, and a high degree of mechanization, among many other characteristics. The rotary drilling rig is the executor of this method. Whether foreign products or domestic equipment, their structural forms and working principles are largely similar, but the formations being constructed vary greatly, and the specific technical requirements of construction differ as well. Therefore, only by selecting different drill bits can the above requirements be met. Scientifically selecting drill bits and using them reasonably can, to a certain extent, enrich the construction techniques of rotary drilling rigs and broaden their construction applications.

　　There are many factors affecting the selection of rotary drilling bits, which can be summarized into three main aspects: formation conditions; rig functions; and specific requirements such as hole depth, hole diameter, sediment thickness, and wall protection measures.

　　1. Classification of currently common rotary drilling bits

　　Common rotary drilling bits include auger bits, rotary drilling buckets, core barrel bits, underreaming bits, percussion bits, grab bucket bits, and hydraulic grabs. Each type is briefly introduced below.

　　1.1 Auger bits

　　(1) Conical type: double-flight double-start, suitable for hard bedrock. Double-flight single-start, suitable for weathered bedrock, cobbles, and ice-bearing frozen soil. The above bits are equipped with various cutting teeth, and through changes in tooth profile, pitch, and helix angle, many types of bits can be derived.

　　(2) Straight auger bits:

　　a. Bucket-tooth straight auger: double-flight double-start, suitable for sand and poorly cemented small-diameter gravel layers; double-flight single-start, suitable for sand and soil layers; single-flight single-start, suitable for poorly cemented large-diameter cobbles, cohesive soil, and hard gumbo.

　　b. Cutting-tooth straight auger: available in double-flight, triple-flight, and quadruple-flight, suitable for hard bedrock or cobble-gravel.

　　1.2 Rotary drilling buckets

　　By installed teeth, they can be divided into cutting-tooth buckets and bucket-tooth buckets; by number of bottom plates, into double-bottom buckets and single-bottom buckets; by number of doors, into double-door buckets and single-door buckets; by bucket taper, into tapered buckets and straight buckets; by bottom plate shape, into pot-bottom buckets and flat-bottom buckets. Combining these structural forms, together with variations in whether vent holes and door-opening mechanisms are included, dozens of rotary drilling buckets can be configured. Generally speaking, double-bottom buckets are applicable to a wider range of formations, while single-bottom buckets are only suitable for strongly cohesive soil layers; double-door buckets are applicable to a wider range of formations, while single-door buckets are only used for large-diameter cobbles and hard gumbo.

　　1.3 Core barrel bits

　　Two types are currently common: cutting-tooth core barrels (suitable for medium-hard bedrock and cobble-gravel) and roller-cone core barrels (suitable for hard bedrock and large boulders). Within these two major categories of core barrel bits, there are versions with and without coring devices, depending mainly on the difficulty of coring. Because roller-cone core bits are mainly used for hard rock drilling and the annular area drilled is large, if conditions permit, reverse circulation drilling can also be added to the bit section to improve drilling efficiency.

　　1.4 Underreaming bits

　　To increase the bearing capacity of a single pile without increasing pile diameter or pile depth, design departments often use underreamed piles. Rotary drilling rigs can construct underreaming without any modification, simply by selecting an underreaming bit. Underreaming bits are commonly mechanical type; this kind of bit is relatively simple to use and maintain, available in upward-opening and downward-opening versions, with the opening mechanism generally being a four-bar linkage, used in soil layers, strongly and moderately weathered formations, and even hard bedrock. Since rotary drilling is non-circulation drilling, after underreaming is completed, a cleaning bucket can be used to remove cuttings.

　　1.5 Percussion bits and grab bucket bits

　　When drilling large-diameter cobbles, large boulders, and hard bedrock, using percussion bits and grab bucket bits in conjunction with rotary drilling is particularly effective. These bits are operated by being suspended from the auxiliary hook of the rotary rig. Because an impact action is required, the auxiliary hook should have a free-fall rope release function for better results.

　　1.6 Hydraulic grabs

　　Currently, construction of diaphragm walls and cutoff walls is increasingly common. With slight modifications to the rotary drilling rig, hydraulic grabs can be used. The opening and closing of the hydraulic grab is hydraulically driven; the grab has only one cylinder, so only two oil lines in and out and one control valve are needed.

　　2. Selecting bits according to geological conditions

　　The main function of a rotary drilling rig is to form holes and trenches in the ground. The working object is rock. Because the pile holes constructed are relatively shallow, the rock has undergone complex changes in structure, grain size, porosity, cementation, occurrence, and compressive strength due to tectonic movements and natural mechanical and chemical actions. In addition, China has a vast territory, so the working objects of rotary drilling rigs are particularly complex. They can be summarized into the following categories.

　　By lithology (common): mudstone-shale, sandstone, limestone, granite, etc.; by origin: igneous rock, sedimentary rock, metamorphic rock; by mechanical properties: competent, plastic, and loose. So how should bits be selected according to formation conditions? They are introduced by category below.

　　(1) Clay: use a single-bottom rotary drilling bucket; if the diameter is relatively small, a two-petal bucket or a bucket with a soil-unloading plate can be used.

　　(2) Silt, weakly cohesive soil layers, sand, and poorly cemented cobble layers of small particle size: a double-bottom rotary drilling bucket can be used.

　　(3) Hard gumbo: use a rotary drilling bucket with a single soil inlet (either single or double bottom), or a bucket-tooth straight auger.

　　(4) Frozen soil: for low ice content, bucket-tooth straight auger buckets and rotary drilling buckets can be used; for high ice content, conical auger bits can be used. It should be noted that auger bits are effective for soil layers (except silt), but they must be used in the absence of groundwater to avoid suction effects that may cause sticking.

　　(5) Well-cemented cobble-gravel and strongly weathered rock: conical auger bits and double-bottom rotary drilling buckets are required (single-port for larger particle sizes, double-port for smaller particle sizes).

　　(6) Moderately weathered bedrock: equipped with cutting-tooth core barrel bits -- conical auger bits -- double-bottom rotary drilling buckets, or cutting-tooth straight auger bits -- double-bottom rotary drilling buckets.

　　(7) Slightly weathered bedrock: equipped with roller-cone core barrel bits -- conical auger bits -- double-bottom rotary drilling buckets; if the diameter is relatively large, staged drilling techniques should also be adopted.

　　3. Selecting bits according to the functions of the rotary drilling rig

　　If the rotary drilling rig has continuous wall capability, hydraulic grabs or double-wheel milling cutters can be selected for continuous construction. If the auxiliary winch has an automatic rope release function, percussion bits and grab bits can be selected to drill large-diameter cobbles or boulders. If equipped with an air chamber and water swivel, air-liquid drilling can be carried out. If equipped with dual power heads or a casing oscillator, casing drilling can be carried out, which is an effective method for solving quicksand, broken cobble-gravel, and silt layers.

　　4. Selecting bits according to other factors

　　If sediment thickness requirements are relatively strict or underreaming drilling is being carried out, a dedicated cleaning bucket is required. When drilling with casing, the clearance between the bit and the casing is small. To prevent the bit from catching or colliding with the casing and to reduce suction effects, the selected bit should have vent holes, be equipped with a guiding device, and have an opening-closing mechanism as close to the bit center as possible, preferably integrated with the machine operator. This special problem can also be solved by selecting combined-form bits.