In recent years, rotary drilling rigs have become a common sight at bridge, railway, and highway construction sites in various specifications due to their high efficiency, environmental friendliness, high hole-forming quality, high degree of mechanization, and wide adaptability to different strata, and are highly favored by construction units. However, because China has a vast territory with variable strata and complex, unpredictable geological conditions, construction technical requirements differ for different stratum structures. Therefore, how to scientifically select and reasonably use drill bits of different structures for different geological conditions is the key to ensuring smooth foundation construction.

The drill bits used by rotary drilling rigs mainly include rotary drilling (rotary) buckets, auger bits, and rock bits. Among them, rotary drilling buckets are the most widely used, mainly for construction in soft strata free of sand and gravel or containing very small particles of sand and gravel. Auger bits are mainly used for weathered bedrock and gravel layers with small particle sizes. Rock bits are used for construction in large boulder layers and hard rock layers. This article elaborates on the structure of the widely used rotary bucket.

The rotary drilling bucket consists of several parts, including connecting seat 1, opening/closing mechanism 3, bucket body 4, bucket bottom 5, and reaming mechanism 2, as shown in Figure 1.

The connecting seat is connected to the drill pipe via a connecting pin. The opening/closing mechanism presses down on the impact rod through the downward movement of the power head's impact body; the impact rod drives the hook plate to open and close the bucket bottom. At the same time, a limit rod restricts the downward travel of the impact body, ensuring the bucket bottom opens fully without causing damage to related components. According to construction experience, the bucket body structure is generally designed as a regular frustum, though straight cylindrical structures are also used under different geological conditions (relatively rare). For different soil conditions (varying sand content), the bucket bottom can be made as a single-bottom drilling bucket or a double-bottom sand-grabbing drilling bucket.

Below the bucket bottom, two symmetrically arranged hinged doors are provided that can only open inward toward the bucket. When the drilling bucket is drilling, the soil and sand cut from the hole bottom are pressed into the bucket through the hinged doors; during lifting, under the gravity of the mud and sand in the bucket, the two doors close downward to prevent sand and soil from leaking back into the hole. This structure is simple to manufacture, saves material, and is convenient to use. However, if the soil layer is loose and the gravity effect is not sufficiently reliable, the hinged doors may not close tightly, causing sand and soil to leak back into the hole, thereby reducing drilling efficiency and adversely affecting the cleaning of the hole bottom before construction completion. To overcome these drawbacks, a double-bottom sand-grabbing drilling bucket was designed, characterized by a bucket bottom composed of a fixed bottom and a movable bottom. After each rotary drilling operation, the movable bottom can rotate relative to an angle to achieve full opening and closing of the sand inlet of the bucket bottom. The rotation angle and door opening size are designed to ensure no soil leakage when the movable bottom is closed. When the drill pipe rotates to drive the drilling bucket clockwise for drilling, the bottom inlet is in an open state, and the opening and closing positions of the movable bottom are determined by limit blocks on the fixed bottom. When the drill pipe is to be lifted, it inevitably drives the drilling bucket to rotate counterclockwise by a certain angle. Due to the resistance of the soil, the movable bottom remains stationary, and the bucket body together with the fixed bottom rotates relative to the movable bottom to close the sand inlet, sealing all mud and sand inside the drilling bucket.

Based on structural differences, during rotary drilling rig construction, drill bits with different bucket bottom forms can be selected according to geological conditions. Single-bottom soil drilling buckets are suitable for construction in clay layers and dry hole conditions. Double-bottom rotary drilling buckets are mainly suitable for quicksand layer operations, sand and soil, and poorly cemented gravel layers with small particle sizes. Under wet hole drilling conditions, double-bottom rotary drilling buckets can also facilitate hole bottom cleaning. Construction practice has proven that in complex strata, the drilling efficiency and hole bottom cleanliness of double-bottom drilling buckets are significantly better than those of single-bottom drilling buckets.

In addition, the distribution, number, and welding angle of bucket teeth are also key to improving the operational efficiency of rotary drilling buckets, directly affecting the wear rate of the teeth and drilling efficiency. The number and distribution of bucket teeth must ensure essentially consistent loads within the drilling cutting face to avoid tooth breakage. Particular attention should be paid to the shape and welding position of the edge teeth, because these teeth are at the outermost layer of cutting and face the harshest load and wear environment; their welding angle needs timely adjustment according to different geological conditions. Only by promptly understanding the specific conditions of the strata being constructed and through continuous exploration and research can more experience be accumulated and the potential of rotary drilling rigs be fully realized.