“With the rapid development of the pile foundation industry, rotary drilling rigs occupy most of the market share thanks to their environmentally friendly, convenient, and efficient construction technology. However, under complex construction conditions, rotary drilling rigs inevitably have some shortcomings, and inclined holes are one of them. Engineering specifications require that borehole verticality be controlled within 1%, and some sites may even require verticality of 5‰.”

　　Inclined holes not only have a severe impact on project quality but also cause huge economic losses and psychological impact for the construction party. In particular, during the process of reaming and correcting deviation, they may cause major secondary accidents, including: severe overbreak, hole collapse and drilling tool burial, and even equipment failures in the rotary drilling rig such as drill pipe key strip fracture, power head oil leakage, and wear. How to effectively prevent borehole deviation accidents is an important task in the construction process of rotary drilling rigs.


　　2.1 Geological Causes

　　Uneven distribution of mechanical strength in strata can easily induce hole deviation. Practice has found that inclined rock strata, limestone karst cave strata, boulder and erratic boulder strata, hard rock strata, strata with alternating soft and hard layers, and pebble layers are all prone to hole deviation accidents.


　　2.2 Equipment Causes and Diagnosis

　　(1) Mast deviation

　　Due to reasons such as equipment, commissioning, or operator handling, the automatic mast verticality adjustment function of some rotary drilling rigs cannot guarantee the verticality of the mast, and the mast may deviate slightly. During construction, uneven settlement of the chassis caused by vibration of the rotary drilling rig, pressure leakage in the oil cylinder, and other reasons can also cause the originally vertical mast to deviate. Mast deviation can be detected by borrowing a theodolite or total station from the construction party and using the instrument to inspect mast verticality.


　　(2) Causes related to the drill pipe

　　When there are problems with the straightness of the drill pipe or the square head of the drill pipe, borehole deviation may also occur.
　　Judgment methods:
　　① Drill pipe problems can be manifested through the wear condition of the drilling tool; frequently used drilling tools will show greater wear on one side and less wear on the other.
　　② Suspend a heavy object with a soft thin rope and fix it near the square head of the drill pipe, then slowly rotate the drill pipe and measure the distance between the four planes of the drill pipe square head and the vertical line to determine whether the drill pipe square head or the drill pipe is deviated.

　　(3) Causes related to the drilling tool

　　The causes related to drilling tools are relatively diverse. For example, the drill bit connection square and the central guide cone are not located at the center of the barrel; the arrangement of drill teeth is unreasonable, causing unbalanced forces during drilling; the barrel taper is too large; and the interior of the barrel is structurally restricted (unlocking structure, connecting shaft, or inclined plate, etc.), resulting in slag accumulation, among other issues. In all cases where hole deviation occurs due to drilling tool reasons, the drilling tool must be subjected to unbalanced forces, and the wear of the barrel wall, guide strips, and drill teeth will also be asymmetric.

　　2.3 Operational Causes

　　Constrained by fixed thinking and operating habits, operators cannot flexibly handle geological changes and uneven settlement of the chassis caused by vibration during construction. For example, at the interface between soft and hard rock layers or at inclined rock locations, continuous pressure application artificially causes borehole deviation. In strata prone to deviation, priority should be given to drilling tools with greater barrel height, and conical drilling tools should be avoided as much as possible to increase the guiding performance of the drilling tool. Reduce the applied pressure, maintain a constant rotation speed, and if necessary, drill with the drill pipe suspended until the deviation-prone stratum is penetrated, then apply pressure for drilling.