The breakthrough strategy for rotary drilling machines in complex geological conditions

        In various infrastructure construction projects, the complexity of geological conditions often becomes a "blocker" hindering the progress of the project. Among them, karst caves and gravel layers, as typical complex geological formations, pose unprecedented challenges to rotary drilling machine construction.

Karst caves are mostly developed in areas of soluble rocks such as limestone. Their internal shapes vary greatly; some are narrow and deep, some are spacious and open, and they may even be interconnected to form a complex underground cave network. During rotary drilling machine construction, if encountering karst caves, leakage of the mud is very likely to occur. The mud quickly drains away, causing imbalance in the hole pressure, and the hole wall loses the support and protective effect of the mud, thereby triggering collapse accidents. In severe cases, it may even cause the drilling rig to sink, bury the drill bit, and other major equipment accidents, forcing the construction to be interrupted. This not only delays the construction schedule but also significantly increases the construction cost. Moreover, the roof of karst caves has inconsistent strength, and the drill bit may get stuck due to uneven force during passage, making it difficult for the drilling tools to be lifted and lowered normally, thereby affecting the construction efficiency.

        The gravel layer is mainly composed of stones of various sizes and shapes that are piled up. It is commonly found in river alluvial plains and pre-glacial fans. The particles in the gravel layer have large pores and a loose structure, with strong water permeability. This makes it difficult for the mud to form an effective mud skin on the hole wall during rotary drilling machine drilling. The stability of the hole wall is poor, and it is prone to collapse. At the same time, the gravel has high hardness and an irregular distribution, and the drill bit will be subjected to strong impact and wear during cutting. The wear of components such as the bucket teeth and drill rods intensifies, and frequent replacement of the drill tools not only increases the construction cost but also slows down the construction progress. Moreover, due to the unevenness of the gravel layer, during the drilling process, the hole skew phenomenon is very likely to occur, making the verticality of the pile body difficult to meet the design requirements, affecting the bearing capacity and stability of the pile foundation. These construction problems in karst caves and gravel layers seriously restrict the smooth progress of rotary drilling machine construction and urgently require scientific and effective construction strategies to be addressed.

Characteristics and Distribution of Caves

        The formation of caves is due to the long-term erosion of soluble rocks such as limestone by groundwater. The formation principle is that the insoluble calcium carbonate in limestone, under the combined action of water and carbon dioxide, transforms into soluble calcium bicarbonate. When water containing calcium bicarbonate is heated or the pressure suddenly changes, calcium bicarbonate decomposes and re-precipitates as calcium carbonate, and this process repeats cyclically, gradually shaping caves of various shapes. In China, regions such as Guangxi, Yunnan, and Guizhou, due to their unique geological conditions, have become areas with widespread distribution of caves. The famous Guilin landscape is a typical example of cave landform, with its strange peaks and unique caves attracting countless tourists, showcasing the divine craftsmanship of nature.

        According to size classification, caves can be divided into small caves less than 1 meter in size, medium-sized caves larger than 1 meter but less than 4 meters, and large caves larger than 4 meters. Although small caves are relatively small in size, during drilling, they may also experience drill bit force imbalance when contacting the cave roof, causing the drill hole to deviate; medium-sized caves have relatively larger internal space and may contain complex fillings such as plastic-like clay and silt, which have poor stability and are prone to collapse or shrinkage problems; large caves are even more complex, not only possibly having connected underground cave networks but also possibly connected to underground rivers. If slurry leaks during construction, the consequences can be unimaginable, and even serious accidents such as drill machine sinking or drilling being buried may occur.

1. Prevention of Inclined Holes

        In the karst strata, the existence of inclined rock faces or half-rock sections makes it very easy to form inclined holes during rotary drilling. This seriously affects the verticality of the pile body and the bearing capacity of the pile foundation. To effectively prevent inclined holes, the method of using a tube drill and an embedded rock drill bit in combination for drilling can be adopted. First, use the tube drill to drill and form a stable guide, creating a free surface for subsequent drilling. At the same time, appropriately raise the tube drill to enhance its guiding effect. Then, the rock drill bit is used for drilling and debris removal. In terms of operation, it is essential to adopt light pressure or floating pressure drilling until the drill bit is fully embedded in the rock layer, and then resume normal drilling to ensure the verticality of the hole and avoid the occurrence of inclined holes.

2. Solution for Stuck Drills

        When the drill bit passes through the karst roof, stuck drill situations often occur, manifested as the collision and deformation of the drill frame. This not only hinders the construction progress but also may cause damage to the drill bit. To address this issue, additional guide plates can be welded onto the front end of the square drill frame to ensure a smooth transition between the drill frame and the karst roof, reducing the risk of stuck drill. For sufficiently strong drill frames, it is also possible to weld cutting teeth on the corresponding side plates and orient them towards the stuck rock mass. By cutting the stuck rock, the space of the karst cavity can be expanded, facilitating the removal of the drill bit and allowing the construction to proceed smoothly.

3. Handling of Collapse Holes

        The overlying strata of karst caves and unstable karst fill layers are prone to collapse holes and reduced diameters during construction. The karst fill material is mostly plastic-like clay or silt, and the stability is extremely poor after drilling. The collapse material will flow along the hole wall to the bottom of the hole. In such cases, it is necessary to repeatedly remove debris at the same depth until the hole wall is basically stable. If after prolonged debris removal, the drilling depth still remains stagnant, backfilling measures should be taken. Fill the severely collapsed area with crushed stones and packaged loose clay or block-shaped cement slurry in a certain ratio, then let the drilling machine perform floating and reversing operations to compact the backfill material and stabilize it for a period of time before re-drilling to ensure the stability of the hole wall.

4. Handling of Excessive Volume

        The presence of karst caves often leads to excessive volume during concrete pouring, increasing construction costs. Different sizes of karst caves require different solutions. For independent low karst caves smaller than 1 meter, after hole formation, concrete can be poured to fill them directly, and the small excess volume is considered normal. For slightly higher karst caves larger than 1 meter but smaller than 4 meters, or when there are karst caves distributed around the surrounding pile holes, crushed stones and packaged loose clay or block-shaped cement slurry can be backfilled to fill the empty karst cavities and then re-drill; for high karst caves larger than 4 meters, since it is difficult to completely fill them with the filling material and the backfilling efficiency is low and the construction is cumbersome, cement mortar or low-grade concrete can be backfilled. Wait for it to set before re-drilling to ensure the complete filling of the connected karst caves and maximize the saving of concrete.

5. Protection of Drill Teeth

        Although the rocks in karst strata are often fragmented and have developed fractures, they are densely compacted and the drill bit is subjected to uneven force during drilling, mostly in an impact force state. This causes severe wear of the drill frame and drill teeth, leading to problems such as tooth breakage and drill bit damage. To protect the drill teeth, the drill teeth should be welded for reinforcement, and the welding joints of the drill head components should be checked frequently for any cracks. In operation control, strictly follow the drilling principle of "low drilling speed and variable pressure", and flexibly adjust the drilling speed and pressure according to different geological conditions and drilling situations to effectively avoid accidents such as tooth breakage and drill bit damage, and extend the service life of the drilling equipment.

The characteristics of the pebble layer

        The pebble layer is widely distributed in China, commonly found in rivers and geological silt plains, etc. It is a complex stratum often encountered during rotary drilling machine construction. It is mainly composed of rock debris with uneven particle size, irregular shape, and varying degrees of weathering, as well as primary minerals such as quartz and feldspar, presenting a single-grain structure, blocky or pseudo-banded structure. This stratum has the characteristics of irregular arrangement, large pores, strong water permeability, low compressibility, and high shear strength. Due to the large pores and loose structure between the pebbles, the mud cannot form an effective casing mud layer on the hole wall, resulting in poor hole wall stability and easy collapse. At the same time, the high hardness of the pebbles causes the drill bit to be subjected to strong impact and wear during drilling, accelerating the wear of components such as the bucket teeth and drill rods, significantly increasing the construction cost. Moreover, the unevenness of the pebble layer is also prone to cause hole deviation during drilling, affecting the quality and bearing capacity of the pile foundation.

Construction Measures

1. Lowering the casing and anti-collapse measures

When encountering a shallow layer of pebbles, problems such as difficulty in lowering the casing and collapse at the hole opening are common. For professional Baomei drilling rigs equipped with casing drivers, by driving the casing down and pressing it down once every 40 centimeters during drilling, these problems can be effectively solved. For domestic drilling rigs without casing drivers, the first step is to position the casing correctly and compact the hole opening to prevent collapse caused by the casing falling during construction, which could lead to large-scale collapses and prevent accidents such as the drilling rig overturning. Additionally, in this geological condition, laying steel plates beneath the drilling rig is also essential, as it can increase the rig's stability and prevent the drilling rig from tilting or sinking due to ground collapse.

2. Leakage control

Leakage is a challenging issue in the construction of pebble layers. Due to the large gaps between pebbles, the solid particles in the mud cannot be promptly blocked, even if the mud consistency is adjusted, it is difficult to provide effective protection for the pure pebble layer. At this time, using yellow clay is an effective solution. Pouring relatively dry yellow clay into the hole and using the drill bit of the rotary drilling rig to descend into the hole and reverse (it is best to use a scraper with a water passage for sand捞) to forcibly smear the yellow clay onto the hole wall. During the operation， let the drill bit rotate forward for one turn， raise the drill 2 - 3 meters， and then dig 40 - 50 centimeters deeper. Repeat this process until the pebble layer is passed. Then， replace the drill bit with a diameter smaller than the designed pile diameter (for a 1.5-meter pile， a 1.25-meter drill bit can be used) to continue drilling to the bottom of the hole and finally use the designed diameter drill bit to enlarge the hole until it is completed. This method can reduce lateral resistance and internal resistance， lower the probability of collapse and excessive filling， and improve drilling speed and safety.

3. Optimization of the drill bucket

The drill bucket is a key component of the rotary drilling rig， and its performance directly affects the construction efficiency and quality. During drilling in pebble layers， the drill bucket will encounter various resistances， including the positive resistance of the bucket teeth during rotary cutting of the geological layer， the side resistance generated by the friction between the drill barrel and the external geological layer， and the internal resistance caused by the weight of the dirt in the drill barrel and the friction between the dirt and the inner wall of the barrel. To reduce these resistances， the drill bucket can be optimized. First， change the length of the bucket teeth， separate the bucket teeth into ordered lengths， so that the inner teeth contact the geological layer first， as their lever arm is the shortest， and they are subjected to the least resistance， making it easier to cut and crush. As the pressure increases during drilling， the longer bucket teeth will follow and drill in， thus separating the drilling resistance， achieving small resistance and fast advancement； second， adjust the position of the tooth seat to ensure a reasonable gap between the two groups of bucket teeth， making them complementary and interlocking， ensuring that the drill bucket can cut all geological layers during a 360-degree rotation， reducing the occurrence of bottoming out caused by uncut remaining geological layers； third， set up discharge slots at the lower part of the drill barrel， opening a semi-elliptical discharge slot and adding edge knives on the side， so that the dirt remaining on the sides of the drill bucket can be acted upon by the edge knives and enter the drill barrel through the discharge slot， effectively reducing lateral resistance.

4. Mud selection

Mud plays a crucial role in rotary drilling rig construction. It is not only a lubricant during the drilling process， but also has multiple functions such as pressure， wall protection， and suspension. When constructing in pebble layers， it is particularly important to select the appropriate mud ratio based on geological conditions. The main problem with leakage in pebble layers is due to the gaps between pebbles. Increasing the mud viscosity and the particle size and content of the solid particles， using yellow soil， red soil， or clay to make the mud， while increasing the mud consistency， viscosity， and particle content， and appropriately ignoring the sand content， can form an effective wall layer， solving the general leakage problem. In addition， using a one-time drilling method can reduce the number of times of wall protection and leakage. During up and down drilling， it is strictly prohibited to rotate the drill bucket to prevent the edge knives， edge teeth， and skirt edge from damaging the wall layer， causing secondary leakage.

5. Operating Points

Operating a rotary drilling rig is a technical task. Experienced operators can adjust the drilling parameters promptly according to different geological conditions to ensure the smooth progress of the construction. When drilling in an卵石 layer， the first step is to control the rotation speed of the drill pipe to avoid excessive speed that causes uneven force on the drill bit， thereby reducing wear and equipment vibration. At the same time， based on the hardness of the卵石 layer and the drilling situation， adjust the drilling pressure reasonably to prevent excessive pressure or insufficient pressure from affecting the drilling efficiency and the quality of the borehole. During the drilling process， closely monitor the conditions inside the hole. If abnormal phenomena such as a drop in the mud level or collapse of the hole wall are detected， stop the drilling immediately and take corresponding measures. During operation， also pay attention to safety to ensure the stability of the drilling rig and prevent accidents caused by improper operation. During the drilling process， ensure the verticality of the mast to avoid the borehole from tilting， which will affect the bearing capacity of the pile foundation.

Pre-construction Preparation

        Before construction, detailed geological surveys are of utmost importance. It is like a doctor conducting a comprehensive examination before treating a patient. Only by understanding the "condition" of the geological formation can one prescribe the appropriate treatment. Through geological surveys, we can obtain detailed information about the strata, including the specific location, size, shape, fill material of the sinkholes, as well as the particle size, distribution, and density of the pebble layers. These pieces of information provide a scientific basis for the formulation of subsequent construction plans, just as an army map is crucial for an army. When formulating the construction plan, we should carefully select the type of rotary drilling rig based on the results of the geological survey, combined with the specific requirements and characteristics of the project. Different models of rotary drilling rigs have differences in torque, lifting force, drilling diameter, and depth, and the appropriate equipment should be selected based on the actual geological conditions and project requirements. At the same time, the construction sequence should be reasonably arranged. For areas with complex distribution of sinkholes and pebble layers, priority should be given to the construction of less-influential pile positions to avoid mutual interference during the construction process. Adequate materials should also be prepared, such as mud materials, casing pipes, concrete, etc., to ensure the continuity of material supply during the construction process.

Construction Process Monitoring

        During the construction process, real-time monitoring of equipment operating parameters is a key link to ensure construction safety and quality. We should closely monitor parameters such as drilling speed, torque, and pressure of the rotary drilling rig. These parameters are like the "vital signs" of the equipment, reflecting the working condition of the equipment and changes in the strata. If any abnormal parameters are detected, such as a sudden decrease in drilling speed or a sharp increase in torque, the drilling should be stopped immediately, and inspection and analysis should be conducted to identify the cause and take corresponding measures. At the same time, we should closely observe the conditions inside the hole. By monitoring changes in the color, smell, and sand content of the mud, we can determine the stability of the hole wall and whether there are any leakage problems. Utilizing advanced hole imaging technologies, such as drilling television, we can visually understand the geological conditions inside the hole and the verticality of the drilling. Timely detect and handle abnormal situations inside the hole. In addition, regular inspections and maintenance of the equipment should be carried out to ensure its normal operation and reduce the impact of equipment failures on the construction progress and quality.

Personnel Technical Training

        The technical level and ability to handle complex situations of construction personnel directly affect the quality and safety of the construction. Therefore, technical training for construction personnel is indispensable. The training content should include the operating skills of the rotary drilling rig, maintenance and maintenance knowledge, safety operation procedures, and construction techniques and response measures in complex geological conditions. Through a combination of theoretical explanations and practical operations, construction personnel should gain a deep understanding of the working principle and performance characteristics of the rotary drilling rig, and master the operating skills and methods for troubleshooting common faults. At the same time, organizing construction personnel for case analysis and experience sharing, sharing successful experiences and lessons learned from construction in complex geological conditions, can enhance their ability to handle complex situations and decision-making skills. In addition, regular assessments should be conducted for construction personnel to ensure that their technical level and operational skills meet the requirements.

        Under complex geological conditions, caves and pebble layers pose numerous challenges for rotary drilling machines. However, through a series of targeted construction strategies, these challenges can be effectively addressed, ensuring the smooth progress of the construction. When constructing in cave strata, measures such as preventing inclined holes, resolving stuck drills, handling collapse holes, dealing with overfilling, and protecting drill teeth need to be taken based on the size, shape, and fillings of the caves. Each step is closely interrelated and indispensable. When constructing in pebble layers, efforts should be made from aspects such as the lower casing, prevention of collapse and leakage, optimization of the drill bucket, selection of mud, and operational key points to comprehensively ensure the safety and quality of the construction. These construction strategies have been continuously summarized and improved through practice, and are of great significance for enhancing the construction efficiency and quality of rotary drilling machines in complex geological conditions. Looking ahead, with the continuous advancement of technology, rotary drilling machine construction technology will also usher in new development opportunities. The application of intelligent technology will enable rotary drilling machines to perceive changes in the strata in real time, automatically adjust drilling parameters, and achieve more precise and efficient construction. The development and application of new materials will enhance the wear resistance and strength of drilling tools, extend their service life, and reduce construction costs. At the same time, with increasingly strict requirements for environmental protection, green construction technologies will also become the development direction for rotary drilling machine construction, such as using environmentally friendly mud, reducing noise and dust pollution during the construction process, etc. We believe that with the promotion of technological innovation, rotary drilling machine construction in complex geological conditions will become safer, more efficient, and more environmentally friendly, making greater contributions to infrastructure construction.