When rotary drilling rigs drill into mudstone or sandstone, slippage often occurs—that is, no resistance yet no ability to apply pressure, making drilling impossible or difficult, thereby affecting construction progress and production efficiency. Through long-term construction and accumulated experience, and having participated in numerous mudstone geological foundation construction projects, I have gained certain understanding and insights into drilling mudstone. By integrating geology, drilling tools, and operation, the challenge of drilling mudstone has ultimately been solved.

　　I. Overview of Mudstone

　　Mudstone, sandy mudstone, argillaceous sandstone, sandstone, shale, and other sedimentary rocks, because they contain abundant oxides, appear red, dark red, or brown; these rocks are collectively called red sandstone. Red sandstone mainly presents two typical structural forms: granular clastic structure and muddy cementation structure. Most red sandstone can disintegrate and break apart, or even become muddy, under the action of the atmospheric environment, so the size of its rock blocks and particle gradation will change with the time process of dry-wet cycles. Mudstone is a sedimentary rock solidified from mud and clay; its composition and structure are similar to shale but it is less likely to break. It is a clay rock with indistinct bedding or foliation. Mudstone has water-absorbing and bonding characteristics.

　　II. Causes of Slippage

　　During construction, if drilling mudstone in a dry hole, using a spiral drilling bucket gives relatively high drilling efficiency; if a double-bottom sand-fishing bucket is selected, drilling is also possible, but the load is relatively large. Once water is encountered in the hole, slippage immediately occurs. Therefore, slippage is directly related to water or mud. Thus, the main causes of mudstone slippage consist of the following points: 1. the hardness of mudstone; 2. the lubricating effect of water or mud; 3. water absorption and softening of mudstone, etc.

　　III. Drilling with Different Tools

　　1. When a spiral drilling bucket drills water-bearing mudstone, because the mudstone softens upon contact with water, the conical head at the front end of the spiral drilling bucket becomes packed with mudstone debris and soil, forming a whole, and the tool loses its drilling function. After it can no longer drill continuously, water or mud enters between the drilling bucket and the mudstone and lubricates it, thus forming slippage.

　　2. When a double-bottom pick drilling bucket is used to drill water-bearing mudstone, because the pick drilling method uses the alloy head at the front end to drill in, the alloy head easily penetrates the mudstone; however, since the alloy head at the front end is very short, the pick body fixing the alloy head must follow and cut in, but the pick body is too blunt, thus creating resistance. Because the pick penetrates the mudstone too shallowly and the mudstone has cementing properties, the crushing effect cannot be achieved. Therefore, continuous drilling is impossible, and water or mud enters between the picks and the mudstone and lubricates it, thus forming slippage.

　　3. From the above analysis, mudstone has good cementation and is not easy to break; sharper tools are needed, with deeper cutting penetration and low penetration resistance, to achieve the effect of cutting and crushing.

　　IV. Methods for Handling Slippage in Mudstone Drilling:

　　Therefore, a double-bottom sand-fishing bucket is selected, and cutting-type drilling conforms to the above analysis. However, in practice, the sand-fishing bucket may also slip, though this involves many factors.

　　1. Defects on the drilling bucket, such as the central positioning tip being too long, too wide, or too blunt; the angle of the tooth tips and the gap between tooth holders; the diameter of the side teeth; the model of the bucket teeth or wear of the tooth tips. It is best to improve these by creating height differences in tooth tip length and reducing the contact area.

　　2. The diameter of the sand-fishing bucket: the larger the diameter of the drilling bucket, the larger the contact area, and the harder it is to overcome slippage. Therefore, for larger pile diameters, a smaller-diameter drilling bucket can first be selected for drilling.

　　3. Analysis of overcoming slippage: as mentioned earlier, the main cause is slippage caused by lubrication by water or mud. Therefore, as long as the softened mudstone and the water or mud on the surface are overcome, mudstone slippage can be overcome. After overcoming it, continuous drilling must be maintained so that water or mud cannot enter the cutting surface; in this way, the tooth tips remain in contact with dry geology and normal drilling can proceed.

　　4. Operational control is very important. Because mudstone is relatively hard and has good cementation, it is not easy to cut into under the lubrication of water. First, raise the power head to the highest position, then control the power head to rotate slowly, with slow and continuous pressure application. The rotation and pressure application speeds of the power head must be well coordinated. If the pressure stroke is completed without cutting into the mudstone, raise the power head again and continue drilling until the geological load increases upon cutting in. After that, continuous drilling must be maintained. When the load is too heavy, the power head needs to be raised to reduce the load, thereby protecting the drilling rig components. When the load decreases, pressure application must continue to prevent water or mud from entering the cut surface and reducing the friction coefficient.

　　5. Classification of slippage: during drilling, because the mudstone is softened, two types of slippage occur. One is bottom-supported slippage, the so-called slippage caused by soil falling when drilling in mud layers; the other is mudstone slippage. Because mudstone is relatively hard, drilling and cutting are inherently difficult, and under the lubrication of water or mud it slips. The two types of slippage are different, so the type of slippage must be clearly distinguished to handle it effectively.

　　6. Distinguishing the two types of slippage is crucial for handling slippage, and the treatment methods are also different. If it is slippage caused by softened mudstone with bottom support, then reverse rotation followed by forward rotation with pressure can overcome it. If it is slippage of relatively hard mudstone caused by water reducing the friction coefficient, it can be overcome through the series of integrated methods described above. Because of differences in cementing materials and degrees of weathering, the strength of mudstone varies greatly; therefore, not all mudstone can be drilled using the cutting method.