Drilling methods of rotary piles in different strata

Drilling in clay layer

Drilling in different strata by rotary drilling rigs requires different considerations. The main contents are nothing more than how to drill effectively, how to drill efficiently, how to prevent construction accidents, how to deal with construction accidents, etc. Specifically, it can include what method to use, what drill rod to choose, what drill bit to choose, how to operate drilling, how to modify the drill bit according to the stratum, how to use mud and casing, etc.

Definition of clay: In engineering, it should be divided into silty clay and clay according to the plasticity index. Soil with a plasticity index greater than 10 and less than or equal to 17 should be named silty clay, and soil with a plasticity index greater than 17 should be named clay.

Although clay is the most suitable stratum for rotary drilling rig construction, the following problems are often encountered in actual construction:

1. Drill suction. It often occurs in soft plastic and fluid plastic clay;

2. Failure to drill. It often occurs in friction rod construction, manifested as slippage or sticking of the drill

- Causes of sucking drill

1) Saturated clay and silty clay flow to the center of the hole, resulting in local necking;

2) Expansive clay absorbs water and expands, causing local necking;

3) Slag sticks to the drill and wraps the side wall of the drill bit.

- Treatment of sucking drill

1) Pay attention to slow lifting and slow release during drilling to minimize disturbance to the hole wall;

2) Pay attention to the single bucket penetration depth not being too deep. For clay layers, the single bucket penetration depth is generally about 1/2 of the barrel height;

3) Add water holes in the hole to ensure convection of upper and lower mud.

Drilling in sand layers

- Characteristics of soft soil and other easily collapsed strata and risks of hole formation

In soft soil, newly accumulated backfill soil, and newly deposited loose fine sand soil layers, if the groundwater level is high, after the rotary drilling rig digs the soil, even if the drilling hole is protected by mud, it is difficult to keep the hole wall stable, which is very likely to cause hole collapse or necking. Since this type of stratum is mostly distributed in the shallow surface layer, as the amount of collapse gradually increases, the ground around the borehole will collapse, often causing the casing to sink. In severe cases, it will also pose a threat to drilling equipment, operators, etc. When encountering such a situation, especially when it is found that the hole casing is already suspended or sunk during construction, you should not continue drilling with a fluke, but backfill the borehole as soon as possible, re-fix the hole casing, and then drill.

- Construction method of rotary drilling

1) Use segmented casing to cross unstable layers, mainly for the situation where the surface encounters a quicksand layer and the short casing cannot protect the wall.

2) Use high viscosity and high specific gravity mud to protect the wall.

- Operation precautions

1) When drilling in the sand layer, it is necessary to lift, drill and release slowly to reduce the flow rate of mud between the drill bucket and the hole wall, reduce scouring and prevent scratches.

2) Appropriately increase the area of ​​the water flow hole in the drill bucket to reduce the negative pressure at the top and bottom during the pull-out process of the drill bucket, thereby reducing the flow rate of mud in the hole

3) Appropriately increase the outer angle of the drill teeth to increase the distance between the hole wall and the side wall of the drill bucket.

4) Configure high-quality mud to protect the wall and measure the sand content of the mud in the hole in time. Take effective measures in time when it exceeds the standard.

5) Check the sealing of the bottom plate of the drill bucket after closing. If it is found that the gap is large due to distortion, it should be repaired in time to avoid sand leakage.

Drilling of pebble formations

Pebble is a mechanically loose formation with large pores, strong water permeability, low compressibility, and high shear strength. The penetration rate is cut into the gaps between the pebbles by the rotation of the drill teeth to squeeze the stones into the drill bucket. The pebbles are arranged irregularly, have low compressibility, uneven force on the teeth during drilling, severe vibration of the whole machine, and the drill rod jumps and swings, which are very harmful to the drill rod and the tool. The operator needs to understand the distribution law of the formation, and at the same time, determine the density of pebbles, the number and location of boulders according to the changes in load and vibration and the swing of the drill rod. The pebble and boulder formation is a river sedimentary phase, with more than 50% of pebbles with a particle size greater than 20mm, and the local boulder particle size is 600-1000mm. The density of pebbles and boulders is medium dense and dense. The pebble composition is mainly sandstone, quartzite and granite, and the filling is mainly sand, silt and clay. During the construction process, pay attention to the following points:

- Ensure the verticality of the mast

- The ground supported by the drilling rig cannot be weak or uneven in hardness and softness, and the site needs to be leveled and compacted. Ensure the verticality of the mast during drilling, monitor it through the display, and make adjustments in time.

- Wall protection: The formation has poor cementation and is prone to leakage or hole collapse, especially in the medium-dense pebble layer. The wall protection should be buried below the groundwater level. When stirring the mud, sawdust, pulp, and cottonseed chips can be added to the mud. It is best to add the above substances to the impact drilling hole to form a high-viscosity mud. When using this high-viscosity mud, the head height is more than 1.5m above the groundwater level. If there is no thick layer of silt sand and silt clay, the specific gravity of the mud should also be higher, reaching more than 1.3. During drilling, pay attention to whether the casing is loose or collapsed around it, and whether the casing is in place. At the same time, pay attention to the bubbles on the surface of the mud.

- Drilling of Pebble Soil

Due to the large variation in the particle size of pebbles, when drilling, it is necessary to select the appropriate construction method according to the size of the pebble particle size:

1) For small-size crushed pebbles between 20mm and 100mm, use a drilling bucket for efficient drilling;

Bauer teeth or B47K teeth

High skirt

Peripheral picks assist in pre-cutting

2) For medium-size crushed pebbles between 100mm and 500mm, develop a double-layer barrel drill or barrel to achieve efficient drilling

Adopt a new drilling idea--compression

Customize the inner and outer barrels according to the particle size of pebbles and crushed stones

High inside and low outside, effectively reducing drilling resistance

3) For crushed pebbles (also known as blocks and boulders) with a particle size of more than 500mm, use barrel or other drill bits to achieve efficient drilling

-Drilling Precautions

1) Lift and lower the drill bit as slowly and evenly as possible to prevent the drill bit from scratching the hole wall, protect the mud skin, and avoid hole collapse. When drilling, low speed, high torque, and clear mud high-speed flushing of the hole wall are required. Strictly control the single bucket footage depth, and strictly prevent the occurrence of drilling. At the same time, the water flow hole is long and thin to prevent stones from coming out of the water flow hole and avoid drilling. Double-layer rock-embedded simple drill can be used for strata with high gravel content. Short spiral drill bits can be used to deal with strata containing isolated stones and boulders. After loosening, use pick-tooth simple drill to fish them out.

2) When lifting the drill and reversing the power head, it is strictly forbidden to start the operation mode quickly, which will cause the resistance to increase instantly, break the wire rope, twist the drill rod, etc. When the drill rod is held by the load, it is strictly forbidden to drill hard, resulting in resistance having nowhere to escape and damaging the drill rod and drilling tools. When the lifting has an abnormal increase in resistance, it is strictly forbidden to lift hard to prevent the drill tool from getting stuck.

3) In the dense pebble layer, the skeleton particles are large in content and staggered, with dense and continuous contact, which makes the drill teeth unable to cut into the pores of the stone, resulting in slippage or drill rod jumping. Drilling should be done at a slow speed. The drill rod should be suspended by the main winch wire rope, and the pebbles should be moved by the gear. The method of alternating forward and reverse rotation should be adopted. When the forward rotation encounters greater resistance, the reverse rotation should be immediately adopted, and then the forward rotation should be repeated. After the pebbles are loosened, the drill rod should be floated down. If the penetration rate is fast, the heavy load drilling state should be maintained by point pressure. The back pressure should not be applied continuously. If the penetration rate is not reached for a long time, in order to prevent the hole from collapsing, the drill bit needs to be lifted for a while, lowered again, and the free surface drilling should be re-established.

4) The hook of the bottom mechanism of the drill bucket opening and closing is easily stuck by pebbles and fails, and there is also the possibility of unhooking at the bottom of the hole. It is best to seal the hook with a steel plate. If it is not closed and a sand bucket is used, the bucket door should be completely closed every time to ensure that the hook is completely hung to prevent the bottom of the bucket from opening in the hole.

Drilling of backfill soil layer

Backfill soil layer is widely distributed in my country, especially in urban areas where old foundations are being renovated and in mountainous areas with limited flat space. Because the surface of the mountain is rugged and the flat space is limited, artificial backfilling of depressions is relatively common. The thickness of the backfill soil layer ranges from more than ten meters to dozens of meters, and the backfill period ranges from a few years to more than ten years. There are many types of backfill materials with very different properties. They are mainly plain fill and miscellaneous fill. Plain fill is mainly composed of clay, sand or gravel, with a small amount of broken bricks, tiles and other debris. Miscellaneous fill is mainly construction waste, domestic waste or industrial waste. In thick backfill soil layers, traditional pile foundation construction basically adopts manual digging, rotary drilling rigs and impact drilling rigs. Rotary drilling rigs and impact drilling rigs are cheap and have low construction costs, but they are very polluting, have poor hole quality and low drilling efficiency; manual digging has low construction costs, but has a large safety risk and low construction efficiency. Therefore, rotary drilling rigs are needed to successfully solve this construction problem and meet the existing urgent needs of users.

Due to the poor stability of thick backfill soil layers, it is very easy to cause hole wall collapse during the construction of rotary drilling rigs, causing leakage, hole collapse, drill stuck and buried drill accidents. In addition, due to the irregular arrangement and complex composition of thick backfill soil, when the rotary drilling rig is drilling, there are often unfavorable working conditions such as uneven force on the drill tool, severe vibration of the whole machine, drill rod jumping and drill rod deflection, which are very harmful to the drill rod and drill tool. To solve the above unfavorable working conditions, the operator is required to understand the distribution law of the stratum and determine the density of the backfill soil, the number and position of the boulders.

There are many types of backfill materials such as large boulders, large concrete blocks, bricks and tiles, with loose structures, uneven sizes and poor stability. Therefore, the hole wall is unstable during the drilling process, and it is very easy to leak slurry or collapse. There are mainly two conventional wall protection methods: mud wall protection and full casing wall protection. Mud wall protection is simple to operate and has low construction cost. It can be used when the large rocks are not too large and there is clay filling; full casing wall protection is more complicated to operate and has high construction cost and low construction efficiency. In the case of widespread distribution of large rocks, full casing wall protection is a more ideal and effective way.

Mud wall protection: The requirements for mud are very high. The mud specific gravity must reach more than 1.3, the viscosity must reach more than 30S, and the head pressure of the mud must be guaranteed. When stirring the mud, sawdust, pulp, cottonseed chips and other materials can be added to the mud. During the drilling process, pay attention to whether the surrounding of the casing is loose or collapsed, and whether the casing is in place. At the same time, pay attention to the situation of bubbles on the surface of the mud to judge the stability of the hole wall in the hole

Full casing wall protection: The method of pre-burying casing is adopted, that is, the rotary drilling rig buries the casing first, and then drills. This function can be achieved by using a casing driver. For thick backfill soil layers, the drilling resistance is very large, and the casing must be buried while drilling, and the two must be carried out in combination.

Drilling in mudstone formations

Mudstone is a sedimentary rock solidified by mud and clay. It has no obvious stratification and is blocky. It is mainly composed of fine-grained sediments. It has a large internal force and low compressive strength, generally between 0.17-10Mpa. It belongs to the category of extremely soft rock and soft rock, and has strong water absorption and cementation. When the rotary drilling rig drills into the mudstone formation, the mud invades the contact surface between the teeth and the native rock formation, reducing the friction between the teeth and the native rock formation, causing the teeth to slide on the surface of the native rock formation and unable to cut into the rock formation, so there is a phenomenon of slippage and difficulty in advancing. Slippage is a major problem in drilling in mudstone formations. This problem must be solved to speed up the construction progress and improve production efficiency.

- Analysis of slippage

There are two forms of slippage in mudstone drilling, namely bottoming slippage and cutting slippage.

1) Bottom slip

During drilling, the drill cuttings absorb water and become a very sticky plastic clay material that sticks to the drill teeth and the bottom of the drill. The bucket teeth lose their cutting effect and cannot continue drilling. When the drill bucket is reversed to close the bottom of the bucket before lifting the drill, the bottom of the bucket will be reversed with the drill bucket and cannot be closed because there is no resistance at the bottom of the bucket. During the process of lifting the drill, the slag in the bucket falls out. After the second drilling, the tooth tip is directly inserted into the slag, and the bottom plate of the drill bucket is directly pressed on the slag. As the drill bucket rotates and drills, the uneven slag and gaps that fall are completely compacted. When the slag can no longer be compressed, the bottom plate is supported by the slag, and the bucket teeth repeatedly rotate in the grooves previously drawn, forming a bottom slip phenomenon.

2) Hard mudstone slip

The friction coefficient of hard mudstone is reduced under the lubrication of mud, and the pressure provided by the drill rod is insufficient. The bucket teeth cannot cut and drill, and a smooth surface is formed on the contact surface between the bucket teeth and the mud layer, resulting in the phenomenon of hard mudstone slip.

- Treatment of slippage

1) Treatment of bottom slippage

If the loose soil in the hole is soft mud, you can quickly and repeatedly operate the power head to pressurize forward and reverse, which will instantly have a huge impact on the drill bucket, causing friction between the bucket teeth and the loose soil. The pressure is instantly transmitted to the loose soil, cutting and squeezing the loose soil into the drill bucket.

If there is a lot of loose soil in the hole and it is hard mudstone, you can replace it with a short spiral drill bit with a slightly smaller diameter to solve the slippage. There is some gap between the small-diameter spiral drill bit and the hole wall. The fallen soil is squeezed to the side under the weight of the drill rod and the pressure. The drill bit can drill normally when it contacts the solid soil. After lifting the drill, the loose soil squeezed to the side falls into the small-diameter hole, and the slag that does not fall into the hole is squeezed into the hole by the large-diameter drill bucket that is drilled twice, so that the large-diameter drill bucket can drill normally.

2) Treatment of hard mudstone slippage

When the ultimate bearing capacity of hard mudstone is above 500kpa, it requires huge pressure to cut and crush. It is necessary to use a interlocking kelly bar with a short spiral drill bit and a double-bottom rock-entering drill bucket for drilling.

- Selection and optimization of drilling tools

Double-bottom sand buckets should be used for drilling in mudstone formations, and V20 drill teeth are preferred. The drill bucket should be optimized through the following points:

1) Adjust the number, position, angle and model of teeth to reduce the contact area and increase the pressure on the rock surface;

2) The positioning cone cutter should be as sharp as possible, with moderate length and width, and contact the rock surface in a linear and point manner;

3) The teeth are staggered and interlaced to avoid overlapping cutting trajectories of the teeth;

4) For rock formations with sticky drill residues, a side soil inlet is set slightly behind the side cutter;

5) In order to achieve the initial pressure, the drill bit sometimes needs to be weighted