Regarding the differences between the rotary drilling tool - steel sheet casing and double-walled casing

When mentioning casing, we first think of the short casing commonly used in the market, which is often made of 8 to 10mm steel sheet rolled into shape. A single section of such casing is typically 1m or 2m long. It is primarily used to stabilize unstable upper strata, prevent the collapse of the borehole opening, and keep objects around the borehole from falling into it. The burial depth of such casings on construction sites is generally not too deep, ranging from 2 to 6 meters, depending on specific engineering requirements. While the casing plays a partial role of a protective tube, due to its material and structural limitations, its burial depth is restricted when using different machinery for sinking, thus limiting its application. The production of the casing should comply with design requirements. If there are no design requirements, the following provisions should be followed:

1. The casing can be made of steel plate or reinforced concrete. The finished casing should meet the requirements of the drilling method for its rigidity and strength. Steel casings are suitable for various construction conditions, are highly safe, and can be reused multiple times. Reinforced concrete casings can be used in drilling with shallow water depths.

2. The thickness of the casing is determined comprehensively based on the casing material, hydrogeological conditions of the borehole location, construction method, design pile diameter, casing diameter, burial depth, and burial method. If necessary, it should be determined through calculation. Generally, the thickness of steel casings for bored pile construction should be 6-12mm, and for reinforced concrete casings, it should be 8-250mm.

3. The inner diameter of the casing should be slightly larger than the design pile diameter, and the specific value should be determined based on the type of drill used.

The burial depth of the casing should comply with design requirements. If there are no design requirements, the following provisions should be followed:

1. When burying the casing in dry land or shallow water, for impermeable bottom layers, the burial depth should be 1.0 to 1.5 times the outer diameter of the casing, but not less than 1.0m. For permeable bottom layers such as sandy soil and silt soil, the burial depth is the same, but it is advisable to replace the soil to a depth of at least 0.5m below the cutting edge of the casing with impermeable soil. The replacement diameter should exceed the casing diameter by 0.5 to 1.0m.

2. In deep water and riverbeds with thick soft soil and silt layers, the cutting edge of the casing should penetrate into the impermeable layer. If there is no impermeable layer, it should penetrate into the gravel or cobble layer for 0.5 to 1.0m.

3. For riverbeds affected by erosion, the cutting edge of the casing should penetrate into the general erosion line for at least 1.0m. For riverbeds severely affected by local erosion, the cutting edge of the casing should penetrate into the local erosion line for at least 1.0m.

4. In seasonal frozen soil areas, the cutting edge of the casing should penetrate into the unfrozen soil layer below the frost line for at least 0.5m. In permafrost areas, the cutting edge of the casing should penetrate into the permafrost layer for at least 0.5m.

The casing used in full casing pipe construction differs from ordinary casings. It can be classified into single-walled casing and double-walled casing based on its structure, and can be divided into single sections of 2m, 3m, and 4m in length. Currently, double-walled casing construction is mainly adopted in the market. Double-walled casing sections adopt a design with inner and outer double-walled tubes, which can effectively improve the rigidity of the casing. Its structural design ensures that the outer surface is flush with the ground and the inner surface is flat, reducing the frictional resistance applied to the casing by the external borehole wall during the process of lowering the casing. Meanwhile, the inner tube wall can provide better guidance for the drilling tool, preventing it from scraping against the inner wall of the casing. In terms of connection methods, the casing adopts a button-type male/female joint form, with the number of joints varying according to the diameter of the casing. This connection method is in line with international standards, ensuring compatibility with both domestic and international markets. Additionally, it allows for quick connections, ensuring a tight and efficient construction process for full casing pipe construction.

When constructing with a rotary drilling machine, it is a highly technical and systematic engineering task. Only by fully understanding the equipment performance, the drilled strata, site conditions, drill tool selection, and operating procedures