Underground engineering pile foundation rotary excavating and bottom expanding construction method suitable for complex geology

Abstract

The invention discloses an underground engineering pile foundation rotary excavating and bottom expanding construction method suitable for complex geology, which comprises the following steps: reasonably adjusting parameters such as water quantity and slurry concentration for a hard interlayer in a drilling process to clean a soil body, placing a pile casing after reaching the position of a bottom expanding hole, and expanding the bottom for the second time; and the bottom expanding quality is guaranteed through the scale marks, the spiral steel sheets and accurate control over the expanding angle and depth of the pile casing. Then wall protection slurry injection, casing outer grouting, casing recovery and secondary hole cleaning are sequentially carried out, then a steel sheath is put in, secondary pipe sinking grouting is carried out, then a concrete guide pipe is put in for hole cleaning, finally high-pressure grouting is carried out, and concrete pouring and pile head treatment are completed; and meanwhile, according to the scheme, the steel sheath is made of a degradable polymer composite material, the construction quality and environmental protection requirements are considered, and a systematic and effective solution is provided for pile foundation construction in a complex geological environment.

Description

Technical Field

[0001] The invention relates to the technical field of building construction, and in particular to a rotary excavation and bottom expansion construction method for underground engineering pile foundations suitable for complex geology. Background Art

[0002] The construction of underground construction pile foundations plays a vital role in the entire construction field. It is the foundation for ensuring the stability and safety of buildings. Pile foundations can transfer the load of buildings to deep and stable foundation soil layers, effectively preventing the settlement and tilt of buildings. Especially in areas with complex geological conditions, such as soft soil foundations and karst areas, suitable pile foundation construction technology is directly related to the success or failure of construction projects. With the acceleration of urbanization, various high-rise buildings and large-scale infrastructures are constantly emerging, which puts higher requirements on the bearing capacity, construction accuracy and construction efficiency of pile foundations.

[0003] In the traditional construction of underground pile foundation rotary excavation and bottom expansion, the construction site is first leveled, the rotary drilling rig is put in place, and the pile position is marked with measuring tools. During the drilling process, the rotary drilling rig relies on the drill rod to drive the drill bit to rotate and cut the soil. When encountering a hard interlayer, it mainly attempts to penetrate by increasing the drilling force and increasing the mud circulation volume. After reaching the bottom expansion position, the pile bottom diameter is gradually expanded by mechanical rotation. In the wall protection link, ordinary mud wall protection is usually used, followed by grouting operations, and the slurry is injected into the pile hole through conventional grouting equipment. The hole cleaning operation often uses a simple pump suction method to remove the sediment at the bottom of the hole.

[0004] From the perspective of construction efficiency, when encountering complex geology such as hard interlayers, traditional construction methods such as increasing drilling force and mud circulation volume not only consume a lot of time, but also have poor penetration effects, seriously slowing down the construction progress and failing to meet the needs of modern engineering for efficient construction.

[0005] In terms of construction quality, ordinary mud wall protection has limited effect and is difficult to cope with groundwater pressure and soil stability problems under complex geological conditions. It is easy to cause hole collapse and affect the quality of the pile body. The mechanical rotary bottom expansion method has insufficient control accuracy on the diameter and shape of the bottom expansion, which reduces the bearing capacity of the pile foundation.

[0006] In terms of environmental impact, if the large amount of waste mud generated during traditional construction is not handled properly, it will cause pollution to the soil and water, which is not in line with the current green and environmentally friendly construction concept.

[0007] The present invention proposes new solutions to the above three problems. Summary of the invention

[0008] In view of the above technical problems in the traditional construction of underground engineering pile foundation rotary excavation and bottom expansion, the purpose of the present invention is to provide a construction method for underground engineering pile foundation rotary excavation and bottom expansion suitable for complex geology. Through the drilling process, the water volume, mud concentration and other parameters are reasonably adjusted for the hard interlayer to clean the soil. After reaching the bottom expansion hole position, the casing is placed and the bottom is expanded for the second time. The scale lines, spiral steel sheets and precise control of the casing expansion angle and depth are used to ensure the quality of the bottom expansion. After that, the wall protection slurry is injected, the casing is grouting outside the casing, the casing is recovered and the hole is cleaned for the second time, and then the steel casing is placed and the sinking pipe is grouting for the second time, followed by the concrete conduit to clean the hole, and finally high-pressure grouting is performed to complete the concrete pouring and pile head treatment;

[0009] At the same time, this solution uses degradable polymer composite materials to make steel sheaths, taking into account both construction quality and environmental protection requirements, and providing a systematic and effective solution for pile foundation construction in complex geological environments.

[0010] In order to achieve the above-mentioned object, the present invention provides a method for rotary excavation and bottom expansion of underground engineering pile foundations suitable for complex geology, comprising:

[0011] Step 1: Level the site, put the rotary drilling rig in place, mark the hole of the rotary drilling pile according to the pile foundation layout design and pile position size in the rotary drilling pile design drawing, and measure the verticality of the rotary drilling pile;

[0012] Step 2: Use a rotary drilling rig to drill and clean the soil in the hard interlayer section at the same time. The specific construction steps of the rotary drilling rig drilling and cleaning include:

[0013] Step 2.1: Drilling with rotary drilling rig;

[0014] Step 2.2: When the drill bit of the rotary drilling rig contacts the hard interlayer, increase the amount of drilling water and mud concentration, and perform uninterrupted hole sweeping;

[0015] Step 2.3: When the drill bit penetrates the hard interlayer section, reduce the amount of slag removal and mud flushing water, and clean the hard interlayer section at the same time;

[0016] Step 3: When the rotary drilling bit reaches the bottom hole expansion position, the rotary drilling rig is stopped, the casing is placed in the hole of the rotary pile, and the rotary drilling machine is started to expand the bottom hole for a second time to form a secondary bottom hole expansion;

[0017] Step 4: After the secondary bottom expansion is completed, the wall slurry injection device is used to inject the wall slurry into the casing and the secondary bottom expansion hole, and the stable observation is carried out;

[0018] Step 5: Grouting is performed outside the casing. After the grouting of the slurry wall is completed, a pressurized grouting device is installed on the casing to perform grouting. After the grouting is completed, the casing is pulled up to complete a wall grouting.

[0019] Step 6: After the primary wall grouting is completed, the pile hole is left to stand to allow the slurry to fully solidify, the casing is pulled up and recovered, and the hole is cleaned for the second time;

[0020] Step 7: The degradable steel sheath is placed in the rotary pile hole, and secondary pipe grouting is performed outside the degradable steel sheath;

[0021] Step 8: When the secondary immersed tube grouting is completed, place the concrete conduit into the degradable steel sheath and clean the hole again through the conduit;

[0022] Step 9: After the secondary hole cleaning is completed, place the high-pressure grouting machine next to the rotary pile hole, put the grouting pipe into the center of the pile position, start the high-pressure grouting machine, and the grouting pipe starts to deliver grout. When cement slurry appears at the mouth of the high-pressure grouting pipe, turn off the high-pressure grouting machine, check the sealing of the pipeline, and start grouting after the inspection is qualified;

[0023] Step 10: Pour concrete into the rotary drilling pile from the concrete guide pipe to the center of the pile position. After the concrete pouring is completed, close the top of the rotary drilling pile.

[0024] Furthermore, before the rotary drilling rig drills in step 2.1, the plane layer where the bottom expansion section and transition section of the rotary pile are located is first determined according to the rotary drilling design drawings, and the bottom expansion diameter is determined according to the bottom expansion design. With the center of the circle as the center, the rotary drilling drill rod is rotated to the bottom expansion design diameter range, the rotary drilling rig is started for drilling, and the hard interlayer section soil is cleaned using a cleaning device.

[0025] Furthermore, in step 3, when the rotary drilling bit reaches the position of the bottom hole expansion, the rotary drilling drill rod is first lifted 3-5 cm away from the bottom of the hole, the rotary drilling rig is stopped at this time, the casing is placed into the hole position of the rotary drilling pile in the vertical direction, and the rotary drilling pile fixes the casing at the same time, the rotary drilling rig is started, and the bottom expansion of the bottom hole is performed for the second time, wherein the vertical direction is consistent with the center direction of the bottom of the expanded hole, the central axis of the casing is consistent with the vertical direction of the center of the expanded hole, and during the secondary expansion of the bottom hole, the casing remains stationary, and the casing length is 1m<casing length≤3m;.

[0026] Furthermore, a spiral steel sheet is provided inside the casing, and scale lines are provided on the casing. The scale lines ensure that the casing vertically enters the secondary bottom expansion hole of the rotary bored pile, and the internal spiral steel sheet is used to enhance the wall protection effect, enhance the bearing capacity of the pile foundation and prevent the hole wall from collapsing.

[0027] Further, the specific steps of the secondary bottom expansion in step 3 are: start the rotary drilling machine, and when the lower part of the casing hits the hard interlayer section, start to expand, the expansion angle of the casing is 30-60°, the casing continues to rotate, and when the upper part of the casing hits the hole of the rotary pile, stop expanding, the upper part of the casing fits the inner wall of the hole of the rotary pile, and use the rotary drilling rig to adjust the position of the casing, so that the central axis of the casing coincides with the central axis of the secondary bottom expansion hole;

[0028] When it is determined that the central axis of the casing coincides with the central axis of the secondary bottom expansion hole, turn on the rotary drilling rig, rotate the casing in the vertical direction, and drive the casing to expand the outer layer of the secondary bottom expansion hole. The expansion depth is 30-80cm, and the upper end of the casing is placed outside the hole opening of the rotary pile. Then stop the rotary drilling rig, take out the spiral steel sheet, and lift the casing through the steel cable lifting device to separate the casing from the secondary bottom expansion hole, and the secondary bottom expansion is completed.

[0029] Furthermore, the degradable steel sheath is composed of a biodegradable polymer composite material steel sheath. After the project is completed, the degradable steel sheath can gradually decompose in the natural environment to reduce the pollution to the soil and groundwater.

[0030] The degradable steel sheath is composed of degradable polymers such as polylactic acid (PLA) and polyhydroxyalkanoate (PHA) as a matrix, with an appropriate amount of natural fiber reinforcement phase added.

[0031] Furthermore, in step 8, the conduit is cleaned by using air lift reverse circulation hole cleaning technology. After the degradable steel sheath is placed in the concrete conduit, high-pressure gas is injected into the conduit to form a gas-liquid mixed flow in the conduit. The rising speed of the gas-liquid mixed flow generates negative pressure to suck the sediment and mud at the bottom of the hole out of the hole.

[0032] Compared with the prior art, the rotary excavation and bottom expansion construction method of underground engineering pile foundation applicable to complex geology provided by the present invention has the following beneficial effects:

[0033] (1) Methods for dealing with complex geology: When drilling into hard interlayers, the amount of water, mud concentration, and slag removal are accurately adjusted according to the contact status between the drill bit and the hard interlayer. When the drill bit contacts the hard interlayer, increase the amount of water and mud concentration to give full play to the mud wall protection and slag carrying functions to assist drilling; after penetration, reduce the amount of slag removal and flushing water to maintain the stability of the hole wall and ensure smooth drilling under complex geological conditions;

[0034] (2) Precise control of casing expansion. Casing plays a key role in construction. Insert casing for secondary expansion. Ensure vertical entry through the scale lines on the casing. Use internal spiral steel sheets to enhance the wall protection effect. The casing is expanded according to the location and geological conditions. The expansion angle and depth are strictly controlled to ensure that the shape and size of the expanded bottom hole meet the design, enhance the bearing capacity of the pile foundation and prevent the collapse of the hole wall.

[0035] (3) Multiple grouting process optimization: multiple grouting process is adopted, wall protection slurry is injected to form a wall protection layer to enhance the stability of the hole wall, grouting is performed outside the casing to reinforce the soil around the pile, and high-pressure grouting is used to make the slurry densely fill the gap between the pile body and the soil, thereby improving the bonding force between the pile body and the soil and enhancing the overall strength and stability of the pile foundation;

[0036] (4) Application of environmentally friendly materials: biodegradable polymer composite materials are used to make steel sheaths. After the project is completed, the material is decomposed into carbon dioxide, water and biomass under the action of microorganisms, avoiding soil and groundwater pollution caused by the residue of traditional steel sheaths, and achieving a balance between construction and environmental protection. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] The present invention is further described below in conjunction with the accompanying drawings and specific embodiments.

[0038] Figure 1 A schematic diagram of the overall process of the rotary excavation and bottom expansion construction method for underground engineering pile foundations applicable to complex geology provided by the present invention;

[0039] Figure 2 A schematic diagram of the process of drilling and cleaning of a rotary drilling rig in a rotary drilling and bottom expansion construction method for underground engineering pile foundations applicable to complex geology provided by the present invention;

[0040] Figure 3 A schematic diagram of the process of secondary bottom expansion in the rotary excavation and bottom expansion construction method for underground engineering pile foundations applicable to complex geology provided by the present invention;

[0041] Figure 4 A schematic diagram of concrete pouring in the rotary excavation and bottom expansion construction method for underground engineering pile foundations applicable to complex geology provided by the present invention. DETAILED DESCRIPTION

[0042] In order to make the technical means, creative features, objectives and effects achieved by the present invention easy to understand, the present invention is further described below with reference to specific diagrams.

[0043] The invention provides a method for rotary excavation and bottom expansion of pile foundations in underground engineering projects with complex geology, which can improve the efficiency and reliability of construction and reduce pollution to the environment. Figure 1 , the construction method comprises the following steps:

[0044] Step 1: Level the site, put the rotary drilling rig in place, mark the hole of the rotary drilling pile according to the pile foundation layout design and pile position size in the rotary drilling pile design drawing, and measure the verticality of the rotary drilling pile;

[0045] Step 2: Use a rotary drilling rig to drill and clean the hard interlayer soil at the same time. Figure 2The specific construction steps of rotary drilling rig drilling and cleaning include:

[0046] Step 2.1: Drilling with rotary drilling rig;

[0047] Step 2.2: When the drill bit of the rotary drilling rig contacts the hard interlayer, increase the amount of drilling water and mud concentration, and perform uninterrupted hole sweeping;

[0048] Step 2.3: When the drill bit penetrates the hard interlayer section, reduce the amount of slag removal and mud flushing water, and clean the hard interlayer section at the same time;

[0049] In this stage, before the rotary drilling rig drills in step 2.1, first determine the plane layer where the bottom expansion section and transition section of the rotary pile are located according to the rotary drilling design drawings, and determine the bottom expansion diameter according to the bottom expansion design. With the center of the circle as the center, rotate the rotary drilling drill rod to the bottom expansion design diameter range, start the rotary drilling rig to drill, and use the cleaning device to clean the hard interlayer section soil;

[0050] Step 3: When the rotary drilling bit reaches the bottom hole expansion position, the rotary drilling rig is stopped, the casing is placed in the hole of the rotary pile, and the rotary drilling machine is started to expand the bottom hole for a second time to form a secondary bottom hole expansion;

[0051] When the rotary drilling bit reaches the position of the bottom hole expansion, the rotary drilling drill rod is first lifted 3-5cm away from the bottom of the hole, the rotary drilling rig is stopped at this time, the casing is placed in the hole position of the rotary drilling pile in the vertical direction, and the rotary drilling pile fixes the casing at the same time, and the rotary drilling machine is started to expand the bottom of the bottom hole for the second time, wherein the vertical direction is consistent with the center direction of the bottom of the expanded hole, the central axis of the casing is consistent with the vertical direction of the center of the expanded hole, and during the secondary expansion of the bottom hole, the casing remains stationary, and 1m<casing length≤3m;

[0052] Furthermore, a spiral steel sheet is provided inside the casing, and scale lines are provided on the casing. The scale lines ensure that the casing vertically enters the secondary bottom expansion hole of the rotary bored pile, and the internal spiral steel sheet is used to enhance the wall protection effect, enhance the bearing capacity of the pile foundation and prevent the hole wall from collapsing.

[0053] like Figure 3 The specific steps of the secondary bottom expansion are: start the rotary drilling machine, and when the lower part of the casing hits the hard interlayer section, it starts to expand, and the expansion angle of the casing is 30-60°. The casing continues to rotate in, and when the upper part of the casing hits the hole of the rotary pile, the expansion stops, and the upper part of the casing fits the inner wall of the hole of the rotary pile. The position of the casing is adjusted by the rotary drilling rig to coincide the central axis of the casing with the central axis of the secondary bottom expansion hole;

[0054] When it is determined that the central axis of the casing coincides with the central axis of the secondary bottom expansion hole, turn on the rotary drilling rig, rotate the casing in the vertical direction, and drive the casing to expand the outer layer of the secondary bottom expansion hole. The expansion depth is 30-80cm, and the upper end of the casing is placed outside the hole opening of the rotary pile. Then stop the rotary drilling rig, take out the spiral steel sheet, and lift the casing through the steel cable lifting device to separate the casing from the secondary bottom expansion hole, and the secondary bottom expansion is completed.

[0055] Step 4: After the secondary bottom expansion is completed, the wall slurry injection device is used to inject the wall slurry into the casing and the secondary bottom expansion hole, and the stable observation is carried out;

[0056] Step 5: Grouting is performed outside the casing. After the grouting of the slurry wall is completed, a pressurized grouting device is installed on the casing to perform grouting. After the grouting is completed, the casing is pulled up to complete a wall grouting.

[0057] Step 6: After the primary wall grouting is completed, the pile hole is left to stand to allow the slurry to fully solidify, the casing is pulled up and recovered, and the hole is cleaned for the second time;

[0058] Step 7: The degradable steel sheath is placed in the rotary pile hole, and secondary pipe grouting is performed outside the degradable steel sheath;

[0059] When the degradable steel jacket is placed in the rotary pile hole, the lower end of the degradable steel jacket is lower than the designed elevation of the bored pile. Preferably, the lower end of the degradable steel jacket is 30-50 cm lower than the designed elevation of the bored pile, and secondary pipe grouting is performed at the same time;

[0060] Specifically, the degradable steel sheath is composed of a biodegradable polymer composite steel sheath. After the project is completed, the degradable steel sheath can gradually decompose in the natural environment to reduce the pollution to the soil and groundwater.

[0061] The degradable steel sheath is mainly composed of degradable polymers such as polylactic acid (PLA) and polyhydroxyalkanoates (PHA) as the matrix, with an appropriate amount of natural fiber (hemp fiber, bamboo fiber) reinforcement added. Polylactic acid has good mechanical properties and processing properties, polyhydroxyalkanoates give the material biocompatibility, and the natural fiber reinforcement phase can improve the strength and rigidity of the material, so that it can meet the mechanical requirements of the steel sheath during the construction process.

[0062] The degradation process of the steel sheath made of degradable materials is based on the action of microorganisms. After the project is completed, microorganisms in the soil and groundwater will gradually attach to the surface of the steel sheath and secrete enzymes that can decompose high molecular polymers. Under the catalytic action of the enzyme, the molecular chains of polylactic acid and polyhydroxyalkanoates are gradually broken and decomposed into carbon dioxide, water and biomass. Compared with traditional steel sheaths, the use of this degradable steel sheath has significant advantages. From the perspective of environmental impact, it avoids the problems of heavy metal pollution in the soil and deterioration of groundwater quality that may be caused by the long-term retention of traditional steel sheaths underground. Although its initial cost is slightly higher than that of traditional steel sheaths, considering the long-term environmental governance costs and sustainable use of resources, from the perspective of full life cycle cost analysis, the cost gap will gradually narrow.

[0063] Step 8: When the secondary immersed tube grouting is completed, place the concrete conduit into the degradable steel sheath and clean the hole again through the conduit;

[0064] Specifically, the air lift reverse circulation hole cleaning technology is used to clean the conduit. After a degradable steel sheath is placed in the concrete conduit, high-pressure gas is injected into the conduit to form a gas-liquid mixed flow in the conduit. The rising speed of the gas-liquid mixed flow generates negative pressure to suck the sediment and mud at the bottom of the hole out of the hole.

[0065] This solution uses air lift reverse circulation hole cleaning technology to replace traditional hole cleaning methods, which can more efficiently remove fine particles and sediment at the bottom of the hole, improve the hole cleaning quality, reduce the settlement risk of the pile foundation, and enhance the bearing capacity of the pile body.

[0066] Step 9: After the secondary hole cleaning is completed, place the high-pressure grouting machine next to the rotary pile hole, put the grouting pipe into the center of the pile position, start the high-pressure grouting machine, and the grouting pipe starts to deliver grout. When cement slurry appears at the mouth of the high-pressure grouting pipe, turn off the high-pressure grouting machine, check the sealing of the pipeline, and start grouting after the inspection is qualified;

[0067] Step 10: pour concrete into the rotary pile from the concrete guide pipe to the center of the pile position. After the concrete pouring is completed, close the top of the rotary pile;

[0068] like Figure 4 , based on the above steps 8, 9 and 10, concrete pouring is completed.

[0069] In summary, starting from leveling the site, positioning the rotary drilling rig, marking the hole position and measuring the verticality, the drilling process reasonably adjusts the water volume, mud concentration and other parameters for the hard interlayer to clean the soil. After reaching the bottom hole position, put in the casing and expand the bottom for the second time. The quality of the bottom expansion is guaranteed by the scale lines, spiral steel sheets and precise control of the casing expansion angle and depth. After that, the wall slurry is injected, the casing is grouting outside the casing, the casing is recovered and the hole is cleaned for the second time, and then the steel casing is put in and the second sinking pipe is grouting, followed by the concrete conduit to clean the hole, and finally high-pressure grouting is performed to complete the concrete pouring and pile head treatment. At the same time, this scheme uses degradable polymer composite materials to make steel casings, taking into account both construction quality and environmental protection needs, and provides a systematic and effective solution for pile foundation construction in complex geological environments.

[0070] The above shows and describes the basic principles, main features and advantages of the present invention. It should be understood by those skilled in the art that the present invention is not limited to the above embodiments, and the above embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, which fall within the scope of the present invention to be protected. The scope of protection of the present invention is defined by the attached claims and their equivalents.

Claims

1. A method for rotary excavation and bottom expansion of underground engineering pile foundations suitable for complex geology, characterized in that: The method comprises the following steps: Step 1: Level the site, put the rotary drilling rig in place, mark the hole of the rotary drilling pile according to the pile foundation layout design and pile position size in the rotary drilling pile design drawing, and measure the verticality of the rotary drilling pile; Step 2: Use a rotary drilling rig to drill and clean the soil in the hard interlayer section at the same time. The specific construction steps of the rotary drilling rig drilling and cleaning include: Step 2.1: Drilling with rotary drilling rig; Step 2.2: When the drill bit of the rotary drilling rig contacts the hard interlayer, increase the amount of drilling water and mud concentration, and perform uninterrupted hole sweeping; Step 2.3: When the drill bit penetrates the hard interlayer section, reduce the amount of slag removal and mud flushing water, and clean the hard interlayer section at the same time; Step 3: When the rotary drilling bit reaches the bottom hole expansion position, the rotary drilling rig is stopped, the casing is placed in the hole of the rotary pile, and the rotary drilling machine is started to expand the bottom hole for a second time to form a secondary bottom hole expansion; Step 4: After the secondary bottom expansion is completed, the wall slurry injection device is used to inject the wall slurry into the casing and the secondary bottom expansion hole, and the stability is observed; Step 5: Grouting is performed outside the casing. After the grouting of the slurry wall is completed, a pressurized grouting device is installed on the casing to perform grouting. After the grouting is completed, the casing is pulled up to complete a wall grouting. Step 6: After the primary wall grouting is completed, the pile hole is left to stand to allow the slurry to fully solidify, the casing is pulled up and recovered, and the hole is cleaned for the second time; Step 7: The degradable steel sheath is placed in the rotary pile hole, and secondary pipe grouting is performed outside the degradable steel sheath; Step 8: When the secondary immersed tube grouting is completed, place the concrete conduit into the degradable steel sheath and clean the hole again through the conduit; Step 9: After the secondary hole cleaning is completed, place the high-pressure grouting machine next to the rotary pile hole, put the grouting pipe into the center of the pile position, start the high-pressure grouting machine, and the grouting pipe starts to deliver grout. When cement slurry appears at the mouth of the high-pressure grouting pipe, turn off the high-pressure grouting machine, check the sealing of the pipeline, and start grouting after the inspection is qualified; Step 10: Pour concrete into the rotary drilling pile from the concrete guide pipe toward the center of the pile. After the concrete pouring is completed, close the top of the rotary drilling pile.

2. The method for rotary excavation and bottom expansion of underground engineering pile foundation applicable to complex geology according to claim 1 is characterized in that: Before the rotary drilling rig drills in step 2.1, first determine the plane layer where the bottom expansion section and transition section of the rotary pile are located according to the rotary drilling design drawing, and determine the bottom expansion diameter according to the bottom expansion design. With the center of the circle as the center, rotate the rotary drilling drill rod to the bottom expansion design diameter range, start the rotary drilling rig to drill, and use the cleaning device to clean the hard interlayer section soil.

3. The method for rotary excavation and bottom expansion of underground engineering pile foundations applicable to complex geology according to claim 1 is characterized in that: In the step 3, when the rotary drilling bit reaches the position of the bottom hole expansion, the rotary drilling drill rod is first lifted 3-5 cm away from the bottom of the hole, the rotary drilling rig is stopped at this time, the casing is placed in the hole position of the rotary drilling pile in the vertical direction, and the rotary drilling pile fixes the casing at the same time, the rotary drilling rig is started, and the bottom expansion of the bottom hole is performed for the second time, wherein the vertical direction is consistent with the center direction of the bottom of the bottom hole, the central axis of the casing is consistent with the vertical direction of the center of the expanded hole, and during the secondary bottom expansion of the bottom hole, the casing remains stationary, and 1m<casing length≤3m;.

4. The method for rotary excavation and bottom expansion of pile foundations in underground projects suitable for complex geology according to claim 1, characterized in that: The casing is provided with a spiral steel sheet inside and scale lines on the casing, which ensure that the casing vertically enters the secondary bottom expansion hole of the rotary bored pile. The internal spiral steel sheet is used to enhance the wall protection effect, enhance the bearing capacity of the pile foundation and prevent the hole wall from collapsing.

5. The method for rotary excavation and bottom expansion of pile foundations in underground projects with complex geology according to claim 4 is characterized in that: The specific steps of the secondary bottom expansion in step 3 are: start the rotary drilling machine, and when the lower part of the casing hits the hard interlayer section, it starts to expand, the expansion angle of the casing is 30-60°, the casing continues to rotate, and when the upper part of the casing hits the hole of the rotary pile, the expansion is stopped, the upper part of the casing is fitted with the inner wall of the hole of the rotary pile, and the position of the casing is adjusted by the rotary drilling rig to coincide the central axis of the casing with the central axis of the secondary bottom expansion hole; When it is determined that the central axis of the casing coincides with the central axis of the secondary bottom expansion hole, turn on the rotary drilling rig, rotate the casing in the vertical direction, and drive the casing to expand the outer layer of the secondary bottom expansion hole. The expansion depth is 30-80cm, and the upper end of the casing is placed outside the hole opening of the rotary pile. Then stop the rotary drilling rig, take out the spiral steel sheet, and lift the casing through the steel cable lifting device to separate the casing from the secondary bottom expansion hole, and the secondary bottom expansion is completed.

6. The method for rotary excavation and bottom expansion of pile foundations for underground projects in complex geology according to claim 1 is characterized in that: The degradable steel sheath is made of a biodegradable polymer composite material. After the project is completed, the degradable steel sheath can gradually decompose in the natural environment to reduce pollution to the soil and groundwater. The degradable steel sheath is composed of degradable polymers such as polylactic acid (PLA) and polyhydroxyalkanoate (PHA) as a matrix, with an appropriate amount of natural fiber reinforcement phase added.

7. The method for rotary excavation and bottom expansion of pile foundations in underground projects suitable for complex geology according to claim 1, characterized in that: In step 8, the conduit is cleaned by using air lift reverse circulation hole cleaning technology. After the degradable steel sheath is placed in the concrete conduit, high-pressure gas is injected into the conduit to form a gas-liquid mixed flow in the conduit. The rising speed of the gas-liquid mixed flow generates negative pressure to suck the sediment and mud at the bottom of the hole out of the hole.

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