A method of construction of a bell head anchor for use in loose ground

Abstract

The application discloses a construction method of an enlarged-head anchor rod suitable for a loose stratum, and comprises the following steps: S1, positioning of an anchor rod and a drilling machine; S2, high-pressure rotary jet grouting hole expansion and grouting; S3, preparation of a rod body assembly; S4, lowering of the rod body assembly; S5, first-time pressure grouting; S6, filling of bean stones and sealing of a hole mouth; S7, second-time pressure grouting; and S8, maintenance and detection. The hole expansion and grouting are combined into one process, the high-pressure rotary jet pipe is lowered to the designed depth, the loose stratum is cut by a high-pressure jet flow to form an enlarged-head space, and cement slurry is immediately injected to form a high-pressure rotary jet grouting body, since the hole expansion and grouting are simultaneously completed, the hole wall at the variable-diameter position of the enlarged head is immediately wrapped and cemented by the cement slurry after being formed, the collapse of the hole after the completion of the hole expansion in the conventional process due to the exposure of the hole wall and the loss of support is avoided, the hidden troubles of the sediment and the anchor body clamping sediment are eliminated, the collapse of the hole at the variable-diameter position of the enlarged head is effectively prevented, and the quality of the anchor body is ensured.

Description

Technical Field

[0001] This invention relates to the field of foundation pit support technology, specifically to a method for constructing enlarged head anchor bolts suitable for loose strata. Background Technology

[0002] Enlarged-head anchor bolts are a commonly used anchoring measure in foundation pit support and anti-buoyancy engineering. By forming an enlarged head at the bottom of the anchor bolt with a diameter larger than that of the bolt body, the pull-out bearing capacity of the anchor bolt is greatly improved. They are widely used in deep foundation pit support, underground structure anti-buoyancy engineering and other projects.

[0003] In conventional methods for constructing enlarged-head anchor bolts, after the borehole reaches the designed depth, a specialized enlargement device is used to enlarge the borehole to a predetermined depth, creating an enlarged-head space. After enlargement, the borehole is cleaned, and then the reinforcing bar and grouting pipe are lowered. Cement grout is then injected into the borehole through the grouting pipe. However, when applied to loose strata such as gravel or sand layers that are poorly cemented and prone to borehole collapse, it is necessary to first drill to the designed depth and then enlarge the borehole at the bottom to create an enlarged-head space. After enlargement, the borehole wall at the front face of the enlarged-head loses the lateral support of the original soil, making it highly susceptible to collapse under its own weight. This results in a large amount of soil debris falling to the bottom of the borehole, increasing sediment and affecting the effective anchorage length and the designed diameter of the enlarged-head. It may also obstruct the subsequent lowering of the reinforcing bar, preventing it from reaching the designed elevation. Moreover, after the borehole is enlarged, a large amount of sediment and collapsed soil accumulated at the bottom of the enlarged head section are difficult to completely remove by conventional cleaning methods due to the limited depth and diameter of the hole. The residual sediment forms a weak interlayer after grouting, which directly affects the effective bonding between the enlarged head and the surrounding soil and reduces the pull-out bearing capacity of the anchor rod. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a method for constructing enlarged-head anchor bolts suitable for loose strata. This method solves the problems of easy hole collapse at the diameter change point after enlarging the hole in the enlarged head section, slag inclusion in the anchor body, and difficulty in removing sediment in existing technologies.

[0005] According to an embodiment of the present invention, a method for constructing enlarged head anchor bolts suitable for loose strata includes the following steps:

[0006] S1: Anchor bolt and drilling rig positioning: Position the drilling rig at the anchor bolt design hole position, adjust the verticality of the drill rod, and use a casing drilling rig to drill to the design depth. After drilling, pull out the casing.

[0007] S2: High-pressure jet grouting with enlarged boreholes involves lowering the high-pressure jet grout pipe to the designed depth of the enlarged head section, starting the high-pressure mud pump, and using high-pressure jets to cut the surrounding loose strata to form the enlarged head section. At the same time, cement slurry is injected into this space to form a high-pressure jet grouting body in the enlarged head section.

[0008] S3: Prepare the rod assembly and fix the primary and secondary grouting pipes onto the rod assembly;

[0009] S4: Lower the rod assembly. Lower the entire rod assembly to the design elevation so that its bottom end is inserted into the high-pressure jet grouting body of the enlarged head section.

[0010] S5: First grouting, grouting is performed through the primary grouting pipe;

[0011] S6: Fill with gravel and seal the orifice. Fill the orifice with gravel and seal the orifice.

[0012] S7: Second grouting, which involves secondary grouting through a secondary grouting pipe;

[0013] S8: Maintenance and Inspection.

[0014] Compared with the prior art, the present invention has the following beneficial effects:

[0015] By combining borehole enlargement and grouting into a single process, after the high-pressure jet nozzle is lowered to the designed depth, cement slurry is immediately injected into the loose strata to form the enlarged head space while simultaneously cutting it with a high-pressure jet to form a high-pressure jet grouting body. Since borehole enlargement and grouting are completed simultaneously, the borehole wall at the diameter change point of the enlarged head is immediately encapsulated and cemented by cement slurry after formation. This avoids the collapse that occurs after borehole enlargement is completed due to the exposed borehole wall and loss of support, as is common in conventional processes. This eliminates the hidden dangers of sediment and slag inclusions in the anchor body, effectively preventing borehole collapse at the diameter change point of the enlarged head and ensuring the quality of the anchor body. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the construction status according to an embodiment of the present invention.

[0017] Figure 2 This is a schematic cross-sectional view of the non-enlarged head section in an embodiment of the present invention.

[0018] Figure 3 This is a schematic cross-sectional view of the enlarged head section in an embodiment of the present invention.

[0019] Figure 4 This is a schematic diagram of the centering bracket in an embodiment of the present invention.

[0020] Figure 5 This is a diagram illustrating the construction steps of an embodiment of the present invention.

[0021] In the above attached figures:

[0022] 1. Precision rolled threaded steel;

[0023] 2. Centering bracket; 201. Nut; 202. Support part;

[0024] 3. Cast iron load-bearing structure;

[0025] 4. Initial grouting pipe;

[0026] 5. Secondary grouting pipe. Detailed Implementation

[0027] The technical solutions of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0028] This invention provides a method for constructing enlarged-head anchor bolts suitable for loose strata, comprising the following steps:

[0029] S1: Anchor bolt and drilling rig positioning: Position the drilling rig at the designed anchor bolt hole position, adjust the verticality of the drill rod, and use a casing drilling rig to drill to the designed depth. After drilling, pull out the casing.

[0030] Specifically, according to the design drawings, the anchor bolt hole positions are marked on the construction site. The drilling rig is moved so that the drill bit is aligned with the center of the hole position. The verticality of the drill rod is adjusted by the drilling rig's own leveling device to ensure that the verticality deviation of the drill rod is no more than 1% to 1.5%. After the drilling rig is in place, a casing drilling rig is used for casing drilling. The drill bit drives the casing to follow synchronously until the design depth is reached. During the casing drilling process, the casing plays a temporary support role for the hole wall to prevent the hole wall from collapsing. After the hole reaches the design depth, the drill bit is pulled out first, and then the casing is pulled out to form a bare hole.

[0031] S2: High-pressure jet grouting with enlarged borehole. The high-pressure jet grouting pipe is lowered to the designed depth of the enlarged head section. The high-pressure mud pump is started, and the surrounding loose strata are cut by high-pressure jet to form the enlarged head section. At the same time, cement slurry is injected into the space to form a high-pressure jet grouting body in the enlarged head section.

[0032] Specifically, the high-pressure jet grouting pipe is lowered to the designed depth of the enlarged head section, and the high-pressure mud pump is started to perform high-pressure jet grouting for borehole enlargement. The high-pressure jet grouting pipe is an existing high-pressure jet grouting pipe with a high-pressure nozzle at its lower end. After the high-pressure jet grouting pipe is lowered to the designed depth of the enlarged head section, the high-pressure mud pump is started, and the high-pressure cement slurry is ejected at high speed from the nozzle through the high-pressure jet grouting pipe to form a high-pressure jet. The high-pressure jet cuts, breaks up and disturbs the loose strata around the enlarged head section, cutting and breaking up the in-situ soil to form the enlarged head space.

[0033] Preferably, the high-pressure jet grouting expansion grouting step includes: lowering the high-pressure jet grouting pipe to the designed depth of the expansion head section and then stopping the lowering while maintaining rotation, spraying grout at the bottom for 30 seconds, then spraying grout while rotating to fully mix the cement grout with the soil, and then spraying grout while rotating in the opposite direction at a uniform speed to raise the high-pressure jet grouting pipe until the top of the expansion head section.

[0034] In practical operation, the high-pressure jet grouting pipe is lowered to the designed depth of the enlarged head section. After the high-pressure jet grouting pipe reaches the designed depth, it stops lowering and continues to rotate. At this time, the nozzle at the lower end of the high-pressure jet grouting pipe is exactly at the designed elevation of the bottom of the enlarged head section. Then, the high-pressure mud pump is started, and high-pressure cement slurry is sprayed out from the nozzle through the high-pressure jet grouting pipe to form a high-pressure jet. The high-pressure jet grouting pipe is kept rotating in place and grouting is carried out for 30 seconds. During these 30 seconds, the high-pressure jet continuously cuts the loose soil at the bottom of the enlarged head section, breaking and dispersing it. At the same time, the high-pressure injected cement slurry is initially mixed with the broken soil debris to form the initial grouting body at the bottom of the enlarged head section, ensuring that the bottom area of ​​the enlarged head section receives sufficient cutting and grouting, and avoiding insufficient grouting or loose compaction at the bottom.

[0035] Secondly, the high-pressure mud pump continues to spray grout, and the high-pressure rotary nozzle continues to rotate. The high-pressure jet from the nozzle continuously cuts the surrounding soil while spraying grout. At the same time, the rotation of the high-pressure rotary nozzle drives the nozzle to move along a circular trajectory, so that the cement grout can fully contact and mix with the cut soil debris from multiple angles to form a uniform mixture.

[0036] While the high-pressure jet grouting pipe rotates, it simultaneously sprays grout and rotates in the opposite direction at a uniform speed until it reaches the top of the enlarged head section. Preferably, the lifting speed of the high-pressure jet grouting pipe is no more than 25 cm / min, preferably 20 cm / min, selected according to the construction schedule. During the process of the high-pressure jet grouting pipe being lifted from the bottom to the top of the enlarged head section, the high-pressure jet continuously cuts the surrounding soil along the full height of the enlarged head section, forming a complete and uniform enlarged head space. At the same time, cement grout is continuously injected into this space and thoroughly mixed with soil debris, ultimately forming a high-pressure jet grouting body of the designed diameter in the enlarged head section. The aforementioned high-pressure cement grout can be prepared using P.O42.5R silicate cement with a water-cement ratio of 0.8. The grouting pressure of the high-pressure mud pump is 25 MPa, and the designed diameter of the enlarged head section is 400~500 mm. Figure 1 As shown, segment A is the enlarged head segment, and segment B is the non-enlarged head segment.

[0037] Furthermore, when the high-pressure cyclone nozzle reaches the front face of the enlarged head, it should rotate and agitate at least once to form a complete front face of the enlarged head. When the high-pressure cyclone nozzle is raised to near the top of the enlarged head section, i.e., at the junction of the enlarged and non-enlarged head sections, such as... Figure 1As shown, C marks the junction. The high-pressure jet grouting pipe continues to spray grout and rotate in place, while the high-pressure nozzle continuously sprays high-pressure jets to perform a final cut and disturbance on the soil wall at the front end of the enlarged head. At the same time, the high-pressure injected cement grout is fully mixed with the cut soil debris at this location, allowing this position to have a longer spraying and mixing time than other locations in the enlarged head section. The cement grout fully penetrates and bonds the hole wall at the diameter change, resulting in a complete grouting body with high strength, which can effectively withstand the end bearing resistance provided by the soil at the front end of the enlarged head during anchor pull-out.

[0038] S3: Prepare the rod assembly and fix the primary and secondary grouting pipes onto the rod assembly.

[0039] S4: Lower the rod assembly. Lower the entire rod assembly to the design elevation so that its bottom end is inserted into the high-pressure jet grouting body of the enlarged head section.

[0040] Specifically, the rod assembly is prepared simultaneously with or after the high-pressure jet grouting and borehole expansion grouting construction, such as... Figure 2 and Figure 3 As shown, the primary grouting pipe 4 and the secondary grouting pipe 5 are fixed to the rod assembly along the length of the rod assembly. Both the primary grouting pipe 4 and the secondary grouting pipe 5 are made of PE pipe with a diameter of 20mm. The lower ends of the two grouting pipes are located at different depths of the rod assembly. The primary grouting pipe 4 is used for the first grouting injection, and the secondary grouting pipe 5 is used for the second grouting injection.

[0041] The prepared rod assembly is lifted as a whole and slowly lowered into the hole. The rod assembly is lowered to the design elevation so that the bottom end of the rod assembly passes through the non-enlarged head section and is inserted into the high-pressure jet grouting body of the enlarged head section. Since the high-pressure jet grouting body has not yet initially set and is in a fluid state, the rod assembly can be inserted into the high-pressure jet grouting body by its own weight, ensuring that the bottom end of the rod assembly reaches the design elevation of the bottom of the enlarged head section.

[0042] Preferably, the rod assembly includes a fine-rolled threaded steel bar 1, a cast iron support body 3 disposed on the fine-rolled threaded steel bar 1, and a centering bracket 2, wherein the cast iron support body 3 is located in the enlarged head section, and the centering bracket 2 is located in the non-enlarged head section.

[0043] Among them, the high-strength threaded steel bar 1 is a single high-strength threaded steel bar that runs through the entire length of the anchor rod. As the main load-bearing reinforcement of the anchor rod, it has the characteristics of high strength and good bonding performance with cement grout. Its surface has continuous threads, which can provide high bonding anchoring force. As a single bar, compared with the conventional rod body which is made of multiple ordinary hot-rolled steel bars tied together, it has the advantages of high rigidity, good integrity, and smooth lowering. The cast iron bearing body 3 is a ring bearing member sleeved on the high-strength threaded steel bar 1. Its function is to transfer the tensile force of the anchor rod reinforcement to the high-pressure jet grout of the enlarged head section when the high-strength threaded steel bar 1 is subjected to pull-out load. The cast iron bearing body 3 transfers the tensile force of the anchor rod reinforcement to the high-pressure jet grout of the enlarged head section, and then to the surrounding soil through the grout, thereby exerting the end bearing resistance of the enlarged head. Preferably, the cast iron bearing body 3 is a conical block with the large end facing upward and the small end facing downward to reduce the resistance of descent. The large end facing upward can maintain the end bearing resistance. Furthermore, there are two cast iron bearing bodies 3, with a spacing of 1m-1.5m between them. The specific spacing value is determined based on the design length and pull-out bearing capacity requirements of the enlarged head section. When the length of the enlarged head section is close to 4.0m or the design pull-out bearing capacity requirement is high, 1.5m is used to make the two bearing bodies more evenly distributed within the enlarged head section. When the length of the enlarged head section is close to 2.5m or the design pull-out bearing capacity requirement is low, 1m is used. Through the graded transmission of the two cast iron bearing bodies 3, the tensile force of the precision-rolled threaded steel bar 1 is evenly transmitted to the high-pressure jet grouting body along the entire length of the enlarged head section, and finally transmitted to the surrounding soil through the grouting body, making full use of the end bearing and side friction resistance of the entire enlarged head section. The cast iron bearing body 3 is located 100mm-120mm below the top surface of the enlarged head section, so that the cast iron bearing body 3 is exactly located in the top area of ​​the enlarged head section, which not only ensures that the bearing body is completely wrapped by the high-pressure jet grouting body, but also makes it as close as possible to the front end face of the enlarged head.

[0044] Moreover, such as Figure 4 As shown, the centering bracket 2 includes a nut 201 disposed on the fine-rolled threaded steel bar 1 and a plurality of support parts 202 arranged circumferentially along the nut 201. Each support part 202 extends radially outward and is used to abut against the hole wall to keep the fine-rolled threaded steel bar 1 centered.

[0045] The surface of the precision-rolled threaded steel bar 1 has continuous threads. The nut 201 is fixed to the precision-rolled threaded steel bar 1 by screwing it into the threaded part of the precision-rolled threaded steel bar 1. By rotating the nut 201, it can be positioned at any position of the precision-rolled threaded steel bar 1. The support part 202 is a support member fixedly set on the outer wall of the nut 201. It can be a support block or a support frame. Preferably, there are three or four support parts 202, which are evenly distributed along the circumference of the nut 201. Each support part 202 extends radially outward from the outer wall of the nut 201. When the precision-rolled threaded steel bar 1 is lowered into the hole, the outer end of each support part 202 abuts against the hole wall. Since the multiple support parts 202 are spaced apart and evenly distributed along the circumference of the nut 201, after each support part 202 abuts against the hole wall at the same time, a centripetal constraint force is generated on the precision-rolled threaded steel bar 1, supporting the precision-rolled threaded steel bar 1 at the center position of the drill hole, ensuring that the precision-rolled threaded steel bar 1 remains centered in the drill space of the non-enlarged head section. The radial extension length of the support part 202 is determined based on the difference between the bore diameter and the diameter of the precision rolled threaded steel bar 1, so as to ensure that the outer end of each support part 202 just abuts against the bore wall, thus achieving the centering function and preventing the rod assembly from being difficult to lower due to excessive length.

[0046] S5: First grouting, grouting is performed through the primary grouting pipe 4.

[0047] Specifically, after the rod assembly is lowered to the design elevation, the grouting pump is started, and cement grout is injected into the hole through the primary grouting pipe 4. The lower end of the primary grouting pipe 4 is located at the bottom of the enlarged head section, filling any remaining pores in the high-pressure jet grout of the enlarged head section, further compacting the grout in the enlarged head section with cement grout. Then, as the grout is continuously injected, the grout level gradually rises and enters the non-enlarged head section, filling the hole space in the non-enlarged head section to form a preliminary grouting anchor body. The grouting pressure is controlled at 0.5~1.0MPa.

[0048] S6: Fill with gravel and seal the opening. Fill the hole with gravel and seal the opening.

[0049] S7: Second grouting, which is carried out through the secondary grouting pipe 5.

[0050] After the first grouting is completed, pebbles are filled into the hole. The pebbles are filled through the hole opening and accumulate in the hole space in the non-enlarged head section. After the pebbles are filled to the designed elevation of the hole opening, the hole opening is sealed. It has grouting holes and venting holes to facilitate the injection of grout and the removal of air from the hole during the subsequent second grouting.

[0051] After the borehole is sealed, a second grouting is performed through the secondary grouting pipe 5. The lower end of the secondary grouting pipe 5 is located in the lower part of the non-enlarged head section. The grouting pump is started to press the cement grout into the borehole through the secondary grouting pipe 5. The grout flows out from the lower end of the secondary grouting pipe 5 and enters the pebbles layer of the non-enlarged head section to further fill the gaps between the pebbles in the borehole of the non-enlarged head section, making the grouting anchor body of the non-enlarged head section more compact.

[0052] S8: Curing and Inspection. During the curing period, the orifice should be kept sealed to prevent moisture evaporation or impurities from entering. After the curing period reaches the design period, pull-out test should be carried out. Construction is completed after the pull-out test is qualified.

[0053] It is worth mentioning that from the completion of the high-pressure jet grouting in S2 to the completion of the lowering of the rod assembly in S4 to the design elevation, the interval time shall not exceed 30 minutes. This ensures that the grout still has sufficient fluidity when the fine-rolled threaded steel bar 1 is lowered, effectively avoiding the problem of the lower reinforcement being blocked and unable to reach the design elevation, and ensuring the effective anchoring length of the anchor rod.

[0054] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for constructing enlarged-head anchor bolts suitable for loose strata, characterized in that, Includes the following steps: S1: Anchor bolt and drilling rig positioning: Position the drilling rig at the anchor bolt design hole position, adjust the verticality of the drill rod, and use a casing drilling rig to drill to the design depth. After drilling, pull out the casing. S2: High-pressure jet grouting with enlarged boreholes involves lowering the high-pressure jet grout pipe to the designed depth of the enlarged head section, starting the high-pressure mud pump, and using high-pressure jets to cut the surrounding loose strata to form the enlarged head section. At the same time, cement slurry is injected into this space to form a high-pressure jet grouting body in the enlarged head section. S3: Prepare the rod assembly and fix the primary and secondary grouting pipes onto the rod assembly; S4: Lower the rod assembly. Lower the entire rod assembly to the design elevation so that its bottom end is inserted into the high-pressure jet grouting body of the enlarged head section. S5: First grouting, grouting is performed through the primary grouting pipe; S6: Fill with gravel and seal the orifice. Fill the orifice with gravel and seal the orifice. S7: Second grouting, which involves secondary grouting through a secondary grouting pipe; S8: Maintenance and Inspection.

2. The construction method according to claim 1, characterized in that, In S2, the high-pressure jet grouting expansion grouting steps include: lowering the high-pressure jet grouting pipe to the designed depth of the expansion head section, stopping the lowering and maintaining rotation, spraying grout at the bottom for 30 seconds, then spraying grout while rotating to fully mix the cement grout with the soil, and then spraying grout while rotating in the opposite direction at a uniform speed to raise the high-pressure jet grouting pipe until the top of the expansion head section.

3. The construction method according to claim 2, characterized in that, The lifting speed of the high-pressure rotary nozzle should not exceed 25 cm / min.

4. The construction method according to claim 2, characterized in that, When the high-pressure rotary nozzle reaches the front end face of the enlarged head, it should be rotated and stirred at least once to form a complete front end face of the enlarged head.

5. The construction method according to any one of claims 1-4, characterized in that, From the completion of high-pressure jet grouting in S2 to the completion of lowering the rod assembly to the design elevation in S4, the interval shall not exceed 30 minutes.

6. The construction method according to claim 1, characterized in that, In S3, the rod assembly includes a fine-rolled threaded steel bar, a cast iron support body located on the fine-rolled threaded steel bar, and a centering bracket. The cast iron support body is located in the enlarged head section, and the centering bracket is located in the non-enlarged head section.

7. The construction method according to claim 6, characterized in that, The centering bracket includes a nut on the fine-rolled threaded steel bar and multiple support parts spaced apart circumferentially along the nut. Each support part extends radially outward to abut against the hole wall to keep the fine-rolled threaded steel bar centered.

8. The construction method according to claim 6, characterized in that, The cast iron support is a conical block with the large end facing upwards.

9. The construction method according to claim 6, characterized in that, There are two cast iron bearings, and the distance between the two cast iron bearings is 1m-1.5m.

10. The construction method according to claim 9, characterized in that, The cast iron support is located 100mm-120mm below the top surface of the enlarged head section.

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