Construction method of mixed pile under saturated silt weak geological condition

Abstract

The invention discloses a construction method of a mixed pile under saturated silt weak geological conditions, which comprises the following steps: S1, according to a construction drawing of a prestressed high-strength concrete hollow square pile, deepening a drawing of a powder spraying pile, arranging the powder spraying pile to ensure that a soil body within a range of 1 meter around a bearing platform pile is reinforced as a standard, improving the bearing capacity of the soil body around the pile, and improving the strength of the pile; the pile head is protected from being damaged; and S2, the construction process of the powder spraying pile comprises the following steps that a, measuring and setting out are conducted, a pile machine is moved to a setting-out point position, a shaft of a mixing pile machine is kept perpendicular, the error should not be larger than 50 mm, and a supporting leg oil cylinder of the pile machine is adjusted so that the inclination degree of a drilling rod of a drilling machine can not be larger than 1%. The control over the perpendicularity of a pile body of the prestressed high-strength concrete hollow square pile in the pile sinking process is enhanced, the problems that pile body displacement and soil body slippage are likely to happen when foundation pit excavation is conducted under the complex geological condition are solved, the quality problems such as pile breakage are reduced to a great extent, and the engineering quality of engineering piles is improved.

Description

Technical Field

[0001] The present invention relates to the field of civil engineering technology, in particular to a construction method of a mixed pile under soft geological conditions of saturated silt. Background Art

[0002] In the riverside areas, the groundwater level is high and the soil is mostly saturated silty soft geology with the characteristics of high water content, high compressibility, low permeability, and low shear strength. The silty foundation pit has strong creep properties, which can easily cause instability of the foundation pit slope, soil flow and collapse, and uplift of the pit bottom.

[0003] The application project of this project is the EPC project of Block 15-1 of Xiwan Garden in Jiangbei New District, Wuhu. It is located on the north bank of the Yangtze River with poor geological conditions. The topsoil is plain fill and the underground is mainly silty clay. The soil characteristics are gray-black, wet to saturated, plastic to soft plastic, with a slightly shiny cross section, containing a small amount of humus debris, locally interbedded with thin layers of silt, no shaking reaction, low to medium dry strength, and low to medium toughness.

[0004] The existing conventional construction method is to use only one type of pile (such as only precast piles as engineering piles). Under weak geological conditions, there are problems such as the verticality of the pile body cannot be controlled during the sinking process of the precast pile, or during the excavation of the foundation pit, the pile body is affected by mechanical dynamic loads, causing displacement or shear damage.

[0005] This project adopts a hybrid pile form of "powder injection piles + prestressed high-strength concrete hollow square piles". Powder injection piles are used to replace and reinforce saturated silt and soft soil to improve the bearing capacity of the foundation. At the same time, it is also beneficial to strengthen the verticality control of the prestressed high-strength concrete hollow square piles during the pile sinking process, and solve the problems of pile tilting, fracture, soil uplift and deformation that are prone to occur during foundation pit excavation under complex geological conditions. The design scheme of hybrid piles has obvious technical and economic benefits. Summary of the Invention

[0006] The present invention adopts a combination of "powder injection piles + PHS prestressed high-strength concrete hollow square piles". The powder injection piles are operated by dry spraying method, with a pile diameter of 500mm, an effective pile length of 5m, an over-pouring of 500mm, and a cement content of 20% (common Portland cement with a strength grade of 42.5). The engineering pile type is prestressed high-strength concrete hollow square pile PHS-AB500 (310). The pile sinking operation is carried out by hammering method. The basement engineering piles are pull-out piles with an effective pile length of 30m. The main building area engineering piles are compression piles with an effective pile length of 45m.

[0007] The present invention proposes a method for constructing mixed piles under soft geological conditions of saturated silt, comprising the following steps: S1: Based on the construction drawings of prestressed high-strength concrete hollow square piles, the drawings of the powder injection piles are refined. The powder injection piles are arranged to ensure that the soil within 1 meter around the pile cap is reinforced as a standard, thereby increasing the bearing capacity of the soil around the piles and protecting the pile heads from damage. S2: The construction process of powder injection pile is as follows: a. Measure and stake out. Move the pile driver to the stakeout point and keep the shaft of the pile driver vertical. The error should not be greater than 50mm. Adjust the outrigger cylinder of the pile driver so that the inclination of the drill rod of the drilling rig is not greater than 1% to prevent inclined pile driving and affect the bearing capacity of the pile foundation. b. Close the ash line valve of the powder spraying machine, open the air line valve, start the pile driver, start the air compressor and slowly open the air line pressure regulating valve to supply air to the pile driver. The pile driver gradually accelerates, rotates forward to pre-stir and sink, observe the pressure gauge reading, and adjust the pressure difference as the drill pipe drilling pressure increases, so that the rear valve is 0.02-0.05Mpa larger than the front valve. When drilling close to the designed depth, use low speed and slow rotation, and rotate the drill rig in place for 1-2 minutes; c. Lifting powder spray mixing: After reaching the designed pile bottom, the drill bit is generally lifted at a speed of about 1.0m / min in the opposite direction. The soft soil and cement are fully mixed while being sprayed and stirred. After being lifted to the designed stop elevation, the mixture should be stirred slowly in situ for 1-2min. The spraying amount should match the lifting speed. The powder spraying amount should be controlled at 55kg / m. If one spraying cannot meet the design requirements, two sprayings can be used. When lifting, choose medium and slow gears to ensure uniform mixing. It is required that the stirring shaft rotates at least 1 circle for every 16mm lifting. When the drill bit is lifted to 0.5m below the ground, the powder sprayer should stop spraying. During the powder spray pile construction, cement pumping must be continuous. The cement dosage and the time of pumping cement should be recorded by a dedicated person. Powder spraying and stirring are carried out during the lifting process. Cement powder is sprayed into the stirred soil through the powder sender to fully mix the soil and cement along the depth direction. d. Remixing: Stop spraying powder, drill while rotating the drill bit until it reaches the designed depth, then reversely rotate while lifting to fully mix the soil and cement. The soil is fully crushed and the cement powder is evenly dispersed in the pile soil. The remixing depth is 1 / 3 of the pile length. Remixing is an effective measure to ensure uniform pile formation and improve pile strength. e. Within 7 days after the pile is completed, randomly select 1% of the total number of piles and no less than 3 powder injection piles, and use the light dynamic penetration method to conduct quality inspection on the cement pile body of the powder injection pile. Generally, within 7 days after the pile is completed, use a light penetration probe to take cement soil samples from the pile body. The uniformity of the powder injection pile mixing is checked by observing whether the color of the cement soil samples is consistent and whether there are lumps of cement or unmixed soil clumps. At the same time, the strength of the powder injection pile body is judged based on the number of penetration blows (N10). The number of blows for N10 penetration of 10cm shall not be less than 10. Sections with less than 10 blows or each blow greater than 10mm are considered to be unsatisfactory and should be handled; S3: Preparation work before construction of PHS prestressed high-strength concrete hollow square piles: a. Pile position setting out; b. Put the hammer pile driver into position; c. Before lifting, mark the length of the pile in units of 0.5m on the pile body and mark the length of the pile from bottom to top in order to observe the depth of the pile into the soil and record the number of hammer blows per meter; S4: The square pile is welded on-site using a cross-shaped steel pile tip. First, weld the upper half of the pile tip. Then, rotate the pile 180 degrees and weld the remaining half of the pile tip. After welding, cool for 10 minutes and apply two coats of asphalt for corrosion protection. S5: Positioning and straightening the piles: a. After the first section of pile is hoisted, the pile tip is aligned with the pile position marker on the ground to accurately insert and position the pile; b. Use two theodolites to cross at 90 degrees and set up about 10 meters away from the pile position. First, observe the verticality of the pile driver's guide rod. After adjusting it to meet the requirements, use the theodolite to observe the verticality of the pile again. Instruct the pile driver to make multiple adjustments until the verticality of the pile body is controlled within 0.1%. The verticality of the first section of the pile should be strictly guaranteed to ensure its vertical guiding function. c. Hammering piles, using a heavy hammer with a low impact, the hammer weight is 12.8t, and the stroke is about 2m; d. Fill the pile bottom with C40 micro-expansive concrete for sealing, with a height of 2m; S6: Pile welding: a. Before connecting the piles, the steel plates at the ends of the upper and lower piles should be cleaned to ensure that there is no rust or pollution. In particular, the grooves of the end plates must be cleaned until the metallic luster is exposed. b. When connecting the upper and lower pile sections, the verticality of the pile body should be corrected in time and the axis should be aligned; c. The misalignment deviation of the upper and lower pile sections should be controlled to be ≤2mm. The two end surfaces of the upper and lower pile sections should fit tightly together without any gaps at the joints. It is strictly forbidden to fill the gaps between the joints with iron wires, welding rod ends, and other debris. When the piles are put into place and corrected, they should not be struck horizontally with a sledgehammer. If there are gaps at the joints, they should be welded securely with wedge-shaped iron sheets. d. Perform symmetrical spot welding at 4 to 6 points around the groove. After the upper and lower piles are fixed, remove the guide hoop and then perform symmetrical welding in layers. e. After the welded pile joints have cooled naturally and been coated with two coats of anti-corrosion asphalt, pile sinking can be continued. The natural cooling time should not be less than 8 minutes. It is strictly forbidden to use water cooling or to sink piles immediately after welding without cooling. S7: Before driving piles, the depth of the piles should be calculated in advance, and a suitable pile driver should be selected. Clear marks should be made on the pile driver. The deviation of the pile top elevation should be strictly controlled with a level, and the construction records of each pile should be recorded.

[0008] Compared with the prior art, the present invention has the following beneficial effects: Compared with traditional engineering piles, this technology improves the bearing capacity of the base soil, strengthens the verticality control of the prestressed high-strength concrete hollow square piles during the pile sinking process, solves the problems of pile displacement and soil slippage that are prone to occur during foundation pit excavation under complex geological conditions, greatly reduces the occurrence of quality problems such as broken piles, and improves the engineering quality of engineering piles. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 This is a detailed diagram of the powder injection pile of the present invention. DETAILED DESCRIPTION

[0010] The present invention is further described below with reference to the accompanying drawings and embodiments. It is obvious that the described embodiments are only part of the embodiments of the present invention. Based on the embodiments of the present invention, other embodiments obtained by ordinary technicians in this field without any creative work are all within the scope of protection of the present invention.

[0011] like Figure 1 As shown, this embodiment proposes a construction method for mixed piles under saturated silt and soft geological conditions, including the following steps: S1: Based on the construction drawings of prestressed high-strength concrete hollow square piles, the drawings of the powder injection piles are refined. The powder injection piles are arranged to ensure that the soil within 1 meter around the pile cap is reinforced as a standard, thereby increasing the bearing capacity of the soil around the piles and protecting the pile heads from damage. S2: The construction process of powder injection pile is as follows: a. Measure and stake out. Move the pile driver to the stakeout point and keep the shaft of the pile driver vertical. The error should not be greater than 50mm. Adjust the outrigger cylinder of the pile driver so that the inclination of the drill rod of the drilling rig is not greater than 1% to prevent inclined pile driving and affect the bearing capacity of the pile foundation. b. Close the ash line valve of the powder spraying machine, open the air line valve, start the pile driver, start the air compressor and slowly open the air line pressure regulating valve to supply air to the pile driver. The pile driver gradually accelerates, rotates forward to pre-stir and sink, observe the pressure gauge reading, and adjust the pressure difference as the drill pipe drilling pressure increases, so that the rear valve is 0.02-0.05Mpa larger than the front valve. When drilling close to the designed depth, use low speed and slow rotation, and rotate the drill rig in place for 1-2 minutes; c. Lifting powder spray mixing: After reaching the designed pile bottom, the drill bit is generally lifted at a speed of about 1.0m / min in the opposite direction. The soft soil and cement are fully mixed while being sprayed and stirred. After being lifted to the designed stop elevation, the mixture should be stirred slowly in situ for 1-2min. The spraying amount should match the lifting speed. The powder spraying amount should be controlled at 55kg / m. If one spraying cannot meet the design requirements, two sprayings can be used. When lifting, choose medium and slow gears to ensure uniform mixing. It is required that the stirring shaft rotates at least 1 circle for every 16mm lifting. When the drill bit is lifted to 0.5m below the ground, the powder sprayer should stop spraying. During the powder spray pile construction, cement pumping must be continuous. The cement dosage and the time of pumping cement should be recorded by a dedicated person. Powder spraying and stirring are carried out during the lifting process. Cement powder is sprayed into the stirred soil through the powder sender to fully mix the soil and cement along the depth direction. d. Remixing: Stop spraying powder, drill while rotating the drill bit until it reaches the designed depth, then reversely rotate while lifting to fully mix the soil and cement. The soil is fully crushed and the cement powder is evenly dispersed in the pile soil. The remixing depth is 1 / 3 of the pile length. Remixing is an effective measure to ensure uniform pile formation and improve pile strength. e. Within 7 days after the pile is completed, randomly select 1% of the total number of piles and no less than 3 powder injection piles, and use the light dynamic penetration method to conduct quality inspection on the cement pile body of the powder injection pile. Generally, within 7 days after the pile is completed, use a light penetration probe to take cement soil samples from the pile body. The uniformity of the powder injection pile mixing is checked by observing whether the color of the cement soil samples is consistent and whether there are lumps of cement or unmixed soil clumps. At the same time, the strength of the powder injection pile body is judged based on the number of penetration blows (N10). The number of blows for N10 penetration of 10cm shall not be less than 10. Sections with less than 10 blows or each blow greater than 10mm are considered to be unsatisfactory and should be handled; S3: Preparation work before construction of PHS prestressed high-strength concrete hollow square piles: a. Pile position setting out; b. Put the hammer pile driver into position; c. Before lifting, mark the length of the pile in units of 0.5m on the pile body and mark the length of the pile from bottom to top in order to observe the depth of the pile into the soil and record the number of hammer blows per meter; S4: The square pile is welded on-site using a cross-shaped steel pile tip. First, weld the upper half of the pile tip. Then, rotate the pile 180 degrees and weld the remaining half of the pile tip. After welding, cool for 10 minutes and apply two coats of asphalt for corrosion protection. S5: Positioning and straightening the piles: a. After the first section of pile is hoisted, the pile tip is aligned with the pile position marker on the ground to accurately insert and position the pile; b. Use two theodolites to cross at 90 degrees and set up about 10 meters away from the pile position. First, observe the verticality of the pile driver's guide rod. After adjusting it to meet the requirements, use the theodolite to observe the verticality of the pile again. Instruct the pile driver to make multiple adjustments until the verticality of the pile body is controlled within 0.1%. The verticality of the first section of the pile should be strictly guaranteed to ensure its vertical guiding function. c. Hammering piles, using a heavy hammer with a low impact, the hammer weight is 12.8t, and the stroke is about 2m; d. Fill the pile bottom with C40 micro-expansive concrete for sealing, with a height of 2m; S6: Pile welding: a. Before connecting the piles, the steel plates at the ends of the upper and lower piles should be cleaned to ensure that there is no rust or pollution. In particular, the grooves of the end plates must be cleaned until the metallic luster is exposed. b. When connecting the upper and lower pile sections, the verticality of the pile body should be corrected in time and the axis should be aligned; c. The misalignment deviation of the upper and lower pile sections should be controlled to be ≤2mm. The two end surfaces of the upper and lower pile sections should fit tightly together without any gaps at the joints. It is strictly forbidden to fill the gaps between the joints with iron wires, welding rod ends, and other debris. When the piles are put into place and corrected, they should not be struck horizontally with a sledgehammer. If there are gaps at the joints, they should be welded securely with wedge-shaped iron sheets. d. Perform symmetrical spot welding at 4 to 6 points around the groove. After the upper and lower piles are fixed, remove the guide hoop and then perform symmetrical welding in layers. e. After the welded pile joints have cooled naturally and been coated with two coats of anti-corrosion asphalt, pile sinking can be continued. The natural cooling time should not be less than 8 minutes. It is strictly forbidden to use water cooling or to sink piles immediately after welding without cooling. S7: Before driving piles, the depth of the piles should be calculated in advance, and a suitable pile driver should be selected. Clear marks should be made on the pile driver. The deviation of the pile top elevation should be strictly controlled with a level, and the construction records of each pile should be recorded.

[0012] The present invention adopts a combination of "powder injection piles + PHS prestressed high-strength concrete hollow square piles". The powder injection piles are operated by dry spraying method, with a pile diameter of 500mm, an effective pile length of 5m, an overfilling of 500mm, and a cement content of 20% (common Portland cement with a strength grade of 42.5). The engineering pile type is prestressed high-strength concrete hollow square piles PHS-AB500 (310). The pile sinking operation is carried out by hammering method. The basement engineering piles are pull-out piles with an effective pile length of 30m. The main building area engineering piles are compression piles with an effective pile length of 45m. Compared with traditional engineering piles, the bearing capacity of the base soil is improved, the verticality control of the prestressed high-strength concrete hollow square piles during the pile sinking process is strengthened, the pile displacement and soil slippage easily generated during foundation pit excavation under complex geological conditions are solved, the occurrence of quality problems such as broken piles is greatly reduced, and the engineering quality of the engineering piles is improved.

[0013] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with the technical field, within the technical scope disclosed by the present invention, who makes equivalent replacements or changes based on the technical solution and inventive concept of the present invention, should be covered by the scope of protection of the present invention.

Claims

1. A construction method for mixed piles under soft geological conditions of saturated silt, characterized in that: The following steps are involved: S1: Based on the construction drawings of prestressed high-strength concrete hollow square piles, the drawings of the powder injection piles are refined. The powder injection piles are arranged to ensure that the soil within 1 meter around the pile cap is reinforced as a standard, thereby increasing the bearing capacity of the soil around the piles and protecting the pile heads from damage. S2: Construction of powder injection piles; S3: Preparation work before construction of PHS prestressed high-strength concrete hollow square piles; S4: Weld the square piles and perform anti-corrosion treatment on them; S5: Positioning and straightening the piles; S6: Pile welding; S7: Calculation, marking and construction records before pile delivery.

2. The construction method of a mixed pile under saturated silt soft geological conditions according to claim 1 is characterized in that: In S2, the construction process of the powder injection pile is as follows: a. Measure and stake out. Move the pile driver to the stakeout point and keep the shaft of the pile driver vertical. The error should not be greater than 50mm. Adjust the outrigger cylinder of the pile driver so that the inclination of the drill rod of the drilling rig is not greater than 1% to prevent inclined pile driving and affect the bearing capacity of the pile foundation. b. Close the ash line valve of the powder spraying machine, open the air line valve, start the pile driver, start the air compressor and slowly open the air line pressure regulating valve to supply air to the pile driver. The pile driver gradually accelerates, rotates forward to pre-stir and sink, observe the pressure gauge reading, and adjust the pressure difference as the drill pipe drilling pressure increases, so that the rear valve is 0.02-0.05Mpa larger than the front valve. When drilling close to the designed depth, use low speed and slow rotation, and rotate the drill rig in place for 1-2 minutes; c. Lifting powder spray mixing: After reaching the designed pile bottom, the drill bit is generally lifted at a speed of about 1.0m / min in the opposite direction. The soft soil and cement are fully mixed while being sprayed and stirred. After being lifted to the designed stop elevation, the mixture should be stirred slowly in situ for 1-2min. The spraying amount should match the lifting speed. The powder spraying amount should be controlled at 55kg / m. If one spraying cannot meet the design requirements, two sprayings can be used. When lifting, choose medium and slow gears to ensure uniform mixing. It is required that the stirring shaft rotates at least 1 circle for every 16mm lifting. When the drill bit is lifted to 0.5m below the ground, the powder sprayer should stop spraying. During the powder spray pile construction, cement pumping must be continuous. The cement dosage and the time of pumping cement should be recorded by a dedicated person. Powder spraying and stirring are carried out during the lifting process. Cement powder is sprayed into the stirred soil through the powder sender to fully mix the soil and cement along the depth direction. d. Remixing: Stop spraying powder, drill while rotating the drill bit until it reaches the designed depth, then reversely rotate while lifting to fully mix the soil and cement. The soil is fully crushed and the cement powder is evenly dispersed in the pile soil. The remixing depth is 1 / 3 of the pile length. Remixing is an effective measure to ensure uniform pile formation and improve pile strength. e. Within 7 days after the pile is completed, randomly select 1% of the total number of spray-injected piles (not less than 3 piles) and conduct quality inspection on the cement pile body of the spray-injected piles using a light dynamic probing method. Generally, within 7 days after the pile is completed, use a light probing device to take cement soil samples from the pile body. The uniformity of the spray-injected pile mixing is checked by observing whether the color of the cement soil samples is consistent and whether there are lumps of cement or unmixed soil clumps. At the same time, the strength of the pile body is judged based on the number of probing blows (N10). The number of blows for N10 penetration of 10 cm must not be less than 10. Sections with less than 10 blows or each blow greater than 10 mm can be considered as not meeting the requirements and should be processed.

3. The construction method of a mixed pile under saturated silt soft geological conditions according to claim 1 is characterized in that: In S3, the preparation work includes the following steps: a. Pile position setting out; b. Put the hammer pile driver into position; c. Before lifting, mark the length of the pile in units of 0.5m on the pile body and indicate the length of the pile from bottom to top so as to observe the depth of the pile into the soil and record the number of hammer blows per meter.

4. The construction method of a mixed pile under saturated silt soft geological conditions according to claim 1 is characterized in that: In the S4, the square piles are welded on-site using cross-shaped steel pile tips. The upper half of the pile tip is welded first, and then the square pile is rotated 180 degrees axially and the remaining half of the pile tip is welded. After welding, the pile is cooled for 10 minutes and two coats of asphalt are applied for corrosion protection.

5. The construction method of a mixed pile under saturated mud and weak geological conditions according to claim 1 is characterized in that: The S5 includes the following steps: a. After the first section of pile is hoisted, the pile tip is aligned with the pile position marker on the ground to accurately insert and position the pile; b. Use two theodolites to cross at 90 degrees and set up about 10 meters away from the pile position. First, observe the verticality of the pile driver's guide rod. After adjusting it to meet the requirements, use the theodolite to observe the verticality of the pile again. Instruct the pile driver to make multiple adjustments until the verticality of the pile body is controlled within 0.1%. The verticality of the first section of the pile should be strictly guaranteed to ensure its vertical guiding function. c. Hammering piles, using a heavy hammer with a low impact, the hammer weight is 12.8t, and the stroke is about 2m; d. Fill the bottom of the pile with C40 micro-expansive concrete for sealing, with a height of 2m.

6. The construction method of a mixed pile under saturated silt soft geological conditions according to claim 1 is characterized in that: The S6 includes the following steps: a. Before connecting the piles, the steel plates at the ends of the upper and lower piles should be cleaned to ensure that there is no rust or pollution. In particular, the grooves of the end plates must be cleaned until the metallic luster is exposed. b. When connecting the upper and lower pile sections, the verticality of the pile body should be corrected in time and the axis should be aligned; c. The misalignment deviation of the upper and lower pile sections should be controlled to be ≤2mm. The two end surfaces of the upper and lower pile sections should fit tightly together without any gaps at the joints. It is strictly forbidden to fill the gaps between the joints with iron wires, welding rod ends, and other debris. When the piles are put into place and corrected, they should not be struck horizontally with a sledgehammer. If there are gaps at the joints, they should be welded securely with wedge-shaped iron sheets. d. Perform symmetrical spot welding at 4 to 6 points around the groove. After the upper and lower piles are fixed, remove the guide hoop and then perform symmetrical welding in layers. e. After the welded pile joints have cooled naturally and been coated with two coats of anti-corrosion asphalt, pile sinking can be continued. The natural cooling time should not be less than 8 minutes. It is strictly forbidden to use water cooling or to sink piles immediately after welding without cooling.

7. The construction method of a mixed pile under saturated mud and weak geological conditions according to claim 1 is characterized in that: In S7, before driving the pile, the driving depth should be calculated in advance, and a suitable pile driver should be selected. A clear mark should be made on the pile driver. The deviation of the pile top elevation should be strictly controlled with a level, and the construction record of each pile should be recorded.

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