Construction method for new foundation pit close to existing subway station deep foundation pit close approach area

Abstract

The application discloses a construction method for a new foundation pit close to an existing subway station deep foundation pit close area, which comprises the following steps: new foundation pit site treatment, close area inclined support support pile construction, shared enclosure pile waist beam construction, support pile pile cap construction, close area inclined support construction, close area earthwork excavation, new foundation pit close area underground structure construction, close area inclined support lower plain concrete backfilling, close area underground structure internal inclined support cutting, support pile used as underground structure uplift pile and close area backfilling soil to a design elevation. The application forms an enclosure pile of a new foundation pit by borrowing an existing subway foundation pit enclosure pile, and forms a support structure of a pile cap-inclined support-waist beam, so that the influence of new foundation pit excavation on a subway project and other surrounding underground buildings is avoided; the whole support system plays a good control role in displacement deformation of the foundation pit and deformation influence on surrounding buildings and environment on the basis of ensuring overall stability and safety of the foundation pit; and the application effectively controls engineering investment and greatly shortens a construction period.

Description

TECHNICAL FIELD

[0001] The present application relates to the construction technology field of foundation pit retaining pile, especially to the construction method of new foundation pit close to existing foundation pit access area. BACKGROUND

[0002] Due to the influence of limited urban planning land resources, some new projects must be constructed close to existing subway stations. The excavation of the existing subway station foundation pit will affect the soil in the access area, causing changes in the original stress of the soil in the access area. Similarly, to prevent the disturbed soil in the access area from collapsing and affecting the stability of the two foundation pits during the excavation of the new foundation pit, the disturbed soil in the access area must be completely excavated, and certain support measures must be taken to prevent excessive displacement of the new foundation pit. For general deep foundation pit support, retaining piles, anchor cables and internal supports are commonly used. For the construction method of the access area between the two foundation pits, steel supports can be installed on the existing foundation pit retaining piles to ensure the stability of the access area. SUMMARY

[0003] The purpose of the present application is to solve the problems existing in the prior art and to provide a construction method for a new foundation pit close to an existing foundation pit access area.

[0004] To achieve the above-mentioned purpose, the present application adopts the following technical solutions:

[0005] The construction method for a new foundation pit close to an existing foundation pit access area comprises the following steps:

[0006] I. New foundation pit site treatment

[0007] Due to its special geographical location and specific regional influence, the project is very important at each stage from foundation pit support to soil excavation and inclined support construction. Foundation pit excavation is the first stage of the project implementation and is particularly important. According to the corner points of the boundary line measured by the positioning pile, the inner boundary line or slope bottom line of the foundation pit is measured and laid out according to the operating face width and drainage ditch width. The first layer of soil is excavated to the bottom position of the shared retaining pile waist beam design elevation (1500mm below the ground surface).

[0008] II. Inclined support pile construction in the access area

[0009] The support pile is constructed by jumping excavation: 1, 3, 5, 7 and 9 piles are constructed first, then 2, 4, 6, 8 and 10 piles are constructed, and so on. The top end of the support pile is located 8750mm below the ground surface, the bottom end of the pile is located 12750mm below the ground surface, the length of the pile is 4000mm, and the diameter of the pile is 800mm. After the construction of the support pile is completed, to prevent workers from falling into the support pile hole, the soil is backfilled, and the excavation is performed again when the support pile pile cap is constructed.

[0010] 1) Positioning the wire

[0011] According to the building positioning axis, the support pile site is accurately laid out on site by professional survey personnel according to the support pile plan, and wooden short piles are used as support pile site markers for site pile laying;

[0012] 2) Pile site hole forming

[0013] After the pile site is accepted, the drilling machine is positioned and the machine body is adjusted, the front and rear vertical markers of the drilling machine tower body are used to check the guide rod, and the position is corrected to make the drilling rod vertically aligned with the center of the pile site, so as to ensure that the pile body verticality deviation is not greater than the allowable deviation;

[0014] 3) Reinforcing cage placement

[0015] In order to ensure that the reinforcing cage does not deform during hoisting and lowering, a φ20 welded reinforcing rib is added every 2m in the reinforcing cage, the reinforcing rib is welded with the main reinforcement, and the thickness of the reinforcement around the pile is determined according to the size of the pile hole, but not less than 5cm; the reinforcing cage is hoisted by the auxiliary winch of the long spiral drill, the site is checked before the reinforcing cage is hoisted to ensure that there are no high-voltage lines and obstacles, to ensure the safety of the reinforcing cage hoisting construction, the reinforcing cage is installed by butt joint at the wellhead, after the reinforcing cage is put into the hole, it must be firmly positioned, the positioning elevation error is ±5cm, and the length of the reinforcing cage anchored into the pile cap is not less than 35d;

[0016] 4) Concrete pouring

[0017] When pouring concrete, the slump of the concrete should be measured, and the slump should be 16-18cm, and the concrete should have good workability; in order to ensure the quality of the concrete pile body, the guide pipe should be continuously up and down during pouring to make the concrete dense. If necessary, a vibrating rod should be used for vibration; in order to ensure the quality of the pile top, the guide pipe should be slowly pulled out of the concrete at the end of pouring to prevent the formation of cavities.

[0018] III. Construction of shared retaining pile waist beam

[0019] When the earthwork is excavated to the height of the surrounding purlin, the inclined support waist beam and the top force seat are constructed, and one side wall of the waist beam and one side wall of the pile cap are required to be parallel to facilitate the subsequent construction of the inclined brace;

[0020] The upper protection layer of the subway shared retaining pile is removed, the stress reinforcement is exposed, the surface of the reinforcement is cleaned, and the reinforcement of the waist beam is welded with the original retaining pile body reinforcement, then the formwork is placed, the concrete is poured, and the maintenance is carried out;

[0021] The waist beam construction process is as follows:

[0022] Excavate the top soil of the waist beam → remove the support pile floating layer to the design elevation of the pile top → cushion concrete → bind reinforcement → support formwork → pour waist beam concrete (C30) → maintenance;

[0023] Four, the construction of support pile cap

[0024] When the support pile concrete reaches the design strength, continue to excavate the foundation pit to the top of the support pile, and slope the soil in the close area of the two deep foundation pits to construct the support pile cap.

[0025] 1) Reinforcement of the cap

[0026] After the completion of the support pile construction, the reinforcement frame should be erected before binding, and the reinforcement of the cap should be bound above the anchor reinforcement reserved at the top. When binding, the upper main reinforcement is placed first, then the stirrup, and the stirrup is bound vertically with the main reinforcement at the interval specified in the drawing. The stirrup spacing meets the design and specification requirements. After the upper binding is completed, the lower main reinforcement is inserted and bound with the stirrup in time. After the lateral main reinforcement is inserted, the inner stirrup and tie reinforcement are placed one by one and bound firmly. The main reinforcement overlap uses single-sided overlap welding, and the length of the reinforcement after welding meets the design requirements. After the reinforcement is bound, the reinforcement cushion is bound in time.

[0027] 3) Formwork support and concrete pouring

[0028] The concrete uses commercial concrete with a strength grade of C30, which is transported to the construction site by a concrete transport vehicle. When pouring concrete, it should be continuous. If it must be intermittent, the interval time should be shortened as much as possible. During the pouring process, it should be vibrated in time. The insert-type vibrator should be inserted quickly and pulled out slowly. The insertion points should be evenly arranged and moved sequentially without omission. The moving distance should not be greater than 1.5 times the vibration radius (usually 30-40 cm). After vibration, the beam surface is initially smoothed with a wooden trowel, and the beam surface is treated with a steel trowel before final setting. After the pouring of the cap is completed, it is covered with straw bags, and a dedicated person is assigned to water and maintain it every day.

[0029] 4) Demolition and maintenance

[0030] The cap should be watered and maintained within 12 hours after the completion of concrete pouring. The number of maintenance times should be based on keeping the surface moist. The cap can be removed after 24 hours. When removing, the removal tool should not touch the concrete to avoid damaging the edges and affecting the overall appearance. The removed formwork should be cleaned and stacked neatly for next use.

[0031] Five, the construction of inclined bracing in the close area

[0032] The steel support of the main body adopts a steel lattice inclined support with a size of 600x600, and a steel plate with a size of 700x700x16 is welded at the top end of the steel support to seal the end. A special jacking seat is arranged at the bottom end of the steel support. The jacking seat is connected with the pre-buried steel bars on the inclined edge wall of the pile cap through anchor pieces, and the jacking is uniformly applied through the jacks. The steel support and the waist beam of the shared retaining pile are welded, and the other end is a movable support. The pre-applied axial force of the steel support is completed by relying on two hydraulic jacks. The capacity of the jacks meets the requirement of the design pre-applied axial force. The jacks are installed between the pre-buried plate 406 and the jacking seat 303. When the steel support is installed, the axes of the waist beam, the end head and the jacks are kept on the same axis. To ensure flatness, the locking pieces on the steel support should be tightened simultaneously in diagonal order. The final eccentricity value of the assembly is not greater than 20mm. Before the inclined support is welded, the jacking should be tightened. When the jacking force reaches the design pre-applied force, the jacks are removed after the locking pieces are installed and welded in the locking section. The pre-applied axial force of the jacks must be symmetrical and synchronous to ensure that the support axis is stressed. Before the inclined support is welded, a pre-jacking force of not less than 400KN must be applied.

[0033] Six, excavation of earthwork in the approach area

[0034] The earthwork under the inclined support is reserved, and the construction of the basement structure of the library is carried out. After the completion of the construction of the basement structure, the reserved earthwork is removed. The earthwork between the steel inclined supports is excavated by an excavator. The earthwork near the steel inclined support part and the bottom of the inclined support is excavated by manual excavation to prevent the instability and damage of the inclined support caused by mechanical excavation.

[0035] Seven, construction of underground structure in the approach area of the newly-built foundation pit

[0036] During the construction of the basement structure, only the structure of the support pile cap to the shared retaining pile part is constructed. The steel bars at the intersection of the side wall and the bottom plate are bound and the concrete is poured. When the strength of the side wall concrete reaches the design strength, the outer surface of the side wall is coated with waterproof material. The remaining part of the basement structure is continued to be constructed after the steel inclined support is cut off.

[0037] Eight, backfilling of plain concrete under the inclined support in the approach area

[0038] After the completion of the construction of the basement structure, the reserved earthwork is removed and the inclined support is cut off. During the removal of the reserved earthwork and the cutting off of the inclined support, the retaining pile completely loses the support and the lateral pressure of the earthwork. Whether it will have an adverse effect on the retaining pile and the subway structure, such as reserving the part of the earthwork and backfilling the upper concrete of the earthwork, which will protect the retaining pile to a certain extent.

[0039] Nine, cutting off of the inclined support inside the underground structure and waterproofing

[0040] After cutting off the steel lattice inclined support, the inclined support holes in the basement floor and the outer wall part need to be plugged and waterproofed, and there are 29 inclined supports in total, which will produce 58 leakage hidden danger points. Since the basement side wall and the common retaining pile are backfilled with plain concrete, which has a certain waterproofing effect, only the steel support hole in the side wall needs to be plugged with concrete. After cutting off the steel support of the basement floor, lay the water stop piece first, then pour the concrete cushion, continue to bind the steel bars on the concrete cushion, pour the concrete, complete the unfinished basement floor structure, and reinforce the basement wall waterproofing to prevent leakage, that is, complete the entire construction.

[0041] Ten, support pile is used as underground structure anti-floating pile

[0042] In areas where groundwater is relatively abundant, after the basement structure internal steel inclined support is removed, the support pile cap is removed, the stressed steel bars are exposed, the stressed steel bars are welded with the basement structure floor steel bars, and concrete is poured, so that the support pile becomes an integral structure of the basement floor and is used as an anti-floating pile of the basement, greatly saving the construction cost.

[0043] Eleven, backfilling soil in the adjacent area to the design elevation.

[0044] After the entire basement structure is completed and the concrete strength reaches the design elevation, backfill the underground soil. Before backfilling the outdoor foundation pit, the underground waterproof layer and protective layer must be inspected, any damage found must be repaired in time, and the garbage and other debris at the bottom of the foundation pit must be cleaned up. Before backfilling, a horizontal line is drawn on the outdoor waterproof protection board to control the height of the backfilling soil.

[0045] Preferably, the construction procedure of step one is: site leveling → foundation pit positioning → pile positioning → support pile construction → compaction grouting → waist beam construction → brick arch retaining wall construction → soil layering and excavation → soil nailing → hanging steel mesh → anchor spraying → maintenance.

[0046] Preferably, the mortar supply and masonry operation in step one are as follows: the support pile concrete uses commercial concrete C30, and the mortar is manually mixed on site. Preferably,

[0047] The pile foundation construction in step one: uses drilling and pouring construction, lowers the steel reinforcement cage, and uses excavators for on-site earthwork and stone transportation, and uses self-unloading trucks for external transportation.

[0048] Preferably, in step two, the drill rod is drilled to the predetermined depth, and the site construction technical personnel analyze whether the soil layer meets the design requirements based on the geological survey report and actual drilling and soil observation; if long spiral dry hole construction is found during construction, long spiral post-positioned steel reinforcement cage construction method will be used.

[0049] Preferably, in step two, before drilling, the drill bit valve is closed, the drill rod is moved downward until the drill bit touches the ground, the drilling machine is started to rotate the drill bit; generally, the speed should be slow at first and then fast, if the drill rod shakes or it is difficult to drill during the hole forming process, the machine should be stopped or the footage should be slowed down, if an obstacle is encountered, drilling should be stopped, the cause should be analyzed and forced drilling should be prohibited.

[0050] Preferably, in step two, the protective layer of the concrete cushion is made in an arc shape on the side close to the hole wall, generally, a vertical interval of about 2 m is arranged, and each interval is symmetrically arranged with four blocks along the circumference.

[0051] Preferably, the concrete cushion can also be made in a cylindrical shape with a hole capable of being worn on the stirrup.

[0052] Preferably, in step two, when the reinforcement cage is placed, the surveyor should first lay a line on the cushion layer, and the reinforcement cage is placed close to the line, the horizontal error should not exceed 3 cm, the vertical error should not exceed 2 cm, and the placed reinforcement cage should be observed to be straight.

[0053] Preferably, in step three, before the concrete is poured, the garbage, soil and other sundries in the formwork and the oil stains on the reinforcement should be cleaned, and whether the cement mortar cushion of the reinforcement is well padded should be checked.

[0054] Compared with the prior art, the application has the beneficial effects that:

[0055] 1. The application forms new foundation pit support piles by borrowing existing subway foundation pit support piles, and avoids the influence of new foundation pit excavation on subway engineering and other surrounding underground buildings by setting a support structure of a bearing platform-inclined support-beam.

[0056] 2. The application effectively controls engineering investment and greatly shortens the construction period. Since the construction site is limited, the starting point of the support scheme is to minimize the influence of foundation pit excavation on surrounding buildings and environment. The entire support system plays a good control role in the displacement deformation of the foundation pit and the deformation influence on surrounding buildings and environment on the basis of ensuring the overall stability and safety of the foundation pit, and also takes into account the economy. BRIEF DESCRIPTION OF DRAWINGS

[0057] Figure 1 The flow chart of the construction method of the new foundation pit close to the existing foundation pit access area proposed by the application;

[0058] Figure 2 The structure diagram of the process of installing inclined supports by grooving in the construction method of the application;

[0059] Figure 3 The structure diagram of the process of excavating earthwork to the bottom in the construction method of the application;

[0060] Figure 4 The structure diagram of the process of backfilling concrete in the construction method of the application;

[0061] Figure 5 Structure diagram of the hoop (trapezoidal) of the bearing platform in the application;

[0062] Figure 6 Structure diagram of the main reinforcement of the bearing platform in the application;

[0063] Figure 7 Section structure diagram of the inclined strut in the application;

[0064] Figure 8 Connection structure diagram of the inclined strut and the waist beam in the application;

[0065] Figure 9 Connection structure diagram of the inclined strut and the bearing platform in the application.

[0066] In the figure: subway foundation pit support pile 1, waist beam 2, pre-embedded plate A201, inclined strut 3, steel support 301, steel plate 302, top force seat 303, bearing platform 4, well-shaped main reinforcement 401, square closed hoop 402, longitudinal reinforcement 403, trapezoidal closed hoop 404, transverse reinforcement 405, pre-embedded plate B406, prestressed reinforcement 407, anchor 408 bearing platform, support pile 5, basement structure 6. DETAILED DESCRIPTION

[0067] The technical solutions in the embodiments of the application will be clearly and completely described below with reference to the drawings in the embodiments of the application. Obviously, the described embodiments are only some of the embodiments of the application, but not all the embodiments of the application.

[0068] The project: the Hefei Central Library is located at the intersection of Qimen Road and Feicui Road, and is adjacent to Feicui Road (Line 3 of the subway) in the west and Qimen Road (Line 4 of the subway) in the south. The place is the intersection and transfer point of Lines 3 and 4 after being put into operation. According to the foundation pit support design drawing, the library foundation pit excavation and the foundation pit support process will share the original support pile of the subway project. In order to prevent the shared support pile from tilting to the side of the newly excavated foundation pit, the following construction method is proposed, referring to Figure 1 .

[0069] 1. Construction of inclined strut support pile, referring to Figure 2 :

[0070] 1.1 Construction preparation

[0071] (1) Before officially entering the site, the entire set of construction equipment should be inspected to ensure that the equipment is in good condition, and no faulty equipment is allowed to enter the site. Assist Party A to complete the water and electricity pipeline layout work related to the support pile construction, so as to ensure that the construction can be immediately put into operation after entering the site. The roads and foundation pit slopes in the construction site should meet the driving requirements of the equipment transport vehicles and automobile cranes to ensure the safety of transportation.

[0072] (2) An isolation area should be set up during equipment assembly, with a designated person in charge. Assembly should be carried out strictly in accordance with the procedure. Non-installation personnel are not allowed in the assembly area to prevent safety accidents.

[0073] (3) Arrange for materials to be delivered to the site, and conduct material re-inspection and concrete mix design tests as required.

[0074] (4) Conduct quality and safety technical briefings before commencement of work and fill out the "Technical Briefing Record".

[0075] 1.2 Positioning and Laying Out

[0076] After the construction unit provides the building positioning axis, both parties should jointly verify it, and both parties should sign and confirm the "Handover Inspection Record". Based on the building positioning axis provided by the construction unit, professional surveyors should accurately lay out the pile positions on site according to the pile plan. Short wooden stakes are used as pile position markers for on-site pile layout.

[0077] After the construction unit, supervision unit, and design personnel jointly inspect and approve the pile positions and sign off on them, the next procedure can be carried out.

[0078] 1.3 Drilling

[0079] After the pile location is accepted, the drilling rig is positioned and the rig body is adjusted. The guide rod is checked and its position is corrected using the front and rear vertical markers of the drilling rig tower to ensure that the drill rod is vertically aligned with the center of the pile location, so as to ensure that the verticality deviation of the pile body does not exceed the allowable deviation.

[0080] Before drilling, seal the drill bit valve and move the drill rod downwards until the drill bit touches the ground. Then start the drilling machine and rotate the drill bit. Generally, the speed should be increased gradually from slow to fast. If the drill rod shakes or becomes difficult to drill during the hole-forming process, the machine should be stopped or the drilling speed should be reduced. If an obstacle is encountered, drilling should be stopped, the cause analyzed, and forced drilling is prohibited.

[0081] Based on the designed pile length, determine the drilling depth and mark it conspicuously at the corresponding position on the drilling rig tower as the basis for controlling the pile length during construction. When the bottom surface of the power head reaches the mark, the pile length meets the design requirements.

[0082] Once the drill rod reaches the predetermined depth, on-site construction technicians will analyze the geological survey report and the actual soil excavated from the borehole to determine whether the soil layer meets the design requirements. In case of special geological conditions, the pile support designer should comprehensively determine the appropriate method based on the drawings and the actual on-site geological conditions, and promptly notify the supervisor.

[0083] During construction, the "Support Pile Construction Record" should be filled out promptly and accurately. If it is found that dry drilling with a long spiral is not feasible during construction, the long spiral post-reinforced cage construction method will be adopted.

[0084] 1.4 Fabrication and installation of steel cages

[0085] (1) Steel cage manufacturing

[0086] a) In order to ensure that the steel cage does not deform during lifting and lowering, a φ20 reinforcing bar is added every 2m in the steel cage. The reinforcing bar is welded with the main reinforcement. The thickness of the reinforcement around the pile is determined according to the size of the pile hole, but not less than 5cm.

[0087] b) The type, model and size of the steel should meet the design requirements. If replaced, it should be replaced according to the specification requirements, and the design and supervision should be signed and recorded.

[0088] c) Steel cage binding sequence: first arrange the main reinforcement at equal intervals, then fix the reinforcing hoop, and then install the hoop according to the specified interval. The joints between the hoop, reinforcing hoop and main reinforcement are fixed by arc welding.

[0089] d) Concrete cushion or other effective measures should be provided on the outside of the steel cage to ensure the thickness of the steel reinforcement. The thickness of the main reinforcement should be ≥50mm, and the thickness of the bottom of the pile cap should be ≥100mm.

[0090] (2) Steel cage stacking

[0091] a) The steel cage manufacturing site can be determined according to the actual situation on site, but it must be within the coverage of the tower crane for the stacking of the steel cage.

[0092] b) Steel cage stacking should be in the order of installation, with the first installation stacked on the upper layer, and the later installation stacked on the lower layer, with a maximum of two layers.

[0093] c) When stacking the lower layer of steel cage, use stones to stabilize the side of the steel cage to prevent rolling.

[0094] (3) Steel cage installation

[0095] a) Steel cage is lifted by long spiral drill auxiliary winch, with a pulling force of 20kN (2T). The longest pile position drill hole support pile Z1 in this project has an effective pile length of 15m, and the single pile weight is 0.932T. The auxiliary winch meets the lifting weight. Before lifting the steel cage, check that there are no high-voltage lines and obstacles in the site to ensure the safety of the steel cage lifting construction. The steel cage is installed by butt joint at the wellhead. After the steel cage enters the hole, it should be firmly positioned. The positioning elevation allows an error of ±5cm. The length of the steel cage anchored into the pile cap is not less than 35d.

[0096] b, reinforcement

[0097] ① Four steel "earrings" are symmetrically arranged on the same section at certain intervals (usually about 2m) on the upper and lower ends and the middle of the steel cage. The steel diameter is generally 10-12mm.

[0098] ② Use concrete cushion to do protection layer. Make arc shape on the side of hole wall, generally every 2m interval arrange a row, every row arrange four blocks along the circumference symmetrically; concrete cushion can also be made into cylinder shape, with hole in the middle to be able to be worn on stirrup.

[0099] 1.5 Concrete pouring, support pile detection

[0100] a) After dry work hole forming acceptance, adopt guide pipe as chute to pour concrete within 1 hour. The distance between guide pipe bottom and hole bottom is about 1m.

[0101] b) When pouring concrete, measure the slump of concrete, and require the slump to be 16-18cm, and the concrete to have good workability.

[0102] c) To ensure the quality of concrete pile body, continuously move the guide pipe up and down during pouring to make the concrete dense. If necessary, use vibration rod to vibrate.

[0103] d) To ensure the quality of pile top, slowly pull out the guide pipe from the concrete when pouring is finished to prevent forming a cavity.

[0104] After support pile construction is completed and the curing age reaches 70% of the strength, carry out support pile integrity detection. The design drawing requires that the number of detected piles is not less than 20% of the total number of support piles, and not less than 5. The site requires that the support pile integrity detection is 100% to ensure construction safety. After detection is qualified, the next process of pile cap construction can be carried out.

[0105] 2. Construction of inclined strut support pile cap, refer to 5 and 6, one side of the pile cap 4 is formed by a cross-shaped main reinforcement 401, the square closed hoop 402 is welded around the main reinforcement 401, the longitudinal reinforcement 403 is bound inside the square closed hoop 402, the other side of the pile cap 4 is formed into a trapezoidal closed hoop 404, the transverse reinforcement 405 is bound inside the trapezoidal closed hoop 404, the formed reinforcement cage is bound with the reinforcement at the top of the support pile 5, and finally the pile cap 4 is poured;

[0106] 2.1 Pile cap construction technology

[0107] Remove the upper protection layer of the subway shared retaining pile, expose the stressed reinforcement, clean the surface of the reinforcement, weld the waist beam reinforcement with the original retaining pile body reinforcement, then place the formwork, pour concrete, and maintain.

[0108] Excavate the top soil of the pile cap → remove the float layer of the support pile to the design elevation of the pile top → cushion concrete → bind reinforcement → support formwork → pour pile cap concrete (C30) → maintenance.

[0109] 2.2 Construction preparation

[0110] 1) The base trench soil has been excavated to the design elevation, and the soil between the piles is excavated clean.

[0111] 2) Mark the pile top elevation position on the pile body with a level and mark it with red paint.

[0112] 2.3 Construction Notes

[0113] 1) When using the machine, check and confirm that the motor, cable, etc. are normal, the grounding device is good, and the protection device is safe and effective before starting the machine.

[0114] 2) During the pile head breaking process, construction personnel should be equipped with appropriate labor protection articles to prevent concrete debris from splashing and causing harm to construction personnel; persons unrelated to construction should not approach the construction site.

[0115] 3) Steel wire rope should be used to tightly bundle the pile head when lifting it away, and the bundle personnel should be removed before lifting.

[0116] 2.4 Reinforcement Tying of Pile Cap

[0117] 2.4.1 Main Materials and Tools

[0118] Reinforcing steel: Should have a factory certificate, and undergo mechanical property retest according to regulations. After passing the test, it can be put into use. Reinforcing steel should be free of old rust and oil stains.

[0119] Iron wire: 20-22 gauge iron wire (fire-burned wire) or galvanized iron wire (lead wire) is used.

[0120] Concrete cover: Mortar cushion

[0121] Tools: Reinforcing steel hooks, crowbars, reinforcing steel wrenches, tying frames, steel wire brushes, trolleys, chalk, rulers, etc.

[0122] 2.4.2 Reinforcing Steel Processing

[0123] Select the correct reinforcing steel grade and size according to the drawing, and process the cutting. The lengthening of longitudinal reinforcement should preferentially use welding, and reinforcing steel with d ≤ 20 is allowed to use lapping (d is the diameter of reinforcing steel). The length of the lapping section should be ≥ 0.7L, and should not be less than 200mm. The area percentage of longitudinal reinforcing steel lapping joints in the same connection section should be ≤ 50%. Before formal welding, perform a test weld, and after passing the test, proceed with batch welding. Reinforcing steel lapping sections should be staggered, and the distance between adjacent joints should be no less than 1m. The number of lapping joints for main reinforcement in the same section should not exceed 25% of the total number of reinforcing steel, and should be staggered by more than 1m at the corner. After straightening, the hoop reinforcement can be cut and processed. The cutting size should strictly follow the drawing size. Hoop reinforcement processing should be performed by skilled workers, and the processing size should be frequently checked during processing to avoid the inability to use due to excessive errors.

[0124] 2.4.3 Reinforcing Steel Tying

[0125] Before binding, steel reinforcement binding frame should be set up, and binding should be done above the anchoring reinforcement of support pile to improve efficiency. When binding, first place the main reinforcement of the top layer of the bearing platform, and then place the stirrup. When binding according to the drawing interval, the stirrup should be perpendicular to the main reinforcement, and the interval of the stirrup should meet the design and specification requirements. After the binding of the top layer is completed, the main reinforcement of the lower layer of the bearing platform is passed through and bound with the stirrup in time. After the lateral main reinforcement is passed through, the inner stirrup and tie reinforcement are placed one by one and bound firmly. The main reinforcement lap joint adopts single-sided lap joint welding, and the length of the steel reinforcement after welding meets the design requirements. After the steel reinforcement is bound, the steel reinforcement cushion block is bound in time.

[0126] 2.5 Bearing platform formwork setting

[0127] First, prepare 2 pairs of bearing platform side formworks according to the design requirements in the processing area, and then drive 48*3 steel pipes with a length of 1.2 m and a soil depth of 500 mm between every two piles. Place the prepared formwork inside the steel pipe, connect the joints with wooden boards, paste the inside joint seams with adhesive tape to prevent slurry leakage, and pad the protective layer cushion block. After confirming that the steel reinforcement and cushion block are all bound, connect the steel pipes between the piles with fasteners using horizontal steel pipes, and connect the steel pipes with the vertical columns at a position 150 mm below the top elevation of the bearing platform. After the installation is completed, reinforce the vertical columns with diagonal braces.

[0128] 2.6 Bearing platform concrete pouring

[0129] Before pouring, clean the garbage, soil and other sundries in the formwork and the oil stains on the steel reinforcement, and check whether the cement mortar cushion block of the steel reinforcement is well padded.

[0130] The concrete adopts commercial concrete with a strength grade of C30, which is transported to the construction site by a concrete transport vehicle. When pouring the concrete, continuous pouring should be performed, and if intermittent pouring is necessary, the intermittent time should be shortened as much as possible. During the pouring process, timely vibration should be performed, the insertion type vibrator should be inserted quickly and pulled out slowly, the insertion points should be uniformly arranged, and the vibrator should be moved point by point in sequence without omission to achieve uniform and compact vibration. The moving distance should be no greater than 1.5 times the vibration radius (generally 30-40 cm). After vibration, the beam surface is initially smoothed with a wooden trowel, and the beam surface is treated for light collection with a steel trowel before initial setting and final setting. After the bearing platform is poured, it is covered with straw bags, and a dedicated person is assigned to water and maintain it every day.

[0131] 2.7 Bearing platform form removal and maintenance

[0132] The beam surface should be watered and maintained within 12 hours after the completion of concrete pouring, and the number of watering and maintenance should be determined according to the requirement of keeping the surface of the bearing platform wet. The bearing platform side formwork can be removed after 24 hours. When removing, the removal tool should not touch the concrete to avoid damaging the corners of the bearing platform and affecting the overall appearance. The removed formwork should be cleaned in time and stacked neatly for next use.

[0133] 3. Grooving and installation of diagonal braces, refer to Figure 2 ,7 -9, the inclined brace 3 is composed of a steel support 301 of the main body, a steel plate 302 at the top end and a top force seat 303 at the bottom end, the steel support 301 is a lattice column welded together;

[0134] The side of the waist beam 2 close to the steel support 301 is chiseled to expose the embedded plate A201 and is fully welded with the steel plate 302;

[0135] The side of the bearing platform 4 close to the steel support 301 is chiseled to expose the embedded plate B406 and the prestressed tendon (407), the prestressed tendon (407) passes through the top force seat 303, one plate of the top force seat 303 is fully welded with the embedded plate B406, and the prestressed tendon (407) is locked with the other plate of the top force seat 303 after tensioning.

[0136] When the earthwork is excavated to the height of the surrounding purlin, the inclined support waist beam and support are constructed, and the inner inclined brace is arranged in the groove reserved on the earthwork.

[0137] 1) The steel support adopts Q235B steel, the welds at the welded joints are all full welds, the weld height hf is greater than or equal to 8, and should meet the design and relevant specification requirements;

[0138] 2) The position of the inclined brace is arranged according to the plan view, which should be rechecked on site and should avoid the main structure beams and columns;

[0139] 3) The earthwork in the section view must be reserved during the construction of the inclined brace, and can be removed in layers and sections after the construction of the inclined brace is completed, and the construction of the inclined brace should be separated by one construction, and the earthwork should be excavated in time after the construction of the inclined brace, and the exposure time should not be more than 12 hours;

[0140] 4) The steel lattice inclined brace passes through the bottom plate and side wall of the main structure, and one or more water stop plates can be arranged at the bottom plate and side wall, the number of the water stop plates is determined according to the waterproof requirements of the foundation bottom plate and side wall. If the water stop plates on the single limb angle steel overlap, they should be staggered.

[0141] 5) The steel lattice inclined brace inside the basement can be cut off after the backfilling of the basement foundation pit.

[0142] 6) The inclined brace should be tightly pressed before welding.

[0143] Inclined brace pressing construction: the end of the steel support is provided with a special top force seat, and a jack is uniformly applied. When the top force reaches the design pre-tensioning force, the jack is removed after the locking piece is installed and welded. The pre-tensioning force of the jack must be symmetrical and synchronous to ensure that the support is subjected to axial force. Before the inclined brace is welded, a pre-tensioning force of not less than 400KN must be applied.

[0144] 4. The earthwork is excavated to the bottom, and the Figure 3 ,

[0145] The earthwork under the inclined support is reserved, and the library basement structure construction is carried out. After the basement structure construction is completed, the reserved earthwork is cleared and transported.

[0146] 5. Backfilling of plain concrete, refer to Figure 4 ,

[0147] The earthwork under the inclined support is reserved, and the library basement structure construction is carried out. After the basement structure construction is completed, the reserved earthwork is cleared and transported, and the inclined support is cut off, and the plain concrete backfilling is carried out. When the reserved earthwork is cleared and transported and the inclined support is cut off, the enclosing pile completely loses the support and the lateral pressure of the earthwork, and whether it will have an adverse effect on the enclosing pile and the subway structure, if the part of the earthwork is reserved, and only the upper concrete of the earthwork is backfilled, the enclosing pile will be protected to a certain extent, and whether the scheme is feasible is researched.

[0148] 6. Cutting off the support inside and waterproofing

[0149] After the steel lattice inclined support is cut off, the inclined support holes passing through the basement bottom plate and the outer wall part are sealed and waterproofed, there are 29 inclined supports in total, and 58 leakage hidden danger points will be generated, the four inclined supports which are constructed first are improved in process, the water stop plates are welded, and the outer wall waterproofing is reinforced, so that the effect of eliminating leakage is achieved.

[0150] The above is only the preferred specific embodiment of the present application, but the protection scope of the present application is not limited to this, any person skilled in the art in the technical range disclosed by the present application, according to the technical scheme and the inventive concept of the present application, equivalent replacement or change, should be covered in the protection scope of the present application.

Claims

1. A construction method for a new foundation pit close to an existing foundation pit, characterized in that, The method comprises the following steps:

1. New foundation pit site treatment According to the corner points of the positioning pile, the inner edge line or the slope bottom line of the foundation pit is measured and placed according to the operation face width and the drain width; the first layer of earthwork is excavated to the bottom position of the design elevation of the common enclosing pile waist beam; 2. Construction of inclined support pile in the adjacent area The support pile is constructed by jumping excavation: first, 1, 3, 5, 7, and 9 piles are constructed, then 2, 4, 6, 8, and 10 piles are constructed, and so on; the top end of the support pile is located at 8750 mm below the ground surface, the bottom end of the pile is located at 12750 mm below the ground surface, the length of the pile is 4000 mm, and the diameter of the pile is 800 mm; after the construction of the support pile is completed, to prevent workers from falling into the support pile hole, the soil is backfilled, and the support pile cap is excavated when it is constructed; 1) Positioning and placing According to the positioning axis of the building, the support pile position is accurately placed on the site by professional survey personnel according to the support pile plan; the support pile position is marked by wooden short piles on the site; 2) Pile position hole forming After the pile position is accepted, the drill is positioned and the machine body is adjusted; the front and rear vertical markers of the drill tower body are used to check the guide rod and correct the position, so that the drill rod is vertically aligned with the center of the pile position, to ensure that the verticality deviation of the pile body is not greater than the allowable deviation; 3) Reinforcing cage placement In order to ensure that the reinforcing cage does not deform during hoisting and lowering, a φ20 reinforcing bar is added every 2 m in the reinforcing cage; the reinforcing bar and the main reinforcement are connected by welding; the thickness of the reinforcing cage around the pile is determined according to the size of the pile hole, but it is not less than 5 cm; the reinforcing cage is hoisted by the auxiliary winch of the long spiral drill; before hoisting, the site is checked to ensure that there are no high-voltage lines and obstacles, to ensure the safety of hoisting the reinforcing cage; the reinforcing cage is installed by butt joint at the wellhead; after the reinforcing cage is put into the hole, it is fixed in position; the allowable error of the positioning elevation is ±5 cm; the length of the reinforcing cage anchored into the cap is not less than 35d; 4) Concrete pouring When pouring concrete, the slump of the concrete should be measured, and the slump should be 16-18 cm; the concrete should have good workability; to ensure the quality of the concrete pile body, the guide pipe should be continuously moved up and down during pouring to make the concrete dense; a vibrating rod is used for vibration; to ensure the quality of the pile top, the guide pipe should be slowly pulled out of the concrete at the end of pouring to prevent the formation of a hollow; Three, common enclosing pile waist beam construction When the earthwork is excavated to the height of the enclosing purlin, the inclined support waist beam and the top support are constructed; one side wall of the waist beam and one side wall of the cap are required to be parallel, to facilitate the subsequent construction of the inclined support; The upper protection layer of the subway common enclosing pile is removed, the stress reinforcement is exposed, the surface of the reinforcement is cleaned, and the reinforcement of the waist beam is welded with the original enclosing pile body main reinforcement, then the formwork is placed, the concrete is poured, and the maintenance is performed; The waist beam construction process is as follows: Excavate the top soil of the waist beam → remove the float layer of the support pile to the design elevation of the top of the pile → cushion concrete → bind reinforcement → support formwork → pour waist beam concrete → maintenance; Four, construction of support pile cap When the support pile concrete reaches the design strength, the foundation pit is excavated to the top of the support pile, the soil is left in the slope of the two deep foundation pits, and the support pile cap is constructed; 1) Cap reinforcement binding After the construction of the support pile is completed, a steel reinforcement binding frame should be erected before binding, and the reinforcement binding of the pile cap should be carried out above the anchor reinforcement reserved at the top end of the frame. When binding, the upper main reinforcement should be placed first, then the stirrup, and the stirrup should be bound vertically to the main reinforcement at the interval shown in the drawing, and the interval of the stirrup should comply with the design and the relevant regulations. After the upper binding is completed, the lower main reinforcement should be passed through and bound with the stirrup in time. After the lateral main reinforcement is passed through, the inner stirrup and tie reinforcement should be placed and bound one by one. The main reinforcement overlap should be welded by single-face overlap, and the length of the reinforcement after welding should meet the design requirements. After the reinforcement is bound, the reinforcement cushion should be bound in time. 2) Formwork support and concrete pouring The concrete should be commercial concrete with a strength grade of C30, and should be transported to the construction site by a concrete transport vehicle. When pouring the concrete, it should be continuously carried out, and if it must be intermittent, the intermittent time should be shortened as much as possible. During the pouring process, the concrete should be vibrated in time. The insert-type vibrator should be inserted fast and pulled out slowly, and the insertion points should be arranged uniformly, moved point by point, and carried out in sequence without omission, so as to achieve uniform vibration and compaction. The moving interval should be no more than 1.5 times the radius of the vibrating action. After the vibration, the beam surface should be initially smoothed by a wooden trowel in time, and the beam surface should be treated by a steel trowel for lightening before the initial setting and final setting. After the pouring of the pile cap is completed, the pile cap should be covered with straw bags, and a special person should be assigned to water and maintain the pile cap every day. 3) Demolition and maintenance The pile cap should be watered and maintained in time within 12 hours after the completion of the pouring of the concrete. The number of maintenance times should be determined according to the maintenance of the surface moisture. The pile cap can be demolished after 24 hours. When demolishing, the demolition tool should not touch the concrete to avoid damaging the corners and affecting the overall appearance. The demolished formwork should be cleaned in time and stacked neatly for the next use. Five, construction of the inclined support in the close approach area The steel support of the main body adopts a 600×600 steel lattice inclined support, and a 700×700×16 steel plate is welded at the top end of the steel support for end sealing. A special top force seat is arranged at the bottom end of the steel support. The top force seat is connected with the pre-buried reinforcement on the inclined edge wall of the pile cap through an anchor, and the top force is uniformly applied by a jack. The steel support and the waist beam of the shared enclosing pile are welded, and the other end is a movable support. The pre-applied axial force of the steel support is completed by relying on two hydraulic jacks. The capacity of the jacks meets the requirements of the design pre-applied axial force. The jacks are installed between the pre-buried plate and the top force seat. When the steel support is installed, the axis lines of the waist beam, end head, and jack should be kept on the same axis line. To ensure straightness, the locking pieces on the steel support should be tightened simultaneously in diagonal order. The final eccentricity value of the assembly should be no more than 20 mm. The inclined support should be tightly pressed before welding, and the jacks should be removed after the locking pieces are welded at the locking section after the top force reaches the design pre-applied force. The pre-applied axial force of the jacks must be symmetrical and synchronous to ensure that the support axis is under stress. Before the inclined support is welded, a pre-applied force of no less than 400 KN must be applied. Six, earthwork excavation in the close approach area The earthwork below the inclined support is reserved for the construction of the library basement structure. After the construction of the basement structure is completed, the reserved earthwork is removed. The earthwork between the steel inclined supports is excavated by an excavator, and the earthwork near the steel inclined support and the bottom of the inclined support is excavated manually to prevent the mechanical excavation from causing the instability and damage of the inclined support. Seven, construction of the underground structure in the close approach area of the newly-built foundation pit The basement structure is constructed by first constructing the support pile cap to the part of the structure of the shared retaining pile, binding the steel bars at the junction of the side wall and the bottom plate, and pouring concrete. When the strength of the side wall concrete reaches the design strength, the outer surface of the side wall is coated with waterproof material. The remaining part of the basement structure is constructed after the steel diagonal brace is removed. VIII. Backfilling of plain concrete under the diagonal brace in the adjacent area After the completion of the basement structure construction, the remaining soil is removed, the diagonal brace is removed, and the plain concrete backfilling is performed. IX. Removal of the diagonal brace inside the underground structure and waterproofing After the removal of the steel lattice diagonal brace, the holes in the basement bottom plate and the outer wall part are sealed and waterproofed. There are 29 diagonal braces in total, which will generate 58 leakage points. Since the basement side wall and the shared retaining pile are backfilled with plain concrete, which has a certain waterproofing effect, only the steel support removal hole in the side wall needs to be sealed with concrete. After the removal of the steel support in the basement bottom plate, a water stop piece is laid and a concrete cushion is poured. The steel reinforcement is then bound on the concrete cushion, and the concrete is poured to complete the basement bottom plate structure that has not been constructed. The bottom plate wall waterproofing is reinforced to prevent leakage, and the entire construction is completed. X. Support pile used as anti-floating pile for underground structure In areas with abundant groundwater, the support pile cap is removed after the removal of the steel diagonal brace inside the basement structure, and the load-bearing steel bars are exposed. The load-bearing steel bars are welded to the basement structure bottom plate steel bars and poured with concrete, so that the support pile becomes an integral structure of the basement bottom plate and is used as an anti-floating pile for the basement, greatly saving construction costs. XI. Backfilling of soil in the adjacent area to the design elevation After the completion of the entire basement structure construction and the concrete strength reaches the design elevation, the underground soil is backfilled. Before backfilling the outdoor foundation pit, the underground waterproof layer and protective layer must be inspected, any damage must be repaired in a timely manner, and the garbage and debris at the bottom of the foundation pit must be cleaned. Before backfilling, a horizontal line is drawn on the outdoor waterproof protection plate to control the height of the backfilled soil.

2. The construction method for a new foundation pit close to an existing foundation pit according to claim 1, characterized in that, In step two, the drill rod is drilled to the predetermined depth, and the site construction technician determines whether the soil layer meets the design requirements based on the geological survey report and actual drilling observations. If the long auger cannot be used for construction, the long auger post-reinforcement cage construction method will be used.

3. The construction method for a new foundation pit close to an existing foundation pit according to claim 1, characterized in that, In step two, before drilling, the drill bit valve is closed, and the drill rod is moved downward until the drill bit touches the ground. Then, the drill is started and the drill bit is rotated. The drilling speed should be slow at first and then fast. If the drill rod shakes or is difficult to drill during the drilling process, the drilling should be stopped or the drilling speed should be slowed down. If an obstacle is encountered, drilling should be stopped, the cause should be analyzed, and forceful drilling should be prohibited.

4. The construction method for a new foundation pit close to an existing foundation pit according to claim 1, characterized in that, In step two, the protective layer made of concrete pads is made into a circular arc shape on one side of the hole wall, with a vertical interval of 2m. Each interval is symmetrically arranged with four blocks along the circumference.

5. The construction method for a new foundation pit close to an existing foundation pit according to claim 1, characterized in that, In step two, when placing the reinforcement cage, the surveyor should first lay a line on the cushion. The reinforcement cage should be placed tightly against the line, with a horizontal error of no more than 3cm and a vertical error of no more than 2cm. The placed reinforcement cage should be straight and level.

6. The construction method for a new foundation pit close to an existing foundation pit according to claim 1, wherein In step three, before pouring concrete, the garbage, soil debris, and oil on the steel bars in the formwork should be cleaned, and the cement mortar pads on the steel bars should be checked to ensure they are properly placed.

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