CN111576431A

CN111576431A - Excavation method for foundation pit of four-layer subway station

Info

Publication number: CN111576431A
Application number: CN202010448088.9A
Authority: CN
Inventors: 杨琼鹏; 胡新朋; 马斌; 李卫; 赵长青; 张存博; 吴文娟; 唐赫
Original assignee: China Railway Tunnel Group Co Ltd CRTG; China Railway Tunnel Stock Co Ltd
Current assignee: China Railway Tunnel Group Co Ltd CRTG; China Railway Tunnel Stock Co Ltd
Priority date: 2020-05-25
Filing date: 2020-05-25
Publication date: 2020-08-25

Abstract

The invention discloses a four-layer subway station foundation pit excavation method, which aims to solve the technical problem that the stability cannot be ensured in the complex geographical environment of the excavation surface of a multi-layer foundation pit in the prior art. The invention comprises the following steps: step 1: manufacturing a crown beam and a first concrete support; step 2: manufacturing a dewatering well, which comprises an in-pit recharge well and an out-pit observation well; and step 3: excavating the earth, wherein the negative layer and the negative layer are constructed by an open-cut forward construction method, and the negative layer are constructed by a cover-excavation reverse construction method; and 4, step 4: arranging foundation pit supports including the 1 st, 5 th and 6 th concrete supports and the 2 nd, 3 th and 4 th steel supports in gaps excavated in the earth; and 5: and (5) constructing a main structure, and finally backfilling a top plate to finish foundation pit excavation. The beneficial technical effects of the invention are as follows: the safety coefficient is high, and the device can adapt to complex landform characteristics.

Description

Excavation method for foundation pit of four-layer subway station

Technical Field

The invention relates to the technical field of subway construction, in particular to a four-layer subway station foundation pit excavation method.

Background

With the rapid development of domestic subway lines, the depth of stations is getting larger and larger, and some subway transfer stations reach four floors. As the depth increases, the stability problems of the pit are increased. In addition, for some stations near water, the soil quality is relatively soft, and complicated topographic features are caused by nearby residential buildings and the like, and how to ensure the safety of the surrounding environment is the key point of construction.

Disclosure of Invention

The invention provides a four-layer subway station foundation pit excavation method, which aims to solve the technical problem that the stability cannot be ensured in the complex geographical environment of the excavation surface of a multi-layer foundation pit in the prior art.

In order to solve the technical problems, the invention adopts the following technical scheme:

a four-layer subway station foundation pit excavation method is designed, and comprises the following steps:

step 1: manufacturing a crown beam and a first concrete support;

step 2: manufacturing a dewatering well, which comprises an in-pit recharge well and an out-pit observation well;

and step 3: excavating the earth, wherein the negative layer and the negative layer are constructed by an open-cut forward construction method, and the negative layer are constructed by a cover-excavation reverse construction method;

and 4, step 4: arranging foundation pit supports including the 1 st, 5 th and 6 th concrete supports and the 2 nd, 3 th and 4 th steel supports in gaps excavated in the earth;

and 5: and (3) main structure construction, namely firstly excavating to a negative two-layer bottom plate by an open excavation method and constructing a bottom plate structure, constructing a negative two-layer structure side wall, a negative one-layer middle plate, a negative one-layer side wall, a top plate and a ground outlet structure from bottom to top after the strength reaches the design strength, backfilling and hardening the top plate, excavating from the position of a reserved soil outlet hole by a cover excavation method, and constructing a negative three-layer middle plate, a bottom plate, a negative four-layer side wall, a negative three-layer side wall and a plugging soil outlet hole.

Furthermore, in the step 1, the top beam and the first concrete support are supported by bamboo glue formworks, reinforcing steel bars are bound on site, commercial concrete is transported to the site for pouring, and then a tamper is inserted for vibrating and compacting.

Further, the manufacture of the crown beam and the first concrete support comprises the following steps: (1) excavating and protecting a side slope; (2) pile head breaking, continuous wall surface cleaning and cushion layer pouring; (3) binding supporting steel bars; (4) installing a template; (5) and (5) pouring concrete.

Further, in the step 2, the dewatering well is constructed by a vehicle-mounted reverse circulation drilling machine, steel pipes and filter pipes are used as well pipes of the dewatering well in the hole, and gaps between the well pipes and the well wall are filled with round or sub-round sand pebbles.

Further, the preparation of the dewatering well comprises the following steps: (1) logging well positions of the well; (2) burying a mouth protecting pipe; (3) installing a drilling machine and drilling a hole; (4) cleaning holes and replacing pulp; (5) lowering a well pipe; (6) burying and filling the filter material; (7) and (5) washing the well.

Further, the construction of the open cut section structure in the step 3 adopts longitudinal segmentation, and the longitudinal segmentation is vertically layered and is carried out layer by layer from bottom to top; the underground excavation section adopts longitudinal segmentation, and a vertical layered and layer-by-layer structure is reversely manufactured from top to bottom.

Furthermore, the earthwork of the first to fourth layers dug out in the step 3 is directly transported to a spoil area, and the fifth layer earthwork is firstly turned upwards to the first layer and then dug out by a ground excavator.

And further, stopping mechanical operation when the fifth layer earthwork to the sixth layer earthwork is excavated to the residual 30cm of the design elevation, adopting manual excavation to the design elevation, then pouring a concrete cushion, laying an isolation layer, erecting a scaffold, installing a template, binding middle plate steel bars and pouring concrete.

And furthermore, in the step 4, the steel support is manufactured in sections, pipe sections are connected through high-strength bolts with flange plates, and one end of the end part of the support is provided with a flexible joint.

Further, the steel supports are pre-assembled on the ground according to the width of the actual measurement foundation pit, whether the steel supports are qualified or not is checked after the steel supports are assembled, and then the steel supports are numbered on the qualified steel supports.

Compared with the prior art, the invention has the beneficial technical effects that:

(1) the main foundation pit of the station is constructed by adopting a negative one-layer open excavation sequential method, a negative two-layer open excavation sequential method, a negative three-layer cover excavation reverse method and a negative four-layer cover excavation reverse method, the bottom of the foundation pit is positioned in a 2-4d2 silt layer and a 2-4b3-4+ d silty clay sand inclusion layer, and the foundation soil is high in strength, poor in uniformity and uneven. The marking section is provided with 6 supports along the direction of the foundation pit, the first support is a concrete support, the second, third and fourth steel supports are phi 609, the fifth and sixth supports are concrete supports, a negative two-layer bottom plate of an open-cut forward-cut method and a negative three-layer bottom plate of a cover-cut reverse-cut method are used as supports in the excavation process of the foundation pit, the south-north end well is additionally provided with one more steel support phi 800 than a standard section at the falling depth position, and other supports are arranged on the same standard section, so that the stability of foundation pit excavation is enhanced through layer-by-layer support.

(2) The invention is provided with the internal and external precipitation wells, so that underground water in soil bodies in the excavation range is drained, and construction operation of excavators and workers in the pit is facilitated; the water content of the soil in the pit is reduced, the strength of the soil in the pit is improved, the bottom uplift of the pit and the deformation of the enclosure structure are reduced, and excessive settlement of the earth surface outside the pit is prevented; the height of the confined water head of the lower confined aquifer is reduced in time, the occurrence of sudden gushing at the bottom of the foundation pit is prevented, and the stability of the pit bottom of the foundation pit during excavation is ensured.

Drawings

FIG. 1 is a flow chart of a manufacturing process of a crown beam and a concrete support of the excavation method of a foundation pit of a four-layer subway station.

FIG. 2 is a flow chart of a manufacturing process of the dewatering well of the excavation method of the foundation pit of the four-layer subway station.

Fig. 3 is a schematic view of earth excavation by the four-layer subway station foundation pit excavation method of the present invention.

FIG. 4 is a process flow diagram of steel support construction of the excavation method of the foundation pit of the four-layer subway station.

FIG. 5 is a process flow diagram of the main structure construction of the excavation method of the foundation pit of the four-layer subway station.

Detailed Description

The following examples are intended to illustrate the present invention in detail and should not be construed as limiting the scope of the present invention in any way.

Example (b): a four-layer subway station foundation pit excavation method is disclosed, referring to figures 1 to 5, and comprises the following steps:

crown beam and first concrete support

A crown beam is arranged on a 1200mm ground connecting wall of the station enclosure structure, and the size of the crown beam is 1400mm multiplied by 800 mm. The first support of the station is made of reinforced concrete with the size of 900mm multiplied by 800mm, and the connecting beam is made of reinforced concrete with the size of 600mm multiplied by 600 mm. The 2 nd, 3 rd and 4 th ways are phi 609, t =16mm steel supports, the 5 th and 6 th ways are concrete wales and concrete supports, the size of the wale is 1200mm multiplied by 1000mm, and the supporting beams are supported by 1000mm multiplied by 1000mm reinforced concrete.

The top beam, the waist beam and the concrete support are constructed according to the excavation progress by adopting a bamboo glue template formwork, binding reinforcing steel bars on site, transporting commercial concrete to the site for pouring, vibrating and compacting by using an inserted type tamper.

The process flow of the construction of the crown beam, the waist beam and the concrete support is shown in figure 1,

digging and slope protection

After the underground continuous wall reaches the strength, a first layer of earthwork can be excavated, and a crown beam and a concrete support are constructed.

Removing pile head, cleaning surface of continuous wall and pouring cushion layer

And measuring and setting out, and determining the center line, the side line and the elevation of the crown beam and the concrete support. And removing the miscellaneous soil and the floating slag on the top of the continuous wall, breaking the wall head above the elevation of the bottom surface of the crown beam by using the manual pneumatic pick to expose fresh concrete, and cleaning the contact surface of the crown beam and the continuous wall.

And after the pile head of the continuous wall is chiseled off, leveling the supporting position of the crown beam and erecting a mold and pouring cushion concrete.

Construction of steel bar

After the pile head is broken, firstly straightening the anchoring reinforcing steel bars on the top of the continuous wall.

The crown beam and the concrete supporting steel bars are processed into semi-finished products in advance in a steel bar processing field according to the design size, and the semi-finished products are sorted, and are stacked in order according to the type numbers. Before construction, the design paper is checked again, and the construction site is transported after the inspection is correct.

And (4) binding the crown beam and the concrete support steel bars on site, and lengthening the main steel bars by lap welding. The length of the welding seam is not less than 10d, and the joint with the same section cannot exceed 50%. And each section of the crown beam steel bar reserves the lap joint length for the construction of the lower section of the crown beam, and the lap joint length is staggered by not less than 1 m.

Construction of shuttering

The template adopts the wood pattern of thickness 15mm, and the keel adopts 100mm square timber in the support system, and the interval is 300mm, and outer keel adopts phi 48 two-way double-deck steel pipe. The bottom die adopts a bamboo plywood. The form is coated with a release agent prior to installation.

Fifthly, concrete pouring

The crown beam and the concrete supporting concrete are C35, and commercial concrete and an inserted vibrator are adopted for vibration. When the concrete is vibrated, the vibrating rod is vertically inserted into the concrete and is inserted into a lower-layer non-initial-setting layer by 50-100 mm to promote the mutual combination of the upper layer and the lower layer, the distance between each inserting point should not exceed 1.5 times of the acting radius of the inserting point, when the concrete is used, the vibrating key point of 'fast inserting and slow pulling' is realized, the vibrating time of each inserting point is preferably 20-30 s, and the concrete surface begins to become slurry and does not generate bubbles. And immediately carrying out watering maintenance after the concrete is solidified, wherein the maintenance time is not less than 14 days. And (3) curing within 12-18 hours after the concrete pouring is finished, wherein if the concrete is poured in hot or dry seasons, the curing time is advanced to 8-14 hours.

(II) precipitation construction

In the embodiment, 45 dewatering wells are totally counted, wherein 25 dewatering wells in the pit are also used as recharging wells, 20 observation wells outside the pit are used, and the well depth is about 45-54 m. And the dewatering well is constructed by adopting a vehicle-mounted reverse circulation drilling machine. The hole diameter of the drilled hole is 650mm, a diameter 273 steel pipe and a filter pipe are used as the well pipe of the dewatering well, and a gap between the well pipe and the hole wall is filled with round or sub-round sand pebbles.

The construction process flow of the dewatering well is shown in figure 2.

Measuring well position

And (4) measuring the well position according to the plane layout diagram of the downcomer well, and marking the well position after the well position measurement is finished, so that the subsequent construction is facilitated. If there is a surface obstruction at the deployed well site, it should be attempted to clear the obstruction to facilitate the well construction. If the ground obstacles are not easy to clear or are influenced by other construction conditions and the well can not be dug at the original well position, the ground obstacles should be communicated with engineers and the first party in time and take other measures, and the well position can be properly adjusted if necessary.

② burying mouth-protecting pipe

When the mouth protecting pipe is buried, the bottom mouth of the mouth protecting pipe is inserted into an undisturbed soil layer, cohesive soil or a straw braid is applied outside the pipe for sealing, the slurry return outside the pipe during construction is prevented, and the upper part of the mouth protecting pipe is 0.10-0.30 m higher than the ground.

Installing drilling machine

When a drilling machine is installed, in order to ensure the verticality of a hole, a machine table is installed stably and horizontally, a hook is aligned with the center of the hole, the hook, a turntable and the center of the hole form a line, an opening is tightly closed, two drill collars are arranged at the joint of a drill bit and a drill rod, and the bent drill rod cannot be put into the hole.

Drilling to form holes

The dewatering well is constructed by a vehicle-mounted reverse circulation drilling machine, and the aperture of the opening of the dewatering well is phi 650 mm. The uniform diameter is the bottom when the holes are formed; when drilling a hole, the steel wire rope with the hook is hung tightly, and the steel wire rope is slightly pressed and slowly rotated to ensure the perpendicularity of drilling the hole.

The hole forming construction adopts artificial slurry making in the hole, the density of slurry is controlled to be 1.10-1.15 in the drilling process, and when the drilling tool is lifted or stopped, the hole must be filled with the slurry to prevent the hole wall from collapsing.

Fifthly, cleaning holes and changing pulp

And after the drilling hole is drilled to the designed elevation, lifting the drill rod to be 0.50m away from the bottom of the hole before lifting the drilling hole, punching to remove impurities in the hole, simultaneously gradually adjusting the density of slurry in the hole to 1.10, wherein the sediment at the bottom of the hole is less than 30cm, and the returned slurry does not contain a mud block.

Sixthly, the pipe goes into the well

After the well pipe enters the field, whether the gap of the filter meets the design requirement or not is checked. The hole depth must first be measured and recorded on a well pipe and water filter pipe by pipe basis. And (3) plugging the bottom of the settling tube, wherein the lower plugging iron plate is not less than 6mm in order to ensure that the bottom of the settling tube is firmly plugged. Secondly, checking the welding of well pipes, wherein the welding joints of the well pipes are in a sleeving connection type, the length of the sleeving connection is 20mm, and the sleeving connection is sleeved into each of an upper well pipe and a lower well pipe by 10 mm; the casing coupling and the well pipe are welded firmly, the welding seam is uniform and has no sand hole, and the height of the welding seam is not less than 6 mm.

And (3) starting to lower the well pipe after the inspection is finished, respectively arranging a set of centralizers (centralizers) with the diameter smaller than the aperture 5cm at the upper end and the lower end of the water filter pipe in order to ensure that the water filter pipe is centered during pipe lowering, wherein the centralizers adopt trapezoidal iron rings, and the upper centralizer iron ring and the lower centralizer iron ring are staggered by 1/2 and are not on the same straight line.

Seventh, burying and filtering material

Before filling the filter material, a drill rod is put into the well pipe until the drill rod is 0.30-0.50 m away from the bottom of the hole, after a choke plug is added to seal the upper opening of the well pipe, slurry is pumped from the drill rod to perform hole punching and gradual slurry mixing, so that the slurry in the hole returns from the inside of the filter pipe to the outside through an annular gap between the well pipe and the wall of the hole, the density of the slurry in the hole is gradually adjusted to 1.05, then the filter material is filled according to the structural design requirement of the well by opening a small pump, and the height of the filter material is measured along with the filling. Until the filter material is put into a preset position.

When the filter material is filled, the pump amount is adjusted according to the water return condition of the orifice. The upward returning height of the filter material is tracked in the process of filling the filter material, and after the filter material is compacted to the designed height, the hole is filled to the well mouth by a clay block between the well pipe and the hole wall and is compacted, so that mud and surface sewage are prevented from flowing into the well from the outside of the pipe.

Washing well

Before the drill rod is lifted, the drill rod in the well pipe is connected with an air compressor to pump water by the air compressor, after the well can discharge water, the drill rod is lifted out and the well is washed by a piston (used when the well is formed by positive circulation). The difference between the diameter of the piston and the inner diameter of the well pipe is about 5mm, and the bottom of the piston rod needs to be provided with a valve. When the well is washed, the piston must be pulled upwards from the lower part of the water filtering pipe to pull water out of the hole opening, the piston can move up and down on the filter part of the well with little water yield to impact mud on the hole wall, and at the moment, the piston is pulled while injecting water into the well. When the water drawn out by the piston does not contain silt basically, an air compressor can be replaced to pump water and wash the well, and the pipe bottom is blown out to sink silt until the water is clear and does not contain the silt. After the well is washed, the pump can be taken down for pumping. The well can be put into use after the completion of the hole forming.

(III) earth excavation

When the underground continuous wall enclosure structure reaches the design strength and the precipitation reaches the expected effect, the foundation pit can be dug formally, and the main body construction needs are considered in the foundation pit excavation. Five main points of layering, step by step, symmetry, balance and time limitation are well mastered in the excavation process, and the construction principle of vertical layering, longitudinal segmentation, transverse block division, support-first excavation and strict over excavation is followed.

A. Vertical stratification

Determining the number of foundation pit excavation layers according to the number of tracks of the vertical supports of the foundation pit, wherein the foundation pit of the station is divided into nine layers by six supports to excavate earthwork; and the end shield well is excavated in ten layers by seven supports in total, and the layering principle is that 1.0m of the bottom of each support is a layer interface.

B. Longitudinal segmentation

The excavation length of each unit is determined according to the production capacity and excavation conditions of excavation equipment, the excavation width of a foundation pit, the framing position of the underground continuous wall and the height of each layer of excavation, the excavation length of each section of earthwork is controlled to be 3-6 m, and 2 steel supports are guaranteed to be installed in each section of excavation.

The excavation process flow is shown in fig. 3.

Firstly, excavating a first layer of soil body by adopting 2 PC200 excavators to a first concrete supporting bottom position (1.2 m), and directly loading excavated earth to a dump truck for transporting to a dump site. The surface soil can be excavated in large area, and the top beam and the concrete support can be constructed.

And secondly, excavating a second layer of earthwork after the supporting strength of the concrete reaches the design strength. The second layer of earthwork excavation adopts 2 PC200 excavators to overturn the soil in the pit to the top of the slope, 1 PC200 excavator is placed on the ground to directly load the soil, and the soil is excavated to 1.0m (excavation 6 m) below the bottom of the second support; and scattered earthwork at the positions of the edges is cleaned by manual cooperation. The excavated earthwork is directly transported to a spoil area (if the earthwork can not be directly transported outward, the temporary stacking area is required to be more than 10m away from the foundation pit, and the height is not more than 2 m). Excavation length of each section of the layer is 5-6 m, excavation time of each section of the two sides of the layer is controlled within 10h after the middle of the layer is excavated, and 2 steel supports are installed within 6h to pre-add axial force.

If the sign of longitudinal slippage appears in the excavation process, a layered excavation and layered supporting mode is immediately adopted, and the inner supporting upright post and the dewatering well are guaranteed to be safe after being supported and excavated.

And thirdly, the third and fourth layers of earthwork excavation methods are the same as the second layer, 4 PC200 excavators are adopted from the fifth layer to sequentially dig the earthwork upwards to the first layer, and then the earthwork is excavated by the ground excavator.

Fourthly, stopping mechanical operation when the fifth layer of soil is excavated to the designed elevation and the residual 30cm is left, and adopting manual excavation to the designed elevation; and excavating to 1.8 meters below the middle plate of the negative two layers, pouring a concrete cushion layer, and paving an isolation layer. And after the cushion layer is of equal strength, erecting a scaffold mounting template, binding middle plate steel bars and pouring concrete. And after the strength of the middle plate reaches the design requirement, a second-layer structure and a first-layer structure are constructed, and the PC60 excavator can be continuously placed from the reserved soil outlet to dig soil after the middle plate is completed.

Fifthly, excavating and loading soil for the sixth layer of earthwork by using 4 PC60 type excavators, directly hoisting and transferring the soil by using two grab buckets, stopping mechanical operation when the left 30cm of the designed elevation of the fifth concrete support is excavated, and excavating manually to the designed elevation; and pouring a concrete cushion layer of the support beam. Binding supporting beam steel bars, installing templates and pouring concrete after the cushion layer has equal strength. After the strength of the support beam reaches the design strength requirement, the PC60 excavator can be continuously lowered from the gap of the support beam.

Sixthly, excavating and loading soil for the seventh-layer earthwork by adopting 4 PC60 type excavators, directly hoisting and transferring the soil by using two grab buckets, stopping mechanical operation when the left 30cm of the designed elevation is excavated, and excavating manually to the designed elevation; and excavating to 1.8 meters below the middle plate of the negative three layers, pouring a concrete cushion layer, and paving an isolation layer. And after the cushion layer is of equal strength, erecting a scaffold mounting template, binding middle plate steel bars and pouring concrete. After the strength of the middle plate reaches the design requirement, the PC60 excavator can be continuously placed from the reserved soil outlet for digging soil.

Seventhly, excavating and loading soil for the eighth layer of earthwork excavation by adopting 4 PC60 type excavators, directly hoisting and transferring the soil by using two grab buckets, stopping mechanical operation when the residual 30cm of the designed elevation of the sixth concrete support is excavated, and excavating to the designed elevation manually; and pouring a concrete cushion layer of the support beam. Binding supporting beam steel bars, installing templates and pouring concrete after the cushion layer has equal strength. After the strength of the support beam reaches the design strength requirement, the PC60 excavator can be continuously lowered from the gap of the support beam.

Eighthly, excavating and feeding earth by adopting 4 PC60 type excavators, directly hoisting and transferring the earth by using two grab buckets, stopping mechanical operation when the earth is excavated to the residual 30cm of the designed elevation, and excavating manually to the designed elevation; and embedding a grounding and pouring concrete cushion layer, and paving a waterproof layer. Binding the bottom plate reinforcing steel bars after the cushion layer has equal strength, and pouring concrete. And constructing the negative four-layer structure side wall after the strength of the bottom plate reaches the design requirement.

(III) Foundation pit support

The support of the embodiment adopts a phi 1200mm diaphragm wall +6 support forms, and the standard section support forms are a concrete support + 2 nd, 3 rd and 4 th steel support forms of the 1 st, 5 th and 6 th concrete supports. The end well support adopts a phi 1200mm underground diaphragm wall plus 7 supporting forms, and the standard section supporting forms are a 1 st, 5 th and 6 th concrete support plus a 2 nd, 3 rd, 4 th and 7 th steel support supporting forms.

The steel support construction process is shown in fig. 4.

Measuring and lofting

a. And (3) marking the designed axis position and the elevation of the steel support on the net spraying plane by a measurer, if the deviation exists between the designed axis position and the elevation and the designed position, properly adjusting the two sides at the same time, and keeping the adjusting distance to be not more than 30 cm.

b. And determining the positions of the bracket, the connecting beam, the steel purlin and the like according to the central point of the support.

c. When the steel support is installed, a gradienter is used for popping up a cross line at the elevation and the center position of the steel support on the plane of the steel enclosing purlin, square frames are drawn according to the diameters of the supports from top to bottom and from left to right of the cross line, the contact surface of each support and the steel enclosing purlin is controlled according to the left position and the right position, and the height difference of two ends is not more than 20 mm.

② acceptance of approach

And establishing a support component part entering acceptance system, accepting the size, specification, material and appearance of each support, marking unqualified component parts, and strictly forbidding the components to be put into use. The support is pre-spliced on the ground before installation and the flatness of the support is checked, the deviation degree of the center connecting line of the two ends of the support is controlled within 20mm, the support which is checked to be qualified is numbered according to positions so as to avoid misuse, and the support is hoisted in place by 2-3 sections.

Mounting steel support

a. Installation of steel purlin

And chiseling the bulge on the wall surface of the enclosure structure and plastering the wall body. And (4) installing the steel enclosing purlin, and filling the back of the steel enclosing purlin after the steel support is erected.

b. Machining of steel supports

The steel pipe supports are manufactured in sections, the pipe sections are connected through high-strength bolts with flange plates, and one end of each supporting end is provided with a flexible head. The steel pipe supports are pre-assembled on the ground according to the width of an actually measured foundation pit, after the assembly is completed, the stay wire is used for checking the straightness and the length measured by the steel tape, and whether the connection of the joints of the supporting pipe is tight, whether the supporting pipe is damaged or deformed and whether two supporting ends are flat or not are checked. After the inspection is qualified, the support is numbered by red paint.

c. Support mounting

And (3) dividing the support qualified by inspection into 2-3 sections, and hoisting the support in place by using a 16+16t gantry crane, wherein two-point hoisting is adopted for the support hoisting. And fixing the middle part of the support by using the temporary upright posts and the connecting beams.

The support must be kept stable, collision-free and deformation-free in the hoisting process. After the steel pipe support is hoisted in place, the lifting hook is not loosened, and the support is adjusted to the designed position by manual assistance and then is temporarily fixed. The eccentricity of the central line of the two ends of the steel pipe support must be controlled within 20mm when the steel pipe support is installed.

After the support is temporarily fixed, pre-pressing axial force is immediately applied according to the design requirement. 2 200t hydraulic jacks are hung and placed at the jacking position of the movable head, and the placement positions of the two hydraulic jacks are necessarily symmetrical and parallel. When the pre-stressed axial force is applied, the two jacks are kept synchronously and symmetrically graded, when the pre-stressed axial force reaches a design value, a steel cushion block (the steel cushion block adopts a steel plate with the thickness of 30 mm) is wedged in the movable head and is firmly welded, and then the oil return is performed, the jacks are loosened, the steel wire rope is untied, and the support is installed. When the prestress is applied, the stress is gradually increased at a constant speed and recorded for future reference.

The steel support installation requirement is as follows: 1. the steel support is installed in the vertical plane of the foundation pit and strictly adheres to the layered excavation, and the support installation and the earth excavation are closely matched according to the principle that the steel support is firstly supported and then excavated from top to bottom. After the foundation pit is exposed, the net spacing of the foundation pit is measured on site, pre-splicing is carried out on the ground to check the straightness of the support, the deviation degree of the center connecting line of the two ends of the support is controlled to be not more than 20mm, and the support qualified through checking is numbered according to the position so as to avoid misuse. 2. The steel support is assembled in advance according to the width of a foundation pit before support erection, quality defects such as welding damage and cracking do not exist after inspection, and the axis deviation and the flexural deformation of the assembled steel support are within an allowable range.

Stress application

a. Two 2X 200T oil pressure jacks are lowered in cooperation with a gantry crane in stress application, the axial force of the steel support is applied by two 200-ton jacks in a grading manner, when the stress is applied, 30% of stress is applied firstly after the support is installed, and when no abnormal condition exists, 40% of stress is applied again, and finally 30% of stress is applied again.

b. And after the support shaft force is applied, all the bolts connected with the steel support are retightened.

c. In the process of applying axial force, graded force application is adopted, the pressure value is read through an oil pump pressure gauge, the pressure is stopped for 30 minutes when the reading number reaches 30% of the pre-applied force required to be erected, the pressure is stopped for 30 minutes when the pressure is continuously applied for 40%, the pressure is continuously applied again to enable the corresponding pressure value to be consistent, the pressure of the jack is stabilized, and the steel wedge is arranged at the adjustable end for limiting. And reinforcing the node when necessary, and locking after the prestress is stabilized.

d. After the support erection is finished, within 12 hours before and after the support prestress is added, monitoring frequency is encrypted, and when the prestress loss or the deformation rate of the enclosure structure is obviously increased, the prestress is added to a design value; when the temperature difference between day and night is too large, which causes the loss of the support prestress, the prestress is immediately added to the design value in the day low-temperature period.

e. After the steel support is erected in place, safety steel wire ropes must be installed at two ends of the steel support, the steel support weighs about 8t according to the longest 21m, and the steel wire rope with the diameter of 14mm and bearing the weight of 8t is adopted in consideration of the possibility that one end of the steel support slides down. And (3) planting bars into the diaphragm wall at an included angle of 30 degrees formed by the top of the steel support and the enclosure structure at the position of 30cm, wherein the diameter of the planted bars is 25mm, the depth of the planted bars is 30cm, and the planted bars are used for connecting the safety steel wire rope.

Dismantling the steel support

And (4) after the construction of the main structure bottom plate and the side wall is finished, the steel supports are removed after the design strength reaches 80%, and the previous steel support is removed upwards step by step.

a. The steel support is dismantled in a segmented and layered mode along with the construction process of the main body structure; before dismantling and propping, the concrete strength of the front section is actually measured by a laboratory (the test piece strength report is actually measured by a resiliometer or maintained under the same condition), and the concrete strength is reported and managed after meeting the requirements, so that the dismantling and propping construction can be carried out.

b. Before the steel pipe support is dismantled, if the steel support exists, the previous layer of steel support is subjected to axial force pre-application for one time, and the steel support is dismantled after the design requirement is met.

c. And a 16+16T gantry crane is used for suspending and protecting the steel support and the movable end of the steel support by using a steel wire rope, so that the falling-off is prevented and the safety of a foundation pit is endangered. For longer steel supports, steel support bolts are disassembled and suspended near the cross beam in advance, 2 200T jacks are arranged at the movable end to apply axial force until the steel wedge blocks are loosened, the steel wedge blocks are taken out, and the steel wedge blocks are unloaded step by step until the steel wedge blocks are taken out. And (3) lightly placing the steel support on the structural slab, decomposing and dismantling, and finally hanging the support out of the foundation pit by using a gantry crane, wherein the antithetical couplet beam must be hung firstly and then cut.

d. And (3) paying close attention to abnormal phenomena nearby in the steel support dismantling process, stopping construction immediately when necessary, and feeding back information in time to ensure safe construction of steel support dismantling.

(IV) construction of main body structure

The embodiment is four layers of frame construction in underground, wherein two-layer is the open cut and is made the way in the same direction as the way, and two-layer is the construction of lid excavation reverse construction method down, when excavating to two layer negative bottom plate, execute and do the bottom plate structure, from bottom to top in proper order do two layer negative structure side wall, one layer negative middle plate, one layer negative side wall, roof and play ground structure, roof backfill sclerosis after intensity reaches design strength, then the lid excavation method is from reserving out the excavation of soil hole position, construction three layer negative middle plate, bottom plate, four layer negative side wall, three layer negative side wall, shutoff soil outlet hole.

The main structure construction process is shown in figure 5,

when the foundation pit is excavated, the clearance size in the diaphragm wall is checked at any time, and the bulge concrete of the diaphragm wall is chiseled off in time so as to ensure the section size of the lining wall in the main structure. When the foundation pit is excavated to the designed elevation 1.8m away from the bottom of the middle slab of the negative second floor, manual matching machinery is used for clearing and leveling the bottom, and a concrete cushion layer with the thickness of 20cm is poured. And (3) erecting a short (about 1.6m high) scaffold, laying a template, binding steel bars and pouring concrete, and ensuring the flatness of the plate.

1, constructing a cushion layer, pouring a first concrete support, reinforcing the inner groove wall of a construction pit, reinforcing a water stop pile outside the pit, an underground continuous wall, a top ring beam, reinforcing a foundation, constructing a stand column pile (a hollow pile is partially backfilled and compacted), a lattice column and a precipitation well, excavating a soil body to the bottom of the 1 st support, and pouring the 1 st concrete support.

2, excavating earth to erect 2 and 3 steel supports, excavating earth to the bottom of the 2 nd support in sequence, and erecting the 2 nd steel support; and excavating the soil body to the bottom of the 3 rd support, and erecting the 3 rd steel support.

And 3, excavating the earth to erect 4 steel supports, excavating the earth to the bottom of the 4 th support in sequence, and erecting the 4 th steel support.

And 4, constructing a negative second-layer bottom plate, excavating soil bodies to the bottom of the 5 th-layer earthwork in sequence, and performing open excavation to construct a negative second-layer cushion layer and a bottom plate.

And 5, constructing the negative second-layer side wall, the column and the middle plate, dismantling the 3 rd and 4 th steel supports after the negative second-layer bottom plate reaches the design strength, and performing open cut to the negative second-layer side wall, the column and the middle plate (performing open cut to the middle plate).

And 6, constructing a first-layer side wall, a first-layer column and a first-layer top plate, dismantling the 2 nd steel support after the first-layer side wall, the first-layer column and the second-layer middle plate reach the design strength, and constructing the first-layer side wall, the first-layer column and the first-layer top plate (manufactured by open excavation).

7, backfilling the top plate, cutting the 1 st concrete support after the top plate reaches the design strength, constructing a soil outlet retaining wall, backfilling the top plate, and (making in the same direction as open cut).

And 8, excavating the earth to construct a 5 th concrete support, after the top plate is backfilled, continuing to lower the excavator from the bottom plate of the negative second layer, excavating a 6 th soil body, and constructing the 5 th concrete support (cover and dig in the same direction).

And 9, excavating a negative three-layer bottom plate for earthwork excavation construction, excavating a 7 th layer soil body after the 5 th concrete support reaches the design strength, and constructing the negative three-layer bottom plate (cover-excavation-forward-doing).

And (5) excavating 10 earthwork to construct a 6 th concrete support, excavating 8 th soil body after the negative three-layer bottom plate reaches the design strength, and constructing the 6 th concrete support (cover-excavation-in-line).

And (4) excavating a negative four-layer bottom plate in the earthwork excavation construction, after the 6 th concrete support reaches the design strength, continuously excavating a 9 th earthwork layer, and constructing a grounding net, a plain concrete cushion layer, a waterproof layer and the negative four-layer bottom plate (cover excavation).

And (4) constructing the side wall with the bottom half of the negative four layers by 12, and constructing the side wall with the bottom half of the negative four layers (cover-digging-down manufacturing) after the bottom plate with the bottom four layers reaches the design strength.

And 13, cutting the 6 th concrete support, constructing the upper half side wall and the column of the negative fourth layer, cutting the 6 th concrete support after the lower half side wall of the negative fourth layer reaches the design strength, and constructing the upper half side wall and the column of the negative fourth layer (cover-and-dig-in).

And (4) constructing the side walls of the lower half part of the negative three layers, and constructing the side walls of the lower half part of the negative three layers (covering and excavating).

And 15, cutting the 5 th concrete support, constructing the upper half side wall and the column of the negative three-layer, and cutting the 5 th concrete support (manufactured by covering and excavating) after the lower half side wall of the negative three-layer reaches the design strength.

16 station tables and rail top air channels, and after the negative three-layer structure reaches the design strength, the station tables and the rail top air channels are constructed.

17 hole sealing, roof waterproofing, backfilling, roof-minus three-layer floor soil outlet hole sealing, roof construction additional waterproof layer, residual earthing and pipeline relocation, and road surface restoration.

And (5) after the foundation pit is excavated to the designed elevation, immediately carrying out basement inspection and grounding grid construction, and carrying out bottom sealing construction after the grounding grid operation is inspected and accepted. When the foundation pit is excavated, the clearance size in the diaphragm wall is detected at any time, and the collapsed hole concrete of the diaphragm wall is chiseled off in time so as to ensure the section size of the lining wall in the main structure. And (4) manually clearing the bottom after the excavation is finished, and timely constructing the cushion layer after the grids are collected empirically.

The construction of the bottom plate structure needs to be poured in sections, and a facility seam is not suitable between the two sections of the bottom plates; if the sections are longer, a post-cast strip can be considered to be arranged; meanwhile, the mixing proportion of the concrete is scientifically adjusted, and the multifunctional intumescent waterproof agent and the fly ash are added so as to compensate the temperature difference shrinkage of the concrete caused by hydration heat and effectively reduce the hydration heat of the concrete; and an effective pouring and curing method is used for assisting, so that the effect of controlling the temperature difference shrinkage cracks of the concrete is achieved.

The concrete pouring is carried out by adopting a mode of commercial concrete supply, concrete pump conveying and manual vibration of an inserted vibrator, and a construction joint is arranged between the two sections of bottom plates. According to the requirement of pouring speed, 1 concrete conveying pump truck is arranged for each pouring, and the pouring adopts a continuous pouring method of 'one-slope, thin-layer pouring, sequential propelling and one-time top reaching' along the longitudinal direction.

Two vibrators are arranged before and after each pouring. The first way is arranged at a concrete discharging point and mainly solves the tamping of the upper concrete; the second way is arranged at the position of a concrete toe to ensure the tamping of the concrete at the lower part. The concrete pouring is pushed forward, and the tamping is followed up, so that the quality of the whole concrete body is ensured.

The cement slurry on the surface of the pumped concrete is thick, and the pumped concrete needs to be carefully treated after the concrete pouring is finished. And scraping the materials for about 4-5 h by using a long ruler according to the elevation, carrying out the processes of 'slurry lifting, compacting and finishing' before final setting, and carrying out secondary slurry collection and finishing to close the water collection cracks. And covering and fully watering for moistening and curing after about 12-14 h. And finishing excavation of the foundation pit of the four-layer subway station.

While the present invention has been described in detail with reference to the drawings and the embodiments, those skilled in the art will understand that various specific parameters in the above embodiments can be changed without departing from the spirit of the present invention, and a plurality of specific embodiments are formed, which are common variation ranges of the present invention, and will not be described in detail herein.

Claims

1. A four-layer subway station foundation pit excavation method is characterized by comprising the following steps:

step 1: manufacturing a crown beam and a first concrete support;

and 5: the main structure is constructed by two layers of open excavation and two layers of cover excavation, firstly, the open excavation method is used for excavating to a negative two-layer bottom plate and applying the bottom plate structure, after the strength reaches the design strength, the negative two-layer structure side wall, a negative one-layer middle plate, a negative one-layer side wall, a top plate and a ground outlet structure are sequentially applied from bottom to top, the top plate is backfilled and hardened, then, the cover excavation method is used for excavating from the position of a reserved soil outlet hole, and a negative three-layer middle plate, a bottom plate, a negative four-layer side wall, a negative three-layer.

2. The excavation method of the foundation pit of the four-layer subway station as claimed in claim 1, wherein in step 1, the top beam and the first concrete support are supported by bamboo glue formwork, reinforcing steel bars are bound on site, commercial concrete is transported to site for pouring, and then a tamping tool is inserted for tamping.

3. The excavation method for the foundation pit of the four-layer subway station as claimed in claim 2, wherein said manufacturing of the crown beam and the first concrete support comprises:

(1) excavating and protecting a side slope;

(2) pile head breaking, continuous wall surface cleaning and cushion layer pouring;

(3) binding supporting steel bars;

(4) installing a template;

(5) and (5) pouring concrete.

4. The excavation method for the foundation pit of the four-layer subway station as claimed in claim 1, wherein in step 2, the dewatering well is constructed by using a vehicle-mounted reverse circulation drilling machine, steel pipes and filter pipes are used as well pipes of the dewatering well, and gaps between the well pipes and the well wall are filled with round or sub-round sand and pebbles.

5. The excavation method for the foundation pit of the four-layer subway station as claimed in claim 4, wherein said dewatering well is manufactured by:

(1) logging well positions of the well;

(2) burying a mouth protecting pipe;

(3) installing a drilling machine and drilling a hole;

(4) cleaning holes and replacing pulp;

(5) lowering a well pipe;

(6) burying and filling the filter material;

(7) and (5) washing the well.

6. The excavation method for the foundation pit of the four-layer subway station as claimed in claim 1, wherein in step 3, the construction of the open cut section structure adopts longitudinal segmentation, and the longitudinal segmentation is carried out vertically and sequentially layer by layer from bottom to top; the underground excavation section adopts longitudinal segmentation, and a vertical layered and layer-by-layer structure is reversely manufactured from top to bottom.

7. The excavation method for the foundation pit of the four-layer subway station as claimed in claim 1, wherein the earthwork excavated in the first to fourth layers in step 3 is directly transported to a spoil area, and the fifth layer earthwork is first turned up to the first layer and then excavated by a ground excavator.

8. The excavation method of the foundation pit of the four-layer subway station as claimed in claim 1, wherein the mechanical operation is stopped when the fifth to sixth layers of earthwork are excavated to the designed elevation and the remaining 30cm is left, the excavation is performed manually to the designed elevation, then a concrete cushion layer is poured, an isolation layer is laid, a scaffold is erected, a template is installed, middle plate steel bars are bound, and concrete is poured.

9. The excavation method for the foundation pit of the four-layer subway station as claimed in claim 1, wherein in step 4, the steel supports are made in sections, the pipe sections are connected through flange high-strength bolts, and one end of each support end is provided with a flexible head.

10. The excavation method of the foundation pit of the four-layer subway station as claimed in claim 9, wherein the steel supports are pre-assembled on the ground according to the actually measured width of the foundation pit, and after being assembled, whether the steel supports are qualified is checked, and then the steel supports which are qualified are numbered.