阅读说明：本技术 用于浅覆盖层硬基岩深水深基坑组合式围堰的施工方法 (Construction method of combined cofferdam for shallow overburden hard bedrock deep water deep foundation pit ) 是由 廖云沼 郑震 王杰 陈杨 容敏 蓝巧新 张峰 邓晓峰 于 2021-09-29 设计创作，主要内容包括：本发明公开了用于浅覆盖层硬基岩深水深基坑组合式围堰的施工方法,包括如下步骤：1)施工前的准备；2)围堰施工；3)止水层施工；4)内撑架组件吊装下放前的准备；5)内撑架组件下放施工；6)第一道内撑架施工；7)第二道内撑架施工；8)第三道内撑架施工；9)第四道内撑架施工。通过采用钢管桩和钢板桩组合而成的组合桩,先将组合桩和钢板桩插打到岩层面固定,再将钢管桩内钻孔浇筑混凝土形成锚固桩,在钢管桩与锚固桩的交界处制作止水层,下放内撑架固定的施工方案,实现了快速施工、成孔,减少钢板桩的入岩深度,钢管桩内成孔泥浆不外流,有效保护河流水体环境的效果。(The invention discloses a construction method of a combined cofferdam for a deep foundation pit of a shallow overburden hard bedrock, which comprises the following steps: 1) preparation before construction; 2) constructing a cofferdam; 3) constructing a water stop layer; 4) preparing the inner support frame assembly before hoisting and lowering; 5) lowering the inner support frame assembly for construction; 6) constructing a first inner support frame; 7) constructing a second inner support frame; 8) constructing a third inner support frame; 9) and constructing the fourth inner support frame. Through the composite pile that adopts steel-pipe pile and steel sheet pile combination to form, it is fixed to insert to beat the rock face with composite pile and steel sheet pile earlier, and it forms the anchor pile to pour concrete with the drilling in the steel-pipe pile again, makes the stagnant water layer in the juncture of steel-pipe pile and anchor pile, transfers the fixed construction scheme of inner strut, has realized quick construction, pore-forming, reduces the income rock depth degree of steel sheet pile, and pore-forming mud does not flow outward in the steel-pipe pile, effectively protects river water body environment's effect.)

1. The construction method of the combined cofferdam for the deep water and deep foundation pit of the shallow overburden hard bedrock is characterized by comprising the following steps:

1) preparation before construction:

removing part of the original construction platform, reserving a plugging space of a cofferdam pile foundation, processing and fixing the steel pipe piles and the steel sheet piles to form combined piles, and conveying the combined piles and the steel sheet piles to a construction site;

2) cofferdam construction:

sequentially inserting and driving the combined piles and the steel sheet piles clockwise or anticlockwise from one end of an inserting and driving space on the downstream side of the construction platform, sequentially interlocking the steel sheet piles, drilling anchoring holes to designed positions from rock strata downwards in the steel pipe piles which are inserted and driven, removing drilling slag, placing a steel reinforcement cage downwards in the anchoring holes, pumping water to the bottoms of the anchoring holes, pouring concrete in the anchoring holes to form anchoring piles, finally completing the closure of the combined piles and the anchoring piles in all the steel pipe piles to form a cofferdam, and surrounding the steel pipe piles by the steel sheet piles after the closure;

3) and (3) water stopping layer construction:

drilling a grouting hole along the edge of the steel sheet pile in the direction of the steel sheet pile far away from the cofferdam, inserting a grouting pipe along the grouting hole for pressure grouting, firstly, shallow-hole drilling and then deep-hole drilling to form a water stop layer at the junction of the combined pile and the anchoring pile, and draining accumulated water in the cofferdam after the soil body of the pressure grouting reaches the design strength;

4) preparation before hoisting and lowering of the inner support frame assembly:

dismantling and adjusting part of an original construction platform, heightening and arranging a pad beam on the construction platform, mounting a hoisting device on the pad beam, processing and fixing an inner support pipe and a ring beam to form an inner support frame, sequentially assembling four inner support frames to form an inner support frame assembly, sequentially arranging a first inner support frame, a second inner support frame, a third inner support frame and a fourth inner support frame from top to bottom, hoisting and connecting the hoisting device with the inner support frame assembly, integrally hoisting and separating the inner support frame assembly from the construction platform, and checking the descending safety and reliability;

5) the lowering construction of the inner support frame assembly:

pumping water in the cofferdam to a position below the designed position of the first inner support frame, and lowering the inner support frame assembly into the cofferdam by the hoisting device to stop when the first inner support frame is positioned at the designed position;

6) constructing a first inner support frame:

welding a bracket structure at the design position of the first inner support frame, pouring and filling concrete in gaps among the bracket structure, the steel pipe piles and the ring beam, and welding and fixing the first inner support frame and each steel pipe pile by utilizing profile steel;

7) and (3) second internal support frame construction:

pumping water in the cofferdam to a position below the design position of the second inner support frame, continuously lowering the other three inner support frames to the design position of the second inner support frame, welding a bracket structure at the design position of the second inner support frame, pouring and filling concrete in gaps among the bracket structure, the steel pipe piles and the ring beam, fixedly welding the second inner support frame and each steel pipe pile by utilizing profile steel, and installing a vertical connecting piece between the first inner support frame and the second inner support frame;

8) and (3) constructing the third inner support frame:

pumping water in the cofferdam to a position below the design position of a third inner support frame, continuously lowering the other two inner support frames to the design position of the third inner support frame, welding a bracket structure at the design position of the third inner support frame, pouring and filling concrete in gaps among the bracket structure, the steel pipe piles and the ring beam, fixedly welding the third inner support frame and each steel pipe pile by utilizing profile steel, and installing a vertical connecting piece between the second inner support frame and the third inner support frame;

9) and fourth construction of the inner support frame:

pumping water in the cofferdam to a position below the design position of the fourth inner support frame, transferring the fourth inner support frame to the designed position, welding a bracket structure at the designed position of the fourth inner support frame, pouring and filling concrete at the gap between the bracket structure and the steel pipe pile and between the bracket and the ring beam, welding and fixing the fourth inner support frame and each steel pipe pile by utilizing profile steel, installing a vertical connecting piece between the third inner support frame and the fourth inner support frame, and finishing cofferdam construction.

2. The construction method of the combined cofferdam for the shallow overburden hard bedrock deep water foundation pit as claimed in claim 1, wherein: in the step 1), the specification model of the steel sheet pile is Larsen IVw, and the steel pipe pile is fixedly connected with the steel sheet pile through a fixing piece, so that part of the outer wall of the steel pipe pile is tangent to one side of the steel sheet pile.

3. The construction method of the combined cofferdam for the shallow overburden hard bedrock deep water foundation pit as claimed in claim 1, wherein: in the step 2), when the combined pile and the steel sheet pile are inserted and driven, the accurate inserting and driving position of the combined pile is ensured by using auxiliary positioning measures, the verticality of the combined pile is ensured to be less than 1%, and the deviation range is-3 cm to 3 cm.

4. The construction method of the combined cofferdam for the shallow overburden hard bedrock deep water foundation pit as claimed in claim 1, wherein: and 2) inserting and driving the combined pile and the steel sheet pile into the rock stratum until the combined pile and the steel sheet pile cannot go deep into the rock stratum.

5. The construction method of the combined cofferdam for the shallow overburden hard bedrock deep water foundation pit as claimed in claim 1, wherein: in the step 2), the height of the superposition from the top of the anchoring pile to the bottom of the steel pipe pile is more than 2 m.

6. The construction method of the combined cofferdam for the shallow overburden hard bedrock deep water foundation pit as claimed in claim 1, wherein: in the step 3), the height of the water stopping layer is 3m, the width of the water stopping layer is 1.5m, and the overlapping height from the top of the water stopping layer to the bottom of the steel pipe pile is 1 m.

7. The construction method of the combined cofferdam for the shallow overburden hard bedrock deep water foundation pit as claimed in claim 1, wherein: and 3), synchronously grouting and sealing the groove between the steel sheet pile and the steel pipe pile.

8. The construction method of the combined cofferdam for the shallow overburden hard bedrock deep water foundation pit as claimed in claim 1, wherein: and 4), integrally lifting the inner support frame assembly to be separated from the construction platform by more than 10cm, and standing for more than 30 minutes to check the descending safety and reliability.

9. The construction method of the combined cofferdam for the shallow overburden hard bedrock deep water foundation pit as claimed in claim 1, wherein: and 5), in the process of lowering the inner support frame assembly by the hoisting device, the vertical displacement deviation of the inner support frame assembly is less than 5 mm.

10. The construction method of the combined cofferdam for the shallow overburden hard bedrock deep water foundation pit as claimed in claim 1, wherein: in the step 7), the step 8) and the step 9), all the section steels adopted for welding and fixing the inner support frames and the steel pipe piles are I-shaped steels 56a, and the top surfaces and the bottom surfaces of the I-shaped steels are fixedly connected with steel plates with the thickness of 20 mm.

Technical Field

The invention relates to the technical field of bridge engineering construction, in particular to a construction method of a combined cofferdam for a deep foundation pit of a hard bedrock in a shallow covering layer.

Background

With the gradual increase of domestic bridge engineering, in the prior art, when the foundation construction of low pile cap, deep water, shallow covering layer and hard rock stratum is handled, the cofferdams are constructed in the conventional forms of box-jacketed cofferdams, steel sheet piles and the like, and are difficult to directly insert and strike to the designed depth. The cofferdam construction method in the prior art has the defects of low basement rock grooving efficiency, low construction speed, high cofferdam water stopping cost, weak cofferdam rigidity and the like.

Disclosure of Invention

The invention aims to at least solve one technical problem in the prior art, and therefore, the invention provides a construction method for a combined cofferdam of a deep foundation pit in deep water of a hard bedrock of a shallow covering layer, which comprises the following steps:

1) preparation before construction:

removing part of the original construction platform, reserving a plugging space of a cofferdam pile foundation, processing and fixing the steel pipe piles and the steel sheet piles to form combined piles, and conveying the combined piles and the steel sheet piles to a construction site;

2) cofferdam construction:

sequentially inserting and driving the combined piles and the steel sheet piles clockwise or anticlockwise from one end of an inserting and driving space on the downstream side of the construction platform, sequentially interlocking the steel sheet piles, drilling anchoring holes to designed positions from rock strata downwards in the steel pipe piles which are inserted and driven, removing drilling slag, placing a steel reinforcement cage downwards in the anchoring holes, pumping water to the bottoms of the anchoring holes, pouring concrete in the anchoring holes to form anchoring piles, finally completing the closure of the combined piles and the anchoring piles in all the steel pipe piles to form a cofferdam, and surrounding the steel pipe piles by the steel sheet piles after the closure;

3) and (3) water stopping layer construction:

drilling a grouting hole along the edge of the steel sheet pile in the direction of the steel sheet pile far away from the cofferdam, inserting a grouting pipe along the grouting hole for pressure grouting, firstly, shallow-hole drilling and then deep-hole drilling to form a water stop layer at the junction of the combined pile and the anchoring pile, and draining accumulated water in the cofferdam after the soil body of the pressure grouting reaches the design strength;

4) preparation before hoisting and lowering of the inner support frame assembly:

dismantling and adjusting part of an original construction platform, heightening and arranging a pad beam on the construction platform, mounting a hoisting device on the pad beam, processing and fixing an inner support pipe and a ring beam to form an inner support frame, sequentially assembling four inner support frames to form an inner support frame assembly, sequentially arranging a first inner support frame, a second inner support frame, a third inner support frame and a fourth inner support frame from top to bottom, hoisting and connecting the hoisting device with the inner support frame assembly, integrally hoisting and separating the inner support frame assembly from the construction platform, and checking the descending safety and reliability;

5) the lowering construction of the inner support frame assembly:

pumping water in the cofferdam to a position below the designed position of the first inner support frame, and lowering the inner support frame assembly into the cofferdam by the hoisting device to stop when the first inner support frame is positioned at the designed position;

6) constructing a first inner support frame:

welding a bracket structure at the design position of the first inner support frame, pouring and filling concrete in gaps among the bracket structure, the steel pipe piles and the ring beam, and welding and fixing the first inner support frame and each steel pipe pile by utilizing profile steel;

7) and (3) second internal support frame construction:

pumping water in the cofferdam to a position below the design position of the second inner support frame, continuously lowering the other three inner support frames to the design position of the second inner support frame, welding a bracket structure at the design position of the second inner support frame, pouring and filling concrete in gaps among the bracket structure, the steel pipe piles and the ring beam, fixedly welding the second inner support frame and each steel pipe pile by utilizing profile steel, and installing a vertical connecting piece between the first inner support frame and the second inner support frame;

8) and (3) constructing the third inner support frame:

pumping water in the cofferdam to a position below the design position of a third inner support frame, continuously lowering the other two inner support frames to the design position of the third inner support frame, welding a bracket structure at the design position of the third inner support frame, pouring and filling concrete in gaps among the bracket structure, the steel pipe piles and the ring beam, fixedly welding the third inner support frame and each steel pipe pile by utilizing profile steel, and installing a vertical connecting piece between the second inner support frame and the third inner support frame;

9) and fourth construction of the inner support frame:

pumping water in the cofferdam to a position below the design position of the fourth inner support frame, transferring the fourth inner support frame to the designed position, welding a bracket structure at the designed position of the fourth inner support frame, pouring and filling concrete at the gap between the bracket structure and the steel pipe pile and between the bracket and the ring beam, welding and fixing the fourth inner support frame and each steel pipe pile by utilizing profile steel, installing a vertical connecting piece between the third inner support frame and the fourth inner support frame, and finishing cofferdam construction.

The construction method has the beneficial effects that the combined pile formed by combining the steel pipe pile and the steel sheet pile is adopted, the combined pile and the steel sheet pile are firstly inserted and driven to the rock surface for fixation, then the hole in the steel pipe pile is drilled and the concrete is poured to form the anchoring pile, the water stop layer is manufactured at the junction of the steel pipe pile and the anchoring pile, and the inner support frame is put down for fixation, so that the construction method realizes the construction and pore forming, reduces the rock-entering depth of the steel sheet pile, prevents pore-forming slurry in the steel pipe pile from flowing out, and effectively protects the river water environment.

Preferably, in the step 1), the specification model of the steel sheet pile is larsen ivw, and the steel pipe pile is fixedly connected with the steel sheet pile through a fixing piece, so that part of the outer wall of the steel pipe pile is tangent to one side of the steel sheet pile.

Preferably, in the step 2), when the combined pile and the steel sheet pile are inserted and driven, the accurate insertion and driving position of the combined pile is ensured by using auxiliary positioning measures, the verticality of the combined pile is ensured to be less than 1%, and the deviation range is-3 cm to 3 cm.

Preferably, in the step 2), when the combined pile and the steel sheet pile are inserted and driven, the combined pile and the steel sheet pile are inserted and driven into the rock stratum until the combined pile and the steel sheet pile cannot go deep into the rock stratum.

Preferably, in the step 2), the height of the superposition from the top of the anchoring pile to the bottom of the steel pipe pile is more than 2 m.

Preferably, in step 3), the height of the water stopping layer is 3m, the width of the water stopping layer is 1.5m, and the height from the top of the water stopping layer to the bottom of the steel pipe pile is 1 m.

Preferably, in step 3), the groove between the steel sheet pile and the steel pipe pile is also synchronously grouted and closed.

Preferably, in step 4), the height of the inner support frame assembly integrally lifted and separated from the construction platform is more than 10cm, and then the inner support frame assembly is stood for more than 30 minutes to check the descending safety and reliability.

Preferably, in the step 5), in the process that the inner support frame assembly is lowered down by the hoisting device, the vertical displacement deviation of the inner support frame assembly is less than 5 mm.

Preferably, in the step 7), the step 8) and the step 9), all the section steels adopted for welding and fixing the inner brackets and the steel pipe piles are i-shaped steels 56a, and the top surfaces and the bottom surfaces of the i-shaped steels are fixedly connected with steel plates with the thickness of 20 mm.

Drawings

The invention is further described in the following with reference to the accompanying drawings, it is obvious that the drawings in the following description are some embodiments of the invention, and that for a person skilled in the art, other drawings can be derived from these drawings without inventive effort.

FIG. 1 is a schematic diagram of preparation and insertion driving of a combined pile and a steel sheet pile before cofferdam construction according to an embodiment of the invention;

FIG. 2 is a top view of FIG. 1;

FIG. 3 is a schematic illustration of cofferdam construction according to an embodiment of the present invention;

FIG. 4 is a top view of FIG. 3;

FIG. 5 is a schematic diagram of the construction of a water stop layer according to an embodiment of the invention;

FIG. 6 is a schematic top view of a water stop layer according to an embodiment of the invention;

FIG. 7 is a schematic illustration of preparation for the inner brace assembly prior to hoisting and lowering in accordance with an embodiment of the present invention;

FIG. 8 is a schematic top view of an embodiment of the present invention in preparation for hoisting and lowering of the inner brace assembly;

FIG. 9 is a schematic illustration of the lowering of the inner brace assembly and the first pass of the inner brace construction according to an embodiment of the invention;

FIG. 10 is a schematic view of the second, third and fourth inner brace construction according to the embodiment of the present invention;

FIG. 11 is a schematic structural diagram of a composite pile according to an embodiment of the present invention;

FIG. 12 is a view from direction H of FIG. 11;

FIG. 13 is a schematic illustration of preparation of the bearing platform prior to construction according to an embodiment of the invention;

FIG. 14 is a schematic illustration of a first section of a bearing platform according to an embodiment of the present invention;

FIG. 15 is a schematic illustration of a first conversion of the cofferdam and a second construction of the cap in accordance with an embodiment of the present invention;

fig. 16 is a schematic diagram of a second system conversion of the cofferdam of the embodiment of the present invention.

In the figure: the construction method comprises the following steps of a construction platform 100, a pile casing 110, a piling space 120, a combined pile 200, a steel pipe pile 210, a steel plate pile 220, a connecting part 221, a bending part 222, a rabbet part 223, a fixing part 230, a groove 240, a cofferdam 300, a rock stratum 310, an anchoring hole 320, an anchoring pile 330, a water stop layer 400, an inner support frame assembly 500, a first inner support frame 510, a second inner support frame 520, a third inner support frame 530, a fourth inner support frame 540, an inner support frame 550, an inner support pipe 551, a ring beam 552, a pad beam 560, a hoisting device 570, a bracket structure 610, a gap 620, a vertical connecting part 630, an anchoring pile top part 710, a steel pipe pile bottom part 720, a water stop layer top part 730, a first pile cap 810, a second pile cap 820, the periphery 830 of the first pile cap, the periphery 840 of the second pile cap, the periphery 850, and a pile base 860.

Detailed Description

The following detailed description of the present invention is given for the purpose of better understanding technical solutions of the present invention by those skilled in the art, and the present description is only exemplary and explanatory and should not be construed as limiting the scope of the present invention in any way.

It will be understood that the terms "upper", "lower", "left", "right", "front", "back", "bottom", "top", "inner", "outer", and the like, as used herein, refer to orientations or positional relationships illustrated in the drawings, which are used for convenience in describing the present invention and to simplify the description, and do not indicate or imply that the referenced device or element must have a particular orientation, be constructed and operated in a particular orientation, and thus, are not to be considered limiting of the present invention. Herein, the terms "first", "second", and the like are used for distinguishing different objects, not for describing a particular order.

It should be noted that, in this document, the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. In the description of the present invention, unless otherwise explicitly limited, terms such as arrangement, installation, connection and the like should be understood in a broad sense, and those skilled in the art can reasonably determine the specific meanings of the above terms in the present invention in combination with the specific contents of the technical solutions.

Reference will now be made in detail to the present preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout.

Referring to fig. 1 to 16, the construction method of the combined cofferdam for the deep foundation pit of the hard bedrock in the shallow overburden layer in the embodiment of the invention comprises the following steps:

continuing to refer to fig. 1 and 2, 1) preparation prior to construction:

and (3) dismantling part of the original construction platform 100, reserving a driving space 120 of the cofferdam pile foundation, processing and fixing the steel pipe pile 210 and the steel sheet pile 220 to form the composite pile 200, and conveying the composite pile 200 and the steel sheet pile 220 to a construction site. With continued reference to fig. 11 and 12, preferably, the specification model of the steel sheet pile 220 is larsen ivw, the steel pipe pile 210 is fixedly connected with the steel sheet pile 220 through a fixing member 230, so that part of the outer wall of the steel pipe pile 210 is tangent to one side of the steel sheet pile 220, further, the steel sheet pile 220 has a connecting portion 221, two ends of the connecting portion 221 are respectively and symmetrically connected with a bending portion 222, the other sides of the two bending portions 222 are connected with rabbet portions 223, the steel sheet piles 220 are occluded through the rabbet portions 223, the connecting portion 221 is tangent to the outer wall of the steel pipe pile 210, the steel pipe pile 210 is fixedly connected with the steel sheet pile 220 through the fixing member 230, preferably, one end of the fixing member 230 is welded and fixed to the outer wall of the steel pipe pile 210, the other end of the fixing member 230 is welded and fixed to the bending portion 222 of the steel sheet pile 220, and the fixing member 230 forms a groove 240 between the steel sheet pile 220 and the steel pipe pile 210. The steel sheet pile 220 and the steel pipe pile 210 are connected through the fixing member 230 to form the combined pile 200, so that the rigidity and strength of the combined pile 200 are increased, and the combined pile 200 is suitable for various piling devices. Preferably, a guide device is installed before the inserting and beating to guide the inserting and beating construction, so that deviation is avoided.

Continuing to refer to fig. 1-4, 2) cofferdam 300 construction:

inserting and driving the combined pile 200 and the steel sheet pile 220 in turn clockwise or anticlockwise from one end of the inserting and driving space 120 at the downstream side of the construction platform 100, sequentially interlocking the steel sheet piles 220, drilling an anchoring hole 320 to a designed position towards a rock stratum 310 in each inserted and driven steel pipe pile 210, removing drilling slag, lowering a steel reinforcement cage in the anchoring hole 320, pumping water to the bottom of the anchoring hole 320, pouring concrete in the anchoring hole 320 to form an anchoring pile 330, finally completing the closing of the combined pile 200 and the anchoring piles 330 in all the steel pipe piles 210 to form a cofferdam 300, and surrounding the steel pipe piles 210 by the steel sheet piles 220 after closing. Preferably, when the combined pile 200 and the steel sheet pile 220 are inserted and driven, the auxiliary positioning measures are utilized to ensure that the insertion and driving position of the combined pile 200 is accurate, the verticality of the combined pile 200 is less than 1%, and the deviation range is-3 cm to 3 cm. Preferably, the insertion beating equipment adopts a crawler crane matched with a vibration hammer. Further, when the combined pile 200 and the steel sheet pile 220 are driven by insertion, the combined pile is driven by insertion into the rock layer 310 until the combined pile cannot be driven further into the rock layer 310. Preferably, the height of the superposition of the anchoring pile top 710 to the steel pipe pile bottom 720 is more than 2 m. Preferably, the device for drilling the anchoring hole 320 is a rotary drill. Further preferably, after the insertion driving of the composite pile 200 and the steel sheet pile 220 on one side is completed, the drilling of the anchoring hole 320 is also performed simultaneously when the insertion driving of the other side is performed, thereby reducing the construction time.

Continuing with fig. 5 and 6, 3) constructing the water stopping layer 400:

drilling grouting holes along the edges of the steel sheet piles 220 in the direction that the steel sheet piles 220 are far away from the cofferdam 300, inserting grouting pipes along the grouting holes for pressure grouting, firstly, shallow-hole drilling and then deep-hole drilling so as to form a water stopping layer 400 at the junction of the combined pile 200 and the anchor piles 330, and after the soil body of the pressure grouting reaches the design strength, draining accumulated water in the cofferdam 300. Preferably, the height of the water stop layer 400 is 3m, the width is 1.5m, and the height from the top 730 of the water stop layer to the superposition of the bottom 720 of the steel pipe pile is 1 m. Furthermore, the slurry for pressure grouting is formed by mixing cement and water glass. Further, the groove 240 between the steel sheet pile 220 and the steel pipe pile 210 is also synchronously grouted and sealed, and after grouting is finished, the soil body on the outer side of the cofferdam 300 reaches certain strength, and then water is pumped into the cofferdam 300.

Continuing with reference to fig. 7 and 8, 4) preparation of the inner brace assembly 500 prior to hoisting and lowering:

dismantling and adjusting part of the original construction platform 100, heightening the construction platform 100 to form a pad beam 560, installing a hoisting device 570 on the pad beam 560, processing and fixing an inner support tube 551 and a ring beam 552 to form an inner support frame 550, sequentially assembling four inner support frames 550 to form an inner support frame assembly 500, sequentially lifting the whole inner support frame assembly 500 to be separated from the construction platform 100 by more than 10cm from top to bottom, standing for more than 30 minutes, and checking the descending safety and reliability. Preferably, the lifting device 570 is a steel strand lifting jack.

Continuing to refer to fig. 9, 5) the inner support frame assembly 500 is lowered for construction:

the cofferdam 300 is pumped to a position below the design position of the first inner bracket 510, preferably, the position below the design position of the first inner bracket 510 is pumped to 100 cm. Hoist 570 lowers inner brace assembly 500 into cofferdam 300 to stop the first inner brace 510 when it is in the designed position. Further, in the process that the hoisting device 570 lowers the inner support frame assembly 500, the vertical displacement deviation of the inner support frame assembly 500 is smaller than 5 mm.

6) The first inner support frame 510 construction:

welding a bracket structure 610 at the design position of the first inner bracket 510, pouring and filling concrete in a gap 620 between the bracket structure 610 and the steel pipe piles 210 and between the bracket structure 610 and the ring beam 552, and welding and fixing the first inner bracket 510 and each steel pipe pile 210 by using profile steel;

with continued reference to fig. 10, 7) second inner bracing frame 520 construction:

and pumping water in the cofferdam 300 to be below the design position of the second inner bracket 520, preferably, pumping water to be 100cm below the design position of the second inner bracket 520. Continuously lowering the other three inner brackets to the designed position of the second inner bracket 520, welding a bracket structure 610 at the designed position of the second inner bracket 520, pouring and filling concrete in a gap 620 between the bracket structure 610 and the steel pipe piles 210 and between the bracket structure 610 and the ring beam 552, welding and fixing the second inner bracket 520 and each steel pipe pile 210 by using profile steel, and installing a vertical connecting piece 630 between the first inner bracket 510 and the second inner bracket 520;

8) and (3) constructing a third inner support frame 530:

the water is pumped in the cofferdam 300 to the position below the design position of the third inner strut 530, and preferably, the water is pumped to the position 100cm below the design position of the third inner strut 530. Continuously lowering the other two inner brackets to the designed position of the third inner bracket 530, welding a bracket structure 610 at the designed position of the third inner bracket 530, pouring and filling concrete in a gap 620 between the bracket structure 610 and the steel pipe piles 210 and between the bracket structure 610 and the ring beam 552, welding and fixing the third inner bracket 530 and each steel pipe pile 210 by using profile steel, and installing a vertical connecting piece 630 between the second inner bracket 520 and the third inner bracket 530;

9) and (3) fourth construction of the inner support frame 540:

and pumping water in the cofferdam 300 and excavating to the position below the design position of the fourth inner supporting frame 540, preferably, pumping water and excavating to the position 100cm below the design position of the fourth inner supporting frame 540. And (3) lowering a fourth inner support frame 540 to a designed position, welding a bracket structure 610 at the designed position of the fourth inner support frame 540, pouring and filling concrete in a gap 620 between the bracket structure 610 and the steel pipe piles 210 and between the bracket structure 610 and the ring beam 552, welding and fixing the fourth inner support frame 540 and each steel pipe pile 210 by using profile steel, installing a vertical connecting piece 630 between the third inner support frame 530 and the fourth inner support frame 540, and finishing the construction of the cofferdam 300. Preferably, the section steel adopted for welding and fixing each inner support frame and the steel pipe pile 210 is i-steel 56a, and the top surface and the bottom surface of the i-steel are both fixedly connected with steel plates with the thickness of 20 mm.

In other embodiments of the present invention, according to the cofferdam of the embodiment of the present invention, the pier foundation bearing platform is constructed in the cofferdam 300 to complete the system conversion of the cofferdam 300, and the construction steps are as follows:

a) excavating mudstone in the cofferdam 300:

a water collection well is provided in the cofferdam 300 to perform drainage and excavation to a designed position.

Continuing with FIG. 13, b) preparation of the platform prior to construction:

and (3) draining accumulated water in the cofferdam 300, removing the hoisting device 570, removing the original pile casing 110 and pile head in the cofferdam 300, and pouring a concrete cushion 860.

With continued reference to fig. 14, c) first section cap 810 construction:

and (4) binding the steel bars of the first section of bearing platform 810, erecting a template, and pouring concrete of the first section of bearing platform 810.

With continued reference to fig. 15, d) first system conversion of cofferdam 300:

the periphery 830 of the first section of bearing platform is filled with medium coarse sand from bottom to top in the height direction, and is compacted and concrete is poured.

e) And (3) constructing a second bearing platform 820:

and (5) binding reinforcing steel bars of the second section of bearing platform 820, erecting a formwork, and pouring concrete of the second section of bearing platform 820.

With continued reference to fig. 16, f) a second hierarchy transformation of cofferdam 300:

and (3) filling coarse sand from bottom to top, compacting, pouring concrete, dismantling the third inner support frame 530, and finally constructing a pier arch seat 850 in the height direction of the periphery 840 of the second section of bearing platform to complete pier foundation construction.

The principles and embodiments of the present invention are explained herein using specific examples, which are presented only to assist in understanding the method and its core concepts of the present invention. It should be noted that there are no specific structures but a few objective structures due to the limited character expressions, and that those skilled in the art may make various improvements, decorations or changes without departing from the principle of the invention or may combine the above technical features in a suitable manner; such modifications, variations, combinations, or adaptations of the invention using its spirit and scope, as defined by the claims, may be directed to other uses and embodiments.