CN113356246A - Construction method of combined cofferdam under hard riverbed - Google Patents

Abstract

The invention is suitable for the technical field of riverbed cofferdam construction, and provides a construction method of a combined cofferdam under a hard riverbed, which comprises the following steps: s1, construction preparation; s2, island filling and pile foundation construction are carried out; s3, conducting hole guiding construction; s4, inserting and driving the steel pipe piles and the steel plate piles; s5, excavating the foundation pit and installing the winged lock supports; s6, excavating the foundation pit to a preset elevation; s7, constructing a pier body of the bearing platform and backfilling the foundation pit; s8, removing the internal supporting of the winged sandalwood, and removing the steel pipe pile and the steel plate pile. The method can be used for cofferdam construction on a hard riverbed, does not need occlusion type hole leading operation, avoids the problem that holes are easy to be stringed in the hole leading process, and has the advantages of high construction speed, high safety and wide application range.

Description

Construction method of combined cofferdam under hard riverbed

Technical Field

The invention belongs to the technical field of riverbed cofferdam construction, and particularly relates to a construction method of a combined cofferdam under a hard riverbed.

Background

In recent years, in the rapid development period of the road network in China, bridge construction technology is continuously innovated, and particularly, a complex bridge design is adopted for adapting to special stratum structures of a plurality of rivers, lakes and soft soil layers in coastal areas, so that the requirement on cofferdam technology is higher and higher. The steel sheet pile cofferdam is mainly of a single-wall structure, and the steel sleeve (hanging) box cofferdam can be divided into a single-wall type, a double-wall type and a single-double-wall type. The steel sheet pile cofferdam is suitable for 20-30m bridge position cofferdams and deep water foundations of riverbeds such as sandy soil, cohesive soil, gravel soil, weathered rocks and the like, and is widely applied to cofferdam engineering during bridge deep water foundation construction due to the advantages of high construction speed, good waterproof performance, reusability and the like. However, according to the characteristics of shallow depth of river and slow water flow of river bed, the existing riverbed cofferdam is designed to be inserted into the rock stratum by 1.5m in the original scheme, but the insertion depth of the steel sheet pile cannot meet the design depth requirement in actual construction, the effect of continuous engaged hole guiding operation is not ideal, and the construction safety is poor.

Disclosure of Invention

The invention provides a construction method of a combined cofferdam under a hard riverbed, and aims to solve the technical problems.

The invention is realized in this way, a construction method of a combined cofferdam under a hard riverbed, which comprises the following steps:

s1, construction preparation;

s2, island filling and pile foundation construction are carried out;

s3, conducting hole guiding construction;

s4, inserting and driving the steel pipe piles and the steel plate piles;

s5, excavating the foundation pit and installing the winged lock supports;

s6, excavating the foundation pit to a preset elevation;

s7, constructing a pier body of the bearing platform and backfilling the foundation pit;

s8, removing the internal supporting of the winged sandalwood, and removing the steel pipe pile and the steel plate pile.

Preferably, the step S3 specifically includes:

positioning the lead hole;

embedding a pile casing;

positioning a hole guiding machine;

drilling a lead hole in a rotary drilling manner;

controlling the pile length;

and backfilling the guide holes.

Preferably, the step S4 specifically includes:

lubricating and anti-seepage processing the locking notch;

inserting and driving the steel pipe piles and the steel sheet piles;

and folding the steel pipe pile and the steel sheet pile combined cofferdam.

Preferably, the step of folding the steel pipe pile and the steel sheet pile combined cofferdam comprises the following steps:

selecting a folding point;

preparing folding;

inserting, beating and folding the steel sheet piles;

and evaluating the construction quality after closing.

Preferably, the step of evaluating construction quality after closing comprises:

the driving time for closing the steel sheet piles is basically consistent with the preset steel sheet pile inserting driving time;

in the piling process, the locking notch of the folded steel sheet pile is well connected with the locking notch of the adjacent pile, so that the phenomenon of damaging the locking notch is avoided;

after the steel sheet piles are folded and driven, the inclination of two adjacent piles is still kept within 0.2 percent and is consistent with the forward inclination of the folded piles.

The method has the advantages that the embodiment of the invention prepares for construction; filling the island and constructing a pile foundation; conducting hole guiding construction; inserting and driving steel pipe piles and steel plate piles; excavating a foundation pit and installing a winged sandalwood support; excavating the foundation pit to a preset elevation; constructing a pier body of the bearing platform and backfilling the foundation pit; and removing the internal supporting of the enclosing sandals, and removing the steel pipe piles and the steel plate piles. The method can be used for cofferdam construction on a hard riverbed, does not need occlusion type hole leading operation, avoids the problem that holes are easy to be stringed in the hole leading process, and has the advantages of high construction speed, high safety and wide application range.

Drawings

FIG. 1 is a flow chart of a construction method of a combined cofferdam under a hard riverbed provided by the invention;

fig. 2 is a flow chart of another construction method of the combined cofferdam under the hard riverbed provided by the invention.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

The embodiment of the invention prepares for construction; filling the island and constructing a pile foundation; conducting hole guiding construction; inserting and driving steel pipe piles and steel plate piles; excavating a foundation pit and installing a winged sandalwood support; excavating the foundation pit to a preset elevation; constructing a bearing platform pier body and backfilling a foundation pit; and removing the internal supporting of the enclosing sandals, and removing the steel pipe piles and the steel plate piles. The method can use a hard riverbed to carry out cofferdam construction, does not need occlusion type hole leading operation, avoids the problem that holes are easy to be stringed in the hole leading process, and has the advantages of high construction speed, high safety and wide application range.

Example one

Referring to the attached drawing 1, fig. 1 is a flow chart of a construction method of a combined cofferdam under a hard riverbed provided by the invention. A method of constructing a combined cofferdam under a hard river bed, the method comprising the steps of:

and S1, construction preparation.

Specifically, the site is leveled after pile foundation construction is completed, and smoothness of a mechanical operation space and a material transportation road is guaranteed. And positioning and lofting the position of the cofferdam.

After the steel sheet pile and the steel pipe pile are transported to a construction site, the steel sheet pile and the steel pipe pile need to be arranged. Removing impurities (such as welding beading slag, waste filler, etc.) in the locking notch, and trimming the defective part.

The ZX650H excavator is matched with PCF450 hydraulic vibration hammer for operation. The vibrating hammer is a key device for driving and pulling the steel pipe pile and the steel plate pile, and special inspection must be carried out before driving and pulling, so that the smoothness of an oil path is ensured, the function is normal, and the teeth of the clamping plate cannot be abraded too much.

And S2, filling the island and constructing a pile foundation.

Specifically, the island is filled and processed, and pile foundation construction is carried out, so that the construction efficiency of the island and the pile foundation is improved, and the efficiency of cofferdam construction is increased.

And S3, conducting hole guiding construction.

Specifically, a 220-type rotary drilling rig is adopted for hole guiding, a full-protection cylinder is adopted for arm protection follow-up, the diameter of a formed hole is 600mm, the distance between the holes (pile centers) is 930mm, and the depth of the holes is controlled at the elevation of the bottom of the steel pipe pile. Therefore, the occlusion type hole leading operation is not needed, the problem that holes are easily mixed in the hole leading process is avoided, and the construction speed is high.

And S4, inserting and driving the steel pipe pile and the steel plate pile. Through inserting and driving the steel pipe pile and the steel sheet pile, the form that the interior support combined together is safe and reliable when the cofferdam is under construction, prevents to overturn, and the security is high.

And S5, excavating the foundation pit and installing the winged lock supports.

Specifically, carry out foundation ditch excavation after steel-pipe pile and steel sheet pile are inserted and are beaten the construction, this engineering foundation ditch excavation adopts open cut method construction, and the earthwork excavation must cooperate the installation that encloses the purlin and support, goes on step by step, guarantees foundation ditch construction safety.

Taking this engineering GJ01 steel-pipe pile + steel sheet pile combination cofferdam as an example, the foundation ditch is about 9m deep, adopts the structural style of twice purlin internal stay to consolidate the steel sheet pile cofferdam. GJ01 island construction ground elevation is 41.5m, and river course month average water level is 40.2m, and the steel sheet pile exposes 1.5m on ground, and the purlin setting is enclosed at 41.0m department for the first time, carries out the purlin internal stay installation procedure of enclosing when waiting for the foundation ditch excavation to 40m department.

Firstly, brackets are welded on a steel sheet pile cofferdam in a foundation pit, and the brackets are 20 channel steel.

The enclosing purlin is made of H-shaped steel (400 x 400) and is hoisted to the bracket in sections, and the H-shaped steel is spliced and welded to form the first enclosing purlin.

The steel support is a steel pipe with the diameter phi 609 being 16, the two ends of the steel support are provided with flanges, the lengths of the flanges are 1m, 2m, 3m, 3.9m, 5.2m and 6m, the steel support with the proper length is spliced according to the size of the purlin and is matched with the loose head for use, and the effect of supporting the inside of the steel sheet pile cofferdam is achieved. The flexible head has two types of round tubes and square tubes, the GJ01 steel sheet pile cofferdam steel support form is "2 m +2m +3.9m +5.2m + round tube flexible head", and 3 steel supports are arranged on each purlin. Because of along with foundation ditch excavation, corresponding deformation can appear in the steel sheet pile cofferdam, and the steel shotcrete both ends can increase the steel sheet cushion as required, compensate the not enough of support length.

Before the steel shotcrete was hoisted to foundation ditch surrounding purlin, "hanging claw" at steel shotcrete both ends welding, when "hanging claw" welded position, need notice both ends position symmetry, prevent that the steel shotcrete installation is uneven, cause rework rewelding. When the steel shotcrete hoisted to enclosing purlin assigned position top, artifical hammering was moved the head of the throne, made it probe enough length, guaranteed that the steel shotcrete closely laminates with enclosing purlin, affirmed the position and probed length qualified after, dropped the steel shotcrete.

And (3) continuing to excavate the foundation pit after the inner support of the first purlin is completed, adopting a 300-type excavator and a PC220-8 type long arm excavator to cooperate to excavate the foundation pit, excavating the foundation pit around the cofferdam by the excavator in a rotating mode, and transporting earthwork by a dump truck. When the long-arm excavator cannot work, 1 30-type small excavator is adopted to excavate soil at the bottom of the foundation pit, the crawler crane is matched with the excavation, and the surrounding purlin support is not collided during excavation. And when the distance is excavated to 36.0m, carrying out second purlin surrounding support installation operation, wherein the operation flow is the same as that of the first purlin surrounding operation.

And S6, excavating the foundation pit to a preset elevation.

The preset elevation may be a designed standard height, a predicted safety standard height, or the like.

Specifically, after the second purlin support is installed, earth excavation is continued to the designed elevation, and the rock stratum excavation size is 1m of the outward expansion of the edge of the bearing platform. The mode that a 300-type excavator is used for replacing a pickaxe is adopted for carrying out rock stratum crushing and chiseling operation, the PC 220-8-type long-arm excavator is matched with an earth and rockfill excavating machine to a foundation pit, a specially-assigned person is needed to command the excavation process, and the situation that the inner support of the enclosing purlin and the steel sheet pile cofferdam are collided is avoided.

Because of the river course average water level of month is 40.2m, receive the great influence of pebble bed water content, the unavoidable condition that seeps water appears in the excavation process, adopt the mode that high-lift water pump drew water and excavate the escape canal, satisfy foundation ditch construction conditions.

And S7, constructing the pier body of the bearing platform and backfilling the foundation pit.

Specifically, the settlement deformation measurement work of the steel sheet pile cofferdam is well done in the construction process of the pier body of the bearing platform, the work is carried out once a day, if the settlement deformation is larger than the deviation, retest is carried out in time, whether the steel sheet pile cofferdam is in a dangerous state or not is judged, and risks are reasonably avoided.

Because the pier body of the project conflicts with the steel support, after the construction of the bearing platform is finished, the backfilling operation of the foundation pit is carried out, part of the steel support is removed, and then the pier body construction is carried out. The foundation pit backfilling process is influenced by the larger water content of the pebble layer and the limitation of the operation space, the soil body cannot be compacted and has larger water content, and the steel support is dismantled after the foundation pit backfilling process needs to be stood and deposited for a period of time.

S8, removing the internal supporting of the winged sandalwood, and removing the steel pipe pile and the steel plate pile.

Specifically, the inner support of the purlin is detached, and the operation is performed according to the sequence from bottom to top. The whole section of steel shotcrete lifts by crane outside the foundation ditch, lifts by crane the steel shotcrete in-process and pays attention to avoiding the collision, prevents the high altitude and falls down the thing, treats that three internal stay demolishs the back, cuts H shaped steel enclosing purlin, and the segmentation lifts by crane outside the foundation ditch, neatly stacks, is convenient for transport and turnover use.

And removing the steel sheet pile after the inner support of the cofferdam is removed. When the steel sheet pile is pulled out, the head of the steel sheet pile is clamped by a vibration hammer to vibrate for 1-2 min, so that soil around the Larsen steel sheet pile is loosened to generate liquefaction, the frictional resistance of the soil to the pile is reduced, and then the steel sheet pile is slowly pulled upwards by vibration. When the load condition of the vibration hammer is noticed during pile pulling, and the pile pulling is stopped when the pile pulling is difficult or impossible, the pile can be driven downwards a little in advance and then pulled upwards, and the pile can be pulled out repeatedly. The steel sheet piles after being pulled out are orderly stacked, and are convenient to transport and use in a turnover mode.

When the method is implemented specifically, construction preparation is carried out; filling the island and constructing a pile foundation; conducting hole guiding construction; inserting and driving steel pipe piles and steel plate piles; excavating a foundation pit and installing a winged sandalwood support; excavating the foundation pit to a preset elevation; constructing a pier body of the bearing platform and backfilling the foundation pit; and removing the internal supporting of the enclosing sandals, and removing the steel pipe piles and the steel plate piles. The invention is suitable for the construction of the steel pipe pile and steel sheet pile combined cofferdam under the geological condition of a hard riverbed. A rotary drilling and skip type lead hole is adopted, the supporting form of the combination of the steel pipe pile and steel sheet pile combined cofferdam and the inner support is safe and reliable, and overturning danger is prevented. The steel pipe pile and steel sheet pile combined cofferdam has good water stopping effect and meets the construction requirement. The occlusion type hole leading operation is not needed, the problem that holes are easily mixed in the hole leading process is avoided, and the construction speed is high. The modularization construction, steel sheet pile and supporting member can have enough to meet the need the use, and the high-usage. According to the invention, a construction process of combining rotary drilling and skip hole leading, and steel pipe pile and steel sheet pile combined cofferdam and inner support is adopted, so that the problem of hole stringing in the occlusion hole leading process is effectively avoided, and the effects of safety, overturn prevention and water stop of foundation pit construction are ensured.

Referring to fig. 2, fig. 2 is a flow chart of another construction method of a combined cofferdam under a hard riverbed according to the present invention. In this embodiment of the present invention, the step S3 specifically includes:

and S31, positioning the lead hole.

Specifically, measuring and lofting, inserting and driving wood piles at two ends to determine the center line of the lead hole, then tying a construction line on the wood piles, and tying a color bar section at intervals of 60cm so as to control the center position of each lead hole.

And S32, embedding a pile casing.

Specifically, a steel casing with the inner diameter of 600mm is adopted, the length of the casing is determined according to geological conditions, and the casing penetrates through a pebble bed. After the pile casing is clamped by the vibrating hammer, the pile casing is pressed down after the position and the verticality of the pile casing are aligned, the pressing depth is about 4-6 m, then a rotary drill is used for taking soil from the pile casing, soil is grabbed while the pile casing is continuously pressed down, and the depth of a bottom opening of the pile casing, which is ahead of an excavation surface, is always kept to be not less than 2.5 m. And stopping pressing after the casing is pressed into the stroke argillaceous sandstone layer. And after the protective barrel is pressed in, detecting the verticality of the protective barrel, if the protective barrel is unqualified, performing deviation rectification adjustment, and if the protective barrel is qualified, continuously pressing and drilling.

And S33, positioning the hole guiding machine.

Specifically, the perpendicularity deviation of the pile frame is adjusted to be smaller than 1/250 by moving the drilling machine to the operation position. The displacement of the drilling machine is uniformly commanded by a constructor on duty, the site condition must be carefully observed before the displacement, and the displacement needs to be stable and safe. After the drilling machine is positioned, a current field technician is responsible for rechecking the hole position, and the deviation is not more than 10 mm.

And S34, rotary drilling a guide hole.

Specifically, the drilling machine in place is guided to be excavated through rotation, so that the construction efficiency of the guided hole is high, and the accuracy is high.

And S35, controlling the pile length.

Specifically, the original ground elevation is measured during drilling construction, and the designed hole depth is calculated. And controlling the depth of the lead hole not to be smaller than the designed Larsen steel plate pile bottom elevation. The design elevation of the pile of each construction section is controlled according to the cofferdam diagram of the underwater bearing platform.

And S36, backfilling the guide holes.

Specifically, after the drainage of each pile hole is finished, the pile hole is backfilled with sandy soil in time, if water exists in the pile hole, the water is pumped by a water pump before the backfilling, and then the backfilling is carried out. And after backfilling is finished, the pile casing can be pulled out, and the next hole leading construction is carried out.

In this embodiment of the present invention, the step S4 specifically includes:

lubricating and anti-seepage processing the locking notch; for the Larsen steel sheet piles and steel pipe piles which are qualified through inspection, the steel pipe piles and the steel sheet piles can be smoothly inserted and pulled out in the construction process, and the seepage-proofing performance of the Larsen steel sheet piles is improved when the Larsen steel sheet piles are used. The locking notch of each sheet pile needs to be uniformly coated with mixed oil, and the volume mixing ratio of the mixed oil to the dry sawdust is 5: 3.

Inserting and driving the steel pipe piles and the steel sheet piles; and driving a first steel pipe pile and a second steel pipe pile into the steel sheet pile from one corner of the cofferdam, then driving steel sheet piles among the steel pipe piles, gradually inserting and driving along the clockwise or anticlockwise direction, and folding at the corner piles. In the inserting and driving process, the measurement work is strengthened, the inclination is found, the adjustment is carried out in time, the pile body is ensured to be vertical, and the splicing amount of the corner piles during the folding is reduced. And positioning piles and drawing contour lines are arranged at four corners of the cofferdam, steel sheet piles are inserted and driven along the contour lines, measurement and correction are carried out once every 3m, and the smoothness of the line type is ensured.

Because the underground rock stratum trends are inconsistent and slightly fluctuate, the ground exposure heights of the steel pipe piles and the steel sheet piles are inconsistent, the inserting and beating depth of the steel pipe piles and the steel sheet piles is qualified by taking the condition that the steel sheet piles do not obviously sink for 2min of inserting and beating by a vibration hammer as a qualified standard, and the steel sheet piles are ensured to be inserted into the surface of the rock stratum. And arranging special-shaped steel sheet piles with corresponding specifications at the folding position, and cutting and welding the special-shaped steel sheet piles according to the actually measured angle and size on site to ensure the sealing property of the whole cofferdam.

And folding the steel pipe pile and the steel sheet pile combined cofferdam. The technology of folding the steel pipe piles and the steel plate piles is the key in the construction process of the steel pipe pile and steel plate pile combined cofferdam, and whether the folding is successful or not determines the construction quality and progress of the cofferdam.

In the embodiment of the invention, the step of folding the steel pipe pile and the steel sheet pile combined cofferdam comprises the following steps:

selecting a folding point; and 4-5 piles are selected near any corner pile as the final folding point.

Preparing folding; in order to control the verticality of the steel pipe piles and the steel plate piles at the closure opening, the inclination of the piles is strictly controlled when 10 piles are left on two sides of the closure opening. The axial and normal inclination of each pile is measured after each pile is inserted and driven, the inclination of the next piles is predicted according to the accumulated increasing rule of the inclination and the existing construction experience, and the subsequent piles are gradually corrected during inserting and driving to ensure that the axial and normal inclination are controlled within 0.2 percent.

Inserting, beating and folding the steel sheet piles; in order to ensure that the closure steel sheet pile can be smoothly meshed with the locking openings of the adjacent piles, the pile heights on the two sides of the closure steel sheet pile are different before inserting and driving, and the height difference is controlled to be 100-200 mm, so that the locking openings of the closure steel sheet pile can be conveniently sleeved with the high pile first and then sleeved with the locking opening of the low pile.

When the closure pile is inserted and driven, the closure pile sleeved with the locking opening of the adjacent pile falls freely, the vibration hammer is vertically descended to press the whole weight of the closure pile, the vibration hammer is started, the resistance is gradually increased along with the increase of the depth of the pile entering the soil body, the current value of the ammeter of the controller is also increased, when the current value is increased to 400A, the pile is lifted for about 2m, and when the current value is reduced to 100A, the pile is driven downwards. Repeating the steps for several times to drive the piles to the preset height.

And evaluating the construction quality after closing. The construction effect, construction efficiency, safety and the like can be known through construction evaluation.

In the embodiment of the present invention, the step of evaluating the construction quality after closing comprises:

the driving time for closing the steel sheet piles is basically consistent with the preset steel sheet pile inserting driving time;

in the piling process, the locking notch of the folded steel sheet pile is well connected with the locking notch of the adjacent pile, so that the phenomenon of damaging the locking notch is avoided;

after the steel sheet piles are folded and driven, the inclination of two adjacent piles is still kept within 0.2 percent and is consistent with the forward inclination of the folded piles.

Specifically, the criteria for closure quality evaluation are as follows: the driving time of the folded steel sheet pile is basically consistent with the inserting driving time of a common steel sheet pile; in the piling process, the locking notch of the folded steel sheet pile is well connected with the locking notch of the adjacent pile, and the phenomenon of locking notch damage is not found; after the folded steel sheet piles are driven, the inclination of two adjacent piles is still kept within 0.2 percent and is consistent with the forward inclination of the folded steel sheet piles.

Compared with the prior art, the embodiment of the invention carries out construction preparation; filling the island and constructing a pile foundation; conducting hole guiding construction; inserting and driving steel pipe piles and steel plate piles; excavating a foundation pit and installing a winged sandalwood support; excavating the foundation pit to a preset elevation; constructing a bearing platform pier body and backfilling a foundation pit; and removing the internal supporting of the enclosing sandals, and removing the steel pipe piles and the steel plate piles. The invention is suitable for the construction of the steel pipe pile and steel sheet pile combined cofferdam under the geological condition of a hard riverbed. A rotary drilling and skip type lead hole is adopted, the supporting form of the combination of the steel pipe pile and steel sheet pile combined cofferdam and the inner support is safe and reliable, and overturning danger is prevented. The steel pipe pile and steel sheet pile combined cofferdam has good water stopping effect and meets the construction requirement. The occlusion type hole leading operation is not needed, the problem that holes are easily mixed in the hole leading process is avoided, and the construction speed is high. The modularization construction, steel sheet pile and supporting member can have enough to meet the need the use, and the high-usage. According to the invention, a construction process of combining rotary drilling and skip hole leading, and steel pipe pile and steel sheet pile combined cofferdam and inner support is adopted, so that the problem of hole stringing in the occlusion hole leading process is effectively avoided, and the effects of safety, overturn prevention and water stop of foundation pit construction are ensured.

The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents and improvements made within the spirit and principle of the present invention are intended to be included within the scope of the present invention.

Claims (5)

1. A construction method of a combined cofferdam under a hard riverbed is characterized by comprising the following steps:

s1, construction preparation;

s2, island filling and pile foundation construction are carried out;

s3, conducting hole guiding construction;

s4, inserting and driving the steel pipe piles and the steel plate piles;

s5, excavating the foundation pit and installing the winged lock supports;

s6, excavating the foundation pit to a preset elevation;

s7, constructing a pier body of the bearing platform and backfilling the foundation pit;

s8, removing the internal supporting of the winged sandalwood, and removing the steel pipe pile and the steel plate pile.

2. The method for constructing a combined cofferdam under a hard riverbed as claimed in claim 1, wherein the step S3 comprises:

positioning the lead hole;

embedding a pile casing;

positioning a hole guiding machine;

drilling a lead hole in a rotary drilling manner;

controlling the pile length;

and backfilling the guide holes.

3. The method for constructing a combined cofferdam under a hard riverbed as claimed in claim 1, wherein the step S4 comprises:

lubricating and anti-seepage processing the locking notch;

inserting and driving the steel pipe piles and the steel sheet piles;

and folding the steel pipe pile and the steel sheet pile combined cofferdam.

4. The construction method of the combined cofferdam under the hard riverbed as claimed in claim 3, wherein the step of folding the steel pipe piles and the steel sheet piles combined cofferdam comprises:

selecting a folding point;

preparing folding;

inserting, beating and folding the steel sheet piles;

and evaluating the construction quality after closing.

5. The method for constructing a combined cofferdam under a hard riverbed as claimed in claim 4, wherein the step of evaluating the construction quality after closing comprises:

the driving time for closing the steel sheet piles is basically consistent with the preset steel sheet pile inserting driving time;

in the piling process, the locking notch of the folded steel sheet pile is well connected with the locking notch of the adjacent pile, so that the phenomenon of damaging the locking notch is avoided;

after the steel sheet piles are folded and driven, the inclination of two adjacent piles is still kept within 0.2 percent and is consistent with the forward inclination of the folded piles.

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