Construction method for improving dam body self-standing and stability of open type shallow buried steel dam body island cofferdam

Abstract

The construction method for improving the self-standing and stability of an open type shallow buried steel structure dam body island cofferdam belongs to the technical field of water conservancy and municipal engineering. In view of the defects of self-standing and stability caused by the fact that the open type steel structure dam body island cofferdam cannot be closed in a ring and the shallow buried anchoring is insufficient, the application constructs a water level natural balance initial state through reserving upstream and downstream ports, and solves the problems of insufficient shallow buried anchoring and uneven initial stress by synchronous filling. Combined with the change law of river tides, layered and collaborative filling is implemented, the sand and gravel layer is naturally compacted by using the advance and retreat of tides, and the dependence on additional ramming equipment is reduced. By reasonably controlling the filling height difference inside and outside the dam body, a compression and tension composite stress system is constructed, a self-balancing system of compression and tension composite stress is formed, and the defects of the open structure closed loop are effectively made up. The method effectively improves the self-standing and stability of the dam body, takes into account the economy and efficiency, adapts to complex geological and hydrological conditions, and is suitable for large-span and long linear construction sites.

Description

Technical Field

[0001] This invention relates to the field of water conservancy and municipal engineering technology, specifically to a construction method for improving the self-support and stability of open-type shallow-buried steel dams by constructing island cofferdams. It is applicable to large-span, long-linear construction sites and large-scale island construction projects in rivers with significant tidal changes. The main focus is on solving the technical problem of insufficient self-support and stability caused by insufficient shallow-buried anchoring of this type of cofferdam. Background Technology

[0002] In water conservancy and municipal engineering construction, island cofferdams are the main facilities for building dry construction platforms in water. Steel-structured dam island cofferdams are widely used in such projects due to their high structural strength and convenient construction. Their typical structure consists of two rows of Larssen steel sheet piles filled with gravel, which are then connected by multiple tie rods to form a steel dam body. The artificial island is then formed by dredging and filling sand inside the dam body.

[0003] Existing steel-structured dam cofferdams for island construction are mainly divided into two types: U-shaped closed-loop and open-loop. Among them, open-loop steel-structured dam cofferdams are increasingly widely used in large-scale river-crossing immersed tunnel projects due to their suitability for construction sites with large required areas and long linear extensions. However, this type of cofferdam has the following technical drawbacks:

[0004] (1) The open end cannot form a ring-closed structure and needs to be closed by sandbag cofferdam, which results in an incomplete overall force system of the dam body and makes it difficult to control the balance.

[0005] (2) Steel structure dams are conventionally driven into the soil layer by vibration. Under shallow conditions, the anchoring effect of the buried rock and soil is poor. If the pre-drilling process is used to increase the burial depth, the pre-drilling operation on the river surface will have problems such as inconvenience and slow progress. Moreover, the rock and soil wrapping of the steel sheet pile buried section will decrease after the pre-drilling.

[0006] (3) After the cofferdam is closed, the method of pumping and sand blowing is used for filling, which is prone to cause the internal and external forces of the steel dam to become unbalanced, further aggravating the stability risk;

[0007] (4) Existing construction methods do not make full use of river tidal conditions, and the density of the gravel layer needs to rely on additional compaction equipment, resulting in high construction costs and low efficiency.

[0008] Therefore, given the structural characteristics and construction difficulties of island cofferdams for open-type shallow-buried steel dams, there is an urgent need to develop a construction method that can balance self-reliance and stability, adapt to shallow-buried conditions, and be economical and efficient. Summary of the Invention

[0009] To address the technical problems existing in the prior art, the purpose of this invention is to provide a construction method for building island cofferdams to improve the self-support and stability of open-type shallow-buried steel dams, effectively enhancing the self-support and stability of the dam while taking into account both economy and efficiency.

[0010] To achieve the above objectives, the present invention adopts the following technical solution:

[0011] The construction method for improving the self-support and stability of open-type shallow-buried steel dams by building island cofferdams includes the following steps:

[0012] S1. When constructing the island cofferdam of the steel dam, first drive double rows of Larssen steel sheet piles. Except for the upstream and downstream ends which are reserved for the time being, all other parts are driven in place.

[0013] S2. The dredging and sand filling operation first involves dredging the steel dam body in layers. Under the premise of maintaining the stress balance between the inside and outside of the steel dam body, the outer side facing the river is simultaneously filled with riprap in layers, and the inner side is filled with sand and gravel in layers. When the three are backfilled, the filling inside the steel dam body is kept higher than the inner and outer sides.

[0014] S3. After the steel dam body is formed and the upstream section is closed, sandbags will be used to close the open section of the downstream cofferdam layer by layer.

[0015] S4. After the steel dam body is formed and the upstream section is closed, the sand transportation method will be changed from sand dredging ships on the river to inland vehicles, and backfilling operations will be carried out in conjunction with excavators to gradually complete the formation of the entire island cofferdam.

[0016] S3 and S4 are flexible in terms of timing and can be carried out sequentially or simultaneously according to the construction organization plan.

[0017] As a preferred option, in step S1, when driving double-row Larssen sheet piles, only the upstream and downstream ends are reserved and not driven for the time being. The reserved port in the upstream section allows river water to flow naturally into the interior of the sheet pile and the inner island construction area. By relying on the natural balance of the water level, the sheet pile is evenly stressed inside and outside, avoiding the displacement of the pile body caused by unilateral stress.

[0018] As a preferred option, step S2 includes: S21 initial stage, S22 development stage, and S23 stabilization stage; the filling operation of the steel structure dam island cofferdam is carried out in layers from upstream to downstream, so that the river water in the cofferdam can be discharged in the direction of downstream flow, avoiding local water accumulation that would cause uneven stress on the dam body, and ensuring the overall stability of the dam body during the filling process.

[0019] As a preferred option, in step S21, the initial backfilling operation of the dredged sand inside the steel dam body is matched with the tidal variation of the river. The initial filling is carried out during the period of lowest ebb tide. The first filling is carried out to half the height of the lowest ebb tide to form a preliminary self-supporting capacity. Subsequent filling is carried out in layers to the lowest ebb tide, and the height of each filling is not greater than 1m.

[0020] As a preferred option, in step S21, after the steel dam body is filled to the preset height, the outer river-facing side is simultaneously filled with riprap and the inner side is filled with gravel. The width and height of the inner gravel filling are consistent with those of the outer riprap filling, achieving coordinated stress distribution between the outer toe protection and the inner filling, ensuring the overall balance of the dam body. Here, the preset height refers to the lowest water level at low tide.

[0021] As a preferred option, in step S22, the dredged sand inside the steel dam body is backfilled in layers. During the development period, the height of each layer is controlled and the backfilling is carried out at intervals of the complete tidal cycle. By utilizing the hydrological characteristics of tidal rise and fall, the sand and gravel layer is naturally compacted through river water infiltration and seepage discharge, reducing the reliance on manual compaction equipment. After the backfilling is completed during the development period, the reserved gap upstream of the cofferdam is closed.

[0022] As a preferred option, in step S23, when the filling height of the inner and outer sides of the steel dam body is at the same level as the normal water level, the method of prioritizing internal filling is switched. After the internal filling height is 2m higher than the outer and inner sides, the filling of the outer and inner sides is carried out simultaneously and the height difference is maintained. The compressive stress formed by the self-weight of the internal gravel is converted into tensile stress of the steel sheet piles through the steel tie rods between the double rows of Larssen steel sheet piles, so that the dam body forms a gravity structure with combined compressive and tensile forces to resist the action of water pressure.

[0023] As a preferred method, before the filling begins in step S21, multiple sand-blocking sandbags are deployed in the downstream area of ​​the cofferdam. These sandbags are arranged in a staggered, layered manner to effectively intercept sand and gravel drifting downstream with the river during the upstream gravel filling operation. This method reduces the loss of gravel during filling, improves the utilization rate of the filling material, and lowers construction material costs.

[0024] As a preferred option, in step S4, after the steel dam structure is formed and the upstream section is closed, the sand transportation method is changed from river surface sand dredgers to inland vehicles, and excavators are used for layered backfilling to gradually complete the formation of the entire island cofferdam. Using this method can accelerate the island construction progress and save on the rental costs of sand transport vessels and sand dredgers.

[0025] As a preferred method, the construction method of building island cofferdams to improve the self-support and stability of open-type shallow-buried steel dams includes the following steps:

[0026] V0. Preliminary preparation: Continuously monitor the tidal cycle and water level changes and draw tidal curves, investigate the engineering geological distribution, and determine the relevant construction parameters for steel sheet pile driving and filling material gradation;

[0027] S1. Steel Sheet Pile Driving: Drive double rows of Larssen steel sheet piles, leaving only the upstream and downstream ends undriven for the time being, and drive the remaining parts in place; drive in the order from upstream to downstream, and after driving is completed, inspect the axis deviation, verticality and leakage. After the standards are met, proceed to the next construction stage.

[0028] S2. Phased filling:

[0029] S21. Initial stage: After the steel sheet piles have passed the acceptance test, sand-blocking sandbags are first laid out in a staggered manner in the downstream area of ​​the cofferdam. When carrying out the dredging and sand filling operation inside the steel dam, construction is carried out during the period of lowest water level during low tide. Sand is blown and water is squeezed in layers to fill to the lowest water level of the river, and the height of each filling is not greater than 1m. The filling is carried out in the order from upstream to downstream. During the dredging process, the downstream sand-blocking sandbags are used to intercept the sand and gravel that drift with the river water to reduce the loss of filling material.

[0030] S22. Development Phase: When the filling height of the inner and outer sides of the steel dam body is equal to the normal water level, maintain the basic balance of the height difference among the three, and continue to fill the dam layer by layer from upstream to downstream by blowing sand and squeezing water, controlling the height of each layer and advancing at intervals of complete tidal cycles, and using the ebb and flow of tides to achieve natural compaction of the gravel layer; during the filling process, check the downstream sandbag placement status, and adjust and reinforce it in time if there is any displacement;

[0031] After the development phase filling is completed, the reserved gap upstream of the cofferdam will be closed, the steel sheet pile driving operation of the remaining steel structure dam body in the area will be completed, and the outer side of the dam body will be filled with boulders and the inner side will be filled with sand and gravel.

[0032] S23. Stabilization period: Carry out filling operations on the inside, outside and inside sides of the steel dam body, giving priority to raising the height of the inside filling so that the height of the inside filling is higher than that of the outside and inside by a certain value and this height difference is maintained continuously; the compressive stress formed by the self-weight of the internal gravel is converted into tensile stress of the steel sheet piles through the steel tie rods between the double rows of Larssen steel sheet piles, so that the steel dam body forms a gravity structure with combined compressive and tensile forces to resist the action of water pressure;

[0033] S3. Closure of the opening: After the steel structure dam body is formed and the upstream closure is completed, the opening of the downstream cofferdam is closed by progressively stacking sandbags in layers. Geotextile is laid simultaneously for seepage prevention.

[0034] S4. Conversion Construction: After the steel structure dam is formed and the upstream section is closed, the sand transportation method will be changed from sand dredging boats on the river to inland vehicles. Backfilling operations will be carried out in conjunction with excavators to promote the gradual formation of the island cofferdam. The sandbag cofferdam at the downstream opening will be stacked in layers in sync with the backfilling height. The sandbags used for intercepting sand do not need to be removed and can be directly covered with gravel.

[0035] The principle of this invention is:

[0036] To address the deficiencies in self-support and stability caused by the inability to achieve rigid closure and insufficient shallow anchorage in open steel dam construction, this invention establishes a natural initial equilibrium state of water level by reserving upstream and downstream ports. This, combined with synchronous filling, resolves the issues of insufficient shallow anchorage and uneven initial stress. Layered and coordinated filling is implemented based on the tidal changes of the river, utilizing tidal advances and retreats to achieve natural compaction of the gravel layer, reducing reliance on additional compaction equipment. By rationally controlling the height difference between the inner and outer filling layers of the dam, a compressive-tensile composite stress system is constructed, forming a self-balancing system that effectively compensates for the lack of closed-loop structure in open dam construction.

[0037] The present invention has the following advantages:

[0038] 1. By using the hydraulically balanced sheet pile driving process, the problems of insufficient anchorage of shallow-buried sheet piles and uneven stress in the initial stage of open structures are solved from the source, achieving stable self-support in the early stage of dam construction; the innovatively constructed compression-tension composite force system effectively compensates for the defects of open structures that cannot achieve circumferential rigidity closure and shallow burial depth of sheet piles, transforming the self-weight of the fill material into structural anti-overturning force, enabling the dam to withstand complex water pressure, adapt to the requirements of high-level flood defense, and significantly reduce the risk of structural instability during construction and use.

[0039] 2. The collaborative layered filling technology, which fully utilizes the natural energy of tides, achieves natural compaction of the gravel layer through the ebb and flow of tides, replacing traditional mechanical compaction operations and reducing the investment, rental, and energy costs of compaction equipment. The construction process eliminates the need for pre-drilling, simplifying the sheet pile driving procedure and shortening the construction cycle. The timely conversion from river-to-land sand dredging to land-based filling not only accelerates the island-building and filling progress but also saves on ship rental, water operations, and other related costs, achieving a dual optimization of construction efficiency and cost control.

[0040] 3. This invention is designed for large-span, long-linear construction sites and is perfectly suited to the structural characteristics of open-type cofferdams. It is applicable to various island cofferdam projects such as immersed tunnels across rivers, cross-sea channels, and river estuary regulation in water conservancy and municipal engineering projects. At the same time, the technology is developed by fully combining tidal hydrological characteristics and shallow buried geological conditions. It can adapt to various strata such as fine sand, silty clay, and strongly weathered silty mudstone, as well as river waters with significant tidal changes. It has outstanding adaptability to complex geological and hydrological conditions.

[0041] 4. A standardized construction process was established, encompassing preliminary preparation, sheet pile driving, phased filling, opening closure, and sand transportation and conversion. Each process was closely linked and logically clear. Phased filling, combined with the upstream-to-downstream advancement sequence, effectively avoided localized water accumulation within the cofferdam, ensuring uniform stress on the dam body. Gradual sandbag closure, combined with geotextile seepage prevention, solved the instability problem caused by sudden changes in water pressure at the open end, improving the closure success rate and seepage prevention effect. From sheet pile driving to fill material, each process step had clear operational points and quality control logic, making on-site construction easy to operate and monitor, and ensuring the overall construction quality of the island cofferdam.

[0042] 5. The core process of this invention fully utilizes the natural energy of tides to achieve self-compacting of the filler, which greatly reduces the frequency of use of engineering machinery and reduces energy consumption, noise and dust pollution during construction. Environmentally friendly fillers such as graded sand and gravel are selected during construction, with no additional construction waste generated, and no harmful pollutants are emitted in each process step, which meets the current green and environmental protection requirements for engineering construction and takes into account both engineering construction and ecological protection.

[0043] 6. All construction processes adopt a modular and progressive approach, which can flexibly adjust the construction rhythm and filling parameters according to the actual tidal changes, water level fluctuations and geological conditions on site; the nodes of opening closure and sand transportation conversion can be dynamically controlled according to the dam body forming, without the need for fixed construction time limits, which can effectively cope with the unexpected situation on the construction site. The flexibility of construction operation and site adaptability are significantly better than traditional processes. Attached Figure Description

[0044] Figure 1 Plan view showing the location of the gaps between the steel dam structure and the upstream and downstream sections.

[0045] Figure 2 This is a schematic diagram of the initial arrangement of dredged gravel and sandbags inside the steel dam structure.

[0046] Figure 3 for Figure 2 Cross-sectional view of the steel structure dam.

[0047] Figure 4 This is a schematic diagram showing the filling of the inside and outside of the steel dam.

[0048] Figure 5 for Figure 4 Cross-sectional view of the steel structure dam.

[0049] Figure 6 A schematic diagram of tidal co-fill self-compacting (high tide immersion state).

[0050] Figure 7 A schematic diagram of tidal co-fill self-compacting (during low tide compaction).

[0051] Figure 8 This is a schematic diagram of the high-density, low-density granular material filling inside the steel dam during the stable phase.

[0052] Figure 9 This is a schematic diagram illustrating the change in sand transportation methods from river surface sand-blowing vessels to inland vehicle sand transportation and filling.

[0053] Figure 10 A schematic diagram of the construction of an island cofferdam for an open-type steel dam.

[0054] Among them, 1-the edge line of the embankment structure, 2-the edge line of the steel dam body, 3-the direction of water flow, 4-the sandbag for interception, 5-the dredged gravel, 6-the dredged sand, 7-the riprap, 8-the steel tie rod, 9-the sandbag cofferdam. Detailed Implementation

[0055] The present invention will now be described in further detail with reference to specific embodiments.

[0056] The construction method for improving the self-support and stability of open-type shallow-buried steel dams by building island cofferdams includes the following steps.

[0057] V0. Preliminary Preparations.

[0058] (1) Tidal cycle and water level change.

[0059] For 15 consecutive days, the tidal cycle and water level changes in the construction area were monitored, a complete tidal change curve was plotted, and key data such as the duration of high tide and low tide, tidal range, period of lowest water level during low tide, normal water level, and multi-year average water level were accurately recorded, providing a basis for the timing and height control of tidal coordinated filling.

[0060] (2) Judgment of tidal window period.

[0061] Based on the tidal change curve, determine the period of lowest water level during low tide each day, and complete the construction preparation work one hour in advance.

[0062] (3) Material preparation.

[0063] Before graded gravel, medium-coarse sand, and boulders are brought to the site, their gradation and mud content must be tested. Unqualified materials are strictly prohibited from use. The graded gravel used for hydraulic filling inside the weir should have a particle size of 0.25-3mm and a mud content of <5%, of which 40% should have a particle size >2mm, 30% >0.25mm, and 30% >0.5mm.

[0064] S1. Steel sheet pile driving under hydraulic balance guidance, such as Figure 1 .

[0065] (1) Setting out the pile positions

[0066] According to the design axis, a total station is used to lay out the line, and a control stake is set every 10m, with a deviation ≤3cm.

[0067] (2) Trial test verification

[0068] Three sheet piles at the end of the cofferdam were selected for trial driving to determine parameters such as excitation force and pile driving speed, and to verify geological adaptability.

[0069] (3) Formal administration

[0070] Double-row Larssen sheet piles are driven in sequence from upstream to downstream, with only the upstream and downstream ends reserved for later driving. The total length of the reserved ends is 5-10% of the total length of the cofferdam. The upstream reserved end is a natural water inlet, allowing river water to flow into the interior of the sheet piles and the inner island construction area, maintaining a water level fluctuation difference of ≤50cm between the inside and outside, and achieving a balance of forces inside and outside the sheet piles. Verticality is monitored in real time during the driving process, and the machine is stopped immediately for correction if the deviation exceeds 1%.

[0071] S2. Tidal coordinated phased filling and upstream closure.

[0072] The backfilling is carried out in layers from upstream to downstream, with a progress rate of 15-20m per day, to ensure that the river water inside the cofferdam flows smoothly out and that no local water accumulation lasts for more than 2 hours.

[0073] S21. Initial filling.

[0074] like Figure 2 After the sheet piles have passed inspection, sandbags are laid out in staggered layers in the downstream area of ​​the cofferdam. During the layered backfilling of the steel dam body, the tidal patterns of the river must be closely monitored. Knowing the changes in river water levels during high and low tides, the initial filling height of the steel dam body should be half the lowest low tide level to give the dam a certain degree of self-support in the initial stage. Subsequently, the filling should be gradually increased to the lowest water level. Figure 3 The filling speed should be controlled at 30 m³ / h.

[0075] like Figure 4 and Figure 5 After the steel dam body is filled to a certain height, riprap is dumped onto the outer river-facing side, followed immediately by dredged gravel filling on the inner side. The width and height of the dredged gravel filling on the inner side are basically the same as those of the riprap filling, ensuring the steel dam body is balanced under stress. The cofferdam construction of the steel dam body involves backfilling in layers from upstream to downstream, allowing the river water within the cofferdam to be discharged downstream.

[0076] S22. Development period.

[0077] The principle of tidal self-closing: The steel dam body is backfilled with dredged sand in layers. During the day-night construction intervals, the river's tidal changes are utilized. Initially, river water covers the sand and gravel; later, as the tide recedes, some water separates from the dredged sand and gravel. Each layer of sand and gravel is initially covered by river water, such as... Figure 6Later, through the flow of water, a certain degree of self-sealing effect is achieved, such as... Figure 7 The repeated advance and retreat of the tides enhances the compaction of each layer of gravel.

[0078] In this embodiment, during high tide, the river water soaks the gravel layer, and the peak pore water pressure is about 0.8 kPa; during low tide, the water flows out, and the pore water pressure dissipates at a rate of about 0.2 kPa / h. The sand particles naturally compact under the action of gravity and seepage, with an average settlement of 5 mm.

[0079] Construction is suspended during high tide, and the sand and gravel layer is soaked by river water. After low tide, the density is tested, and if it does not reach 65%, the work is extended by one tidal cycle.

[0080] Development and filling: advance in layers with a thickness of ≤1.0m, with each layer spaced one complete tidal cycle apart.

[0081] Upstream port closure: After the development phase filling is completed, the reserved gap upstream of the cofferdam will be closed, the remaining steel sheet piles of the steel dam body in the area will be driven, and the outer side of the dam body will be filled with boulders and the inner side will be filled with gravel.

[0082] S23. Stable period.

[0083] After the upstream closure is completed, the stabilization phase begins. When the filling height of the inner and outer sides of the steel dam is basically level with the normal water level, the change is made from the previous approach of ensuring the filling height of the inner side of the steel dam is higher than 2 meters before starting the filling of the outer and inner sides of the dam. This process maintains the height difference between the inner and outer sides within 2 meters. Figure 8 The tiered advancement speed is 18m per day.

[0084] Construction principle of composite stress structure in steel dams:

[0085] During the filling of the outer and inner sides of the steel dam, the height difference between the inner and outer sides is maintained within 2 meters. Utilizing the compressive stress formed by the dredged gravel inside the steel dam, the steel sheet piles on both sides are clamped by steel tie rods in the steel dam body. Under the action of compressive stress, a certain tensile stress is generated in the steel dam body. Through the combination of compressive and tensile stress, a gravity steel dam is formed to resist water pressure.

[0086] By using quantitative height difference control and stress transfer design, the deficiencies of insufficient closed-loop structure and shallow steel sheet pile embedment are compensated for, thereby improving the overturning resistance of the cofferdam. Based on the mechanical equilibrium formula H=k·(γ_w / γ_s)·h, where H is the internal filling height of the steel dam body; k is the mechanical equilibrium safety factor (taken as 1.3); γ_w is the natural unit weight of water (taken as 10kN / m³); γ_s is the natural unit weight of the filling gravel (taken as 19kN / m³); and h is the current working water depth. Considering construction feasibility, a stable height difference of 2.0m higher than the outer side is determined; the self-weight of the internal gravel generates vertical compressive stress (σ=19×2=38kPa), which is transferred to the steel sheet piles on both sides through two horizontal steel tie rods, and is transformed into horizontal tensile stress T=38×0.8=30.4kN (stress transfer efficiency coefficient taken as 0.8), forming a self-balancing system of compressive stress-tensile stress-steel sheet pile reaction force.

[0087] S3. Opening and closing.

[0088] After the downstream filling reaches the predetermined height, the cofferdam opening is closed by progressively stacking sandbags in layers, and geotextile is laid simultaneously for seepage prevention.

[0089] S4. Conversion construction.

[0090] like Figure 9 After the steel dam structure is completed and the upstream section is closed, the sand transportation method will be changed from sand dredgers on the river to inland vehicles, and backfilling operations will be carried out in conjunction with excavators to gradually form the island cofferdam. The sandbag cofferdam at the downstream opening will be stacked in layers in sync with the backfilling height. The sandbags used for intercepting sand do not need to be removed; they can be directly covered with gravel. The construction effect is as follows: Figure 10 This optimized sand transportation method not only significantly accelerates the island reclamation construction progress but also saves on the rental costs of sand transport vessels and dredging vessels. Backfilling is carried out in layers, with each layer controlled to a thickness of 0.8m. After leveling and initial compaction by an excavator, a vibratory roller is used for final compaction to ensure the backfill soil meets the required density.

[0091] Verification of construction results and technological advantages:

[0092] The cofferdam involved in this invention is designed for high-level flood control and adopts an open-type layout to adapt to the needs of large-area, long-linear construction sites. The structural form is a composite structure consisting of double-row steel sheet piles filled with sand and gravel between the piles, sand and gravel dredged inside the cofferdam, and riprap placed outside the cofferdam. The construction faces complex geological and hydrological conditions. To address the construction difficulties, a special study on the key construction techniques of open-type shallow-buried steel structure dam island cofferdams has been conducted, resulting in four core technological achievements. These achievements have been verified through engineering practice and have yielded significant construction results, providing a mature and reliable construction method for similar projects. The details are as follows:

[0093] (1) Steel sheet pile driving technology under hydraulic balance guidance.

[0094] By reserving ports to maintain dynamic balance of internal and external water levels and avoid unilateral stress, and in conjunction with synchronous filling control between piles, inside and outside the weir, the problems of insufficient shallow anchorage and uneven initial stress are solved, and the dam body is stabilized and self-supporting. Practical verification shows that the maximum horizontal displacement in the initial stage after the steel sheet piles are driven is ≤8mm, which completely solves the industry problem of insufficient shallow anchorage.

[0095] (2) Tidal coordinated layered filling technology.

[0096] By optimizing the timing and cycle of filling in accordance with tidal patterns, the initial filling is carried out at appropriate times, and the filling is advanced in layers with sufficient time reserved to utilize the tidal effect to achieve self-compaction of the gravel layer. At the same time, the self-compaction effect is enhanced by optimizing the material gradation, reducing the reliance on additional compaction equipment and effectively improving the filling quality and construction efficiency. In actual construction, the density of the gravel layer after tidal self-compaction is ≥85%, the subsequent settlement is ≤3%, and the construction efficiency is 35% higher than that of traditional processes.

[0097] (3) Construction technology of compression-tension composite stress system.

[0098] By rationally controlling the filling height difference between the inside and outside of the dam body, and the compressive stress generated by the self-weight of the internal filling material is converted into tensile stress through steel tie rods, a self-balancing force system is formed. This effectively compensates for the deficiencies of insufficient closed loop in the open structure and the shallow embedment depth of the steel sheet piles, significantly improving the overturning resistance of the cofferdam and enabling it to withstand complex water pressure and high-level floods. After the system is formed, the maximum horizontal displacement of the cofferdam is ≤30cm, and the overturning resistance is improved by 38% compared with the traditional process.

[0099] (4) Core process and technical details of standardized cofferdam construction.

[0100] The entire process of cofferdam construction was clearly defined and standardized. First, long-term tidal monitoring and geological surveys were conducted to prepare the groundwork. Then, steel sheet piles were driven from upstream to downstream, with pre-reserved openings. Filling was carried out in three phases: initial, development, and stabilization, with progressive sandbag sealing to close the opening. Finally, the sand transportation method was changed at the appropriate time, and onshore filling and compaction techniques were used to complete the cofferdam's formation. In practice, the change in sand transportation method significantly reduced costs associated with maritime operations such as ship leasing, and the overall construction period was shortened by 20% compared to traditional methods.

[0101] This invention, through technological innovation and process optimization, effectively solves the core technical problems in the construction of open-type shallow-buried steel dam island cofferdams, such as uneven stress, insufficient anchorage, low filling density, high construction costs, and weak overturning resistance. It significantly improves the self-support and stability of the dam body while also ensuring economic efficiency and high performance. The specific implementation method of this invention has clear process parameters, quantifiable formulas, and controllable operation. The related technical solutions are fully adaptable to complex geological and hydrological conditions, avoiding many drawbacks of traditional construction methods. It can be directly applied to open-type shallow-buried steel dam island cofferdam projects in water conservancy and municipal engineering projects with various large spans and long linear construction sites, providing a safe, reliable, economical, efficient, replicable, and feasible construction paradigm for similar projects.

[0102] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A construction method for improving the self-support and stability of an open-type shallow-buried steel dam by constructing an island cofferdam, characterized in that... Includes the following steps: S1. When constructing the island cofferdam of the steel dam, first drive double rows of Larssen steel sheet piles. Except for the upstream and downstream ends which are reserved for the time being, all other parts are driven in place. S2. The dredging and sand filling operation first involves dredging the steel dam body in layers. Under the premise of maintaining the stress balance between the inside and outside of the steel dam body, the outer side facing the river is simultaneously filled with riprap in layers, and the inner side is filled with sand and gravel in layers. When the three are backfilled, the filling inside the steel dam body is kept higher than the inner and outer sides. S3. After the steel dam body is formed and the upstream section is closed, sandbags will be used to close the open section of the downstream cofferdam layer by layer. S4. After the steel dam body is formed and the upstream section is closed, the sand transportation method will be changed from sand dredging ships on the river to inland vehicles, and backfilling operations will be carried out in conjunction with excavators to gradually complete the formation of the entire island cofferdam.

2. The construction method for improving the self-support and stability of an open-type shallow-buried steel dam by constructing an island cofferdam according to claim 1, characterized in that: In step S1, when driving double-row Larssen sheet piles, only the upstream and downstream ends are reserved and not driven for the time being. The reserved port in the upstream section allows river water to flow naturally into the interior of the sheet pile and the inner island construction area. By relying on the natural balance of the water level, the sheet pile is evenly stressed inside and outside, avoiding the displacement of the pile body caused by unilateral stress.

3. The construction method for improving the self-support and stability of an open-type shallow-buried steel dam by constructing an island cofferdam according to claim 1, characterized in that: Step S2 includes: S21 initial stage, S22 development stage, and S23 stabilization stage; the filling operation of the steel structure dam island cofferdam is carried out in layers from upstream to downstream, so that the river water in the cofferdam can be discharged in the direction of downstream flow, avoiding local water accumulation that would cause uneven stress on the dam body, and ensuring the overall stability of the dam body during the filling process.

4. The construction method for improving the self-support and stability of an open-type shallow-buried steel dam by constructing an island cofferdam according to claim 3, characterized in that: In step S21, the initial backfilling operation of the dredged sand inside the steel dam body is matched with the tidal variation of the river. The initial filling is carried out during the period of lowest ebb tide. The first filling is carried out to half the height of the lowest ebb tide to form a preliminary self-supporting capacity. Subsequent filling is carried out in layers to the lowest ebb tide, and the height of each filling is not greater than 1m.

5. The construction method for improving the self-support and stability of an open-type shallow-buried steel dam by constructing an island cofferdam according to claim 4, characterized in that: In step S21, after the steel dam body is filled to the preset height, the outer side facing the river is simultaneously filled with riprap and the inner side is filled with sand and gravel. The width and height of the sand and gravel filled on the inner side are consistent with those of the riprap on the outer side, so as to achieve coordinated stress between the outer toe protection and the inner filling of the dam body and ensure the overall balance of the dam body.

6. The construction method for improving the self-support and stability of an open-type shallow-buried steel dam by constructing an island cofferdam according to claim 3, characterized in that: In step S22, the dredged sand inside the steel dam body is backfilled in layers. During the development period, the height of each layer is controlled and the backfilling is carried out at intervals of the complete tidal cycle. By utilizing the hydrological characteristics of tidal rise and fall, the sand and gravel layer is naturally compacted through river water infiltration and seepage discharge, reducing the reliance on manual compaction equipment. After the backfilling is completed during the development period, the reserved gap upstream of the cofferdam is closed.

7. The construction method for improving the self-support and stability of an open-type shallow-buried steel dam by constructing an island cofferdam according to claim 3, characterized in that: In step S23, when the filling height inside the steel dam body is at the same level as the normal water level on the outside and inside sides, the method of prioritizing internal filling is switched. After the internal filling height is 2m higher than the outside and inside sides, the filling on the outside and inside sides is carried out simultaneously and the height difference is maintained. The compressive stress formed by the self-weight of the internal gravel is transferred to the tensile stress of the steel sheet piles through the steel tie rods between the double rows of Larssen steel sheet piles, so that the dam body forms a gravity structure with combined compressive and tensile forces to resist the action of water pressure.

8. The construction method for improving the self-support and stability of an open-type shallow-buried steel dam by constructing an island cofferdam according to claim 3, characterized in that: Before the filling begins in step S21, multiple sand-blocking sandbags are set up in the downstream area of ​​the cofferdam. The sand-blocking sandbags are arranged in a staggered manner in layers to effectively intercept the sand and gravel that drift with the river water when the dredging and filling operation is carried out upstream.

9. The construction method for improving the self-support and stability of an open-type shallow-buried steel dam by constructing an island cofferdam according to claim 1, characterized in that: In step S4, after the steel dam body is formed and the upstream section is closed, the sand transportation method is changed from sand blowing boats on the river to inland vehicles. In conjunction with excavators, layered backfilling operations are carried out to gradually complete the formation of the entire island cofferdam.

10. The construction method for improving the self-support and stability of an open-type shallow-buried steel dam by constructing an island cofferdam according to claim 1, characterized in that, Includes the following steps: V0. Preliminary preparation: Continuously monitor the tidal cycle and water level changes and draw tidal curves, investigate the engineering geological distribution, and determine the relevant construction parameters for steel sheet pile driving and filling material gradation; S1. Steel Sheet Pile Driving: Drive double rows of Larssen steel sheet piles, leaving only the upstream and downstream ends undriven for the time being, and drive the remaining parts in place; drive in the order from upstream to downstream, and after driving is completed, inspect the axis deviation, verticality and leakage. After the standards are met, proceed to the next construction stage. S2. Phased filling: S21. Initial stage: After the steel sheet piles have passed the acceptance test, sand-blocking sandbags are first laid out in a staggered manner in the downstream area of ​​the cofferdam. When carrying out the dredging and sand filling operation inside the steel dam, construction is carried out during the period of lowest water level during low tide. Sand is blown and water is squeezed in layers to fill to the lowest water level of the river, and the height of each filling is not greater than 1m. The filling is carried out in the order from upstream to downstream. During the dredging process, the downstream sand-blocking sandbags are used to intercept the sand and gravel that drift with the river water to reduce the loss of filling material. S22. Development Phase: When the filling height of the inner and outer sides of the steel dam body is equal to the normal water level, maintain the basic balance of the height difference among the three, and continue to fill the dam layer by layer from upstream to downstream by blowing sand and squeezing water, controlling the height of each layer and advancing at intervals of complete tidal cycles, and using the ebb and flow of tides to achieve natural compaction of the gravel layer; during the filling process, check the downstream sandbag placement status, and adjust and reinforce it in time if there is any displacement; After the development phase filling is completed, the reserved gap upstream of the cofferdam will be closed, the steel sheet pile driving operation of the remaining steel structure dam body in the area will be completed, and the outer side of the dam body will be filled with boulders and the inner side will be filled with sand and gravel. S23. Stabilization period: Carry out filling operations on the inside, outside and inside sides of the steel dam body, giving priority to raising the height of the inside filling so that the height of the inside filling is higher than that of the outside and inside by a certain value and this height difference is maintained continuously; the compressive stress formed by the self-weight of the internal gravel is converted into tensile stress of the steel sheet piles through the steel tie rods between the double rows of Larssen steel sheet piles, so that the steel dam body forms a gravity structure with combined compressive and tensile forces to resist the action of water pressure; S3. Closure of the opening: After the steel structure dam body is formed and the upstream closure is completed, the opening of the downstream cofferdam is closed by progressively stacking sandbags in layers. Geotextile is laid simultaneously for seepage prevention. S4. Conversion Construction: After the steel dam body is formed and the upstream section is closed, the sand transportation method will be changed from sand blowing boats on the river to inland vehicles. Backfilling operations will be carried out in conjunction with excavators to promote the gradual formation of the island cofferdam. The sandbag cofferdam at the downstream opening is stacked in layers in sync with the backfill height; the sandbags for interception do not need to be removed, and are directly covered with filling gravel.

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