Hard clay layer steel pipe pile cofferdam construction method

Abstract

This invention relates to the field of bridge construction technology and provides a method for constructing a steel pipe pile cofferdam in a hard clay layer, comprising the following steps: S1, driving steel pipe piles at the designed location to form a cofferdam; S2, determining the ratio of sodium silicate solution and oxalic acid solution injected into the support layer and the Baumé degree of the sodium silicate solution based on the pH value of the hard clay layer, the compressive strength required for the underlying support layer during bridge abutment construction, and the shortest time for the support layer to maintain this compressive strength; wherein the support layer is the hard clay layer from below the abutment to the bottom of the steel pipe piles; S3, injecting oxalic acid solution and sodium silicate solution sequentially into the support layer at a predetermined location within the cofferdam according to the stated ratio. This method utilizes the inherent high bearing capacity of the hard clay layer, and by injecting sodium silicate solution and oxalic acid solution of predetermined concentration and ratio, its compressive strength is improved, enabling it to possess sufficient bearing capacity for a period of time to withstand the construction load of the abutment.

Description

Technical Field

[0001] This invention relates to the field of bridge construction technology, specifically to a method for constructing a steel pipe pile cofferdam in a hard clay layer. Background Technology

[0002] In modern bridge construction, cofferdams are often used for the construction of pier and abutment foundations in rivers, lakes, and seas. With the development and improvement of science, technology, and construction materials, steel cofferdams have gradually become the preferred construction method for bridge cofferdam construction due to their excellent bending resistance, good airtightness, and mature and stable construction technology. In steel cofferdam construction, depending on the characteristics of the bridge construction and the water depth, steel pipe pile cofferdams and steel sheet pile cofferdams are frequently used. The main principle is to drive a ring of steel pipe piles or steel sheet piles at the bridge pier location, connecting the piles with welding or interlocking to form a good sealed space. Then, the water inside the cofferdam is pumped out, creating a dry construction environment that meets the construction conditions for the bridge piers and abutments.

[0003] Taking a steel pipe pile cofferdam as an example, during cofferdam construction, the steel pipe piles are mainly subjected to forces from two directions. First, the horizontal thrust caused by the pressure difference between the inside and outside of the cofferdam causes it to tend to collapse inwards. Second, the buoyancy of the water on the outside and the vertical pull-out force caused by the upward piping of the soil and water causes it to tend to rise upwards. For the first type of external force, the horizontal force is often counteracted by adding internal supports and walers during construction. For the second type of external force, it is necessary to calculate the embedment depth of the steel pipe piles to ensure that they have sufficient lateral friction, relying on their own weight and lateral friction as vertical pull-out resistance. In addition, a layer of sealing concrete is often injected at the bottom of the steel pipe piles during construction. This concrete binds itself to the surrounding soil and the steel pipe piles, relying on its own weight and lateral friction to improve the pull-out resistance of the steel pipe piles. Simultaneously, the solidified bottom layer of concrete can also serve as a construction platform to support the construction load of the pier foundation. However, the sealing concrete also needs to take into account various factors such as construction temperature and geological conditions, which has significant limitations.

[0004] For geological formations where hard clay layers and water-rich fine sand layers alternate, if the cofferdam steel pipe piles need to penetrate multiple layers of hard clay and fine sand and finally be fixed in the hard clay layer, the conventional construction process involves excavating the soil inside the cofferdam after the steel pipe piles are driven to the designated elevation, then pouring a layer of concrete at the bottom of the cofferdam to seal it. The concrete covers the bottom of all the steel pipe piles, and after the concrete solidifies, it consolidates with the surrounding soil. The resulting consolidated body can serve as a direct force transmission path for the steel pipe piles, and its own weight can help the steel pipe piles resist upward pull-out forces. The consolidated body itself also has sufficient strength to provide support for the tying of the pier foundation reinforcement and the erection of formwork. However, in actual construction, due to the dense structure of hard clay, it is not easy to form a solid whole when directly penetrated into concrete; at the same time, because concrete disrupts the soil-water balance at the bottom, it is easily carried away by fine sand and water flow, and it is not easy to solidify in the designated position; finally, if the project is close to the winter construction period, the concrete is more easily disturbed due to temperature stress, and it cannot solidify quickly under relatively low temperature conditions, thus failing to achieve the intended construction effect. Summary of the Invention

[0005] One of the objectives of this invention is to provide a construction method for steel pipe pile cofferdams in hard clay layers, which can enable the construction of steel pipe pile cofferdams relatively quickly in hard clay layer environments and shorten the construction period.

[0006] To achieve the above objectives, the present invention provides a method for constructing a steel pipe pile cofferdam in a hard clay layer, comprising the following steps:

[0007] S1. Drive steel pipe piles at the designed location to form a cofferdam;

[0008] S2. Based on the pH value of the hard clay layer, the compressive strength required for the supporting layer below during bridge abutment construction, and the shortest time for the supporting layer to maintain the compressive strength, determine the ratio of sodium silicate solution and oxalic acid solution injected into the supporting layer and the Baumé degree of the sodium silicate solution, wherein the supporting layer is the hard clay layer from below the abutment to the bottom of the steel pipe pile.

[0009] S3. At a predetermined location within the cofferdam, oxalic acid solution and sodium silicate solution are injected sequentially into the support layer according to the stated ratio to enhance the compressive strength of the support layer.

[0010] Preferably, before determining the ratio of sodium silicate solution and oxalic acid solution, a compressive strength test needs to be conducted on the hard clay layer, including the following steps:

[0011] S201. Select 12 sample points at the bottom of the cofferdam according to the grouting sequence. These points are located at the four corners of the cofferdam, the midpoints of the four sides of the cofferdam, and four sampling points evenly distributed in the middle of the cofferdam. Sampling is carried out at a depth of 1m.

[0012] S202. Cut 12 soil samples into cylindrical test blocks with a diameter of 50 mm and a height-to-diameter ratio of 2:1.

[0013] S203. The constant stress flexural and compressive strength integrated machine was used to conduct compressive strength tests on each specimen and determine its compressive strength.

[0014] Preferably, all test blocks are tested under the same ambient temperature and humidity, and the total test time is controlled within one hour to eliminate errors caused by environmental changes.

[0015] Preferably, the concentration of oxalic acid in the oxalic acid solution is 75%.

[0016] Preferably, determining the ratio of sodium silicate solution and oxalic acid solution based on the pH value of the hard clay layer includes the following steps:

[0017] S211. In the hard clay layer, select 6 different locations as sampling points, and make 6 groups of samples, with 5 samples in each group;

[0018] S212. Oxalic acid solution and sodium silicate solution are injected into the 6 groups of samples in sequence, wherein the volume ratio of sodium silicate solution to oxalic acid solution in each group of 5 samples is 0.5, 0.75, 1, 1.25 and 1.5 respectively.

[0019] S213. Measure the pH value of the sample after grouting, and select the ratio of sodium silicate solution and oxalic acid solution corresponding to the required pH value.

[0020] Preferably, determining the Baumé degree of the sodium silicate solution includes the following steps:

[0021] S221. In the hard clay layer, select 6 different locations as sampling points, and make 6 groups of samples, with 6 samples in each group;

[0022] S222. Oxalic acid solution and sodium silicate solution in a 1:1 ratio are injected into the 6 groups of samples, wherein the Baume degrees of sodium silicate solution in the 6 groups of 6 samples are 36, 38, 40, 42, 44 and 46 respectively.

[0023] S223. Prepare each sample into a cylindrical specimen with a diameter of 50 mm and a height-to-diameter ratio of 2:1 for compressive strength testing, and select the Baumé degree of the sodium silicate solution corresponding to the required compressive strength.

[0024] Preferably, the cofferdam is rectangular and is divided into three equal regions along its length. The preset position includes the center point of the three regions and grouting points evenly distributed on the sides.

[0025] Preferably, when injecting oxalic acid solution and sodium silicate solution into the support layer, the grouting sequence is performed in the order of first the middle and then the perimeter.

[0026] Preferably, before injecting oxalic acid solution and sodium silicate solution into the support layer, the support layer and the preset depth below it are first detected to see if there are any voids. If there are voids, the voids are filled first.

[0027] Preferably, after injecting oxalic acid solution and sodium silicate solution into the support layer, six samples of the support layer are taken every three days for compressive strength testing to detect the time it takes to maintain the required compressive strength.

[0028] Based on the above description and practice, the steel pipe pile cofferdam construction method for hard clay layers described in this invention utilizes the high bearing potential of the hard clay layer itself. By injecting sodium silicate solution and oxalic acid solution of preset concentration and ratio, the structural density of the hard clay layer is increased, thereby improving its compressive strength. This allows it to have sufficient bearing capacity for a period of time to withstand the construction load of the cofferdam pier. Therefore, subsequent construction work can be carried out directly without the need for bottom sealing concrete, saving the construction period, improving the economic benefits of the project, and solving the construction problem that bottom sealing concrete cannot effectively exert its strength under special geological conditions of hard clay layers and water-rich fine sand layers. At the same time, this construction technology is not limited by winter construction and can effectively ensure the smooth progress of the project. Attached Figure Description

[0029] Figure 1 This is a flowchart illustrating a construction method for a steel pipe pile cofferdam in a hard clay layer, as described in one embodiment of the present invention.

[0030] Figure 2 This is a schematic diagram showing the distribution of grouting points in a cofferdam according to one embodiment of the present invention.

[0031] Figure 3 This is a graph showing the relationship between load and time in the compressive strength test of hard clay samples 1-6 in one embodiment of the present invention.

[0032] Figure 4 This is a graph showing the relationship between load and time in the compressive strength test of hard clay samples 7-12 in one embodiment of the present invention.

[0033] Figure 5 These are the compressive strength values ​​of various groups of samples at different Baumé degrees in one embodiment of the present invention.

[0034] Figure 6 This is a graph showing the relationship between the compressive strength of the hard clay layer after grouting and the time required in one embodiment of the present invention. Detailed Implementation

[0035] Exemplary embodiments will now be described more fully with reference to the accompanying drawings. However, exemplary embodiments can be implemented in many forms and should not be construed as limited to the examples set forth herein; rather, these embodiments are provided so that this disclosure will be more comprehensive and complete, and will fully convey the concept of exemplary embodiments to those skilled in the art. The described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

[0036] Furthermore, the accompanying drawings are merely illustrative of this disclosure and are not necessarily drawn to scale. The same reference numerals in the drawings denote the same or similar parts, and therefore repeated descriptions of them will be omitted. It should be noted that in this disclosure, the terms "comprising," "configured with," and "set in" are used to indicate an open-ended meaning of inclusion, and refer to the presence of additional elements / components / etc. besides those listed.

[0037] Unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to fixed connections, detachable connections, or integral connections; they can refer to mechanical connections or electrical connections; they can refer to direct connections or indirect connections through an intermediate medium; and they can refer to the internal connection between two components. Those skilled in the art will understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0038] In this embodiment, a construction method for steel pipe pile cofferdams in hard clay layers is disclosed. This method utilizes the high strength and bonding properties of the hard clay layer itself at the construction site, and takes measures to improve the performance of the hard clay layer at the bottom of the cofferdam underwater, so that it can support the construction of the pile cap, thereby achieving the goal of directly constructing the pile cap without the need for concrete sealing. Figure 1 The flowchart of the construction method for the steel pipe pile cofferdam in the hard clay layer is shown, mainly including the following steps S1-S3:

[0039] Step S1: Drive steel pipe piles at the designed location to form a cofferdam. The steel pipe piles are placed around the perimeter of the foundation. After the cofferdam is formed, the water inside can be pumped out to create construction space for the foundation construction.

[0040] Step S2: Based on the pH value of the hard clay layer at the cofferdam location, the compressive strength required for the supporting layer below during bridge abutment construction, and the shortest time for the supporting layer to maintain the compressive strength, determine the ratio of sodium silicate solution and oxalic acid solution injected into the supporting layer and the Baumé degree of the sodium silicate solution. The supporting layer is the hard clay layer from below the abutment to the bottom of the steel pipe pile.

[0041] Step S3: At predetermined locations within the cofferdam, inject oxalic acid solution and sodium silicate solution sequentially into the support layer according to the stated ratio to enhance the compressive strength of the support layer. This step effectively improves the performance of the hard clay layer at the bottom of the cofferdam, enabling it to support the foundation construction and allowing for direct foundation construction without the need for concrete sealing.

[0042] Specifically, before determining the ratio of sodium silicate solution to oxalic acid solution, a compressive strength test needs to be conducted on the hard clay layer, including the following steps:

[0043] Step S201: Select 12 sample points at the bottom of the cofferdam according to the grouting sequence. These points are located at the four corners of the cofferdam, the midpoints of the four sides of the cofferdam, and four sampling points evenly distributed in the middle of the cofferdam. Sampling is carried out at a depth of 1m.

[0044] Step S202: Cut the 12 sample soil blocks into cylindrical test blocks with a diameter of 50 mm and a height-to-diameter ratio of 2:1.

[0045] Step S203: Use a constant stress flexural and compressive strength integrated machine to conduct compressive strength tests on each specimen and determine its compressive strength.

[0046] By testing the compressive strength of this section of hard clay, it can be determined whether it can replace concrete sealing to support the upper foundation construction. For example, if the minimum compressive strength required for the lower support layer in foundation construction is 30 MPa, and the tested compressive strength of the hard clay layer is 25 MPa, the difference is small. After injecting oxalic acid solution and sodium silicate solution, the performance of the hard clay layer can be significantly improved, making its compressive strength sufficient to support the upper foundation construction. Conversely, if the tested compressive strength of the hard clay layer is much lower than the minimum compressive strength required for the lower support layer in foundation construction, then grouting the hard clay layer cannot replace concrete sealing.

[0047] Furthermore, in this embodiment, all 12 test blocks for the compressive strength test of the hard clay layer were tested under the same ambient temperature and humidity, and the total test time was controlled within one hour to eliminate errors caused by environmental changes. This ensures that the final test data is more accurate.

[0048] Furthermore, in this embodiment, the concentration of oxalic acid in the oxalic acid solution injected into the hard clay layer is 75%. Since oxalic acid is mainly used to alter the original structure of the water-rich fine sand layer and the hard clay layer, adjust the soil pH, loosen the soil to allow it to fully integrate with the steel pipe piles, and simultaneously extract some metal salt ions, creating an environmental basis for the subsequent grouting process to produce precipitates, an appropriate oxalic acid concentration is sufficient. Actual testing has determined that an oxalic acid concentration of 75% achieves the best grouting effect.

[0049] Furthermore, based on the pH value of the hard clay layer, the determination of the ratio of sodium silicate solution and oxalic acid solution includes the following steps:

[0050] Step S211: In the hard clay layer, select 6 different locations as sampling points, and make 6 groups of samples, with 5 samples in each group;

[0051] Step S212: Oxalic acid solution and sodium silicate solution are injected into the 6 groups of samples in sequence, wherein the volume ratio of sodium silicate solution to oxalic acid solution in each group of 5 samples is 0.5, 0.75, 1, 1.25 and 1.5 respectively;

[0052] Step S213: Measure the pH value of the sample after grouting, and select the ratio of sodium silicate solution to oxalic acid solution corresponding to the required pH value.

[0053] Because oxalic acid solution is acidic, its injection will lower the pH value of the hard clay layer. Therefore, it is necessary to control the ratio of sodium silicate solution to oxalic acid solution to ensure that the pH value of the hard clay layer does not change significantly after the injection of these two solutions. By following these steps, the optimal ratio of sodium silicate solution to oxalic acid solution can be determined.

[0054] In addition, in this embodiment, determining the Baumé degree of the sodium silicate solution includes the following steps:

[0055] Step S221: In the hard clay layer, select 6 different locations as sampling points, and make 6 groups of samples, with 6 samples in each group;

[0056] Step S222: Inject oxalic acid solution and sodium silicate solution in a 1:1 ratio into the 6 groups of samples, wherein the Baumé values ​​of the sodium silicate solution in the 6 groups of 6 samples are 36, 38, 40, 42, 44 and 46 respectively.

[0057] Step S223: Prepare each sample into a cylindrical specimen with a diameter of 50 mm and a height-to-diameter ratio of 2:1 for compressive strength testing, and select the Baumé degree of the sodium silicate solution corresponding to the required compressive strength.

[0058] These steps will determine which Baume degrees of sodium silicate solution can achieve the required minimum compressive strength.

[0059] Furthermore, in this embodiment, the cofferdam is rectangular, divided into three equal regions along its length. The aforementioned preset locations include the center point of each of the three regions and grouting points evenly distributed along its sides. Grouting at these locations ensures a relatively uniform distribution of the oxalic acid solution and sodium silicate solution in the hard clay layer, guaranteeing that the compressive strength at each location does not vary significantly, thus forming a relatively stable support layer.

[0060] Furthermore, when injecting oxalic acid solution and sodium silicate solution into the support layer, the grouting sequence should be performed from the center to the perimeter. This can further improve the uniformity of the distribution of oxalic acid solution and sodium silicate solution in the hard clay layer.

[0061] Furthermore, in this embodiment, before injecting oxalic acid solution and sodium silicate solution into the support layer, it is necessary to first detect whether there are voids in the support layer and at a predetermined depth below it. If voids are found, they need to be filled first. This ensures that the hard clay layer can ultimately form a stable support structure.

[0062] In addition, in this embodiment, after injecting oxalic acid solution and sodium silicate solution into the support layer, six samples of the support layer are taken every three days for compressive strength testing to detect the time it takes to maintain the required compressive strength. Only when this time is greater than the time required for constructing the foundation can the above method and proportion be used to improve the compressive strength of the hard clay layer; otherwise, the above method should not be used to replace concrete sealing, or the above proportion needs to be further adjusted to further improve the time the hard clay layer maintains its improved compressive strength.

[0063] The following example illustrates the construction method of the steel pipe pile cofferdam in the clay layer.

[0064] First, the geological conditions of the location of the steel pipe pile cofferdam in this construction project will be explained.

[0065] The geological conditions at the location of the steel pipe pile cofferdam primarily consist of silt, fine sand, and hard clay layers. The silt layer is mainly distributed on the riverbed surface, while the underlying soil layers are predominantly fine sand and hard clay, with these two soil layers generally alternating. According to the construction design, the steel pipe piles for the cofferdam must ultimately be fixed within the hard clay layer. Furthermore, due to construction load disturbances, the fine sand and hard clay layers do not have a clear boundary in some areas. Directly injecting concrete to seal the bottom in this environment would result in the concrete being impacted and flowed by sand and gravel and the turbulent flow at the bottom, failing to achieve the intended sealing effect. Simultaneously, the relatively dense structure of the hard clay cannot form a stable and effective bond with the concrete, thus compromising the sealing quality of the cofferdam bottom. After dewatering, this could leave behind safety and quality hazards such as bottom water and sand inrush, affecting construction safety and progress. During winter construction, concrete is difficult to solidify and its strength increases slowly, which cannot meet the construction cycle requirements. Furthermore, its semi-liquid state makes it easy for water flow to impact and displace it, which cannot effectively help steel pipe piles transmit stress. Consequently, it cannot help steel pipe piles resist buoyancy and other pile pulling forces. In fact, it may even increase the water and soil pressure difference inside and outside the cofferdam, bringing additional load to the steel pipe piles of the cofferdam.

[0066] Furthermore, this portion of hard clay exhibits strong cohesion in water, but retains high hardness and compressive strength after water evaporation. Therefore, the hard clay layer itself possesses the potential to directly bear the construction load of the foundation. Consequently, for this project, it was decided to adopt the aforementioned cofferdam construction method using steel pipe piles in a clay layer. This method enhances the performance of the hard clay layer at the bottom of the cofferdam underwater, thereby achieving the goal of directly constructing the foundation without the need for concrete sealing.

[0067] Based on the project's construction plan and schedule, it is necessary to rapidly improve the compressive strength of the soil at the bottom of the cofferdam, ensuring that the improved soil maintains a compressive strength of no less than 30 MPa within 15 days. This is crucial to guarantee that construction processes such as reinforcement binding, formwork erection, and concrete pouring can be completed within a suitable construction period. Following on-site geological exploration and considering the project's geological conditions, it was decided to inject oxalic acid solution and sodium silicate solution, in a specific ratio, into the hard clay layer at the bottom of the cofferdam to improve the soil's mechanical properties.

[0068] The soil in this project is mainly composed of fine, soft sand and gravel and hard clay. Hard clay is a type of refractory clay that is not easily dispersed in water and has low plasticity. Its main constituent minerals are quartz, feldspar, kaolinite, and diaspore, with silica and alumina accounting for over 50%. To improve the soil's mechanical properties, a certain concentration of oxalic acid solution is first injected into the soil layer. This effectively degrades and decomposes the soil. As a strong organic acid, oxalic acid solution reacts with most minerals in the soil to produce various salts such as sodium oxalate, silicates, and calcium oxalate. Simultaneously, it releases a large amount of gas to loosen the soil, helping the bottom clay of the cofferdam to fully contact the steel pipe piles and creating space for the subsequent injection of sodium silicate solution. After the oxalic acid solution has reacted with the bottom soil layer for a certain period, a certain concentration of water-soluble sodium silicate solution is injected according to a predetermined ratio. Sodium silicate solution is an inorganic cementing material with high binding and hardening properties. When injected into the soil, it reacts with various salts to form hard, water-insoluble precipitates such as calcium silicate and magnesium silicate. These precipitates fill the voids in the soil. Secondly, sodium silicate solution hydrolyzes with water to form silica gel. The gel's cementing effect effectively binds soil particles, forming a relatively hard structure and aiding in the bonding between the soil layer and the steel pipe piles, creating a closed construction environment. The soil's own weight also increases the pull-out resistance of the steel pipe piles. Finally, due to its high alkalinity, sodium silicate solution effectively neutralizes excess oxalic acid and other acidic substances in the soil, altering the soil's acid-base environment. The slightly alkaline environment stimulates the electrochemical activity of soil particles, promoting adsorption and cementation between particles and accelerating soil solidification. Finally, a layer of cement mortar is injected on the surface and smoothed, creating a level construction environment for the superstructure. The cement mortar also strengthens the bond between soil particles, further consolidating the soil structure.

[0069] Before grouting, ground-penetrating radar is used to detect any voids in the underlying soil layer. Voids are prioritized for backfilling before grouting to prevent them from becoming channels for water seepage and sand inrush after grouting. Through project testing and field practice, this construction method has proven effective in improving the compressive strength and other mechanical properties of the bottom soil within the cofferdam. Combined with the inherent compressive potential of the hard clay layer, the reinforced soil layer can be used directly as a working platform for the foundation construction without the need for bottom-sealing concrete. This significantly shortens the construction period and improves the project's economic benefits.

[0070] According to the construction plan and design drawings, the construction of the cofferdam pier cap requires a foundation bearing capacity of at least 30 MPa. With a construction period of 10 days, the treated soil needs to maintain a foundation bearing capacity of no less than 30 MPa for 15 days. Therefore, it is necessary to design appropriate grouting concentrations and ratios of sodium silicate and oxalic acid solutions, taking into account the project's specific geological conditions, to ensure the treated foundation meets the requirements. Considering the grout diffusion mechanism, the overall grouting sequence follows the attached... Figure 2 The grouting point location and sequence control described in the figure follow the principle of grouting the center first and then the perimeter to ensure complete soil coverage inside the cofferdam. The cofferdam in the figure is divided into three equal areas along its length. The points evenly distributed at the center and sides of each area are the final grouting points. The serial numbers indicate the location and sequence of the grouting points. Grouting is performed in the order of grouting point ①, grouting point ②, grouting point ③, grouting point ④, and grouting point ⑤, following the principle of grouting the center first and then the perimeter.

[0071] Furthermore, before grouting to improve the performance of hard clay layers, it is necessary to test their compressive strength to verify whether they possess sufficient compressive strength. If the compressive strength before performance improvement is far lower than the required compressive strength, it will be difficult to increase the compressive strength to the required value using the aforementioned methods, and consequently, the methods cannot be practically applied. In addition, this test can also be used to determine specific improvement targets, facilitating the subsequent determination of the ratio of sodium silicate solution to oxalic acid solution.

[0072] Specifically, 12 sample points were selected at the bottom of the cofferdam according to the grouting sequence, located at the four corners of the cofferdam, the midpoints of the four sides of the cofferdam, and grouting point ③ inside the cofferdam (see [reference]). Figure 2 The sampling depth was 1m. Twelve soil samples were cut into cylindrical specimens with a diameter of 50mm and a height-to-diameter ratio of 2:1. A constant stress flexural and compressive strength testing machine was used to uniformly test the specimens and determine the compressive strength of the soil. All specimens were tested under the same ambient temperature and humidity, and the test time was controlled within one hour to eliminate errors caused by environmental changes. Each group consisted of six specimens, divided into two groups. The final test results are shown in Tables 1 and 2 below. Figure 3 and Figure 4 Table 1 shows the experimental compressive strength values ​​of hard clay samples 1-6. Figure 3 Table 1 shows the relationship between load and time in the compressive strength tests of stiff clay samples 1-6. Table 2 shows the compressive strength test values ​​of stiff clay samples 7-12. Figure 4 The figure in the middle shows the relationship between load and time in the compressive strength test of hard clay sample 7-12. The test results show that the average bearing capacity of the hard clay at the bottom of the cofferdam after weathering and hardening can reach 24.4 MPa, indicating high bearing potential. The difference between this and the required compressive strength is small, making the aforementioned grouting method suitable for improving its performance.

[0073] Table 1:

[0074]

[0075] Table 2:

[0076]

[0077] Before grouting, it is necessary to determine the concentration and ratio of sodium silicate solution and oxalic acid solution.

[0078] Specifically, since oxalic acid solution is mainly used to change the original structure of water-rich fine sand layer and hard clay layer, adjust soil pH value, loosen soil so that it can fully combine with steel pipe pile, and extract some metal salt ions to create an environmental basis for the next step of grouting to produce precipitates, the concentration of oxalic acid can be within a suitable range. The project laboratory determined that an oxalic acid concentration of 75% can achieve the best grouting effect.

[0079] The higher the modulus of sodium silicate in a solution, the higher its viscosity and strength, but the lower its solubility in water. Sodium silicate solutions are often expressed using density or Baumé degree (or similar measurement) to indicate their sodium content. A higher Baumé degree indicates higher viscosity and strength, but also lower solubility in water. Conversely, a lower Baumé degree indicates better solubility in water, but also lower viscosity and strength. To ensure that sodium silicate is both easily soluble in water and has sufficient fluidity for grouting, while also possessing high strength, the project laboratory used sodium silicate with a Baumé degree range of 36-46 to prepare a sodium silicate solution for soil improvement tests.

[0080] According to relevant hydrological data and on-site testing, the pH value of the river water and soil in the project area ranges from 8.54 to 9.52. After injecting oxalic acid solution into the soil, the soil pH value reached 5.64, indicating an overall acidic environment, which does not meet environmental protection requirements. Furthermore, the acidic environment reduces the electrochemical activity of soil particles, hindering particle cementation and adsorption. Therefore, the ratio of sodium silicate solution to oxalic acid solution needs to be determined based on the pH value of the treated soil. Specifically, soil samples were taken from different locations in the hard clay layer, and six groups of five samples were prepared. The pH value of the treated soil was measured according to a sodium silicate solution to oxalic acid solution ratio of 0.5-1.5, as shown in Table 3. Based on the experimental results, a 1:1 ratio of sodium silicate solution to oxalic acid solution can meet the grouting requirements, creating a slightly alkaline environment without excessively altering the pH value of the river water and soil.

[0081] Table 3:

[0082] Soil pH value of sample group 1 6.13 7.05 8.41 10.05 11.22 Soil pH values ​​of sample 2 groups 5.99 7.13 8.61 10.32 11.65 Soil pH values ​​of sample 3 groups 5.87 7.22 8.68 10.98 11.58 Soil pH values ​​of 4 sample groups 6.05 6.99 8.72 9.79 11.98 Soil pH values ​​of 5 sample groups 6.34 7.35 8.32 10.16 11.05 Soil pH values ​​of 6 sample groups 6.13 7.51 8.18 10.01 11.34 Mean value of soil pH measurement 6.09 7.21 8.49 10.22 11.47

[0083] After determining the two-component ratio, it is necessary to conduct experiments to determine the Baumé degree of the sodium silicate solution. Specifically, experiments were conducted on sodium silicate solutions with a Baumé degree range of 36-46, divided into 6 groups, with 6 samples in each group. After two-component grouting treatment, cylindrical specimens with a diameter of 50 mm and a height-to-diameter ratio of 2:1 were also prepared for compressive strength testing. The test results are shown in Table 4 below. Figure 5 , Figure 5 The values ​​in Table 4 represent the compressive strength of each group of samples at different Baume degrees. In Table 4, the serial numbers 1-1, 2-1, 3-1, 4-1, 5-1, and 6-1 represent the six samples in the first group, with the last digit of the serial number indicating the group number.

[0084] Table 4:

[0085]

[0086]

[0087] The experimental results show that the soil strength after grouting does not increase linearly with the increase of the Baumé degree of the sodium silicate solution; the maximum value occurs between 38 and 40 Baumé degrees. At lower Baumé degrees, increasing the Baumé degree leads to higher viscosity and strength of the water glass solution. However, as the Baumé degree continues to increase, the water solubility of sodium silicate decreases, preventing the complete precipitation of silicate ions, which is detrimental to the precipitation of metal particles in the soil layer, thus reducing the soil improvement effect. At a Baumé degree of 38, the water glass solution is sufficient to improve the soil strength to meet the requirements of subsequent construction, and the water solubility of sodium silicate is also satisfactory. Therefore, the project ultimately decided to use a 1:1 mixture of sodium silicate solution with a Baumé degree of 38 and 75% oxalic acid solution for two-component grouting to improve the performance of the soil layer at the bottom of the cofferdam, achieving the goal of completing subsequent construction without the need for bottom sealing concrete.

[0088] When using the above-mentioned grouting method to improve the soil quality at the bottom of the cofferdam, the grouting depth is 6 meters from the bottom of the foundation to the bottom of the steel pipe pile. Before grouting, check for any voids at the bottom, fill the voids, and then begin grouting. The grouting points and sequence are as follows: Figure 2 As shown, ensure complete coverage of the soil layer at the bottom of the cofferdam. After the two-component grouting is completed, take 6 soil samples every three days for compressive strength testing. The test results are as follows. Figure 6 As shown, the hard clay layer has relatively stable compressive strength after grouting. The compressive strength only shows a significant decrease 10 days after grouting, and the compressive strength is still mostly above 30MPa after 16 days of grouting. The standard construction period for the pier cap is 10 days. Therefore, the construction technology of improving the compressive strength of the hard clay layer using the above method can fully meet the construction requirements of the pier cap.

[0089] It should be noted that this embodiment is illustrated using a specific construction project as an example. It is based on the unique geological conditions of the project area, characterized by alternating layers of hard clay and water-rich fine sand. Its purpose is to address the practical construction difficulties of the pier and foundation construction in this project, and therefore has certain geological limitations. However, it can provide a novel construction approach for engineering projects with similar geological environments. Furthermore, this solution has achieved excellent construction results after being used in this project, saving construction time and improving economic efficiency.

[0090] Furthermore, the core idea of ​​this scheme is to utilize the oxalic acid solution to react with most of the minerals in the fine sand and hard clay layers at the bottom of the cofferdam, generating various salts such as sodium oxalate, silicates, and calcium oxalate. Simultaneously, a large amount of gas is released, loosening the soil and creating space for the next step of solution injection. After the oxalic acid solution reacts with the bottom soil layer for a certain period, a sodium silicate solution of a specific concentration is injected in proportion. This reacts with various salts to form a precipitation reaction, generating a hard, water-insoluble precipitate that fills the voids in the soil layer. Secondly, sodium silicate can undergo hydrolysis with water to generate silica gel. The gel's gelling effect effectively binds soil particles, forming a relatively hard structure that helps bond the soil layer to the steel pipe piles, creating a closed construction environment. The soil's own weight also increases the pull-out resistance of the steel pipe piles. The sodium silicate solution injected in a suitable ratio neutralizes excess oxalic acid and other acidic substances in the soil, maintaining the soil's original slightly alkaline state. This slightly alkaline environment stimulates the electrochemical activity of soil particles, promoting adsorption and cementation between particles and accelerating soil solidification. Finally, a layer of cement mortar is injected and smoothed on the surface to create a flat construction environment for the superstructure. The cement mortar also strengthens the bond between soil particles, further consolidating the soil. The treated soil layer can maintain a compressive strength of over 30 MPa for at least 15 days, fully meeting the requirements of subsequent pier construction. Therefore, there is no need to pour cofferdam bottom sealing concrete, overcoming the difficulties of winter construction, improving construction efficiency, and reducing construction time.

[0091] Furthermore, using a sodium silicate solution with a Baume degree of 38 and a 75% oxalic acid solution in a 1:1 ratio to prepare the grouting fluid is a feasible solution based on the actual project conditions. The ratio in this solution can also be adjusted according to different geological conditions. For example, replacing the sodium silicate solution with a potassium silicate solution may achieve similar or better results. Similarly, changing the concentration and ratio mentioned above may also achieve similar or better results.

[0092] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention, and no reference numerals in the claims should be construed as limiting the scope of the claims.

Claims

1. A construction method of a hard clay layer steel pipe pile cofferdam, characterized by, Includes the following steps: S1. Drive steel pipe piles at the designed location to form a cofferdam; S2. Based on the pH value of the hard clay layer, the compressive strength required for the supporting layer below during bridge abutment construction, and the shortest time for the supporting layer to maintain the compressive strength, determine the ratio of sodium silicate solution and oxalic acid solution injected into the supporting layer and the Baumé degree of the sodium silicate solution, wherein the supporting layer is the hard clay layer from below the abutment to the bottom of the steel pipe pile. S3. At a predetermined location within the cofferdam, oxalic acid solution and sodium silicate solution are injected sequentially into the support layer according to the stated ratio to enhance the compressive strength of the support layer.

2. The construction method for steel pipe pile cofferdams in hard clay layers as described in claim 1, characterized in that, Before determining the ratio of sodium silicate solution and oxalic acid solution, a compressive strength test needs to be conducted on the hard clay layer, including the following steps: S201. Select 12 sample points at the bottom of the cofferdam according to the grouting sequence. These points are located at the four corners of the cofferdam, the midpoints of the four sides of the cofferdam, and four sampling points evenly distributed in the middle of the cofferdam. Sampling is carried out at a depth of 1m. S202. Cut 12 soil samples into cylindrical test blocks with a diameter of 50 mm and a height-to-diameter ratio of 2:

1. S203. The constant stress flexural and compressive strength integrated machine was used to conduct compressive strength tests on each specimen and determine its compressive strength.

3. The construction method for steel pipe pile cofferdams in hard clay layers as described in claim 2, characterized in that, All test blocks were tested under the same ambient temperature and humidity, and the total test time was controlled within one hour to eliminate errors caused by environmental changes.

4. The construction method of steel pipe pile cofferdam in hard clay layer as described in claim 1, characterized in that, The concentration of oxalic acid in the oxalic acid solution is 75%.

5. The construction method for steel pipe pile cofferdams in hard clay layers as described in claim 4, characterized in that, The determination of the ratio of sodium silicate solution and oxalic acid solution based on the pH value of the hard clay layer includes the following steps: S211. In the hard clay layer, select 6 different locations as sampling points, and make 6 groups of samples, with 5 samples in each group; S212. Oxalic acid solution and sodium silicate solution are injected into the 6 groups of samples in sequence, wherein the volume ratio of sodium silicate solution to oxalic acid solution in each of the 5 samples in each group is 0.5, 0.75, 1, 1.25 and 1.5 respectively. S213. Measure the pH value of the sample after grouting, and select the ratio of sodium silicate solution and oxalic acid solution corresponding to the required pH value.

6. The construction method for steel pipe pile cofferdams in hard clay layers as described in claim 5, characterized in that, The determination of the Baumé degree of the sodium silicate solution includes the following steps: S221. In the hard clay layer, select 6 different locations as sampling points and make 6 groups of samples, with 6 samples in each group; S222. Oxalic acid solution and sodium silicate solution with a volume ratio of 1:1 are injected into the 6 groups of samples in turn. The Baume degrees of the sodium silicate solution in the 6 groups of 6 samples are 36, 38, 40, 42, 44 and 46, respectively. S223. Prepare each sample into a cylindrical specimen with a diameter of 50 mm and a height-to-diameter ratio of 2:1 for compressive strength testing, and select the Baumé degree of the sodium silicate solution corresponding to the required compressive strength.

7. The construction method for steel pipe pile cofferdams in hard clay layers as described in claim 1, characterized in that, The cofferdam is rectangular and is divided into three equal regions along its length. The preset positions include the center point of the three regions and grouting points evenly distributed on the sides.

8. The construction method for steel pipe pile cofferdams in hard clay layers as described in claim 7, characterized in that, When injecting oxalic acid solution and sodium silicate solution into the support layer, the grouting sequence should be carried out in the middle first and then around the perimeter.

9. The construction method for steel pipe pile cofferdams in hard clay layers as described in claim 1, characterized in that, Before injecting oxalic acid solution and sodium silicate solution into the support layer, first detect whether there are voids in the support layer and the preset depth below it. If there are voids, fill them in first.

10. The construction method of steel pipe pile cofferdam in hard clay layer as described in any one of claims 1 to 9, characterized in that, After injecting oxalic acid solution and sodium silicate solution into the support layer, six samples of the support layer are taken every three days for compressive strength testing to detect the time it takes to maintain the required compressive strength.

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