Research on Optimization Design of High Strength Prestressed Grooved Sheet Piles

Through theoretical calculations, the cracking moment, bending moment and section bending coefficient of high-strength prestressed grooved sheet piles and high-strength prestressed corrugated sheet piles were compared and analyzed; theoretically, the optimized high-strength prestressed grooved plates were obtained. And through this method we optimize the grooved sheet pile section. The optimized mechanical properties of the section have been greatly improved, making the mechanical properties of the grooved sheet piles approach or better than that of the corrugated sheet piles of the corresponding model. Combined with production experience, a series of optimization measures have been proposed for the section of the grooved sheet piles.


Introduction
In the construction of foundation pit excavation and support in urban underground construction of high-rise buildings, river courses, bank dikes, road abutments, and railway highway slopes, it is necessary to construct retaining structures for retaining soil. Choosing the appropriate form of support structure is the key to safe and economic construction and timely construction of the enclosure. The methods for supporting foundation pits in traditional projects include drilled cast-in-place piles, deepmixed cement-soil retaining walls, high-pressure jet-grouting pile retaining walls, underground continuous walls, etc. All these supporting structures require the completion of concrete pouring on site. The construction cost is high and the working hours are long [1] . In recent years, steel sheet piles have gradually been used as foundation pit support structures in some important projects, but the overall cost is high. Compared with traditional materials and construction methods and steel sheet piles, concrete sheet piles have the advantages of beautiful appearance after piled walls, good durability, reliable quality, fast construction speed, and a significant reduction in construction costs while shortening the construction period [2] . Although the cost is higher than that of conventional masonry, the project has a long service life and it is safe and reliable. Due to the reasonable structure adopted, a large amount of materials are saved, and the materials have good corrosion resistance and no maintenance. Compared with traditional construction methods, concrete sheet piles have obvious advantages in terms of performance, economic benefits, energy saving and environmental protection.
The countries that use sheet piles abroad mainly include Japan, the Netherlands, Vietnam, and other Southeast Asian countries. In 1989 and 1993, Prof Japan made reports on the successful development of prestressed concrete sheet piles at different international concrete academic conferences [3] . The Netherlands successfully applied sheet piles to various fields in construction projects. The country not only applied sheet piles to river revetments, but also successfully applied road and railroad engineering slope protection, urban building foundation pit engineering, dock engineering, municipal engineering, and bridge and culvert engineering,etc. [4] . At present, prestressed concrete sheet piles have three types of cross sections: flat piles, U-shaped sheet piles, corrugated sheet piles, and Japan is keen to develop corrugated sheet piles [5] . In 2009, U-shaped sheet piles were used in China and cooperated with foreign companies to optimize sheet pile technology [6] .
Guo Liting [5] through the preparation of "sheet pile revetment components general map", explored the practical and economic rational structure of the sheet pile components, and carried out analysis and calculation, in order to improve the design of sheet piles. Zhang Jingjing [7] proposed an economical analysis of the height of the relative compression zone of a reinforced concrete single-reinforced rectangular cross-section member by means of controlling the height of the compression zone in the cross-section to achieve optimal design. Hu Shaowei [8] established a calculation model for the flexural bearing capacity of prestressed concrete corrugated sheet piles based on section equivalent equivalence assumptions, and obtained calculation methods for the neutral axis height and crosssection bending moment. Lou Tiejiong [9] ,etc. Based on the nonlinear constitutive relation between concrete and steel, different formulas for the internal force of the section due to the movement of the neutral axis during the loading of the prestressed concrete section of the double-strand section are deduced, and a coupling between two variables into a set of nonlinear equations.is established. For the U-shaped and corrugated sheet piles used in the current project, the height of the cross-section of the U-shaped and corrugated sheet piles is small, and the pile heads are easily damaged when the piles are piled up; the connection between the piles and the piles is too small, and the cracks easily occur in the piles. , leading to pile position deviations and other issues. Prof. Huang Jianhua has developed a new type of prefabricated pile-high-strength prestressed concrete grooved sheet piles [10,11] (hereinafter referred to as grooved sheet piles). The overall section of the sheet pile is in the form of a groove, and the connector of the sheet is arranged on the left and right side webs, as shown in Figure. 1 . The author compares and analyzes the mechanical properties of grooved sheet piles and corrugated sheet piles through theoretical calculations. At the same time, an optimized section of the grooved sheet piles is proposed by calculation.

Sheet pile calculation section simplified
The section of a single pile of grooved sheet piles is shown in Figure. 2. In engineering applications, a single sheet pile is connected to each other through a groove (ie, a groove on one side and a tongue on the opposite side) to form an integral sheet pile wall, as shown in Figure.3. In order to simplify the calculation in theoretical force calculation analysis, the chamfer and other details in the groove have little effect on the calculation of the overall mechanical properties of the grooved sheet piles. In the calculation, neglecting the chamfer and interface misalignment visual misalignment grooves are adopted. The section is equivalent to two regular rectangular spliced T-section, as shown in Figure 4.

Grooved sheet pile crack resistance bending moment calculation method
The total nominal area of the prestressed steel bars in the tension zone and compression zone.
n n 、 -Prestressed steel bar reinforcement in tension zone and compression zone.     Calculating the moment of inertia of transition section of prestressed reinforcement.
Calculate section bending coefficient.  Considering the plasticity of the concrete in the tension zone, the cross-section resisting moment shaping factor of the concrete component is introduced.
Cross-section bending moment.

Calculating method of normal section ultimate bending moment for grooved piles
x -Concrete pressure zone height.

Groove sheet pile parameters
In order to compare with corrugated sheet piles with more specificity, we chose the grooved sheet piles with the same section size as the corrugated sheet piles. The specific dimensions of the grooved sheet piles are shown in Table 1. The prefabricated steel bars of the same kind and quantity are arranged on the grooved sheet piles and the corrugated sheet piles. The difference is that the corrugated sheet piles are asymmetrical in cross section. Therefore, the calculation model using.six prestressed steel bars are arranged in the  Table 2. Both types of sheet piles are made of concrete with a strength grade of C60. According to the "Code for Design of Concrete Structures" (GB50010-2010) [12] , the mechanical properties of concrete are shown in Table 3.

Corrugated sheet pile parameters
The cross-section of a traditional corrugated sheet pile is shown in Figure. 5 [8] . The corrugated sheet piles with the same cross-sectional area as the grooved sheet piles are used. The cross-sectional dimensions are shown in Table 4, and the mechanical performance parameters are shown in Table 5.

Compare with corrugated sheet piles
The principle of optimum design of grooved sheet piles is that the grooved sheet piles in each model have the same cross-sectional area as the current corrugated sheet piles. By redesign the cross-section, calculation optimization. Improve the mechanical properties of the grooved sheet piles. The results of the initial design of the sheet pile section are compared with the corrugated sheet piles. The detailed comparisons are shown in Table 6. The comparative analysis of the mechanical properties of the two types of sheet piles results in the following results: In the case of the same cross-sectional area and similar section dimensions, the initial design of the grooved sheet piles is smaller than the corrugated sheet piles in terms of cracking bending moment, ultimate bending moment, and section bending resistance. On the basis of ensuring the innovative design of the slot-grooved sheet piles, it is necessary to further adjust and optimize the cross-section dimensions of the new grooved sheet piles.

Grooved sheet piles optimized section
Based on the above calculation method of cross-section bending coefficient, the calculation can be drawn: In the same sheet pile area, increasing the web height 0 t has more influence on the section bending coefficient than increasing the web width b ; increasing the web width b has more influence on the section bending coefficient than increasing the Flange height 1 t ; The relationship between the three is 0 Therefore, under the same cross-sectional area, the web height is the size that has the greatest influence on the cross-section bending coefficient. According to the results obtained, the cross-section dimensions of the grooved sheet piles are optimized. At the same time, when the prefabricated grooved sheet piles are prefabricated, air will accumulate at the edges and corners during the pouring process, and the piled surface will have pit problems after forming. In the inner side of the sheet pile, a round design is made to remove the outer edges and corners of the sheet pile. This design has three advantages: 1) Solve the problem of air accumulation when pouring into piles; 2) Effectively remove the stress concentration at the edges and corners of sheet piles; 3) Reduce the amount of concrete used. The optimized 600mm height grooved sheet pile design cross section is shown in Figure. 6. After the optimized cross section, the high strength prestressed grooved sheet pile cross section dimensions are shown in Table 7. Optimized high-strength prestressed grooved sheet piles, original-sized grooved sheet piles and corrugated sheet piles, the three mechanical parameters such as shown in Table 8. Compared with the original 450 mm high-profile grooved sheet piles, the optimized high-strength prestressed grooved sheet piles have a 26%, 32%, and 32% increase in cracking moment, ultimate bending moment, and bending resistance, mechanical properties are still worse than corrugated sheet piles. Compared with the original 600 mm high-profile grooved sheet piles, the optimized highstrength prestressed grooved sheet piles have 84%, 28%, and 28% increase in cracking moment, ultimate bending moment, and bending resistance, stronger mechanical properties than corrugated sheet piles. From this, it is deduced that with the successive increase of the height of each type of sheet pile, the mechanical properties of each type of groove-shaped sheet piles over 600 mm are better than the existing corrugated sheet piles.

Conclusion
(1) The method for calculating the mechanical properties of the sheet piles is reasonable and feasible. After the optimization, The width of the the mouthpieces increased, and the mouthpieces are arranged on both sides of the web. Pile sinking is not prone to damage, and the guidance of the webs on both sides is conducive to the guidance during the pile driving, so that the pile position is not easy to be displaced, and the connection between the pile and the pile is tighter.
(2) In the inner side of the sheet pile, a round design is made to remove the outer edges and corners of the sheet pile, and a misalignment visual misalignment groove design is made for the interface. The design can solve the problem of air accumulation when pouring piles and effectively remove the stress concentration at the corners of sheet piles. It can not only reduce the amount of concrete but also improve the mechanical properties of sheet piles.
(3) By increasing the web height and reducing the width of the web, the cross section optimization scheme can effectively ensure that the section bending coefficient is increased under the same crosssectional area. Thereby, the bending moment and cracking moment of the section are improved, and the mechanical properties of the sheet pile are improved, which provides a basis for the optimization design of other types of grooved sheet piles.