1.0 Introduction

Steel sheet pile is commonly used in the construction of bridges, piers and docks. It has the advantages of high intensity, short construction period, good waterproof performance, and flexible size. For the frequent use of steel sheet piles, double-row steel sheet pile cofferdam is more and more preferred in the cofferdam structure. This is because double-row steel sheet pile have the characteristics of simple structure, convenient construction and wide adaptability.  

However, the application of double-row steel sheet pile cofferdam is still in its infancy stage, and there are two following main questions: 

(1) The use of double-row of steel sheet pile cofferdam is restricted by fluvial geomorphology.

Steel sheet pile cofferdam takes cantilevered double-row steel sheet pile structure in water conservancy projects. To meet the requirement of stability, there is a need to have enough distance between two rows of piles. Under this case, steel sheet piles are often used in wide rivers with large span structure.  The application of steel sheet pile cofferdam in the narrow river channel is restricted by its structural characteristics.

(2) The calculation theory of double-row steel sheet pile cofferdam is not well-developed. 

Although double-row steel sheet pile cofferdam is widely used, the current detailed design is still based on the semi-empirical and semi-theoretical calculation method. The double-row steel sheet pile cofferdam is used as gravity cantilever retaining structure to calculate the overall anti-slip stability and anti-overturning. This calculation method can meet the needs of general engineering, but it is difficult to accurately predict the displacement and deformation of cofferdam in the construction process. It means that there is a significant limitation for the reliability analysis of the structure. 

2.0 Project overview

Yangguantun barrage is an important part of the first phase of the east route of the south-to-north water transfer project. The main task of the project is to control and manage the water resources of Nansi Lake, as well as navigation functions. For the detailed information, the designed quantity of free discharge for this project is 156m3/s, and the Navigation standard is Ⅶ a (grade 3). The brake shaft is 130m away from the inlet, and there are two holes with 8m and 12m net widths in the lock chamber. 

Yangguantun barrage is located at the entrance of Yangguantun River to Nansi Lake, and the channel is narrow. The project is located at the border of Shandong and Jiangsu provinces, and its geographical location is exceptional, so the construction area is susceptible. Additionally, the project location is generally low-lying, so it is challenging to get earth nearby to build dykes. Because the project is limited by site and land acquisition, the cantilevered small span double-row steel sheet pile cofferdam is taken.

3.0 Geological conditions

The strata in the field area belong to Huaibei stratigraphic sub-area in the north China stratigraphic area. The soil in this layer is mainly silty loam, medium and coarse sand, etc. Also, sand mining in this area is widespread, so there is no better choice for the construction site. Before construction, a detailed geological survey was carried out again on the site of the gate. For this area, the foundation of the main building is located in the second layer of clay, and its limit bearing capacity is 160 kPa, which is a capable bearing stratum for construction foundation. 

However, during the geological investigation, it was found that there were holes in the third layer silty clay and the eighth layer of medium-coarse sand. After analysis, these holes are caused by sand mining, and a large number of these holes make the geological conditions of this project very complicated. The soil structure is damaged, and mechanics index is much lower than the standards shown by the uneven strength of the soil. 

4.0 Layout of steel sheet pile cofferdam and selection of equipment 

Yangguantun River is narrow, and the width in the location of this project is only 40m. Also, this river needs to meet the navigation requirements during the construction period, so double-row steel sheet pile cofferdam with large span is not applicable for this case. After considering all the factors, cantilevered double-row steel sheet pile cofferdam with a small span is taken. In this plan, the distance between two rows of steel sheet piles is 6m, and the space between these piles are filled with soil. Steel sheet pile in this project is wrul3-575 Lassen steel sheet pile made by Nanjing Wanhui Steel Sheet Pile co., LTD. This steel sheet pile is 58cm wide, 36cm high, and 1cm thick. Also, the weight of this pile is 78g per meter. 

5.0 Design and construction of steel sheet pile  cofferdam combined with cast-in-place pile

Between the geological drill YG7 and YGl 1(42 m long), the exploration results demonstrate there are no only sand mining holes in the foundation, but also black split above the bottom elevation of 26.5m. The stability calculation shows that the safety factors of anti-slip and anti-dip of steel sheet pile in this section do not meet the design requirements. This is because the depth for the insertion of a steel sheet pile is relatively small. At the same time, this double row steel sheet pile cofferdam shows characteristics of the flexible structure of the filling gravity body better. In case of such complex geological conditions of this project, the problem of deformation is often even more serious, so the most effective way to solve this problem is to increase the overall stiffness of the structure. After repeated discussion, it is decided to set cast-in-place pile in the double-row steel sheet pile cofferdam during construction to ensure the stability of cofferdam.

In this section, steel sheet piles with a length of 15 m are used, and the spacing between double rows of piles is 6 m. Cast-in-place piles are set between double-row of steel sheet piles, and the centre of Cast-in-place pile is placed on the centerline of the cofferdam. The pile is 30m long with the diameter of 1.2m, and the pile spacing is 3m. A cap is set at the top of the pile, and connecting beams are arranged between these caps to increase the overall stiffness of the pile. The spacing of channel steel is 1 m connected with the connecting beam evenly.

6.0 Conclusion

Based on the experience of this project, the construction technology of double-row steel sheet pile cofferdam with a small span under complex geological conditions of the narrow river is studied in detail. In the case of the narrow river channel, limited construction period and complicated geological conditions, the project meets the requirement of traffic and construction safety technical complex situations successfully. The successful implementation of the steel sheet pile cofferdam in Yangguantuen barrage project establishes a foundation for main projects completed on schedule. It has achieved excellent economic and social benefits. Also, it provides a great deal of valuable information for similar engineering design and construction.