Sheet Pile Design in One of Beijing-Shanghai High-Speed Railway Projects

Based on the hydrological and geological conditions around the main bridge of Jinan Yellow River Bridge of Beijing-Shanghai High-Speed Railway Project in China, for sheet pile cofferdam design and construction, we introduced the thinking of design, comparison of two sheet pile design proposals and reasons for selection.

Project Description

The length of Jinan Yellow River Bridge, one of sections of Beijing-Shanghai High-Speed Railway project,  is 5.143km. The main bridge is 112+3*168+112m supported by continuous steel with same height. A  stiffened arch was set in the middle, height of truss is 16m and width 30m, distance between sections is 14.0m. The main piers are No. 0 ~ 5, in which No.1,2,3 and 4 Pier all use 28 pieces of Φ2.5m bored piles, the bearing platform is 6.0m in thickness and 42.5m*23.3m in plane size. While No.0 and 5 Pier use 21 pieces of Φ2.0m bored piles, bearing platform is 4.5m in thickness and size 34.6m*13.8m. Concrete of bearing platform is C45 type.

Bearing platforms of No.1 and 2 pier of main bridge are in the main river cap (year-round water), others are in the south shallow beach area. Elevation of top surface of pier No.0 pier is +33 m (ground elevation is +33.50 m), No.1 pier +27 m, No.2 Pier +27 m, No.3 +27 m (ground elevation +30.14 m), No.4 +27 m (ground elevation +29.91 m), No.5 +30 m (ground elevation +30.92 m).

Thinking of Design

 

According to detailed information on geological exploration, No.3,4 and 5 Pier Yellow River main bridge is located in the floodplain south of Yellow River, silt and silt layers are rich in water within 19m below the ground surface, and there is a a hard plastic-like clay layer 2m below (elevation is about +9 ~ + 11 m) with relative complete and uniform distribution. The main bridge’s No. 1 and 2 pier is located in main channel of Yellow River, the riverbed’s elevation is + 17 ~ 24 m, the surface is covered by loose silt,  between +9 ~ +11 m there is a 2m thick layer of hard plastic-like clay. And this layer is confirmed in the piling or driving construction of steel casing and bored pile, the soil samples removed are sticky and large hard plastic-like clay block. So based on detailed construction surveys, the bearing platform of main Bridge’s No.1,2,3 and 4 Pier will use steel sheet pile cofferdam program. For No.0 and 5 pier, because of shallow excavation depth, method of open-excavation and protection in porcess will be used.

 

Comparison and Selection

 

Option One : Long Steel Sheet Pile Program

The flat surface of steel sheet pile cofferdam is rectangular for the base of the main bridge, the inner mouth size is 46.5m*28.3m, steel sheet pile is 24m each.

Mouth elevation at the end of steel sheet pile cofferdam for No.1 and 2 pier in water is 7.0m, top elevation 31m (top elevation of bearing platform add 27 m, flood elevation is +30.5 m), Mouth elevation at the end of steel sheet pile cofferdam for No.3 and 4 pier at beach is +6.5 m, the top surface elevation +30.5 m.

Use German Larsen IV (or shunli cold formed type) steel sheet pile . The cofferdam is made by single layer steel sheet pile, 3 layers of steel tubes and inside steel support formed by beams. Inside ring-support (purlin) for steel sheet piles use I56 compsite structure and internal support use 630mm steel pipe.

For sheet pile wall construction on land, pile by hydraulic vibratory hammer with the help of 50t crawler crane; for sheet pile wall construction in water, use 40t gantry crawler crane for water platform and hydraulic vibratory hammer.

For excavation and soil clear inside cofferdam on land,  use long arm excavation machine and high-pressure water jetting for soil suction method; for the ones in water, use method of high-pressure water jetting for dredging. Considering  the end of steel sheet pile will be piled or drove into the hard plastic-like clay layer between +9 ~ +11 m deep, after soil clear or dredging, sheet pile wall or cofferdam could not do bottom sealing, instead directly drain the water inside the cofferdam, and then pour 20 cm thick concrete pads, then remove steel casing, chisel trailer coupling and start construction of bearing platform.

Advantages : No bottom sealing reduces the risk of concrete construction in large area and construction investment, reduces the dredging depth of the cofferdam and saves lot of sealing concrete. At the same time, within the cofferdam, excavation, dredging, construction of concrete pads are all in dry place, improves the efficiency and speed up the construction schedule.

Disadvantages : Length of steel sheet pile added 6m increases the cost in itself.

Option two : Short Sheet Pile Program

Use 18m long steel sheet pile, the others are same as Option One. Considering the steel sheet pile cofferdam is in relative loose silt layer, bottom sealing in water will be carried out after soil clear within cofferdam, then until strength of the sealing concrete meet requirements drain the water, and then remove the steel casing, chisel pile head, and start construction of bearing platfrom.

Advantages : Length of steel sheet pile reduced 6 m will lower investment in itself.

Disadvantages : Because of bottom concrete sealing in water, it increases the construction difficulty of excavation in water and dredging within the cofferdam, reduces construction efficiency. And bottom concrete sealing construction in large area and generous amount would bring high risk and difficulty, a lot of extra sealing concrete also increases the uncertainty and costs on construction.

 

Program Selection

 

Through repeated investigations on site and programs’ comparison, option one is determined to use. From the construction technology, safety and reliability considerations, if by option one,  though water can not be drained inside the cofferdam, can also continue with Option two, there is not much risk. Also from an economic point of view, construction cost of the option one is much lower than Option two, the successful implementation would mean significant cost savings.