Sheet Pile Pit-Retaining Structure Design

Design of  sheet pile retaining structure in foundation pit can be divided into the following three categories:

1. Cantilever Support Structure – when the excavation is not very deep, the cantilever support structure can be used to block soils behind the wall.

2. Single Support Structure – when the excavation depth is larger, to ensure the retaining structure safety and reduce distortion, often set a support or an anchor near the top of the supporting structure.

3. Multi-support Structure – when the excavation depth is large, we can set multi-layers of support or anchors.

Design Contents of Steel Sheet Pile Retaining Structure

1. Select the sheet pile type and model cross-section;

2. Calculate and check sheet pile insertion depth, cross-section strength and deformation;

3. Consider the corresponding precipitation plan and its impact on the surrounding environment (including the vibration impact in pile recycling or removing)

(Excerpted From Steel Sheet Pile Pit-Retaining Structure Design Classification )

Sheet Pile Wall Design

Generally in sheet pile wall design, normally we need to calculate all kinds of loads, scour depth, length of sheet pile (including pile embeded depth, the concrete cover thickness) through test algorithm. According to different construction conditions in process aselect sheet pile sections, supporting layers and structure, at last check the overall stability of sheet pile wall.

1. Data Collection
Collect and analyze geological maps, the highest water level, the highest level of construction and so on.

2. Determine Geometry Size
Plane size is often dermined by upper structure and its foundation size. Sheet pile wall construction should be in accordance with the whole project construction and template installation, so the wall’s size should be at least 1.5m greater than the basic outline. It must also consider the needed size of pumping equipment and wells’ installation.

Dimensional size is usually influenced by water level, scour depth, the depth of excavation inside wall and factors such as stability in the pit end. But the most direct impact is water level.

3. Calculate Sheet Pile Wall Lateral Pressure

Assume sheet pile’s embedment depth first, then calculat lateral pressures hydrostatic pressure, water impact, ice pressure, wind, wave power and earth pressure.

4. Force Checking by Constructin Conditions

5. Stability Checking
Need to analyze wall’s overall stability and soil stability at the end of the foundation such as arch, water well-out and sand-cast.

Article Source:   Sheet Pile Wall Design Steps

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.

Sheet Pile Cofferdam Design with 6 Layers of Inside Support

Construction process of sheet pile cofferdam with six-layer inside support is generally divided into six steps of pumping and excavation down and five steps of concrete pouring and support demolition. The most dangerous condition is excavation to the end. For design, key points are as follows:

1. First set the guide frame of sheet pile, insert and play, then install the first layer purlin and support (to avoid high tide). Then try pumping water and plug pile joints in water over the riverbed. 

 
2. From layer 2 to 6, pump or excavate one by one to the position 0.5m lower than the elevation of support to be installed, then install purlin and support.

 
3. From the 5th layer, gradually narrow the excavation surface layer to form slope. 

 
4. Excavate by divided area to the end of cushion, pouring concrete pads, and ultimately form a complete flat construction surface. 

 
5. Pouring cap by four-layer, backfill sands from nearby and remove a support layer as soon as each layer’s pouring completed. 

 
6. For the last 2 layers’ support, should consider the construction situation for support removal or change when pouring the upper parts of bridge piers.

In addition, in the calculation and design, select datas of drilling holes in the place with poor geological conditions. Then refer to section diagram, analyze and calculate the displacement and bending moment of sheet pile in the six steps’ pumping and excavation.

Sheet Pile Wall Structure Designs on Seriously Weathered Rock Layer

For sheet pile wall or cofferdam construction, normally we need to drive piles to the required depth. But for shallow layer especially seriously weathered rock layer, it is very difficult to drive piles. For instance, top surface of highly weathered rock and sand layer is 1-4m lower than cushion base, and some part is even near the cushion base. Because the driving depth is not enough as required, the displacement at the pile-end will be larger, sheet piles in some project sections would easily get damaged like "kicking damage".

For this problem, we must take some measures on structural design first.

For structral desgin of sheet pile wall, there are mainly three plans to solve the problem :

1. Enlarge the wall or cofferdam and put slope inside;

2. Increase inside support layers;

3. Reinforce the bottom of sheet piles;

Sheet Pile Wall or Cofferdam

Comparing three plans above, if only by plan 1, because the wall or cofferdam is normally large enough, it will cost extra money or time. And if there are more wall or cofferdams in construction at the same time, it will influence other projects too; Just by plan 2, if we increase inside support layers when excavation near the end, supports' changing process will be more complicated when it comes to the back concrete cover; If just by plan 3, although it is applicable in theory, it is not enough for pratical construction.

The best way actually is to combine the strong points of the three plans. For instance, firstly, we can set the axis line of sheet pile wall near the cap side which will proive a certain slope space and is useful for sheet pile reinforcement.Secondly, set 6 layers of inside support,make the space wider at the upper 4 layers. At last, at the location higher than layer elevation of 7.0m, set steel tubular pile (or called steel pipe pile) as support foot right near the driving-hole of sheet piles.