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.

Rockfill Cofferdam and Steel Sheet Pile Cofferdam

Rockfill Cofferdam, that is, use local available building materials as the main filling material;

It is a traditional earth-rock cofferdam. The local earth and rock materials can be used, taking into account the main pumping station was built after the project finished as a permanent sea wall, so that it could be a part of eastern  north berm project.

But if we use solutions of ordinary rockfill cofferdam and rockfill cofferdam in the heart of sheet piling wall , there does not have enough space in such a narrow range of grading. 

Secondly, cofferdam construction required fast speed in building, but in these types of programs, large amount of rock and serious slippery preloading can not be too high too fast. 

Thirdly, as pit depth is down to -20.8m, according to requirements from excavation design unit , the seepage control depth of cofferdam is better to deep clay layer to -13m,  slope can not be stabilized within the ordinary rockfill cofferdam and excavation can not meet requirements. 

Furthermore, outside the cofferdam, the water is deeper so that velocity and wave is more serious and erosion is more severe, so outside of the slope, it is not stable either. Especially if cofferdam foundation is not treated on soft ground, combining with the ebb and flow, it easily results in land shifts, slides and collapse. Then we have to fill and repair frequently because of poor security.

Steel Sheet Pile Cofferdam, that is,use cold-formed steel sheet pile wall to form a cofferdam, and then fill the clay.

Use cold-formed steel sheet pile wall to form a cofferdam. When leaving an appropriate distance from the excavation lines of pumping station, dig out the original loading layer of riprap and drive two rows of cold-formed steel sheet pile . 

The program has advantages of high strength in material, arbitrary length, lower price, light weight per unit, fast construction speed, short project cycle, good security and stability and effectively guard against moisture,waves and seepage.

Steel Sheet Pile Cofferdam by Deep Truss Method

Construction Process of Deep Truss in Steel Sheet Pile Cofferdam

Picking Steel Sheet Pile→Driving Pile Lead → Construction of Inside Support and Purlin → Sheet Piling Insert and Hit→Excavate the Excessed Stuff Inside → Pouring Back Cover Concrete → Pumping Inside Cofferdam → Cutting Steel Casing → Pile Head Chisel → Steel Rod Tie → Pouring Concrete on the Flat → Pier Construction → →Removal of Steel Sheet Pile

Notice : during the process, we should always prevent leakage, plugging and deformation.

Advantages of Deep Truss Method in Steel Sheet Pile Cofferdam

Deep truss method can be safely applied to large-scale steel sheet pile cofferdam in deep water and bad geological condition. It is able to overcome difficulties by the traditional steel sheet pile cofferdam construction technology when in the pumping mode and large spacing between the soil stays and cofferdam inside support, like steel sheet pile can not afford to the external water pressure in pumping mode. Deep truss method has a wide range of applications. In this article, we elaborates on the new truss method and its calculations in steel sheet pile cofferdam construction and construction process. We can also accumulate more new technical information on the application of steel sheet pile cofferdam.

Project Overview

Changsha Harbor Bridge is located in the south of Huizhou-Chaozhou section of Xiamen-Shenzhen Railway in China. The main bridge 55 # -88 # pier is in the water, 63 # -67 # pier is at the deepest level and has worst geological condition. The flat caps there is at the maximum size of 8.2 * 12.5m, a thickness of 2.5m. The end caps from the deepest water is calculated 9.0m; The water level of the Bridge construction is +1.528 m, consider the water level as +3.0m in calculation.

Using the German Larsen IV locking type steel sheet pile, to improve the state of force when steel sheet piles in deep water , use Deep Truss Method for steel sheet pile cofferdam construction：

Firstly make the inside support of steel sheet pile cofferdam as truss system, use decentralization device on steel casing to place it to designed elevation, and then use truss support system as the frame guide to play and insert play steel sheet pile.

This method can be applied to large-scale cofferdam in deep water and with bad geological conditions, and it is very safe. The cofferdam is using the structure of purlin and internal support. (see Figure ), for the purlin, use beam 2145b, internal support use Φ300mm (thickness δ 6mm) steel, diagonal steel connection use channel 18 . Between layers of circuit purlin use beam 2136b.

Pic Note: Three-dimensional and floor plans of steel sheet pile cofferdam