Soldier Pile

Soldier piles and lagging walls, commonly known as soldier piles, are some of the oldest forms of retaining wall systems used in deep excavations. Soldier pile walls have successfully been used since the late 18th century in metropolitan cities like New York, Berlin, and London. The method is also commonly known as the “Berlin Wall” when steel piles and timber lagging is used. Alternatively, caissons, circular pipes, or concrete piles can also be used as soldier piles but at an increased cost. Timber lagging is typically used although reinforced concrete panels can be also utilized for permanent conditions. 

Soldier pile walls are formed by:

Moment resistance in soldier piles and lagging walls is provided solely by the soldier piles. Passive soil resistance is obtained by embedding the soldier piles beneath the excavation grade. The lagging bridges retain soil across piles and transfer the lateral load to the soldier pile system.

Soldier pile and lagging walls are the most inexpensive systems compared to other retaining walls. They are also very easy and fast to construct. The major advantages of soldier pile walls are:

Secant Piles

Secant pile walls are formed by constructing intersecting reinforced concrete piles. Secant pile walls are reinforced with either steel rebar or with steel beams and are constructed by drilling under mud. Primary secant piles are installed first with secondary reinforced secant piles constructed in between primary piles once the latter gain sufficient strength. Pile overlap is typically in the order of 3 inches (8 cm). In a tangent pile wall, there is no pile overlap as the piles are constructed flush to each other. The main advantages of secant pile walls are:

Very often unreinforced secant piles are constructed with weaker concrete to allow for easier drilling of the reinforced secant piles. Other times, two or three unreinforced secant piles are utilized to achieve greater economy. In all cases though, a secant pile wall designer needs to consider compression arching stresses in the concrete as well as shear at the intersections. For deeper secant piles, installation tolerances might have to be also taken into consideration.
In circular secant pile shafts, it might be possible to only have unreinforced piles if a compression ring action is ensured throughout the secant pile wall perimeter.

Contiguous Pile

Series of adjacent piles constructed to form earth retaining structure as a cantilever or a propped cantilever wall. Contiguous piled walls consist of a row of successive unconnected cast-in-situ concrete piles constructed with small gaps between the adjacent piles, formed using CFA or rotary bored piles (replacement piles techniques). Compared to other lateral soil supporting systems, contiguous pile walls are a more simple and economical supporting system due to the reduction in cost and construction period when considering small to medium scale depths of excavations. The diameter of each pile in a contiguous piled wall is usually not less than 300mm diameter.

Contiguous piles are suitable where the groundwater table is below excavation level. It is normally the most economic and rapid option. The wall consists of discrete piles typically installed at centers 150mm greater than their diameter, leaving gaps where the soil is exposed during excavation.

Common uses