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SECANT PILE WALLS - SECANT PILE WALL DESIGN
FOR DEEP EXCAVATIONS:
Secant Pile Walls
Secant pile walls are formed by constructing intersecting reinforced concrete
piles. The piles are reinforced with either steel rebar or with steel beams and
are constructed by either drilling under mud or augering. Primary piles are
installed first with secondary 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 or tangent
pile walls are:
1. Increased construction alignment flexibility.
2. Increased wall stiffness compared to sheet piles.
3. Can be installed in difficult ground (cobbles/boulders).
4. Less noisy construction.
The main disadvantages of secant pile walls are:
1. Verticality tolerances may be hard to achieve for deep piles.
2. Total waterproofing is very difficult to obtain in joints.
3. Increased cost compared to sheet pile walls.
Secant pile wall design when steel beams are used involves the
use of weaker than normal concrete. The pile that is lagging the
wall between two main beams has to be examined for shear and compression
arching.
Tangent Pile Wall
Tangent pile walls are a variation of secant pile walls and soldier
pile walls. However, tangent pile walls are constructed with no
overlap and ideally one pile touches the other. Compared to secant
pile walls, tangent pile walls offer the following advantages:
1. Increased construction alignment flexibility.
2. Easier and quicker construction.
The main disadvantages of tangent pile walls are:
1. They are can not be used in high groundwater tables without
dewatering.
2. Each pile is independent from adjacent piles
Soldier Pile and Lagging Wall (Berliner Wall)
Soldier pile and lagging walls are some of the oldest forms of
retaining systems used in deep excavations. These walls have successfully
being used since the late 18th century in metropolitan cities
like New York, Berlin, and London. The method is also commonly
known as "Berliner 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 and lagging walls are formed by:
1. Constructing soldier piles at regular intervals (2 to 4m, typical)
2. Excavating in small stages and installing lagging.
3. Backfilling and compacting the void space behind the lagging.
Moment resistance in soldier pile 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 and retains soil across piles and transfers 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 disadvantages of soldier pile and
lagging systems are:
1. They are primarily limited to temporary construction.
2. Cannot be used in high water table conditions without extensive
dewatering.
3. Poor backfilling and associated ground losses can result in
significant surface settlements.
4. They are not as stiff as other retaining systems.
5. Because only the flange of a soldier pile is embedded beneath
subgrade, it is very difficult to control basal soil movements.
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