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154 Barry’s Advanced Construction of Buildings
Basement walls
There is often insufficient space to provide for the basement excavation and adequate batter
so that the basement walls can be constructed out of the ground. It is becoming more
common for deep basements to be constructed using diaphragm walls, with the aid of large
clam grabs and bentonite slurry, and auger bored piled walls using secant, interlocking and
contiguous piles.
Contiguous piles
With the development of CFA piling rigs, bored piles are becoming more common as a
form of permanent wall and foundation. Contiguous piles (Figure 3.74 and Figure 3.75a)
are formed by drilling bored piles at close centres. Piles vary in diameter from 300 to
2400 mm, although piles greater than 1200 mm are rarely used. A small gap is left between
each pile; typically this ranges between 20 and 150 mm (Photograph 3.17). The size of the
gap depends on the soil strength. The gaps may be filled to provide a more water-resistant
structural concrete facing wall. The use of CFA rigs to form the piles limits the depth of
pile to 30–55 m depending on the type of rig. In practice walls are usually constructed to
a maximum of 25 m, although some piles may be driven much deeper to provide vertical
load capacity. A ring beam is cast along the top of the piles linking all of the piles together;
this provides extra rigidity and strength and helps to distribute any loads placed on top of
the piles. Ground anchors can also be used to help resist the overturning forces caused by
the surrounding strata and hydrostatic pressure (Photograph 3.18). Grout is normally
forced through the anchor to tie it securely into the ground. Where the area surrounding
the retaining structure accommodates roads, structures or other property, grout may be
forced into the ground to produce a positive pressure on the ground, which counteracts
the possibility of settlement caused by the excavation of the basement and movement of
the retaining wall. The excavation of the basement may cause settlement due to the vibration,
which causes the surrounding ground to compact. Also, as the surrounding ground
applies its load on the retaining wall, some slight movement will occur. Vibration and
movement of the retaining wall will result in settlement of the surrounding ground; the
pressure exerted by pressurised grout can be used to remove the potential of settlement.
Secant piles
Secant piles consist of overlapping and interlocking piles (Figure 3.75b–e and Photograph
3.19). Female (primary) piles are bored using CFA rigs and cast first. The secondary male
piles are then drilled, secanting (cutting into) into the female pile. The system is often used
in the construction of deep basement walls. Secant walls are often considered to be a more
economical alternative to diaphragm walls.
Depending on the type of secant pile construction, either one or both piles are reinforced
to resist the lateral loads. When the secant wall is in place, the excavated face can be covered
with a layer of structural concrete. The concrete can be either sprayed or cast against the
wall, providing a fair-faced concrete finish. A reinforced concrete ring beam connects all
of the piles together, improving the structural stability of the wall. The beam will also help
to distribute any loads placed on top of the wall. The depth of the wall is usually limited
to 25 m; however, it is possible to construct secant walls to a depth of 55 m. Multi-storey