There are many reasons a geotechnical engineer would recommend a
deep foundation over a shallow foundation, but some of the common reasons are very large design loads, a poor soil at shallow depth, or site constraints (like property lines). There are different terms used to describe different types of deep foundations including the pile (which is analogous to a pole), the pier (which is analogous to a column), drilled shafts, and caissons. Piles are generally driven into the ground in situ;
other deep foundations are typically put in place using excavation and
drilling. The naming conventions may vary between engineering
disciplines and firms. Deep foundations can be made out of timber, steel, reinforced concrete and prestressed concrete.
Contents
- 1 Driven foundations
- 1.1 Pile foundation systems
- 1.2 Monopile foundation
- 2 Drilled piles
- 2.1 Under reamed piles
- 2.2 Augercast pile
- 2.3 Pier and grade beam foundation
- 2.4 Drilled Pier vs. Drilled Pile
- 3 Speciality piles
- 3.1 Micropiles
- 3.2 Tripod piles
- 3.3 Sheet piles
- 3.4 Soldier piles
- 3.5 Suction Piles
- 3.6 Adfreeze Piles
- 3.7 Vibrated stone columns
- 4 Piled walls
- 4.1 Secant piled walls
- 4.2 Slurry walls
- 5 Deep mixing/mass stabilization techniques
- 6 Classification of pile with respect to type of material
- 6.1 Timber
- 6.2 Steel
- 6.3 Prestressed concrete piles
- 6.4 Composite piles
Driven foundations
Pipe piles being driven into the ground
Illustration of a hand-operated pile driver in Germany after 1480.
Prefabricated piles are driven into the ground using a pile driver. Driven piles are either wood, reinforced concrete, or steel. Wooden piles are made from the trunks of tall trees. Concrete piles are available in square, octagonal, and round cross-sections (like Franki Piles). They are reinforced with rebar and are often prestressed. Steel piles are either pipe piles or some sort of beam section (like an H-pile). Historically, wood piles used splices to join multiple segments end-to-end when the driven depth required was too long for a single pile; today, splicing is common with steel piles, though concrete piles can be spliced
with mechanical and other means. Driving piles, as opposed to drilling
shafts, is advantageous because the soil displaced by driving the piles
compresses the surrounding soil, causing greater friction against the
sides of the piles, thus increasing their load-bearing
capacity. Driven piles are also considered to be "tested" for
weight-bearing ability because of their method of installation; thus the
motto of the Pile Driving Contractors' Association is "A Driven
Pile...Is a Tested Pile!".
Pile foundation systems
Foundations relying on driven piles often have groups of piles connected by a pile cap
(a large concrete block into which the heads of the piles are embedded)
to distribute loads which are larger than one pile can bear. Pile caps
and isolated piles are typically connected with grade beams
to tie the foundation elements together; lighter structural elements
bear on the grade beams, while heavier elements bear directly on the
pile cap.
Monopile foundation
A monopile foundation utilizes a single, generally
large-diameter, foundation structural element to support all the loads
(weight, wind, etc.) of a large above-surface structure.
A large number of monopile foundation have been utilized in recent years for economically constructing fixed-bottom offshore wind farms in shallow-water subsea locations.For example, the Horns Rev wind farm from 2002 in the North Sea west of Denmark utilizes 80 large monopiles of 4 metres diameter sunk 25 meters deep into the seabed,while the Lynn and Inner Dowsing Wind Farm
off the coast of England went online in 2008 with over 100 turbines,
each mounted on a 4.7-metre-diameter monopile foundation in ocean depths
up to 18 metres of water.
The typical construction process
for a wind turbine subsea monopile foundation in sand includes driving a
large hollow steel pile, of some 4 m in diameter with approximately
2-inch-thick walls, some 25 m deep into the seabed, through a 0.5 m
layer of larger stone and gravel to minimize erosion around the pile. A
"transition piece (complete with pre-installed features such as
boat-landing arrangement, cathodic protection, cable
ducts for sub-marine cables, turbine tower flange, etc.)" is attached
to the now deeply driven pile, the sand and water are removed from the
centre of the pile and replaced with concrete.
An additional layer of even larger stone, up to 0.5 m diameter, is
applied to the surface of the seabed for longer-term erosion protection.
Drilled piles
A pile machine in Amsterdam, the Netherlands.
Also called caissons, drilled shafts, drilled piers, Cast-in-drilled-hole piles (CIDH piles) or Cast-in-Situ piles. Rotary boring techniques are larger diameter piles than any other piling method and permit
pile construction through particularly dense or hard strata.
Construction methods depend on the geology of the site. In particular,
whether boring is to be undertaken in 'dry' ground conditions or through
water-logged but stable strata - i.e. 'wet boring'.
For end-bearing piles, drilling continues until the borehole has
extended a sufficient depth (socketing) into a sufficiently strong
layer. Depending on site geology, this can be a rock layer,
or hardpan, or other dense, strong layers. Both the diameter of the
pile and the depth of the pile are highly specific to the ground
conditions, loading conditions, and nature of the project.[citation needed]
Drilled piles can be tested using a variety of methods to verify the pile integrity during installation.
Under reamed piles
Underreamed piles have mechanically formed enlarged bases that have
been as much as 6 m in diameter. The form is that of an inverted cone
and can only be formed in stable soils. The larger base diameter allows
greater bearing capacity than a straight-shaft pile.[citation needed]
These pile are suited for expansive soils which are often subjected
to seasonal moisture variations, as also filled up ground and loose or
soft strata. They are used in normal ground condition also where
economics are favorable.
Augercast pile
An augercast pile, often known as a continuous flight augering (CFA) pile, is formed by drilling into the ground with a hollow stemmed continuous flight auger to the required depth or degree of resistance. No casing is required. A cement grout mix is then pumped
down the stem of the auger. While the cement grout is pumped, the auger
is slowly withdrawn, conveying the soil upward along the flights. A shaft of fluid cement grout is formed to ground level. Reinforcement can be installed. Recent innovations in addition to stringent quality control
allows reinforcing cages to be placed up to the full length of a pile
when required. A typical reinforcing cage will consist of 4 to 8 bars
from #5 to #8 bars typically 1/3 the length of the pile with
longitudinal circular ties spaced along the length of the cage. Where
tension loads are present it is typical to see a single full length bar
placed at the center of each pile.
Augercast piles cause minimal disturbance, and are often used for
noise and environmentally sensitive sites. Augercast piles are not
generally suited for use in contaminated soils, due to expensive waste
disposal costs. In cases such as these however a displacement pile may
provide the cost efficiency of an augercast pile and minimal
environmental impact. In ground containing obstructions or cobbles and
boulders, augercast piles are less suitable as refusal above the design
pile tip elevation may be encountered. In certain cases drill motors
that produce more torque and horsepower may be able to mitigate these events.[citation needed]
Pier and grade beam foundation
In drilled pier foundations, the piers can be connected with grade beams
on which the structure sits, sometimes with heavy column loads bearing
directly on the piers. In some residential construction, the piers are
extended above the ground level and wood beams bearing on the piers are
used to support the structure. This type of foundation results in a crawl space underneath the building in which wiring and duct work can be laid during construction or re-modelling.
Drilled Pier vs. Drilled Pile
There is a difference between the terms "drilled pier" and "drilled pile".
Drilled Pier
- Consists of concrete and a rebar cage
- CSL testing is performed
- May or may not have permanent/temporary casing
Drilled Pile
- Consists of concrete, a pile, and may or may not have a rebar cage
- CSL tests are not typically performed
- May or may not have permanent/temporary casing
