Structures may be founded on rock, on strong or weak soils,
cohesive or noncohesive soils, above ground level, below water level, etc. The
type of foundation used to support a structure depends on local conditions.
After obtaining a general evaluation of the subsurface
conditions the engineer should attempt to identify all potential useful
foundation alternatives for a structure. Three basic types of foundations are
available: soil-founded, various types of piles, and piers or caissons.
Each of these foundation types has many subcategories. The
following paragraphs provide a short description and evaluation of the various
pile types.
The purpose of a pile foundation is to transfer and
distribute load through a material or stratum with inadequate bearing, sliding
or uplift capacity to a firmer stratum that is capable of supporting the load
without detrimental displacement.
A wide range of pile
types is available for applications with various soil types and structural
requirements. A short description of features of common types of piles follows:
(1) Steel H-Piles. Steel H-piles have significant
advantages over other types of piles. They can provide high axial working
capacity, exceeding 400 kips. They may be obtained in a wide variety of sizes
and lengths and may be easily handled, spliced, and cut off.
H-piles displace little soil and are fairly easy to drive.
They can penetrate obstacles better than most piles, with less damage to the
pile from the obstacle or from hard driving. The major disadvantages of steel
H-piles are the high material costs for steel and possible long delivery time
for mill orders. H-piles may also be subject to excessive corrosion in certain
environments unless preventive measures are used. Pile shoes are required when driving in dense
sand strata, gravel strata, cobble-boulder zones, and when driving piles to
refusal on a hard layer of bedrock.
(2) Steel Pipe Piles. Steel pipe piles may be driven
open- or closed end and may be filled with concrete or left unfilled. Concrete
filled pipe piles may provide very high load capacity, over 1,000 kips in some
cases. Installation of pipe piles is more difficult than H-piles because
closed-end piles displace more soil, and open-ended pipe piles tend to form a
soil plug at the bottom and act like a closed-end pile. Handling, splicing, and
cutting are easy. Pipe piles have disadvantages similar to H-piles (i.e., high
steel costs, long delivery time, and potential corrosion problems).
(3) Precast Concrete. Precast concrete piles are
usually prestressed to withstand driving and handling stresses. Axial load
capacity may reach 500 kips or more. They have high load capacity as friction
piles in sand or where tip bearing on soil is important. Concrete piles are
usually durable and corrosion resistant and are often used where the pile must
extend above ground.
However, in some salt water applications durability is also
a problem with precast concrete piles. Handling of long piles and driving of
precast concrete piles are more difficult than for steel piles. For prestressed
piles, when the required length is not known precisely, cutting is much more
critical, and splicing is more difficult when needed to transfer tensile and
lateral forces from the pile head to the base slab.
(4) Cast-in-Place Concrete. Cast-in-place concrete
piles are shafts of concrete cast in thin shell pipes, top driven in the soil,
and usually closed end. Such piles can provide up to a 200-kip capacity. The
chief advantage over precast piles is the ease of changing lengths by cutting
or splicing the shell. The material cost of cast-in-place piles is relatively
low. They are not feasible when driving through hard soils or rock.
(5) Mandrel-Driven Piles. Mandrel-driven piles are
thin steel shells driven in the ground with a mandrel and then filled with
concrete. Such piles can provide up to a 200-kip capacity. The disadvantages
are that such piles usually require patented, franchised systems for
installation and installation is not as simple as for steel or precast concrete
piles. They offer the advantage of
lesser steel costs since thinner material can be used than is the case for
top-driven piles.
The heavy mandrel makes high capacities possible.
Mandrel-driven piles may be very difficult to increase in length since the
maximum pile length that can be driven is limited by the length of the mandrel
available at the site. Contractors may claim extra costs if required to bring a
longer mandrel to the site.
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