EI 02C097
01 Jul 97
heave of expansive soil. Shafts subject to pullout loads or
local labor rates, fuel, tools, supplies, cost and freight of pile
uplift thrust must have sufficient reinforcement steel to
materials, driving resistance, handling, cutoffs, caps, splicing,
absorb the tension load in the shaft and sufficient skin
and jetting. Jetting is the injection of water under pressure,
friction and underream resistance to prevent shaft uplift
usually from jets located on opposite sides of the pile, to
movements.
preexcavate a hole and to assist pile penetration. Costs are also
influenced by downtime for maintenance and repairs,
(b) The shaft may pass through relatively soft,
insurance, overhead, and profit margin. An economic study
compressible deposits and develop vertical load capacity
should be made to determine the cost/capacity ratio of the
from end bearing on hard or dense granular soil (Fig. 1-7b)
various types of piles. Consideration should be given to
or rock (Fig. 1-7c). End-bearing capacity should be
including alternative designs in contract documents where
sufficient to support vertical loads supplied by the structure
practical.
as well as any downdrag forces on the shaft perimeter caused
by negative skin friction from consolidating soil (Fig. 1-7b).
(2) Drilled shafts. Drilled shafts are usually cost effective
in soil above the water table and installation in cohesive soil,
(c) Single drilled shafts may be constructed with large
dense sand, rock, or other bearing soil overlaid by cohesive soil
diameters, typically 10 feet or more, and can extend to
that will not cave when the hole is bored. Drilled shafts,
depths of 200 feet or more. Drilled shafts can be made to
particularly auger-placed, pressure-grouted shafts, are often
support large loads and are seldom constructed in closely
most economical if the hole can be bored without slurry or
spaced groups.
casing.
f. Length.The length of the deep foundation is generally
(d) Drilled shafts tend to be preferred compared with
driven piles as the soil becomes harder. Pile driving
dependent on topography and soil conditions of the site.
becomes difficult in these cases, and the driving vibration
can adversely affect nearby structures. Also, many onshore
(1) Driven piles. Pile length is controlled by soil
areas have noise control ordinances which prohibit 24-hour
conditions and location of a suitable bearing stratum,
pile driving (a cost impact).
availability and suitability of driving equipment, total pile
weight, and cost. Piles exceeding 300 feet have been installed
(e) Good information on rock is required when drilled
offshore. Piles up to 150 feet are technically and economically
shafts are supported by rock. Drilled shafts placed in
acceptable for onshore installation.
weathered rock or that show lesser capacity than expected
may require shaft bases to be placed deeper than anticipated.
(2) Drilled shafts. Shaft length depends on the depth to a
This may cause significant cost overruns.
suitable bearing stratum. This length is limited by the
capability of the drilling equipment and the ability to keep the
d. Location and topography. Location and topo-graphy
hole open for placement of the reinforcement steel cage and
strongly influence selection of the foundation. Local practice
concrete.
is usually an excellent guide. Driven piles are often
8. Site and Soil Investigations
undesirable in congested urban locations because of noise,
inadequate clearance for pile driving, and the potential for
damage caused by vibration, soil densification, and ground
The foundation selected depends on functional requirements of
heave. Prefabricated piles may also be undesirable if storage
the structure and results of the site investigation. Site
space is not available. Other variables may restrict the
investigation is required to complete foundation selection and
utilization of deep foundation:
design and to select the most efficient construction method.
The first phase of the investigation is examination of site
(1) Access roads with limited bridge capacity and head
conditions that can influence foundation performance and
room may restrict certain piles and certain construction
construction methodology. The seond phase is to evaluate
equipment.
characteristics of the soil profile to determine the design and
the construction method. These phases are accomplished bythe
(2) The cost of transporting construction equip-ment to
following:
the site may be significant for small, isolated structures and
may justify piles that can be installed using light, locally
available equipment.
performed to determine the requiriements of a deep
e. Economy.
(1) Driven piles. Costs will depend on driving rig rental,
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