Transcript Factors to Consider in Foundation Design Chapter # 02

Factors to Consider in
Foundation Design
Chapter # 02
Lec. # 04
key Factors
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8.
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12.
Foundations in Sand and Silt deposits
Foundations on clays and clayey silts
Foundation on Loess and other
collapsible soils
Foundation on Residual Soils
Foundation on unsaturated soils subject
to volume change with change in water
contents
Environmental Considerations
7-Foundations in Sand and Silt
deposits
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Foundation in sand and silt will require consideration
of the following
Bearing capacity
Densification of loose deposits to control settlement.
Placing the footing at a sufficient depth that the soil
beneath the footing is confined. If silt or sand is not
confined, it will roll out from the footing perimeter
with a loss of density and bearing capacity. Wind
and water may erode sand or silt from beneath a
footing that is too near the ground surface.
Uncontaminated glacial silt
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Uncontaminated glacial silt deposits can have
a large capillary rise because of the small
particles sizes. Sometime these deposits can
be stabilized by excavation to a depth of 0.6
to 1 m, followed by placement of a geotextile
water barrier. The silt is then backfilled and
compacted to provide a suitable foundation.
Foundations on clays and clayey silts
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Clay and clayey silts may range from very
soft, normally consolidated, to very stiff,
highly over consolidated deposits.
Major problems are often associated with the
very soft to soft, deposits from both bearingcapacity considerations and consolidation
settlements. Silt with a large IP (PL) and /or
wL (LL)may be called plastic silts.
Remarks
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Consolidation test should be made to determine the
expected settlement if the structure has a relatively
high cost per unit area. For smaller or less important
structures, some type of settlement estimate based
on the index properties might be justified.
The net ultimate bearing pressure for vertical loads
on clay soils is normally computed as a
simplification of either the Meyerhof or Hansan
equations;
qult = cNc Sc dc+ q Nq Sq dq-q
9-Foundation on Loess
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Loess having particle size range 0.01 to
0.1mm, specific gravity 2.6 to 2.8, in situe dry
densities ranges from 10 to 16.5 kN/m3 and
Atterberg limits 25-55 & 15-30 percents is
predominant collapsible soil.
Foundation on Loess and other collapsible
soils
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Collapsible soils are generally wind-blown
(Aeolian) deposits of silts, dune sands and
volcanic ash.
Typically they are loose but stable,
Certain conditions of load + wetting produce
a collapse of the soil structure, resulting large
settlement.
Improvement of site conditions
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Compact (excavate and replace) the soil to
γdry > 15.5 kN/m3
Use an admixture during compaction.
Admixtures may lime, lime/fly ash, or Portland
cement.
Use of piles through the collapsing soils to a
more competent underlying stratum.
10-Foundation on Residual Soils
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A residual soil is produced from physical and
chemical weathering of rock. i.e sedimentary,
metamorhic or igneous. Soils produced in this
manner tend to be sandy silts or silty sands
often with some mica particles and clay
contamination.
In many cases, to make a reliable design,
some soil exploration is necessary in all
residual soils.
11-Foundation on unsaturated soils subject to
volume change with change in water contents
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Expansive soils undergo volume changes
upon wetting and drying. For a volume
changes to occur these soils must be initially
unsaturated at some water content w0.
when water content changes to a new value
w1,the volume increase if w1> w0 or
decreases if w1< w0 unless w0 is the
shrinkage limit where w0 = ws.
Remarks
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These soils occur in an active zone, which
starts at the ground surface and goes down
to the saturated part of the zone of capillary
rise above the ground water table.
Other approaches related with volume
changes
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Volume-changes related to consolidation
Volume change related to the expansion
index E1
Volume change based on soil suction
Volume change correlations using soil index
properties
Designing structure on soils susceptible to
volume change
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Alter the soil (using lime cement, or other
admixtures)
Compact the soil well on the wet side of the OMC.
Control the direction of expansion. By allowing the
soil to expand into activities built in the foundation,
the foundation movements may be reduced to
tolerable amounts. A common practice is to build
“waffles” slabs so that the ribs support the
structure while the waffle voids allow soil
expansion.
•Control the soil water.
The soil may be excavated to a depth such that
the excavated overburden mass of soil will
control heave, lay a plastic fabric within the
excavation, and then backfill.
The rising water vapor is trapped by the geotextile and any subsequent volume change is
controlled by the weight of overlying material and
construction. The surface moisture will also have
to be controlled by paving, grading, etc.
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Check whether a granular blanket of 0.3 to 1m or
more depth will control capillary water and
maintain nearly constant water content in the
clay.
Ignore the heave. By placing the footings at a
sufficient depth and leaving an adequate
expansion zone between the ground surface and
the building, swell can take place without causing
detrimental movement. A common procedure is
to use belled piers with the bell at sufficient depth
in the ground that the soil swell produces pull-out
tension on the shaft of the whole system heaves.
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Load the soil to sufficient pressure intensity
to balance swell pressure. Using spread
footing or replacing granular material with
soil may treat the swell.
Environmental Considerations
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Foundation engineers have the responsibility to
ensure that their portion of the total design does not
have a detrimental effect on the environment.
Examples;
Soil boring through sanitary landfills can pollute the
ground water via seepage through the boreholes.
Soil boring logs should be checked for indication of
effect of site of excavation on the environment in
terms of runoff, pollution in runoff, odor problems,
dust and noise.
Environmental Considerations
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One should investigate means to salvage
topsoil for landscaping.
Pile driver noise and vibration can be
objectionable.
It should be determined whether soil borings
near streams cause piping problems during
high water periods. These may be avoided by
careful plugging of the boreholes.
The effect of river and marine structures on
aquatic life must be minimized.
Improving site soils for foundations
use
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Mechanical stabilization
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Compaction
Pre-loading
Drainage
Densification using vibratory equipments
Use of in-situ reinforcement
Use of geo-textile
Chemical stabilization