Download Primary Considerations for Building Material Selection

Chapter 2 a
Soils & Excavation
Building Foundations
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The purpose of foundations is to
provide stability and to safely transfer
building loads to the earth.
Three load considerations occur:
Vertical weight of the building
Horizontal forces above ground such
as wind
Horizontal forces below ground as in
earthquakes.
Foundation Loads
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Dead Load
Weights of building material due
to gravity
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Live Load
Those type loads that can
change, such as weight of
furniture, people, wind, snow, ice,
and in certain areas of the world,
horizontal forces below grade,
such as movement caused by
earthquakes.
Foundation Requirements
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Foundations Must Be:
Safe From Structural Failure
Free of Settlement that does not
impair Function, such as unsightly cracks and
damage to the building that would allow
deterioration due to weather.
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Foundations Must Be Technically &
Economically Feasible
Types of Settlement
No Settlement
Uniform
Differential
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Of the two types of settlement, Differential
Settlement is the worst, as structural failure
often occurs when one part of a structure
moves and another part doesn’t.
Differential settlement is due basically to two
conditions:
Poor bearing capacity of soil in differing
areas of the building.
Shrinking and swelling of the soil which
allows one part of a building to sink due to
load – or forces of swelling soil causing an
upward movement in parts of a building.
The
Leaning Tower of Pisa
One of the Most Famous
Examples of
Differential Settlement
Differential Settlement
Between an
Areaway & Sidewalk
Differential Settlement
Next to
A Building Column
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Differential Settlement happens when one
part of a building changes elevation relative to
another part of the building.
Differential settlement is usually caused by
an inconsistency in the soil bearing, or a change
in the soil volume.
Types of soil range from the most stable -from hard granular particles -- to clay - - - to
somewhere in between – which may contain
sound, structural particles and some that are
not.
Types of Soil
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Soil types are identified by analysis in a
laboratory by Geotechnical Engineers.
Soil that is basically stable are comprised of
hard particles that touch and do not move when
subjected to loads or inclusion of water.
Soil that is mostly clay has extremely small
particles. Water will force the particles apart,
causing the mass to swell – in all directions.
When water migrates out of the mass, the soil
will shrink.
Porous
(sandy)
Clays
Soil Particle Distribution
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Soil is rarely one type - but a mixture of
different particles due to years of migration
of particles, caused by wind and water.
Distribution of soil type, particle size, and
particle strength is important in Predicting:
– Load Bearing Capacity
– Stability
– Drainage Characteristics
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A type of soil that is available in nearly all
locations is called loam, which is a rather
generic name for common, ordinary soil that
does not have characteristics of unstability in
some form.
Loam is very good farm type soil, as it
contains sand, some clay, silt, and organic
matter. Since it does not contain an
overbalance of clay or silt it is a stable
foundation soil when compacted to a dense
formation.
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Two types of soil unique to the West Texas area
have varying characteristics:
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Caliche – actually is soil with a high content of
calcium carbonate. Is strong when dry but
unstable when wet. Good for a sub layer for
concrete and asphalt roads when protected
from absorbing moisture. Not a good base for
bearing capacity under buildings.
Playa Lakes – Soil found in playa lakes has
extremely small particles, structurally
unsound, and very unstable. It is composed of
drainage silt over years of washing downhill
from someplace else moving to lower levels
until it ends at a playa lake.
Soil Investigation
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Who needs to know soil characteristics?
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When do they need to know them?
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How is that knowledge obtained?
SOIL BEARING CAPACITY
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Since the weight of a building ultimately must
rest on the earth, the capacity for the soil to
resist loads – forever - is of major concern
before any appreciable advancement is made
to formulate a design.
Geotechnical Testing laboratories take soil
samples at construction sites at varying
depths and test the soil composition for
strength and stability. Generally, the
engineers will recommend a type of
foundation design for a specific type of
building.
Pictured here
Is a geotech
drilling team
making a test
boring.
As the auger
extends into
the ground,
the hollow
tube gathers
soil at all
depths.
o Multistory buildings will require deep footings that
extend to a rock strata for support.
o Medium height (2 to 4 story) buildings may be built
on soil with spread type footings, but must be
protected from adverse conditions, such as
inclusion of water and freezing.
o Light load buildings (1 & 2 story) generally have
foundations near the ground surface. These also
must be protected from adverse conditions.
PRIMARY ADVERSE CONDITIONS
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For low to medium height buildings,
sufficient bearing capacity can generally be
found within the soil structure, without the
need for rock bearing.
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But even with sufficient bearing capacity,
unstable soil can be a detriment to buildings
due to uneven settlement.

Shallow footings spread to areas required for
bearing capacity work against bearing design
when clay soil absorbs moisture and swells,
causing upheaval in varying parts of the
building. Cracks occur and increase the
potential for structural stress.
SOLUTIONS TO DIFFERENTIAL SETTLEMENT
IN SINGLE STORY BUILDINGS
• 1 Remove the soil in the immediate building
area to a satisfactory depth (usually around 4
feet) and install stable, compacted soil.
• 2 Stabilize the soil:
Mix stabilizing ingredient into the soil
such as cement or fly ash.
Inject lime slurry under pressure into the
soil
• 3 Where concrete floors are to be placed,
expand the soil to maximum volume by
saturating it with water, then seal with a
moisture barrier such as plastic sheeting.
SOIL FILL and EXCAVATION
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After sufficient information is known regarding
foundation type due to soil, and preliminary
corrections are made;
– Soil fill is added to or taken away from the site as
necessary to establish proper levels of surface
elevation for drainage and general terrain design.
– Soil fill is made and compacted in the area of the
building to establish finish soil grade before
preparations for installation of concrete foundation.
– Soil fill is always made in “lifts” – layers of not more
than 8” to 12” at a time, then compacted to assure
that all soil is sufficiently dense.
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The compaction of soil is done in order to
reduce the space between particles of soil as
much as possible so the density of the soil will
resist subsequent movement.
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Water is added to the soil as compaction is
done in careful amounts in order to densify the
soil to a percentage of maximum, with optimum
water content.
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Compaction is done with a variety of
equipment specific for the soil types, including
pronged ‘sheep’s foot’ rollers, flat rollers,
rubber-tired rollers, and vibrating rollers.
Hand-held compactors are used in small areas.
Vibrator compactor for small areas
Soil Placement & Compaction
Backfilling in “Lifts” – Compacted & Tested
Compactor - Dozer
Water Wagon
Sheeps Foot
Compactor
Vibratory Roller
Vibratory Roller
Sheepsfoo
Compaction Equipment
Walk - Behind Roller
Plate
Trench Sheeps Foot
Tamper
o Preparatory requirements by the Architect of a
project will determine how construction
procedures develop. The Architect will state in
the General Conditions and Specifications how
results of construction processes are verified.
For instance:
 After
placement and compaction of the soil,
compaction tests will be made by the Geotechnical
Laboratory that did the soil analysis.
 Engineers will take samples of the soil after
compaction is complete, then make tests in the lab
to determine compaction percentage results.
 Reports of the compaction tests are delivered to
the Architect, stating the moisture content and the
percentage of compaction.
Sample Location Selected
Sample Removed & sent to the
testing ‘Lab’
Sand Replaces Sample ‘Material’
‘In Place’ Volume of Sample Determine
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Maximum Density
Of soil with the
Optimum moisture
Content
Density test
results for
three soil
samples.
EXCAVATION FOR FOUNDATION
 After all dirt work is done, trenches are cut for
under floor piping such as plumbing, electrical,
fire sprinkler pipes etc., for installations that
must be made before concrete is placed.
 Then trenches are cut for foundation grade
beams, spread footings, and areas where the
floor slab will be made thicker for load bearing
walls.
 The surface of excavations that will form
concrete are sprayed with a material that will
not allow the dry soil to draw water from the
concrete.
o Required reinforcing steel is then installed at
the proper places and secured.
o Concrete is then placed in the excavation under
and around the reinforcing steel and tamped to
eliminate voids in the structural concrete.
o In areas where the soil is not stiff enough to
form the sides of beams & footings, and in
areas where appearance and finish of concrete
is essential, wood or steel forms will be set to
determine the limits of the finish concrete
structure.
OPEN TRENCH FOR STRUCTURAL GRADE BEAM
GRADE BEAMS AFTER CONCRETE HAS BEEN PLACED
EXCAVATION FOR A SWIMMING POOL AND INTEGRAL HO