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Contrasting Design Approaches forContrasting Design Approaches forSlab-on-Ground and Raised FloorSlab-on-Ground and Raised Floor
Foundations on Expansive SoilsFoundations on Expansive Soils
Robert L. Lytton
Houston Foundation Performance AssociationHouston, Texas
December 9, 2009
OutlineOutline
• Elements of design• Site conditions• Slabs-on-ground• Raised floor foundations• Costs
1
Elements of DesignElements of Design
• Structural requirements Moments Shear Deflection
• Costs
2
As determined by site conditions
Slabs-on-Ground: SoilsSlabs-on-Ground: Soils
• ym: differential movement of soil Perimeter swelling Perimeter shrinking Center swelling Center shrinking
4
Slabs-on-Ground: SoilsSlabs-on-Ground: Soils
• em: edge moisture variation distance Not a cantilever distance A property of the soil mass Depends on soil activity and cracking
7
Slabs-on-Ground: SoilsSlabs-on-Ground: Soils
• zm: depth of the moisture active zone Depth of shrinkage cracks created by roots (log for root
fibers) No deeper than soil at the wilting point of plants
11
Slabs-on-Ground: SoilsSlabs-on-Ground: Soils
• zA: depth of the movement active zone Always shallower than zm
Large factor in ym
15
Raised Floor Foundations: SoilsRaised Floor Foundations: Soils
• ym: differential movement Center drying, total shrinkage Perimeter total swelling
• Drainage• Flower beds
Perimeter total shrinking: trees Downhill creep
18
NOTE
AM OUNT OF SHRINKAGE DEPENDS ON THE LEVEL OF RELATIVE HUM IDITYIN THE AIR BENEATH THE FLOOR. THE LOWER THE RELATIVE HUM IDITY, THEGREATER WILL BE THE AM OUNT OF SHRINKAGE. INITIALLY, THE SOIL WILLBE AT OR ABOVE 98% RELATIVE HUM IDITY. e m IS THE EDGE M OISTUREVARIATION DISTANCE WHICH VARIES TYPICALLY BETWEEN 3 AND 9 FEETDEPENDING ON THE TYPE OF SOIL.
NOTE
AM OUNT OF SHRINKAGE DEPENDS ON THE LEVEL OF RELATIVE HUM IDITYIN THE AIR BENEATH THE FLOOR. THE LOWER THE RELATIVE HUM IDITY, THEGREATER WILL BE THE AM OUNT OF SHRINKAGE. INITIALLY, THE SOIL WILLBE AT OR ABOVE 98% RELATIVE HUM IDITY. e m IS THE EDGE M OISTUREVARIATION DISTANCE WHICH VARIES TYPICALLY BETWEEN 3 AND 9 FEETDEPENDING ON THE TYPE OF SOIL.
FLOORFLOOR
DRY AIR SPACEDRY AIR SPACE
SHRINKAGESHRINKAGE
AIR SPACE BENEATH THE FLOORAIR SPACE BENEATH THE FLOOR
emem emem
ORIGINALGROUND LEVELORIGINALGROUND LEVEL
TOTAL MOVEMENT PATTERNSWITH RAISED WOOD FLOOR FOUNDATIONS
TOTAL MOVEMENT PATTERNSWITH RAISED WOOD FLOOR FOUNDATIONS
NOTE
AM OUNT OF HEAVE DEPENDS ON THE AM OUNT OF WATER THAT IS M ADEAVAILABLE TO THE SOIL AT THE EDGE OF THE BUILDING AND THE TYPEOF SOIL. e m IS THE EDGE M OISTURE VARIATION DISTANCE WHICH VARIESTYPICALLY BETWEEN 3 AND 9 FEET DEPENDING ON THE TYPE OF SOIL.
NOTE
AM OUNT OF HEAVE DEPENDS ON THE AM OUNT OF WATER THAT IS M ADEAVAILABLE TO THE SOIL AT THE EDGE OF THE BUILDING AND THE TYPEOF SOIL. e m IS THE EDGE M OISTURE VARIATION DISTANCE WHICH VARIESTYPICALLY BETWEEN 3 AND 9 FEET DEPENDING ON THE TYPE OF SOIL.
VERTICAL AND HORIZONTALMOISTURE BARRIERS
VERTICAL AND HORIZONTALMOISTURE BARRIERS
FLOORFLOORP OORDRAINAGEP OORDRAINAGE
WATE RWATE R
WATE RFLOW
WATE RFLOW
SLOPESLOPE
WATE RFLOW
WATE RFLOW
WATE RINGWATE RING
FLOWE R BE DFLOWE R BE D
emem e me m
HE AV EHE AV E HE AV EHE AV E
NOTE
AM OUNT OF HEAVE DEPENDS ON FREQUENCY OF LAWN IRRIGATION AND TYPEOF SOIL. e m IS THE EDGE M OISTURE VARIATION DISTANCE WHICH VARIESTYPICALLY BETWEEN 3 AND 9 FEET DEPENDING ON THE TYPE OF SOIL.
NOTE
AM OUNT OF HEAVE DEPENDS ON FREQUENCY OF LAWN IRRIGATION AND TYPEOF SOIL. e m IS THE EDGE M OISTURE VARIATION DISTANCE WHICH VARIESTYPICALLY BETWEEN 3 AND 9 FEET DEPENDING ON THE TYPE OF SOIL.
LAWN IRRIGATIONLAWN IRRIGATION
FLOORFLOOR
LAWNIRRIGATION
LAWNIRRIGATION
LAWNIRRIGATION
LAWNIRRIGATION
emem e me m
HE AV EHE AV E HE AV EORIGINALGROUND LE V E LORIGINALGROUND LE V E LHE AV E
NOTE
AM OUNT OF SHRINKAGE DEPENDS ON T HE DEPTH OF THE TREEROOT Z ONE, HOW FAR T HE ROOT S PENET RATE BENEATH THEBUILDING, AND T HE T YPE OF SOIL .
NOTE
AM OUNT OF SHRINKAGE DEPENDS ON T HE DEPTH OF THE TREEROOT Z ONE, HOW FAR T HE ROOT S PENET RATE BENEATH THEBUILDING, AND T HE T YPE OF SOIL .
TREE ROOT ZONETREE ROOT ZONE
WATER IS DRAWN INTOTHE ROOT ZONE DURINGDRY WEATHER
WATER IS DRAWN INTOTHE ROOT ZONE DURINGDRY WEATHER
TREESTREES
FLOORFLOOR
ORIGINALGROUND LEVELORIGINALGROUND LEVELSHRINKAGESHRINKAGE
NOTE
DOWNHILL CREEP OCCURS ON SLOPES THAT ARE STEEPER THAN 1 FOOT RISE IN 10 HORIZONTAL FEET. CREEP RATE IS GREATER THESTEEPER THE SLOPE AND THE WETTER THE SOIL IS .
NOTE
DOWNHILL CREEP OCCURS ON SLOPES THAT ARE STEEPER THAN 1 FOOT RISE IN 10 HORIZONTAL FEET. CREEP RATE IS GREATER THESTEEPER THE SLOPE AND THE WETTER THE SOIL IS .
DOWNHILL CREEPDOWNHILL CREEP
FLOORORIGINALGROUND LEVELORIGINALGROUND LEVEL
FLOOR
NO MOVEMENT PLANE
NO MOVEMENT PLANE
DOWNHILL CREEPM OVEM ENT
DOWNHILL CREEPM OVEM ENT
DOWNHILL CREEPM OVEM ENT
DOWNHILL CREEPM OVEM ENT
BUILDINGDISTORTION
BUILDINGDISTORTION
Raised Floor Foundations: SoilsRaised Floor Foundations: Soils
• em: edge moisture variation distance Not as important as with slabs on ground except around
the perimeter
24
NOTE
AM OUNT OF SHRINKAGE DEPENDS ON THE LEVEL OF RELATIVE HUM IDITYIN THE AIR BENEATH THE FLOOR. THE LOWER THE RELATIVE HUM IDITY, THEGREATER WILL BE THE AM OUNT OF SHRINKAGE. INITIALLY, THE SOIL WILLBE AT OR ABOVE 98% RELATIVE HUM IDITY. e m IS THE EDGE M OISTUREVARIATION DISTANCE WHICH VARIES TYPICALLY BETWEEN 3 AND 9 FEETDEPENDING ON THE TYPE OF SOIL.
NOTE
AM OUNT OF SHRINKAGE DEPENDS ON THE LEVEL OF RELATIVE HUM IDITYIN THE AIR BENEATH THE FLOOR. THE LOWER THE RELATIVE HUM IDITY, THEGREATER WILL BE THE AM OUNT OF SHRINKAGE. INITIALLY, THE SOIL WILLBE AT OR ABOVE 98% RELATIVE HUM IDITY. e m IS THE EDGE M OISTUREVARIATION DISTANCE WHICH VARIES TYPICALLY BETWEEN 3 AND 9 FEETDEPENDING ON THE TYPE OF SOIL.
FLOORFLOOR
DRY AIR SPACEDRY AIR SPACE
SHRINKAGESHRINKAGE
AIR SPACE BENEATH THE FLOORAIR SPACE BENEATH THE FLOOR
emem emem
ORIGINALGROUND LEVELORIGINALGROUND LEVEL
TOTAL MOVEMENT PATTERNSWITH RAISED WOOD FLOOR FOUNDATIONS
TOTAL MOVEMENT PATTERNSWITH RAISED WOOD FLOOR FOUNDATIONS
Raised Floor Foundations: SoilsRaised Floor Foundations: Soils
• zm: depth of the moisture active zone Same factors as with slab on ground Interest is in the total movement
26
Raised Floor Foundations: SoilsRaised Floor Foundations: Soils
• zA: depth of movement active zone Important to determine movement of supports
• Pads• Spread footings• Posts• Piers
Which to use? Determined by costs
28
Objective:design for expected movement
Objective: small support movement
Slab-on-Ground: Site HazardsSlab-on-Ground: Site Hazards
• Trees Shrinkage Affects zm, zA, em, ym
• Slopes Like differential shrinkage Rate depends on moisture Affects zm, ym
• Drainage Swelling Affects em, ym
29
Raised Floor Foundations:Raised Floor Foundations:Site HazardsSite Hazards
• Trees: shrinkage reduces total differentialmovement
• Slopes Like differential shrinkage Rate depends on moisture Affects zm, ym
• Drainage Swelling Increases total differential movement Affects ym
30
Slab-on-Ground: Building GeometrySlab-on-Ground: Building Geometry
• Shape factor affects structural design quantities Moment Shear Differential deflection
• Shape factor
Circle = 12.56 Square = 16 Rectangle = 3.7 ~ 1 SF = 24
31
Raised Floor Foundations:Raised Floor Foundations:Building GeometryBuilding Geometry
With adjustable floor elevation, does not matter
32
Slab-on-Ground:Slab-on-Ground:Structural RequirementsStructural Requirements
This is a soil-structure interaction problem.
• Soil is not a uniform pressure• em is not a cantilever distance• Structural properties of slab and soil are creep
(long-term) properties
35
Slab-on-Ground:Slab-on-Ground:Structural RequirementsStructural Requirements
• Ground pressures never come close to bearingcapacity
• zm is dictated by site conditions and not byregional conditions
• ym for both edge shrinking and edge swelling (> 1inch) must be estimated accurately
36
Slab-on-Ground:Slab-on-Ground:Structural RequirementsStructural Requirements
• em: for both edge shrinking and edge swelling isthe most sensitive predictor of design moments,shears, and required stiffness, EI.
• zA: is smaller than zm and is dictated by siteconditions and not by regional conditions
37
Slab-on-Ground:Slab-on-Ground:Structural RequirementsStructural Requirements
• β – length = relative stiffness length
• Dictates what is a long- and short-slab (> 6β islong)
• Controls design quantities for long slabs
48
Ec = creep stiffness of concreteEs = creep stiffness of soil
Design Criteria:Design Criteria:Deflections / ToleranceDeflections / Tolerance
• Sensitivity of foundation and super structure (Δ/L)• Differential (Δc+ Δs)
As built (Δc) Soil movement (Δs)
• Total• Twisting
49
Design Criteria:Design Criteria:StiffnessStiffness
• Substitute for deflection tolerance
• Enough concrete section to handle soil movement
50
Raised Floor Foundations:Raised Floor Foundations:Structural RequirementsStructural Requirements
This is NOT a soil-structure interaction problem.
• Moment• Shear• Deflection
are the same as for column-supported structures.
51
criteria
Raised Floor Foundations:Raised Floor Foundations:Structural RequirementsStructural Requirements
• Allowable deflections before adjusting supportelevations dictate the design quantities of the floorsupports
• Design of supporting footings, posts or piers is tomake rough estimate of total movement Vertical Rotation
52
DRILLED PIER REINFORCEMENT
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
0 2000 4000 6000
Perc
ent o
f Ste
el d
ue to
Sid
e Fr
ictio
n St
ress
Side Friction Stress, αc, (psf)
d = 1.0 ’
d = 1.0 ‘
d = 1.5 ‘
d = 2.0 ‘
d = 1.5 ‘
d = 2.5 ‘
d = 1.0 ‘d = 2.0 ‘
d = 2.5 ‘
d = 1.5 ‘
d = 2.0 ‘
d = 2.5 ‘
L = 15 ’ L = 10 ’
L = 10 ’
L = 15 ’
L = 10 ’
L = 15 ’
L = 5 ’L = 10 ’
L = 10 ’
L = 5 ’
L = 5 ’
L = 5 ’
Temperature and Shrinkage Reinforcement
Concrete Tensile
Strength Governs
Steel Yield
Stress Governs
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
0.000 1.000 2.000 3.000 4.000 5.000
Soil Suction, pF
Copyright John T. Bryant (2008)
Empirically Measured SuctionsEmpirically Measured SuctionsBCI 2002 to 2008 = 26,000+ Data PointsBCI 2002 to 2008 = 26,000+ Data Points
Reasonable Suction Change RangeReasonable Suction Change Range
2003 Total Soil Suction Data (4776 Observations)
0
200
400
600
800
1000
1200
Unsaturated Soil MechanicsTypical Suction Envelope
Initial Profile
EquilibriumProfile
Final Profile
Typical “Trumpet”shape based onMitchell and Lytton.
Exponential Suction Profile for Extreme WettingExponential Suction Profile for Extreme Wettingand Drying Conditionand Drying Condition
0
2
4
6
8
10
12
14
2 3 4 5
Suction (pF)
Dep
th (f
t)
Wet
Equil.
Dry
0
2
4
6
8
10
12
14
0 0.2 0.4 0.6
Volumetric Water Content
Wet
Dry
0
2
4
6
8
10
12
14
0 1 2 3
Vertical Movement (in)
Wet
Dry
Fort Worth Interstate 820
Mitchell (1979)
MoistureActivezone
( , ) exp - cos 2 -e on n
U Z t U U Z nt Z! !
!" "
# $ # $= + % & % &% & % &
' ( ' (
( ) exp -e on
U Z U U Z!"
# $= + % &% &
' (
NOTE
AM OUNT OF SHRINKAGE DEPENDS ON THE LEVEL OF RELATIVE HUM IDITYIN THE AIR BENEATH THE FLOOR. THE LOWER THE RELATIVE HUM IDITY, THEGREATER WILL BE THE AM OUNT OF SHRINKAGE. INITIALLY, THE SOIL WILLBE AT OR ABOVE 98% RELATIVE HUM IDITY. e m IS THE EDGE M OISTUREVARIATION DISTANCE WHICH VARIES TYPICALLY BETWEEN 3 AND 9 FEETDEPENDING ON THE TYPE OF SOIL.
NOTE
AM OUNT OF SHRINKAGE DEPENDS ON THE LEVEL OF RELATIVE HUM IDITYIN THE AIR BENEATH THE FLOOR. THE LOWER THE RELATIVE HUM IDITY, THEGREATER WILL BE THE AM OUNT OF SHRINKAGE. INITIALLY, THE SOIL WILLBE AT OR ABOVE 98% RELATIVE HUM IDITY. e m IS THE EDGE M OISTUREVARIATION DISTANCE WHICH VARIES TYPICALLY BETWEEN 3 AND 9 FEETDEPENDING ON THE TYPE OF SOIL.
FLOORFLOOR
DRY AIR SPACEDRY AIR SPACE
SHRINKAGESHRINKAGE
AIR SPACE BENEATH THE FLOORAIR SPACE BENEATH THE FLOOR
emem emem
ORIGINALGROUND LEVELORIGINALGROUND LEVEL
TOTAL MOVEMENT PATTERNSWITH RAISED WOOD FLOOR FOUNDATIONS
TOTAL MOVEMENT PATTERNSWITH RAISED WOOD FLOOR FOUNDATIONS
Raised Floor FoundationRaised Floor FoundationBoundary ConditionsBoundary Conditions
OUTSIDE pFWET
FLOWER BED 2.9LAWN WATER 3.0TREE 4.5
pF DRY R.H.BENEATH,%
6.50 10 6.35 20 6.22 30 6.10 40 5.98 50
NOTE
AM OUNT OF SHRINKAGE DEPENDS ON T HE DEPTH OF THE TREEROOT Z ONE, HOW FAR T HE ROOT S PENET RATE BENEATH THEBUILDING, AND T HE T YPE OF SOIL .
NOTE
AM OUNT OF SHRINKAGE DEPENDS ON T HE DEPTH OF THE TREEROOT Z ONE, HOW FAR T HE ROOT S PENET RATE BENEATH THEBUILDING, AND T HE T YPE OF SOIL .
TREE ROOT ZONETREE ROOT ZONE
WATER IS DRAWN INTOTHE ROOT ZONE DURINGDRY WEATHER
WATER IS DRAWN INTOTHE ROOT ZONE DURINGDRY WEATHER
TREESTREES
FLOORFLOOR
ORIGINALGROUND LEVELORIGINALGROUND LEVELSHRINKAGESHRINKAGE
Typical Clay Properties
Property Range ExampleLiquid Limit 55-90 86Plasticity 30-60 59Plastic Limit 25-30 27Percent Passing #200 80-99 97Percent Passing 0.002mm - 65
NOTE
VERTICAL AND HORIZONTAL M OISTURE BARRIERS DECREASE THE AM OUNTOF M OISTURE THAT GETS BENEATH THE WALLS OF THE BUILDING. EFFECTIVEDEPTHS OF VERTICAL M OISTURE BARRIERS ARE BETWEEN 2.5 AND 4 FEET.EFFECTIVE WIDTHS OF HORIZONTAL M OISTURE BARRIERS ARE BETWEEN 4 FEETAND e m, THE EDGE M OISTURE VARIATION DISTANCE.
NOTE
VERTICAL AND HORIZONTAL M OISTURE BARRIERS DECREASE THE AM OUNTOF M OISTURE THAT GETS BENEATH THE WALLS OF THE BUILDING. EFFECTIVEDEPTHS OF VERTICAL M OISTURE BARRIERS ARE BETWEEN 2.5 AND 4 FEET.EFFECTIVE WIDTHS OF HORIZONTAL M OISTURE BARRIERS ARE BETWEEN 4 FEETAND e m, THE EDGE M OISTURE VARIATION DISTANCE.
VERTICAL AND HORIZONTALMOISTURE BARRIERS
VERTICAL AND HORIZONTALMOISTURE BARRIERS
FLOOR
ORIGINALGROUND LE V E LORIGINALGROUND LE V E L
FLOOR
WATE RFLOW
WATE RFLOW
WATE RFLOW
WATE RFLOW
V E RTICALM OIS TURE BARRIE RV E RTICALM OIS TURE BARRIE R
HORIZONTALM OIS TURE BARRIE RHORIZONTALM OIS TURE BARRIE R
NOTE
AM OUNT OF HEAVE DEPENDS ON THE AM OUNT OF WATER THAT IS M ADEAVAILABLE TO THE SOIL AT THE EDGE OF THE BUILDING AND THE TYPEOF SOIL. e m IS THE EDGE M OISTURE VARIATION DISTANCE WHICH VARIESTYPICALLY BETWEEN 3 AND 9 FEET DEPENDING ON THE TYPE OF SOIL.
NOTE
AM OUNT OF HEAVE DEPENDS ON THE AM OUNT OF WATER THAT IS M ADEAVAILABLE TO THE SOIL AT THE EDGE OF THE BUILDING AND THE TYPEOF SOIL. e m IS THE EDGE M OISTURE VARIATION DISTANCE WHICH VARIESTYPICALLY BETWEEN 3 AND 9 FEET DEPENDING ON THE TYPE OF SOIL.
VERTICAL AND HORIZONTALMOISTURE BARRIERS
VERTICAL AND HORIZONTALMOISTURE BARRIERS
FLOORFLOORP OORDRAINAGEP OORDRAINAGE
WATE RWATE R
WATE RFLOW
WATE RFLOW
SLOPESLOPE
WATE RFLOW
WATE RFLOW
WATE RINGWATE RING
FLOWE R BE DFLOWE R BE D
emem e me m
HE AV EHE AV E HE AV EHE AV E
NOTE
DOWNHILL CREEP OCCURS ON SLOPES THAT ARE STEEPER THAN 1 FOOT RISE IN 10 HORIZONTAL FEET. CREEP RATE IS GREATER THESTEEPER THE SLOPE AND THE WETTER THE SOIL IS .
NOTE
DOWNHILL CREEP OCCURS ON SLOPES THAT ARE STEEPER THAN 1 FOOT RISE IN 10 HORIZONTAL FEET. CREEP RATE IS GREATER THESTEEPER THE SLOPE AND THE WETTER THE SOIL IS .
DOWNHILL CREEPDOWNHILL CREEP
FLOORORIGINALGROUND LEVELORIGINALGROUND LEVEL
FLOOR
NO MOVEMENT PLANE
NO MOVEMENT PLANE
DOWNHILL CREEPM OVEM ENT
DOWNHILL CREEPM OVEM ENT
DOWNHILL CREEPM OVEM ENT
DOWNHILL CREEPM OVEM ENT
BUILDINGDISTORTION
BUILDINGDISTORTION
Shallow Slope Failure
GRASS
ROOTS
During dry periods roots extract waterFrom the soil and cause shrinkage cracks
Foundation soil propertiesFoundation soil properties
Moisture Conditions Beside Raised Floors Beneath Raised Floors
Differential MovementsSoil PropertiesSite Investigation
Raised Floor FoundationRaised Floor FoundationBoundary ConditionsBoundary Conditions
OUTSIDE pFWET
FLOWER BED 2.9LAWN WATER 3.0TREE 4.5
pF DRY R.H.BENEATH,%
6.50 10 6.35 20 6.22 30 6.10 40 5.98 50
OutlineOutline
• Elements of design• Site conditions• Slabs-on-ground• Raised floor foundations• costs
109
Elements of DesignElements of Design
• Structural requirements Moments Shear Deflection
• Costs
110
As determined by site conditions