slab on grade on shrink swell soil: a new ......slab on grade on shrink swell soil: a new method the...
TRANSCRIPT
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SLAB ON GRADE ON SHRINK SWELL SOIL: A NEW METHOD
The Cross‐USA Lecture
Professor Jean‐Louis BRIAUD
Distinguished ProfessorTexas A&M University
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CONTENT1. Shrink Swell Soils: a Background
2. Development of the method
3. The design charts
4. Superposition of loads
5. Verification
6. Case history
7. Example by hand
8. TAMU‐SLAB: Excel spread sheet
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Water content – volume change model
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WATER NORMAL STRESS
(SUCTION)
(pF )
TENSION COMPRESSION
0 uw (kPa)
(PORE PRESSURE)
UNSATURATED SATURATED
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Foundation solutionsfor shrink swell soils
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EXISITNG METHODS
1. BRAB (Building Research Advisory Board, 1968)
2. WRI (Wire Reinforcing Institute, 1981)
3. AS2870 (Australian Standard, 1996)
4. PTI (Post Tensioning Institute, 2012)
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Development of the Method
1. Quantify the effect of the weather on the soil 2. Obtain a realistic shape of the soil mound3. Place the foundation on top of that mound,
find Mmax, Vmax, and Δmax4. Parametric simulation study to identify main
parameters.5. Develop simple design charts6. Develop spread sheet TAMU‐SLAB
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Weather Effect on the Soil
College
Station
San
AntonioAustin Dallas Houston Denver
log (uf max / uf min) 0.788 1.392 0.866 1.295 1.283 1.374
log (ue max / ue min) 0.394 0.696 0.433 0.648 0.642 0.687
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Water content vs Time (2 years)
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Realistic Shape of Mound
2 211 12 12
2 1221 21122
2 1 exp 2 cosh2 2 1
cosh22
n
nfield field
h edgen
field
H H xn nHx f H ULH nnnH
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Mesh for numerical simulations
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Example results of Abaqussimulation
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Most Important Input Parameters
1. Shrink‐swell index ISS2. Depth of the active zone H
3. Change of soil surface water tension (suction)
ue max /ue min
4. Slab stiffness represented by the equivalent
flat slab thickness d
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Soil Weather Index
max
min
1 log2
e
eSW SS
uu
I I H
SW eI H w
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Equivalent Thickness
3 3
12 12D db S
1/3bd DS
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EDGE DROPUNIFORM LOAD
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
0.00 0.50 1.00EQ
UIV
AL
EN
T L
EN
GT
H L
eqv
(m)
SOIL WEATHER INDEX Isw = H.∆we
p = 10kpa
0.63
0.51
0.38
0.25
0.13
d (m)
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EDGE DROPLINE LOAD (EDGE)
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Superposition of uniform load and line load
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Predictions vs mean prediction
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Case history
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Soil movement vs depth (5 meters)
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Soil movement vs time (2 years)
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INPUT: Depth of active zone H = 3.0 m,Water content Δwf = 20%,Slab 20 m x 20 m,Beams S = 3.0 m, D = 1.2m, b = 0.3 m,Load p = 10 kPa, q = 10 kN/m.
CRITERION: L/Δ > 360
CALCULATION: Water content at edge of the slabΔwe = 0.5Δwf = 0.5 x 0.2 = 0.1,Soil weather index Isw = H x Δwe = 3 x 0.1 = 0.3m,Slab stiffness EI = EbD3/12 = 2 x 107 x 0.3 x 1.23/12 = 8.64 x 105kNm2,Equivalent depth d = D (b/S)1/3 = 1.2(0.3/3)1/3 = 0.56m
CHARTS: Uniform pressure: Leqv = 5.11m, Fv = 0.70, and FΔ = 2.21Line load: Leqv = 7.07m, Fv = 1, FΔ = 2.12
DESIGN EXAMPLE
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MOMENT, SHEAR, DEFLECTION CALCULATIONS
Uniform pressure Spacing of 3 m: P = 10 x 3 = 30kN/mMmax = PLeq
2/2 = 30 x 5.112/2 = 392kNmVmax = Fv P Leq = 0.70 x 30 x 5.11 = 107.31kNΔmax = PLeq
4/FΔEI = 30 x 5.114/2.21x 8.64x105 = 1.07 x 10-2m.
Line load Spacing of 3 m: Q = 3 x10 = 30kNMmax = Q Leqv = 30 x 7.07 = 212kN.mVmax = Fv Q = 1 x 30 kN = 30kNΔmax = Q Leqv
3/FΔEI = 30 X 7.073/2.12 X 8.64 X 105 = 5.80 X 10-3m.
Combined load Mmax(p) + Mmax(q) = 392 + 212 = 604kNmVmax (p) + Vmax(q) = 107.3 + 30 kN = 137.3kNΔmax (p) +Δmax (q) = 1.07 x 10-2+ 5.80 X 10-3 = 1.65 x 10-2m.
CHECK DISTORTION Leqv(ave) /Δmax = (5.11+7.07)/2 / 1.65 x 10 -2 = 369 > 360.
DESIGN EXAMPLE
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TAMU‐SLAB
• Excel SPREAD SHEET
• AUTOMATES THE DESIGN PROCESS
• INPUT SOIL, WEATHER, SLAB, LOAD
• CONTAINS ALL DESIGN CHARTS
• CALCULATES MOMENT, SHEAR, DEFLECTION
• CHECKS DISTORTION CRITERION
• TRIAL AND ERROR APPROACH