foundry sands as structural fill for embankments and
TRANSCRIPT
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Foundry Sands as Structural Fill for
Embankments and Retaining Walls
Craig H. Benson, PhD, PE
Wisconsin Distinguished Professor
Recycled Materials Resource Center
University of Wisconsin-Madison [email protected]
www.recycledmaterials.org
© University of Wisconsin-Madison 2007
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What is foundry sand?
Foundry sands are sand-bentonite mixtures.
Base Sand
85%
Organic 3%
Water 5%Bentonite 7%
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Index Properties
• Fine Sand
• Fines: typically 10 – 12%
• 2 mm Clay: typically 3 to 10%
• PI: typically NP-5
• SC, SP, or SP-SM or A-2-4 or A-3
• Gs: 2.52 to 2.73 (Base Sand = 2.66)
• Roundness: Subrounded to subangular (R
= 0.5 to 0.7)
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0
20
40
60
80
100
0.010.1110
Pe
rce
nt
Fin
er
(%)
Particle Diameter (mm)
14 Foundry Sands from
WI, IL, MI, & IN
Particle Size Distribution
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Foundry sand
being spread as
sub-base at STH
60 field site.
Foundry sand being
compacted at STH 60
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Conventional
Wall MSE Wall
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Embankments
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Engineering Analyses for Retaining
Walls and Embankments
• Overturning (RW)
• Sliding (RW)
• Global stability (RW and embankment)
• Settlement (RW and embankment)
• Practical issues (drainage, raveling, etc.)
Methodology: same approach used with conventional
earth fills
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Issues for the Engineer
• What is the shear strength of foundry sand?
• What is the interface shear strength with
geosynthetics?
• What are the pullout characteristics of
geotextiles and geogrids?
• What is the compressibility?
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0
20
40
60
80
100
0 10 20 30 40 50 60
Sh
ear
Str
ess (
kP
a)
Normal Stress (kPa)
ca (kPa)f
aSoil
28.050oFoundry Sand A
16.739oFoundry Sand B
19.243oFoundry Sand C
28.042oFoundry Sand D
5.542oPortage Sand
8.342oBase Sand
A
DC
B
BaseSand
PortageSand
Direct Shear Strength of Foundry Sands
Soaked
f ~ 40o
c’ ~ 0
Unsoaked
f ~ 40o
c’ varies
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Mirafi Geogrid
Belton 113
Geotextile
Textured
Geomembrane
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LVDT
Load Cells
Porous Stone Substrate
Soil
Pressurized
Air Bladder
LVDT
Geosynthetic
Direction of
Displacement
Track
Lower
Shear Box
Clamp Geosynthetic
Clamp
Large-Scale (D 5321) Direct Shear Machine
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Interface Direct Shear Box (300 mm x 300 mm)
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Frictional Efficiencies
E(%) = tand/tanf’ x 100
Geotextile:
Base Sand - 83%
Foundry Sands - 61 to 74%
Geogrid:
Base Sand - 96%
Foundry Sands - 51 to 71%
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Geosynthetic Pullout Tests
Pullout behavior measured in a pull-out box.
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Geosynthetic Pullout Box
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Interaction Coefficients - Geotextile
0.0
0.5
1.0
1.5
2.0
0 10 20 30 40 50 60
Base Sand (Fines = 1.1%)Sand A (Fines = 6.5%)Sand C (Fines = 11.3%)Sand D (Fines = 19.2%)
Inte
ractio
n C
oe
ffic
ient
Normal Stress (kPa)
Woven Geotextile
'tan'A2
PC
if
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Interaction Coefficients - Geogrid
0.0
0.5
1.0
1.5
2.0
0 10 20 30 40 50 60
Base Sand (Fines = 1.1%)Sand A (Fines = 6.5%)Sand C (Fines = 11.3%)Sand D (Fines = 19.2%)
Inte
raction C
oeff
icie
nt
Normal Stress (kPa)
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Hydraulic Conductivity of Foundry Sands
10-9
10-8
10-7
4 6 8 10 12 14 16 18 20
Lab - Mod.ProctorLab - Std. ProctorLab - Red. ProctorField - TSBField - SDRIField - Shelby Tubes
Hyd
rau
lic C
on
du
ctivity (
cm
/se
c)
Compaction Water Content (%)
Optimum water content
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Effect of Bentonite Content
10-9
10-8
10-7
10-6
10-5
10-4
10-3
10-2
10-1
0 5 10 15 20
Laboratory Data
Mean of Field Ks
SDRI
Hyd
raulic
Co
nductivity (
cm
/sec)
Bentonite Content - By Weight (%)
13
2
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Drainage Issues - 1
• Green sand backfills will not drain like a
clean sand backfill. Core sand usually will
drain satisfactorily.
• For retaining structures, provide drainage
along the back face of the retaining wall to
prevent pore pressure build up. Use
geosynthetic drainage material.
• Check filtration criteria for geotextile
separating drainage layer and foundry
sand.
for pore water pressure
•
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Conventional (AASHTO) Filter Criteria
For soils with < 50% fines (nearly all foundry
sands):
O95 (GT) < 0.6 mm
AOS (GT) > No. 30 sieve size
where O95 or AOS is provided by geotextile
manufacturer.
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Luettich’s Filter Criteria
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Drainage Issues - 2
• Compact green sands dry of optimum
water content to increase hydraulic
conductivity.
• Compaction dry of optimum will also result
in higher undrained strength and lower
compressibility.
• Select sands with lower bentonite content
if practical.
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Raveling and Erosion
• Green sands will erode internally and
externally in the presence of flowing water.
• Geotextile filters on drainage mats will
prevent internal erosion in retaining walls.
• Foundry sand surfaces should be covered
with surface layer soils and vegetated to
minimize surface erosion.
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Retaining Wall and Structural
Fill Design Recommendations
• f’ = 34o, c’ = 0 (conservative)
• E = 50% (geogrid) or 60% (geotextile)
• Ci = 1.0 (low normal stresses)
• Ci = 0.5 (higher stresses)
• Compact dry of optimum
• Provide drainage on the back face of
retaining walls. Evaluate stability with moist &
saturated unit weight in calculations.
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Construction Issues
Foundry sands behave like
natural sands with fines.
Foundry sands can be very
dry when delivered.
Adding moisture improves
compaction and reduces
dust.
Foundry sands can be
contaminated with other
debris. Be sure the
foundry follows good waste
separation practices.
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Conventional moistening methods work well.
Stockpiling over winter season particularly
effective.
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Disking or roto-
tilling sand
improves
moisture
distribution
significantly, but
is only required if
strict control on
water content is
required.
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Compacting green
sand with a footed
roller improves
compaction.
Padfoot compactors
work best.
Vibration is not
required or
beneficial unless
working with core
sand.
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Moistened sand tracks in and compacts well. However,
wet sand will compact poorly and rut significantly. Avoid
standing water by grading surface at close of work day.
A smooth drum seal is very beneficial.
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Concluding Comments
• Check with sand supplier beforehand to determine
typical characteristics of sand and known ranges for
key sand properties: d50, Cu, fines content,
bentonite content. Compare against data in this
presentation.
• Run a drained direct shear test (ASTM D 3080) on
the sand if concern exists regarding sand quality.
• Run a compaction test (ASTM D 698 or D 1557) to
define maximum density and optimum water content
for compaction control.
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Concluding Comments
• Measure water content of sand (ASTM D 2216)
from supplier to determine level of moisture
conditioning required in the field.
• Compare water contents measured during
construction using the nuclear gage (ASTM D 6938)
to oven-dry water contents (ASTM D 2216) to check
suitability of nuclear gage for sand being used in the
field. Conduct this test at the start of construction to
validate nuclear method.
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Acknowledgements
Federal Highway Administration
United States Environmental Protection Agency
Wisconsin Recycling Market Development Board
Wisconsin Solid Waste Research Program
Wisconsin Department of Natural Resources
Wisconsin Cast Metals Association
Wisconsin Department of Transportation