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Draft Subject To Revision 3/7/2012
1
Compaction Grouting Consensus GuideShort Course
Compaction Grouting DesignDr. Dawn Shuttle, PE
Objectives of the Design Module
1.Become familiar with standard CGdesign.
2.Know what can be densified, and why.
3.Understand why slower injection isbetter…..
4.Understand the limitations of standardanalysis of compaction grouting.
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Slides 5 to 8 have been relocated to‘advances’
Start with the conceptual model ….Soil Behavior 101
Loose soil contracting during shear….
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3
Soil Behavior 101
Dense soil expanding during shear….
BUT we don’t want the soil to expand
Soil Behavior 101
-5-4-3-2-1012345
volu
met
ric s
train
: %
contraction
expansion
0
200
400
600
0 5 10 15axial strain: %
devi
ator
stre
ss, q
: kPa
Loose
-5-4-3-2-1012345
volu
met
ric s
train
: %
contraction
expansion
0
200
400
600
800
1000
0 5 10 15axial strain: %
devi
ator
stre
ss, q
: kP
a
Dense
Easy tocompactiongrout Impossible to
compactiongrout
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4
0.500
0.600
0.700
0.800
0.900
10 100 1000 10000
Mean effective stress, p' (kPa)
Void
ratio
, e
emax
CSL
G667
G665G662
G681G682
G685
cee
Erksak
Loose
Dense
Relationship between Dr and SoilDescription
RelativeDensity (%)
DescriptiveTerm
Approximate
0 – 15 Very loose +0.1
15 – 35 Loose 0.0
35 – 65 Medium Dense -0.05
65 – 85 Dense -0.1
85 – 100 Very Dense -0.3
minmax
max
eeeeDr
0 3
3 10
10 18
18 30
>30
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Density matters
• CG only works for‘looser’ soils
• Generally > 0.05• If soil is ‘dense’ CG canactually make looser….
• If in doubt, considersome shear box (ortriaxial) testing
• Or consider CPT testing-5-4-3-2-1012345
volu
met
ric s
train
: %
contraction
expansion
0
200
400
600
0 5 10 15 20 25 30 35 40axial strain: %
devi
ator
stre
ss, q
: kPa
Dense
What about the increase of meaneffective stress?
By far the most “bang for your buck” from shearing
mean stress increase
shearing
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6
Drained vs. Undrained
Drained loading
Undrained loading +consolidation
Modified from Kovacevic et al., 2000
contraction
Drained or Undrained ?
ALWAYS DRAINED !
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7
Practically Injection Rate
• Typical injection rate is on the order of2ft3/min. ( 57 litres/min)
• Slower rates may be needed for sensitiveapplications.
• Higher rates may be used under certainconditions (but likely needs advancedmonitoring and/or analysis).
CG Mechanics – Injection Rate• However, grouting at high injection ratesmay be harder to control.
• High injection ratesmay produce undrainedbehavior.
– Densification is reduced significantly.
– Injection pressure is greatly increased.
• Rule of thumb, 8 psi/min; 1 to 2 ft3/min.55 kPa; 28 to 57 litres/min
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CG Mechanics – Injection Rate• Injection rates should be slow enough toallow pore pressure dissipation consistentwith the surrounding soils’ permeabilityrate. => successful compaction
• Unbalanced injection rates can producehydraulic fracturing.
• Hydrofracture formation can preclude thecontrolled intergranular shearing necessaryto successful densification.
Soils types suitable for CG
• Need the grout to displace the grout, notpermeate into the soil’s pore space
• Need pore pressures to dissipate….
GRAVEL USUALLY NOT SUITABLE
But low mobility grout is good for filling voids
CLAYS NOT SUITABLE, SILTS ????
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Pressure at depth
• The pressure that matters is the pressure atgrout injection – not at surface
• Measuring this will be covered in detail in“Field Advances”
• Simpler (although less accurate) approach isthe offset approach
gaugepressure
length
Atmosphericpressure
Pressure drop down groutpipe = pressure drop / length
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Design – questions before we start
• Is the soil suitable ? (density, type)
• Layering ?
• Nearby structures or topography changes ?
Design considerations
• Layering ?
from Byle (2000)
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Design considerations
• Nearby structures ?
• Topography change ?
Building foundations
Utilities
Slope
Cliff
Effect of confinementvery loose
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Effect of confinementloose
With confinement = +0.05
Very Loose Silty Sand
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0 0.25 0.5 0.75 1 1.25 1.5
Radial Distance from Grouthole (m)
Initial Void Ratio1 ft3 ( 0.03 m3)2 ft3 (0.06 m3)3 ft3 (0.09 m3)4 ft3 (0.11 m3)5 ft3 (0.14 m3)7 ft3 (0.20 m3)
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with confinement = -0.10
Medium Dense Silty Sand
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0 0.25 0.5 0.75 1 1.25 1.5
Radial Distance from Grouthole (m)
Initial Void Ratio1 ft3 ( 0.03 m3)2 ft3 (0.06 m3)3 ft3 (0.09 m3)4 ft3 (0.11 m3)5 ft3 (0.14 m3)7 ft3 (0.20 m3)
CG Mechanics – Confinement• Soil densification more effective if confinedby stiffer soil zones, either above or adjacentto the expanding grout mass.
• Enables development of higher localstresses, forcing particles closer togetherduring shear.
• Importance of drilling/injecting in sequenceon a grid pattern.
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Grout quantity estimate
• To reliably estimate grout quantities you needreliable in situ soil properties– Sadly, there really is no way round this– Will the soil dilate or contract ?– Density– Layering
• Need a good low mobility grout• Control of injection• Slow injection• Remember some ‘leakage’
10 m
8 m
Plan
Layer 1: Dense sand
Layer 2: Fill
5 m d = 1360 kg/m3
Layer 3: Rock
Soil Profile
Proposed Building
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Example ‘grout volume estimate’calculation
• Current density (kg/m3) = 1360
• Target density (kg/m3) = 1700
• Required reduction in volume (%) 20 %mass of soil remains constant
Mass = Volume * Density
Convenient to assume initial volume = 1
1 * 1360 = Final Vol * 1700
Final Vol = 0.80
1 – 0.8 = 0.2
Example ‘grout volume estimate’calculation
• Volume of soil to be grouted (m2) = 5*(8+10) * (10+10) = 1800
• Volume of grout to be injected (m2) = 0.2 * 1800 = 360
• Volume per metre depth (m2) = 360 / 5 = 72
• Spacing = 2.5 m (usually about 2 – 3.5 m)
• Number of holes 18 /2.5 * 20/2.5 72
• Grout per hole per m depth (m3) = 360/(5*72) = 1.(radius = 0.56 m)
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Layout of the holes
row
row
row
row
row
Typical spacing 6 12 ft (1.8 – 3.6m)
Confine
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Fill in
Remember the columns will move…
so need to “round up” volumes
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Controlling Grout Injection(and the things to watch…)
• Pressure limit at a given injection rate– The injection rate matters !
• Volume of grout limit– This is usually quoted per stage
• Monitor movements– Buildings, utilities, etc
• Maximum lift at surface– Raising the ground surface has little effect on thein situ soil density (i.e. avoid!!)
Points for the contract
• Remember you’re paying to have the soilmade denser – NOT to pump grout.
• So either– Specify the final soil density and how this densitywill be confirmed, or
– Specify RATE as well as volume and limitingpressure for refusal
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Points to take away
• Compaction grouting works by shearing thesoil to very high strains
• Loose soils densify during shearing (but densesoils can loosen)
• Undrained soils don’t change volume duringshearing (so CG won’t work)
• Compaction grouting is not difficult to design,but does need a good knowledge of the soiland the site
Thank you for your kind attention
The Presenters for the Compaction Grouting Consensus Guide Short Course, their employers, affiliates,successors, and anyone acting under their authority disclaim any and all liability for any personal injury, propertydamage, financial loss, or other damages of any nature whatsoever, including without limitation any direct,indirect, special, exemplary, or consequential damages, resulting from any person’s use of, or reliance on any andall materials presented at the aforesaid short course, which is presented for information purposes only.