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AN INTRODUCTION TO MICROPILES
CIVL 141Spring 2011
A Local Situation:5
to 1
2 fe
et(v
arie
s)
35
fee
t
Fill (SM /SP-SM /SW -SM ) = 101 pcf'C S = 32°c = 0
F ine sand w ith s ilt(SP-SM ) = 95 pcf'C S = 32°c = 0
S urface (E levation = 1200 ft m sl)
W ater Table(as observed at the tim e of drilling)
A Shallow FootingDefinition:
B
D f
Df/B < 2
Capacity of Spread Foundations(cohesionless soils)
F
qq ult
allow
γγqqult CNBγ0.5CNσq
Capacity of Spread Foundations(cohesionless soils)
F
qq ult
allow
γγqqult CNBγ0.5CNσq
THIS ONLY CONSIDERS THE SHEAR STRENGTH OF THE SOIL;WHAT ABOUT SETTLEMENTS?
Settlement-Based Capacity
• Use a bearing pressure that results in meeting some allowable settlement.– Less than 1 inch of total settlement; or
– Less than ½ inch of differential settlement.
Settlement-Based Capacity
• Use a bearing pressure that results in meeting some allowable settlement.– Less than 1 inch of total settlement; or
– Less than ½ inch of differential settlement.FOR COHESIONLESS SOILS (SANDS),
THIS CRITERIA GENERALLY CONTROLS. THE LOST CAPACITY FROM
SHEAR STRENGTH CAN BE SIGNIFICANT.
FOR MANY PROJECTS, THERE IS MOTIVATION TO INCREASE THE BEARING LOAD WITHOUT ADDITIONAL
SETTLEMENTS
OR SIMPLY REDUCE THE EXPECTED SETTLEMENTS ALTOGETHER.
FOR MANY PROJECTS, THERE IS MOTIVATION TO INCREASE THE BEARING LOAD WITHOUT ADDITIONAL
SETTLEMENTS
OR SIMPLY REDUCE THE EXPECTED SETTLEMENTS ALTOGETHER.
HOWEVER, A CHALLENGE EXISTS TO DO SOWHILE MAINTAINING CONSTRUCTION BUDGETS
Lower the Bearing Pressure
Lower Bearing Pressures Do Not Always Provide Less Settlement
2B
Hard bearing layer
Com pressib le layer
Hard bearing layer
B
qallow = 2,000 psf
Lower Bearing Pressures Do Not Always Provide Less Settlement
B2B
Hard bearing layer
Com pressib le layer
Hard bearing layer
qallow = 4,000 psf
More-Often, This is the Observed Design Situation
2B
Com pressib le layer
Hard bearing layer
B
Currently Available Alternatives For Reducing Settlement
• Densify Bearing Soils– Over-Excavate and Recompact– Rapid Impact Compaction
• Deep Foundations (drilled shafts or piles)• Intermediate Solutions
– Rammed Aggregate Piers– Helical Piers – Micropiles
Over-Ex and Recompact
3 '
2 '5 '5 '
Micropiles
Micropiles
• Micropiles are high capacity, small diameter (typically 5" to 12") drilled and grouted in-place piles designed with steel reinforcement to primarily resist structural loading.
Micropiles
• Micropiles are high capacity, small diameter (typically 5" to 12") drilled and grouted in-place piles designed with steel reinforcement to primarily resist structural loading.
• Micropiles do not necessarily use hollow-core bars but this is what I will present today.
Micropile Types
Hollow-Core Bar (Fy = 75 ksi)
Pressure-Grouted Micropiles
• This discussion will be limited to hollow-core, pressure-grouted micropiles because they can be readily adapted to shallow foundations.
• Too, flow of grout is ensured over the complete length of the micropile.
Structural Limitations
Ref: FHWA Manual, Section 5
(Through full-scale tests, elastic buckling does not appear to be a design concern and is not considered by FHWA.
(note: IBC uses 0.33 and 0.40 instead of 0.40 and 0.47, respectively)
Compression:
BARBARyGROUTGROUTC AreaFAreaf ,, .. 470400P AllowC,
Structural Limitations
Ref: FHWA Manual, Section 5
(note: IBC uses 0.6 instead of 0.55)
Tension:
BARBARy AreaF ,.550P AllowT,
Geotechnical Limitations
Capacity of a Micropile
TIPLult PPP
Capacity of a Micropile
Ldiam.bit3.141
F
αP soilgrout
L
grout-soil can be 3 to 10 kips per square foot, found from a test;
F typically taken as 2.5;
bit diameter is conservative; and
L = bond length of micropile.
Capacity of a Micropile
qtip; taken as 32 ksf (Reese et al. 1976)
F typically taken as 3;
Rf = 0.6B
2
f
tipTIP diam.bit
4
3.141
RF
qP
Example
qtip = 32 ksf for medium dense sands;
Bit diameter = 0.5 feet;
L = 10 feet
F is 3 for end bearing and 2.5 for side friction; and
Rf = 0.6(0.5 feet);
grout-soil = 7 ksf
Example
kips44feet 10feet 0.53.1412.5
ksf 7PL
kips7feet 0.54
3.141
feet50*60*3
ksf32P 2
TIP
..
Pult = 51 kips
Estimating grout-soil
20 f
t
PV C sleeve
10
ft lo
ng
bon
ded
zo
ne
grout
e lectron ic load ce ll
d ia l gaugereference beam
hydrau lic ram
1-in th ick bearing p la te
1-in th ick bearing p la te
nutR 38 m icrop ile
load ing beam
w ood cribb ing as support
Estimating grout-soil
Estimating grout-soil
BARBARy AreaF ,AllowT,P
BARBARyGROUTGROUTC AreaFAreaf ,,. 850P AllowC,
Structural Limitations During Testing
Grout-Soil Shear Strength from a Vertical Tension Test
10-ft long bonded zone
0 10 20 30 40 50 60 70
L o a d (k ip s)
-0 .2
0
0.2
0 .4
0 .6
0 .8
De
form
atio
n (
inch
es)
Grout-Soil Shear Strength from a Vertical Tension Test
;ft10ft 0.53.141
2.5
αkips50 soilgrout
psi55kips/ft8α 2soilgrout or
Word of Caution (or a Strong Reason to Use Hollow-Core Bars)
Example situation
0 10 20 30
-80
-40
0
40
80
Sh
ea
r (k
ips)
0 10 20 30
-300
-200
-100
0
100
200
300
Mo
me
nt (
kip
-fe
et)
0 10 20 30
w = 7 kips/foot
35 k ips 35 k ips
70 k ips
Loa
d
Example situation
0 10 20 30
-80
-40
0
40
80
Sh
ea
r (k
ips)
0 10 20 30
-300
-200
-100
0
100
200
300
Mo
me
nt (
kip
-fe
et)
0 10 20 30
w = 7 kips/foot
35 k ips 35 k ips
70 k ips
Loa
d
Assumes no support from soil
Example situation
0 10 20 30
-80
-40
0
40
80
Sh
ea
r (k
ips)
0 10 20 30
-300
-200
-100
0
100
200
300
Mo
me
nt (
kip
-fe
et)
0 10 20 30
w = 7 kips/foot
35 k ips 35 k ips
70 k ips
Loa
d
Shear in concrete – acceptable?
Example situation
0 10 20 30
-80
-40
0
40
80
Sh
ea
r (k
ips)
0 10 20 30
-300
-200
-100
0
100
200
300
Mo
me
nt (
kip
-fe
et)
0 10 20 30
w = 7 kips/foot
35 k ips 35 k ips
70 k ips
Loa
d
Negative moment
Connection to Spread Footing
South Mountain Community College,Phoenix, AZ
Double-reinforced footing (grade beam)
Connection to Spread Footing
• Proof loading is typically completed on about 5% of the production micropiles.
• Loading is completed to 167% of the Design Test Load.
• Creep test may or may not be completed.
Proof Loading
Proof Loading Mt. Lemmon HighwayMilepost 9.8
Tucson, ArizonaDec. 2009
Proof Loading Mt. Lemmon HighwayMilepost 9.8
Tucson, ArizonaDec. 2009
Proof Loading Mt. Lemmon HighwayMilepost 9.8
Tucson, ArizonaDec. 2009