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