Micropiles – Applications & DesignTom Richards, PE, Chief Engineer

&Metrolink Case History

Roger Baldwin, Vice PresidentNicholson Construction Company

Micropile NamesMicropile ( DFI & FHWA)= Pin PileSM ( Nicholson)= Minipile ( Hayward Baker)= Bored-in Pile ( NYSDOT)= Small Diameter Grouted Piles ( Mass.

Building Code)= <12” diameter drilled and grouted

Why use Micropiles?

Physical Factors• Difficult Access• Low Headroom• Can be Drilled Through Virtually

Any Ground Condition

Nicholson Crew ensured that the mall was cleaned and functional for normal business

Inside work completed at night

Our Most Popular Picture

Physical Factors• Vibration/Noise Sensitivity• Settlement Sensitivity• Underpinning

NIPSCO - Chesterton, IN

MANDALAY BAY, LAS VEGAS, NV

Two reaction beams, four bars grouted into the existing mat, and one jack transferred the load to each pile. A bond breaker was used to assure that the pile did not bond to the existing mat.

Geotechnical Factors• Karstic Limestone• Bouldery Ground• Glacial Till• Variable and/or Random Fill• Man-Made Obstructions• Rock Formations with Variable

Weathering• Soils under High Water Table

Pin Piles Ideally Suited For:• New Construction• Reconstruction Over Old Substructures• Underpinning Existing Foundations• Rehabilitation for:

• Deteriorated Wood Piles• Increasing Load Capacity• Seismic Upgrades

Construction Steps

Installation Drilling Methods

• Hollow Stem Augers (N/A)• External Flush• Duplex• Rotary Eccentric Percussive

Duplex

External Flush Drilling•Opening larger than casing size•Risk of ground loss in cohesionless soils•Good cleaning of borehole•Tremie grouted through casing - then pulled with tremie head or excess pressure - enhances lateral confinement

Duplex Drilling•Often specified - least risk•Minimal loss of ground in cohesionless soils•Grouted through the casing - then pulled with tremie head or excess pressure

Duplex Drill Casing and Roller Bit

Water Flush thru Duplex Ejector

Rotary Eccentric Percussive Duplex DrillingWe call it Tubex - Some call it a mouthful

Materials• Steel Casing

• Use Mill Secondary Oil Field Casing

• Typically Flush threaded joints • 80 ksi minYield Strength

Materials

• Grout• Neat Cement with

Water/Cement Ratio of 0.45• Compressive Strength of 4-6 ksi• Improved Stiffness

w/Confinement

Materials• Reinforcing Bars

• Grade 60• Grade 75• Grade 150• Centralizers are used

PILE DESIGN

0 50 100 150 200 250 300 350 400 450 500

ACI with LF = 1.55

FHWA Micropiles

AASHTO Caisson

AASHTO Driven Unfilled

AASHTO Driven Concrete Filled

AREA Driven CIP Concrete

AREA Drilled Shafts

MASS BLDG CODE

IBC2000 & BOCA Drilled uncased piles

IBC2000 Concrete filled pipe piles > 8"

IBC2000 Concrete filled pipe piles

IBC2000 Caisson Piles > 18"

BOCA Concrete filled pipe piles > 8"

UBC 1808.2.2 Uncased CIP Concrete Piles

UBC 1808.3.2 Metal Cased Concrete Piles

UBC 1808.7.2 Concrete-filled Pipe Piles

UBC 1808.7.2 Concrete-filled Pipe Piles

Allowable Load in KipsCODE

7”OD x 0.5” wall casing with 5 ksi grout

Allowable Strength Formula - Cased Section

Allowable load

* Note 0.40/0.47 = 0.85 agrees with Whitney Stress Block for USD Concrete

0.40 is per AASHTO 8.15.2.1.1; 0.47 is per AASHTO, i.e. 1 \ 2.12 = 0.47

Detail of load transfer through the casing plunge length.

Pile Uncased Length

DFI Guide Spec Allowable Compression

Pallc = (0.33 * fc * Agrout + 0.4 * fycasing * Acasing + 0.4 * fybar * Abar)

where: Pallc = allowable working load (compression)fc = Unconfined Compressive Strength of groutAgrout = area of groutfycasing = yield strength of casing up to 80 ksiAcasing = area of steel casing (with allowance for corrosion if appropriate)fybar = yield strength of rebar/core steel up to 80 ksiAbar = area of rebar/core steel

The maximum useable strength of the steel of 80 ksi is based on the typical ultimate concrete strain of 0.003 (29000 ksi * 0.003 = 87 ksi). 80 ksi is also the maximum steel strength used in ACI 318. In the cased section, additional confinement of the casing yields higher grout strength due to triaxial effects.

Buckling“Most pile designs that have soil surrounding the pile have no reduction for buckling”, except piles :•extending above ground, •piles subject to scour, •piles through mines/caves, and •piles through soil that will liquefy.

Buckling may be evaluated using:•assigned buckling length say 5 feet into soft soil•LPILE ( new version of COM624)•Poulus & Davis “ Pile Foundation Analysis & Design”•ADSC White Paper

Geotechnical Design• Tip Resistance Neglected• Typical Friction Pile• Pall = σπdLwhere:σ = Allowable Bond Stress of Soil/Rock in Bond

Zoned = Diameter of Bond ZoneL = Length of Bond Zone

Geotechnical Aspects

• Non-Plastic Clays or Silts• Sands & Gravels• Rock Formations• Combination Materials

Deflection Analysis

E*AL*P=δElastic Theory for Simple Column

ii

ii

E*AL*P∑=δElastic Theory for Column with Multiple Cross-Sections

dz)z(E*)z(A)z(PL

0∫=δ

Therefore, Elastic Deflection = Area under P/AE vs depth curve

Load Test Acceptance Criteria -FHWA Micropile Guidelines and DFI Guide Spec

• The pile shall sustain the compression and tension design loads (100% DL) with no more than ______ inches total vertical movement at the top of the pile.

• The slope of the pile deflection curve at twice the allowable design load is less than a slope of 0.15 mm per kN (0.05 in / Ton) of applied load.

• FHWA & DFI Creep at Test Load of 0.04 inches 1 to 10 min or 0.08 inch/log cycle

Lateral Load Analysis

Batter PilesNAVFAC f procedureLPILE to determine bending momentGROUP5 considers effect of batter

Combined Stress = Axial Load + Bending

Pin Pile Case Study St. Clair County Metrolink

Extension

Roger Baldwin,Vice President

St. Clair County Metrolink Extension• Metrolink is a light rail commuter train

beginning at St. Louis International Airport that travels through Metro St. Louis east to college station in Shilo Il.

• An extension was to be added connecting Scott Airforce Base along with adjoining Mid-America Airport.

St. Clair County Metrolink Extension• The proposed path crossed over a

Norfolk Southern Railroad line underlain by a five foot subsiding mine at approximately 126’ from the surface.

Project Team• Owner - Bi-State Development Agency• Construction Managers - STV/PB Booker• Engineer - URS Inc.• Foundation Engineer - Sverdrup Civil, Inc.• General Contractor -

Kilian/Baxmeyer/Illinois Excavating JV• Pin Pile Subcontractor - Nicholson Const.

Co.

NCC Scope of Work• Pin Pile Design (Value Engineering)• Testing of Bond Breaker• Venting of Pressurized Methane• Installation of Pin Piles• One Pile Load Test (on production pile) • Pile Cut-offs

Down Drag

Value EngineeringNCC VE Design

• 9 5/8” Outer Casing• 7” Core Stl. Casing• Bond breaker

coating on core casing

• No grouting of outer casing

• Threaded connections

Original Design• 16” Outer Casing• 10” Core Stl. Casing• Annulus C/B Grout

w/ welded centralizers

• OD of outer casing grouted

• Welded joints

Bond Breaker Testing• Prepared 4 types of samples for

analysis• With out bond breaker• Carboline Bitumastic 50 as a bond breaker• Carboline Bitumastic 50 w/ DouGard form

oil as a bond breaker• Jet-Lube 21 rod grease as a bond breaker

Pile Specifics

• 119 Piles at 10 locations• 9 5/8” OD x .545 wall N80 casing to

mine floor• Core steel consisted of 7”OD x .500 wall

casing full length• Outside of inner casing coated with

Bitumastic 50 to act as bond breaker during potential mine subsidence

NCC Pile Design

Design Criteria• Design Load

• Compression Load 90 ton @ Piers• Compression Load 65 ton @

Abutments• Tension Load 20 ton @ All

locations

Bond Breaker Testing

Construction of Samples

9 5/8” outer casings cut 12” long had

1-1/2” plugs inserted in the

bottom

7 in. inner casings cut 12 in. long received their

various coatings

Construction of Samples

7” casings were centralized

inside the outer casing

Samples were grouted using

a 5gal/bagType II cement

Construction of Samples

Samples were moved inside with a thermocoupler inserted to record curing temps & verify a testing temperature of 55F

Testing Procedure

• Samples were compressed with 60,000lb and 300,000lb testing machines, with a strain rate of 0.02”/min to a total deflection of 1”, at Non-Destructive Testing Group in Pittsburgh, PA.

• Machines plotted load versus crosshead movement.

• Along with machine plotted movement was verified and recorded using a dial gauge.

Testing Procedure cont.

• Samples were compressed in the reverse direction following the same steps as above.• This was done to simulate real world

conditions as the pile was loaded with the structure, with impending ground settlement.

Test ResultsIL Metrolink Uncoated Plain Casing

0

10000

20000

30000

40000

50000

60000

70000

80000

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Cross Head Deflection (in)

Load

lb

N-11

Test Results cont.IL METROLINK, ST. CLAIR NSRR BRIDGEBondbreaker Testing Result Summary

Sample B-1 B-2 B-3 BO-4 BO-5 RG-7 RG-8

Coating Bitumastic 50 Bitumastic 50 Bitumastic 50Bitumastic 50 &

DuoGard form oilBitumastic 50 &

DuoGard form oilJet-Lube 21 rod grease

Jet-Lube 21 rod grease

126Max Load (kips) 120 137 269 95 56 285 493Defl = 0.01" 119 69 262 52 51 204Defl = 0.1" 75 97 178 32 25 195 368Defl = 0.25" 60 80 147 23 20 225 372Defl = 0.5" 45 62 112 16 14 246 325Deflection = 1.0" 30 36 81 9 10 285 237% decrease in Maximum Load from no coating

98.9% 98.8% 97.6% 99.1% 99.5% 97.4% 95.6%

Downdrag Load on Pile with Length of ft

Test Results cont.IL METROLINK COATED SAMPLES

0

500

1000

1500

2000

2500

3000

3500

0 0.05 0.1 0.15 0.2

Cross Head Deflection (in)

Figure 1

Load

lb

B-1B-2B-3BO-4BO-5RG-7RG-8

Test Results cont.Bondbreaker Testing Result SummaryReverse Loading

Sample B-1 B-2 B-3 BO-4 BO-5

Coating Bitumastic 50 Bitumastic 50 Bitumastic 50

Bitumastic 50 &

DuoGard form oil

Bitumastic 50 &

DuoGard form oil

Max Load (kips) 238 255 365 205 182Defl = 0.01" 7 6 7 13 1Defl = 0.1" 215 220 287 177 163Defl = 0.25" 157 167 216 126 105Defl = 0.5" 122 126 160 91 74Deflection = 1.0" 77 78 101 78 53% decrease in Maximum Load from no coating

97.9% 97.7% 96.7% 98.2% 98.4%

126Downdrag Load on Pile with Length

Test Results cont.IL METROLINK COATED SAMPLES

0

500

1000

1500

2000

2500

3000

0 0.05 0.1 0.15 0.2

Cross Head Deflection (in)

Figure 4

Load

lb

B-1 RB-2 RB-3 RBO-4 RBO-5 R

Test Results cont.

• 2.90kips/ft for 365kips downdrag load (DDL) for a 126ft free length

• 180kips design load (DL)• Test load =2*DL+DDL = 725kips

Schedule• Mobilized April 25, 2001• 50 hour work week• Total 72 working shifts• Target completion date of 8/1/01 per

contract• Coordination with Kilian’s operation• Project Completed on 8/3/01

Safety Concerns• Venting of pressurize methane• Working adjacent to active railroad track

(60 mph)• Working along side Scott Airforce base• Our work in general