micropiles for roadway support, bridge abutments … for roadway support, bridge abutments and metal...

Micropiles for Roadway Support,

Bridge Abutments and Metal

Signal Poles on US-158 from

US-17B to East of Pasquotank

River, in Elizabeth City, NC

Schnabel Foundation Company

Presenter: Stephen Dimino, P.E.

Team

• Owner -NCDOT

• Prime Contractor-Archer Western Construction

• Micropile Subcontractor-Schnabel Foundation

Company

Project Overview

• Install micropiles to support the roadway, metal

signal poles (MSP), bridge abutment and pump

station

• The roadway and approach micropiles were

installed through the concrete roadway deck

• Bridge micropiles were installed in the river

• MSP micropiles and the pump station piles were

installed in foundation excavation.

Project Overview

• Install 258 Roadway Micropiles

• Install 16 Bridge Approach Micropiles

• Install 10 ea. Bridge Abutment Micropiles

• Install 4 Micropiles at 8 separate MSP foundations

for a total of 32 piles.

• Install 8 Pump Station Support Micropiles

• Install 4 Generator Slab Support Micropiles

• Install 4 Electrical Building Micropiles

Roadway/Approach Piles

• Install 258 Roadway Micropiles

– 8-5/8” x 60 ft. casing with casing tip elevation

of -52 ft.

– Bond length of 28 ft.

– Full length of corrosion protected Center bar

– Three sacrificial demonstration piles

– Fifteen proof tests on production piles

Bridge Piles

• Install 10 Bridge Abutment Micropiles

– 10-3/4” x 60 ft. casing with casing tip elevation

of -58 ft.


– Full length of corrosion protected center bar

tied into pile cap

– One sacrificial demonstration piles

– Two proof tests on production piles

MSP Piles

• Install 32 Bridge Abutment Micropiles

(4 each at 8 separate pile caps)

– 10-3/4” casing with casing tip elevation varies

from -40 ft. to -58 ft.



tied into pile cap

– One proof tests at each of the 8 pile caps

Pump Station Piles

• Install 16 Pump Station Micropiles

– 7” casing with casing tip elevation of -40 ft.



tied into pile cap

– One proof tests at each of the 8 pile caps

– One demonstration test for the 7” piles

Design

• The permanent micropile design is in accordance

with

– FHWA Implementation Manual for Micropile Design

and Construction Guidelines (FHWA-SA-97-070),

dated June 2000 using the Load Factor Design (LFD)

Method;

– AASHTO LRFD Bridge Design Specifications 2010;

and The Recommendations of Prestressed Rock and

Soil Anchors, PTI, 2004.

– In this design the micropiles are “Type A” micropiles

Design

• Design loads and test loads given in Contract

Documents

– Roadway/Approach DL=25T (comp) TL=36T

– Bridge DL=50T (comp) TL=72T

– MSP DL=24T (comp) ;14T (tension)

• Specified casing diameter and thickness based on

method of installation

Design

• The overburden soils were very soft/weak.

• The grout to ground bond strengths are per Table

6.3 in The Recommendations of Prestressed Rock

and Soil Anchors, PTI, 2004, and AASHTO Table

C10.9.3.5.2-1. The bond strengths of the soils of

the pile above the minimum tip elevation shown

the plans were neglected.

– Roadway and Approach Piles 3.8klf

– Bridge Piles 4.5klf

– MSP Piles 4.5klf

Issues

Design

• Grout mix design Use either of the two

grouts:

– A neat mix of Portland Cement (Type I/II) and

potable water at an approximate mix ratio of 5

to 6 gallons of water per cubic foot of grout.

Minimum compressive strength at 28 days,

fc=3,500 psi.

– An approved ready mix grout with a minimum

compressive strength at 28 days, fc=3,500 psi

Testing • Tested per ASTM D3689 8.1.2 Quick Test

• Roadway Piles-The contract called for 3

Demonstration Tests and 15 Proof Tests. Roadway

micropiles test load is 36T (72 kips)

• Bridge Piles-The contract called for 1 Demonstration

Test and 2 Proof Tests. Bridge micropiles test load is

72T (144 kips)

• MSP Piles-The contract called for 1 Proof Test at

each pile cap. MSP micropiles test load is 24T (48

kips)

Testing

Issues

• We had to stabilize the hole and the environmental

requirements affected our planning

• Original method was not productive

• Constraints of working on road deck limited our

work process

• Tight schedule due to MOT

• Changing methods is time consuming

• Working near the water

Overall view of jobsite

Construction Method 1

• 100 lf x 11” dia. Continuous Flight Auger

• Drill down to depth and pump grout through auger

• Push casing and center bar down through plastic

grout

• Main issue was that the auger walked and we

could not put the casing in to depth

• Cleaning the “fat” clay off the auger took a long

time.

Method 1 CFA installation

Method 1 CFA Installation


• Drill down to pile tip elevation using the casing as

the drill string

• Pump grout through the casing as the casing is

pulled up to so that the bottom of casing matches

pile tip elevation. The casing and grout stabilize

the hole.

• Push bar through plastic grout.

• Hooking the casing up at the rotary head took a

long time.

Method 2 Drilling with Casing


• Vibrate casing down to casing tip elevation

• Drill through casing and down to pile tip

elevation.

• Pump grout through drill string as we retracted the

inner rod.

• Very successful in the soil type. We were able to

use smaller equipment.

Drilling through the Vibrated Casing

Drilling through Vibrated Casing

Vibrated Piles at MSP Pile Cap

Completed MSP Micropiles