complete replacement of railroad bridge using … replacement of railroad bridge using micropiles...

Complete Replacement of Railroad Bridge using Micropiles

Richard D. Payne, P.E., S.E ESCA Consultants, Inc. 2008 Linview Avenue

Urbana, IL 61801 Phone: (217) 384-0505

Fax: (217) 384-0506 [email protected]

Curt Fitzgerald, P.E. Nicholson Construction

5945 West Main Street, Suite 102 Kalamazoo, MI 49009 Phone: (269) 353-8421

Fax: (269) 353-8435 [email protected]

Rodney Nagel, P.E. CN

17641 Ashland Avenue Homewood, IL 60430-1339

Phone: (708) 332-3556 Fax: (708) 332-3514 [email protected]

Payne, Fitzgerald & Nagel 2

ABSTRACT CN Bridge 82.23 is located on the South Bend Subdivision near Mill Creek, Indiana. It carries

CN over the Little Kankakee River. The bridge is situated on a vital double track corridor

connecting Chicago, Illinois and Port Huron, Michigan. At Port Huron, the line crosses into

Ontario, and is the primary link between Chicago and Eastern Canada.

The existing structure was a seven span double track timber trestle built in 1928.

Extensive repairs had been made throughout the bridge’s 78 years of service. The structure had

deteriorated to the point that complete replacement was necessary.

The alternative structure type study for this site was challenging, due to constraints

provided by CN as well as hydraulic and environmental considerations. CN required that the

only interruption to 60-MPH rail traffic throughout construction of the project would be during

removal of the timber superstructure and erection of the new superstructure. Two track windows

(one for each track) of eight (8) hours each were allowed to facilitate the superstructure erection.

The entire substructure had to be constructed without any slow orders or traffic interruptions.

Analysis of the feasible alternatives bounded by these constraints resulted in selection of

a substructure that consisted of cast-in-place concrete pier and abutments supported on micropiles

below grade. Micropiles were selected primarily due to their low headroom installation

characteristics and controlled drilling methods for working adjacent to and below existing

structures.

Although the use of micropiles is relatively new to the railroad industry, the technology

has been developed and used in commercial, industrial, and highway applications for the past 30

years.

This paper addresses the challenges associated with designing, competitively bidding,

and constructing a railroad bridge for all AREMA recommended forces, utilizing micropiles as

the sole deep foundation element.

Keywords: micropile, railroad, bridge


INTRODUCTION Bridge South Bend 82.23 is a double track bridge carrying CN’s mainline across the Little Kankakee River near Mill Creek, Indiana. The South Bend Subdivision is situated

on a vital corridor connecting Chicago, Illinois and Port Huron, Michigan. At Port

Huron, the line crosses into Ontario, and is the primary link between Chicago and Eastern

Canada. The bridge carries about 30 trains per day.

The existing structure was a seven span, double track timber trestle. It was built in 1928.

Extensive repairs had been made throughout the 78 years that the bridge was in service.

The structure had deteriorated to the point that complete replacement was necessary.

FIGURE 1 – EXISTING BRIDGE PHOTO

This paper addresses the challenges associated with designing, competitively bidding,

and constructing a railroad bridge under traffic with no slow orders or track time required


during substructure construction. The replacement bridge is designed for all AREMA

recommended forces, and uses micropiles as the sole deep foundation element.

PRELIMINARY ENGINEERING

Hydraulic Considerations

The bridge spans the Little Kankakee River. Scour was a concern due to relatively high

flow velocity and the proximity of the bridge to a bend in the channel. Drift was also a

concern due to the wooded nature of portions of the floodplain upstream of the site. The

existing seven span, double track timber trestle had the potential to significantly impede

the flow, especially since the bents for each track were not lined up with each other.

They were staggered such that they took up twice as much flow area as a single-track

trestle would.

FIGURE 2 – EXISTING BRIDGE PLAN


FIGURE 3 – EXISTING BRIDGE ELEVATION

Hydraulic analysis indicated that the new bridge could be constructed entirely between

the existing end bents. The additional waterway opening provided by removing all of the

timber bents allowed the bridge to be shortened enough to construct both of the new

abutments just inside of the existing end bents. Riprap was provided to protect the end

slopes from scour.

Environmental Considerations

Permits were required from the U.S. Army Corps of Engineers, the Indiana Department

of Natural Resources, and the LaPorte County Drainage Board. A sign-off was also

obtained from the Indiana Department of Environmental Management. The State of

Indiana Department of Natural Resources would not allow any work between April 1 and

June 30 (fish spawning season). The Contractor had to submit a sketch showing his

proposed method of diverting the flow of the river in order to build the pier. The


Contractor’s cofferdam design was a significant factor in one of the issues that developed

during construction. That issue will be addressed later in this paper.

Alternative Structure Type Study

Due to the constraints established by the Railroad, the Alternative Structure Type Study

for this site was challenging. CN desired that the only interruption to 60-MPH train

traffic throughout construction of the project would be during removal of the timber

superstructure and erection of the new superstructure. Two single-track windows (one

for each track) of eight (8) hours each were allowed to facilitate superstructure erection.

Single-track traffic had to be maintained during the windows. This meant that for options

built on the existing alignment, either:

1. Traffic needed to be detoured on a shoofly runaround during construction, or

2. All of the piling had to be driven outside of the existing deck, or

3. The entire substructure had to be built from below the existing bridge deck, with

limited headroom.

Options built on a permanently offset alignment were also considered, as well as hybrid

options utilizing the existing alignment for one track and an offset alignment for the

other.

The idea of driving all of the piling outside of the deck was eliminated very early in the

study, due to the double track nature of the site. The cross beam would need to be

massive in order to span the distance out to out of deck.


Temporary Shoofly

This alternative involved a great deal of ROW acquisition, primarily due to the need to

maintain two 60 MPH tracks. ROW acquisition can be very costly, not to mention time

consuming. The length of trackwork as well as the volume of borrowed fill material also

made this option very expensive. Environmental permitting for the large quantity of fill

to be placed on the floodplain would also have been very time consuming.

Permanent Offset Alignment

Figures 4 and 5 are schematic depictions of the offset alignment alternative.

FIGURE 4 – STAGE I OFFSET ALIGNMENT


FIGURE 5 – STAGE II OFFSET ALIGNMENT

The Permanent Offset Alignment options have some of the same problems as the shoofly

ideas. ROW/easement, environmental permitting, cost of trackwork and embankment

were all concerns. They also would involve working above track level very close to the

existing track, which would require frequent work stoppages (as every train approached).

It was decided that low headroom options to install the new substructures on the existing

alignment should be investigated.

Existing Alignment (Under Traffic)

Several options were evaluated for installing deep foundation elements under the existing

bridge. Drilled piers, augercast piles, and micropiles were all considered. In order to

construct the new abutments immediately in front of the old end bents, it was obvious

that shoring would need to be installed to allow headroom for any of the options. The


headroom available under the end spans was about two to three (2 to 3) feet. The

maximum headroom available was at the flowline of the River, where the clearance from

ground to low beam on the existing bridge was about seventeen (17) feet. Drilled piers

were eliminated from consideration early on, since they were not economically feasible

with this amount of headroom. Augercast pile installers told us that they need about

fifteen to twenty (15 to 20) feet of headroom. Micropiles could be installed with a

minimum of about eight (8) feet of headroom at this site. After extensive evaluation of

the design parameters and the cost impacts of the various options, it was decided to

proceed with micropiles supporting cast-in-place concrete substructure units. In order to

use CN’s standard PPC spans, not infringe upon the design high-water elevation, and not

have to raise the track, a two-span configuration was chosen, with a solid shaft pier in the

river channel.

FIGURE 6 – BRIDGE LAYOUT


FIGURE 7 – BRIDGE LAYOUT

FINAL DESIGN, PLANS, AND SPECIFICATIONS

Micropile Design

Although the use of micropiles is relatively new to the railroad industry, the technology

has been developed and used successfully in commercial, industrial, and highway

applications for the past thirty (30) years. Micropiles were selected for this project

primarily due to their low headroom installation characteristics and controlled drilling

methods for working below and adjacent to existing structures.

The Federal Highway Administration’s (FHWA) Reference Manual Titled “Micropile

Design and Construction,” December, 2005, Publication No. FHWA NHI-05-039 was


used for design of the micropiles for gravity loads. This publication was used as a guide

for the structural design of both the cased and uncased lengths of the micropiles as well

as for determining the required length of the bond zone. The soil profile consisted of fine

to coarse sand, varying in density from very loose near streambed (about 20’ below base

of rail), becoming medium at around five (5) feet below streambed, and gradually

becoming very dense with depth. Hard silty clay was encountered about seventy-five

(75) feet below base of rail.

Base of Rail 100.00 Streambed 80.0 Very loose sand N=2 75.0Fine to Coarse Sand N=24 to N=52

25.0Hard Silty Clay Qu > 4.0 TSF

FIGURE 8 – SUBSURFACE PROFILE

After the micropiles were sized for gravity loads, they were modeled for AREMA

mandated transverse and longitudinal forces using GROUP software. GROUP was

developed as a result of Special Publication No. 29, Bureau of Engineering Research,

University of Texas – Austin, by Reese and Matlock. The relative stiffness of the pier

and abutments was modeled with multiple pile sizes and layouts. Several micropile

options for both the pier and the abutments were developed, with various sizes,

capacities, and layouts. Any options not meeting the allowable stress criteria as well as


the longitudinal deflection criteria mandated by AREMA were eliminated from

consideration.

Contract Plans

Since the economy of micropiles is somewhat dependent upon the availability of casing,

it was decided to present several options for casing size and associated micropile layout

in the form of a table included in the plans (see Figure 9). The Contractor was then

allowed to select the most economical option. Figure 11 shows the micropile detail that

appeared in the plans.

FIGURE 9 – MICROPILE TABLE

One of the primary design goals of the project was to complete the substructure work

without any slow orders or track time. In order to achieve the minimum eight (8) feet of

headroom required at the abutments, it was necessary to excavate in front of the end bent,

and shore it. A plan for shoring was developed and included with the plans. The plan


involved installing additional bulkhead timbers as excavation proceeded. The new

timbers were drift bolted to the existing piling, and strutted across to the adjacent bents.

Figure 10 is a typical section showing the shoring that was included with the plans.

FIGURE 10 – END BENT SHORING

After the bidding phase, the selected Contractor requested to substitute sheet piling

driven behind the end bents for the shoring shown in the plans. This option was not

considered in the design, since it required track time. However, the Railroad approved

the Contractor’s shoring submittal.

Often, the design of temporary facilities that are required to facilitate the work are left up

to the Contractor. The plans for this project depicted end bent shoring that was ultimately

not used. However, the plans did leave the means and methods for diverting the stream

flow up to the Contractor. The Contractor submitted his means and methods to the State


of Indiana for environmental permitting; however, neither the Consultant nor the Railroad

received a copy of this submittal. This would later result in a problem that could have

been avoided.

FIGURE 11 – TYPICAL MICROPILE DETAIL

Specifications and Bidding Documents

AREMA currently does not provide any guidance in its Manual for Railway Engineering

for design or construction of micropiles. Therefore, the Consultant wrote specifications

for this contract using the “Guide to Drafting a Specification for Micropiles” published

by the Deep Foundations Institute and The International Association of Foundation

Drilling. The compression load test that was required for this project closely follows the

test recommended by this Guide. It was decided to test to 200% of design load (or

failure) on this project since this was the first bridge CN had built supported entirely on

micropiles. The micropile installer recommended a pile that was five (5) feet shorter than

the Consultant’s design calculations indicated. Drill casing was specified as either


ASTM A252 Grade 3 (50ksi) or API N80 Structural Grade (Mill Secondary). Grout was

specified at 4000 psi at 28-days. The micropile portion of the project was paid for

“Lump Sum.”

BIDDING

We required that all interested Micropile Contractors attend the Pre-Bid Meeting. It was

also strongly recommended that the Railroad prequalify Micropile Contractors before

allowing them to bid on this project. Most Railroads have a list of preferred General

Contractors that they invite to bid. These General Contractors may or may not be

familiar with quality Micropile Contractors. It only makes sense to expect the same or

better level of quality from the Micropile Contractor as the Railroad consistently gets

from its General Contractors. On our project, we required all potential bidders to provide

brochures outlining their qualifications, details of projects with similar or greater work

scopes, and a list of references. Two of the three Micropile Contractors who attended the

Pre-Bid Meeting were ultimately prequalified by the Railroad.


CONSTRUCTION – Consultant’s Perspective

One problem occurred during construction that merits discussion from the Consultant’s

perspective. Micropile installation at the Pier commenced on August 11, 2006. The last

micropile at the Pier was grouted on August 18th. The micropile crew immediately

moved to the West Abutment.

FIGURE 12 – COFFERDAM

On August 27th, the existing timber trestle experienced settlement at Bents 4 and 5 (see

Figure 7). The timber piles at these locations were relatively short friction piles. These

two bents are each side of the new pier. Bent 4 settled about 3” and Bent 5 settled about

2”. As a result, the bridge was slow ordered. The settlement was immediately preceded

by a large storm event which resulted in a rapid rise of the Little Kankakee River.

Although the micropiles at the pier had been completed nine days before, the concrete


pile cap had yet to be installed when the storm hit and the river rose. Figure 12 shows the

method used by the Contractor to divert the stream flow. It can be readily seen that the

rise of the River created an unstable, “quick” condition in the area around Bents 4 and 5.

If the sheeting had only been driven a few feet further into the ground, this would not

have happened. The situation was stabilized when the Pier pile cap was poured and

backfilled on August 29th. This provided enough weight to offset the hydrostatic uplift

that caused instability in the existing timber friction piles. This experience underscores

the need for the Railroad and the Consultant to check all Contractor designs, irrespective

of how pressing the project schedule becomes.

CONSTRUCTION – Micropile Contractor’s Perspective

Development of the micropile industry over the past 30 years has shown the application

to be viable and cost effective for construction projects with many different constraints.

The most well known constraint is limited headroom and tight access conditions with

equipment capable of installing elements with design loads exceeding 200 Tons of axial

load and lateral loads exceeding 10 kips per element in areas with as little as 8 feet of

headroom and access ways as little as 4 feet wide. Micropiles are also very effective for

difficult drilling conditions with drilling methods capable of penetrating just about

anything (including steel with the use of special bits). Most conditions can be drilled

through without obstructed drilling costs that are typically associated with other deep

foundation applications. Micropile applications are also well suited for sensitive drilling

conditions adjacent to and below adjacent structures (such as this project) where duplex

drilling methods can be used to limit the risk of undermining the structures.


Drilling Method

Duplex drilling methods provide a stabilized drill hole by advancement of the outer drill

casing which maintains near intimate contact with the insitu soils as the hole is drilled.

The drill flushing medium is introduced through the drill rods and bit and returns up the

annulus between the drill rods and casing, as shown in figure 14 to a diverter swivel

which then discharges the cuttings to a contained location (earthen pit or dumpsters).

FIGURE 13 – DUPLEX DRILLING METHOD

Drilling for this project extended below the water table, therefore water flush was chosen

for the drill flush medium as the use of air below the water table can create hydro-

fracturing of the formation.

The plans and specifications for the project provided several options for casing size and

pile layout. This allowed for selection of the most readily available and cost effective


casing size. Nicholson chose a 7-inch O.D. x 0.453-inch wall casing which also limited

the number of piles. Nicholson also developed a creative solution for the core

reinforcement of the pile for a more efficient installation operation.

Grouting Method

Once the tip of pile elevation was reached, the pile was tremie grouted by lowering a

tremie tube to the bottom of the pile and grout injected until good quality grout returned

at the top of the casing. The outer casing was then withdrawn until the tip of casing was

20 feet below the bottom of footing as required by the project plans and specification. A

final step of pressure grouting through the core steel until grout returned through the

annulus between the core steel and outer casing was completed to confirm intimate

FIGURE 14 – ABANDONED TIMBER PILES AT CENTER PIER LOCATION


contact between the outer casing and surrounding soil had been maintained. This last

step proved beneficial for some of the center pier locations as removal of old abandoned

timbers at the center pier location had loosened the upper 10 to 15 feet of soils and the

pressure grouting permeated the very loose soil formations as evidenced by very high

grout takes at several locations. Figure 15 shows the old timbers prior to removal by the

General Contractor.

The grout was mixed in a high shear colloidal mixer at a 0.44 water/cement ratio. The

high shear colloidal mixer provides complete hydration of the cement particles to achieve

the highest quality grout. The consistency of the grout from batch to batch is maintained

since the grout plant has a holding tank for the water with an overflow port sized for

exactly 15 gallons of water. The set volume of water is mixed with three 94 pound bags

of portland cement to produce consistent grout for every batch made.

Verification Test Pile

In order to confirm the final design configuration of the micropile, a sacrificial

verification test pile was specified for the project. Since the verification test pile would

be completed prior to production pile installation, the submitted design used an

aggressive grout to ground adhesion value. Some challenges were encountered during

the test pile installation including a zone of coarse sand and gravel approximately 25-feet

to 30-feet which required minor modifications to the planned installation procedures. As

the initial test pile location was significantly disturbed while the drilling procedure was

modified, a second test pile location was determined and installed. It should be noted

that the planned drilling method often requires modification during project start-up, the


key is the method needs to be determined so the verification test pile is installed using the

actual drilling method that will be used for the production piles.

FIGURE 15 – VERIFICATION TEST PILE SET-UP

Once the verification test pile was installed, the reaction beam and reference frame was

set-up in accordance with ASTM D-1143 as shown in Figure 16, and the reaction anchors

prestressed to limit deflection of the reaction beam during testing. The test was started

after the grout had reached design strength and a cyclical loading schedule was used as

specified. We did find that the schedule for the load hold readings at 130 percent of

design load, indicated in the specification as based on the DFI guidelines mentioned

above, have the times at 1, 2, 3, 4, 5, and 10 minutes, with additional readings at 20, 30,

40, and 60 minutes if required. This author found the schedule troubling since creep

movement is evaluated by looking at deflection over log cycles of time (i.e. 1 to 10, 2 to

20… …, 6 to 60 minutes). This was discussed with the inspector at the time of the test

but he insisted the schedule in the specification be followed, whether it was technically

correct or not. Fortunately the soils for this site were not creep susceptible and it was not


an issue, but this item should be reviewed for any future projects particularly if the soils

are susceptible to creep failure.

Acceptance criteria for the load test including reaching 200 percent of the design load

without failure of the pile. Failure was defined as a slope of the load versus deflection

curve exceeding 0.025 inches/kip. This criteria was exceeded at 185 percent of the

design load, therefore the bond length was revised from 38 feet to 43 feet based on the

results of the pile load test. A graph of the load versus deflection curve from the test is

shown in Figure 17.

FIGURE 16 – TEST PILE LOAD VERSUS DEFLECTION CURVE


Production Pile Installation

After acceptance of the revised pile length based on the verification test pile results,

installation of the production piles was started at the center pier location, with the west

abutment and east abutment following. There were three issues during construction that

warrant discussion; the settlement of the structure during construction, high grout takes

due to loosened soil from removal of old abandoned timbers, and location of the piles and

alignment to miss hitting the existing timber piles. The first two have already been

addressed above and do not need to be revisited.

Pile Location and Alignment

The project plans provided dimensions for each line of piles from the edge of pile cap and

indicated “equally spaced” for the remaining piles in each line. This item was discussed

at the pre-bid meeting as the battered piles would be crossing the line of existing timbers

as Figures 6 and 7 above illustrate. Adjustments to the spacing would be required to miss

the existing piles and was noted on the submitted shop drawing shown in Figure 18.

FIGURE 17 – SHOP DRAWING PILE LOCATION PLAN


Although no limit or tolerance was provided for the adjustments it was apparent that

keeping the spacing close to equal was necessary to not impact the pile cap

reinforcement. Nicholson anticipated that each line of piles would be adjusted

independently and only minor adjustments would be needed. However, during

construction the on-site inspector indicated that it was critical to keep each pair of

micropiles along the same alignment (parallel with the tracks). As the timber piles were

not lined up and battered in two directions it became very challenging to find an

alignment that worked for both sides of the center pier locations. The two abutments

were much easier as the micropiles were only battered in one direction, and only one bent

of timber piles needed to be missed. Initially the micropiles were spaced at five foot

centers, but the adjustments ended up creating spacing as much as seven feet on center.

If the lines could have been adjusted independently, the maximum spacing between piles

could have been maintained at less than 5 ½ feet.

CONCLUSION – Micropile Contractor’s Perspective

One factor that contributed greatly to the success of this project was an open and

cooperative line of communication between the Micropile Contractor, the Engineer, and

the Railroad during construction to address questions and issues as they developed.

It is agreed that pre-qualification of the Micropile Contractor is essential as there are any

number of details that can go awry if the Contractor is not experienced in the micropile

application. To address all of the details in a specification is not practical, and often the

Micropile Contractor can provide a more cost effective end result when the parameters

and performance criteria are defined, and the Contractor is able to apply their own


experience and knowledge. Through years of experience installing micropiles in power

facilities, Micropile Contractors have developed ways of installing deep foundation

elements without interrupting operations or risking the integrity of existing structures.

This project is an example of how the micropile application can be applied to the railroad

industries effort of upgrading the infrastructure without losing track time.