APT-Structural Asessment of Aging Timber Piles

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1/9

Structural Assessment of Aging Timber Piles on the Banks of the Mississippi

Author(s): MARK R. CHAUVIN and ARNE P. JOHNSONSource: APT Bulletin, Vol. 44, No. 2/3 (2013), pp. 15-22Published by: Association for Preservation Technology International (APT)Stable URL: http://www.jstor.org/stable/41982401.

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2/9

Structural Assessment of

Aging

Timber Piles

on the Banks of the

Mississippi

MARK R. CHAUVINAND ARNE P. JOHNSON

Knowledge

f he

typical

erformance

and deterioration echanisms or

timber

iles,

ombinedwith he

strategic

se of modern cientific

testing

nd

analysis

methods,

was

used to assess the

expected

performance

f railroad tructure

supported yhistoric imber iles n

Saint

Paul,

Minnesota.

Introduction

Many

historictructuresre

supported

by imber-pile

oundations.he condi-

tion f

the imber

iles

s

generally

unknown

nd difficulto

assess,

ince

these lements

re buried elow

grade.

In

some

ases,

buried imber

iles

may

remain

n

good

conditionfter un-

dreds

f

years

f ervice.nother

ases,

theymay

havedeterioratedr even

disintegrated.

his ase

study

llustrates

how advancedmethods ffield

esting

and

engineering

nalysis

an be

used o

evaluate

ging

imber

iles

till

n

ser-

vice,

with he

goal

of

aving

istoric

structuresnd

minimizing

ntervention.

The Union

Depot

n

Saint Paul

The Union

Depot

railroad

erminal,

located

long

hebanks f

theMissis-

sippi

River

n

downtown aint

aul,

Minnesota,

as constructed

etween

1917 and 1926. The architect as

Charles umner

rost,

he

designer

f

Fig.

.

Union

epot,

aint

aul, inn.,

ooking

orth,

ate nknown.

ourtesy

f he

Ramseyounty

Historical

ociety.

many mportant

rain tations

on-

structed

cross heUnited tates. he

structural

ngineer

frecordwas the

Toltz

Engineering

ompany.

he

pro-

ject

ncluded headhouse

uilding

nd

an elevated rack-deck

tructure,

s well

as a concourse

nd

waiting-room

truc-

ture

hat acilitated

assenger

ove-

ment

etween hose eatures.he track

deck ccupied ix acres nd accommo-

dated 0 railroad

racks

Fig.

1).

Decline nd Revitalization

Union

Depot

was a

very

ctive rain

station rom 926

through

hemid-

1960s. At ts

peak

nthe1920s there

were 82 trainmovements

nd

20,000

passengers

erved

aily.

As the ailroad

era

declined,

rain ervice

eased

n

1971,

and theUnited tates ostal er-

vice

USPS)

subsequently

cquired

most

of he rack-decktructure.

n1978 the

USPS

reconfigured

he rack eck

nto

postal

distribution

enter,

emoving

he

ballast,

ailroad

racks,

nd

platforms.

In

1974 Union

Depot

was isted n the

National

Register

fHistoric

laces.

Ramsey ounty egional

ailroad

Authority

RCRRA)

finalizedts

cquisi-

tion f he

property

n

2009.

RCRRA's

vision

was to rehabilitatehe

acility

nd

create multi-modalransitnd trans-

portation

ub

n

downtown

aint aul.

RCRRA

envisionedhe levated rack

deck nce

gain ccommodating

m-

trak

assenger-rail

ervices,

ith om-

muter nd

high-speed

ail ines future

goal.The rail ineswillbe ntegrated

with

acilities

or

ntracity

nd transit

buses,

assenger

ehicles,axis,

nd

bicycles,

ll on the urface f

he eha-

bilitatedrack eck.

RCRRA commissioned

design-

build eam o

perform

he

ehabilitation

work.1

he nitial

hase

f he

project

included etailed onditionssessments

15

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3/9

16 APT

BULLETIN:OURNAL

F PRESERVATIONECHNOLOGY

44:2-3,

013

Fig.

.

Sectionf he

ypicaleometry

f he rack-deck

tructure.

magesy

he

uthors,

nless

otherwiseoted.

of he

headhouse,

rack

eck,

nd con-

course nd

waiting-room

tructureso

define

scope

f

necessaryepairs

or

each lement. iss

Janney

lstner as

retainedo

assess he

xisting

rack-deck

structure.2

easibility

f

he

depot

eha-

bilitation

inged

n whetherhe

xisting

concrete

uperstructure

nd

timber-pile

foundationsad sufficient

apacity

r

couldbe

effectively

epaired

o

reliably

supporthe nticipatedoads for

desirederviceife

f50

years.

Track-Deck

tructure

The track-deck

tructure,

hichmea-

sures

pproximately

50,000

square

feet

n

plan

rea,

onsists f flat

1-

inch-thick

einforced-concretelab

sup-

ported

n

approximately

00 circular

concrete

olumns nd various

erimeter

concrete

etaining

alls.The columns

and

retaining

alls re

supported

n

below-grade

oncrete

ile aps

and

groups f imberiles Fig.2). Fourteen

timber

iles upport ypical

ile aps;

24

or 30

timber

iles upport

he

pile

caps

belowboth

he rack eck nd

overheadoncourse tructure.n

total,

the rack eck

s

supported

n

approxi-

mately

,000

timber

iles.

According

o

magazine

rticles

ub-

lished

uring

he ourse f he

riginal

construction,

he

rack eck nd ts

timber-pile

oundations ere

esigned

for

ooper

-60 train

oading,

ith 5

percent

ddedfor

mpact.3

he maxi-

mum

esign earing apacity

f ach

timber

ile

was

specified

s 20 tons.The

timber

iles

were

eported

s

being

0

feet

ong,

riven

hrough

ill

nd

outly-

ing

muck o river and.4No additional

informationas

available

egarding

he

as-built

eometry

f he

imber-pile

groupings

r thediameternd wood

species

f he imber

iles.

Signs

of Deterioration

Approximately

0

years

f

exposure

o

a harsh orthernlimate ad taken

tollon the

oncretelements

f

the

track-deck

uperstructure.ultiple

areasofdeteriorationere

present

n

the

rack eck nd the olumns hat

supported

t,

ncluding

reasofcrack-

ing, palling,

orrosion fthe einforc-

ing

teel,

nd freeze-thaw

amage.

The

deck xhibited

idespread

ot-

tom-side

eterioration,

articularly

along xpansionoints

nd at

exposed

edges f he tructure.eteriorationt

thedeck

opside

was

suspected

ut

unknown,

ince hedeckwas covered

y

3 feet f

overburden.

Loss of

imber-pilentegrity

as also

suspected.

rom heir

ast xperience,

the

geotechnical

nd structural

ngineers

on the

project

eam elieved hat he

original

onstructionf he rack eck

predated

he ommon se of

preserva-

tive-treatedimber

iles

n

the

Min-

neapolis-Saint

aul area

by pproxi-

mately

0

years.

n

addition,

limited

structuralssessmentf he rack eck

performedy

others or heUSPShad

exposed

ne timber

ile

hat xhibited

significantecay.

herefore,

ignificant

concern as warrantedegardinghe

existing

nd future

ntegrity

f heun-

treated

imber

iling.

Because he imber

iles

were on-

cealedbelow

grade,

he xtentnd

severity

f he

imber-pile

eterioration

across he itewas difficulto ascertain.

Visual

nspection

f substantial

er-

centage

f

piles

was notfeasible ue

to

the

high

ost,

n

both ime nd

money,

associated ith he xcavationf est

pits.

Nondestructive

esting

NDT)

techniques

o facilitatessessment

f he

timber

iling

rom

position

tor

above hepile apswere onsideredut

notused.

Although

ertain

DT

tech-

niquesmight

avebeen bleto ocate

the nds

f he

piles

within

he

pile aps

(e.g., mpact-echo

r

ground-penetrating

radar)

r to estimateheir

engthe.g.,

parallel

eismic

methods),

o

NDT

methodsxisted o

practically

r

reliably

evaluate he onditionnd

remaining

structural

ntegrity

f

he

buried imber

piles.

Therefore,

n

nvestigationlan

was

carefully

evised o obtain

nformation

regarding

he

imber-pile

oundations

thatwould llowreasonablengineering

analysis

nd conclusions

egarding

u-

ture

erformance.

he

primary

bjective

of hisworkwastoevaluatewhetherhe

remainingtrength

f he

piles

was ade-

quate

for he

nticipatedesign

oads

over hedesired

0-year

erviceife f

the ehabilitatedrack-decktructure.

Investigation

fTimber iles

Timber-pile

oundations

ontinuously

submerged

elow hewater abledo not

generallyecay

o a

significant

egree

over ime ue to the

xygen-starvednature ftheir nvironment.

owever,

timber-pile

oundations

hat avenot

been reated ith

reservativesay

be

vulnerableo

decay

when hewater

table s below he

ops

ofthe

piles.

All

ofthe

ngredientsecessary

o

promote

wood-decay ungi namely

moisture,

oxygen,

nd a suitable

emperature

couldbe

present

t and above hewater

line.Deteriorationf

imber-pile

oun-

dations an occur

n

such

ettings.



4/9

STRUCTURAL

SSESSMENT F AGING

IMBER ILES

17

Fig.

. Overall

iew f xcavationorest

it

Two

general

ypes

f

fungi

ause

deterioration

f

wood:

typical

rown-

andwhite-rot

ungi

nd soft-rot

ungi

related o molds.

rown- nd white-rot

fungi

re hemost ommon

ypes

f

decay

bserved

n

above-grade

truc-

tures. ecause hey equire ighmois-

ture ontent

n

thewood

typically

above20

percent)

nd sufficient

xygen,

brown- nd white-rot

ungi ypically

o

not ccur

n

wood that s

submerged

n

water r buried

eep

below

grade.

However,

hey

an exhibit

apid

rowth,

and thus

hey

re

ypically

hemore

destructive

orms f

fungi.

oft-rot

ungi

can tolerate

igh

moistureevels nd

require

ess

xygen,

o

they

an be more

significant

n

wood

that s

submerged,

very

et,

r below

grade.

acterial

degradation

an also occur

nwood that

is

submerged;

owever,

he eduction

n

strength

uetobacterialction s usu-

ally

minor

ompared

o the ther

ypes

of ttack.5

lthough

bove-waterimber

pile

an also'be

degraded y

nsect

attack,

uch s

termites,

his

orm f

attack

was not

xpected

o bean issue

n

the oldclimate f

Minnesota.

The

nvestigation

as

organized

nto

sequential

teps

hat llowed nforma-

tion

o be

collected,eviewed,

nd con-

sidered

efore he

ubsequent

asks

were

performed.

he

chronology

f hework

is

detailed

elow nd

consists f ix

tasks:

eviewing

ite

onditions,

nspect-

ing

for vidence

f

ettlement,

xposing

and

nspecting

imber

iles, onducting

field nd

aboratory

oad

testing,er-

forming

wood-pathology

nalysis,

nd

performingtructuralnd statistical

analyses.

he

nvestigation

as

carried

outfrom

anuary

o

April

010.

Task

1:

reviewing

ite conditions.

Geotechnical

urveys

fthe ite

were

conducted

o define he

general

osition

ofthewater

able elative

o the imber

piling

nd to

dentifyeneral

atterns

n

the oil

or

groundwater

onditions

hat

may epresent

ore

r ess

potential

or

timber-pile

ecay

ncertainonesofthe

site.6

The

groundwater

levation as

highestlong

henorth

dge

f he rack

deck, raduallyoweringo the outh

toward he iver.

y orrelating

he

reported

ater-tablelevations

ith he

specified

ile-cap

levations,

twas

estimatedhat he

op

2

to

12

feet f

he

timber

iles

were

ikely ositioned

bove

thewater able.

However,

he

position

of hewater able

was also known

o

fluctuate

easonally

ith he levation

f

the

Mississippi

iver.

looding

f he

river esulted

n

standing

ater nthe

roadways long

he outh

nd south-

west

dges

f he

rack-decktructure

on

many

ccasions.

Borings

dentified

he oils

long

he

southern

ortion

f he ite s a

pre-

dominantlyoose ndgranular aterial

(sand)

t and below

he

pile-cap

leva-

tions.

ecay

of imber

iles

s more

likely

n

granular

oilconditionshan

n

cohesive

oilsbecause

f he

potential

for ncreased

oisture

xposure,

ois-

ture

luctuation,

nd

greater

xygen

concentrations.

As noted

bove,

limited

tructural

assessmentf he rack eck

was

per-

formed

n

1991

by

others. wo small

test

its

were xcavated t

thenortheast

and southwestorners

f he rack

eck

as a

component

f his

work.7

he

timberile xposed t thenortheast

corner

f hedeckwas

reported

o be

sound nd

exhibited o evidence

f

decay.

However,

he

imber

ile

xposed

at the outhwestorner

f hedeck

was

characterized

s 90

percent

bsent.

In

summary,nterpretation

f he

geotechnical

urveysuggested

hat

he

topregions

f ll of he imber

iles

were

ulnerableo

decay.

he timber

piles

ocated

long

he

outh ide

f

he

site

ppeared

o be

more

usceptible

o

decay

han

he

piles

o thenorth ue

to

the ower ndmore ariable

round-

water levationnd themore ranular

soil

ypes

n

that

egion.

his

hypothesis

was

supported

y

he

indings

f

he

past

est-pit

ork t the

ite.

Task

2:

inspecting

or vidence

f

settlement.

reas f

xisting

ettle-

ment-related

istress arranted

areful

consideration,

s

theymay

ndicate

possible

imber-pile

eterioration

elow.

To

identifyny

uch

reas,

both

isual

inspections

nd elevation

urveys

ere

performed

cross

heunderside

f he

Fig.

.

Example

f wood ore emoved

rom

timber

ile.



5/9

18

APT

BULLETIN:

OURNAL

FPRESERVATION

ECHNOLOGY

44:2-3,

013

Fig.

. Plan f heUnion

epot

rack

eck,

llustrating

he est

it

ocations

nd he

nticipated

esign

loads

ollowing

ehabilitation.

track eck.

Thiswork

ttempted

o

locate

ny

tructure

istress,

ow

spots,

abrupt

hanges,

r unusual

radients

n

elevation

hat

may uggest

imber-pile

degradation

elow.

he

survey

mea-

surements

ere sed o

generate

on-

tour

lots

f hedeck's

nderside

leva-

tion,

whichwere orrelated

ith

he

visual

nspection.

The vastmajorityf he rack eck

was

found o be

evel,

with

o evidence

offoundationettlement.

owever,

distress

onditionsonsistent

ith

ettle-

mentwere ound

nthree

articular

zones f he outhwest

orner f

he

track-deck

tructure,

roximate

o

the

river. he elevation

urvey

ecorded

very

harp

ownward

lope

f he rack

deck

n

each

of hese reas.

These

mea-

surements

ere

orroborated

y

field

observations

f tructure-distress

ondi-

tions,

hich

were lso consistent

ith

foundationettlement.

he distress

included ide racksnthe rack eck,

retaining

alls,

nd column

roppanels

and

arge

erticalffsets

t certain

oints

inthedeck.

Task3:

exposing

nd

inspecting

im-

ber

piles.

Test

its

were xcavated

o

expose epresentative

roups

ftimber

piling.

he

excavationsxtended

elow

the

pile

aps

to

expose

he

full ircum-

ferencef he

op

3

to

4

feet f

he

timber

iles

long

he uter ides

f

neighboring

ier aps Fig.

3).

This

worknecessitated

emporary

onstruc-

tion-safety

easures,

ncluding

he

installation

f rench

oxes

within he

excavationsnd

tensioned-cable

ling

tiebacks

o brace

he

deck olumns.

Within ach

test

it,

he

following

asks

were

erformed:

Documenting

he

ize,

position,

nd

visual

ondition

f

xposed

imber

piles.

Awl

probing

nd drill-resistance

testing

t

multiple ositions

long

he

exposed

ile

height

o evaluate

he

wood's

ntegrity

nd

to

quantify

he

depths

f

decay.

Core

ampling,

ncrementally

rom

surface

o

pith,

or

microscopic

x-

amination

y

wood

pathologists

o

identify

ood

species

nd the

pres-

ence,

ype,

nd

severity

f

decay ungi

(Fig.

).

Core

holes

were illed ith

hardwood owels.

Removing

ull-diameter

ections

f

select imber

iles

by

hainsaw or

pathology

nd

aboratory

esting.

Reinforced-concrete

ollars

estored

pile

ontinuity

t

removal

ocations.

Moisture-content

esting

n

represen-

tative

iles

n

each

pit,

noting

he

proximate

oil

type.

Examining

he

pile aps

at each est

pit,

whichwere

onsistently

ound

o

be

n

good

condition.

The RCRRA

had allotted

or he

excavation

f hreeest

its

or

he

timber-pile

ssessment.

onsidering

he

vast

izeof he

rack

eck,

he election

of he ocationsf hesehreeest its

was a

highly

trategic

nd

critically

important

xercise.

Test

it

1

Based

on

the

indings

f asks

1

through

,

theworst

imber-pile

condition

as

suspected

o be

n

the

south

egion

f

he rack-deck

tructure,

particularly

nthe outhwest

orner.

Unfortunately,

his

ection

f he

rack

deck

was

intended

o

carry

he

heaviest

train

oading Fig.

5).

To

investigate

these ombined

worst ase

condi-

tions,

he

firstest

it

was ocated

n the

southwest

orner

f he

rack

eck.

Twenty-nine

imberileswere x-

posed

below

four

djacent ile

aps

n

test

it

1

(Fig.

3).

The diameters

f he

timber

iles

hroughout

he

xcavation

ranged

rom

to

15

inches,

ithmost

measuring

etween

1 and

14 inches.

On the

north ide

f he

est

it,

where

the oils

were

more

ohesive,

ll 16

exposed

iles

were

ound o

be

n

good

condition,

lthough robing

etected

surface

oft-rot

ecay p

to

1 inch

eep.

On the

outh ide

of he xcavation

he

soil

conditions

ere

redominantly

granular.

even

f he13

piles

n

this

regionxhibitedrown-rotecay hat

extended

eep

nto he

ross

ection f

the

pile,

ncluding

wo

piles

with

more

Fig.

.

Heavily

ecayed

imber

iling

iscovered

at est

it



6/9

STRUCTURAL

SSESSMENT F AGING

IMBER

ILES

19

Fig.

.

n-situoad

esting

f timber

ile. Fig.

.

Laboratory

oad

esting

f

imber-pileegmentsxhibitingecay.

than 0

percent

ross-sectionoss

Fig.

6).

Preliminary

tructural

alculations,

which onsideredhe

omplete

oss of

the

ully ecayed iles

nd

a

very p-

proximate

stimate

f

remaining

ood

strength

t the ther

iles, uggested

that he

emainingile-cap apacity

as

marginal

ith

espect

o the

Cooper

E-

80

design

ail oad of he ehabilitated

facility.

his est

it xposed nly

limited umber f

piles;

however,

he

results f asks to 3 indicatedhat he

timber-pile

oundations

n

the outhwest

quadrant

f he rack eckwere

most

likely

ot eliable or uturerain oad-

ing.

Test

its

and 3. The conditionsn-

counteredt test

it

1

were

iscourag-

ing,

ut

hey rompted refocusing

f

the

est-pit

ffortso thenorth alf f

the ite. he

goal

was to determine

whetherhe imber

iling

o thenorth

couldbe relied

pon

o

support

he

lighter

ommuter

ail, uses,

nd vehi-

cleson that ection fthe rack eck.

Test its and 3 were ositionedust

north f he

nticipated eavy

ail

loads,

long

he outh

dge

of he

pro-

posed

ommuterail

ines,

s

shown

n

Figure

.

Twenty-five

imber

iles

were x-

posed

below ixdifferent

ile aps

n

test

its

and 3. The diametersf he

timber

iles anged

rom 0 to 16

inches,

ithmost

measuring

etween

1

and

14

inches.

ll

15

piles

bserved

t

test

it

3 were

n

very ood

condition,

with

robingetecting

ess hanV2 nch

of urfaceoft-rot

ecay.

n

test

it

2,

however,

eeppockets

fbrown-rot

decay

were

resent

t the

ops

f hree

of henine

xposed

imber

iles.

hese

decay ockets

measured

n

excess

f 3

inches

eep

nd

encompassed

signifi-

cant

portion

f he

pile

ircumference.

The

remaining

ix timber

iles

n

test

it

2

were

n

good

condition,

ith n aver-

age depth

f urface

ecay

f ess han

inch.

Although

he ondition f he

timber

iles

was found o be

very ood

at test it3,the onditionf hepiles t

test

it

was mixed ndof oncern.

Task

4:

conducting

ield nd

labora-

tory

oad

testing.

he

capacity

f

timber

ile

s a functionfthewood

strength

nd the

esistancef he oil to

pile

movement,

ncluding

oth nd-

bearing

nd side-frictionffects.

he

design apacity

f

he imber

iles

was

specified

n

the

riginal

rawing

s 20

tons,

r

40,000

pounds.

As a

compo-

nent f he ssessment

ork,

oad test-

ing

was

performed

n

thefield

nd

n

the

aboratory

o evaluate he ctual

n-

situ apacity fthe imberiles, valu-

ating

othwood

strength

nd

soil

resistance.

Field oad

testing

as

performed

n

two imber

iles

n

situ.

he

objective

f

this

esting

as to evaluateoilresis-

tance or

ypical

ndamaged

imber

piles.8

he selected

iles

were ocated

n

test

its

and 3 andexhibited o

appre-

ciablewood

decay.

A

large

ydraulic

ram,

oad

cell,

nd deflection

nstrumen-

tationwere itted ithin

gap

cut nto

the

piles Fig.

7).

One

pileplunged

t

approximately

0,000

pounds,

while he

other

xperienced

ittle

movemento

150,000

pounds

when he oad test

was

stopped.

hese

est esultsndicatedhat

the 0-ton-rated

apacity

f he

imber

piles

was

appropriate,

f

not onserva-

tive.As

such,

wood

decay,

ather

han

soil

resistance,

as therefore

argeted

s

the

primary

oncern.

Laboratory

oad

testing

as

per-

formedo assess

he ffectfwood

deterioration

n

pile

trength.

he

tops

of hree

eeply ecayed iles

rom est

pit

1

and

2

were emoved

sing

chain-

saw and

shipped

o

the

aboratory

or

full-size

estingFig.

8).

The

specimens

were

tored

n

a chamber

maintainedt

100

percent

umidity

nd tested

o

failure

n

compressionsing

load rate

roughlyquivalent

o that

pecified

y

ASTM

for

esting

f mall

lear

peci-

mens

n

compression.

llthree

eeply

decayed ile

ections

ere bleto

sup-

port

ltimate

ompression

oadsover

100,000

pounds,

nd none

xperienced

significant

nelastic eformation

crush-

ing)

until

oading

eached ver

0,000

pounds.

hese esults

rovided quali-

tativendicationhat

iles

with

eep

decay

ould till

upport

he

ated

ile

capacity

f20 tons.

However,

ignificant

deflection

ouldoccur

n

advance

f

failure.

Portions

fother ull-diameterec-

tions

f

imber

ile

were ut

nto

mall,

clear

pecimens

nd tested

n

compres-

sion

parallel

o the

grain.9

he

objective

was to estimate

he esidual

trength

f

each

pecies

fwood

n

areas

with o



7/9

20

APT

BULLETIN:OURNALF PRESERVATION

ECHNOLOGY

44:2-3,

013

Fig.

. Smalllear

amples

ere

ut romull-

size ectionsf imber

ile

or

ompressive-

strengthesting

n

he

aboratory.

visual ndicationsf dvanced

ecay.

The test

pecimens

easured inch

y

1

inch y inches all nd were ocated

along

adial

ines

f

ach

pile

ross

section

Fig.

).

An

average ompression

parallel

o

grain trength

as calculated

for ach

pileusing

he

rea-weighted

average trengths

romenter

inner),

middle,

nd outer

ones.

Thesevalues

were hen

onverted

o allowable om-

pressive-stress

alues.10

epending

n

pile pecies,

he

alculated

llowable

stresses

ere

pproximately

to 40

percent

ess han he llowable alues

calculated

sing

urrent

ublished

al-

ues.These estsndicatedhat 0

years

of

xposure

o moist

round

onditions

and a

fluctuating

ater ablehavere-

sulted

n

moderate

trengthegradation

of hewood.

Task

5:

performing

ood-pathology

analysis.

Wood-core

amples

nd

por-

tions ffull-diameter

ile

ections ere

sent o wood

pathologists

ormicro-

scopic

xamination.

total

f 15 differ-

ent

piles

were

ampled,

ncluding

5

wood-core

pecimens

nd

5 cross-

sectional lices. he

analyses

ncluded

identificationf hewood

species,

microbiologicalnalyses

f he

ype

nd

severityf nywood-destroyingungi,

inspections

or videncef

ny

bacterial

or nsect

ttack,

nd review fthe

general

ondition f hewood'scellular

structure.

Fourwood

species

were dentified

among

he

15

sampled iles:

ed

pine,

bald

ypress,

hite

ine,

nd

southern

pine.

All

of

he dentifiedood

species

are

considered

ondurable

xcept

ald

cypress,

hich s classifieds durable.

Nondurable oodsdo nothave natu-

ralresistanceo the ction

f

wood-

destroyingungi,

orers,

nd

nsects. s

expected,

one

f he

piles

had been

treated ith

reservative.

Soft otwas

the

predominate

ype

f

wooddecay etectednthe pecimens.

The soft

otwas

generally

hallow

r

superficial,arelyxtending

ore han

inch nto he

pile.

Advanced rown-rot

decay

was detected

n

several

ampled

timber

iles, ncluding

ll three f

he

full-diameter

ile-top

ections

hatwere

removed rom est

its

and 2. The

extent f

decay

was

greatly

nfluenced

by

he oil

type;

llof he

amples

hat

had

decay eeper

han

inchwere

extractedrom

imber

iles

t ocations

where he

urrounding

oil

ppeared

somewhat

andy.

Themicrobiologicalxpertsdvised

that uture

rowth

f

decay

was

difficult

to

predict

ith

ccuracy

ue to the

number

fvariables nd unknowns

n

this ase.

However,

n

their

udgment,

they xpected

he

growth

f oft-rot

decay

o

be

approximately

inear ver

time,

r

essentially

omparable

o the

past

rate. he rate

ffuture rown-rot

decay

ould cceleratever

ime.

Task6:

performing

tructuralnd

statistical

nalyses.

The

field

nvestiga-

tion

xposed

4 individualimber

iles

below10 different

ile aps,

or

about

one-halff one

percent

f he

pproxi-

mately

,000

timber

iles upporting

the ntire rack-deck

tructure.he

small ize

of he

ample

etwas further

complicated

y

he xtreme

ariability

ofthe dentified

onditions,

ncluding

pile

diameters

anging

rom

to 16

inches,multiple oodspecies fdis-

parate ompressivetrengths,epths

f

wood

decay anging

rom

ero o essen-

tially

00

percent

ross-section

oss,

nd

soil

types arying

ithin he ame

est

pit.

To evaluatend contextualize

ll of

the ollected

ata,

MonteCarlo tatisti-

cal simulations

ere

erformed

a

MonteCarlo imulations a

computa-

tional

lgorithm

hat

andomlyamples

the

probability

istributionf

nputted

values or ertain ariables o

determine

a

range

f

possible

utcomes,

s well s

the

probability

hat hose utcomes

ill

occur).11he results f heseimula-

tionswere

ncorporated

nto struc-

tural eview

nd

oad-ratingnalyses.

Thereaders cautioned

hat he esults

ofthis

method re

highly ependent

n

input ssumptions;

herefore,

s

such,

range

f

possible

ssumptions

hould e

studied,

nd the inal esults

hould e

interpreted

ith

ppropriate

ngineer-

ing udgment.

For

his

roject,

he

apacity

f

particular

olumn

oundation,

r

pile

cap,

s defined

y

he umulative

apac-

ity

f he imber

iles

n

which t s

supported. onteCarlo imulations

were sed o estimatehe

probable

capacity

f he oundations

hroughout

the rain

eck,

with ue

consideration

Fig.

0.

Graphicalummary

fMontearloimulationesults

or

ile-capapacities

n

0

years.

he

curve

epresents

he

requency

r

ercentage

f imulationshich

redictedapacities

t r bove

the alues

hownn he -axis.



8/9

STRUCTURAL

SSESSMENT

F AGING

IMBER ILES21

for

ossible

ariations

n

the

geometry

and conditionf he imber

iles.

The

measuredonditionsf he 4

piles

examined re

sed o

statistically

predicthe onditionf he est f he

approximately

,000

unexposed

imber

piles.

he

objective

f hese

nalyses

was to determine

he

probability

hat

the llowable

ile-cap apacities

ould

be

greater

han he

equired

oad de-

mands or oth he

heavy

ail oad and

the

ight

ail nd bus oad.

Threemain ariables ere

ncluded

in

theMonte

Carlo imulations:he

diameterf

single ile,

hewood

species

f he

pile,

nd the

percent

f

cross-sectionalreareduction

ue to

decay.

or ach

variable,

he

esults f

the nvestigationere sed odevelop

suite f54

possible ile

haracteristics

thatwere

andomly

ombined

o create

thousandsf virtual

iles

n

the tatis-

tical

modeling.

irtual

ile aps

were

then etermined

yrandomly

ombin-

ing

he irtual

iles

nto housands f

groups.

he

capacities

f hevirtual

ile

caps

were alculated nd

analyzed

tatis-

tically. grouping

iaswas ncluded

n

the imulationo account

or he en-

dency

or

ecay

o

occur

n

groups

f

piles.

he columns ere lso checked

for

ossible ending

orces hat ould

result romccentricityf he enterf

resistancef he

pilegroup.

Simulations ere

erformed

o evalu-

atethe

xistingapacity

f he

imber

piling

nd

pile aps

and were

epeated

to estimatehe

nticipatedile apaci-

ties 0

years

rom ow.The atter

imu-

lationwas ntendedo evaluate he

effectsffutureeteriorationf he

piling uring

he ife f he ehabilitated

facility.

he

analysis

f

possible

uture

conditions

onsidered linear

xtrapola-

tion f oft-rot

ecay

nd

rough

sti-

mates f

possible

rown-rot

ecay.

The resultsf he tructuralnd

statistical

nalyses

emonstratedhat

the

xistingiles

hould otbe relied

upon

o

support

eavy

ail

oading,

particularly

f

futureeterioration

s

considered

Fig.

10).

The simulations

predicted

hat

bout30

percent

f he

pilegroups

ould

reliablyupport

he

heavy

ail

oading

ow

but hat

nly

about

percent

oulddo so after 0

more

ears

f

xposure.

n the ther

hand,

imulations

ndicatedhat

oughly

80

percent

f he

pilegroups

ould

reliablyupport

he

ight

ail

nd bus

loading

n50

years.

ecause hese

e-

sults

were

ery pproximate

nd

highly

dependent

n the

underlyingssump-

tions, hedevelopmentf onclusions

also

required

ngineeringudgment

nd

considerationf

ranges

or he

various

assumptions

nd the

possible

ffectsf

inaccuracies.he reader

hould ote

that

pproximate

nalysis

echniques

f

this

ype

re

a toolfor n

experienced

engineer

o

use

n

the ontext

f

broader

ssessmentnd

might

otbe

appropriate

n

some

ituations.

Findings

Based

on the esultsfthe

nvestigation,

the imber-pileoundationsupporting

the

outh ne-third

f he rack eck

werenotreliable o

support

he ntici-

patedheavy-rail

oading.

oundation

underpinning

r removal nd recon-

structionf he rack eckwere

ecom-

mended. he timber

iling

elow he

north wo-thirds

fthe rack

eck,

however,

asfound o be

n

somewhat

betterondition.

he results

fthe

investigation

nd

statisticalimulations

indicated hat

most fthefoundations

in

thatnorth one should

have ufficient

capacity

o

support

he

nticipated

ight

rail, us, ndvehicle oadsfor henext

50

years;

owever,

heres a

relatively

low risk hat ertain

ile aps

willnot

have ufficientesidual

trength

uring

thenext 0

years.

he

consequence

f

such

deficiency

ouldbe slow nd

localized

oundation

ettlement,

imilar

to whathasmanifested

n

the outhwest

region

fthe rack

eck,

nd not ud-

denmovement

r

collapse.

Accordingly,

it

was recommendedhat he

imber-

pile

foundations

n

the

northernone

be

monitored

yregular

levation

urvey-

ing

f he rack-deck

olumns,

nd that

remedial ction

e.g.,underpinning

f

select

ile aps)

be taken hould oun-

dation ettlemente detected.

Current tatus

Rehabilitationf

heUnion

Depot

track

deck s

complete,

nd the

acilitypened

to

the

public

nDecember 012.

In

the

south hird fthe

tructure,

nderpin-

ning

was installedt the

pile aps,

which

upported

oth he rack eck

and thehistoric

oncourse tructure

above.The

underpinning

onsistedf

concrete

rade

eams

ast cross

he

top

ofthe

xisting ile aps,

keyed

nto

the

xisting

olumns,

nd

supported

t

theirnds n newmicropiles. ostof

the

emaining

rack

eck nd ts

foun-

dations

nthis reawere emolished

and

reconstructed

o

replicate

he

rigi-

nal

historic

ppearance.

owever,

nthe

north wo-thirdsfthe

tructure,

he

existing

oncrete

eckwas retained

nd

repaired,

nd

the

xistingimber-pile

foundations

ere eft ndisturbedo

support

he ehabilitated

acility.

Summary

The track-deck

tructure

f hehistoric

UnionDepotrailroaderminalssup-

ported

n

aging

imber

iles.

The

ex-

pected

erformance

f hese

imber-pile

foundations

as assessed

sing

nowl-

edge

of

ypical

erformance

nd

deteri-

oration

mechanisms

or imber

iles,

careful

eview f

he

upported

truc-

ture nd soilcharacteristics

t

the

ite,

exposure

nd

n-depth

esting

n a

limited umber

f

piles,

nd

analytical

studies

sing pproximate

tatistical

methods

o estimate

ikely

uture

er-

formance.

he

findings

llowed

n-

formed

ecisionso be made

by

he

design-buildeam ndowner egarding

the ehabilitation

pproach

nd future

monitoring

fthe tructure.

his olu-

tion

provided

reat

alue o the

ehabil-

itation

roject

nd enabled

hehistoric

fabric

f he rack-decktructure

o be

successfully

aintainednd

ntegrated

into

henewUnion

Depot

multi-modal

transportation

acility.

MARK .

CHAUVIN,

n

ssociate

rincipal

n

the

Minneapolis

fficef

WJE,

erveds

the

projectngineer

or he

nion

epot

rack-

deck ssessment.

e s structural

ngineer

and as

erformed

umerous

tructuralvalua-

tions

nd

nvestigations.

recent

roject

f

note as henvestigationf he -3Wbridge

collapse.

ecan e eachedtmchauvin@

wje.com.

ARNE

.

JOHNSON,principal

n

he

orth-

brook,llinois,

fficef

WJE,

erved

s the

project anager

or he nion

epot

rack-

deck ssessment.

e has

articularxpertise

evaluating

nd

esigningepairs

or

istoric

structures,

ncluding

everal

istoric

tadiums

andmuseums

ocated

crosshe

ountry.

e

can e

reachedt

[email protected].



9/9

22 APTBULLETIN:

OURNALF PRESERVATION

ECHNOLOGY

44:2-3,

013

Notes

1

The

esign-build

eam

as

ed

y

Mortenson

Construction

ompany

f

Minneapolis,

inn.

2.

Wiss,

anney,

lstner

ssociatesas etained

byURS orporation,he tructuralngineerf

recordor ehabilitation

nd

econfiguration

f

the rack-decktructure.

3. G.

H.

Wilsey,

St. aul niontation

ork

Entershird

tage, ailwayge

1,

no. 5

(Dec. 7, 921):

202.

v

4. G.W.

Wilsey,

St.

aul nion

epot

om-

pletes

hird

ection,

ailwayge

6,

no.

7

(Mar.

9, 924):

29.

5. Kris .

Altiero,

Evaluationf

xisting

Wood

iles,

Wood

esign

ocus

Summer

1996):

-11.

6. The

eotechnical

ork as

erformedy

American

ngineeringesting

f

t.

aul,

Minn.

7. The 991

est-pit

ork as

erformed

or

the SPS

y

oltz,

ing,

uvall,

nderson

nd

Associatesf t. aul.

8.These ests ere

erformed

n

general

accordanceith STM 1143, tandardestMethodsor

eep

oundationsndertatic

Axial

ompressive

oad.

9. These ests

ere

erformed

n

ccordance

with

STM 143

09,

tandardest

Methods

for

mall lear

pecimensf

Timber.olume-

weightedreenpeciestrengths

erehen

calculated

sing

lear

trength

ood aluesnd

standing

imberolumes

ublished

nASTM

D2555-06,

tandardractice

or stablishing

ClearWood

trength

alues.

inally,

llowable

compressionarallel

o

grain

tressesere

calculated

or

ach f he

ile peciessing

he

area-weightedverage

est

trengths

nd

ro-

cedures

iven

n

ASTM

2899-03,

tandard

Practice

or stablishing

llowabletresses

or

Round imberiles.

10.Theseonversionsere

erformedsing

the

roceduresiven

n

ASTM

2899-03,

Standardractice

or stablishing

llowable

Stresses

or

ound imberiles.

11Nicholas etropolisnd .Ulam,TheMonte arloMethod, ournalf he meri-

can tatisticalssociation

44,

o. 7

Sept.

1949):

35-341.

H

I

I

j

M

The

Association

ogy

organization

International,

APT

ulletin

of

dedicated

Preservation

s

an

published

interdisciplinary

to he

Technol-

prac-

by

he

I

Association

f reservationechnol-

j

ogy

nternational,

n

nterdisciplinary

I

organization

edicated

o he

rac-

INTERationl

tcaj

pplication

f he

rinciples

nd

techniquesecessary

orhearend ise se f

the uiltnvironment.

ubscription

o he

Bulletinnd reenlineccesso

ast

rticles

re

memberenefits.ormore

nformation,

isit

www.apti.org.

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APT-Structural Asessment of Aging Timber Piles