apt-structural asessment of aging timber piles
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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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7/24/2019 APT-Structural Asessment of Aging Timber Piles
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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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.
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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.
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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
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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
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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.
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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
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7/24/2019 APT-Structural Asessment of Aging Timber Piles
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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