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Missouri University of Science and Technology
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Performance of a stone column supportedembankment
R.R . Goughnour
R.D. Barksdale
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Performance
o
a Stone Column Supported Embankment
R. R Goughnour
Vice President Vibroflotation Foundation Company
R.
D Barksdale
Professor Georgia Insti tute o Technology
The proposed
expansion of
ramps
connect ing
In te r s t a t e Route
664
with In te r s t a t e
Route
64 a t
Hampton, Virg in ia involved
numerous
high embankments and bridge s tructures over marshlands.
Potent ia l problems
of embankment s t ab i l i t y and
excessive
long term, post
construction
sett lements were fur ther
complicated
by very s t r i c t
environmental
cons t ra in t s on acceptable
c o n s t r u c t i o n
methods.
The so l u t i o n chosen was s t ab i l i za t i o n of the
in
s i t u
so i l s by t he
ins ta l l a t ion
of
stone
columns.
A
descr ip t ion is given of stone column design, cons t ruc t ion
,
f i e ld
embankment instrumentation,
and
embankment
performance for the f i r s t
two
years of operat ion .
Four
theories for
predict ing
set t lements
of stone
column
re in fo rced
ground
are br ie f ly
reviewed.
Calcu la ted se t t lements of
the
embankment
are
then compared
with
the measured sett lements.
Although the set t lements p red ic ted
by
each
method
d i f f e r , they
genera l ly
give
good resu l t s .
INTRODUCTION
Construct ion of the in terchange expansion a t
Hampton,
Virginia connect ing In te r s t a t e I -64
with
I-664
involved numerous
high
embankments
and bridge s t ruc tu res constructed over very
so f t
marshland
depos i t s . Approximately
134,000 f t . (40,900 m
of
stone
columns
were
used
to support
port ions of
the in terchange
embankments. Important factors in deciding to
re inforce
the
ground
with s tone
columns
included (1) s t r i c t environmental cons t r a in t s ,
(2) the
presence
of
Newmarket
Creek
immediately ad jacen t to one in terchange ramp,
and
(3)
achieving
acceptable
post
const ruct ion
set t lements without delaying the pro jec t .
Stone columns were
selected
over
(1)
excavat ion
and
replacement
and (2) surcharg ing
due
primarily
to environmental and economic
considerations.
Before
construction
of
the in te rchange , a
long term,
v e r t i ca l
load t e s t program was
conducted
to
ver i fy
the
des ign p r i n c i p l e s .
This
t e s t
program, which
has
been descr ibed
in
de ta i l
by
Goughnour and Bayuk
( l979a) , gave
valuable information
concerning
ul t imate
column load,
group set t lements ,
pore pressure
development
and
s t r es s concen tra t ions in the
stone
columns.
INTERCHANGE CONSTRUCTION
A
plan
view of
the
I-64/I-664 in terchange i s
shown
in
Figure 1.
The
major
port ion of
the
in te rchange i s loca ted in
a
sha l low t i d a l
marsh area having
a
ground surface elevat ion
of approximately +2 f t .
(0.6
m above
mean
sea
level . Brush
up
to 8
f t . (2.4
m
in
height i s
present.
735
Stone
columns were placed under portions of
t h e
e a s t and west bound l anes of
I -64 ,
and
portions of Ramps A, B, C
and
D (Figure 1).
The embankments placed
above
the stone column
improved ground var ied
in
height from 7 to 28
ft (2 .1 to
8.5
m). All embankments
were
constructed on a 2
(horizontal) to
1
(ve r t i ca l )
s ide
s lope.
1 - 6 6 m \ Route
1-664
_ Connector
I
lnterchanoe
I
Figure 1 .
General
Location Plan.
SUBSURFACE
CONDITIONS
A
f i rm
to
very s t i f f
marsh
mat
2
to
4 f t .
(0 .6
to
1.2 m
th ick
occurs a t the surface in
the
v i c i n i t y
of the interchange. Immediately
beneath the
marsh
mat,
10 to
16 f t .
3
to
5 m
of
e r ra t i c marine depos i t s were encountered
inc luding very sof t brown s i l t s with sand and
very sof t to
f i rm,
dark
gray and blue
clays
with
very th in
seams of f ine sand
and s i l t .
Organics were of ten
present .
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This st ratum
was
underlain a t a
depth
of 10
to
16 f t .
(3
to 5 m) by loose to very f i rm clayey
and s i l t y sands , f i n e to
medium
sands , and
f ine sandy c l ay s .
The
median v a lu e o f the
undrained
shear
s t rength in the upper 10 to 16
f t . (3
to 5 m),
as determined by
f i e l d
vane
shear
t e s t s ,
was between about 500 and
600
psf
(24 to
29 kN/m
2
) ,
while
the median value
fo r
the sof ter zones
was about
380 psf
(18 kN/m
2
) .
The embankment fill for
the
eas t approach to
the Ramp C br idge
was located in the
vic in i ty
of some of the poorest so i l s encountered along
the route . Near Ramp C
the
very so f t
so i l s
were about
8
to 9ft (2 .4 to
2 .7
m)
in
t h i ck n ess .
The
lowes t
two
undra ined sh ear
strengths measured in th i s area (and on the
s i te)
were
140
and 180
psf
(6.7
to
8.6 kN/m
2
)
a t depths o f 3 and 6 ft ( 0. 9
and l
8 m)
respec t ive ly .
SOIL PROPERTIES
The
highly compressibi le
gray and
blue clays
(CH)
had void ra t ios
varying
from
about 1.5
to
2 .6 , low
wet
u n i t
weights of about 85 p cf
(13.3
kN/m
3
) , and
water
contents
around 110 .
The
l i qu id
l i m i t o f
these s o i l s was around
118,
and the p las t i c l imi t
39,
with a
corresponding l i q u id i t y
index
of
0.75 to 0.90.
The Compression Index, Cc, var ied from 0.9 to
1 .1
as
summarized in Table . I
E f f e c t i v e
s t ress
s t rength
parameters were
es tab l i shed
from consol idated
undrained t r i ax i a l t e s t s
with pore
pressure measurements
CU
t e s t s ) .
The
resu l t s of
these t e s t s
indicated
t h a t the
very sof t
clays have
effec t ive s t re s s s t rength
parameters of c =
50 psf
(2.4 kN/m
2
) and
=
26.
c
eo
y
w DEPTH
DESCRIPTION
c
C
rlf)
( f t . )
1
1. 06
2.6 84.2
l lO
Dark
gray
organic
clay (OH)
2
1. 07
2.6
87.6
109
Dark gray
highly
p l a s t i c
c lay (CH)
3
0.86
1.5
84
l l2
7-9
Gray
s i l t y
clay
4
0.27
0.8
l l7
39
7-9
Gray
s i l t y
clay
5
0.203
0.75
122.5
30
17-19
Gray
s i l t y
f ine
sand
with she l l
fragments
6
0.050 0.93
128
50
22-24 Si l ty
sand
with
some
clay
Table
I -
Summary of
Projec t .
Soi l
Proper t ies on
Tota l
736
The
l e s s
compressible
s i l t y sands
SM)
organic
s i l t s
(ML-01)
and low p l a s t i c i t y clay
(CL)
had l iqu id
l im i t s
typ ical ly
varying fro:
17
to 45, and plas t i c l im i t s varying from 0
t
27;
many of
the
sample t e s t ed
were nonplast ic
The void ra t io t y p i ca l l y var ied from about
0.
to
1 .0 , and
wet
weight
from
115
to 128 pcf (1
to
20 kN/m3), with wate r co n ten t s o f
30
t
40 .
The Compression
Index, Cc, var ied fro
about 0.05 to
0.3 .
Peak and remolded undrained shear s t rength
were
obtained
by f i e ld
vane
t e s t s .
The media
value
of sens i t iv i ty for the s i t e , taken a
the
ra t io
of peak to remolded shear strength
was about
2. The
s e n s i t i v i t y
v ar i ed f ro
approximately 1
to 3.
According
to
th
c las s i f i ca t ion
system of Bjerrum (1954),
thes
s o i l s f a l l
wi th in t h e
i n se n s i t i v e (1-2) t
moderately
sensi t ive
(2-4) range. Ston
column exper ience has been l imi ted to s i te
having s e n s i t i v i t i e s not exceeding
about
(Baumann aJ,d Bauer , 1974) .
STONE COLUMN
DESIGN AND
CONSTRUCTION
Approximately 134,000
l inea r
f t . (40,900
m)
c
stone columns
were
placed beneath
6,300
l inea
f t . (1920 m) o f interchange embanlanent.
Tl:
s tone
columns were cons t ruc ted using
eq u i l a t e ra l t r i an g u la r p a t t e rn
with
sic
dimensions
varying from 5
to
8 f t .
(1.5
to 2.
m .
Column
spacing
The column
spacing se lec ted depended on t r
he igh t
of
embankment and column loca t ic
with in the embankment.
Zone
A was
the
centre
p a r t
of
the embankment, and Zone C e
t h a t
par t
of the s ide s lope < 12 f t . ( 3. 7 n
in he igh t
(see Figure
2). Zone
B
was in te r
mediate to Zones A
and
C.
In low embankment
Zone
B did
not
exis t .
This
was
the
case
E
the i n s t ru me n te d
s e c t i o n s .
In Zone A tl:
l imi t ing
design
c r i t e r ion was
se t t l ement
while
in
Zones B and C
s t ab i l i t y cons idera
t i o n s determined column
sp ac in g . F ig u re
i l lus t ra teR how design
spacings
were chose
for
Zone
A.
The design
curve in
Figure 3 was based on tr
r esu l t s
of
the f u l l
scale ,
long term, ver t ic
