Missouri University of Science and Technology
Scholars' Mine
International Conference on Case Histories in
Geotechnical Engineering
(1998) - Fourth International Conference on Case
Histories in Geotechnical Engineering
Mar 8th - Mar 15th
Soil Improvement in Flood Plain of the Mississippi
River Using Pre-load Fill
Mike Alizadeh
Geotechnology, Inc., St. Louis, Missouri
Sanjeev Kumar
Geotechnology, Inc., St. Louis, Missouri
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Recommended Citation
Alizadeh, Mike and Kumar, Sanjeev, "Soil Improvement in Flood Plain of the Mississippi River Using Pre-load Fill" (1998).
International Conference on Case Histories in Geotechnical Engineering. 18.
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=
-
Proceedings: Fourth International Conference on Case Histories in Geotechnical Engineering, St. Louis, Missouri
March 9-12, 1998.
'
1273
-
SOIL IMPROVEMENT IN FLOOD PLAIN OF THE MISSISSIPPI RIVER
USING PRE-LOAD FILL
Mike Alizadeh, P .E.
Geotechnology, Inc.
2258 Grissom Drive
St. Louis, Missouri-USA. 63146
Sanjeev Kumar, Ph.D., P.E.
Geotechnology, Inc.
2258 Grissom Drive
St. Louis, Missouri-USA, 63146
Paper No. 7.37
ABSTRACT
The pre-load method of improvement of in-situ, fine grained soils, is frequently used in practice particularly in flood plain
environments. A case history is presented in which the same method was used to induce expected settlements in the
existing soils before the start of construction. Total settlements and rate of settlement were monitored at two sites for
a project in Jefferson County, Missouri. The results presented show that total settlements predicted using one dimensional
consolidation theory are in reasonable agreement with the measured settlements in field. The results also show that in
a flood-plain environment having compressible layer consisting of silty clays and silts, most of the settlement is likely
to take place during placement of the fill.
KEYWORDS
Flood Plain, Preload Fill, Settlement, Soil Improvement
INTRODUCTION
With the rapid pace of industrialization, structures are
often being constructed in coastal areas and in the flood
plains of major rivers.
However, because of the
presence of soft clays in the flood plain areas, soil
improvement techniques are frequently used to resist
heavy loads from industrial structures, e.g, electric
power plants, airports, storage tanks, cement plants etc.
There are a number of techniques used to improve the
in-situ soft soils, which in their existing conditions are
not suitable to support foundation loads. Pre-loading of
the site with or without sand drains is frequently used to
induce settlement of in-situ soils before the start of
construction. This results in a significant reduction in
post-construction settlements of soils. The magnitude of
the preload pressure and time required to induce the
desired settlements significantly depends on the
consolidation characteristics of the underlying soils.
This paper presents a case history of in-situ soil
improvements accomplished using preload fill without
sand drains. Total settlements and rates of settlement
were predicted and measured in the field at two sites for
a project in the flood plain of the Mississippi River, in
Jefferson County, Missouri. Results are presented to
show that one dimensional consolidation theory (Lambe
and Whitman, 1969) predicted total settlements within 30
percent of those measured in the field. However, the
time rate of settlement estimated using the onedimensional consolidation theory are significantly higher
than those observed in the field. Results show that insitu
soils consisting of silty clays and silts in a flood plain
environment are likely to consolidate significantly faster
than the residual soils or wind blown deposits (modified
loess).
SITE AND PROJECT DESCRIPTION
The project consisted of construction of a steamgenerated, electric power plant in Jefferson County,
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Missouri University of Science and Technology
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1274
Missouri. The site is located on the west bank and in
the alluvial valley of the Mississippi River. Site location
of the project is shown in Fig. I. The existing ground
surface elevations at the time of subsurface investigation
were in the range of El 380 to 388 feet msl.
Approximate! y 22 to 30 feet of new fill was required to
achieve the proposed elevation of El410 to raise the site
above 200 year flood stage.
390
390
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350L-~--
S1d. Penetration._
Blows/Ft(Typ.l
SITE I
J
350
SITE 2
Fig. 2 Upper stratum soil profiles at Sites I and 2
Fig I. Site location of the project
FIELD INVFSTIGATION
CONDITIONS
AND SUBSURFACE
Field investigation for the total project consisted of
drilling ninety-seven borings ranging in depths from 16
to 145 feet. The surface of the area indicated typical
ridge and swale topography approximately parallel to the
river. Based on the soil conditions observed in the
borings, the foundation soils at the site can be divided
into two strata. The upper stratum consisted of clays
and silts, approximately 10 to 30 feet thick, except in
certain sloughs where the thickness approaches 50 feet.
The stratum of clays and silts was underlain by a stratum
consisted of fine to coarse sands approximately 80 to 120
feet thick. Some buried silt and clay lenses were also
present in the sand formation. Limestone bedrock was
encountered at depths of approximately 120 to 140 feet
from the existing ground surface, near the river. The
depth to rock was observed to decrease rapidly to a
depth of 30 to 40 feet near the western edge of the site,
away from the river. Soil profiles of the top stratum at
the preload fill test sites, Sites I and 2, are shown in
Fig. 2.
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The consistency of the upper stratum of clays and silts
was observed to be soft to medium stiff (s,. in the range
of 0.20 to 0.40 tst), typical for the flood plain of the
Mississippi River in Jefferson County, Missouri.
Moisture contents at the preload fill test sites generally
varied between 30 and 35 percent. Liquid limits of the
silts and clays were observed to be between 37 and 48 at
Site 1 and between 30 and 38 at Site 2. Plastic limits at
both sites were observed to be between 23 and 26.
From the consolidation tests, the over consolidation ratio
(OCR) of the soil was estimated to be between 1.5 and
2 except for samples obtained near the ground surface
where over consolidation ratios above 6 were measured,
The Compression index of the existing soils was
estimated to be between 0.28 and 0.34 except for a
sample near the ground surface where a compression
index of 0.16 was measured. High over consolidation
ratios and low compression indices of the existing soils
near the ground surface show some desiccation of the
surfacial soils. The in-situ sands were observed to be
medium dense to dense with measured blow counts in
the range of 20 to 40.
PRELOAD TEST FILLS
Two preload test fills were constructed on the site to
measure settlement characteristics of the existing soils at
the project site. Plan and elevation dimensions of the
preload fills at Sites 1 and 2 are shown on Figs. 3 and
4, respectively.
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ELEVATION
ELEVATION
Fig. 3. Plan and elevation dimensions at Test Site 1
Fig. 4 Plan and elevation dimensions at Test Site 2
Two settlement plates were installed at each preload site.
The settlement plates are referred to as SP (Shallow
Plate) and DP (deep Plate). Details of the settlement
plates used for measuring settlements in the existing soils
are shown in Fig. 5.
Density tests taken from the fill indicate an average wet
density of approximately 115 pounds per cubic foot
(pet).
Preload Test Fill at Site 1. The top stratum of the
existing soils at the location of preload test fill, Site 1,
predominantly consisted of 20 feet thick layer of silty
clay. The preload fill was constructed of uncompacted
loose fill to a height of 24 feet. The fill was 35 feet
square at the top and had side slopes of 1 Vertical to 1.5
Horizontal (1V:l.5H). A 5 foot high and 10 foot wide
stabilizing berm was constructed around the bottom of
the preload fill after placement of preload fill.
Fourth International Conference on Case Histories in Geotechnical Engineering
Missouri University of Science and Technology
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Preload Test Fill at Site 2. The top stratum of the
existing soils at the location of preload test fill, Site 2,
essentially consisted of 16 feet thick stratum of silts with
clay seams. The preload fill at Site 2 was constructed of
compacted fill from a base dimension of 90 feet square
with a side slope of IV: 1.25H up to a height of 13 feet.
Loose fill was placed from 13 feet to the maximum
height of 19 feet with a slope of 1V: l.5H making the fill
approximately 35 feet square at the top. No density tests
were made of the preload fill material at Site 2 but a
density of 115 pounds per cubic foot (pet) was assumed
from loose preload fill.
1276
~
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---jO.P
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Top of preload fill
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8
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TYPICAL COUPLING DETAIL
~--SANDs----
Fig.5
Details of shallow and deep settlement plates
SE'ITLEMENT OBSERVATIONS
Fig. 6 shows settlements observed and elevation of fill
with time at Site I. Similar observations for Site 2 are
shown on Fig. 7. As shown in Fig. 6, total settlements
of 0.12 feet and 1.25 feet were observed at shallow and
deep settlement plates, respectively, resulting from the
weight of the fill (increase in pressure of 1.4 tsf). Based
on the laboratory test results, settlement of 0.9 feet was
computed which is approximately 72 percent of the
measured settlement of 1.25 feet. Readings from the
deep settlement plates appear to be in error. Fig. 6 also
shows that approximately 90 percent of the total
settlement in the clay stratum occurred as the load was
placed. Fig. 7 shows that the total settlements of 0.05
feet and 0.55 feet were observed at the deep and shallow
plates, respectively. Total settlement of 0.60 feet was
computed using data from consolidation tests performed
in laboratory. Fig. 7 shows that approximately 90
Fourth International Conference on Case Histories in Geotechnical Engineering
Missouri University of Science and Technology
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percent of the total settlement took place within 2 days
after placement of the fill.
Time required for 90 percent of the total settlement to
take place was computed to be between 30 and 50 days
using one-dimensional consolidation theory.
A
coefficient of consolidation, C., of I ft2/day was used to
compute the time required for completion of 90 percent
of the settlement (Holtz and Kovacs, 1981 and
NAVFAC, 1971). However, as shown in Figs. 6 and 7,
consolidation of the existing soils was significantly faster
than that predicted and most of the settlements were
complete within 2 days of placement of the preload fill.
This shows that in flood plain environments, the time
rate of settlement estimate using one-dimensional
consolidation theory may be conservative.
1277
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ELAPSED TIME IN DAYS
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Computed settlement for SP\
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40
top of sot (EI 366.5)
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30
- SP on top of ground (EI 387.5)
Noto DP "odioos fcom 0 to II
appeared to be In error; read_ ings were adjusted based on
..J 1.0 f-Iedr~polation of subsequent
I-
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Fig. 6 Settlements observed and elevation of pre-load fill with time, Site 1
ELAPSED TIME
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7 International
Settlements
observed and elevation of pre-load fill with time, Site 2
Conference on Case Histories in Geotechnical Engineering
Missouri University of Science and Technology
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40
50
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Computed Isettlement for S P)
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0
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Note: Initial settlement
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IN DAYS
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1278
The faster rate of consolidation in flood plain
environments may be because of the presence of thin
layers or lenses of relatively large permeability silts and
sands within the compressible stratum which acts as
drainage paths for expulsion of water during
consolidation, thus increasing the rate of consolidation.
Therefore, it is the authors' opinion that when estimating
the time rate of settlements in the flood plain
environments where the compressible soil stratum
consists of silty clays and silts, consideration should be
given to the likelihood of the presence of thin layers or
lenses of silts and sands in compressible layers, and an
effective length of the drainage path should he used to
compute time rate of settlements.
REFERENCES
CONCLUSIONS
Holtz, R.D and Kovacs, W.D. (1981). "An Introduction
to Geotechnical Engineerin~." Prentice Hall, Inc.
Based on the theoretical estimates and measured
settlements it was concluded that settlements in the range
of 0.5 to 1.5 feet may occur at the site under the weight
of the plant fill. The total settlement will depend on the
thickness and characteristics of the compressible stratum
at any given location. It was concluded that most of the
settlements due to plant fill will be completed within 5
to 10 days after completion of the fill.
Fourth International Conference on Case Histories in Geotechnical Engineering
Missouri University of Science and Technology
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Results presented here show that in a flood plain
environment where the compressible soil layer consists
of silty clays and silts, most of the expected settlements
are expected to take place during the placement of the
fill. This may be because of the presence of thin layers
or lenses of relatively large permeability silts and sands
within the compressible stratum.
Total settlement
measured at Site I was approximately 30 percent higher
than that predicted using one-.limensional consolidation
theory. Total settlement measured at Site 2 was close to
the predicted settlements.
Lambe, T.W. and Whitman, R.V (1969).
Mechanjcs." John Wiley & Sons.
"SQil
NAVFAC (1971). "Soil Mechanics, Foundations, and
Earth Structures." Design Manual DM-7, U.S.
Navy, Washington, D.C.