Missouri University of Science and Technology
Scholars' Mine
International Conference on Case Histories in
Geotechnical Engineering
(2004) - Fifth International Conference on Case
Histories in Geotechnical Engineering
Apr 13th - Apr 17th
Unique Foundation Solution for Organic Soil Site
Keith E. Robinson
EBA Engineering Consultants Ltd., Vancouver, B.C., Canada
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Recommended Citation
Robinson, Keith E., "Unique Foundation Solution for Organic Soil Site" (2004). International Conference on Case Histories in
Geotechnical Engineering. 54.
http://scholarsmine.mst.edu/icchge/5icchge/session01/54
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UNIQUE FOUNDATION SOLUTION FOR ORGANIC SOIL SITE
Keith E. Robinson, M.S., P.Eng.
Principal Geotechnical Consultant
EBA Engineering Consultants Ltd.
Vancouver, B.C. Canada
ABSTRACT
A soft soil site, located near Vancouver, BC, presented a unique challenge for the Owner and geotechnical engineer. The relatively narrow
but long site was covered by a variable thickness of silty sand and gravel fill overlying about 3 m of peat, organic silt to 12 m depth and
very soft, sensitive clay to about 35 m depth. A three/two-story, 27 by 200 m, concrete tilt-up warehouse type structure was proposed for
the site. The two foundation options initially considered included the full structure and slab supported on piles driven down to refusal at
about 40m depth or conventional spread foundations after preloading the site with up to 6 m of fill for at least one year.
An alternative and chosen design was to use a thick mat of Lightweight Cellular Concrete (LCC) to offset building and fill loads as well
as to provide a method of redistributing foundation stresses to reduce potentially adverse differential movements. Because of a history
of variable fill thicknesses placed over the property, with resulting variable magnitude of preconsolidation of the soft soils, it was necessary
to design a variable thickness for the LCC along the length of the building and provide a nominal preload for three months prior to building
construction. The project was completed on budget and has performed as predicted over the past four years since construction. This paper
presents the results of settlement monitoring of the preload and warehouse, and provides a summary of building performance.
INTRODUCTION
The owners of a property in Port Coquitlam, a suburb of
Vancouver, BC, Canada required an innovative foundation
design for a long, narrow warehouse. The 27 by 200 m structure
was to be the equivalent of 3 levels in the western (front) 60 m,
with the remaining structure consisting of 10 m wide warehouse
units with mezzanine levels over part of each unit. Allowable
floor loading was 12.5 kPa.
half. A small office was located at the western end. In 1992 a
preload was placed, as indicated in Figure 1, in preparation for
a planned small office structure that was eventually not
constructed. The preload was left in-place for some time but the
removal date was unknown. A long, very narrow, block wall
storage building abuts the northern property line about 17.5 m
from the proposed building as shown in Figure 1.
Because of the very soft soils at the site and the variable past
usage of the lot (loading history), the optimization of the
foundation design required significant innovation. The structure
was completed in mid 1999. This paper describes site
conditions, foundation options, details of the chosen design and
monitoring performance over the past 4 years.
At the time of the site investigation the general grade sloped
down from about Elevation 5.3 m at the east end to Elevation
3.5 m near the Schoolhouse St. at the west end. There were some
piles of fill covering parts of the eastern end of the lot to heights
of 1.5 m. Apparently the eastern part of the site and a small west
central area (see Figure 1) had some fill loading history which
resulted in partial preconsolidation of the soft native soils.
SITE CONDITIONS
Soil conditions, as revealed by 3 boreholes and 32 test pits,
consisted of 0.6 to 2.6 m of fill increasing from west to east
across the site. The fill consisted of river sand and silty sand and
gravel (glacial till excavated from nearby sites), overlying about
The long, narrow lot had historically been used for storage of
equipment and, occasionally, miscellaneous fill in the eastern
Paper No. 1.92
1
15
15
0
30
5.2
ALL DISTANCES ARE IN METRES
CREST OF OLD PRELOAD AREA
0
5.
4.0
LCC Elevation
Change
LCC Elevation
Change
3.8
4.4
LCC Elevation
Change
LCC Elevation
Change
LCC Elevation Change
5.0
4
5.
5.2
5.
0
2
4.
Building
4.8
Building
5.0
6
4.
4.4
2
4.
Building
4.8
0
4.
Building
5.
0
4.6
BUILDING PERIMETER
8
3.
2
5.
6
3.
LCC Elevation
Change
8
3.
SCHOOLHOUSE STREET
Existing Storage Building
Building
Fig. 1. Site Plan & Contours
0.3 m of hog fuel (wood chips). Below the fill, native soils
consisted of the following:
Soil Description
Thickness
(meters)
Average Moisture
Content in Percent
Peat to silty peat
Peaty organic silt
Soft organic silt
Stiff silt
Sensitive silty clay
Dense gravel over
glacial till
2 to 3.4
0.6 to 3.4
6
3.5 to 4.0
18
>2
300
100
65
38
55
10
differential settlements of the building within tolerable levels.
The preload would have to be placed in stages and bermed to
prevent slope failures. A further problem for a high preload of
long duration would be the impact on adjacent structures,
particularly the long block wall building to the north.
Settlements of this building would be significant and require
remedial treatment.
Based on previous experience with Lightweight Cellular
Concrete (LCC), it was recommended that another option be
considered which would combine LCC and a nominal preload.
This would be less costly than the pile option and meet the
Owner's construction schedule. The added benefit was to have
the yard and building move together as long term settlements
develop.
FOUNDATION OPTIONS
FOUNDATION DESIGN
The upper 12 m of soils are very compressible. The lower 18 m
of sensitive clay is also compressible but would not be
significantly impacted by the relatively light, narrow building.
The non-uniform load history of the site provided a complicating
factor relative to settlement estimates for various options. A
further complication was the requirement for a slab grade at
elevation 4.7 m, up to 1.2 m above the existing grade at the west
end.
The two standard foundation options for similar soil conditions
in the Greater Vancouver region are piles, or preloading followed
by spread footings. There are two significant issues related to
piles that eliminated them from consideration. The soft soils at
the site would continue to settle differentially under the influence
of previous fills and the 1.2 m raised grade at the west end.
Maintaining access to loading bays would be difficult, costly and
unsightly. Also, the extensive depth to firm bearing for the piles
would result in a very expensive solution as the structure and
slab would have to be pile supported.
The preload option provided a number of difficulties. Because
of the extensive thickness of compressible soils and their low
permeability, preload time would long even if a very high
preload was used. It was estimated that up to 6 m of preload
would be required for at least 1 year to keep long term
Paper No. 1.92
LCC is a mixture of cement, ash, water and foam. The LCC is
used to reduce the total weight of the structure by offsetting soil
loads. For this project, the LCC was designed to have a
maximum wet cast density of 5 kN/m3. To provide a cured
strength of 1 MPa for the LCC, the cement/ash portion of the mix
was 1/1. To protect the LCC it was wrapped in 12 mil PVC.
The foundation design required an iterative process based on the
changed grade at various positions under the building, average
building loads, preload impacts and thickness of LCC. The
impact of building loads and raised grade fills was minimized by
varying the thickness of LCC as well as preloading the building
area with a nominal height of fill for 3 to 4 months.
The final foundation design included a variable thickness of
preload fill, followed by excavation to place a variable thickness
of LCC. The thickness of both were a maximum at the west end
of the building where soil conditions were least favorable,
existing grade was lower and there was no history of previous fill
or preload placement.
To complete the analysis, consolidation parameters for the peat,
organic silt, silt and sensitive clayey silt, summarized in the form
of Cc / 1+℮o values of 0.44, 0.25 and 0.19 and 0.17, respectively,
2
were used to assess primary settlements under various loading
conditions. An additional secondary settlement component was
also included. The LCC was varied in thickness to balance the
net increase in soil pressures resulting from the fill, LCC and
average building loads against estimated long term settlements.
For example, in the western 30 m of the building, the net
increased pressure below the LCC based on the average dead
plus long term live loads plus 2.4 m of LCC less 0.6 m of the
original site grading fill, was estimated to be about 20 kPa.
length of that section. The effect was estimated to be a gentle
tipping of the building rather than abrupt differential offsets. The
unique and attractive feature of this design was the thick mat of
1 MPa material (LCC) below footings that spread loads over the
soft organic soils. The LCC was designed to extend 3 m beyond
the building perimeter. Settlements to the east were predicted to
be gradually less in total magnitude to a minimum of 135 mm at
the eastern end of the building.
The long term settlement of the building with this net pressure
increase, without preloading, was estimated to be about 950 mm.
The preload was expected to reduce the maximum postconstruction, building settlement to 380 mm which would be a
combination of primary and secondary consolidation to 10,000
days or the year 2026.
PRELOAD SETTLEMENTS
Because site grades and relative preconsolidation of the
underlying soils increased toward the east, the thickness of LCC
and preload fill was reduced. To provide an acceptable
foundation cost to ensure the viability of the project, the Owner
accepted that settlements could not be balanced along the
building length. The design, therefore, provided for differential
settlements that could be tolerated by the structure. The design
resulted in settlement estimates that would be significantly
greater at the west end than at the east end of the building.
The design section, shown in Figure 2, included 1.2 to 2.1 m of
preload above final slab grade and 0.8 to 2.4 m of LCC below the
base of slab. The LCC was placed in a series of cells, excavated
and filled in sequence from east to west. For the third cell from
the west, two thicknesses of LCC were used to offset the effect
of the previous preload in the north part of the building. The
north half had 0.9 m of LCC, while 1.8 m was used for the south
half.
The building was divided into 3 independent sections with
separate walls at 60 and 120 m from the west end. The LCC was
continuous across the sections although steps in thickness
coincided with the sections. The intention was to allow the
building to move relatively freely in sections to prevent or
minimize potential cracking, should adverse settlements develop.
This was considered an additional safety factor.
Based on the anticipated building and site grading fill loads, and
the short term preload, settlements in various sections of the
building were estimated. For the western 60 m section, the higher
building loads and the proximity of the old preload to its eastern
end, resulted in settlement estimates that ranged between 150 and
380 mm with differential movements of up to 230 mm along the
Paper No. 1.92
Preload commenced in mid April, 1998 with removal between
mid August and mid September 1998. LCC placement started
toward the end of August progressing west from the east end as
the preload was removed. The preload was monitored by
surveying 24 gauges spaced around the building as shown in
Figure 3. At the west end, the preload remained in place almost
5 months with a minimum time at the east end of 4 months.
Projected settlements of each gauge to the removal date has been
shown in Figure 3 with movements ranging from 0.08 to 1.11 m.
The low readings were from areas of previous preload activities
and the area at the east end where the recent site grade had been
higher.
A few typical settlement plots are provided in Figure 4. The
plots are typical for the Vancouver region where these types of
organic soils are encountered, with an almost straight line
projection of settlements on a semi-log scale. The importance of
preloading in combination with LCC is evident from the
monitoring results.
BUILDING PERFORMANCE
The LCC installation was completed by the end of 1998.
Building construction could then be initiated without concerns
regarding soil conditions during inclement weather. The
building was effectively completed by the end of summer 1999.
For purposes of projecting building settlements, it was assumed
all loads were instantaneously in place on June 30, 1999. Figure
5 shows the building as it looked in June 2003, 5 years after
preloading and 4 years after completion.
Settlements of the building were monitored by surveying the slab
at the entrance to each of the small warehouse bays. Settlement
readings were taken in December 1999, February
3
EL. 6.22m
EL. 5.91m
EL. 5.30m (1.52m BENCH)
PROPERTY LINE
PRELOAD
EL. 6.83m
EDGE OF BUILDING
PROPERTY LINE
1.5m
EXISTING GRADE
EL. 4.69m
EDGE OF
BUILDING
EL. 4.54m UNDERSLAB
LIMIT OF EXCAVATION
EL. 3.78m
EL. 3.63m
EL. 3.63m (NORTH HALF)
EL.3.32m
3m
EL. 2.41m
EL. 2.71m (SOUTH HALF)
EL. 2.10m
EXCAVATION
1.5m
30.5m
30.5m
30.5m
46.0m
30.5m
30.5m
SECTION A - A'
0
7.5
15
22.5
30
37.5m
0
0.75
1.5
2.25
3.0
3.75m
HORIZONTAL
3m
3m
25.8m
EL. 6.83m
1.5
1
PROPERTY LINE
PROPERTY LINE
VERTICAL
6%
1.5
6%
BLDG. SLAB EL. 4.69m
1
EL. 3.66m - APPROX. EXISTING GROUND ELEVATION
EL. 2.10m
SECTION B - B'
0
2
4
6
NOTE - See Figure 1 and 3 for Plan View
8
10m
Fig. 2 Foundation Design Section
15
0
30
15
ALL DISTANCES ARE IN METRES
Note:
11 - Gauge Number
104 - Settlement at the end of preload (cm)
Existing Storage Building
A
SCHOOLHOUSE STREET
B
24
81
23
71
22
71
21
32
20
10
19
8
18
17
17
25
16
20
15
14
13
13
12
111
11
104
10
84
9
46
8
22
7
32
6
32
5
25
4
13
3
17
2
14
1
17
B'
PRELOAD OUTLINE (AT CREST)
Building
Building
A'
BUILDING PERIMETER
Building
Building
Building
Fig. 3. Preload Settlement Gauge Location.
Paper No. 1.92
4
Settlement (m)
0.0
Legend
Point 9
Point 12
Point 20
Point 21
Point 22
Point 24
-0.5
-1.0
-1.5
1
10
100
1000
Time Since Construction (days)
Fig. 4. Typical Preload Gauge Settlements
total settlement of 420 mm at Bay 29 by 2026. Settlements then
reduce to the west to about 300 mm at Bay 32. It is likely that
the more dramatic settlement gradients are the result of a lighter
preload to the east and the transition to the west where the old
preload had been located. In hindsight, the recent preload and
LCC thickness should have been increased to the east of the old
preload area.
Fig. 5. Photo, June 2003.
2001 and May 2003, and compared to the finished floor grade
effectively completed with full building load in mid 1999. The
locations of monitor points are shown in Figure 6 along with
total settlements to May 21, 2003 and projected total settlements
to the end of 2026, the effective date for estimated long-term
settlements. A few representative settlement plots are shown in
Figure 7. Currently, settlements range between 30 and 320 mm.
Using a straight line projection of the semi-log plots, the
estimated long term settlements are expected to range between
35 and 420 mm, with maximum differential movements of about
130 mm in 10 m (1 in 75). The current maximum differential
movement is 90 mm in 10 m (1 in 110).
These maximum differential movements occur between Bays 27
and 28 (see Figure 6) along a section of the north side of the
building, where the settlements increase steadily to an estimated
Paper No. 1.92
With the exception of Bay 29, maximum ultimate settlements are
now predicted to be 380 mm which is equivalent to the maximum
settlements estimated during the design phase of the project. A
detailed inspection of the building revealed no signs of distress.
However, the slight dip adjacent to Bay 29 could be observed
but was not causing any problems for the Owners, or building
cracking. The overall functioning of the building is excellent
considering the current and expected level of settlements.
CONCLUSIONS
This project demonstrates the benefits of an integrated design
approach involving the owners desire to control construction
costs and time, while recognizing the potential risks associated
with innovative foundation designs. The engineer had to predict
the performance of the building after a variable history of loading
conditions over very soft compressible soils. Combining a
unique building material (LCC) with variable thicknesses of
short term preloading and the LCC, it was possible to design a
foundation system that satisfied the Owner’s needs while
performing at an acceptable level. The Owner accepted the need
to balance acceptable building settlements against foundation
construction costs.
5
15
30
15
0
ALL DISTANCES ARE IN METRES
Note: 34/24 Building settlements projected through 2026 - 34cm
Actual building settlements to May 21, 2003
- 24cm
T
R
S
5/4
6/4
5/4
9/7
19/14
32/23
Q
3.5/3
P
O
42/32
34/24
30/20
30/19
30/20
32/21
33/23
35/26
35/25
35/24
36/25
Existing Storage Building
C
24
23
22
21
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
11/8
11/8
14/11
9/7.5
25
9/7
26
9/6.5
27
9/7
28
12/9
29
23/17
30
26/20
31
32/25
32
34/26
33
32/24
34
30/21
35
33/24
36
32/23
37
35/25
38
35/25
39
33/24
40
37/27
SCHOOLHOUSE STREET
B
5/4
D
A
38/28
U
BUILDING PERIMETER
Building
E
H
I
L
M
F
G
J
K
N
Building
Building
Building
Building
Fig. 6. Building Settlement Monitors and Results.
Originally estimated long term settlements of 135 to 380 mm
compare favorably with the settlements extrapolated to 2026 (27
years after completion), from 4 years of monitoring, of 35 to
420 mm. Differential movement projected along the western 60
m section of the building is now predicted to be 70 mm
compared to original worst case estimates of up to 230 mm. Only
a short section to the east of an old preload area has undergone
significant differential movements. Based on current
observations and prediction for the future, it is likely that long
term differential movements can be readily tolerated by the
building with no adverse impacts to individual warehouse users.
The thick mat of LCC helps spread the stresses of the building
over the soft organic soils and prevent abrupt differential
movements.
Of interest to structural engineers will be the significant
differential movements that the building has been able to tolerate
without any signs of structural or architectural distress.
Settlement (m)
0.0
Legend
Point 1
Point 6
Point 11
Point 19
Point 28
Point 32
Point 39
-1.0
-2.0
-3.0
-4.0
1
10
100
1000
10,000
Time Since Construction (days)
Fig. 7. Typical Building Settlements.
Paper No. 1.92
6