MARCH 2019 PROJECT AQUA
STACKBO, GÄVLE, SWEDEN OVERALL GEOTECHNICAL INVESTIGATION
GEOTECHNICAL INTERPRETATION REPORT (GIR)
MARCH 2019 PROJECT AQUA
STACKBO, GÄVLE, SWEDEN OVERALL GEOTECHNICAL INVESTIGATION
GEOTECHNICAL INTERPRETATION REPORT (GIR)
ADRESS COWI AB
Södra Förstadsgatan 2
211 43 Malmö
Sverige
TEL 010 850 10 00
FAX 010 850 10 10
WWW cowi.se
PROJECT NO. DOKUMENT NO.
A070065 A070065-03-04-GIR-STACKBO-003
VERSION DATE OF ISSUE DESCRIPTION PREPARED CHECKED APPROVED
3.0 19.03.2019 GEOTECHNICAL INTERPRETATION REPORT
SICO JEHA
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CONTENT
1 Introduction 6
2 Site description 6
2.1 Overall 6
2.2 Topography and surface conditions 7
3 Basics for the design 8
3.1 Conducted surveys 8
3.2 Other basis 8
4 Governing documents 9
5 Geotechnical category 9
6 Geotechnical condition 9
6.1 Overall 9
6.2 Soil conditions 10
6.3 Groundwater conditions 12
6.4 Soil parameters 12
7 Radon in the ground 13
8 Overall geotechnical recommendation 13
8.1 Foundation 13
8.2 Excavation 13
8.3 Groundwater management 14
8.4 Filling 14
9 Continued design 14
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1 Introduction MS is considering acquiring a site north of Stockholm in the municipality of
Gävle. The area is called Stackbo and is planned for construction of Data halls.
COWI AB have been asked to conduct the Due Diligence for MS. In order to get
some information about the geotechnical conditions COWI have conducted an
overall geotechnical survey on the site. The survey has been performed in two
different areas on two different occasions.
The main purpose of the investigation was to create an overview of the soil
layers, the groundwater conditions and the technical properties of the soil.
Furthermore, the purpose was to identify possible adverse geotechnical issues
and recommendations for additional site investigation.
The following report presents a description of soil and groundwater conditions as
well as recommendations for foundation, groundwater management, excavation
and filling.
2 Site description
2.1 Overall The site is located in the municipality of Gävle, approximately 15 km south west
of central Gävle and approximately 170 km north of Stockholm. For location, see
Figure 1.
Figure 1. Location of site Stackbo.
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2.2 Topography and surface conditions The site consists of undeveloped, forested land. The investigation area is divided
into two smaller areas; Area 1 and Area 2. For orientation, see Figure 2.
There are no known buildings on the site. Some gravel roads are crossing the
site, for example the road “Kabelvägen” and the road “Farfarsvägen”. The creek
“Valsjöbäcken” crosses the western part of the site from south to north.
Figure 2. Map of Stackbo site. The larger black boundary roughly corresponds to the site
limit. The red boundary lines represent the investigated areas.
The topography within the investigated areas is generally flat. The investigation
areas are shown as red boundary lines in Figure 2. Ground levels at the location
of conducted boreholes in Area 1 range from about +65 to +68 in west to about
+70 to +74 in east. In the southern part, Area 2, the ground levels vary between
+68 and +76.
The forest is dominated by pine and spruce. The investigated area consists of a
larger wetland area in the eastern part of the area and there is a lot of large
block and stones in the surface. Figure 3 shows photos from the area.
There is a power line crossing the site from southeast to northwest.
Kabelvägen
Farfarsvägen
Area 1
Area 2
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Figure 3. Photos from the site.
3 Basics for the design
3.1 Conducted surveys Geotechnical field investigations have been conducted within the site in two
different areas on two different occasions. The results are presented in the
following document:
› Soil investigation report (SIR), Overall geotechnical investigation, Project
Aqua, Stackbo, Gävle, Sweden, project no A070065, date of issue
19.03.2019 and prepared by COWI AB.
3.2 Other basis As the basis for planning, conducting and reporting of geotechnical field
investigation, the following basis has been used:
› Geological maps from Swedish Geological Survey (SGU)
› Digital substrate with existing wires and cables
› Digital basic map obtained by the municipally of Gävle
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4 Governing documents This report connects to SS-EN 1997-1 and associated national annex and TK Geo
13 (The Swedish Transport Administration's technical requirements for
geotechnical constructions).
5 Geotechnical category The surveys are conducted in accordance with the requirements for applying
Geotechnical Category 2 (GK2).
6 Geotechnical condition
6.1 Overall According to the geological map from the Swedish Geological Survey (SGU), the
upper soil layer within the site is dominated by glacial till and sand till. In the
western parts of the site postglacial sediments of sand and locally even small
parts of sediments like clay and silt are overlaying the till. In the central and
eastern part of the site the map also shows wetland areas consisting of organic
deposit of different type of peat, see Figure 4
Figure 4. Geological map from SGU showing different soil layer of the site. The larger black
boundary roughly corresponds to the site limit. The red boundary lines represent the
investigated areas. Blue area is till and sand till, orange/yellow area is sediment of sand
and clay/silt, brown/light brown area is different type of peat and red area is top of rock
outcrops [www.sgu.se].
Area 1
Area 2
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Within the investigated areas, see red lines in Figure 2, geotechnical field
investigations have been carried out in total 22 boreholes. These field
investigations have included weight sounding (Vim), ram sounding (HfA),
percussion sounding with registration (Slb), soil-rock probing (Jb) and disturbed
soil sampling with auger drilling (Skr).
The soundings and auger drillings have stopped at a depth varying between 0.3
and 3.6 m below ground surface, due to the probe could not be driven deeper
down by normal procedure or stop against stone or boulder.
6.2 Soil conditions
6.2.1 Stratigraphy in Area 1
The upper most unit consist generally of humus with a thickness of 0.1-0.3 m.
Organic deposit of somewhat sandy fibrous peat has been found at the top in
the boreholes 18CW303B and 18CW306 with a thickness of 0.3-0.4 m. The peat
has a natural water content of 130 and 228 % and an ignition loss of 35 and 49
%.
Just below the humus in the borehole 18CW308 a thinner layer of gyttja-bearing
clay is found with a thickness of approximately 0.2 m. The clay has a natural
water content of 55 % and an ignition loss of 9.8 %.
The extent of peat and clay has not been clarified but are generally expected to
be found in the wetland areas.
The deposit of humus, peat and clay are underlain either by sediments of silt,
sand and gravel or glacial till.
Sediments of sandy silty gravel is found in the boreholes 18CW302 and
18CW305 to an estimated depth of 1.4 m below ground surface. The sediments
may contain residues of humus and plant remains in its top layer. The conducted
soundings indicate a loose to medium dense state to a depth of 0.5-0.6 m below
ground surface. The gravel layer is underneath medium dense to dense. The
sandy silty gravel is seen as some frost-susceptibility soil.
In the boreholes 18CW303, 18CW303B and 18CW308, sediments of silt and
sand have been found to a depth of 1.0-1.7 m below the ground surface. The
sediments are classified as silty sand, sandy silt and gravely silty sand. The
sediments are according to the conducting soundings loose to medium dense.
The sediments are mainly seen as some to very frost-susceptibility soil.
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Sieving test have been carried out on 2 soil samples from the upper 0.3 m of the
soil layer in the boreholes 18CW302 and 18CW310. The soil samples were
classified as humus-bearing somewhat silty sandy gravel with plant remains
respective humus-bearing sand with plant remains.
Glacial till has been found underlain either the layer of humus and peat or
sediments of silt, sand and gravel. According to the soil sampling, the top of the
till is found, at the depth varying between 0.3 and 1.7 m below ground surface.
The glacial till is classified as gravely sandy silty till, sandy silty gravel till and
sandy silt till. The conducted soundings indicate medium dense to dense till. The
till contains a large amount of stones and boulders. The glacial till is seen as
moderately to very frost-susceptibility soil.
6.2.2 Stratigraphy in Area 2
The top soil layer consists generally of humus with a thickness of 0.1 m.
Under the top soil sediments of sandy silty gravel is mainly found with a
thickness of 0.9-1.6 m. In borehole CW3202 and CW3206 the sandy silty gravel
is also humus-bearing. In some of the boreholes there were also plant remains.
The conducted soundings indicate a loose to medium dense state. The sediments
are mainly seen as some to very frost-susceptibility soil.
Sieving test have been carried out in the sediment in borehole CW3208 at the
depth of 1.0 to 1.6 m. The soil sample was classified as sandy gravel.
In borehole CW3211 humus-bearing sandy silty clay is found directly under the
humus with a thickness of about 0.5 m. The clay layer contains plant remains.
The undrained shear strength of the clay has not been able to be determined on
the basis of conducted investigation methods. The clay is seen as very frost-
susceptibility soil.
The deposit of gravel and clay are underlain by glacial till, mostly gravely silty
sand till. The auger drill has stopped at a depth around 1.1 to 3.0 m in the till.
According to performed ram soundings and soil/rock probing the underlain till is
boulder-bearing. The conducted soundings indicate medium dense to dense till.
The glacial till is seen as moderately to very frost-susceptibility soil.
Sieving test have been carried out in the till in borehole CW3206 at the depth of
2.0 to 3.0 m. The soil samples were classified as gravely silty sand till.
6.2.3 Bedrock
The till is underlain by the bedrock.
According to the SGU rock map the bedrock in the area is dominated by gneiss
forming rocks like granite, granodiorite, monzonite etc. The map further shows
that the estimated depth to the bedrock surface is mainly varying between 5
and 10 m.
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Within the investigated area in Area 1, soil-rock probing has been carried out in
a total of 2 boreholes (18CW304 and 18CW307). The bedrock was found at
about 5.2 m and about 10.1 m below the ground surface.
In Area 2 soil-rock probing was performed in all the boreholes. The bedrock was
found at the depth varying between 1.3 and 3.2 m below the ground surface.
No top of rock outcrops was observed during the geotechnical field
investigations.
6.3 Groundwater conditions During the geotechnical field investigation, a free groundwater table was
observed in 2 open boreholes; 18CW303B and 18CW304, at the level of 0.6-1.0
m below the ground surface.
Groundwater is affected by season and precipitation. Given that the observation
time has been short, a stabilized groundwater level may occasionally be higher
than reported above.
6.4 Soil parameters The soil properties given below, in Table 1, should be considered as cautiously
selected values in the investigated area. The η factor can be set to η = 1.0, thus
the selected values can be seen as characteristic values.
Table 1. Cautiously selected values of soil properties in the investigated area.
Soil layers Depth below ground
surface
γ /γ´
ϕ´
E
Clay (boreholes
18CW308 and
CW3211)
From 0.1 m to 0.6 m 17/7 kN/m3 NA NA
Sediment of
silty sandy
gravel
Down to 0.5-0.6 m 18/11 kN/m3 30-32 ° 8-12 MPa
From 0.5-0.6 m to
1.4 m 18/11 kN/m3 32-34 ° 10-12 MPa
Sediments of
silt and sand Down to 1.0-1.7 m 18/11 kN/m3 32-34 ° 10-12 MPa
Glacial till From 0.3-1.7 m 20/11 kN/m3 35-37 ° 28-30 MPa
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7 Radon in the ground No radon measurements have been performed in this assignment.
8 Overall geotechnical recommendation
8.1 Foundation Within the investigated area there are mainly good conditions for construction.
The extent of peat and clay has not been clarified but are generally expected to
be found in the wetland areas.
Based on the conducted investigations, the foundation of future buildings is
recommended to be founded in the existing glacial till. The top of the till is found
at a depth varying between 0.3 and 1.7 m below ground surface.
The foundation can be carried out as a shallow foundation in a conventional
/normal way without any extra technical reinforcement efforts and greater risk
for large deformation. The foundation can be carried out either with concrete
slab or with spread footing.
All foundation should be carried out frost free and on dry, firm and undisturbed
excavation bottom. Prior to foundation and possible filling, all organic soil
(humus and peat) and clay must be excavated before all kind of foundation, as
for buildings, roads and cables.
A well-functioning drainage around the building is important. All foundation
should be carried out with drainage and capillarity braking layer of for example
gravel or macadam (coarse aggregate).
Drainage pipes shall be laid out around the buildings.
Ground surfaces should be designed with fall from the building.
8.2 Excavation The terrain conditions in the area can lead to both excavation and filling.
Earth work shall be adapted to the existing soil layer, the weather, the
excavation depth and the proximity to the groundwater. The conditions for
excavation can be difficult due do the large amount of stones and block
(boulder). Rock blasting may also be required in some parts.
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The excavation may be carried out with slope. Side slope for temporary
excavations above the groundwater table, can mainly be carried out with the
slope 1:1.5. Under the groundwater table the slope must be flattened.
8.3 Groundwater management Due to a groundwater level near the ground surface, a minor local temporary
groundwater reduction in connection with excavation and foundation will be
required.
In order to avoid soil erosion and flood phenomena of the soil layers at the
excavation bottom and along the sides of the slope, the groundwater level
should be lowered to a level corresponding to at least 0.5 m below the
excavation bottom before excavation. Minor reductions are expected to be
carried out by pumping into deepened ditches in the shafts.
8.4 Filling Excess soil from excavation can be used for filling below buildings and roads or
for balancing the ground surface within the site.
Large blocks and boulders must, however, first been removed before using
excess soil for filling.
Current soils within the site contain silt. Filling with silty soils places great
demands on the execution, since the packing properties of silty soils are very
sensitive for the soils natural water content. Relatively small changes of the silty
soils water content, the soil properties can greatly change.
Filling below building should not be carried out with frost-susceptibility soil.
9 Continued design This survey is of a general nature.
When the design and location of future buildings are determined, supplementary
geotechnical surveys should be performed to better clarify stratigraphy and
groundwater condition, depth to the bedrock and the soils technical properties
for each individual object.
Furthermore, the geotechnical surveys should include supplementary radon
measurements in the ground within the area for future buildings.
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Due to the large area and varying soil- and bedrock condition, the geotechnical
investigation is further recommended to include seismic investigation.
It has not been possible to investigate the thermal conductivity in the soil. A
considerable amount of soil samples are needed for laboratory tests and due to
the presence of dense till, soil samples need to be taken using an excavator. In
addition, these tests should preferably be carried out in the area for planned
pipelines.