MARCH 2019 PROJECT AQUA STACKBO, GÄVLE, SWEDENold.gavle.se/PageFiles/338156/09_Geoteknik_GIR_nord...

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

STACKBO, GÄVLE, SWEDEN

GIR

http://projects.cowiportal.com/psc/A070065/Documents/03 Project documents/04_Geotechnical/Dokument/Stackbo/Kompletterande undersökning/A070065-03-04-GIR-STACKBO-003.docx

5

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

6 STACKBO, GÄVLE, SWEDEN

GIR


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.

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