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BRIEF
GEOLOGICAL AND GEOTECHNICAL DESCRIPTION OF SWEDEN
Topography
Sweden is a long, narrow country, 1,500 km long and 300 -
400 km wide, covering a total of 450,000 km2. The
topography of Sweden is varied. Almost all the land area has
an elevation between 200 and 1,000 m above the sea level.
The highest peaks reach about 2,100 m.
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| Figure
1. Map of Sweden |
Soil formation
During the last glaciation, the whole of Sweden was covered
by the Continental' ice sheet. The ice started to retreat
from southern Sweden about 14,000 years ago and about 8,500
years ago the inland ice had practically disappeared.
Figure 2. Map showing the highest marine coastline
In connection with the retreat of the ice, Sweden was covered
by alternating fresh and brackish water. The highest water
level - the highest shoreline - occurred at different times
in different parts of the country. As the pressure of the
inland ice decreased during and after the melting of the ice,
the land was uplifted from the water due to the elastic rebound
of the depressed crust of the Earth - isostatic uplift - a
process that continues today. The absolute maximum uplift
is calculated at about 800 metres. The maximum uplift between
the highest shoreline and the recent sea level is about 285
m above sea level in the north of Sweden. In the south, the
highest shoreline is uplifted about 50 m above sea level.
Together
with the shorelines of the former ice lakes, the highest shoreline
mentioned above is very important from the geotechnical point
of view. Fine-grained sediments, especially clay, have a very
limited extent above these levels and below them there are
only wave washed materials. Thus the soil profiles are principally
different above and below these levels.
Soil and rock composition
The bedrock in Sweden is dominated by crystalline basement,
pre-Cambrian hard rock, such as gneisses and granites. There
are a few areas with Cambro-Silurian sedimentary rock, such
as sandstones, limestone and clay shales. In the mountain
area, the Cambro-Silurian rock covers a very large area.
The
soils are geologically young and belong with a few exceptions
to the Quaternary period. The soils were formed in connection
with the movements and melting of the last Continental land
ice (glacial soils) and subsequent processes (post-glacial
soils).
Figure 3. Black areas indicate distribution of clay,
silt and boulder clay
The glacial soils can be divided into tills and glacial sediments.
Till material - as uppermost layer - covers about 75 % of
the land area of Sweden and normally underlies other soils.
The composition of the tills is highly varied, ranging from
fine grained boulder clay to coarse grained gravel till. The
glacial sediments consist of coarse grained sediments - sand,
gravel and cobbles - in eskers and deltas, and of fine grained
sediments - clay and silt - deposited outside the edge of
the ice.
Figure 4. Black areas indicate distribution of sand
and gravel (glacio-fluvial sediments)
The
postglacial soils can be divided into re-worked and redeposited
soils and organic soils. Most of the postglacial processes
are still in progress, although with rather small magnitude.
The glacial soils on slopes exposed to waves were washed during
the land uplift. These wave washed sediments were transported
down the slopes, coarse grained material a short distance
and clay particles long distances, and deposited upon glacial
soils. Post-glacial silts and clays are normally found as
a relatively thin top cover on glacial clay. Lower lying glacial
material was eroded by large rivers during the uplift process.
Significant volumes of fluvially eroded material, mainly silt
and sand, were thus transported downstream to form new (post-glacial)
sediments covering glacial soils. Of special geotechnical
interest is that wave washed and eroded gravels and sands
in many places cover clay and silt in the lower parts of slopes
and valley floors. The distribution of coarse and fine-grained
sediments is shown in Figures la and Ib (it should be observed
that the bedrock in the mountain range in the north-western
part of Sweden is also shown on the maps).
After
the glacial period, there was a strong increase in the production
of organic material. Organic material was mixed into the fine
grained material to form more or less organic fine grained
soils, e.g. organic clay (gyttja clay) and gyttja. Peat bogs
and fens were formed in many places in Sweden.
The
till is in general a few meters thick, but may in some places
reach depths of several tens of meters. The thickness of sand
and gravel in eskers and large deltas is often a few tens
of meters and not infrequently up to 50 m or more. The thickness
of clay deposits is in general 5-10 m, but thick layers of
clay, up to 100 m-are encountered on the west coast (Gothenburg
area) and north of Stockholm (city of Uppsala). Sedimentary
layers consisting of sand and silt and some clay with thicknesses
up to 50 m or more occur in river valleys in Varmland, Dalama
and in river valleys along the eastern part of northern Sweden.
A
typical soil profile in the middle of southern Sweden is shown
in Figure 5. The profile illustrates the soil layering principles
below the highest shoreline.
Figure 5. Schematic profile from the middle of southern
Sweden below the highest shoreline
Thickness of soil cover
Rock outcrops are common in Sweden and the thickness of the
soil cover is therefore in general relatively modest, normally
a few meters to some tens of meters. The thickness may change
rapidly within a short distance. The greatest known soil depth
is about 200 m.
The
till is in general a few meters thick, but may in some places
reach depths of several tens of meters. The thickness of sand
and gravel in eskers and large deltas is often a few tens
of meters and not infrequently up to 50 m or more. The thickness
of clay deposits is in general 5-10 m, but thick layers of
clay, up to 100 m, are encountered on the west coast (Gothenburg
area) and north of Stockholm (city. of Uppsala). Sedimentary
layers consisting of sand and silt and some clay with thicknesses
up to 50 m or more occur in river valleys in Varmland, Dalama
and in river valleys along the eastern part of northern Sweden.
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