Mining has been effectively developed in Finland during the last three decades. Over 25 new mines have been opened, the greatest part of them underground. The a- mount of hoisted ore has grown from 3.1 Mt in 1950 to 18.6 Mt in 1980.
Drift sizes have grown, too. In the fifties, the most common drift size was about 8m2 today it is about 20 m2. The increased Size points out more and more the need for correct planning and excavation of drifts.
The bedrock of Finland belongs mainly to the Baltic Shield (Fig. 1). Its geological age varies between 1500...2300 My with the exception of Caledonides (400 My) in the most northwestern part of Finland.
The general features of the Finnish bed-rock have been formed in the Middle Precambrian during several mountain deformation Phases. Today erosion has uncovered the roots of those old mountain chains.
Over 50% of the Finnish bedrock consists of plutonic rocks, mainly granites. Schistose, metamorphic rocks represent about 40%. Unmetamorphosed, younger sedimentary rocks are found in some places only.
The last glacial period for some 10 000 years ago transported the alteration pro- ducts away and left the rock basement to be found near the surface. The depth of bedrock surface is on the average 9 m, mostly 3...4 m.
The boundary between soil and rock is u-sally sharp, in contrast to the younger formations where there is a continuous alteration zone from soil to rock.
(Figure in full paper)
Structurally the bedrock of Finland can be divided into five different regions (Fig. 2). These correlate partly with different rock types.
Tectonically the main controlling feature is a deep crushed belt, which extends diagonally across Finland from Lake Ladoga to the coast of the Gulf of Bothnia over a breadth of 40 to 150km. This belt is nowadays believed to be a border between two continental plates. No movements have been registered along it.
Uniaxial compressive strengths have been measured in the laboratory for Finnish rocks from 40 MPa for some limestones and serpentinites up to 260 MPa for some granites. Young's moduli have been measured from 40 GPa for limestones and serpentinites up to 160 GPa for some sulphide ores.
Stress measurements have been carried out in Finland since 1961, altogether over 40 different locations. In the beginning' only Hast's method was used but later also Leeman-NTH three-dimensional method and solid soft-inclusion method were taken into use (FAMME, 1981).
A lateral stress field is acting in the Earth's crust also in Finland. The major primary horizontal stresses and primary vertical stresses are presented in relation to depth in Figs. 3 and 4. It can be seen that the horizontal stresses exceed the vertical ones, the ratio σh/σvbeing normally between 2...2.5. The stresses increase with the depth by where is the major horizontal stress (MPa), σhl is the vertical stress (MPa) and σHV is the depth from surface (m).
