Geological Survey of Finland, Special Paper 46 - arkisto.gsf.fi

GROUNDWATER LEVEL FLUCTUATIONS AND SUBSIDENCE OF
BUILDINGS
Groundwater level fl uctuations were reconstructed
for the old part of the city based on the data from
114 drillholes made between 1963-2002 (Karro
et al. 2004). The monitored area (fl oodplain) lies
mostly between 32-40 m a.s.l., while the average
water level in the Emajõgi River (in 1947-1995)
next to the monitored area was 30.7 m a.s.l. In 1963,
the Toomeoru water intake started to operate some
400-800 m south-southeast of the city centre with
fi ve 50-60 m deep wells. The amount of groundwater
withdrawn from the Quaternary aquifer
varied from 200 to 3,750 m 3 /d between 1963-2000
(Fig. 2). Due to intensive consumption, the groundwater
level in the water intake lowered 3.5 m
by 1974-75 and the groundwater level in the old
city area followed that trend (Karro et al. 2004). The
lowering of the groundwater level lead to the decrease
in pore pressure in unconsolidated soils and to land
subsidence (Poland 1985, Chen et al. 2003), which
in the Tartu old city area was the highest in 1973
– average of 38 buildings = 4.87 mm/yr (Lokotar
2005). A decrease in groundwater extraction started
in 1976 and over the next 20 years, the groundwater
level has steadily recovered in the old city area
(Fig. 2). According to data from the last general
levelling of buildings in 1993, the subsidence has
decreased to a rate of 1.5-2 mm/yr (Lokotar 2005).
Fig. 2. Average subsidence of buildings located on the fl oodplain of the Suur-Emajõgi River in relation to water extraction from
the Toomeoru water intake and the level of groundwater in monitored wells.
DECAY OF TIMBER IN THE CULTURAL LAYER
Since the 13 th century, spruce and pine timber have
been placed on peaty soils to support foundations of
buildings constructed on peaty soils. Today the timber
pavements are ca 2-3 m below the surface under
the cultural layer. Due to intensive water extraction
and the groundwater level lowering described
above, ancient timber pile foundations and rostferk
became exposed to aeration. Soon the damage to
timber piles by Cossonus parallelepipedus worms
Geological Survey of Finland, Special Paper 46
The urban geology of Tartu, Estonia
was determined (Oll 1967) and 14 different species
of fungi were detected from the wooden rostferk,
which was situated above the groundwater level
(Oll 1976). While the foundations of a few historical
buildings have been reinforced since 1966, it has
not been possible to estimate the contribution of the
decay of timber into the rate of total subsidence of
the buildings.
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