Geotechnical News • September 2012
29
GROUNDWATER
the end of winter, and about 1.5 m
deep in summer. Frost penetration is
in the 1.2-1.5 m range. Temperature
measurements in the MW static water
column, in early May, indicate that the
soil around the MW pipe is still frozen
at depths in the 0.5-1.0 m range.
For training and research activities,
the elevation of the top of the MWs
has been surveyed yearly using a
local relative datum. However, the
elevations were surveyed relative to
a distant absolute reference in 2004
and 2008. In addition, almost yearly, a
team of students have taken elevation
surveys of the ground surface, using
the local datum.
The changes in elevations between
2004 and 2010 are presented in Fig. 2.
After six winters, the MWs have risen
by up to 18 cm. Since the climatic and
soil conditions (fine sand layered with
fine silty clay layers, shallow water
table) are favourable to frost heaving
of the upper frozen layer, the upward
expansion of freezing soil has slowly
but surely jacked the MWs from the
ground. Dur-
ing freezing,
the upper part
of the MW pipe
is squeezed by
frozen soil, and
jacked upwards
by swelling
freezing soil
underneath. The
upward move-
ment is resisted
by friction forces
along the unfro-
zen lower part of
the MW casing.
At the end of
winter, the MW
casing moves
downwards, but
its drop is always
smaller than its
winter lift.
Since the MW total heave has
exceeded 10 cm for many MWs, one
may wonder whether the stress condi-
tion in the MW pipes may have broken
joints or separated pipe sections. Over
the years 2005-2009, the insides of
the MWs were cleaned yearly, but the
mass of recovered sediments (usually
a few grams to less than 100 g) was
not measured. In 2010, the sediments
were recovered when cleaning the
MWs, and the dry masses were mea-
sured. Figure 3 plots the dry recovered
mass versus the MW heave. It appears
that 7 MWs contained between 1500
and 6200 g of solid sediments. These
MWs are considered as broken: solid
particles and also groundwater can
enter into the MW pipe through the
cracks or separations. As a result, the
data obtained using these MWs were
considered as unreliable for ground-
water studies. However, it seems that
there is no correlation between heave
and pipe damage (Fig. 3) for these
MWs, as assessed using the dry mass
of sediments found in the MW pipe.
What are the consequences of having a
broken MW due to frost jacking? First,
the monitored hydraulic head is modi-
fied close to the MW, due to upper
aquifer layers providing water directly
into the broken MW pipe. This modifi-
cation yields an incorrect seepage flow
net, if the data of the damaged MW
are used. As a result, the hydraulic
gradient and the groundwater veloc-
ity are also incorrectly evaluated.
Second, the MW cracks or separation
may bring some contaminated water
from shallow depths directly into the
MW pipe. For example, if an upper
unconfined aquifer has been polluted,
the polluted water can easily reach the
broken MWs and pollute deeper water
around the MW screen, which was
installed to monitor a lower confined
aquifer. Being unaware of a broken
MW may yield the incorrect conclu-
sion that the confined aquifer is pol-
luted, whereas the pollution may exist
only around the damaged MWs and
their screens.
To conclude this short paper, consider-
ing the importance of frost heave and
resulting damage to a MW pipe, it is
important to regularly check moni-
toring well elevations, which may
fluctuate throughout the year. Eleva-
tions are usually higher in winter than
in summer, but usually do not return to
their previous positions, of exactly one
year before: as a result, on average,
the elevations can raise over years.
Acknowledgments
The author thanks the Régie de l’eau
de Sorel-Tracy and Aquatech, for their
authorization to use the Sorel site.
References
Dallaire, V. 2004. Study of scale
effects in an alluvial aquifer (in
French). MSc Thesis, École Poly-
technique, Montreal, 194 p.
Figure 3. Correlation of MW heave to the dry mass of
sediments recovered when cleaning the insides of the
monitoring wells.
