Geotechnical News • September 2017
35
GROUNDWATER
Fully grouted piezometers in a soft Champlain clay deposit
Part I: Piezometer installation
Vahid Marefat, François Duhaime, Robert P. Chapuis, and Vincent Le Borgne
Introduction
The fully grouted method has been
used since the 1970s to install piezom-
eters for several geotechnical and
mining applications. The method has
several advantages including ease in
installing and reduced costs, especially
when boreholes are shared with other
geotechnical instruments. Several
authors have noted that grout perme-
ability is the most crucial factor influ-
encing the piezometric error (Vaughan,
1969; McKenna, 1995; Contreras et
al., 2008; Marefat et al., 2014). For
steady-state seepage, Vaughan (1969)
proposed that the error may be negligi-
ble for a grout with permeability up to
two orders of magnitude greater than
the adjacent formation permeability.
Moreover, based on numerical model-
ling, Contreras et al. (2008) found a
negligible error when the grout had a
hydraulic conductivity within 3 orders
of magnitude of the surrounding
formation. Based on a new analyti-
cal solution and numerical modelling
results, Marefat et al. (2014) found
that pore pressure measurements are
reliable when grout permeability is up
to one order of magnitude greater than
the adjacent clay permeability. Field
measurements reported by McKenna
(1995) indicated that the grout must
be less permeable than the formation
to reduce the piezometric error for
most soil conditions. In addition to
the results of McKenna (1995), there
is very little published information on
the field performance of fully grouted
piezometers. It is worth noting that
there is no consensus on the accept-
able permeability contrast between
the soil and the grout, which is only
one of the parameters influencing the
piezometric error. The grout physical
stability also is an important parameter
for successful piezometer installation.
The main objective of the project was
to appraise the performance of fully
grouted piezometers under natural
field conditions. The paper introduces
a new field site that was equipped in
collaboration with GKM Consultants
for this purpose. The paper also pres-
ents preliminary results regarding pore
pressure measurements.
Site description and stratigraphy
The study area is located in Sainte-
Marthe, near Montreal, Canada. The
intact Champlain clay is about 10 m
thick. It is located under a layer of stiff
clay, which is fractured and sometimes
oxidized. The fractures can reach
down to 6 m below ground surface.
The intact clay deposit is soft and sen-
sitive at depths between 6 and 12 m.
Sensitivity can reach 200 at a depth of
around 10 m (Figure 1). Falling-head
laboratory tests provided an average
hydraulic conductivity of 1.08×10
-9
m/s for the intact clay. In the lower
portion, the clay is mixed with sand,
silt, and coarser material including
erratic blocks. The silty layer is under-
lain by the bedrock. Figure 1 presents
a preliminary geotechnical profile for
the study site.
Figure 1: Geotechnical pro le for the study site: w
L
, liquid limit; w
p
, plastic
limit; w
n
, natural water content; C
u
, undrained shear strength.
