An Evaluation of the In-Situ Hydraulic Conductivity of Cement-Bentonite Walls

Abstract

Cement-Bentonite
(CB) walls are low permeability vertical cut-off barriers that are mainly used
to prevent groundwater flow and isolate contaminated areas. The hydraulic
performance of the CB walls depends on the flow rate (discharge) through the
wall. Therefore, the walls must achieve very low hydraulic conductivity. Poor
in-situ hydraulic performance of these walls due to construction and
post-construction defects may lead to surface settlements, groundwater
contamination, and instability of the construction. This thesis investigates
the range of in-situ hydraulic conductivity values of CB walls installed within
the Netherlands. To achieve this, pump data for the various projects (Westerschelde
Tunnel, A2 Motorway at Best, Motorway A4 Delft-Schiedam, Griftpark Utrecht, and
Richard Hageman Akwadukt) were analyzed to determine the discharge through the
walls. The in-situ hydraulic conductivity of the walls was calculated by
Darcy’s law using groundwater level data and dimensions of the walls. The CB
walls must be "keyed" into an underlying low permeable layer
(aquitard), so that seepage of water through the aquitard is prevented. It was
difficult to compute the in-situ hydraulic conductivity of the CB walls for
Motorway A4 Delft-Schiedam and Griftpark Utrecht. The reason was that the walls
in these projects were embedded in a permeable aquitard, which affected the
total discharge value. The calculated in-situ hydraulic conductivity values
were compared with the required hydraulic conductivity values and laboratory
test results. In most of the cases, the in-situ hydraulic conductivity values
of the walls were larger than the values acquired from the laboratory samples.
The reason is that the estimation of the hydraulic conductivity of the CB walls
from laboratory tests was based on small sample areas. This remains unreliable
to evaluate the field performance of the CB walls. The average hydraulic conductivity
of the CB walls at Richard Hageman Akwadukt was required to be less than or
equal to 1·10-9 m/s. The calculated hydraulic conductivity values of the CB
walls for the northern and southern polders were approximately 2.5·10-9 m/s and
2·10-8 m/s, respectively. This study also shows that the hydraulic performance
of the CB walls is mainly affected by insufficient connections between the
panels. Also, the impact of the defects on the discharge through the wall has
been investigated by developing a threedimensional numerical groundwater flow
model using MODFLOW. Simulations were performed for different types of defects,
such as fully and partially penetrating windows, insufficient embedment, and
connection between the panels of the CB walls. The results of the simulations
indicated that fully penetrating windows have more impact on the discharge
through the wall compared to partially penetrating windows. Partially
penetrating windows with a hydraulic conductivity lower than 1·10-6 m/s are
often insignificant. The size of a fully penetrating window also plays an
important role. The difference in discharge (ΔQ) was calculated by subtracting
the discharge through a perfect wall and imperfect wall (Qimperfect -
Qperfect). The (ΔQ) through a CB wall with a window whose area is equal to 1 m2
can increase by a factor of 10 compared to a window whose area is equal to 0.1
m2. The location of the defects has no impact on the discharge through the
wall. The simulations also show that the flow rate is unaffected by the
embedment depth of the wall, but it must be ensured that the wall is in direct
contact with the aquitard (low permeability soil layer). The vertical deviation
of panels also affects the discharge through the wall. The MODFLOW results can
be used to get an indication of the increase in discharge caused by various
types of defects. From this, the possible defects for a particular project can
be determined.