Geosynthetics April/May 2020 - 22

Construction water flow dynamics of the leak detection layer

FIGURE 5 Watering cover material during
dipole method electrical leak location survey

Any spike in the
flow corresponding
with rain events
should be treated
as a probable leak
and ELL methods
should be applied.

and continuing the few days preceding
primary geomembrane placement, the LDS
material was encapsulated wet and dirty.
Of course, in dry climates the material
will be irrelevant. If the moisture content
never gets above the Sr, the material will
not drain. Note that if ELL testing is performed on the secondary geomembrane,
water is added to the LDS material during
testing, as shown on Figure 5.
Potential solutions to this problem
include washing the LDS material to
remove fines, applying some kind of surfactant to break the surface tension before
the LDS layer is covered, using larger
stone for the LDS material, keeping rainwater out of the LDS layer using temporary tarps, waiting a potentially extended
period of time for cell permitting and
regulators allowing a higher leakage rate
if it can be demonstrated that the primary geomembrane is not the source of
leakage. Some of these solutions are less
practical (and more expensive) than others. The least disruptive to construction
would be the allowance of leakage above
a site's ALR if it can be attributed to construction water.
It could certainly be dangerous for a
regulatory board to allow sites to exceed
a mandated leakage rate by claiming the
issue is caused by construction water,
since every site could point the blame on

this phenomenon. However, true leaks in
the primary geomembrane will cause a
spike in the leakage after a rain event. A
site could have both construction water
flow and leaks in the primary geomembrane. Any spike in the flow corresponding with rain events should be treated as a
probable leak and ELL methods should be
applied. However, if the flow is unresponsive to rainfall and exhibits a flow rate
that corresponds with this phenomenon,
it is likely construction water and should
not be considered leakage through the
primary geomembrane if evidence can
be generated to support this. Drainage
caused by the slow release of construction water from the LDS material can be
accurately quantified with the following
approach during cell construction.
Immediately before the LDS material is
covered by the overlying geosynthetics, a
sample should be taken of in situ material.
The sample should be taken all the way
down to the surface of the secondary geomembrane for a representative cross section. This location can be backfilled with
material from the stockpile. A partially
filled five-gallon bucket of material will
suffice. The following material properties
must be evaluated: moisture content, dry
bulk density, porosity, hydraulic conductivity and specific yield. Flow monitoring
of the LDS layer should start on the day
the sample is taken. Daily measurements
of flow from the LDS, rainfall and ambient temperature should be recorded. The
general trend indicating that flow is from
construction water drainage will be less
flow each day, no response to rainfall, and
likely an increase in flow with increasing
temperatures. Also, the volumetric moisture content of the sample must be larger
than the value of Sr. Calculate the difference between the volumetric moisture
content and the Sr and multiply that by
the volume of the LDS material. This will
be the total quantity of flow that should
come out of the LDS, starting on the day

Geosynthetics | April May 2020

0420GS_p16-23.indd 22

3/23/20 4:32 PM

Geosynthetics April/May 2020

Table of Contents for the Digital Edition of Geosynthetics April/May 2020