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56
Geotechnical News • December 2012
www.geotechnicalnews.com
i. The W. Sichart and W. Kyrieleis
(1930) empirical relationship [5]
that gives R
o
(in m) as a function of
D
d
(in m) and hydraulic conductiv-
ity (K-value in mm/s);
R
o
= 3D
d
√K
(7)
ii. A pumping test for the required
drawdown as compared with that
of the actual dewatering wells; or
iii.A numerical ground water modeling
by using a finite-element software
where R
o
is determined equivalent
to the distance from the dewatering
wells where the drawdown is insig-
nificant.
It is important to note that con-
struction dewatering can only be
terminated after the foundation and
perimeter subdrains and sump pump
are installed and functional to under-
take the depressurization and collec-
tion of ground water for discharge
to an approved facility in lieu of the
construction dewatering wells. The
dewatering wells should be abandoned
in accordance with the regulatory
requirements.
The sizing and layout of the free
draining aggregates, filter fabric
wrapping, perforated pipes, header
solid pipes and the drainage pump
for the subdrains are usually finalized
after a review of the in-construction
ground water conditions. The per-
manent drainage zone-of-influence
under gravitational flow can also be
determined by one of the methods
described above.
Potential dewatering adverse
effects
As shown on Table1, one of the
PTTW principles is prevention and
resolution of acceptable interferences
or adverse effects. The adverse effects
of the ground water level lowering by
temporary dewatering or permanent
drainage are postulated as follows:
i. Potential ground water level lower-
ing in the existing (functional) wa-
ter wells
ii. Reduction of baseflow in the creeks
adjacent to the subject site (ecolog-
ical effect)
iii.Movement of contaminant plume(s)
iv. Potential ground subsidence in the
adjacent structures
The adverse effects under Items (i),
(ii) and (iii) above can be assessed by
a background information review, site
observations and baseflow evalua-
tion in comparison with the proposed
temporary dewatering or permanent
drainage rates and zones-of-influence.
If a serious migration of contami-
nant plume into the excavation is
anticipated, a barrier system should
be incorporated into the shoring and
retaining walls. In this case a secant
concrete caisson shoring system may
be preferable due to its advantages
described above.
The ground water level drawdown-
induced settlement underneath the
structures existing within the dewater-
ing zone-of-influence can be geo-
technically evaluated by assessing
the drawdown-induced increase in
effective stresses, total and differential
settlements which are to be reviewed
by a structural engineer with respect to
the structures deformation tolerance.
PTTW application and
monitoring
Based on the PTTW objectives and
principles, the PTTW application
comprises 12 parts and 3 schedules
[5]. In addition to the administrative
and processing fee payment informa-
tion, the project technical information
and source(s), volumes and rates of
water taking should be provided. The
water conservation, best manage-
ment practices and QP’s certification
are included in the schedules. More
importantly, the application should be
supported geoscientifically by a com-
prehensive report of hydrogeological
site assessment (HSA).
The PTTW processed, reviewed
and issued by the MOE will contain
site-specific conditions that should be
implemented during dewatering by the
PTTW holder under the QP’s supervi-
sion. One of the conditions is about
the site-specific monitoring require-
ments and contingency/compensatory
measures that are usually included
in the HSA report and are generally
outlined as follows:
i. Measuring (by a flow meter) and
recording daily dewatering rate
and volume for annual reporting to
the MOE’s water taking reporting
system (WTRS) before the end of
March following dewatering com-
pletion.
ii. Periodical water level readings and
quality assessment of the samples
taken from monitoring wells and
discharge pipe.
iii. Periodical surveying of the settle-
ment monuments installed at the
structures existing within the de-
watering zone of influence.
Prior to implementation of a construc-
tion dewatering system or a post-
construction subdrainage system, the
plans and details of these systems
proposed by the project contractor
should be reviewed and accepted by
the project hydrogeologist or geotech-
nical engineer for compliance with
the HSA recommendations and the
PTTW’s terms and conditions.
In conclusion, it is hoped this article
provides a better understanding of
the water-taking regulatory criteria,
scientific information of the ground
water flow principles and technical
requirements of typical hydrogeologi-
cal and dewatering conceptual models.
Once the hydrogeological site assess-
ment report is prepared and the water-
taking permit is obtained during the
design phase, prior to construction and
concurrently with the geotechnical site
investigation, potential construction
complications, delay and liabilities of
the ground water control measures can
be avoided.
References
[1] The Ontario Ministry of the Envi-
ronment Website, Water Section: