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Geotechnical News • December 2012
51
GEOTECHNICAL INSTRUMENTATION NEWS
GEO-INTEREST
Hydrogeological and geotechnical considerations of
construction dewatering and drainage systems
Mori H. Mortazavi
In-construction and post-construction
dewatering like other water takings in
Ontario is regulated under the Ontario
Water Resources Act, and a Permit-
To-Take-Water (PTTW) should be
obtained from the Ministry of the
Environment (MOE) if the discharge
rate is greater than 50,000 L/day
(or 0.6 L/s). The PTTW application
should be supported by a hydrogeo-
logical site assessment prepared for
the shoring, excavation, temporary
dewatering and/or permanent drain-
age needs of a development project.
Presented here are regulatory objec-
tives and principles, ground water
flow principles, site investigation
needs, a typical conceptual model,
potential dewatering interferences
and the pertinent design and construc-
tion considerations of development
projects. Keywords: temporary dewa-
tering, permanent drainage, aquifer,
aquitard, ground water strike, hydro-
static ground water level, hydraulic
conductivity and gradient, dewatering
discharge rate and zone-of-influence,
baseflow, and ground subsidence.
Introduction
Water is the world’s most life-giving
and critical resource. The future world
security lies on the water quality,
quantity, conservation and democracy.
Is water a human “right” or a “com-
modity”? If water is a commodity and
traded, then who will pay the nature’s
water needs!?
Only 2.5% of the “blue planet” earth
water is fresh of which only 30% (less
than one percent of the total earth
water) is available for use and 70% is
locked in permafrost. About 30% of
the fresh water is ground water (Figure
1). About 20% of the world fresh
water exists in Canada and one third
of the Canadians rely on ground water.
Since 2000, thanks to the Ontario
Walkerton water tragedy (Justice
O’Connor’s two-part report of 2002)
regulations, policies and guidelines
have been developed under the Clean
Water Act (CWA) [1] to protect drink-
ing water from the source to the tap
in a multibarrier approach including
source protection plans, wellhead
protection areas and intake protection
zones, vulnerable areas and potential
threats for municipal water systems
and watersheds.
For surface water and ground water
source studies, nine guidance mod-
ules have been developed under the
CWA for watershed characterization
including water budget and vulnerabil-
ity analyses, threat inventories, water
quality and quantity risk assessments
as well as municipal water supply
strategy and information management
[1].
Under the CWA and Ontario Water
Resources Act (OWRA), regulations
(O.Reg. 387/04 and O.Reg. 327/07)
and guidelines have also developed for
water takings more than 50,000 L/day
(0.6 L/s) and the PTTW. These regula-
tions and guidelines describe objec-
tives, principles, permit categories, the
hydrogeological studies required for
Category 3, permit application, appli-
cation review process and the permit
conditions including monitoring and
contingency/mitigation requirements.
The MOE has prepared a PTTW
Manual [2] and Technical Guidance
Figure 1. World hydrologic cycle (R. Clarke & J. King, 2004).