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Geotechnical News • December 2012
www.geotechnicalnews.com
or perturbation in the water regime are
compared with those of the existing
(baseline) conditions to assess poten-
tial adverse effects of the proposed
water taking.
For a water-taking quality assess-
ment of the dewatering and drainage
projects, the primary concern is the
discharge receiving facility available
near the project site, such as a natural
sink (a creek, a river or a lake) or a
city sewer for which water quality
objectives or sewer-use bylaw criteria
should be met. The water quality
assessment should be based on histori-
cal and existing land use activities and
discharge point quality standards,
objectives or criteria in relation to the
physical or aesthetic, chemical, micro-
biological and radiological parameters.
For a water quality risk assessment,
the concerns are contaminant loading
of the receiving natural or man-made
sinks and perturbance of any contami-
nant plume existing within the dewa-
tering or drainage zone of influence
and the pertinent environmental and
liability issues.
Hydrogeological field investiga-
tions and laboratory testing
To conduct a hydrogeological site
assessment and prepare a report as
the geoscientific support to the PTTW
application, the following main tasks
are usually undertaken under a QP’s
supervision:
i. A desk-top review of the site back-
ground information including topo-
graphic, physiographic and geo-
logic maps, water well records and
other previously compiled data.
ii. A site reconnaissance to survey
the historical and functional wa-
ter wells within 500 m radius of
the site boundaries and recording
of the existing features and poten-
tial on-site and off-site sources of
ground water contamination.
iii. Subsurface exploration by installing
boreholes, multi-level piezometers
and monitoring wells, concurrently
with the geotechnical field inves-
tigation to assess soil stratigraphy
and ground water conditions, in-
cluding records of ground water
strike level (first encountered dur-
ing drilling) and hydrostatic ground
water level by subsequent monitor-
ing for summer low and annual av-
erage flow.
iv. Borehole permeability testing and
ground water sampling in selected
monitoring wells, and a typical
pumping test, if required.
v. Laboratory testing of representative
soil samples for grain-size distribu-
tion and ground water samples for
determination of the ground water
baseline quality.
Hydrogeological conceptual site
model
In general, a conceptual site model
(CSM) is an assessment tool which
represents qualitative and quantitative
field date to understand how the real
system under study is likely to work
under certain assumptions. The simpli-
fied model is usually both descriptive
and pictorial (plans and profiles). The
hydrogeological CSM usually consists
of the following components:
i. Proposed development features,
construction dewatering and per-
manent drainage needs.
ii. Physiography (including biosys-
tems), topography, geology and
soil stratigraphy.
iii. The site and surrounding hydro-
geologic setting, recharge and dis-
charge areas.
iv. Existing land and water uses (water
well records).
v. Potential on-site and off-site sources
of ground water contamination.
vi. Ground water regime characteristics
such as ground water strike level,
hydrostatic ground water level,
flow direction and hydraulic gradi-
ent.
Dewatering and drainage
conceptual model
The proposed development features
related to shoring, excavation and
ground water control needs are super-
imposed on the hydrogeological CSM
to represent the construction dewater-
ing or permanent drainage model to
quantify the following:
i. The height of the ground water level
to be lowered for purposes of dry
working condition and excavation
base and sides stability;
ii. The temporary (construction) dewa-
tering or permanent (post-construc-
tion) drainage discharge rates and
zone-of-influence for the required
ground water level lowering;
iii. The ground water discharge quality
control measures, such as decanta-
tion of suspended solids, on-site or
off-site treatment of contaminants,
a monitoring program and contin-
gency/mitigative measures for po-
tential adverse effects; and
iv. Considerations of ground water
level lowering dewatering or drain-
age discharge quantity and quality
to provide information for selecting
the shoring type, dewatering scope
and excavation sequence of events
as well as installation of the drain-
age facilities.
The scope and cost of a construction
(temporary) dewatering task depend
very much on the size and depth of
excavation, excavation support or
shoring type, soil stratigraphy, hydrau-
lic conductivity and gradients, ground
water strike and hydrostatic levels,
ground water quality and discharge
receiving facility.
The shoring type, depending on the
excavation and ground water condi-
tions, may vary from box trenching or
hydraulic shoring or cross-trenching
with walers and struts to sheet piling,
soldier piles with tiebacks and lagging,
rakers, or secant concrete caisson
walls with tiebacks. Among these
excavation support types, the secant