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Geotechnical News • March 2016
GEO-INTEREST
Deep excavations adjacent to
existing structures
This type of construction is important
from the standpoint of risk, not only
from a safety standpoint, but also in
terms of possible damage to adjacent
deformation-sensitive structures.
Of fundamental importance in this
respect, in addition to adequate geo-
technical data, is a good knowledge of
such adjacent facilities and the imple-
mentation of appropriate construction
measures in timely fashion.
Deep shaft excavations subject to
bottom heave
This type of problem is not uncom-
mon. It may result from the presence
of artesian pressures at depth or weak
ground at the base of an excavation. It
is important to ensure that exploratory
boreholes are extended deep enough
and that piezometers are installed to
identify these conditions prior to exca-
vation to prevent a “blow-out” or base
failure during construction.
Regional groundwater drawdown
This type of problem is also not
uncommon.
A deep Municipal Sewer was con-
structed beneath a street in a built-up
City area, where the subsoil was
granular in nature with a high ground-
water table. Deep educator wells were
installed to temporarily depress the
groundwater to beneath the invert
level. This drawdown had an adverse
lateral impact on an adjacent housing
development, where settlement and
cracking of homes occurred due to
consequent consolidation of the foun-
dation soil. This, as might be expected,
ended up in litigation proceedings. It
is important to take this situation into
consideration, by providing some pro-
tective form of counteraction, such as
a recharge system during construction.
Settlements of floor slabs on grade
This type of problem and the resultant
distress of cracking, uneven surface
(with mobility problems for in-house
equipment, etc.) is unfortunately fairly
common because of lack of attention
to design and construction details. It
is important therefore to know where
problems could occur. Slab on grade
type of construction should only be
considered if some settlement can be
tolerated. But to accommodate settle-
ment, without distress, the concrete
slab(s) on grade should be placed
structurally separate from any portion
of the building walls and columns,
with construction joints at spacings
determined by established experience.
Slabs on grade should also be placed
on an engineered base course and
designed for the wheel loads which
they have to carry (in Warehouse type
structures for example). If settlement
reaches unacceptable levels, it may be
necessary to replace the slab, although
in some cases such slabs can be raised
and relevelled by low pressure grout-
ing methods (or “mud-jacking”).
This type of problem is of particular
importance to recognize from the
standpoint of its varied pattern of
distress and its common occurrence as
the subject of either a claim or litiga-
tion.
There are other case histories which
could be quoted from the standpoint
of lessons learned. Space restrictions
(and confidentiality matters) do not
permit their coverage herein. To some
extent, however, lessons associ-
ated with them are embodied in later
sections in this paper. As a general
statement, make a point of learning
from the experiences of others, not
only from successful case histories
in the published technical literature,
but also from situations where things
have gone wrong and were resolved
through some form of resolution pro-
cess. And keep in mind that geotech-
nical problems which have become
subjects of litigation are, understand-
ably, not common in the geotechnical
literature.
Dispute resolution – unexpected
consequences
Unfortunately there are instances
when despite all efforts to resolve a
dispute by negotiation, resolution has
to be sought by other means such as
Alternate Dispute Resolution (ADR)
methods, with resort to litigation
being generally the least preferable.
The advantages of ADR methods over
litigation are alluded to in the next
section herein. Several case histories
are presented in this section which
describe situations where unexpected
adverse consequences resulted from
litigation procedures.
Settlement experienced by a hockey
arena
This involves a hockey arena in Rus-
sell Township, Ontario, which experi-
enced unacceptable settlements.
The Arena was of conventional design
and located in an area characterized by
soft, lightly preconsolidated sensitive
clay, (known as Leda clay) overlying
granular till and limestone bedrock.
The clay has a reputation for dramatic
consolidation and resultant settlement
when loaded above the preconsolida-
tion pressure (e.g. Burn and Hamilton,
1968). Based on geotechnical studies
carried out initially in 1974, the foun-
dation support selected was end-bear-
ing piles for the building with interior
concrete floor slabs carried on a thin
lift of engineered granular fill used to
raise grade. Construction was com-
pleted in 1975 and up to about 1979
the grade-supported elements experi-
enced settlements which were accept-
able. However, by 1984, differential
settlements of floors relative to the
pile-supported elements had signifi-
cantly exceeded design expectations.
In the course of a mandated structural
inspection of the Arena by a structural
team which included a geotechnical
engineer, the Owner requested an opin-
ion on the cause of the settlement. The
initial assessment by the geotechnical
“inspector” focussed strongly on only
the clay and surcharge loading from
fill used to raise the grade. This set off
a train of events which progressively
fed on each other and unfortunately led
to initiation of litigation by the Owner
against the original design Geo-Con-
sultant.
