52
Geotechnical News • June 2016
GEO-INTEREST
parties the court was asked to deter-
mine the percentage share of fault, if
any, properly attributable to the defen-
dant - as opposed to that attributable
to the plaintiff and the contractor for
the settlement and failure of the floor,
and to assess the amount of dam-
ages, if any, properly chargeable to
the defendant on the basis of any such
apportionment of fault. The terms on
which the claim against the contractor
had been settled were not disclosed
The Background
The plaintiff, Blank Developments
Ltd., is a company belonging to Mr.
John Doe and a partner whose affairs,
at least in the context of the project
in question, have been managed by
Mr. Doe.
The building sitewas acquired for the
company in 1975.The intention was
to build a warehouse or workshop
building there, and to rent out space
in the building to a tenant or tenants
engaged in light industrial or com-
mercial businesses. Neither Mr. Doe
nor his partner had any significant
previous experience in construction.
They made enquiries about a cement-
and-wood building, to be constructed
on awholly-piled foundation, but
found the cost-in the neighbourhood
of $300,000— too high for the
project to pay its way. During 1978
they heard of building alternatives
which might make the project eco-
nomically feasible. They also found
out something of the dangers inherent
in the use of less costly methods of
foundation design.
Early that year Mr. Doe learned that
there had been settlement in the floor
of a building on the next-door prop-
erty, and also something of its cause.
This building had been constructed
with a piled perimeter foundation sup-
porting the walls and a cement floor
“floated” inside on unpiled ground.
This foundation design had been
adopted against the recommendations
of a soils engineering firm retained
by the owner. Mr. Doe was shown the
soils engineering report in question. Its
most significant passage reads:
We understand that you intend to
pile support the structure and were
intending to “float” the floor. Based
on the depth of peat encountered we
do not recommend that the floor be
supported by any means other than
pile support. Site conditions such as
these warrant total pile support for
the building.
This reference to a “mixed” founda-
tion is significant in the present con-
text. It was this very technique which
the plaintiff was ultimately to adopt
for its own building. It was to do so
with knowledge of the consequences
which had flowed from the use of
that design in the case of the building
next door.
Some knowledge of the way in which
“preloading” works is essential to an
understanding of the problems which
lay ahead for the plaintiff.
In such peaty soil conditions, preload-
ing is generally a less expensive but
more time-consuming method of
foundation preparation than piling.
Done carefully it will eliminate, or
at least minimize, the risk of settle-
ment taking place after a building
has been erected on the prepared
site. The compromise adopted for the
neighouring building, and for which
the plaintiff was to opt in the end,
involves a pile-supported concrete
perimeter foundation for the walls
with a “floated” slab poured on
preloaded soil inside. Engineering
opinion is divided as to the wisdom of
adopting this mixed foundation design.
The evidence suggests that a relatively
small settlement, which might be
tolerable were the whole building on
a “floating” slab, can play havoc if
the walls are stabilized on piles and
the floor alone is floated on unpiled
preloaded soil.
The technique of preloading, while
neither particularly complicated nor
exclusively within the province of
the soils engineer, calls for certain
expert attention both in the planning
stage and in application.
The amount of sand required for
preloading a peaty soil must exceed
by an appropriate margin the greatest
weight which will subsequently be
imposed on the ground which it is
to compress. The preload is usually
a sand pile shaped, very roughly, in
this manner:
The crown of the pile has to extend
beyond the boundaries of the actual
building site, or “envelope”. The
sand must be uniformly shaped, so
that the site will be uniformly com-
pressed. The load must be kept in
place until all settlement has ceased.
In calculating the amount of preload
applied the engineer must exclude
any part of the material which is to
be left on site to restore the origi-
nal ground level after compression,
or to raise it to a new elevation. That
constitutes part of the weight which
the soil must be prepared to carry,
not part of the preload. The preload is
that portion only of the added material
which will be taken off the building
envelope after settlement has ceased.
Thus the design of an appropriate
preload requires calculation of the
weight of the proposed building and
contents, the weight of the material to
be left in place as fill, and the weight
of the material to be removed. The
preload must be properly shaped and
so placed that this crown overlaps
the building envelope. Settlement
must thereafter be completed — sta-
bility must be achieved before the
preload portion of the material can
safely be removed.
These are some, at least, of the
matters to which the mind of an
engineer must be directed in design-
ing and supervising a preload.
When Mr. Doe was looking for an
economic solution to his construc-
tion problem early in 1979 he must
have known that a partially-piled
foundation with a floated slab floor on
preloaded grade would probably be
cheaper than an all-piled foundation.
