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 A Foundation Failure in Compressible Soils, M. Jones
CASE G4 -
Problem: Inadequate foundation design and improper pile placement.
Differential settlement
Symptoms:
Collection of Facts: In designing and building a structure on soft
compressible soil, a number of errors were made. Some of these errors were:
Designing and building a 4 inch non-structural floor slab on top of 7
1.
feet of densely compacted fill overlying a deposit of soft silt and
clay varying in thickness from 4 feet to 70 feet.
Failure to consider negative skin friction in determining required
2.
pile capacity.
Piles were permitted to bear in the compressible clay soil rather
3.
than being driven to underlying limestone.
As a result of these errors, large differential settlements were experienced
in the floor and in the columns. The distress was so severe that part of the
structure ultimately had to be demolished and rebuilt.
The seven feet of compacted fill on this site surcharged the compressible
soils and induced differential settlements. Within two years, 8 inches of
differential settlement was observed. Although the Architect-Engineer had
specified "Fill material shall be placed as soon as possible to consolidate
the subsoil---", the plans did not call for an engineered surcharge with
settlement plates and a formal monitoring program. Settlement resulting from
the fill also induced a negative skin friction on the piles which exceeded the
specified pile capacity.
The plans required eight piles to be test driven with capacities calculated by
pile driving formula. Six piles failed to meet the required driving
resistance; thus additional length of pile and pile splicing packages had to
be ordered. The A/E permitted the 65 foot long piles which were on site to be
driven full length while awaiting delivery of the additional piles and
splicing kits. Upon resumption of driving, two to three weeks later, the
piles could not be driven deeper due to a soil freeze effect. Driving
resistances were very high and consequently the A/E accepted the frozen piles
and approved the driving records.
Solution/Alternative: For this site, a structurally supported floor was
necessary. Another alternative may have been an engineered surcharge with
sand drains.
Negative skin friction on piles results from the downward movement of the soil,
relative to the pile. Failure to recognize the presence of negative skin
friction can result in overloaded piles and unacceptable settlement. Methods
for assessing this "downdrag" are given in DM 7.2. For this structure, the
downdrag loads could have been accommodated in the design, however, the A/E
failed to recognize its presence.
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