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 CASE G10 - Foundation on Compacted Fill, A. H. Wu
Problem: Economic design of foundation for buildings located at the Anacostia
Naval Station.
Background: In 1976, a $2.2 million automotive maintenance facility was
constructed on a compacted structural fill placed over a very thick
compressible clay layer. The facility, designed by the Naval Facilities
Engineering Command, is located east of where the Anacostia River meets the
Potomac River.
A large land area has been reclaimed here since 1884. The subsoil conditions
of this filled land consist of deep deposits of soft silty clay extending
typically to a depth of 60 ft. Because of the weak soil conditions, pile
foundations had been used exclusively to support structures built in the fill
areas in the past.
Use of conventional pile foundations at the new facility would have been very
costly. Furthermore, they would have resulted in the recurring operational
and maintenance problems associated with the differential settlement between
the pile-supported buildings and the adjacent fill supported side-walks,
driveways, and landscaping.
The project consisted of three separate one-story buildings. The maximum wall
load was 5 kips per linear foot, and the maximum bearing wall span is about
140 ft. Since the project site is located in a flood plain area, the master
plan required the building sites to have a finish grade at about 6 ft above
the original surface, which was at about elev, +4.0. Parts of the building
areas were located over an old concrete pavement.
At first, the project faced two major foundation problems: (1) the
probability of large overall settlements along with differential settlements,
and (2) the possibility of excessive foundation cost.
Symptoms:
High cost of foundation.
Collection of Facts: The soil boring indicated that the site subsoils
consisted of 4 ft of recent medium dense silty sand fill, 55 to 60 ft of soft
silty clay with slight organic content, and then a dense silty sand extending
to 120 ft depth. The results of soil testing showed the silty clay material
to have been normally consolidated and to have high compressibility
characteristics.
Consolidation test results were used to predict the rate and magnitude of the
settlement which would occur from the weight of the fill required to raise the
grade. Computer settlement analyses indicated that the ultimate settlement
resulting from the fill load alone was approximately equal to 33% of the
height of the fill placed. In other words, to raise the grade 6 ft, about 9
ft of fill would be necessary (allowing up to 3 ft of settlement). The rate
of consolidation would be so slow that preloading the site with surcharge to
accelerate the consolidation settlement would not have been an effective
solution.
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