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 CASE G9 - Landfill Stabilization, Naval Academy, A. H. Wu
Problem: The U.S. Naval Academy is engaged in modernization and expansion of
its educational facilities which included construction of the multimillion
dollar Nimitz Library and Rickover Hall. To Support the Nimitz Library, a
landfill of about 3.5 acres was constructed along the Dorsey Creek in 1970.
The Nimitz Library is founded on cast-in-place concrete pipe piles which are
partially embedded in the landfill.
Before the filling, the elevation of the area ranged from -3 to -20 ft.
Soil borings showed a deep layer of soft organic clay averaging about 55 ft
thick underlying the fill site. Analysis indicated that this clay soil, which
has low shear strength and high compressibility characteristics, would not
safely sustain the weight of ordinary fill necessary to raise the grade to
about +5 ft. The measured shear strength of the clay was 150 to 350 psf.
To complete the expansion on schedule, the landfill material used was
lightweight oyster shell to lessen instability of the embankment and to reduce
settlement. Vertical sand drains were then installed to accelerate
consolidation of the underlying clay material. The landfill was completed in
September 1971.
Symptoms: The fill surface showed large and rapid subsidence. The rate of
subsidence was far greater than the settlement anticipated from consolidation
of the soft clay soil. A careful inspection of the Nimitz Library structure
revealed cracks in some of the pile caps and girders.
Collection of Facts: Subsequent investigation of the problem revealed that
excessive fill movements were aggravated by overloading of the upland area
during construction of Nimitz Library and Rickover Hall. The lateral movement
appeared to have caused additional earth pressure to the piles embedded in the
fill area, thus creating a potential for structural damage to the library
building. The large settlement which occurred in the landfill made the area
unusable. Fig. 1 shows the potential of instability with existing site
conditions.
As an emergency solution, about 2 to 3 ft of upland fill was removed in
March 1974 to prevent general slide of the landfill embankment. After removal
of the fill, the vertical settlement declined from about 1.25 inches a month
to 0.25 inches a month, while the maximum lateral displacement slowed from
about 0.30 inch/month to 0.10 inch/month. The rate of movement, however,
remained the same through 1975.
Lowering the fill grade resulted in the area being flooded whenever a high
tide in the creek occurred, and vehicle access was lost. However, without
first improving the stability of the entire landfill, it could not be restored
to its original level.
Solution: In February, 1977, remedial work to stabilize the landfill
started. The stabilization method included application of several techniques:
1. A counterweight was applied at the tow of the fill slope and the slope
was flattened to confine the entire fill area. A stone berm was constructed
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