To use pile foundations to avoid large settlement of the buildings would have
cost $400,000 just for the piles alone - too high for the $2 million budget.
Furthermore, support of these buildings on piles, while allowing the
surrounding ground to subside, would create operation and maintenance problems.
To use sand drains along with preloading by surcharge to accelerate settlement
would require a long waiting period for a substantial settlement to take
place. Furthermore, this method was not economical. To promote a uniform
settlement of the building areas, a reinforced-earth application of
geotextiles was considered for footing foundation reinforcement. However,
there was insufficient experience with this technique to warrant adoption at
Solution: A foundation system consisting of strictly controlled engineered
fill placed on a firm stone base and inverted "T" shaped footings was then
considered for the support of the buildings. Detailed computer assisted
settlement analyses for this compacted structural fill scheme further
indicated that: (1) the compacted fill would impose the most significant
settlement-inducing load, and subsequent building loads would have much less
influence on the settlements; (2) the post-construction settlements would
probably maintain a uniform rate; and (3) about 75% of the estimated total
settlement would occur over a period of 20 years. It was concluded that a
compacted structural fill, which would act as a relatively rigid mat, could be
placed over the thick clay soil to support the proposed buildings.
To promote a relatively uniform settlement of the compacted structural fill, a
2 ft thick layer of crushed stone, ranging from 2 to 6 inch in size was placed
on the existing ground to increase the overall rigidity of the fill. In the
areas where the concrete apron existed, the concrete was left in place to
substitute for the crushed stone base. The 7 ft high controlled fill was then
placed in layers and compacted to the final grade (elev. +13).
Fig. 1 shows the compacted structural fill scheme and the typical subsoil
Earthwork specifications required that the fill material should not have a
liquid limit exceeding 25 or a plasticity index exceeding 8, and should not
have any organic matter, or any brick or stone larger than 6 inches in size.
This fill material was mostly available within a quarter of a mile from the
project site, as the Naval Station was once used for stockpiling earth
excavated from the Washington Metro Subway construction. According to the
Unified Soil Classification system, this fill material was generally
classified as SC (clayey sand) or SM (silty sand), with some amounts of CL
(low plasticity clay) material. A substantial cost saving was made by using
this material as the structural fill. The specifications called for the
structural fill to be compacted to at least a dry density of 117 pcf. The
fill was placed in 8 inch loose-lift thickness and uniformly compacted by
means of a combination of a sheepfoot roller and a vibrating smooth drum
roller. Each layer of the compacted fill was tested by the sand cone method.
Field density test results showed that about 87% of the test results met the
compaction requirements, which was considered acceptable. Thus the compacted
structural fill was able to support 2,000 psf footing pressures.