Soil-Structure Interaction Seminar - Foundation Performance ...

Inspections of the foundation had been made in 1988 and 1989 to assess cracking in
the floor t i1 es. An inspection made in 1991, however, showed the presence of a
significant amount of damage. The contour height measurements that were made during
the 1991 inspection are shown in Figure 3. Obviously, the foundation had heaved
upward along the rear wall. Testing showed that the contractor had failed to use a
select fill, as specified. Of more significance, it was learned that the owner had
a drain sump installed in the back yard. The drain pipe was routed to the drain
line that the builder had previously installed at the edge of the foundation;
however, the new pipe was never connected to the ori gina 1 pipe. As a result, any
rainwater that was collected in the yard sump was routed directly to the edge of the
fouhdation and hence down the sides of the piers.
There were two errors which contributed to the problems with this foundation. One
was the placement of the non-select fill; however, since the heaving was limited to
the area where the non-select fill existed, once can but conclude that the-error in
the placement of the fill was not the major contributor. To the best of our knowledge
this foundation has yet to be repaired.
CASE NO. 4 - CONSTRUCTION DEVIATIONS
This was a two story residential building which was constructed in the early 1990's.
The buHding plans showed the foundation to be a reinforced concrete, grade-beamstiffened,
slab type supported on drilled piers. The soils were lcnown to be very
expansive (3).
The building began to develop some craclcing early in its life and, based on a recommendation
from a foundation repair contractor, pressed piles were place under one of
the front corners. The craclcing continued. The results of the foundation height
measurements we made on this project are shown in Figure 4. The owner then hired an
engineer and excavations were made at several locations around the perimeter of the
building. Among other things, it was discovered that there were significant gaps
between the top of the piers and the bottom of the grade beams - some gaps being as
wide as one foot. The foundation grade beams were also found to be of a height that
was somewhat less than specified. No meaningful data were acquired that would lead
to any conclusions with regard to the placement of the reinforcing steel.
The builder then agreed to have the foundation repaired. Bids were acquired to have
the foundation underpinned using pressed piles with the interior piles being placed
using tunneling techniques. A question was raised concerning the potential vertical
rise of the soil near the center of the foundation. The soH testing showed the
moisture content of the soil to be 6 to 10 percent above the plastic limit. It was
concluded that the soil under the slab was near an equilibrium moisture content; however,
there was also a concern that the potential vertical rise could be as much as
4 inches. The foundation repair contractor was aslced to revise his bid to include
raising the entire foundation 4 inches. This bid was equal to approximately 50% of
the cost of the original construction. It was then decided that stabilization would
be achieved using a vertical moisture barrier. The design of this barrier is shown
in Figure 4. The builder agreed to also underpin the foundation around the perimeter
and under the interior of the foundation where point loads existed; however,
no serious attempt was made to raise the foundation any significant amount. The
vertical moisture barrier was installed with the total cost being approximately 50%
of the bid cost to raise the building 4 inches. The long-term results are yet to be
assessed.
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