Soil-Structure Interaction Seminar - Foundation Performance ...

FOUNDATION REPAIR TECHNIQUES PAGE 7 OF 7
PIPE TRENCHES
20) Pipe trenches have traditionally been installed using bank sand as the backfill material. Sand
is dumped into the open trench and water-compacted to achieve the necessary Proctor density.
Sometimes these same pipe trenches become a channel for transmitting excessive water into the
interior of the building, and heaving of the clay soils and cracking of the interior finish materials
may result. Although seldom addressed by structural engineers, care should be taken to specify
that the· ends of pipe trenches be sealed off at the perimeter of the foundations. Suggested
corrections to existing problems of this type include: water barriers, clay fill, French drains,
polyurethane injection or other methods.
CHECKING TENDON STRESSES
21) How do we know if post-tensioned tendons have been stressed? A clue to stressing not
having been done is an unusually high incidence of shrinkage cracks, especially on the sides of
the exterior grade beams. Chipping out at least two live end anchorages on each side and
inspecting the shims, one can see if they are properly seated or not. If it is not clear whether or
not they are, then the tendons should be tensioned to 25 to 50% of the maximum design stress
using a calibrated hydraulic ram jack. If the shims do become not unseated, it may be assumed
that at least a percentage of the design tensile force is present. We do not recommend that the
full design force be applied, since should the gripping shims slip, the cable end may end up
several inches inside the grade beam and very costly structural repairs would then be required.
SOIL STABILIZATION
22) Available soil stabilization methods for existing buildings:
a) Condor SS injection (an ionization treatment for clays)
b) Lime injection
c) Water barriers
d) Polyurethane injection
e) Improved moisture control using deep sump pumps
f) Barriers at pipe trench entrances to foundation system
g) "Leaky pipe" systems
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