January/February 2021 - 91

To better grasp the extent of karst
and potential voids, an electrical
resistivity (ER) survey was performed.
High grout takes under the equipment
pit mud slabs confirmed that very soft
or raveled soil conditions were present,
and those conditions were associated
with karst and ongoing internal soil
erosion. These conditions were found to
be very localized.
During excavations for utilities
close to the equipment pit areas, several
voids were exposed. The exposed voids
were initially filled with flowable fill
and surveyed for possible future
remediation with pressure grout.
Lessons learned - The following
lessons were learned during Phase 4
grouting:
1) Clients may be reluctant to spend
m o n e y to p ro t e c t a g a i n s t a n
unknown risk such as karst. This
case study demonstrates, though,
that construction over karst geology
can involve an unknown magnitude
of risk due to the high variability of
subsurface conditions.
2) Low mobility grouting allows more
control of the area and depth being
remediated. At this site, the deepest
injection pipes took most of the
grout. When highly variable lengths
and pressure grout takes occur
during a project, extra grout
injection points should be added to
properly treat the area.
3) Pressure grout take should be
tracked as total cubic yards and
cubic yards per linear foot. Tracking
grout take per foot treated tends to
normalize the results, leading to a
better understanding of conditions.

and damage similar to Phase 4, a
decision was made to support the Phase
5 equipment pits on ACP. Their
installation at the equipment pits went
smoothly, with few redrills and only
moderate grout takes. A small-diameter
sinkhole appeared later between two of
the new equipment pits. Grouting at
that sinkhole remediated the slab on
grade area.
Lessons learned - The following
lessons were learned during Phase 5
construction:
1) Unexpected conditions may occur
during construction over karst
geology, regardless of mitigation
steps taken. As an example, ACP
were installed without issue at the
equipment pits, but a sinkhole later
formed less than 5 ft (1.5 m) from a
previously installed pile, requiring
remedial repair measures.
2) Moderate- to high-pressure grout
takes can occur during sinkhole
repair, even in an area adjacent to a
previous ACP installation.

Conclusion
Construction over karst geology is
challenging. Due to highly variable
subsurface and rock conditions, expect
the unexpected. For instance, expect to

remediate with pressure grout in areas
that show signs of ground relaxations,
voids or sinkhole formation.
Some clients may want consultants
to prove opinions and illustrate that
quality is built into the project through
testing protocols. A trusting client still
allows geotechnical engineers to push
the envelope and truly become better
consultants. In this project and in
general, the use of ACP as a foundation
system in karst can be messy. However,
high grout takes can be, and were,
effective in treating underlying karst
conditions; use of ACP can also save
money overall.
In the case of the Georgia tire plant,
average grout takes in ACP declined by
roughly 50% (303% to 154%) from Phase
3 to Phase 5. Though higher than
initially anticipated, this reduced grout
take was in part due to site-specific
experience and to generally better soil
conditions in later phases.

Acknowledgments
Berkel & Company Contractors installed
piles in challenging karst conditions.
We would also like to thank KBD Group
for its support and understanding
during design and construction of
Phases 3 to 5 of this project.

Some clients may want consultants to prove opinions
and illustrate that quality is built into the project
through testing protocols. A trusting client still allows
geotechnical engineers to push the envelope and truly
become better consultants.

Phase 5
Phase 5 construction included an
additional tower and equipment pits,
and a warehouse addition. Although the
equipment pits were lightly loaded,
they were critical to operations. To
mitigate the risk of ground relaxations

Bob Goehring, P.E., D.GE, F.ASCE is a chief engineer with ECS Southeast. He has 40 years of
practical geotechnical consulting experience and has published technical papers and
presented case histories at numerous conferences.
Fred Munene, P.E., is a senior project engineer with ECS Southeast. He has over 7 years of
geotechnical experience related to shallow and deep foundation solutions.

DEEP FOUNDATIONS * JAN/FEB 2021 * 91

January/February 2021

Table of Contents for the Digital Edition of January/February 2021