May/June 2022 - 110

location. The foundations had to be
altered to fit the new conditions during
construction, at an expense far more
than additional borings. Discovery of
this feature resulted in construction
delays and additional borings to
appropriately modify the design. Figure
3 shows photos taken in one of these
karst channels.
Communication Tools
Project geotechnical risks and considerations
may be well known within the
geotechnics community, but these risks
need to be more explicitly tied to project
outcomes to ensure the team, client and
owner are making risk-informed
choices. This is where sharing risk
register tables as well as geotechnical
data visualizations come into play.
For the example case of the bridge
on karst, Table 1 offers an example use
of a risk register.
In this case, a record of a potential
project issue, qualitative likelihood and
consequences (cost or schedule impact)
are readily communicated to the whole
project team. The table not only lists the
potential impacts and consequences to
the project, but also details some
specific ideas on how to reduce the
uncertainties. The estimated cost of
this mitigation measure can then be
weighed against the potential impacts
to the project. The risk is now described
in concrete terms and the team, client
or owner can make informed decisions
on how to best deal with this risk.
This risk register should follow
through the various stages of a project
from feasibility through design and
construction. As risks are mitigated,
they can be " archived " or removed from
the register. It is a " living document, "
revisited and updated as project
understanding changes. If team
members change, the risk register also
serves as a communication tool to get
the new team member quickly up to
speed on why certain decisions may
have been made and what risks remain.
It is also an excellent resource for
reviewers of the project.
110 * DEEP FOUNDATIONS * MAY/JUNE 2022
Potential Risk
Potential Impacts
Mitigation Measures
Vertical karst feature in
the rock underneath a
pier or abutment is not
identified in the one
hole per foundation
site investigation.
Foundation rock is
known to have
substantial vertical
karst features that
could have been
missed.
Remediation of
karst feature may
be needed to
install designed
foundation
leading to cost
increases and
delays.
Foundation type
or design may
need to be
changed.
Geophysics survey of
area can be added to
the investigation
during the design
process to reduce
uncertainty in areas
surrounding existing
boreholes and provide
targeted locations for
any supplementary
drilling prior to
construction.
Table 1: Example of a risk register table
Consideration should also be given
on how, when and whether to communicate
some of the identified risks in
the risk register to the contractor. USACE
piloted a joint Cost and Schedule Risk
Register between the contractor and
USACE at the Chickamauga Lock
construction project in Chattanooga,
Tennessee. While the project is still
under construction, results from the
pilot have been very positive. Risk
registers cannot overcome every project
issue, but they are quite valuable in
communicating ground risks.
Getting to " X-ray Vision "
Sometimes tables and charts are not
enough to communicate risks. We often
need to show with images what we are
trying to convey in 2D and 3D. For an
example of this, consider the geologic
foundation of Chickamauga Lock.
Extensive exploration was done prior to
lock chamber excavation because of
problematic geology. The relationship
between bentonite layers and the local
geology with the lock monoliths was
critical to good design.
Rock anchors, drilled shafts and a
small secant pile wall were added
during design. The models helped
communicate both the need for these
features and how they relate to the
overall design.
Typical boring log layouts are very
helpful and have been used for many
years. However, consider the impact of
taking these 2D models and putting
them in 3D. Figure 5 shows a typical
core log sheet from the Kentucky Lock
project. Figure 6 shows a 3D view along
with the actual encountered condition
photos. Figure 6 is much more useful for
communication to nonspecialists.
Figure 4: Layout of downstream Chickamauga Lock construction along with installation of rock anchors
Schedule Risk Level*
Schedule Impact*
Cost Risk Level*
Cost Impact*
Likelihood
Moderate
Moderate
Moderate
High
High

May/June 2022

Table of Contents for the Digital Edition of May/June 2022