Author: Professor, Dr. Dietrich Stein - TrenchlessOnline

POINT OF VIEW
The Importance of
Subsurface Investigations in
New Trenchless Installations
It is said that when it comes to
underground construction, the
owner owns the ground and the
contractor owns the means and
methods. This ultimately means that
the owner (in conjunction with the
designer) is responsible for providing
sufficient information on the subsurface
conditions so that the contractor
can have a reasonable chance of constructing
the new underground facility
as desired by the owner at a fair
price.
Without a sufficient understanding
of the subsurface conditions, the contractor
is at risk of failing to complete
the construction based on the construction
methods that the contractor
planned and priced its work on,
and the owner risks additional costs
and delays due to change orders
resulting from differing conditions. In
other words, the owner is responsible
for providing biddable contract
documents with sufficient subsurface
information so that the contractor
can anticipate the ground conditions
and select the proper procedures,
equipment and tooling.
In the case of microtunneling, the
contractor will need to have an
understanding of the subsurface conditions
so that the configuration and
tooling of the microtunnel head
most appropriate for the ground conditions
can be selected, production
rates can be anticipated and handling
and disposal of excavated material
can be determined.In the case of horizontal
directional drilling, the contractor
needs to estimate production
rates, in addition to determining
potential drill mud lost rates, the
potential for inadvertent surface
returns (frac-out), and again, handling
and disposal methods for the spoil.As
can be seen, the subsurface information
provided by the owner is key for
the contractor to plan his means and
methods and pricing.
Knowing the underground subsurface
conditions, also allows the contractor
to better anticipate ground
lost or subsidence and/or ground
heave based on the anticipated
behavior of the ground based on the
planned construction method.
Nothing ruins an owner’s and contractor’s
day more than having unanticipated
earth movement that causes
surface settlement or heave, having
other utilities move and possibly
break, or perhaps worst, damaging a
nearby structure or building.
Not only should the owner
provide the contractor with
sufficient subsurface information,
the owner should also consider
using a geotechnical baseline
report (GBR).
In addition to pricing the work,
both the owner and the contractor
are at risk due to unknown subsurface
conditions such as large boulders,
buried former building foundations
and contaminated soils and
groundwater. The key word here is
“unknown.” They are obviously
unknown until discovered. However,
to limit claims due to changed conditions,
it is better for all if the owner
can discover such conditions and
owners reduce the amount of
unknowns during the planning and
engineering phase rather than to
have the contractor discover
unknowns during construction.
Discovering the unknowns during
the planning and engineering phase
allows the owner to mitigate the
issues associated with the unknowns
such as handling and disposal of contaminated
soils, changing the vertical
and/or horizontal alignment of the
proposed pipeline to avoid obstructions
and/or locating jacking and
receiving shafts at locations of the
previously unknown obstruction to
avoid installing one there later as the
infamously known “911” hole.
Without a sufficient understanding
of the subsurface conditions, the con-
By
Dennis J. Doherty, P.E.
tractor often needs
to make a number of assumptions on
the subsurface conditions and evaluate
the risk associated with the
assumed underground subsurface
conditions. These assumptions
include conditions that will affect
production rates, methods of controlling
groundwater and earth movement,
construction methods and
potential risk for failure of the construction
method. The risk of failure
is the greatest unknown and could be
costly and potentially economically
devastating to the contractor.
To account for the potential risk, a
contractor may increase his bid price
to cover contingency in case the
unknown subsurface conditions do
in fact affect his production rates and
thus labor and rental cost. In other
words, he is passing the cost of the
risk on to the owner. If the contractor’s
assumptions are correct, the
money not used for the contingency
becomes a windfall for the contractor
and the owner does not get a fair
price for the installed work.
Therefore, it is in the interest of the
owner to provide as much information
on the subsurface conditions as
possible to minimize the assumptions
and guessing that the contractor is
forced to use when pricing the project.This
generally results in lower bid
prices by the contractors.
Not only should the owner provide
the contractor with sufficient subsurface
information, the owner should
also consider using a geotechnical
baseline report (GBR). The GBR is a
risk sharing tool, allowing both the
owner and contractor to share in the
risk. It helps allocate the risk of
underground construction to those
best able to handle the risk.As noted
above, the owner owns the ground
and the contractor owns the means
and methods. Sharing the risk is in
the best interest of all parties.
Dennis J. Doherty, P.E., is senior project
manager-trenchless technologies
at Jacobs Engineering Group Inc.
66 TRENCHLESS TECHNOLOGY September 2007 www.trenchlessonline.com