HYDROCARBON
ENGINEERING
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foundation options, such as ground improvement. This
sophisticated analysis offers cost effective foundation
possibilities that are not readily apparent when reviewing
the raw information in the soil data report.
Site specific seismicity study
The final component of a geotechnical survey is a site
specific seismic investigation and report, which outlines
the magnitude of the ground motions from seismic activity
and the influence of the soil.
The seismicity study defines the ground motion hazard
for the project location, determined by analysing many
variables, including but not limited to regional seismicity,
subsurface soil conditions, and proximity to known seismic
faults. The study also assigns a classification from A - E,
where Class A represents a site on hard rock and Class E
represents a site on soft clay soil. As soils transition from
stiff to loose, they are increasingly more prone to seismic
instability and lateral motion.
Additionally, the study provides information to
quantify the liquefaction potential of the site, or the
ability of the soil to shift and consolidate due to seismic
activities. This calculation, derived from the provided
acceleration rates and raw soil data, is then used to finalise
the foundation design in conjunction with the settlement
calculations.
Timing of a geotechnical
investigation
The best timing for a complete geotechnical investigation
is after the conceptual project design has been finalised,
but prior to inviting tank contractors to bid designs. Early
in a project development lifecycle, during the project
conceptualisation and feasibility analysis, a preliminary
geotechnical assessment will be performed as shown in
Figure 4. At the most rudimentary level, these assessments
commonly consist of a visual inspection of the site and a
review of any previously documented information.
While this assessment assists in determining plausible
terminal or facility layouts, it is inadequate as the basis for
storage tank designs or firm pricing during the bid stage.
Furthermore, prematurely performing an investigation will
result in data not specific to the tank’s size, location, and
zone of influence, thereby rendering it marginally useful.
Rather, the complete geotechnical investigation, as
indicated in Figure 4, should be performed so the results
can be included with other customer specifications within
the bid package. Delaying the availability of this
information, to either later in the bidding process or even
after awarding the contract, severely limits the ability to
develop innovative technical solutions or pursue value
based engineering.
For example, soil improvement strategies, such as vibro
compaction, can eliminate the need for a deep foundation,
shortening the foundation construction schedule and
decreasing material and equipment costs. However, to
pursue this option, time during the bidding stage is needed
for the tank contractor to locate and obtain estimates
from specialty geotechnical subcontractors.
Therefore, these potentially cost saving alternatives
can only be achieved when complete geotechnical
information is made available as part of the bid package to
allow sufficient time to create accurate designs.
Utilising one geotechnical
contractor
To maximise the benefits from this early and complete
investigation, a single geotechnical engineer should be
used to conduct the full investigation. When third party
geotechnical engineers must be located and contracted to
perform the specialised skill of analysing the soil data,
precious schedule time is lost while waiting for the
supplementary reports. If a single party is used, fewer
design basis assumptions are made, which results in more
time to pursue value engineering options that benefit both
the owner and tank contractor.
Additionally, this approach improves consistency
across the investigation and reduces the number of
assumptions within each bid’s design basis. Because third
parties will produce reports and base recommendations on
data which they did not originally collect, these
recommendations may not be accurate, being either overly
conservative or overly liberal. This can translate to a
design inadequate for the location.
The impact of assumptions
on design, decisions, cost and
schedule
These incorrect assumptions can arise from incomplete
investigations, late availability of information, and
components generated from multiple parties. For example,
if borings are not spaced adequately to cover the zone of
influence, the soil profile can become overly generalised,
resulting in a risk prone liberal design or a costly
conservative bid. Incomplete seismicity studies can
indicate a higher classification rating, such as D or E, and
limit the tank geometry to a more expensive, yet sturdier,
short and wide tank. Problematic to owners, these
assumptions cloud the waters when deciding between
bids. As layers upon layers of assumptions are made, the
ability to identify what is assumed and what is known is
Figure 3.
Suggested Boring spacing.
