July/August 2021 - 87

Scour Analysis
The central and northern piers are
perched on the south and north
riverbanks, respectively, and required
the design-build team's hydraulic
engineer, Northstar Hydro, to conduct a
scour analysis. The scour analysis
controlled for the foundation elevations
required to adequately protect the
central and northern piers. A geophysical
investigation using seismic refraction
and ground penetrating radar was
performed to supplement boring data
obtained adjacent to the river channel
and assist in characterizing the inchannel
materials. The geophysical
investigation proved to be an efficient
and cost-effective alternative to
performing soil borings within the river
from a barge or from the bridge decks.
Pile Design
Driven steel H-piles were selected as the
primary foundation type to support the
bridge substructures based on cost,
schedule and the general contractor's
ability to drive the piles with its own
equipment and personnel. The new
abutments were located behind the
existing abutments. This required some
piles for the south abutment to be driven
through the blast-rock fill shell of the
south embankment and not the granular
fill core that was constructed in the
1960s (to allow easier driving of the
original south abutment piles). The
design-build team reviewed an overlay
of the new piles on a subsurface profile
to get an indication of the length of pile
that would need to be driven through the
blast-rock fill. The character of the blastrock
fill was explored in the test borings,
and the borings typically were able to
extend through the blast-rock fill
without excessive difficulty. Based on
this information, the design-build team
moved forward with driven steel H-piles.
An interactive design procedure
was utilized and generally consisted of
the geotechnical engineer developing a
subsurface design profile for a given
substructure element. The bridge
engineer developed a pile layout,
substructure load cases and analyzed
maximum pile axial and shear loads,
bending moments and depth to fixity.
The geotechnical engineer analyzed
pile loading and depth to fixity as a
check of the bridge engineer's analysis.
The pile analyses were used to determine
highest allowable tip elevations
to develop pile fixity for lateral stability
and foundation stiffness values to be
used for superstructure design.
The seventh edition of AASHTO
LRFD Bridge Design Specifications
specifies that piles driven through
embankments must extend a minimum
of 10 ft (3 m) into original ground,
unless refusal on bedrock or suitable
bearing strata is encountered at a lesser
penetration (American Association of
State Highway and Transportation
Officials, 2014). The existing embankments
were placed in a controlled
manner over prepared subgrades more
than 50 years prior. Boring data
indicated a dense and fairly consistent
composition of the embankment and
showed a lack of organics or other
deleterious material at the interface
with the naturally deposited subgrade
soils below. However, for the initial pile
design, the bottoms of the embankments
were conservatively considered
to be the top of the original ground, and
pile tip elevations were specified to be
10 ft (3 m) or deeper below the existing
embankment (unless refusal on
bedrock was encountered at a lesser
penetration).
HP14X117 (HP360X174) and
H P 1 4 X 8 9 ( H P 3 6 0 X 1 3 2 ) g r a d e
50 ksi (345 MPa) piles were specified
fo r t he p i e r s and abu tmen t s ,
respectively. Design-factored axial
resistances ranged from 410 to 540 kips
(1823 kN to 2402 kN) for piers, and from
245 to 325 kips (1090 kN to 1446 kN) for
the abutments. The abutment and
wingwal l pi l e layout typi cal ly
consisted of two rows of piles, with the
front row battered to assist in resisting
lateral loading. The pier foundations
were typically supported with between
35 and 45 piles, which were generally
concentrated toward the exterior of the
pile cap and were oriented in both
orthogonal directions to assist with
lateral loading resistance.
Abutment pile layout had to take into
account the existing abutment concrete
and steel piles to avoid interference with
the battered front row of the new
abutment piles. Potential interference
between piles at the new northbound
and southbound abutments was also
analyzed. That is, some piles for the
southbound abutment (that was
constructed second) were driven past
piles from the northbound abutment
due to batter of the wingwall piles.
Mitigation measures included specifying
a driving sequence that allowed
flexibility in relocating piles that
interfered with existing piles while
maintaining minimum pile spacing
requirements, performing investigative
test pits to verify the existing foundation
location and utilizing pile driving
templates.
Soil boring and air track rock probe
data were utilized to evaluate bedrock
surface elevations and determine
whether prescribed minimum pile
lengths could be achieved for the
central and northern piers, given the
foundation elevations required for
scour protection.
The subsurface data indicated
shallow (less than 10 ft [3 m]) bedrock
elevations at the southbound central
pier. Here, a concrete reinforced spread
footing bearing directly on bedrock was
constructed. To do so, a braced sheet
pile cofferdam was used. Bedrock
elevations at the northbound central
pier and at both of the northern piers
were deep enough to allow the use of
driven steel H-piles to meet the
minimum length requirements.
Based on the subsurface conditions,
required axial pile resistances and
minimum pile lengths to develop fixity,
it was apparent that the piles would
generally need to be driven into dense
DEEP FOUNDATIONS * JULY/AUG 2021 * 87
July/August 2021

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