TIMBER
consisted of good, sand fill material to a
depth of six-feet. Below the sandy fill were
non-uniform, sandy soils of varying density
with occasional soft clay layers to a depth
of 29-feet. Below 29-feet, a stiff sandy clay
stratum, locally called the Cooper Marl,
was encountered.
Due to the nature of the sandy soils
above the Cooper Marl, there was concern
the hotel would be subject to differential
settlement if constructed on shallow foundations.
Therefore, a driven pile foundation
was selected to accommodate the new structure.
The piles would be driven to bypass
the inconsistent, upper soils and bear in
the Cooper Marl layer. The Cooper Marl
is typically an excellent bearing stratum
for driven piles and provides a great deal
of capacity through long-term setup. This
layer is often more than 100 feet thick in the
Charleston area.
In addition to its soils, Charleston’s
location provides challenges for the designers
of deep foundations. A coastal city,
Charleston is subject to tropical storm
systems and related high winds. The city
took a direct hit from Hurricane Hugo in
1989, and suffered extensive flooding during
Hurricane Matthew in 2016. Stormdriven
winds can result in significant uplift/
overturning loads on buildings, particularly
a tall, broad, but relatively slender building
like the new Home2 Suites hotel.
Charleston is also near an active seismic
fault. A strong earthquake in 1886,
caused considerable damage to the city and
resulted in numerous incidents of soil liquefaction.
During a seismic event, saturated,
unconsolidated sands can act as a liquid,
losing bearing capacity. Another aspect of
liquefaction is that previously unconsolidated
sands become consolidated, displacing
groundwater. As groundwater makes its
way to the surface, it can bring subsurface
sands with it. Soil liquefaction can contribute
to differential settlement at the surface.
A geotechnical analysis determined the soils
at the site above the Cooper Marl would be
Bergerson Construction, Inc.
Marine & Heavy Civil Construction
55 Portway • Astoria, OR 97103
(503) 325-7130 • CCB# 63328
www.bergerson-const.com
prone to liquefaction during a seismic event,
resulting in excessive differential settlement
of the new hotel.
Coastal Engineering & Testing
designed and conducted a test pile program
to finalize the foundation design.
Pile Drivers, Inc., of Hollywood, S.C., was
selected to install the piles. Two types of
piles were considered for the foundation, a
12-inch square prestressed concrete pile and
a timber pile. The piles were driven to different
tip elevations within the Cooper Marl
to determine the most appropriate option.
A total of four test piles were driven:
two timber piles, 55 feet in length; and
two 12-inch prestressed concrete piles, 50
feet and 55 feet in length. Two piles, one
prestressed concrete and one timber, were
driven to a depth of 45 feet below grade and
the other two piles were driven to a depth of
50 feet below grade.
A Junttan PM-16 with a variable energy
HHK 4A hydraulic hammer with a
maximum rated energy of 34,718 ft-lbs
was selected to install the piles. The test
piles were easily driven to the prescribed tip
elevations. Due to the characteristics of the
Cooper Marl, it was decided to conduct a
re-strike test after a six-day waiting period,
utilizing a Pile Driving Analyzer (PDA) to
better assess each pile’s bearing capacity.
The six-day re-strike tests provided
excellent results. All four piles gained significant
capacity from that calculated at
the end of initial drive. The re-strike tests
revealed the timber piles outperformed the
prestressed concrete piles at this site, with
the timber pile driven 50 feet below grade
providing a PDA-measured ultimate capacity
of 300 kips (150 tons).
The comprehensive test pile program
proved that not only had timber piles performed
very well at this site, they had
exceeded expectations. Additional analysis
demonstrated timber piles would also
accommodate the uplift and lateral loads
that could be expected at the site.
After additional research and consideration
of other successful projects utilizing
high-capacity timber piles, the decision was
made to utilize 55-foot long treated timber
piles with 10-inch diameter tips to accommodate
a design load of 70 tons.
Timber piles not only provided adequate
capacity, they also proved to be the
most economical option, providing a substantial
cost savings over prestressed concrete
piles.
Continued on page 93
90 | ISSUE 2 2018
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