MARL SURFACE ELEVATION
– 0 TO 10 FT
– 10 TO 20 FT
– 20 TO 30 FT
– 40 TO 50 FT
– 50 TO 60 FT
– 60 TO 70 FT
– 70 + FT
from as shallow as 45 feet to isolated
pockets and holes in which the Marl depth
plunged to 100 feet below the ground surface.
An isopach illustrating the variability
of the Cooper Marl Formation across
the site is presented in Figure 1.
As a result of the challenging subsurface
conditions, the site was surcharged
with eight to 20 feet of crushed granite for
multiple years, amounting to approximately
4 million tons of stone in conjunction
with approximately 33 million vertical feet
of wick drains, which could stretch from
Charleston to Hawaii and still have 1,500
miles of material remaining. Even with the
surcharge, the highly compressible nature
of the underlying dredge and marsh deposits
in conjunction with the required fill
amounts and container loads, the postconstruction
settlement is anticipated to be
up to 12 inches over 25 years, mainly due to
secondary settlement.
In order to support the variety of buildings
and canopy structures, a driven pile
solution was selected – driven piles were
ideal due to their inherent quality control
and ability to provide axial and lateral
support needed. The structures were supported
using 14-inch square pre-stressed
concrete (PSC) piles. The piles were supplied
by Parker Marine Contractors and
inspected by Insight Group personnel in
order to ascertain that quality control measures
were met per project specifications.
A total of 40 test piles and 500 production
piles have been installed on site to
support the buildings and canopy structures.
The piles were designed to overcome
the high seismic loads predicted in the
Charleston region as well as the axial loads
from the buildings and canopies. The axial
pile design includes the substantial drag
forces from the consolidating dredge spoils
and soft marine clays. As a result, the piles
designed and originally installed during the
test pile program are some of the longest
ever in the Charleston area. Several of the
piles were 130 feet long and this exceptional
length caused problems with transportation
and installation.
Prior to the test pile installation, Dr.
Susheel Kolwalkar of Insight Group performed
drivability analyses using the
GRLWEAP software program, on four different
hammer options submitted by the
pile driving contractor (Parker Marine) as
a part of the pile installation plan package.
Because of the easy driving characteristics
of the Cooper Marl, a small hammer
is used to install piles during the initial
drive. This keeps driving stresses below the
PROJECT SPOTLIGHT
Figure 1. Hugh Leatherman Terminal Isopach indicating the variability to the top of the Cooper Marl
Formation across the site and the buildings superimposed
PDA testing Pile pick-up, showing the need for pre-augering Setting a pile in the pre-augered hole
60 | ISSUE 5 2020 www.piledrivers.org
/www.piledrivers.org
