Beneficial Uses of Dredged Sediments from Narragansett Bay
Armand J. Silva, Professor
Departments of Ocean and Civil Engineering
Armand J. Silva earned MS (1956) and PhD (1965) degrees in civil/geotechnical
engineering from the University of Connecticut. He was a professor and head
of civil engineering at Worcester Polytechnic Institute. In 1976, he came to
URI where he was chair of the Department of Ocean Engineering for eight years.
He is director of the Marine Geomechanics Laboratory.
Christopher D.P. Baxter, Assistant Professor
Departments of Ocean and Civil Engineering
Christopher D.P. Baxter earned a BS (1990) from Tufts University, an MS
(1994) from Purdue University, and a PhD (1999) from Virginia Tech in civil
engineering. He is the manager of the Marine Geomechanics Laboratory. His research
interests include the stability of submarine slopes, in situ testing of soils
and sediments, and ground modification techniques.
Victor V. Calabretta, Executive Vice President
Maguire Group, Inc.
Victor V. Calabretta earned his BS (1968) and MS (1970) degrees in civil/structural
engineering from the Worcester Polytechnic Institute. After serving as an officer
in the Civil Engineer Corps, U.S. Navy, in Vietnam, he joined Maguire Group,
Inc., as an entry-level engineer. He is one of four principal owners of the
firm.
Since the last major maintenance dredging project
in Narragansett Bay almost 30 years ago, many of the navigation channels, ports,
and marinas have gradually shoaled to the point that large vessels cannot navigate
up the Bay to the Port of Providence. Large tankers have to transfer oil products
to smaller craft or barges that are then towed to the petroleum facilities in
Providence. In addition to being costly, this procedure increases the possibility
of discharges or spills into the Bay. The need for dredging the main shipping
channel is critical. The channel must be deepened, widened, properly configured,
and restored to a safe condition. In addition, several port and marina facilities
that are vital to the socioeconomic health of the region need rehabilitation.
During the last few years, efforts have been
made to better understand the sediment situation in the Bay to determine how
to rectify shoaling problems. The U.S. Army Corps of Engineers (USACE) and the
State of Rhode Island plan a major dredging project to rehabilitate approximately
nine miles of the upper Providence River shipping channel (see Fig. 1). There
are also proposals for major changes to the port at Quonset Point/Davisville
(QPD) in North Kingstown that will require dredging large volumes of sediment
from the two basins adjacent to the piers and from the channels that lead to
the main navigation channel. Lesser, but still significant, quantities of sediment
will be dredged from other commercial ports and marinas around the bay.
Plans for modifying the QPD port have not been
finalized, therefore it is not possible to determine exactly the quantity of
sediment that will be dredged. The initial proposals to construct a megaport
for very large ships would require a dredged depth of 52 feet (16m) below mean
low water (MLW) and would necessitate dredging and disposing of more than 12
million cubic yards (MCY) of sediment. A more modest plan would require a dredged
depth of at least 40 feet (13m) and would create about 8 MCY of dredged sediment.
To put this in perspective, 8 MCY piled vertically (i.e., no side slopes) on
four football fields (a "standard" measure of 300 feet by 160 feet)
would be 1,125 feet high, or about the height of the Empire State Building (another
standard measure). This is a lot of sediment. The goal of the Marine Geomechanics
Laboratory (MGL) of the College of Engineering is to identify uses for this
material.
A project funded by the URI Transportation Center
and the Maguire Group, Inc., is investigating the feasibility of using dredged
sediments for beneficial purposes and converting some of the waste product into
useful commodities. Throughout the Bay, the upper surficial sediments in channels
and basins are usually muddy, fine-grained organic silt with varying amounts
of shelly material. However, data from a few soil borings near the Davisville
piers suggest that there are also substantial quantities of sandy sediments
within the projected dredging depths. These are clean and coarser materials
that could be used for construction fill for highways, replenishment for eroded
beaches, and perhaps as aggregate for concrete products. The organic silts are
more problematic because of their high water content, low strength, and in some
cases, the presence of contaminants. This material could be used for capping
brownfield remediation projects (where large industrial or commercial areas
have been abandoned) and landfills. Development of economically viable beneficial
use alternatives have obvious economic appeal and, perhaps even more important,
would reduce the need for aquatic dredge material disposal and the possible
attendant environmental disadvantages that in-water disposal might have.
Two field investigations have been completed.
Core samples of the sediments in the Davisville turning basin and the approach
channels were collected (see Fig. 2). The samples were obtained with the URI/MGL
Large-diameter Gravity Coring system on the ocean engineering coastal research
vessel, CT-1. The corer consists of a tube (either PVC or steel with a plastic
liner) with a driving weight and stabilizing fins at the top (see Fig. 3). This
assembly is lowered on a cable until it is about three meters above the sediment/water
interface, at which point it is released and free-falls into the seabed. After
the sample is secured, a check valve at the top and a special core-catcher in
the bottom hold the sediment in the tube. The samples are brought to the MGL
and the physical properties of the sediments are determined.
When the coarser sand was encountered, the gravity
coring equipment could not penetrate to the target depth of 42 feet below MLW.
The first two efforts were not able to sample the majority of the sandy materials
(see Fig. 4). To reach the deeper sediments, we will use a commercial vibracoring
system that is designed especially for sand and gravel. Once we have analyzed
these samples, we will be able to determine which uses are most appropriate
for the various sediment types.
The gravity core samples (see Fig. 2 and Fig.
4) revealed silty sand beneath a surficial organic silt layer at several locations.
This material has too much silt for construction fill and is called "common
borrow" by the State of Rhode Island. However, the fine silt/sand can be
mixed with coarser materials to produce an acceptable gradation. Many of the
old masonry buildings, concrete foundations, and large paved areas at QPD are
being demolished and removed. The rubble is crushed to make it useful for construction
and is similar in size to sand and coarse gravel. There is also some larger
rock-sized material. This crushed rubble can be blended with various amounts
of silty sands obtained from the channel. By varying the mix, we can combine
two waste materials (i.e., construction debris and dredged sediments) and create
products that meet the specifications for a variety of uses.
Preliminary tests investigated the possibility
of mixing Portland cement (the same cement used to make concrete) with the unprocessed
sand/silt beneath the organic silts (i.e., the sediments were not washed to
remove salts or otherwise altered to improve the properties). The hardened samples
showed a significant strength of 1,200 lb/in2, indicating that this treatment
could render them suitable for wharf construction or similar applications that
use lower-strength concrete. We anticipate that coarser sediments will be encountered
as we dredge deeper, especially in the turning basins. Coarser materials can
probably be used directly for most of the beneficial applications. Should this
be the case, and if there is a market, these materials can be overdredged, essentially
quarried, for commercial use.
The volume of the upper organic silt layer is
estimated at 2.5 MCY. Various analyses such as chemical, compaction, and permeability
tests will be used to assess which uses will be appropriate, how the sediments
behave during construction, and if the sediments prevent migration of contaminants.
The most promising application appears to be as capping material for the remediation
of brownfields along the Providence River. The strength and stiffness of the
organic silts can be improved by adding cementitious materials like lime and
Portland cement. In preliminary tests, Portland cement was mixed with unprocessed
organic silt and the cured samples showed a strength of about 500 lb/in2. Organic
silts, thus stabilized, can be used for cell ballast-fill in wharf construction
or similar applications. The use of flyash as an additive is particularly attractive
because it is itself a waste product of burning coal and is commonly disposed
of in landfills.
Dredging operations in the Providence River Channel
will probably begin within a year, but plans for dredging the QPD port area
have not yet been scheduled. The removal and disposal or storage of large quantities
of dredge material must be orchestrated. The QPD area encompasses more than
3,000 acres, some of which could presumably be used to store dredged materials
that are not contaminated or otherwise objectionable. If half of the 8 MCY of
dredge material is to be stored for later use, and if the piles are 100 feet
high, approximately 25 acres will be sufficient to sequester this material.
There could be several storage areas with different locations and configurations.
Perhaps a little topographic relief in this otherwise flat landscape would be
welcome?
We are convinced that beneficial uses can be
found for most of the dredged materials from the QPD turning basins and approach
channels. Some questions remain, one of which is the economic viability of the
various techniques being studied. Economic analysis should take into account
long-term environmental concerns of alternative offshore disposal. The project
continues, with additional field work to probe to the proposed dredge depth,
to perform more bench-scale laboratory tests, and to study the economic viability
of the proposed uses.