silva

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.