Placing and Compacting 400,000 Yards of Fill in the Thames River for the Connecticut State Pier

Abstract: The $310 million project involves the redevelopment of the State Pier complex in New London, Connecticut. The intent of the project was to provide a purpose-built facility for marshalling of heavy cargo, specifically offshore wind turbine components. The existing facility consisted of two adjacent piers with a deepwater berth between them, known as “Central Wharf”. The existing bulkheads and marine structures were replaced and the area between the solid-filled State Pier and Central Vermont Railroad Pier was filled to provide additional staging area. Filling Central Wharf required about 400,000 cubic yards of material to be placed through about 40 feet of water (the tidally influenced Thames River). The fill then needed to be compacted to mitigate liquefaction potential and provide bearing capacity for the intended use of the port. One of the challenges with filling Central Wharf was a layer of very soft organic silt present at the mudline. Dredging and off-site disposal of the organics would have been cost-prohibitive, so the intent of the project was to immobilize the organics in-place and fill over the layer. However, placement of heavy granular fill over the soft organic soils could mobilize and “mud wave” the silt out of the project area, impacting the adjacent federal navigation channel in the Thames River.

The project team developed an innovative and low-cost method to immobilize the organic silt layer in place. This was accomplished by using discrete sand piles, referred to as “moguls”, in lieu of more complicated structural solutions, such as soil-mix grout columns. The “moguls” consisted of 20- to 30-cubic-yard piles of on-site granular material, placed on a roughly 40-foot grid across Central Wharf. The moguls were constructed by placing fill with a telebelt or clam shell bucket that would penetrate into the organic layer. The intent was to immobilize the soft organic sediment, ultimately preventing a “mud wave” from forming during mass filling of Central Wharf. Periodic bathymetric surveys were completed to monitor the mogul placement and ensure a mud wave was not generated by mogul construction, and as the mass filling operations were performed. Mass filling was completed by end-dumping and dozing on-site and off-site granular material from land into the Central Wharf area.

Once the mass filling of Central Wharf was completed, deep densification of the granular fill material was needed to meet the performance requirements of the facility. Several methods of deep densification were designed and utilized on the project including variations of Vibroflotation and Vibratory Probe Compaction (VPC). The project team designed and field tested several compaction strategies to determine the most efficient probe spacings to meet the project specifications. A tandem Vibroflot set-up was used to complete a majority of the deep densification. This was the first time a tandem method was used in the U.S.

The design and construction approaches followed were innovative and ultimately successful. This project will be presented in a way that promotes future innovation and creative design approaches that could be used for other port development projects.

How does this project / topic contribute to the advancement of the industry and profession?: The design approach that was used on this project was a low-tech alternative to a highly technical design solution. The current state of our profession draws heavily on established design procedures. This project will showcase that new and creative designs can be used to complete a project, provided the proper quality assurance and quality control procedures in place.

Does the project / topic implement new and innovative techniques, materials, technologies, and delivery methods?: One of the main design challenges of this project was immobilizing the very soft layer of organic silt at the mudline to allow for the filling of the Central Wharf area. The innovative technique utilized on this project was the “moguls”. Utilizing sand was cost effective as the material was readily available on site and could be installed using conventional construction equipment. The mogul approach could be replicated on future projects that are facing similar challenges.

What was the most challenging aspect of your project / topic and how did you handle it to ensure success?: The most challenging aspect of the project was immobilizing the soft organic silt layer prior to filling Central Wharf. Although the “moguls” stabilization was successfully executed and ultimately performed as intended, to our knowledge this solution had never been implemented on a project. The project team completed risk assessments, developed quality control and contingency plans, and carried out a monitoring program during the execution of the work. This solution required careful observation so that the approach could be adjusted if necessary to meet the intended performance. This solution needed to have built-in flexibility to accommodate any necessary changes during implementation.

Who is the target audience for this paper?: This paper is intended for port owners, port operators, port developers, design engineers, and contractors. This paper will highlight the innovative engineering solutions that can be implemented in the unique and challenging field of port facility engineering. All target entities can benefit from exposure to design approaches such as this by demonstrating how creative thinking can overcome significant technical challenges.