Anchored Precast Gravity Breakwater Pier in Challenging Geotechnical and Coastal Conditions

Abstract: Following the 1995 volcanic eruptions, the main port facility serving the approximately 4,500-person Eastern Caribbean Island of Montserrat was rendered significantly damaged. After a planning and financing process, the Government of Montserrat (GOM) and associated project stakeholders embarked upon the design-build reconstruction of a new, multipurpose, climate-resistant port facility at Little Bay that provides safe harbor to both cargo and cruise vessels. The primary component of the new port involves dredging and construction of a unique 425-foot-long breakwater pier capable of providing a safe harbor during storm events. This paper describes the design and logistic considerations for construction of the pier which will serve as a useful guide for similar port reconstruction projects in remote locations subjected to environmental exposure.

The drivers for implementing the unique gravity structure design for the pier include logistic challenges triggered by the remoteness of the site, significant open-ocean wave heights of almost 20-feet which limited the construction window, and the presence of loose, non-plastic, liquefiable sand/silt overlying weathered rock. The pier consists of two sets of vertically stacked, hollow, precast reinforced concrete blocks enclosing a fill area and forming the berthing faces of the 60-foot-wide structure topped by an in-situ concrete deck. Rather than designing the pier for liquefaction effects, the design removed the poor materials and replaced it with igneous granular fill procured from the island. Similarly, the inside and space between the stacked blocks are also filled with fill material procured from the island’s quarries which significantly mitigated material import costs. Load transfer is provided via the thick reinforced concrete deck slab which tie the four walls of the precast units while global lateral stability against wave and seismic induced loading is provided via the rock anchors drilled into the competent andesite bedrock. The weight-and-size-controlled design eliminated the need for floating equipment and lent itself to execution via a top-down scheme consisting of a 200-ton crawler crane working from the stacked blocks. Formwork for the precast stacked blocks were shipped to the island to allow site fabrication using local labor and materials. The pier is anticipated to be in operation by 2025.

How does this project / topic contribute to the advancement of the industry and profession?: The project implements a first-of-a-kind gravity-based resilient breakwater pier structure and illustrates how designers can navigate difficult coastal and geotechnical solutions to develop cost-and-schedule effective solutions. Conventional filled cells and bulkheads which are standard in the industry is costly and risky to achieve the project’s goals. The design team’s involvement in the means and methods of construction was fundamental to the design process and advances the industry for delivering small projects in remote locations.

Does the project / topic implement new and innovative techniques, materials, technologies, and delivery methods?: The design team developed an interlocking, modular and open box design to be erected by one crane. The boxes are filled with locally sourced granular materials to provide the required gravity structure. This innovative approach enabled top-down construction without cost penalty and eliminated the need for floating marine equipment.

What was the most challenging aspect of your project / topic and how did you handle it to ensure success?: A combination of the wave exposure and geotechnical soil liquefaction within the poor footprint are the most challenging aspects. Numerical modeling (in design) optimized wave loading and soil replacement (in construction/phasing) addressed liquefaction. To provide stability for seismic and wave forces, the units were connected to each other and anchored to bedrock through rock anchors. Perceived damage to the partially built structures and equipment were eliminated by the gravity structure and top-down construction.

Who is the target audience for this paper?: The target audience is multipurpose terminal operators, port marine/civil/structural/geotechnical designers, and marine contractors.