Offshore Geotechnical Investigation for Beach Stabilization Design

Opportunity

Due to rising sea levels, increased shoreline development, and a desire for more natural alternatives to steel bulkheads and stone revetments, the use of constructed dunes is fast becoming the preferred method of beachfront protection. When a groin and constructed dune were proposed for use in Union Beach, New Jersey, as part of a beach replenishment project, the U.S. Army Corps of Engineers (USACE) wanted to estimate the amount of consolidation settlement that would rest beneath the structures.

These proposed structures were to be located in the intertidal zone at the mouth of a small creek, where highly compressible sediments had accumulated to depths of up to 60 feet.

Challenge

Montrose team members were tasked with reaching each of the test locations, drilling borings to collect samples for geotechnical testing, and conducting in-situ measurements of soil parameters with a dilatometer. Having to work in the intertidal zone and navigating the presence of manmade submarine obstructions would make access to the drilling sites challenging. The team would have to drill to the test locations without damaging their equipment, while also maintaining position in open water under changing tides and weather conditions.

Solution

The Montrose team transported barges mounted with a drill rig to each test location during high tide. Each barge was anchored in place with spuds that allowed it to rise and fall with the water levels. When the water was deep enough, personnel were transferred to and from the barges via boat. The work proceeded as planned, with the results of the investigation showing that the compressible layer was not as thick as previously expected. The investigation demonstrated the feasibility of the proposed groin and constructed dune structures.