Protection and erosion control of embankments in a sediment basin

SintexMattress® and SintexDrain® for the port of Gaeta

SintexMattress®: woven polyester shuttering for erosion control of embankments.

Problem

Gaeta port currently has 900 m of berths and a 12,000 m² yard. In addition, the port is building an additional 500 m of quays and a 60,000 m² area for handling goods. The project also includes structures for handling and storing containers, as well as road works covering an area of 20,000 m².

The port’s ongoing expansion includes excavation works in the area facing the south side quay and the head of the Salvo d’Acquisto pier. Three main interventions make up the project. First, a 314 m extension of the existing Cicconardi quay. Second, excavation of the existing sea bed down to -10 m below sea level. Finally, construction of a sediment basin completes the plan.

For this reason, two interventions were required. The first concerns protection of the perimeter and internal embankments of the sediment basin, built from quarry material. These embankments, in particular, need protection from external wave action and from the longitudinal current, which laps a section of the embankments and comes from the drainage channel (Arzano ditch). The second intervention concerns reducing the consolidation time of the sediment basin itself.

Solution

To protect the embankments, engineers chose the geosynthetic product SintexMattress®, a woven shuttering capable of containing cement or bentonite mixes. The material also features filter points that reduce hydraulic uplift caused by the cyclical loads generated by wave motion.

Calculations to determine the mattress’s stability considered both the incident wave and the longitudinal current. As a result, they showed that a 13 cm thick mattress provides adequate protection from erosion. Engineers specifically designed the cement mix used to fill the SintexMattress®. The goal was to obtain a blend that maintains adequate fluidity and workability until the woven shuttering is completely filled.

The sediment basin is built using material from the seabed excavation. To reduce consolidation time, engineers decided to install horizontal drains. Consequently, the drainage geocomposite chosen is SintexDrain® GNG5. In terms of layout, the project includes a first drainage layer at the bottom, followed by a layer roughly every 4 meters. Overall, the basin has a height ranging from 6 to 11 m.

The core of this drainage geocomposite consists of an extruded geonet. The geonet withstands the high compression the product may experience during installation. In addition, this geosynthetic meets the flow rates specified in the project with a gradient of 0.019 (hydraulic gradient i≈0.019). The material withstands loads of 200 kPa at the bottom and 100 kPa on the intermediate layers.

A drainage pipe, positioned in the backfill behind the quay, conveys the drained water. In addition, the use of this drainage geocomposite reduces the load in favor of slope stability. It also allows drainage of water across the entire surface, preventing the formation of neutral pore pressure.