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Mud Cat Technical Papers

Dredged Material Filled Geotubes®
San Antonio River Containment Island
Buenaventura Bay, Colombia

Introduction

One of the first Geotube® applications in Colombia was for construction of confined disposal area islands that will be used for containment and dewatering of fine-grained maintenance dredged materials.

The project is located on the San Antonio Inlet, Buenaventura, Colombia. The dredged material containment area is the first of two oval shaped islands planned in this riverine and tidal environment. This new and innovative construction methodology involved hydraulically filled Geotubes® with a sandy fill material.

Plan view of dredged material containment island

The Geotubes® were positioned end to end to provide a perimeter dike for dredged material containment in tidal variations of 4.0 m twice a day.

After the oval shaped islands are completed they will serve as dredged material containment facilities until they are filled and stabilized. After they are stabilized they will be planted in Mangrove trees and other native vegetation and will be used exclusively for environmental purposes.

Background

The dredged material containment project is located about 8.0 km inland from the Pacific Coast adjacent to a 10 m deep navigation channel that serves the south part of the island of Buenaventura. New port developments have caused a need for new work deepening, maintenance dredging, and additional disposal areas. Existing dredged material disposal areas have either been filled to a capacity or have deteriorated because of inadequate maintenance and design.

An attempt to construct a disposal area using cement and rock filled gabions adjacent to the city of Buenaventura on the San Antonio Inlet has been abandoned because of environmental and foundation conditions. Another disposal area located across the river in an area planned for Port of Solo was constructed using small sand bags but it has been completely filled with maintenance dredged materials and is no longer in service.

Profile view of Geotube® design

Environmental Concerns

Permission to construct dredged containment facilities for fine grained dredged materials in Colombia has become very difficult because of environmental constraints by local and state governments. The navigation channels leading into this area are constantly dredged. Disposal alternatives are limited to non-existing upland disposal areas or transport 80 km offshore in the Pacific Ocean.

Transport and disposal in the ocean was not considered to be economically feasible when compared to disposal islands constructed with Geotubes®.

Several alternatives such as construction of upland disposal sites and dumping in the river adjacent to the navigation channel were met with environmental concerns and were not allowed.

An upland dredged material disposal area was not an option because of potential damage to Mangrove trees, poor foundation conditions and limited space. Construction of dikes on poor foundation materials, in open water with 4.0 m high tides twice a day was not considered to be technically feasible without the use of Geotubes®.

Design

The 1100 m circumference of the oval shaped island design was oriented to minimize obstruction to river currents. The oval shaped dredged material disposal areas were designed using 20 m circumference, 3.0 m high Geotubes® fabricated with a TC Mirafi GT500 woven polypropylene geotextile fabric.

This fabric has an area opening size of 0.425 mm and a wide width tensile strength in the machine and cross direction of 70.0 and 96.3 KN/M respectively. Seam strengths were about 50 to 60 % of these values.

At a height of 3.0 m the Geotubes® contained about 22 m³ per linear meter of dredged material. The first disposal area was about 200 m wide and 300 m long and the second one will be about 300 m wide and 600 m long and they were estimated to contain about 200,000 and 600,000 m³ of maintenance material respectively.

Construction

The construction plan was to fill eleven 100 m long Geotubes® hydraulically with sandy dredged material from the navigation channel to form the perimeter dike for the ocean shaped disposal areas.

The Geotubes® positioned along the proposed alignment were temporarily anchored during filling with 4.0 m long metal poles that were attached to nylon straps presewn along each edge of the Geotubes®.

Initial filling attempts resulted in too many light weight fine materials in the far end of the 100 m long Geotube® and tidal fluctuations caused the first Geotube® to get slightly off the proposed alignment. To minimize this effect the Geotubes® were filled in increments of 25 m with sand with the unfilled portion resting on a small barge until the next 25 m of Geotube® was needed.

Mud Cat Series 370 "Dragon" Dredge

Initially the contractor used an Mud Cat Series B890 dredge, the "Don David", but experienced some problems keeping the Geotubes® in place against a 1.2 m/second tidal flow. This was caused by the Geotubes® being inflated with 2.200 m³ of water in one hour, before sufficient solids were retained to weigh them down. 

Mud Cat Series B890 Dredge

This was solved by replacing the Series B890 dredge with a 12 inch (305mm) Mud Cat Series 370 cutterhead dredge, the "Doña Sara" with 100 m of discharge line. With the smaller dredge all the excess water was drained through the fabric and the remaining filling ports maintaining the Geotubes® deflated until sufficient sand weighed them down.

View of Geotube® being filled

Conclusions

Construction of the Geotube® perimeter containment dike for the first oval shaped dredged material containment facility was successfully completed this spring and is currently being used as a dredged material disposal area. Construction of the second oval shaped disposal area will begin soon.

The use of Geotubes® to form the perimeter dike for the dredged material containment facility has proven to be constructable, and technically and economically feasible. Construction of the disposal area perimeter dike would not have been successful without the use of Geotubes®.

(Geotube® is a registered trademark of Ten Cate Nicolon)

Source: Excerpted from World Dredging Mining & Construction

By Jack Fowler, Ph.D., PE, Geotec Assocs., Vicksburg, MS
Eduardo Martinez, Pres., Dragados Hidraulicos, Bogota, Colombia
Nicolas Ruiz, Civil Engineer, TC Mirafi, Bogota, Colombia
Carlos Ortiz, Bogota, Colombia