SLUDGE CONTAINMENT IN GEOTUBES
(DEWATERING)
Dewatering offers a quick and definitive solution for managing sediments, even in the presence of potential contaminants. A mixture of sediment and water is pumped into tubular geotextile structures using a suction-discharge system. The weave of the geosynthetic material is designed to retain the solid part inside, allowing water to escape due to the pressure difference created between the inside and outside of the geotube. This also enables the placement of geotubes in underwater environments at considerable depths.
Filtering weave detail of geotube
The right combination of high permeability and low porosity of the fabric composing the geotube allows the solid component to be retained inside while permitting water to exit and drain.
Aqueous phase drainage – Monguelfo site 2018-2019
Generally, within a few weeks, a suspended solids concentration of over 70% is achieved. The process can also be improved and accelerated by adding flocculating agents into the pumping line, potentially reaching suspended solids concentrations of over 90%.
Any substances present in the sediments and sludge (including contaminants) remain trapped inside the geotube, as they are primarily adhered to the solid matrix. At the end of the drying process, the entire structure can be sent for disposal.
Case Studies
The Monguelfo Dam
In August 2019, a new dewatering intervention was completed at the Monguelfo Dam in the municipality of Valdaora (BZ), involving the removal of approximately 3,000 m³ of sediment and storage in geotubes placed on the left bank of the reservoir.
Aerial view of dredging and filling
The project, prepared by Alperia Greenpower S.r.l., involved the flotation of the dredged material through a floating pipeline approximately 250 m long, capable of conveying the mixture into the geotubes placed on the left bank near the barrier, through the following phases:
- Launching and positioning on the surface of a pontoon of suitable dimensions to accommodate the equipment necessary for the operation of the pump and its movement to reach all areas affected by dredging (bottom outlets on the right bank);
- Use and positioning of a pump (with related vertical piping for the ascent of the mixture) on the pontoon for the suction of the water/sediment mixture. The pump has a “mixture” flow rate of about 500-600 m³/h over an average head of about 20-25 m;
- Positioning of a horizontal floating pipeline approximately 200-250 m long for pumping the mixture with adequate pressure into the geotubes with a solid content of about 6-8% of the mixture;
- Construction of a terrace on the left bank to host the geotubes, including the laying of a separation geotextile and a draining layer of about 10 cm of aggregate material;
- Positioning and filling of the geotubes into which the water-sediment mixture will be introduced;
- Installation of an in-line conditioning system for the addition of polyelectrolyte;
- Restoration of the bank area by covering the geotubes with soil material.
The area hosting the geotubes was subjected to stability verification both in the transient phase (construction phase) and in the definitive phase (operational phase). In the latter case, both the presence of a snow load and seismic action were considered. All verifications were conducted at Ultimate Limit States (ULS) according to current regulations.
The geotubes were arranged on two levels:
- A first level consisting of 4 geotubes arranged in two parallel rows, each 30 m long and approximately 7.1 m wide and 1.7 m high;
- A second level consisting of 2 geotubes resting on the underlying level, with a width of approximately 11.6 m, a length of 30 m, and a height of 1.9 m.
At the end of the dredging and geotube filling operations, a renaturalization intervention was carried out by covering them with a layer of vegetative soil with an average thickness of 50 cm.
GEOTUBE COVERING PHASE
The Amelia Dam
In 2013, one of the first dewatering interventions in Italy was carried out at the Amelia Dam (TR), involving the removal of 11,000 m³ of sediment and its subsequent containment within geotubes placed on the left bank of the Amelia Torrent downstream of the dam. The material was removed using a suction-discharge pump operated by the arm of an excavator positioned on Soilmec pontoons and discharged into MACTUBE® geotubes, each equipped with three filling nozzles.
DREDGING EQUIPMENT WITH EXCAVATOR-OPERATED PUMP
GEOTUBES ALONG THE BANK OF THE AMELIA TORRENT
At the end of the drying process, the material became shovelable and was subsequently removed for reuse in the bank stabilization of the Amelia Torrent.
