As Irish harbours and ports expand and vessel draughts get deeper, there is an increasing demand for dredging. One issue that has arisen on some of the more recent dredging projects is the disposal of contaminated dredge spoil. Heavy industrial activity has historically been located at or nearby Ireland’s major ports. The emissions from these industries, combined with heavy maritime traffic, have resulted in elevated contaminant levels in harbour sediments. Contamination is also present in smaller fishing ports and typically results from vessel antifouling paints and general fishing activity. The most prevalent contaminants are petroleum oils, heavy metals, organotins, polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs). Oil, especially petrol contamination, is a result of emissions from shipping traffic. PAHs contamination is related to such traffic as they are released during the combustion of petroleum hydrocarbons. The presence of organotin contamination is usually from the use of antifouling paint on vessels. The use of these types of antifouling paints has been prohibited in Ireland since 1987, but traces still remain in harbour sediments. PCBs were used in a variety of products including pesticides and paint; these PCBs have contaminated harbour sediments by general atmospheric deposition.

Dumping at sea alternatives

solidification-stabilisation-s/s-dredgeContamination levels on recent projects in Bantry, Dunmore East, Dingle and the proposed works in Dublin’s Alexandria Basin, to name but a few, have been such that the preferred disposal method, dumping at sea, is not possible. When the proposed sediments to be dredged do not meet the criteria for dumping at sea according to Marine Institute guidance (‘Guidelines for the assessment of dredge material for disposal in Irish waters’, Cronin et al (2006)), there are only alternatives to disposing of the material, including in principle:
  • Onward transport to a licensed facility (landfill), or
  • Reuse on site through solidification and stabilisation (S/S).
The onward transport of dredge spoil option is complicated by the lack of landfill space for the large quantities of contaminated material involved. There may also be a requirement to stabilise the material for road transport, depending on the seabed composition. If the material is classified as hazardous, international transhipment is the only option. The onward transport option has a greater associated cost than disposal at sea, with international transhipment potentially costing greater than €200/m3. There is also the impact on traffic to consider and the number of truck movements when hauling large amounts of dredge spoil to landfill. An alternative to international transhipment or using a terrestrial waste facility is to treat the material on site and reuse it within the project. This approach fulfils several key criteria in modern construction projects: it is cost effective by replacing imported fill with a site’s own material and does not incur the large transport costs associated with the other alternatives to dumping at sea. Environmentally, treating material on site complies with the principles of self-sufficiency and proximity. In addition, from a programme perspective, it is usually the quickest method of dealing with dredge spoil.

Stabilisation and solidification

[caption id="attachment_38999" align="alignright" width="300"]solidification-stabilisation-s/s-dredge-bantry CLICK TO ENLARGE Fig 1: Bantry Inner Harbour Development Phase 1[/caption] Stabilisation/solidification (S/S) involves the binding of sediment (usually contaminated) with a cementitious binder, to create a stable monolithic type material. The aim of the cementitious binder is to reduce the contamination potential of the material by chemically stabilising the reagents. Solidification refers to the physical stabilisation of the material; this not only reduces exposure of contaminated material to external elements, but also gives the S/S material physical properties that may serve an engineering use such as structural fill. Substantial in-situ S/S has been undertaken in Valencia harbour (Spain) and also extensively in Scandinavia – with the ports of Gavle and Kokkola (both Finland) subject to several studies including Fursman et al (2012) and Holm et al (2014) – and in the UK, notably in Cornwall by Envirotreat in 2001. Although the material is being treated and reused on site, it is still considered a waste process and therefore needs to be licensed. In the Irish context, there are several waste-permitting routes that a project can be directed on, depending on the level of contamination and amount of material (waste) being excavated (created/processed) as outlined below. If the material is deemed hazardous, an Industrial Emissions Licence from the Environmental Protection Agency (EPA) is required. If material is contaminated but deemed non hazardous and is of a sufficient quantity, it is most likely a Waste Licence is required from the EPA. A Waste Permit, which is applied for at local-authority level, is usually only applicable when dredge quantities are low and contamination levels are non hazardous or inert. Guidance on how to undertake S/S mix design is available – see ‘Guidance on the use of Stabilisation/Solidification for the Treatment of Contaminated Soil’, Environment Agency (2004). However, due to the variable nature of seabed composition, both in terms of type and contamination level, S/S mix design is best served by adopting a pragmatic and iterative approach. The two most common tests to determine the contaminant binding properties of the S/S mix are the Batch test, BS EN 12457 and the Monolithic tank test, NEN 7375:2004. As well as chemical analysis, the permeability of the S/S material should be recorded using the Tri-axial Tests, BS 1377 Part 6. This is particularly relevant when a permeable structure (such as within a rock revetment) is the proposed receptor for the treated material. Strength testing such as Unconfined Compressive Stress BS 1377 Part 7 is required if the S/S material is destined as a structural or load bearing fill.

Bantry Inner Harbour Development

[caption id="attachment_39241" align="alignright" width="300"] Left: Fig 2A) S/S treatment cells. Right: Fig 2B) ALLU system[/caption] The recently completed Bantry Inner Harbour Development Phase 1 is the first project in Ireland to reuse treated contaminated dredge material on site using S/S under an EPA Waste Licence. The project was commissioned by the Port of Cork Company Ltd and undertaken by BAM Civil Contractors Ltd. Malachy Walsh and Partners (MWP) provided preliminary design, detailed design, procurement, contract administration, environmental and project safety supervision design duties on the project. MWP also completed the Waste Licence Application to the EPA with assistance from Viridius Consulting. The overall project scope included:
  • Town pier improvements- widening of pier and extension to pier head;
  • Construction of an amenity area adjacent to Railway Pier, including a seawall and rock revetment;
  • Quayside reclamation adjacent to the town pier;
  • Installation of a floating breakwater at the head of the Railway Pier; and
  • Installation of marina pontoons alongside pierside reclamation.
[caption id="attachment_39003" align="alignright" width="300"]solidification-stabilisation-s/s-dredge-bantry Fig 3: Revetment and amenity area to backfilled with S/S dredge material[/caption] The project included dredging of approximately 40,000m3 of mildly contaminated silts overlying sands and gravels using a customised back-hoe dredge vessel with internal storage hopper. Soil analysis had shown that it was not suitable for disposal at sea, but was suitable for reuse as engineering fill as part of reclamation works if S/S was undertaken. The granular material was reused as core material for a revetment that forms the outer boundary of a newly reclaimed amenity area. This amenity area was filled with treated dredge material. The S/S process was undertaken ex-situ in treatment cells adjacent to the proposed amenity area (Fig 2A) using the ALLU stabilisation system – a proprietary system used to undertake soil stabilisation that consists of an injection head that fits on conventional excavators. This injection head is fed by an independent self-propelled hopper and pump that delivers the binder into the treatment cell via the injection head (Fig 2B). Further details on the S/S mix design, testing and performance of the treated material can be found in the recently published paper at the ICE Breakwaters conference, ‘Recent Approaches to the Solidification and Stabilisation (S/S) of Contaminated Dredge Material in Ireland’. Acknowledgments The Port of Cork Company Bantry Bay Port Company Bam Civil Contractors Ltd. Viridus Consulting ESI-Consulting Feehily Timoney & Company References
  • Allu (2016),
  • BS EN 12457-2:2002.Characterisation of waste. Leaching. Compliance test for leaching of granular waste materials and sludges. One stage batch test at a liquid to solid ratio of 10 l/kg for materials with particle size below 4 mm (without or with size reduction)
  • Cronin M. et al., (2006) Guidelines for the assessment of dredge material for disposal in Irish waters, Marine Institute
  • Forsman et al,(2012). F d test in Port of Kokkola Longtime, Sustainable Management of Contaminated Sediments
  • Environment Agency, (2004). Guidance on the use of Stabilisation/Solidification for the Treatment of Contaminated Soil
  • Holm et Al., (2014). Field test in Port of Gävle, Sweden, Sustainable Management of Contaminated Sediments
  • Lasheen, M.R., Azza M.A., Hana, S.I., and Shimaa, M.(2013),Pozzolanic-based materials for stabilization/solidification of contaminated sludge with hazardous heavy metal: case study, Desalination And Water Treatment Vol. 51,Iss. 13-15
  • NEN 7375:2004, 45Leaching characteristics - Determination of the leaching of inorganic components from moulded or monolitic materials with a diffusion test - Solid earthy and stony materials
Author: Dr Michael O'Shea is a chartered engineer and has over ten years’ experience in coastal, civil and offshore engineering. As a design engineer for Malachy Walsh & Partners, he is currently working on several S/S dredging projects. He holds a PhD degree in coastal engineering from University College Cork as well as a master's degree in engineering from the National University of Ireland, Galway and a bachelor's degree in civil engineering also from University College Cork.