Úna Fitzgerald and David Sutton write that problematic waste can be removed and, with the correct regulatory approach, can be recycled into further useful products
Civil

 

Authors: Úna Fitzgerald, associate, RPS and David Sutton, graduate engineer, RPS

Pyrite is an iron sulphide mineral that is a commonly-occurring but minor constituent of sedimentary rocks, particularly mudrocks and shales. Pyrite occurs in two main forms: cubic crystals and framboidal crystals.

Cubic pyrite, which is colloquially known as ‘fool’s gold’, is relatively stable in weathering environments. Subject to the correct environment, framboidal pyrite is problematic in confined construction and within hardened concrete, such as under-floor filling and blockwork. This is due to a chemical reaction that can cause pyritic backfill to expand.

This expansion can lead to significant structural problems, including cracking and heaving of floor slabs. The Pyrite Panel, established last year by Phil Hogan, Minister for the Department of the Environment, Community and Local Government, estimated that up to 12,500 homes are affected by expanding pyritic infill, although many industry experts consider this figure to be conservative.

Pyritic expansion in the infill of a home can be a slow process; it may take up to ten years before damage is perceived. However, if oxidation can occur easily and/or starts during construction, then a building may show signs of damage within four years. Cases of pyritic damage in Canada have taken ten years before damage was observed. In Ireland, cases of pyritic damage have taken as little as three years to appear.

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It is very difficult to predict the amount of swelling that will occur in a particular situation. Volume increase will be limited by the amount of pyrite present, rock characteristics and temperature, as well as the amount of calcium carbonate available and the groundwater conditions. Even though the scale of the pyrite problem in the greater Dublin area is such that the authors anticipate the cost of remediation to run to hundreds of millions of euro, very little experimental work has been carried out to investigate this problem.

In recent years, NUI Galway has conducted several laboratory experiments in which the environment of the underfloor fill was simulated. Controlled experiments on ‘active’ fill have successfully measured pyritic expansion and have identified some factors on which the expansion depends.

Factors identified that affect the rate and extent of the expansion include: depth of the backfill, percentage of pyrite present, density of the fill, water content, position of the water table, grading of the fill, lithology of the rock and temperature. The results of the experiment proved that a definite relationship exists between temperature and expansion rate over a temperature range liable to be experienced beneath buildings in Ireland. Previously, temperature had been considered a minor factor affecting the rate of expansion (Sutton et al, 2013).

Figure 1: Pyritic expansion and temperature readings

The difference in average annual temperature is put forward as a possible explanation for why pyrite causes damage in homes much sooner in Ireland than, for example, in Canada, although as well as climatic effects, differences in construction methods, groundwater conditions and the mudstone may also be influential. This research has lead to the publication of a paper entitled, ‘A laboratory study of the expansion of an Irish pyritic mudstone/siltstone fill material’.

Further research is ongoing to determine the effects of variation in the pyrite content of the mudstone on the rate and amount of swelling. It is expected that ongoing research in NUI Galway, including modelling of the effects of mineralogical changes on the behaviour of the fill and the performance of the floor slab and supported structure, will facilitate prediction of the effects, if any, of variations in these factors.

Research is also required to determine the impact of variation in the grading of the fill and its state of the compaction on both the magnitude of heave pressures and the effects of physical and mineralogical changes on the geotechnical properties of the fill.

PYRITE INFILL – WASTE OR RESOURCE?

Several hundred of the estimated 12,500 homes affected by pyrite in north Dublin have already been remediated by the developer in question, Menolly Homes. The remediation involved the removal and replacement of the affected infill, giving rise to significant quantities of excavated material, for which an appropriate end destination had to be determined in the context of waste regulation.

This raised the question of whether or not the material should be considered a waste, and thereby handled accordingly under waste legislation – or was an alternative means of regulation appropriate? At the outset of the remediation work in May 2011, Menolly Homes sought advice from RPS in this regard.

Figure 2: The conveyance of the excavated infill out of the houses

Whilst the presence of pyrite within the fill was detrimental in a confined situation beneath concrete floors, it was still seen as a valuable material with viable re-use options in unconfined applications such as temporary haul roads, and infill for quarry voids – as a resource, not a waste.

Menolly Homes provisionally identified a destination for the excavated infill; it had a private arrangement with a local quarry that could use the material to backfill the void during restoration works, subject to obtaining the necessary regulatory consents.

The Waste Framework Directive (2008/98/EC) introduced  the concept of ‘by-product’ under Article 5, defining four conditions that must all be met in order to classify a material as by-product:

(a)             Further use of the substance or object is certain;

(b)             The substance or object can be used directly without any further processing, other than normal industrial practice;

(c)             The substance or object is produced as an integral part of a production process;

(d)             Further use is lawful in that the substance or object fulfils all relevant product, environmental and health protection requirements for the specific use and will not lead to overall adverse environmental or human health impacts.

The Directive was transposed into Irish law in April 2011 through S.I No. 126 of 2011, European Communities (Waste Directive) Regulations 2011. Article 27 specifies the requirements with respect to by-products.

RPS co-ordinated the preparation of a notification to the Environmental Protection Agency, on behalf of Menolly Homes, under Article 27. This involved sampling and analysis of the material to characterise its chemical composition and assess the potential for harm to human health or the environment within the setting of the quarry destination.

Figure 3: Stockpile of recently excavated infill

The excavated hardcore was a naturally occurring material, a calcareous mudstone, which following extraction at source, was crushed and subsequently placed as infill beneath the houses. Because of the established pyrite content, the primary concern related to the presence of sulphide sulphur, and thereby the potential for ARD (acid rock drainage).

The excavated infill was characterised in accordance with Commission Decision 2009/359/EC completing the definition of inert waste in implementation of Article 22(1) of Directive 2006/21/EC of the European Parliament and Council concerning the management of waste from the extractive industries. This decision prescribes a maximum allowable content of sulphide sulphur and takes account of the acid neutralising potential. It therefore provides a useful framework for determining the risk of sulphur-containing material to cause ARD.

Decision 2009/359/EC also lists eleven potentially harmful metals which may be prudent to consider in the context of material or waste arising from extraction. A suite of parameters for testing of the material was drawn up to include those parameters listed in Commission Decision 2009/359/EC. Interpretation of the results indicated that the infill met the criteria set out in the Decision.

The notification was accepted by the Agency in December 2011. The Agency agreed that the material satisfied all four conditions and could therefore be classified as by-product. In parallel, the quarry operator secured approval under their planning permission for use of the excavated infill as backfill during the first phase of restoration of the quarry.

This project demonstrates that a very problematic waste structurally can be removed, and with the correct regulatory approach can be recycled into further useful products. A problem waste became a resource and still complies with EU and national legislation.

 

REFERENCE

Sutton, D, McCabe, BA, O’Connell, A and Cripps, JC (2013) ‘A laboratory study of the expansion of an Irish pyritic mudstone/siltstone fill material’. Engineering Geology, 152 :194-201.

http://www.engineersjournal.ie/wp-content/uploads/2013/04/Pyrite-1024x785.jpghttp://www.engineersjournal.ie/wp-content/uploads/2013/04/Pyrite-300x300.jpgDavid O'RiordanCivilEnvironmental Protection Agency,NUI Galway,RPS Group,waste
  Authors: Úna Fitzgerald, associate, RPS and David Sutton, graduate engineer, RPS Pyrite is an iron sulphide mineral that is a commonly-occurring but minor constituent of sedimentary rocks, particularly mudrocks and shales. Pyrite occurs in two main forms: cubic crystals and framboidal crystals. Cubic pyrite, which is colloquially known as ‘fool's gold’,...