BRE BR509 PDF

EXECUTIVE SUMMARY

Since the 1970s, hundreds of thousands of cubic metres of disused underground mine workings have been successfully stabilised by filling with grouts based on pulverized-fuel ash (PFA) and Portland cement (PC) in order to prevent surface collapse. Similar volumes of disused workings will need to be filled in coming years as pressure grows on land for development. PFA is particularly favoured for such remediation work as:

• It is cost-effective (especially where the volumes to be filled are very large).

• It is technically effective (extensive experience exists of producing low permeability materials which can be readily pumped into partially collapsed cavities).

• It utilises a nationally available and abundant material (PFA will normally be regarded as a waste material unless it is compliant with the Environment Agency/WRAP waste protocol to be published in 2010 (End of waste criteria for the production of pulverised fuel ash (PFA) and furnace bottom ash (FBA) for use in construction and manufacturing).

However, concerns have been expressed over the potential for contamination of controlled groundwaters by bleed water and leachate released from PFA grout, disruption and dust arising from the work and the effect on land and property prices, and the risk of subsequent classification of the land above the mine workings as being contaminated.

This Code of Practice on the use of PFA grouts for filling disused underground mine workings seeks to address these concerns by providing guidance on the selection of environmentally compatible and cost-effective materials and techniques for stabilisation of underground cavities, together with authoritative guidance on good practice. This new edition of this Code of Practice, which was originally published in 2006, incorporates revisions to make it consistent with the activities of the PFA technical advisory group which has developed a quality protocol for PFA.

The information and guidance in this Code of Practice are based on:

information in the literature

laboratory studies carried out at BRE under a research programme funded by the (then) DTI and industry

data on the use of PFA grouts

expertise from an industry steering group, and advice from the Environment Agency concerning the revisions in 2009

a generic risk assessment (Graham R. The use of pulverised fuel ash and furnace bottom ash generic risk assessment for the waste protocols project, RPS, Chepstow, 2008).

Available field data on the use of PFA grouts do not indicate that there are significant effects on groundwater. However, laboratory studies show that leachates from PFA grouts, without further dilution, exceed water quality standard values for certain elements. The risk of contaminating groundwater through the use of PFA grout can be reduced through:

• appropriate materials selection

• appropriate design of the PFA grout

• measures such as controlling the groundwater regime and using engineered barriers.

The appropriate approach will be dependent on the nature and size of the site. Guidance is given in this document to allow the site and the materials being considered to be characterised, and for appropriate measures for safe filling to be taken.

PFA grouts can be used effectively under a range of circumstances as an effective and cost-effective means of stabilising old mine workings. Nevertheless, any environmental risks need to be properly assessed and mitigated. In relation to this, the principal amendments made in the 2009 revision of the Code of Practice are:

A presumption against the use of grouts in groundwater source protection zones designated by the Environment Agency as source protection zone 1 (SPZ 1).

Grouts should not be used below the water table without an appropriate site-specific risk assessment. Specifically, a site specific risk assessment should be completed for:

o all uses of PFA grout below the water table

o all extensive uses of PFA grout in surface protection zones 2 (SPZ 2) and 3 (SPZ 3) above the water table.

In addition to this Code of Practice a detailed Technical Report is included in pdf format on a CD Rom included with this publication. It describes an in-depth laboratory study carried out at BRE to assess the leaching characteristics, permeability and physical properties of grouts made using different levels of PFA addition, and the effects of using PFA from different sources. Also included on the CD Rom are four supplementary documents:

• a review of groundwater risk assessment models

• leaching tests

• long-term effects of PFA grouts on the environment

• a sample specification for mine infilling works.

The Technical Report and supporting documentation on the CD Rom have not been revised or updated.