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Pyrite (FeS2-Iron Sulphide) is a very common mineral.
Traces of it are found in the sedimentary rock used to make crushed stone for backfill. In the presence of humidity and oxygen, pyrite oxidizes and produces sulphuric acid.
Pyrite oxidation also leads to the formation of gypsum crystals which increase in volume. This increase in volume can lead to backfill swelling which in turn may damage any concrete slabs, foundations or other structures that are laid on top of the backfill.
There are no specific guidelines for visual detection of pyrite in hard-core filling. The qualitative method of detection is by chemical analysis.
The designer shall ensure the suitability of the ground conditions for the purpose of supporting the development, by carrying out necessary site investigations including chemical analysis of soils to ensure that there are no harmful contaminants or hazards, which could cause deterioration of any element of the development.
Further information can be found within the 'Ground Conditions' section of our Technical Manual.
Where backfill is specified it should be certified by a competent laboratory that they are of a suitable nature and quality in relation to the purpose and conditions of their use. They should be chemically analysed to check if such materials contain any chemicals, which should also include the petrographic indicator of swelling potential.
The Petrographic Swelling Potential Indicator (PSPI) varies from 0 to 100 but is not a percentage. It is intended to be a visual evaluation of the sulphatic swelling potential of the materials used.
The following figures present the petrographic swelling potential that may generally be associated with the different PSPI values:
0 – 10 Negligible
11 – 20 Low
21 – 40 Low to medium
41 – 60 Medium to high
61 – 80 High
81 – 100 Very high
Any materials with a PSPI in excess of 20 will not be acceptable for warranty purposes.
Building Regulations: Regulation 7: Materials and Workmanship.
Approved Document C
Technical Manual: Ground Conditions section.
BRE Digest DG 522: Hard-core for supporting ground floors of buildings:
Part 1: Selecting and specifying materials
Part 2: Placing hard-core and the legacy of problem materials.
(Replaces BRE Digest 276 which has been withdrawn.)
Every care was taken to ensure the information in this article was correct at the time of publication. Guidance provided does not replace the reader’s professional judgement and any construction project should comply with the relevant Building Regulations or applicable technical standards. For the most up to date Premier Guarantee technical guidance please refer to your Risk Management Surveyor and the latest version of the Premier Guarantee Technical Manual.