OR/14/001 Appendix 2 Creation of the corrosivity dataset

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Dearden, R A, Tye, A M and Marchant, A. 2013. User Guide for the corroded Asset failure. British Geological Survey Internal Report, OR/14/001.

The dataset was created by scoring five corrosive properties and assigning scores to the BGS Parent Material polygons according to an adapted Cast Iron Pipe Research Association dataset (CIPRA, 1964). The five parameters that are scored are:

Soil pH

Soil-parent materials pH scores have been divided into three classes.

  1. Soil pH values > 8.5
  2. Soil pH values < 4.5
  3. Soil pH values > 4.5 and < 8.5.

Moisture

Soil moisture was based on its texture as stated within the Parent Material database. Three classes were scored

  1. Poor drainage, continuously wet
  2. Fair drainage, generally moist
  3. Good drainage, generally dry

Soil Redox status

Soil redox status was largely based on soil texture information contained within the Parent Material Database. Three classes were outlined for this soil property.

  1. Well oxidised soils,
  2. Soils prone to seasonal anaerobic conditions
  3. Very wet or waterlogged soils such as peat or saltmarsh soils.

Resistivity

Information was obtained from a resistivity model developed by BGS. Resistivity modelling has been undertaken for all the LEX_RCS codes in the Midlands of England as part of an earlier study. This data was then used to inform other LEX_RCS codes outwith this area. Five factors have been identified from which resistivity values have been modelled. The factors are:

  1. Clay mineral content
  2. Porosity
  3. Saturation
  4. Pore water resistivity Ohm.m
  5. Material factor based on the shape of the particles and pores.

Presence of Sulphides and Sulphates

This was based on data in the BGS LEX_RCS database, the National Geotechnical Properties Database and through extensive expert knowledge within the BGS and the literature (e.g. Czerewko & Cripps, 2006; Forster et al. 1995). The presence of primary sulphides in soils and parent materials are typically associated with mudstones, Cretaceous and Tertiary clays, coal seams as well as waterlogged soils such as peat and estuarine and river alluvium. Primary sulphates are found typically in the evaporite deposits of the Permo-Triassic and part of the Jurrasic periods. Secondary sulphates are found where sulphides have been oxidised leading to formation of sulphate ions. Further information on sulphide and sulphate relating to concrete is in the Building Research Establishment publication SD1 (BRE, 2001).

The scoring system in Table A has been used to assign values to the polygons represent the different parent materials held within the BGS Parent Material dataset.

Table A. Scoring system applied to the Parent Material polygons.
Corrosion Property

Value

Score

pH

< 4.5

3

4.5-8.5

0

>8.5

3

Moisture conditions

Sand

0

Sand > loam

0

Sand > loam >clay

1

Loam

1

Loam > sand

1

All

1

Loam > clay

1

Clay + loam

1

Clay > loam

1

Clay

2

Peat

2

Resistivity,

<7

10

Ohm.m

7 – 10

8

10 – 12

5

12 – 15

2

15 – 20

1

>20

0

Redox Status Well oxidised soils e.g. course and highly permeable soils

0

Soils prone to seasonal waterlogging e.g. clay soils

4

Very wet or waterlogged soils e.g. peat and salt marsh soils

5

Sulphides / Sulphates

Primary

3.5

Secondary

3.5
Not present or unlikely

0

The final score for each polygon within the dataset are created by summing all the individual scores for all five corrosive properties (as shown in the table above).

The original CIPRA scoring scheme suggested that any soils with a combined score exceeding 10 were likely to be corrosive. Soils were thus given one of two scores, < 10 or >10. However, a third category has been included for those values around the score of ‘10’ as parent materials that may require additional examination. Table B describes the colour coding and recommendations.

Table B Scoring system used in the final corrosivity dataset.
CLASS SCORE Ground Conditions Recommendation Backfill Colour coding Typical Material Description
CLASS SCORE LEGEND RECOMMENDA BACKFILL
1 <9 Ground conditions beneath topsoil are unlikely to cause corrosion to iron Special protection probably not required, unless the ground is clay or peat or likely to contain saline water (estuarine or marine) if so see class 3. Do not use peat or salty materials for backfill. Only use clay materials if they do not contain sulphide or sulphate crystals or are of low pH Yellow Most rocks e.g. sandstone, limestone, chalk, igneous and metamorphic rocks, boulders, cobbles, gravel, sand and silt
2 9–11 Ground conditions beneath topsoil may cause corrosion to iron Special protection probably required if materials at site are clay, peat or likely to contain saline water (estuarine or marine). If so see class 3

Do not use peat or salty materials for backfill. Only use clay materials if they do not contain sulphide or sulphate crystals or are of low pH

Do not use peat or salty materials for backfill. Only use clay materials if they do not contain sulphide or sulphate crystals or are of low pH Green Mostly ‘clays’ and mudstones with relatively low clay size content and do not contain iron sulphide or calcium sulphate.
3 >11 Ground conditions beneath topsoil are likely to cause corrosion to iron Special protection probably required if materials at site are clay, mudstone, peat or likely to contain saline water (estuarine or marine). If so, further ground investigation is required to assess whether the hazard exists.

Do not use peat or salty materials for backfill. Only use clay materials if they do not contain sulphide or sulphate crystals or are of low pH
Do not use peat or salty materials for backfill. Only use clay materials if they do not contain sulphide or sulphate crystals or are of low pH Blue A variety of material types depending on the lithostratigraphical classification.

The following indicate corrosivity hazard


i) Grey to black clay, brown near surface. May be mudstone at depth. May contain white or translucent ‘soft’ crystals (gypsum) at a few metres depth

ii) Red clay or mudstone, may contain white or translucent ‘soft’ crystals (gypsum) at a few metres depth

iii) Peat

iv) Contains saline water