21 Ground chemically aggressive or prone to volume change

21.1 General


Elements of the natural environment can damage man-made materials and structures. Groundwater, soil and rock can contain constituents, mainly sulfates, in amounts sufficient to cause damage to Portland cement concrete, particularly thin members. Soils that contain iron sulfide might oxidize to produce more aggressive ground when disturbed. Some types of natural ground have a corrosive action on metals, particularly on cast iron, owing to electrolytic or other chemical or bacteriological agencies. Damage can also be caused by differential volumetric changes in the ground related to water content variation without chemical change, and heave resulting from oxidation of iron sulfides to form sulfates.

In industrial areas, corrosive action might arise from industrial waste products either in situ or that have been dumped on the site or used as fill, or from liquid sources such as leaking tanks, sumps and chemical drains. There might also be bulk materials formed during industrial and mining processes, which are subject to volume change.

The potential for attack by chemically aggressive ground on any buried concrete, metal or other construction materials should be taken into account when planning an investigation. Similarly, the potential for the ground to damage the proposed structure due to its volume change should be taken into account.

In river and maritime works, the possible corrosive action of fresh water, sea (saline) water and of trade effluents should be taken into account.

NOTE In a marine environment, the most severe corrosion is found in the zone that is intermittently wetted or splashed with sea water due to the increase in salinity as the water evaporates. This effect is increased in waters with a high tidal range. In estuarine situations, there might be an adverse condition because of changes in salinity.

21.2 Investigation of potential deterioration of concrete


The principal agents deleterious to concrete are acidic waters and sulfates. Acidic water is commonly found in organic soils (principally peat) or results from oxidation of sulfide-rich soils and rocks; calcium carbonate acts as a buffer to such reactions and thus calcareous soils and rocks of sufficient calcareous content do not tend to form acidic environments, but conditions might remain potentially aggressive in terms of sulfate attack. Calcium sulfate occurs as gypsum (CaSO4.2H2O) including crystalline forms (e.g. selenite) in a variety of rocks. Magnesium sulfate also occasionally occurs. Sulfates are also formed by the oxidation of iron pyrites, which occur in grey clays, mudstones and some sands, and many other rocks.

The agressivity of the ground to Portland cement concrete should be determined in accordance with BRE Special Digest 1 [N1]. The laboratory tests carried out should include determination of pH value, sulfide and sulfate content and such other determinands as indicated in BRE Special Digest 1 [N1] as appropriate in the context of the site under investigation.

21.3 Investigation of potential corrosion of metals

The likelihood of corrosive ground conditions should be assessed from geophysical testing, from in-situ measurement of redox potential and electrical conductivity (see BS 1377-9), or from laboratory tests on undisturbed soil specimens and groundwater samples.

When designing a testing programme, the correct conditions should be modelled as disturbance to the ground and the use of imported backfill materials can have a significant impact on the evaluation. Local enquiry should be carried out to ascertain whether corrosion of metals has previously occurred.

21.4 Investigation of chemically-induced expansive ground


Some slags are expansive as they contain calcium oxide and hydroxide and magnesium oxide and hydroxide; steel furnace slags are noted as being expansive, as are some old blast furnace slags. However, modern blast furnace slags are not expansive. A change in the ground conditions, such as an increase in the availability of water or in the water level, could be sufficient to cause expansion of materials that were previously stable.

The occurrence of bulk materials, which are a by-product of industrial processes, particularly those involving high temperatures, should be treated with caution. Slags should not be used beneath foundations unless their nature is confirmed as stable. Slag expansion testing should be performed if there is doubt.

21.5 Industrial causes of aggressive ground


A wide range of contaminative substances might be present in the ground of a former industrial site, and which often occur in localized concentrations. Some chemicals, such as acids, are highly aggressive to both concrete and metal in underground structures. Other substances, such as solvents attack epoxy cements, and even low concentrations of phenols permeate plastic water mains and taint the supply. A number of slags and other burnt materials might contain high sulfate contents.

A thorough investigation of potentially aggressive chemicals should be carried out as part of any ground investigation on a former industrial site (see 16.8 and 16.9).

21.6 Ground prone to volume change due to water content changes


All clay soils, to some degree, change their volume dependant on their water content. In general terms, the plasticity index can be used as an indicator of the shrink/swell potential of a clay.

The potential for soil to shrink and swell due to changes in water content and thus cause damage to buildings or other structures should be investigated; changes in water content might be caused by the presence, planting or felling of nearby trees or shrubs, soakaways, drains or sources of heat or cold.

NOTE See NHBC standards, Part 4 [23] for guidance.

21.7 Ground prone to dissolution


Some rocks are prone to dissolution and voids might occur beneath apparently solid material. Calcareous rocks such as chalk and limestone, gypsiferous and salt-rich rocks are prone to dissolution over time and might form depressions at the surface masked by superficial materials, or voids at depth that migrate towards the surface. In the case of chalk, the existence and thickness of superficial deposits does to some degree influence the risk of cavities. Further guidance is included within Annex F.

Investigations in areas where there is ground prone to dissolution should examine whether the potential for the problem has been realized; this process starts in the desk study and field reconnaissance. The voids or areas of weaker ground might only be picked up by a closely spaced investigation or geophysical techniques.