SITE INVESTIGATION SHOULD BE FOR CONCRETE DURABILITY IN ADDITION TO SOIL STRENGTH PARAMETERS

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All too often Site Investigation work is seen solely to provide soil strength parameters to enable economical foundation design. There is a need for the site investigation industry to make clients more aware that the aggressive nature of the ground should also be accurately determined if adequate precautions are to be taken in the design of a durable concrete for use in the foundations.

The problems associated with the thaumasite form of sulfate attack (TSA) have been well documented and in August 2001 BRE Special Digest 1 was published. Part 1 of the digest is particularly relevant to the site investigation industry. Without the necessary soil and ground water testing to determine the extent of those aggressive chemicals present at a particular site the concrete cannot be designed in accordance with best practice. It is where possible beneficial to have results from both groundwater and soil samples.

Many Site Investigation reports are issued without fundamental site-related parameters to enable the adequate design of the concrete.

The site assessment procedures should vary depending on whether the site can be defined as natural ground, brownfield containing industrial wastes or pyritic ground, reference to BRE Special Digest 1 should be made for full details.

In general it will be necessary to determine the water soluble sulfate in 2:1 water/soil extracts and the pH in 2.5:1 water/soil extracts. Many Site Investigations where they report any chemical testing only show an occasional soluble sulfate result which is often inadequate to determine the Design Classification for the concrete mix. Where the sulfate in the soil extract exceeds 3.7 g/l SO4 or in the groundwater sample exceeds 3.0 g/l SO4 it is necessary to also determine the Magnesium content. The mobility or otherwise of the groundwater on site also has an affect and should be established.

Where a site is brownfield it will generally be necessary to obtain the Chloride and Nitrate content in both the soil and groundwater samples if the aggressive chemical environment for the concrete is to be accurately determined. Where Pyritic ground conditions are anticipated more substantive testing is required to enable the total potential Sulfate and hence the concrete design requirements to be determined, for full details reference should be made to BRE Special Digest 1.

It should be apparent from the above that greater consideration needs to be given to determining the aggressive chemical environment at the site investigation stage than is currently the case, to determine site-related parameters for strength in one site investigation and then undertake further work at a later date to enable the Aggressive Chemical Environment for Concrete to be determined is no way for the industry to improve its standards or its advice to clients.

It should also be noted that BRE Special Digest 1 has superseded BRE 363.

D.Brightman Technical Manager, Rock & Alluvium