Topsoil standard not enough for many schemes, soil experts say

Soil scientists Tim O'Hare Associates (TOHA) are warning specifiers and contractors not to rely solely on the British Standard for Topsoil - especially on difficult schemes.

A soil profile constructed by Tim O'Hare Associates at Queen Elizabeth Olympic Park. Image: Supplied
A soil profile constructed by Tim O'Hare Associates at Queen Elizabeth Olympic Park. Image: Supplied

The latest version (BS3882:2015) was released in April 2015 and complements the British Standard for Subsoil (BS8601:2013).

Although there are some positive changes in the new standard - such as amendments to parameters for Low Fertility Topsoil and the introduction of more detailed soil management advice - there is also a lot of small print, warned TOHA principal consultant Tim O'Hare.

There is also a series of notes to consider at the end of Table 1. Compliance with the parameters in Table 1 does not mean a topsoil is suitable for every landscape situation or application, O'Hare added.

"This standard is intended for use throughout the UK but will not necessarily pick up regional variations in rainfall or temperature or site-specific issues. We suggest using BS3882:2015 as a starting point for assessing topsoil, but possibly consider narrowing some of the tolerances of certain parameters if the project requires it.

"For example, if a free-draining topsoil is required, we would recommend specifying the topsoil to have a high sand content, and possibly include a percolation test in the specification as an additional test parameter."

Potential contaminant tests

Additionally, and with reference to Table 1, Notes 3 and 4, there is a definite requirement to determine the levels of potential contaminants that pose a threat to human health and the environment. These must be analysed for in addition to the parameters set out in Table 1.

Potential contaminant tests are often required to comply with planning conditions (e.g. a site's remediation strategy) or to satisfy the requirements of associated third parties such as the NHBC.

O'Hare said: "We strongly recommend always testing topsoil for contaminants to demonstrate its wider compliance with industry standards. This is something that is often missed by many soil testing companies and ultimately means that their tests are not fully compliant with the standard."

Additional tests

In many cases, other laboratory tests are needed along with those required for BS3882:2015. For example, a common topsoil test is 'detailed particle size analysis' where the portion of sand is further split into five size classes (very fine, fine, medium, coarse, very coarse sands). This enables a more detailed assessment of how the topsoil will perform once laid with respect to drainage, aeration and compaction.

A topsoil with a 'wide particle size distribution' can suffer from particle interpacking, which results in compaction and poor drainage. Conversely, a topsoil with a 'narrow particle size distribution' (i.e. most sand particles falling into one or two of the sand classes) is likely to have free-draining properties and be more compaction resistant.

O'Hare concludes: "The important thing to remember when requesting topsoil analysis is to make sure the testing covers all aspects of your specification and to consider the specific use of the topsoil. The likelihood is that testing just to the BS may not be sufficient for many schemes. "

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