Topsoil as an engineering material – A laboratory analysis

In the construction of flood retention embankments, the general guidance (from the CIRIA International Levee Handbook) is that any topsoil underneath the proposed embankment footprint should be removed prior to construction of the embankment.However, in several recent coastal management schemes within the UK design proposals have instructed the topsoil to remain in-situ with the aim of:

  1. Reduce the quantities of excess material at the site which cannot be used as engineering fill.
  2. Reduce the costs of working and re-working the topsoil.
  3. Reduce the amount of traffic movements on site.

While cost has undoubtable been a driver in these recent developments a larger instigator for change is the more stringent calculations of embodied carbon within ground engineering. If the UK is to meet it’s 2050 target of reducing greenhouse gas emissions to net zero, practices which are commonplace in ground engineering such as the removal, transportation disposal and replacement topsoil are going to need to come to an end. This is especially true of flood embankments, many of which are required dure to rising sea-levels or extreme weather events caused by a warming climate.

It is not however as simple as just leaving the topsoil in place because often the reason for removal is it does not meet the engineering specifications (e.g. density, compaction, moisture content) required for construction.  Through a series of laboratory tests utilising the Geotechnical Engineering Laboratories at the University of Leeds and Rock Mechanics, Engineering Geology and Geotechnics (RMEGG) large shear box, material obtained from coastal and estuary sites will undergo consolidation testing in an attempt to understand the engineering properties of the topsoil and how it will behave under a constructed flood embankment. Natural topsoil is considered a partially saturated soil (three phase system consisting of solids, water and air) and despite several decades of study there is not yet a unified theory for the behaviour of this type of material particullarly for materials with high organic content, which makes planning construction projects built directly on to topsoil a risk.

The project will address that risk, and in addition to investigating the effect of a continuous or discontinuous gas (air) phase, in particular, will aim to characterise how the organic content or clay mineralogy in partially saturated, natural topsoil affects their performance as an engineering material, focusing on compaction, settlement and/or seepage.

It is unlikely that all natural materials will be suitable to build upon so the latter half of the project will also consider potential ground improvement methods (e.g., chemical stabilisation) that can be used to improve the properties of the ground without undermining the aim of reducing the carbon footprint of these projects.