This guide covers the stabilisation of naturally occurring soils or other materials to improve their mechanical properties and performance for use in capping layers, sub-bases and bases. This document, to be in line with European standards and Highways Agency documents, covers treatment with cement and the full range of hydraulic combinations based on fl y ash, granulated blastfurnace slag, gypsum and lime. The resulting materials are known as hydraulically-bound mixtures (HBM). After the introduction, which describes HBM and what they do, the guide describes the following aspects of HBM:

- binder selection

- soil/aggregate selection

- site investigation and preliminary assessment

- mixture design

- production and construction

- construction control.
Each part is designed to be stand-alone and self-contained. Thus for example, should the reader be familiar with the capabilities of the various hydraulic and pozzolanic materials, the fi rst part can be glossed over, or, if guidance is sought solely on construction, then reference need only be made to the fi fth and sixth parts. A construction summary is also included in chapter 6 where time does not permit digestion of the whole construction section. Intentionally, the guide does not cover thickness design and specifi cation, but should provide the background for the formulation of such application documents. Much of what follows is of direct relevance to the treatment of contaminated materials where a process called stabilisation and solidifi cation can be used to ‘immobilise’ contaminated materials as well as improving their engineering properties. Similarly, much is of direct relevance to pavement recycling work. However, these techniques are not the subject of this publication.