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GEOTECHNICS MADE SIMPLE

PUBLICATIONS - Andrew Charles explains how his book, Geotechnics for building professionals, gathers BRE guidance in a unified format and approach for non specialists.

It has been said that soil is the most complex construction material. Although the cynic may respond that a claim of this sort was probably made by a geotechnical engineer, few people with any knowledge of construction would dispute that of all the components making up a building or civil engineering structure, the ground will often be the least well known and appreciated. Furthermore, foundation deficiencies are often more expensive to rectify than problems with the superstructure.

Lack of attention to foundation design and construction can have undesirable and expensive consequences.

In his paper 'The influence of modern soil studies on the design and construction of foundations' to the Building Res- earch Congress in London in 1951, Karl Terzaghi commented: 'The fact that there is no glory attached to the foundations, and that the sources of success or failure are hidden deep in the ground, building foundations have always been treated as step children; and their acts of revenge for the lack of attention can be very embarrassing' This analogy may seem a little harsh on attentive step parents, but anyone familiar with Mr Murdstone, the unprepossessing step father in Charles Dickens' David Copperfield, will know the sort ofsituation Terzaghi had in mind.

The historical development of soil mechanics was primarily related to large scale civil engineering applications.The geo-technical design aspects were concerned with the behaviour of saturated soil at considerable depth below ground level, under relatively large applied stress.

However, it is usually the behaviour of partially saturated soil under low applied stress at shallow depth which controls the foundation behaviour of low rise buildings - not well covered in many accounts of basic soil mechanics.

The influence of climate, topography and vegetation on the soil near the ground surface and the presence of made ground are of particular significance for shallow foundations.

While it is undeniable that much building work was successfully completed before the theories of modern soil mechanics were properly understood, there are good reasons why the well-tried and tested methods of the past are no longer adequate: In an industrialised country like the UK much of the 'good' land has already been built on.

Growing use is made of marginal and brownfield land hitherto considered too poor to use for building. Such sites can contain a wide variety of geotechnical and geoenvironmental hazards.

Modern construction is far more sensitive to foundation movements than are buildings built in Victorian times. Where foundation problems occur, they can be expensive to rectify and litigation is likely.

Most houses are now owner-occupied, and despite the occasional collapse of property values, owners quite understandably perceive their homes as major financial investments.

Subsidence, which can be a significant threat to the value of the home, is a highly emotive issue.

Although foundations affect how the main structure of a building behaves and can be a major factor in determining whether or not it performs satisfactorily in the long term, decisions about foundations for low-rise buildings are often made by people who have only limited understanding of the engineering of the ground.

Geotechnics for building professionals has been written for such people and their professional advisors, including architects, surveyors, loss adjusters, planners, insurance underwriters and property developers.

BRE has published guidance over a number of years on various aspects of foundations for low rise buildings in the form of some 30 digests, together with a number of Information papers, Good building guides and reports.

Geotechnics for building professionals brings together this guidance into a single, unified format and approach. It gives an overview of ground behaviour and geotechnics but focuses primarily on shallow foundations for low rise buildings. Much of the emphasis is placed on near surface ground conditions.

Although the material in the book is based on the work of specialists, an attempt has been made to see geotechnics through the eyes of a non specialist.

The book follows a logical sequence of information, from the viewpoint of someone who is confronted, perhaps for the first time, with having to consider, for example, a new housing development on a site about which little is known.

Geotechnical engineering is a specialised subject requiring considerable experience and knowledge, but building professionals can benefit from a better understanding of the most important elements of geotechnics, which should enable them to better appreciate the limitations of their knowledge, to ask the geotechnical specialist informed questions, and to have greater confidence in the answers they receive.

Total foundation failures may be rare, but inadequate foundation performance is not uncommon and may render a building unfit for its intended purpose even where there is no danger of structural collapse.

It is appropriate therefore that the book begins with a review of the ways in which controls and safeguards are imposed so that buildings are located on stable ground and do minimal damage to the local environment.

Many of these safeguards are beneficial to the developer, as well as the subsequent owner of the building, in helping to avoid expensive mistakes in the ground.

Planning constraints, building regulations, special features of regulations for contaminated land, codes and standards, insurance and litigation are reviewed in relation to the UK. For many years there have been just half a dozen or so relevant British standards and codes of practice relating to ground engineering, but soon these will be replaced by over 30 European standards and technical specifications.

The next three chapters of the book provide an introduction to ground behaviour and site investigation. The importance of the processes involved in the investigation of the ground before construction begins is emphasised.

Extensive and costly problems can be encountered both during and after construction because of inadequate ground information. The greatest uncertainty for the building designer is associated with the behaviour of the ground and, therefore, considerable risks arise from inadequate ground investigation. The focus is on those features of ground investigation that are particularly relevant to low rise buildings.

Four chapters are concerned with foundations. A chapter on the basic elements of foundation design is followed by one dealing with foundations on various types of difficult ground, including shrinkable clay and fill.

The particular hazards found on brownfield sites are given separate attention and commonly used ground treatment techniques, which are of increasing importance in the development of marginal land, are described. The last three chapters of the book deal with foundation movement and damage, remedial treatment and underpinning and various types of ancillary works.

It is hoped the book will serve as a simple introduction to a complex subject for those who need to have some knowledge of the performance of the ground for low rise developments.

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