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Creative engineering

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We rummage through the bookshelves in a round-up of how to engineer sub-soil, night soil and suburban sprawl

Rammed Earth: Design and Construction Guidelines Walker, Keable, Martin, Vasilos, BRE, 2005. 146pp. £35 This is a self-proclaimed 'first guidance document for rammed earth construction published in the UK'. However, it is also a panegyric to promote the construction technique of rammed earth and to encourage its use.

It is something of a purists' manual too, in that the use of cement as an additive is frowned upon because of the CO 2 emissions caused by its manufacture.

This book thus focuses primarily on 'non-stabilised' rammed earth.

There is no great secret to rammed earth 'technology'. Formwork is set up to delineate either side of the wall of given thickness, and moist soil is deposited between the formwork and compacted in 50-100mm layers, the formwork removed and the structure allowed to dry out. It is an ancient - or contemporary Third World - construction method.

Victorian and post-war experiments in rammed chalk still survive to this day and there are some interesting photographic records here.

Under the heading 'drawbacks, ' the authors list the fact that 'rammed earth is susceptible to decay in the presence of water'. An interesting way of saying 'earth + water = mud'. Curiously, under the 'benefits' section, it says that the rate of construction is typically between 5 and 10m 2/day for a 300mm-thick rammed earth wall for a team of three or four workers.' In other words, 2m 2 per worker per day.

After some waffle, the book describes soil characteristics, compressive and shear strengths, shrinkage and thermalexpansion coefficients, but there is a decidedly annoying avoidance of categorical construction and structural information. Sub-headings re-occur throughout the book, tortuously presenting information in stages - just like building up the layers of the rammed earth wall - but it would be so much easier and more helpful if the full information had been located in one place.

One example of this is information on movement joints. After many tormenting references threatening to clarify when and where they should occur, we find, in the recommended specification, that 'the contractor shall prepare a detailed layout of construction joints for approval by the engineer prior to any rammed earth being placed'. This hardly constitutes guidance. The book is oversimplified in parts but when the text requires definitive answers, it shirks its responsibilities and muddies the water.

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