May 16, 2016

BRE Soakaway Guidance Updated

Designers of soakaways should take a more holistic approach to site investigation, including consideration of contamination and ground risk, according to the 2016 update of BRE Digest DG365 Soakaway design.


Jon Archer, Principal Geoenvironmental Engineer at Harrison Group Environmental, said: “While pollution was mentioned in the 2007 edition of the guidance, it was not part of the site investigation recommendations. In contrast, the new publication states: ‘Prior to designing the soakaway, the potential for contamination on the site needs to be established’. It also recommends that remedial measures need to be included in design.”


John added that the new guidance emphasises the importance of desk studies, stating they are preferred even for smallest developments. “It recommends ‘local’ advice is sought on ground and slope stability risk, including flooding- and wash-out-induced settlement, and that these need to be considered in the design,” he said.


“It is clear that developers and their design teams must ensure they commission comprehensive site investigations when designing soakaways,” John continued. “This should include a desk study, a well-planned and targeted ground investigation, plus insitu and laboratory testing. Data gathered can be used to develop a ground model that will help mitigate design risk.”


According to the introduction to the new edition, Digest 365 is one of the most widely used BRE publications. It describes design and construction procedures for soakaways and explains how to calculate rainfall design values and soil infiltration rates, along with design examples.


The new edition replaces BRE Digest 365 (2007) and includes recommendations from the Environment Agency on climate change and consideration of 1 in 100 year flood events.


BRE Digest 365 (2016) can be purchased from the BRE Bookshop ( Harrison Group Environmental has extensive experience in delivering site investigations for developments across the UK; for geotechnical and contaminated land advice, please contact us on 01603 613111 or email