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Technical paper: Partial factor design of polymer reinforced soil walls

By T S Ingold. This paper was first published in GE’s June 1991 issue.


The draft code of practice BS8006:1991Code ofpractice for strengthened/reinforced soils and other fills, BSI(1991), is shortly to be issued for public comment by the British Standards Institution.

The current document most widely referred to by British designers is the Department of Transport Technical Memorandum BE3/78 Reinforced earth retaining toalls and bridge abutments for embankments, DTp (1987). This document, which was first issued in 1973, was revised in 1987and is presently undergoing further revision.

One major difference between these two documents is that draft BS8006 has introduced a partial factor approach to design, in which partial materials factors and partial load factors are employed, as opposed to the permissible stress and lumped factor of safety approach adopted in BE3/78.

Other innovations presented in the draft code include the accommodation of service lives less than the 120 years required by BE3/78 and the facility for self certification of non standard reinforcing materials. The option of self certification by the design engineer or specifier is in addition to the option of third party certification which in BE3/78 was limited to certification by the British Board of Agrement.

Standard materials are limited in the draft code to plain black steel, galvanised steel and stainless steel, consequently all polymeric soil reinforcement must be certified to comply with the draft code. To enable a uniform basis for certification the draft code sets out a list of material properties which must be investigated and the end result of such investigation is formulated within the framework of two principal partial materials factors.

Although the draft code specifies a minimum value of the partial material factor to be applied to plain steel reinforcement no corresponding value is presently enumerated for polymeric wall reinforcement. Suggested values are presented below and calibrated against established values of partial factors of safety. 

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