Better Understanding Of The Intent of AS/NZS 3725:2007 – Case Study

The joint Australian – New Zealand Standard for design and installation of buried concrete pipes AS/NZS 3725:2007 states in one of its application Clauses that “this standard shall not be interpreted so as to prevent the use of materials or methods of design or construction not specifically referred to herein, provided that such materials or methods can be shown to meet the intent of this Standard”. This clause allows users to divert from the use of the specified materials and methods to meet site specific conditions.

The Concrete Pipe Association of Australasia (CPAA) published a “Guidelines for Selecting Materials for Bedding SRC Pipes”. This guideline addresses the intention of the Standard to achieve certain levels of pipe support for each installation condition, while allowing the use of materials outside the Standard grading limits. However, the CPAA guidelines exclude conditions where long term stability of installation might be affected.

In a recent pipe culvert installation, the contractors used gap graded materials outside the limits of the AS/NZS 3725:2007 to achieve both good compaction and free drainage conditions. These conditions allowed for the installation of the large diameter pipe culverts during the wet winter season of 2016. The Engineer requested a proof that this diversion met the intent of the Standard before signing off the work for further embankment construction.

A comprehensive study was made to evaluate the background behind the specified grading requirements, and their possible effect on performance of the existing installation. Results indicate that most of the culverts are satisfactory, while a few needed minor remedial works. All recommendations were accepted by the asset owner engineer and successfully implemented.

This paper clarifies the intent of the material grading requirements of AS/NZS 3725:2007 using the first principles of soil mechanics and the historical background of pipe installation theories and practices. The case study provides an example of how engineers could evaluate compliance of any diversion of the Standard.