Installation of Reinforced Concrete Pipes: Theory and Practice

The classic theory of earth loads on buried concrete pipes was developed in the 1930’s and was based on the concept of a bedding factor to relate the supporting strength of buried pipe to the strength obtained in a three-edge bearing test. Four types of bedding were developed and successfully used for years using conservative bedding factors. The later developments in engineering concepts and construction methods highlighted the limitations of the classical approach which developed standard beddings to fit assumed theories for soil support, rather than ease of and method of construction.

In the 1970’s the American Concrete Pipe Association (ACPA) used state of the art finite element computer program results to replace the historical bedding methods with four new standard installations that more correctly reflect actual conditions. In developing the AS/NZS 3725:2007 standard, the committee appears to have developed bedding methods that combine both classical and new design concepts. The combination of dual design concept in AS/NZS creates the potential for ambiguity or uncertainty in certain industry applications.

A review of many TA specifications in NZ and the local practices used in design and bedding pipes presented in this paper, indicates that current specifications and practices are in many cases not complying with either the classical or the modern design theories. One of the causes of this departure is that selected bedding materials from AS/NZS 3725:2007 are not readily available in most NZ areas, with properties that do not fit the local work conditions. This paper will present case studies which indicate that installation based on the new design theories, rather than AS/NZS 3725:2007 may provide more reliable and economical results.

The paper will conclude with a specification proposal based on both modern theory and NZ proven construction and design practices.