A study of pipe-soil-climate interaction of buried water and gas pipes

2017-02-23T00:30:43Z (GMT) by Chan, Derek Chun Chuen
In Australia, buried water and gas pipes are reported to have more frequent failures in hot and dry summers, which suggests that soil shrinkage and thermal effects are the main factors associated with pipe failure. Shrinkage and swelling are common behaviours of soils especially for reactive clays due to seasonal variation of soil moisture content. As a result, the differential soil movement beneath buried pipe can lead to flexural bending and circumferential fracture of the pipe. In addition, seasonal variation of soil moisture content can cause cyclic upward and downward pipe bending and ultimately lead to fatigue failure. However, the relationships between pipe failure, soil behaviour and seasonal climatic change have not been studied in detail. Consequently, the effect of soil moisture content variation on the performance of buried pipes is not clear. This research study is aimed at extending the current knowledge on the interaction between pipes, soils and climate. Field instrumentation was undertaken to study the behaviour of buried cast iron pipes in the natural environment. The collected data confirmed that significant flexural stress was induced by the soil movement on the buried pipe due to the seasonal variation of soil moisture content. Upward and downward bending of the pipe was found to occur at wet and dry seasons, respectively. Regional field measurements of soil moisture content were undertaken in conjunction with field instrumentation to study the seasonal variation of soil moisture content at sites with different soil profiles and geological formations. The 23 sites considered in the study were classified into three categories based on the geological profiles. The study showed that change in soil moisture content was associated with rainfalls and the geological profiles of the site. Variations of soil moisture content can lead to swelling and shrinking of soils and corresponding bending of buried pipes. These data provided the necessary information to understand the likely behaviour of pipes buried in different types of soils. The ground-atmosphere models were used for prediction of soil moisture content variation of the field sites. The Winkler spring models were used to simulate the pipe-soil interaction, and in conjunction with the long term moisture content variation, pipe flexural stress was calculated. Taking into account the deterioration of cast iron pipe strength with time, the relationship between pipe failure probability and pipe age was obtained. This research provides a prediction tool for pipe failure associated with soil behaviour and seasonal climate change. The prediction model can be used to plan for asset management and rehabilitation of the buried pipe networks, thus providing savings on repair of unexpected pipe failures and consequence loss of water and gas services to the communities.