ABSTRACT:

Managers of underground assets, principally sewers and pipelines rely on condition assessments which are based principally on internal inspection and limited geotechnical information. These assessments are inadequate because they fail to effectively sample, test and account for external ground conditions. Unexpected catastrophic ground collapse above ageing infrastructure due to sewer or pipe collapse does occur. Excessive expenditure on new construction, rehabilitation and foreshortened repair life are consequences of the failure fully understand and evaluate the influence of ground conditions above and below buried infrastructure. Geophysical methods such as radar, electromagnetic and sonic technologies have been applied to examine the ground conditions external to the opening, however, these methods have limited range and often produce ambiguous information. Recently, specially developed seismic imaging technologies such as SRTÓ, SEWREELÓ and SUBSÓ which are analogous to Medical CT scanning have demonstrated the ability to illucidate external ground conditions and to locate significant voids or weak ground. These technologies may be implemented from the conduit or boreholes according to site requirements. The SRTÓ technique does not require entry to the length of pipe and has been used as a rapid scanning method of the major inteceptor sewers in Melbourne. This method has been exported to the USA and an example of its application to condition assessment of a sewer in Portland, Oregon which was to be re-lined is described. The SEWREELÓ technique requires pipe entry and can provide detailed information between the pipe and the ground surface. Examples of the use of this technology to the monitoring of void growth resulting from surcharging of a deep sewer section in Melbourne is provided. The SUBSÓ technique is implemented from boreholes and may be use to examine ground conditions above and below sewers or the conditions prior to construction during route investigations.

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