Case studies

• Customer: Portsmouth City Council
• Project: Conor Bridge Replacement
• Date: 2004
• View survey results (pdf)

Portsmouth City Council, as highway authority, was responsible for the replacement of a 100year old road over rail bridge structure. Portsmouth is a densely populated city and as such the highway areas contain numerous statutory undertakers apparatus. In undertaking the bridge replacement the City Council was aware that diversion of utility apparatus [1No 33kv and 2No 11kv electricity cables, 9No telecommunication ducts (including fibre optic cables)], whether temporary or permanent, would be required. Previous experiences gave the council concerns over the ability to accurately determine the location of the apparatus to be diverted and suitable locations for the apparatus to be diverted to.

The cost of diverting the apparatus was the responsibility of the City Council as part of the bridge replacement scheme and therefore the affected statutory undertakers were required to provide estimates for the works. Initially the estimates were based on record drawings and because of the unknowns of the other statutory undertakers apparatus in the immediate area these estimates included a considerable allocation against risk.

GEOTEC Surveys Ltd were employed by Portsmouth City Council to carry out a detailed underground utility mapping survey of the main road, side roads and bridge area. Combining day and night work to keep disruption to the public to an absolute minimum, GEOTEC Surveys employed several techniques and technologies most suited to this particular environment utilising ground probing radar, radio-frequency detection, cover lifting, threading, induction, sondes and dyes as well as drawing on any existing records and local knowledge, enabling them to gather as much information as possible during the time on site. Detailed AutoCAD drawings of the survey results were then produced.

The detailed survey allowed the individual statutory undertakers to re-assess their initial diversion routes and estimates based on the accurate information, and proved more accurate estimates for the diversion works due to being able to reduce the allocation against risk.