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Case Study: The Missing Manholes

A housing development client got in touch with us after receiving some asset plans from the local water authority, and they needed our expertise. Urgently.

They showed that there were both Foul and Storm sewers running along the southern edge of their development – highlighted below – but there was no evidence of any manholes on site. They needed to know the position, level (cover and invert) and pipe sizes of each to enable them to properly plan a proposed drainage scheme for the site.

 

 

In order for us to satisfy the client’s needs, we had to try and locate a manhole first. The easiest way to do this was to follow the plan upstream from the area the manholes were missing from, until we had located a manhole we could gain access to. If this wouldn’t work, the other options would be to try and locate the manholes via GPR (ground penetrating radar) or by using metal detectors.

In this case we were able to locate the manholes upstream. By using a CCTV crawler system, our surveyors were able to survey the drain run until the next manhole was visible on the video footage. Then, using a sonde transmitter, they could locate the manhole with a hand held receiver.

Once they had established the position of the manhole, our surveyors were able to hand dig the buried manhole. They then lifted it using manhole lifters and took details of the chamber including Invert levels, pipe sizes and flow direction.

Following this procedure carefully, we successfully located all of the missing manholes, and traced out the drainage lines for both the foul and storm sewers running parallel to the site in question.

Once this was completed, the team were able to position all the manholes using an accurate global positioning system (GPS) to provide accurate positions and cover levels that could be added to the existing topographical survey, which we had previously carried out for this client.

Once back in the office, our surveyors could overlay their findings onto the detailed topographical survey

(sample of DWG drawing below)

As we had located the manholes by CCTV, we were also able to provide a CCTV condition report to show the client that the existing sewers were in a good condition.

The result of this survey was that we were able to successfully satisfy the clients needs and not only provide an accurate Autocad (DWG) drawing showing the information that they requested, but also the condition of any pipes that they would be connecting into.

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