Barney's Blog No 1- Locating Missing Manhole Covers

One of the difficulties facing drainage engineers is finding and linking up new drains and sewers to old systems that have acceptable capacity and still function well. Sometimes CCTV inspections show a missing chamber where they can connect, but its cover is missing on the surface so it cannot be located accurately enough to plan excavations.


The first attempt without accurate knowledge is to use a metal detector to find missing covers, and usually results are successful if the missing manhole cover is relatively shallow and the approximate position is known. However much time is wasted if the cover is beyond the depth of conventional metal detectors, and/or the surrounding area is reinforced, making discrimination between lumps of metal cover and reinforcing bars difficult.

The Vivax vCam 5 CCTV pushrod camera system includes a locating sonde that is detected by the vLoc Cam 2 and vLoc Pro 2 locator, so the route of the sewer/drain can be accurately marked out, then by walking that line with the metal detector, the lost cover can be located and location time is reduced, providing that the cover can be detected by the metal locator.

Simple enough; but what if the cover is deeper and outside the range of conventional metal detectors? There are several special magnetometer type locators that only detect ferrous metals, such as the Metrotech FM880B and Schonstedt that have great depth capability and these types work very well for lost covers perhaps down to 2M depth; but in reinforced routes, these are difficult to use as they also detect the reinforcing bars.

OK, so how do we solve the problem if the cover is deep buried in a reinforced area?
There is a little known technique using the drain sonde, either in the CCTV system, or one of the Vivax drain sondes.

Everyone knows that the normal sonde with the 33kHz frequency will not radiate any signal if it is inserted into a cast iron sewer pipe, and the user needs to switch to a low frequency sonde to have any chance of successful location; so in the same way, if the normal sonde passes into a manhole chamber with a iron or steel cover, the signal from the sonde will be lost on the ground above.

So the successful location technique will be to have one operator feeding the flexrod with sonde or camera rod along the drain/sewer and the other locating the sonde on the surface. Good cooperation and a system of hand signals or 2-way radios is needed between locator user and operator feeding-in the sonde. The sonde feeder pushes about 2-3 Metres at a time, and the locator user locates the sonde after each push. When the sonde signal reduces, the sonde is somewhere under the missing cover. By moving the sonde backwards and forwards to get the best cut off of the signal, it is possible to "focus the shadow" created by the manhole cover.
Generally a cover closer to the sonde than the surface will cast a wider signal shadow, and a sonde closer to the surface will cast a narrower signal shadow, but of course that depends on the size of the cover.

And that reminds me of a story....
Some years ago I was visiting Taiwan, and the telecom company had purchased a number of the Radiodetection RD315 metal detectors. These were different from usual metal detectors and they produce maximum response at the edge of a lost metal cover, so with a bit of careful location it was possible to determine the shape of the cover.
The telecom company were not happy because they could not find lost covers, and the distributor and I were obliged to visit the telecom director to offer explanation for the apparent equipment failure.
In the directors' office also filled by blue overalled engineers, I was told that they had used a fibreglass pushrod up a telecom duct to look for a lost telecom chamber, next to an 8 lane highway. At 95M they hit the blockage, so they pulled out the pushrod, laid it along the footpath, and started to search for the missing cover 95M along the footpath where they assumed the chamber to be. And found nothing.
They searched around a bit, still found nothing and blamed the equipment.
Not wishing to cause any difficulty, I asked to see the locator. And as far as I could establish it was working correctly. Of course the blue overalls do not want to "loose face", so I suggested that the cover may be very deep, beyond the detection depth and suggested that we use the sonde technique to help solve the problem, and explained the application. The director and blue overalls went into a huddle and shortly afterwards announced that we would be taken to the site immediately.
In the building basement we prepared our sonde, attached it to their glass fibre pushrod and 4 medium size vans and a team of people set off across Taipei with orange lights flashing like a CTU team in 24 to find the missing cover.

At the site we pushed the sonde up the duct and located it. We had instructed the pushing team what to do. To everyone's surprise the duct route changed about 45M from the start and went diagonally into the first lane of the 8 lane highway. So 2 of the vans and the men waving warning flags were positioned to protect me as we crossed from lane 1 to 2 to 3 and into the centre of the intersection. All much to the surprise and consternation of the blue overalls.
Shortly afterwards I lost signal, and at the same time the rod pusher hit the blockage. We marked the point, pulled back to sonde, and then used the RD315 metal detector to make out the shape of the cover. This promised to be the answer to the problem. They got out their petrol powered jack hammer and jerked it into life. After a few minutes they hit metal, and dug out enough blacktop to see TTC emblem on the buried cover. Chief blue overall was on his radio to the director. We were summoned back to the headquarters, where the director invited us out to lunch...and that is another story...

Email barney@mansls.co.uk for more info on this subject.