Barney’s Blog no 6 - Locating Deep Sewers

Drain location to depth of 6-8 Metres is relatively easy using a suitable battery powered drain sonde propelled through the sewer and located on the surface using a conventional pipe and cable locator with a sonde depth mode like the VlocPro, Vloc Cam and RD4000.

Greater depths to 15 Metres are locatable with a higher powered sonde, but pinpointing accurately is much more difficult. Deeper than 15 Metres – that's a completely different story.

In 2005 I had a request from Drain Services Solutions (now DSS environmental) to locate a sewer that was 18 – 20 Metres deep – beyond the range of any easily obtainable sonde.

The story was that the football ground for Bolton Wanderers football club at Burden Park in Bolton was sold and the stands demolished in 2003 to make way for an Asda Superstore and a new regional police headquarters. By 2005 the Asda superstore was completed and opened, but the Police Headquarters was partially built, but awaiting connection to the drains and sewers. The constructors, Laing O Rourke had enquired about the drains before construction started, and all the building services had been designed to connect to drains at the rear of the building, recorded on the maps at 6-7 M deep. So they called in DSS to find the sewer before digging.
Working from the maps DSS lifted the sewer covers to start to trace the drains, only to discover that instead of the recorded 6-7 M, there was a very deep chamber more than 15 M deep, and initial man entry inspection showed that the sewer pipe did not run in a straight line to the next deep chamber about 150M downstream.
They tried their 15 M sonde but could not detect it on the surface, and as the building design relied on connection to this sewer, so location was imperative, otherwise the building would require redesign, not easy on a publicly funded contract.

When DSS requested a more powerful sonde and none were manufactured as a standard product, it required a new approach to solve their problem.

One possible solution was to use a datasonde that was part of the RD i-track no dig boring tool locator system. Used with internal batteries this would not usually have the depth range, but when used with external battery and powerpack it was supposed to reach to 21M. The itrack no dig locator could also provide the route and depth to the sonde. Usually this sonde would be installed in a rotating boring head, pushed forward by the drilling string. In our case this needed to be propelled through the sewer.

As a backup method my second solution was to use a null sonde. It is a little known technique using the sonde vertically, instead of horizontally. As the Supersonde was not powerful enough, we made up a large coil housed in a plastic suitcase and powered by a 10 watt transmitter.
Both systems could be expected to have enough battery life for the 3 hours needed to carry out the location work.

Next task was to invent a suitable waterproof housing, in cooperation with DSS whose man-entry team would have to launch it into the sewer. The best solution we evolved was a sealable plastic food barrel. A plywood "shelf" inside was the mounting board for either sonde type, and the barrel ends were reinforced with a circle of plywood, drilled to take the eyebolt to be used for the draw rope attachment. Stainless Steel radial V clamps secured the ends of the barrel. Finally we needed 2 lengths of nylon rope to haul the barrel along the sewer and strong enough to hold it against the flow, and reels to wind up the slack at both ends.
DSS were very familiar with man entry procedures.

A few days later we were all at Burden Park with all the equipment tested and ready to go. DSS prepared their hoists for their 2, 2 man entry teams, one pair upstream and one pair downstream.

The first task was to float through the nylon rope to be used to pull the barrel through the sewer and retrieve it at the downstream end and bring it up to the surface.

Next the Datasonde for the iTrack was assembled, switched on and sealed into the barrel, checked and lowered down the chamber into the sewer. When it was drawn 3 or 4 M into the sewer we stopped movement and checked the locate position and depth. All seemed good, and the line seemed to be heading off in the correct direction.
The upstream man entry team let the barrel/sonde drift gently and held it every 3 or 4 metres so it could be located. All went well for 30 or 40 M, but then the output signal stopped. So we abandoned that method, pulled out the barrel and refitted the barrel with the big coil and 10Watt transmitter.
Later on I discovered that the Datasonde performed a "go to sleep" shutdown; normally a horizontal boring sonde is rotating except when steering but if stationary for 10 minutes it "goes to sleep". When powered from external battery it was supposed to transmit continuously, but a software bug prevented it.

Using a "big coil" produces a very strong magnetic field, over a wide area and is difficult to pinpoint when it is very deep, but with the axis of the coil vertical, there is a null point that can be located very precisely over the centre of the coil. For greatest accuracy it is vital to ensure that the coil's axis is vertical, so we had installed some simple outrigger floats on our barrel to keep it correctly orientated.

Once relaunched into the sewer this method proved to be reliable and the locate was completed in about an hour keeping in touch with the man entry crews by walkie-talkies to keep the tension on the draw ropes.
Location technique required communication between topside and man-entry crews to extend and move on the barrel in the sewer's water flow. At the same time the barrel needed to be held in place every 4 or 5 M to pinpoint locate the position. Before retrieving the barrel we verified the accuracy visually and using survey poles across the site the bend in the pipe could be shown.
On the surface a fairly standard survey type locator can be used, like a RD4000 or Vloc Pro or Vloc Cam.

The main difference between conventional sonde location and this method is that the peak antenna will produce a sharp null point directly above the coil axis. If a null antenna is used, it provides a peak result directly over the sonde. There are no "false peaks" experienced with normal sondes. In any case these would be very far apart using a conventional sonde.
Locators like the Vloc Pro and Vloc Cam that show peak and null results simultaneously make the pinpointing and location easier.

In due course the new shaft down to the sewer was constructed, and the drains connected and the Police headquarters opened.

The vLoc Pro and vLoc Cam locators have a "compass" that leads to operator to the sonde position. They also have a pair of vertical antennas so they can produce a sharper result than a standard survey locator used in this application.

I am happy to discuss the technique and any location applications for locating deep sewers, just leave an email at