CCTV survey, chainage and coding

Numbering the system before the camera goes inNodes

Not to scale. Run A is the 24.0 m length graded on the front sheet; the camera entered at MH01 and worked up it. Runs B and C were surveyed from MH02 on the same visit, each with its own log and its own pair of grades.

Nothing is surveyed until the system has been numbered. Covers are lifted across the site and each chamber is given a node reference, starting at the lowest point — usually the chamber inside the boundary where the private drain meets the public sewer. Each length of pipe between two nodes is a run, and a run is the unit everything else is reported in: one chainage log, one pair of grades, one recommendation.

Depth to invert is measured at every node with a rod, cover levels are taken with a level and staff where the drawings will need them, and the direction of flow at each chamber is recorded from the benching rather than assumed from the plan. A chamber with three inlets and one outlet is drawn that way even when the drawing in the file shows two.

The chainage log and the clockLog sheet

The log is a list of chainages and what was at each. Chainage is taken off the cable, not estimated from the picture, and every entry carries a clock reference so the defect can be found on a second visit: 12 o’clock is the crown, 3 and 9 the springings, 6 the invert. A fracture at 4.20 m at 2 o’clock is a different repair from the same fracture at 6 o’clock, because one is above the water line and one is under it.

Water level is logged as a percentage of bore wherever it is standing, and the camera is stopped and the picture held at every coded feature so the still is square on rather than a frame grabbed in passing. Where a lateral comes in, the connection is logged as a feature too: its chainage, its clock position and whether it is cut in square or hammered through.

Codes, and the two grades they produceCoding

Features are coded, not described in prose, so that two surveys of the same run five years apart can be compared line by line. Structural codes cover the fabric — cracks, fractures, broken pipe, deformation, displaced and open joints. Service codes cover what is in the way — settled and attached deposits, roots, obstructions, water level, infiltration. The codes carry a letter string and the grade carries the number.

Each run therefore leaves the site with two grades, not one: a structural grade and a service grade, each the peak of its own set of features. Both are reported, and both are reported with the mean alongside them, because a single worst feature and forty metres of light fault are different jobs that would otherwise share a number.

What the report has to containDeliverable

A report that cannot be used by a third party is not finished. The sheets below are what leaves this office on a survey job, and an insurer, a water company or a buyer’s surveyor should be able to work from them without ringing up to ask what was meant.

Contents of a survey report issued from this office
Node plan	Chambers, runs, gullies, direction of flow, the connection to the public sewer, and the node references used everywhere else in the report.
Chainage log	One row per feature: chainage, code, clock reference, water level, and the still number.
Stills	One image per coded feature with the chainage on the frame, plus the start and finish of each run.
Grade summary	Peak and mean, structural and service, per run.
Level sheet	Cover and invert levels at each node, and the depth measured rather than scaled.
Recommendation	The remedy for each graded run, and what has to happen before it can be priced.
Limitations	Every length not surveyed, with the reason and the chainage at which the camera stopped.

What a survey cannot tell youLimits

A camera sees the inside of a pipe, and three things fall outside that.

Not seen Outside the bore
Bedding, backfill, the outside face of a joint and a void that has not yet broken in are all out of the picture.
Not reached Past the blockage
Nothing beyond an obstruction is reported at all, however short the remaining length looks on the plan.
Not measured Depth and position
A depth comes from a sonde or a rod at the node, not from the picture, and it goes on the level sheet.

Two things change what the camera can reach: jetting, which clears deposits and sometimes the obstruction with them, and flow control, which stops a live run long enough to see the invert. Both are planned before the visit, because both change how long the visit takes and whether the building can stay in use while it happens.