Drainage Investigation Methods
What Lie`s Beneath
There are many reasons for having drainage systems investigated such as home buyer surveys, investigating why a system regularly blocks, water ingress to cellar or sub-floor areas, subsidence investigations or you may need to know where a system runs before you build your new extension. Some of the methods used are basic and have been used for hundreds of years whilst modern drain camera systems let you view the inside of the pipe work for a detailed inspection.
If you have a reoccurring problem with the section of drain from your front boundary line through to the main sewer and your contractor is retrieving tree roots or broken pipe work then you will require a CCTV Drain Survey, this will tell you the extent of any damage that may have occurred and the best method of repair available. If you have waste water ingress half way up a cellar wall adjacent to a waste gully then simple dye testing may well tell you all you need to know, likewise if you need to know the line of a main line sewer before you install your extension foundations a simple sonar trace will confirm the route and depth of the drain in question.
What's The Question ?
I looked at a property some years ago for a chap buying a house, i was recommended to this guy by a structural engineer who i had carried out subsidence investigation works for in the past, the survey showed misaligned joints and fracturing particularly on the branch lines downstream of the gullies and soil vent pipe adjacent to the gable end of the building. I therefore recommended several repairs on the basis that there would be water loss to the sub-soil local to the foundations, which in turn would cause further damage to the drainage system and possibly to the foundations themselves. The client then got a second opinion and was told that the system was unlikely to block and that an annual clean by high pressure water jetting would suffice.
Now in fairness to the second drainage contractor i don`t know what they were asked to look at, if like so many contractors they pushed the camera up the main line without checking the branch lines then yes they may well have come to the conclusion that the system was sound and unlikely to block, and to my knowledge it never did, however i do know i was back there seven years later renewing the drainage for a piling contractor after extensive underpinning works had been carried out on the gable end.
The client had of course done the right thing in getting a second opinion, the second contractor was either sticking the knife in as happens in any trade and profession with regard to the integrity of another contractors report, or they were clueless when it came to surveying or quite possibly they were asked the simple question will this drainage system block on a regular basis. What ever the question was they made the right noises and temporarily saved the client a couple of grand which he probably could have negotiated of the price of the property.
Whether this couple of grand was later paid out as his policy excess on his insurance (if he was insured) i do not know, but with hindsight the upheaval and disturbance of structural underpinning not to mention the affect of having a history of subsidence on a property and how this can affect any future sale could easily have been avoided by sticking to the golden rule, ask three contractors the same question and if works are required get three like for like quotes. This is so easy with regard to CCTV Surveys as the results are on video or disc so get a second or even third opinion, you do not have to get the system re-surveyed unless of course you chose a real lame duck of a contractor to undertake the survey in the first place.
A CCTV Survey that finds broken & misaligned pipe work can not be faulted, but i have also surveyed systems with sound alignment of joints and no obvious defects that when excavated were found to have leaked like a sieve with great chasms found under each joint, the result of years of soil erosion.The following are several ways that drainage systems can be and are surveyed and tested, once again i can not say that there is a definitive way to survey or test a system, but there are processes that you can follow and combine.
Drain Flush Testing
As simple as it sounds you flush a toilet, run a tap or pour a bucket of water down a gully and then stand at the manhole (if there is one) and wait for the water to come through, not overly scientific is it but people buy houses on the back of such investigations. If the water does not come through is the system blocked, are there two separate systems or is that storm gully on a soak-away ?, if the water does come through is it the full amount you put in or are you losing 20% to the sub-soil on the way. If you want to really push the boat out you can put toilet paper in the toilet before its flushed too see if it come through, just to add that extra bit of tension to proceedings.
One of the simplest and most efficient methods of investigation when your dealing with water ingress to cellars and sub-floor areas, methodically dye test different sections of the system using several colours to identify the problem section. The drain dye comes in powder and tablet form, it is environmentally friendly and will not stain bathroom fittings.
Another simple tool when investigating water ingress to cellars or sub-floors by taking a sample for analysis you can determine if your dealing with foul water, potable water or ground/storm water, if the results are not definitive they will normally exclude at least one of the above options which allows you to concentrate your investigations in the right area.
You will have to find a local laboratory to do the analysis and you could try your local water company who if your lucky may even do the test for you, if not you will have to collect a sample of water in a steralised bottle and provide it to the lab within 24 hours making sure you keep it out of direct sunlight, cost wise you should be looking at around £70 to £90.
Water or Air Pressure Testing
Bag testing or water testing is not only used on new build drainage but as an investigation tool, many of the older systems would probably not have held water under pressure when they were put in, if the system does leak under pressure is it leaking from the crown of the pipe or the base ?.
Not the most efficient way of testing a system but a bloody good way of generating work i suspect.
A typical water test will involve installing a bung at one end of a line and then filling the upstream section with water usually measuring the water level from within an upstream manhole, this of course can be flawed if you do not bung off any incoming branch lines or the chamber itself is not water tight.
Another method is to install a bung on a line entering a manhole and then filling it with water until the level rises on the lowest point of the system whether that be an external gully or an internal WC.
A pressure test involves installing a bung at each end of a run and pumping air through one of the bungs with an air line, the air line is connected to a gauge that has a small amount of water sitting in a u bend, as the pressure builds the water rises and holds in position if the system does not leak.
Drain Camera Inspection Systems
Another subject that will fill pages further on in the site but basically a visual survey of the inside of the system , there really is no hiding place any more. Ideal for locating those defects that lead to blockages such as root ingress, broken and misaligned pipe work and generally testing old and new drainage systems.
Sonar Drain Tracing
Surely the holy grail of drain tools for any man who has hand dug an eight foot deep hole only to find he`s missed the pipe by three foot. You push a transmitter down the system and follow the signal above ground with a receiver, this will not tell you the condition of a system but it will tell your where and how deep it is, ideal for pin pointing defects found in camera surveys prior to excavation works, for locating buried chambers, septic tanks and your CCTV Drain Camera when it gets stuck.
Nothing to do with CSI Miami this is a tool that in layman's terms scans a ring of light onto the internal wall of the drain and software is then used to measure any deformation of the pipe. Increasingly called for as newer materials take the place of the traditional clay and concrete pipes which show obvious defects such as cracking and fractures when under pressure, the powers that be now set a tolerance for deformation of new sewer systems made of modern flexible materials and this is the tool of choice for checking the systems prior to hand over.
Ground Penetrating Radar
Increasing used to locate drains and services from ground level, legend has it that somebody developed a unit that travels down the drainage system and gives readings on voids and soil erosion around pipe work and joints, a clever tool if it is out there and of particular use to pipe bursting contractors i would have thought.