Drainage Dropshafts & Drainage Backdrops
A drop shaft is a vertical length of pipe work taking a drain or pipe down to the level of a deeper drain, pipe or sewer, given that most of the early systems were hand dug it is no surprise that they took the path of least resistance. So a drain running from a property would be installed at the minimum workable depth before passing through a drop shaft which would take the waste or storm water down to the level of the main line drainage.
The image to the right shows a vertical drainage line as it enters a drop shaft and then discharges into a main sewer through a lateral junction, though it is not uncommon to find drop shafts discharging directly into the crown of a sewer dependant on its size, construction and age.
If the drop shaft is prior to a particularly deep sewer several properties at a time would be connected so as to save on pipe work, labour and unnecessary deep excavation works, the top of the drop shaft would be either a 90 degree bend as shown or a square junction would be installed with the shaft being extended up to surface level for future access.
These shafts are not confined to connections onto main sewers and they are often found around domestic properties, a typical property with a cellar will have a deep drain run from a cellar gully running directly into a main sewer, all the ground level gullies and toilet runs would then be installed at a minimum depth before entering the deeper drainage via a shaft within the boundaries of the property.
Its not uncommon to find drop shafts on properties without cellars if the main line drainage from the house is a metre or so deep waste and storm water gullies at ground level will often outfall directly into a small shaft before discharging into the main run as shown in the image to the right.
Problems Associated With Drainage Dropshafts
From a drain clearing point of view they are restrictive to drain rods as you typically have a 90 degree bend at the top and base of the shaft, drain jetters and mechanical rods however should pass through the shaft easily enough.
Structurally drop shafts are prone to the usual defects such as root ingress, undue loading from vehicular movement and its common to find the back of the top bend broken and damaged from somebody attempting to get a set of drain rods around that first bend.
If the shaft is prior to the connection onto a main sewer beneath a highway or road this can cause major problems, the sheer weight of the shaft along with vehicle movement and any water loss or soil erosion can cause the shaft to drop, often leaving the rest bend at the base of the shaft lower than the level of the sewer.
Repairing Defective Dropshafts
If a shaft is accessible from a manhole and the problem is due to root ingress of fracturing then there are a couple of drain relining methods available, if however there has been severe movement then excavation is often the only answer.
A backdrop is a drop shaft situated prior to a manhole or chamber the main difference from a standard drop shaft being the telescopic arm that continues through into the chamber wall so as to allow access back into the incoming pipe work as shown in the drawing to the right.
This type of construction allows the higher level drainage to be installed with minimum fall before being connected to the lower level system, however unlike the drop shafts shown at the top of the page all lines are accessible for clearing, surveying and future maintenance.
You do occasionally find a standard or blind drop shafts without the telescopic prior to chambers which are bloody annoying if you are trying to survey or clear a system.
Defective Manhole Backdrops
Just like any drain system section these shafts can be prone to tree root ingress, if there is too much fall on the incoming pipe work or the shaft becomes blocked for some reason then paper and solids can over shoot the shaft and enter the chamber via the telescopic. This usually results in a build up of waste in the base of the manhole which in time falls into the channel pipe causing a blockage.
For this reason the telescopics are sometimes fitted with a cap or sealing plate to prevent any over shooting of waste and you often find the telecopic to be constructed from an upward facing bend so that any over shooting waste matter falls back into the shaft.
The backdrop shaft becomes part of the rigid construction of the chamber as it is anchored to the brick work by the telescopic arm and the rest bend at its base. For this reason the most common defect we find on these system is due to settlement of the pipe work that enters the back drop itself.
Typically a large and deep excavation would have been undertaken to install the chamber which means that the last couple of higher level pipes are sat on infill, if this infill was not compacted properly some settlement can occur and because the shaft is rigid the pipe that enters the junction at the top is the first to drop as shown in the image to the left.
The resulting step against the flow of the system causes paper and solids to build up and block the system, the water loss into the sub-soil can undermine the remainder of the shaft and the manhole itself.
Many years of steady and constant water loss can have a serious affect on the brickwork that forms the manhole, washing out the sand a cement mortar and causing walls to bow and move, depending on the ground type the water loss can undermine the chamber causing it to rotate and settle usually damaging the interceptor or pipe work on the outlet.
Repairing Defective Backdrops
If the shaft has root ingress or fractures then it can usually be relined, if the structural damage is severe and the integrity of the the construction of the chamber itself is in question then a full excavation would be required.
Greatly frowned upon these days, this is where you basically construct a backdrop or drop shaft within an existing manhole or chamber, there is a good argument for them in the right circumstance and good reason for not allowing them on most occasions.
You have just installed 10mtrs of 100mm pipe work at a depth of 400mm and the main line drainage is 2mtrs deep and you need to get the two connected do you; a) hand dig down the full depth, install a junction into the main line drainage and bring up a new shaft, or b) knock a hole through the existing chamber and poke your new pipe through it ?. It is of course human nature or at least drainman nature to take the easy option and knock lumps out of an existing chamber as opposed to digging a 2mtrs deep hole.
The image to the right shows a 100mm internal drop shaft installed in a concrete sectional chamber, the chamber is more that big enough to take the shaft without affecting access however a hole has been punched through the side of the manhole for the pipe work and it has not been sealed afterwards. The bend at the base of the shaft is also considerably higher that the main line drainage and there are no brackets holding the shaft in place, as it is formed from a push fit plastic pipe the whole lot could quite easily become detached and fall into the channel causing a major blockage.
When done correctly in a big enough chamber these things work, the problem is they were seldom installed correctly and they can cause many problems. The main one is safety, modern specifications require and rightly so that if it is necessary to enter a chamber their should be sufficient clearance for the guy and his escape kit to get out quickly if needed, the older manholes are in general tight enough without some clown sticking a 150mm drop shaft in the 600 x 450 neck of the chamber.
Then there is the affect of the works on the manhole itself as usual the lump hammer got preference over the core drill and great chunks of brickwork would go missing, usually down the outlet pipe. The shaft itself was seldom fitted to the brick work with brackets and the bend at the base of the shaft if there was one had nothing in common with the flow of the main line channel and would spray waste and solids indiscriminately over the sand and cement benching, often eroding it in the process.
I have had permission to install internal shafts in recent history, it was a 5mtr deep manhole with a shelf at 2mtrs where it opened up into a good size chamber, we were therefore allowed to install an internal shaft away from the neck of the chamber as long as it was a ductile iron pipe with its outlet entering the flow of the main line drainage at the correct level and angle.
Wall brackets were specified and a stopper in the telescopic were also required with the stopper being anchored to the chamber via a fixed chain. The alternative was a deep dig in a narrow lane in running sand so common sense prevailed.
Having said that i have tried my luck on a couple of occasions since and had a straight ` We do not allow internal shafts. Full Stop !` thrown back at me. It just comes down to who your asking and in what area of the country you are in.
The image to the right an example of how internal shafts hinder access into manholes and chambers, the chamber was poorly installed in the first place as it is way to narrow for its depth and the step irons are installed in opposite corners from each other.
Then presumably at a later date somebody turns up and sticks an internal dropper bang in the middle of the shaft restricting access to such a degree that even a drainman with the figure of a racing snake would struggle to get in there. You will also note the lack of brackets holding the thing in place once again.