This technical article provides additional guidance on drainage and sanitation. It is important that all workmanship carried out during construction is completed in accordance with the relevant tolerances.
In recent years the principle failures for both foul and storm water drainage are inadequate falls in underground pipework.
The main issue appears to be with 100mm polythene pipes. For example, Diagram 9 of Approved Document H supporting the England and Wales Building Regulations, indicates that the minimum permissible fall for 100mm pipework is 1:80, provided that there are adequate flow rates above 6.3 litres/second. This would normally be achieved by at least 1 WC on the system.
In practical terms achieving a constant gradient of 1:80 with 100mm pipes is very difficult. This is not helped by the popularity of using 6m long plastic pipe which, because of its length, can acquire a bend or set during long-term storage either on site or in builders’ merchants. It’s not uncommon when inspecting 6m long sections of pipework laid to minimum falls to have an acceptable fall at one end of the individual pipe yet a back-fall at the other end.
The introduction of plastic pipe work has inevitably led to a skill set loss of the groundworker. Common indications of likely poorly laid pipework are operatives laying pipes between inspection chamber positions without using a taught line to ensure that the pipes are laid straight and to a constant gradient between inspection chambers.
The invert levels of the inspection chamber should be determined by a setting out engineer or other trained operative using a ‘Dumpy’ or ‘Quickset Level’. These levels are determined by ascertaining the total difference in levels from the highest point of the pipework at the start of the run to the lowest level where the pipe run connects to the public sewer or the main pipe run for the site. This should then be used to calculate the falls between inspection chambers to divide the total available fall to achieve a constant gradient from start to finish.
It is not uncommon to observe some sections of a pipe run between inspections chambers with much greater falls than that achieved between other sections. This inevitably results in some sections toward the end of the run with less than a minimum fall where adequate falls were actually achievable. To put it another way, the available falls have not been divided up evenly between changes of direction or junctions in the pipework.
Areas relating to above ground drainage that should also be checked are:
Every care was taken to ensure the information in this article was correct at the time of publication. Guidance provided does not replace the reader’s professional judgement and any construction project should comply with the relevant Building Regulations or applicable technical standards. For the most up to date Premier Guarantee technical guidance please refer to your Risk Management Surveyor and the latest version of the Premier Guarantee Technical Manual.