Holdfast Training Services Trial Blog

This blog aims to give those heading out on phase 2 another reason to keep annoying people by asking ‘why’.

Civil students might remember John Moran talking about the Specification for the concrete on a bridge being built in Australia in 2014 (I think it was Pete Mackintosh?).  His joke was that the only thing that would see the bridge was a dingo, and that the aesthetics were irrelevant.  I am also with JHG, and may have spotted a dingo of my own.

My thesis is centred on the use of GRP over concrete jacking pipes in medium diameter (1720mm OD) drainage schemes.  Amidst the mad panic of downloading and attempting to speed read every document imaginable off the IHS, I stumbled across the Technical Design Guide from the CPSA (Concrete Pipeline Systems Association).

Interestingly, it states that for design purposes, Water UK recommends a Ks (roughness) value of 0.6mm for storm water, and 1.5mm for foul sewers – irrespective of pipe material.  The Client’s specification on my project (and the Client is a local water authority) states the internal surface roughness of the sewer pipe must have a Colebrook White Roughness Coefficient of 0.01mm or less.  I would understand (sort of) if it was a storm water drainage scheme, but we are constructing a foul sewer.  Fortunately, the GRP pipes we are using do comply with the 0.01mm Ks, but why should they?

The sewer system we are building is in a greenfield site – the land will be progressively developed and expects a population growth of 260,000 over the next 25 years.  The CPSA notes that when there is a small flow, it is unwise to select too large a pipe ‘to allow for possible development’ as it may lead to settling out of solids, long retention periods, blockages and build-up of septicity.  Obviously the 110,000 homes and their 260,000 residents are not going to magically appear when the sewer is completed next year – the demands placed on the sewer will increase gradually.  Perhaps the ‘over-specified’ roughness coefficient is the result of clever design for the entire life cycle of the sewer, with greater emphasis placed on the hydraulic performance of the pipes in its early years to ensure that the risk of blockages etc. is mitigated without the need to install a greater number of smaller diameter pipe networks at an increased cost.

My opinion is that the Client is focussed on delivering sustainable solutions through design.  Maybe it was not a dingo after all.

Have a good Christmas break.