Holdfast Training Services Trial Blog

I learnt a few things about drainage last week.

Firstly – When Richard Farmer said that you need to plan your drainage and external levels before you build the structure, he was actually right.

Secondly – No-one in the office knows anything about designing drainage layouts other than reciting age old rules of thumb.

Thirdly – Never admit to having done a module on something unless you’re prepared for it to come your way.

Finally – Never trust Thames Water.

The problem

The new office at Chambers Wharf consists of a three storey stack of 96 cabins. The internal layout for this was practically written up on a fag packet for approvals purposes and not really looked at since.  No designs were ever completed for the outside of the offices so the drainage runs, street furniture, steps and ramps, and access control now need to be shoehorned into a small gap between the office and the hoarding. Rather than working from 1 level, these features must tie in between external footpath level, internal FFL, and new office threshold level.  The office must open on 31 October or 200 people will have nowhere to work.

We have also just learnt that the manhole that the site has been pumping it’s waste into for the last 2 years, and for which we have a discharge consent from Thames Water, is not, and never has been connected to the main sewer, but has been flowing back into site through an abandoned connection.

With less than a month to opening the office, everyone has just decided that they have a crucial requirement for the outside of the office (including bike racks, money gates, artist studio, and parking), and that now is a reasonable time to request it, although they don’t quite have all the details yet.

The solution

It seems impossible to keep everyone happy but turning to my trusty copy of Civil Engineering Procedure and the Joint Board of Moderators guidance (2009), it seems that I have a chance to use this to “demonstrate an ability to cope with the uncertainties of a multitude of factors making up the design brief.”

I’ve prioritised the things that we need to provide for office opening. Namely access and drainage. All of the other stuff is important but we can still occupy the office without it.

The drainage pipe had already been specified (by rule of thumb) as 150mm. Being a competent engineer I knew that using a rule of thumb was risky so I spent a morning looking into the Building Regs and speaking to the cabin manufacturers, and calculated the required pipe diameter.  Turns out we need a 150mm pipe.  Unfortunately there is no plan for greywater recycling. This would have greatly reduced the load on the system.

I then designed the external drainage runs (see attached pdf) and access points in accordance with the guidance in Part H, and the surrounding floor levels will be set to suit. Some compromises have had to be made. Most notable that in some areas the pipe will be laid with less than the recommended backfill, and in others it will be on the surface and boxed in. But that’s what you get for not thinking about it beforehand.

After a number of investigations, Thames Water have finally accepted that it’s their fault we’ve been draining our poo into an abandoned manhole and have agreed to make a connection, but the timescale is uncertain. In the meantime we will be installing a new manhole inside the site and if they can’t meet our timescales then we can install a macerator pump within the manhole to pump our sewage around the site to an alternative discharge point.

Now I just need to hope the showers don’t fill with poo before I go to Phase 3.

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