11 April 2011

PIpeline, Lagging and Sand

The latest contribution from James...

Well, the noise from the Tesco pipeline is no longer a problem. They achieved this by
  • extending the pipe lagging up, over and down the other side of Harbour Road (see pictures);
  • replacing gravel with sand.
The smaller sand particles can be lifted over the roads at lower flow rates than the coarser gravel, which reduces the noise generated. And the lagging - although actually loft insulation designed to stop the flow of heat, not sound - seems to muffle what noise remains. The overall noise level is now so low that I did not believe the pumps were working until I saw the slurry flowing onto the site. This must be a great relief to the residents of Trevelyan Road; but what are the implications for the project?

The first implication is that Tesco and EDDC have lost all credibility in predicting the outcomes of this project. In July last year we pointed out that pumping gravel would be much noisier than they predicted; but we were ignored. Then, in March this year, we pointed out to EDDC that the pipe was unlagged in the noisiest section going over Harbour Road - but this was ignored, also. So, instead of lagging the pipe on the ground, they took time out for a photo session and then had to do the lagging in mid-air (see pictures).

The next problem is getting hold of the sand. The gravel came from the Isle of Wight every 24 hours or so. The sand has to come from Wales, requiring 36 hours for the round trip. So, the infill time will be extended by 50%. As well as extending the nuisance time for the town, this will cost Tesco a fortune in extra project time. Isn't that a shame.

The third problem is the structural implications of using sand instead of gravel to raise the site level. Will a heap of sand be as stable under flood conditions as a heap of gravel ? No doubt Tesco and/or EDDC will tell us that it will; but they made such a mess of the noise issue that nothing they say can now be left unchecked.

Our soils consultant needs to see the particle size analysis of the sand before commenting on stability. We have asked for this data, and will report back when we have more information.


  1. The sand also retains more of the salty water than the sand and gravel mix. I asked the Environment Agency what this would do to the ground water or the impact of surface water runoff. I thought they would tell me that this had been covered. Apparently not! They are investigating. Hope to have a full response later this week ( not holding my breath ).

  2. Still no news from the Environment Agency, perhaps they don't want to answer my query.

    From Geotechnics who are consulting on the fill material, it is a Class 1B material, which is a Silty Fine Sand ( that is why it is so much less noisy in the pipeline - as featured on BBC Spotlight ).

    From what I understand, Class 1B material has a California Bearing Ratio (CBR) of 5%. This compares with Sandy gravel which is about 15% ( did the original material sound like sandy gravel? ).

    How this affects what you can build on it without pile driving I don't know. Would it be more expensive to build on than the gravel and how would that affect the proportion of affordible housing ( what ever that is )?