Re-activation of consolidation in peat 5

[Mudman57]

I'm working on an electrical substation on a river estuary site in England. The ground investigation has found that the site is underlain by around 5m of pulverized fuel ash fill, over about 6m of alluvial soils, the top 3m of which is fibrous peat. Below the alluvium are river gravels and chalk. Water level appears to be below the peat layer, although this was only observed during drilling (the Client wouldn't pay for long term monitoring!)

Some background research suggests that the PFA fill was placed around the time of WWII, when an adjacent power station was constructed. The filling was intended to protect against flooding.

The PFA fill, even though it is lightweight, will obviously have caused consolidation in the peat, but due to the length of time since its placement, one would hope that most or all of the primary consolidation is complete, and a good deal of the secondary/creep.

The main substation structures are to be piled - i.e. the heavy transformers and switchroom. However the idea is that the smaller, lightweight ancillary structures are constructed on shallow foundations in the PFA (which the driller records as cemented, and had high SPT N values). Now this wouldn't give me too much concern, but for the fact it is also proposed to raise levels generally on the site by around 1.5-2m. I'm worried that this will set off new primary consolidation in the peat layer, with the result that the shallow founded structures could move excessively, not to mention of course that the external ground could settle away from the piled parts of the facility!

I have suggested the use of lightweight fill (further PFA for example) to mitigate this, but would welcome any thoughts on this from others.

I haven't posted before but have seen some very helpful posts and thought why not? I'm pretty much the only geotechnical engineer in my company and haven't got too much experience in dealing with peat. I know enough to be worried though!

 

[eric1037]

Mudman57:

You are correct to be concerned about this. Additional fill on the site will start up primary consolidation again. Depending on the construction schedule, you may consider surcharging (AKA preloading) the site before driving piling. You should also be concerned about downdrag on the piles due to the consolidation.

The amount and rate of consolidation will depend on the properties of the peat.

 

[SlideRuleEra]

Mudman57 - You may also run into problems with the fly ash, despite what the driller's report says. Unstabilized fly ash, from modern electrostatic precipitators, is thixotropic (maybe not the case from WWII vintage fly ash). Probably worth checking into. See comments from BigH and me at this thread158-128849

http://www.SlideRuleEra.net

 

[Ron]

Run some additional consolidation tests on the peat, including maybe more than one reload cycle to check the response of the peat.

 

[oldestguy]

Don't forget to install setlement platforms before any filling work. Reference elevation readings to stable BM and read before filling, while filling and periodically thereafter. This is the "proof of the pudding".

Plot on semi-log chart (log of time). Project out to several cycles to see if you need surcharge work.

I usually go to 27 years to predict total settlement for planned fill only.

 

[BigH]

With the amount of time that the 'peat' has undergone settlement means that it is quite likely into several cycles of secondary settlement - still, 1.5 to 2 m of fill atop the existing material will lead undoubtedly to additional primary and subsequent seconddary consolidation (in peat the secondary can be significant). If there was no fill added, you might be able to 'get away' with the use of shallow foundations - say a grillage type with no overlapping bulbs. But this is really thrown for a loop with the added areal upfill; if the settlement is likely to be uniform, then it might not make any 'in use' problems for the light structures, but if not, then unless the foundation system is rigid, you will have some issues. The light cementation of the flyash may be broken down under the added loads and may not, therefore, act in practice. I doubt that additional consolidation testing and the like will change the basic premise - the added fill will lead to added settlements overall.
Downdrag on piles has undergone some revisions over the years from the original concept to one of the neutral plane (do a search on Fellenius) who has several papers on this. This concept, in my view, is that if the pile tip can move (i.e., it isn't supported on rock), then added loads aren't nearly as large. On rock, see some of the classic papers written in the late 60s of very long piling to rock. This is reinforced in Poulos and Davis' Pile book.