Pyrite Pressure on foundation wall

[Francis Malenfant]

Hello, I am a structural engineer. I need to conduct an assessment on a wall that has undergone a deformation of approximately 50 mm at mid-height. There are several cracks in the center, suggesting a weakness in flexion. The back of the wall is composed of a fill made of sand and rock containing pyrite (see sketch).

1- I am wondering if this deformation could be caused by additional pressure exerted by the pyrite on the wall. I am also questioning whether this could have generated additional uplift pressure beneath the footing.

2- If so, is there a way to calculate the additional pressure caused by the pyrite?

3- I plan to remove the sand behind the wall to reduce the earth pressure and stabilize the wall. However, this would expose the footing to more oxygen. Do you think the gravitational load would be sufficient to prevent the uplift of the footing?

4 - Finally, is it possible to stop the expansion of the pyrite and thus prevent additional pressure on the wall and the footing?

I know this is a lot of questions, but I hope you can help me. Thank you in advance!

 

[EireChch]

50mm is a lot and Pyrite swelling can lead to huge displacements. In the UK/Ireland there have been major issues with blocks that were made with aggregate that contained pyrite. Whole houses are being knocked down because the pyrite is swelling and the blocks are crumbling.

So yes it can lead to huge swelling pressures.

In saying that, it is typically worse when its soil that has pyrite in it. If you have rock that contains pyrite but is strong then I cant see it swelling that much. If its a very soft rock like mudstone, that is more akin to a stiff CLAY then maybe it could be swelling.

So the question is, what rock do you have, how strong is it, how weathered is it etc.

Also, when was the displacement observed, how long after construction. As you alluded to pyrite only swells when there is a change in environmental factors such as exposure to air and water.

If the wall is sitting there for a few years with no issues and then one year it starts to displace, you have to ask what the change in environment was.

There is no way to quantify the magnitude of pyrite swelling. The way to stop it is to remove the sand backfill and expose it to the atmosphere and leave it for a few months. You could also put a void former along the vertical stem of the wall to absorb any pressures.

Re your footing, if it has not swelled by not then I don't think it will. Just don't expose the rock in the vicinity of the footing to the atmosphere. Leave the bottom 0.5m of sand in place. Could even pour a thin 100mm of concrete above your 0.5m thickness to make it a bot more air tight.