bored micropiles in calcareous rock

[AK92]

Hi,

I am designing foundations for a structure using bored micropiles which extend to the rock about 30m deep. The borehole log showed hard residual soils right at the top with significant softening at around 10m depth (from N>100 blows to N=5) which continued up to the rock. We suspected that it was a clay infill in a calcareous rock formation, and when we tested the rock, the carbonate content was 12%. It doesn't help that there are hydrochloric acid tanks situated right at the site, and the pH of soil samples near the surface are about 3.5. Since acid reacts readily with carbonate, I can only assume that there is a risk of the rock around the pile getting corroded by the acid as the boring might create flow pathways to the rock.

What precautions should I be taking when designing these foundations?

 

[Ron]

Using micropiles for this application will require that you consider the corrosion potential of the soil on your micropiles and also the effect of the acidic soils on the calcareous materials.

A 12% calcium carbonate content is quite a bit lower than I would expect for a calcareous formation. Any chance you have a gypsum or dolomite formation?

 

[AK92]

Hi Ron,

Thanks for your reply and sorry for coming back so late, I was so incredibly busy with work these few weeks.
The rock is the typical sandstone we find in tropical countries, and I don't think that it is from a gypsum or dolomite formation.

I believe it is highly likely that the acidic soils at the surface are caused by occasional leaks in the HCl tank.

I have discussed this matter with my supervisor, we agreed that the capacity of the rock shouldn't be downgraded, instead we would warn the structural designers to take steps to ensure that any further leakages of the HCl tank are fully contained, and to inform them of the dangers of having the acid leaking to the rock which would lower the capacity of the rock significantly which would have a big negative influence on the geotechnical capacity of the pile.

Regards,
Andrew