The researcher Dagmar Říhová http://mollusca.sav.sk/malacology/Rihova.htm made some experiments about land shell degradation.

Yes, look for  Rihova's work. The Czech workers have done good work in this area.

Like others answering, I am not clear what you mean by crippling. Although some shells can start to decay when the snail is still alive, this is usually because other snails have been chewing them. In long-lived species, the periostracum often wears off in older specimens, again dependent on the environment. After death, the shells decay at different rates in different conditions, and there are differences between species too. In soils with much calcium and high pH, the calcareous lamellar layer is usually well-preserved, often for thousands or more years as (sub-)fossils. In acid soils the shell decays rapidly, and in some thin-shelled species the periostracum lasts longer than the lamellar layer (eg vitrinids) ( this is based on Rihova's work).

For me, the reason for interest here is that when doing an inventory of a site, one can find empty shells of species which I don't find alive. Can I include them in an inventory? This is a hot topic for some, eg Paolo Albano. I usually include shells which appear to be "fresh", i.e. to retain most of the periostracum, but in calcium rich sites there are undoubtedly some problems. I do not think they are very severe, but I think others might disagree!

 Robert Cameron