In the US, I think "bracing block" would be a more common phrase for this, although usage does vary somewhat by region, & also by the particular application involved: i.e., a bridge builder may use a different term than a trenching specialist contractor would use.
In this context - construction, especially of foundations - "arriostramiento" would mean bracing (noun); "arriostrante" would mean bracing (adj.). A "bracing block" would be a block, usually of concrete, designed to resist usually lateral forces. It could be temporary - say for jacking a bridge component into place - or permanent, say as a component of a foundation system.
The "Vocabulario de mecánica de suelos" lists "riostra (Es; Ch, Ec)" as an equivalent for English "brace." (As far as I know, this useful dictionary (English, Spanish, Portuguese, & French) is not available in digital form, & is nowhere to be found on the internet.)
See also a discussion on forumwordreference.com where "bracing block" is given as an English equivalent for "Losa arriostrante", although with uncertainty indicated: http://forum.wordreference.com/threads/cimentación-superfici...