public works, according to the Penguin Civil engineering dictionary, is civil engineering construction, so it is clearly a possiiblity to say public works. However, texts oten talk about "obra civil" and "obra publica" (sorry, no accents), clearly differentiating between the two. (Please see some Google examples), so personally I would not be happy translating "obra civil" by public works.

This page gives you an idea of what civil work covers:

http://www.babcock.co.uk/index.cfm/recordid.27/operatingco.2...

And again:

Simpson's provide all aspects of civil engineering works, which include:construction, roadworks, groundworks,
excavations, water mains, drainage, sewers, water treatment works, reservoir refurbishment.

http://www.simpsoncivil.co.uk/


A construction project (obra) usually always involves some civil engineering work (probably drainage and sewerage in your case), as well as the building work itself.

According to the same dictionary cited above, building (definition 2) means the act of creating a building

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Note added at 16 hrs (2007-03-01 09:35:20 GMT)
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Found a good definition for public works from the Wiley dictionary of Civil Engineering: works owned by the members of a community for their collective benefit and enjoyment, constructed and maintained from tax revenues.