"Positive administrative silence" is a literal translation whose meaning is understood by "insiders", but I have never been able to find this expression used in any true Anglo-American legal sources (US or UK government or court websites, etc.) It's always used in translations from Spanish or in English texts that explain what "administrative silence" is by rendering it literally. For this reason, depending on the context, I prefer to translate "silencio administrativo positivo" as "constructive grant/consent/approval, etc." and "silencio administrativo negativo" as "constructive rejection/refusal, etc.", since this is really what the expressions mean.

You may certainly word this otherwise, but (loosely) your translation might be expressed as follows:

Una vez vencido el plazo de contestación que las normas especiales establezcan para los expedientes de autorización, se entenderá otorgado el mismo por silencio administrativo positivo.

Once the legally-established term for responding to applications for authorization has expired, the Administration's failure to respond shall constitute a constructive grant of authorization.

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Note added at 1 hr (2011-04-05 16:58:18 GMT)
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Perhaps "implied grant of authorization", "tacit grant of authorization" or "apparent grant of authorization" would also work in your text.

In other respects, here are some examples in which a failure to respond or act was deemed as “constructive authorization”:

Failure by the court to respond in time would be construed as an act of constructive authorization. The judicial bypass requires a sworn statement from the ...
housemajority.org/print.php?id=37&t=spon26

The cable provider's negligence in not correctly disconnecting the service operates as constructive authorization for the former customer to ...
www.dslreports.com/.../r17930237-Will-I-get-some-channels-i...

... the case law emerges as the claimant's effort to prove either value to the landlord or the landlord's actual or constructive authorization in order to ...
books.google.com/books?isbn=0735505462...

The Court did not address the issue of "constructive" authorization -- that is, authorization as a matter of law in certain circumstances, whether or not ...
en.wikipedia.org/.../Quanta_Computer,_Inc._v._LG_Electronics,_Inc.

believe that an abbreviated flight plan must be expressly authorized if the facts demonstrate an implied or constructive authorization, ...
forums.jetcareers.com/pilot.../114219-procedure-turn-required-4.html