Cordesman:
''…It was not until 2008, however, that more specific claims about silo construction, hardened sites, locations, and imagery, began to surface in public…Two different blogs, gemint.com and arms controlwonk.com, wrote about the same time in early 2008 of a possible silo missile base near Tabriz
If the Iranian military official was quoted correctly, it would mean that work would have started on these silos around 1996. In fact, perhaps the first public account of Iranian interest in such came in 1993 when then Iranian Defense Minister Foruzandeh led an economic delegation to the DPRK and may have discussed the possibility of joint production of the Nodong missile and construction of underground missile shelters at 18 sites across Iran.
Construction may have been underway by 1998 when Israeli Prime Minister Netanyahu said Iran is “building an enormous infrastructure [including] hardened missiles silos,” which can house Iranian ballistic missiles and protect them against U.S. or Israeli preemptive strikes.
…The next time there was any apparent mention of Iranian missile silos came in 2006. Former head of Israel’s missile defense agency and missile engineer Uzi Rubin wrote that, “there are indications they [Iranians] are now constructing fixed silo-like hardened sites to make their missiles even more survivable.” Rubin mentioned this in the context of Iran’s MRBMs.
…Based on these previous assessments and using imagery obtained through the Library of Congress, this report locates what appear to be those missile silos in northwest Iran near Tabriz. …Imagery obtained through the Library of Congress dated around 2000 and examining additional imagery dated about once every year or two at this location shows what appears to be gradual construction of the second silo site referenced by O’Connor. This second silo pair may have been completed by 2011 from looking at that imagery…
Since then, only a few other experts have mentioned these silos until the Iranian videos and interviews in June 2011. To date, there does not appear to have been any official U.S. public assessment or acknowledgment of these silos. It is unclear what the absence of any such assessment might mean.
To the extent such silos are operational, they offer the potential advantage of allowing Iran to emplace substantially larger missiles as it develops them. While its current MRBMs are fully road-mobile - both the Sejjil and the Shahab-3 and its variants can be launched from movable transporter-erector-launchers - later missiles may be in the range of 40-80 tons (from around 20 for the Sejjil) and hence will not be able to use current TELs. Mobile launchers are generally considered to be more secure than missiles in silos, so constructing them now only makes sense if Iran seeks to streamline construction techniques for later use.
While silos might be a vulnerability in a conflict with the US in the near future, more advanced designs could serve as the home of any future Iranian ICBM force if Iran had sufficient numbers to make a first strike uncertain. They also present added risks from any missile system if Iran chooses to act on the basis of launch on warning or launch under attack.
Some recent reports indicate that Iran has approximately 4-6 active missiles for every TEL (this ratio does not hold for Sejjils), with Iran expected to build more launchers as it produces additional missiles to avoid bottlenecks in its firing rate. In a conflict in which Iran seeks to use its SRBMs, MRBMs, and IRBMs, it will likely rely exclusively on mobile TELs to avoid US and GCC airstrikes and maintain its missile deterrent.''
http://csis.org/files/publication/141007_Iran_Rocket_Missile_forces.pdf