>>508415295GBU 57s can dig 60 meters of rock and earth but only 18 meters of reinforced concrete/granite.
Fordo is 80-90m deep in mountainous terrain, but it's important to note that the mountain range Fordo is buried under is mostly comprised of sedimentary rock like limestone and shale which is only about half the hardness of granite. Only the larger mountains in that region like the Alborz mountains have granite cores, but are still mostly comprised of limestone, sandstone, and shale.
A single GBU57 is not going to penetrate 80-90 meters, even sedimentary rock with a lower hardness. That is why they deployed so many. If you look at the satellite images, you can see 3 distinct holes right next to each other.
The first GBU hits and penetrates 50-60 meters, roughly, before exploding, displacing and/or vaporizing sediment, creating a cavity as well as a strong seismic wave which propagates outward. Because granite is so dense, it actually transfers shock very well, meaning even the mountain itself had a granite core, and the bunker was built into it, the shock from the initial explosion would likely transfer to the reinforced concrete, causing stress fractures and disrupting vital systems.
The second GBU follows up in roughly the same location. Since the first one displaced or vaporized sediment, the second can penetrate even deeper; likely up to 70 meters. The detonate this time also displaces/vaporizes sediment, but the shockwave is much more damaging. At this point, if the bunker doesn't have adequate shock-absorbing reinforcements, the load-bearing walls will likely start to buckle, and critical systems like power and ventilation along with sensitive machinery will be severely damaged.
The third GBU will deliver the coup de grace. If the bunker is 90 meters, it doesn't penetrate fully, but the shockwave will collapse the facility. That's why they deployed 3 in the same spots.
Source: /k/ommando who likes bombs and shit