What kind of fortification can can be used in modern warfare. I'm talking everything except nukes(I know bunkers are a thing).
Nuke the incoming missiles before they nuke you.
Any building sturdy enough to take an RPG. And then ridiculously deep bunkers to hole up in. Anything in between is pointless because it'll just get bombed. Fortification is an outdated tactic, our ability to blow shit up outpaced our ability to fortify things a long time ago.
The important thing to note on tests like that is that they use a standard grade of concrete, steel reinforcing lattice, and grade of steel rebar. Think of it as a stranded test medium like ballistic gelatin, and it was based on the most up to date means that the US ( not the best in the world by any means) would use at the time to make fortifications. The standards were last updated due to considerations over the effect of 'new' high strength concrete... in 1967.
The issue with missile specs is that the US has never been the field leader in quality concrete nor has it really been into the idea of fixed fortifications in since the civil war. Also just about every one uses the US standards on penetration of concrete or standards closely based on US standards ( like the post WWII soviet/Russian ones).
For some reading on the subject....
Also there is another of Saddam's bunkers (one of his Swiss built ones) that was hit with 22 GRU-28 that is rumored of survived.
There is in fact a number of bunks that were hit with various types of bunkers buster multiple times during the second Iraq war either are known to survived or rumored to have survived. All of those bunkers that did so were built after the first Iraq war by a mix of Swiss and North Korea outfits.
>What kind of fortification can can be used in modern warfare.
To get a meaningful discussion on the subject you are going to have to be more narrow on the subject. Ideally into 19th century warfare, early 20th century warfare, or post WWII warfare.
based on what subject matter it is covering it does follow the 25 year rule. Also is a poor place to discus the matter due to having a large number of America posters. Been from a country with by far the largest and most modern air force, a highly level of investment into bunker buster and other anti-fortification weapon systems, and a strong disinterest in building permanent fortifications cause a biased outlook in most of my countrymen on the subject.
An old crusader castle called Krak des Chevaliers is still being used as a fortress in the ongoing Syrian civil war, and is apparently standing up remarkably well to modern weapons. Machine gun and RPG fire have no effect, and even airstrikes have caused only superficial damage.
Obviously it couldn't withstand a concerted assault from a major modern military, but against terrorists and the remnants of a shattered third-world army, it's unassailable.
good old fashion, digging in. a shell scrape will protect you from most artillery strikes save a direct hit
or HESCO for walls and small arms/RPG protection, simple, quick and effective
A airbrust or proximity fuse make that not fully true anymore.
They have been in use since the Battle of the Bulge. You are still much safer from artillery inside a foxhole then outside a foxhole. Especially if you end up under fire from man portable mortar firing rounds made in locally or cheap export versions. Those normally do not have proximity fuses.