It's always hard to tease out 'intentions' in these kinds of situations, but some of the evidence certainly points to the collapse of the towers as a goal.

First, there's the 1993 World Trade Center bombing carried out by Ramzi Yousef, a Pakistani. In that attack, which killed 6 people and injured more than 1,000, a bomb was hidden in a stolen Ryder van that was parked in the parking garage under the North Tower. Apparently, he parked the van where he did because he wanted the North Tower to collapse and the debris to knock down the South Tower. So it's clear that in this case, the goal of the plot was to collapse the towers. Yousef was arrested in 1995 in Pakistan and put on trial in the United States and apparently told federal agents this directly, which is how we know his intentions. There are hundreds and hundreds of pages of court documents--lots available for free via the CourtListener archive. He was found guilty of a conspiracy to bomb the WTC in 1998 in federal court in (I believe) the Southern District of New York. Yousef and the 1993 bombing are also discussed extensively in the 9/11 Commission Report.

It's worth pointing out that Yousef was arrested and tried in the 1990s, in the U.S. federal court system, and therefore was not subject to the same "enhanced interrogation techniques" and military tribunals as post-9/11 terrorism detainees.

Yousef's uncle was Khalid Sheikh Mohammed, who later linked up with UBL and al-Qaeda and is often described as the mastermind of the "planes operation," which is how al-Qaeda referred to 9/11. After the 1993 bombings, KSM and Yousef allegedly began planning "Operation Bojinka," which is kind of a fun name for a plan to assassinate Pope John Paul II, blow up 11 airliners, and crash a plane into CIA headquarters. Bojinka, according to KSM, is when he started thinking about hijacking planes and using them as weapons against other targets, and that the 1993 bombing showed him that bombs and explosives could be problematic.

Bojinka never came to fruition beyond a test explosive Yousef planted on a single flight that killed 1 person, and Yousef was arrested shortly after this. KSM, however, was not, and, in 1996, went to Afghanistan, where he met up with Osama bin Laden, who had recently returned to Afghanistan from Sudan. (KSM was not, at this time, a member of al-Qaeda, but he wanted bin Laden's financial support.) KSM presented Osama bin Laden with a plan to hijack 10 airliners, 9 of which would be flown into various targets on both the East and West coasts. (KSM planned to be on the 10th himself. He would, apparently, kill all the male passengers, and then land the plane after alerting the media, and he'd give a grand speech denouncing the U.S. support of Israel, repressive Arab regimes, etc.) Bin Laden and al-Qaeda were, apparently, skeptical about this grand plan.

A few years later, however, KSM formally joined al-Qaeda and bin Laden decided to support a scaled-down version of his planes operation. This was the 9/11 attacks.

KSM was arrested in 2002. Unlike Yousef, who was treated as a criminal and given the protections afforded criminal suspects, KSM was subjected to enhanced interrogation techniques (torture) and detained in Guantanamo. Legal proceedings are still ongoing against him. (I mention this because it's worth noting that the information he relayed in his interrogations, as then reported in the 9/11 Commission Report and court documents, was obtained under those circumstances, though I can't say if those particular facts were obtained under torture.)

So, by looking at the evolution from the 1993 Ramzi Yousef bombing to the 9/11 attacks, I think it's fair to say that the collapse of the WTC was a goal and an intended, hoped-for outcome. Whether or not they EXPECTED the Towers to collapse is a different story, however.*

I'm drawing from the 9/11 Commission Report (long, but very accessible and readable), court documents in various cases including the trials of Ramzi Yousef and KSM (many of which are available for free via the CourtListener archive). It's also discussed in journalist Lawrence Wright's book The Looming Tower, which discusses the road to 9/11 and won the Pulitzer Prize.

​

*Edit to add: There's a tape dating from October 2001 that the U.S. government calls "the smoking gun" tape, and it features Bin Laden discussing the attacks. According to a translation of the tape released by the DoD in December 2001, UBL claims he expected that only the floors above where the planes hit would collapse.

"We calculated that the floors that would be hit would be three or four floors. I was the most optimistic of them all. (...Inaudible...) due to my experience in this field, I was thinking that the gas in the plane would melt the iron structure of the building and collapse the area where the plane hit and all the floors above it only. This is all that we had hoped for."

This certainly seems to suggest that they were likely not expecting the full collapse that did occur, but were at least hoping for a partial collapse.