Sheffield Wednesday's consultant engineer Dr Wilfred Eastwood said restricted entry at the Leppings Lane end led to a build-up of fans.
In a 1990 statement read to the inquests into 96 Liverpool fans' deaths, Dr Eastwood said 12 turnstiles on Penistone Road were shut.
It led to 6,000 more fans using the Leppings Lane entry.
The inquest hearing in Warrington is looking into the deaths of 96 Liverpool football fans following a crush at the Liverpool v Nottingham Forest FA Cup semi-final at the stadium on 15 April 1989.
Dr Eastwood said the extra spectators "had to be catered for which was a factor in the build-up of the crowd".
"The police argument will no doubt be that they considered it important to avoid confrontation between Liverpool and Nottingham Forest fans."
He added it would have been possible to separate the fans by closing other turnstiles on Penistone Road, and keeping those for the North Stand open.
He said he was "completely unaware" of the closure of the North Stand turnstiles until he read about it in the newspapers on 17 April 1989.
"This was a decision with which my firm was in no way involved and about which we were not consulted or informed," he added.
Legal representatives at the new inquests in Warrington are reading out transcripts of Dr Eastwood's evidence as he is too ill to attend the inquests.
The hearing continues.
The Hillsborough disaster "could have been avoided" if police had not closed 12 turnstiles, the inquests heard.