Motorists travelling on Sydney's M4 spent hours "gridlocked" after the tunnel between Haberfield and Strathfield was shut because of fears the concrete roof could fall onto vehicles.
Transport for NSW coordinator general Howard Collins said engineers found "a couple of large bulges" in the shotcrete, otherwise known as sprayed concrete lining, of the tunnel roof on Wednesday night.
The M4 tunnel has reopened in both directions between Haberfield and North Strathfield ahead of the peak period on Friday morning.
The section of the tunnel, which is part of the WestConnex toll network, had been closed in both directions overnight as emergency crews worked on infrastructure repairs.
While diversions have been lifted and traffic conditions returned to normal, Transurban was yesterday criticised by NSW Minister for Roads Jenny Aitchison for its "frustrating" lack of communication from the company over the issue.
"We will investigate and work with the timelines, and go through this with Transurban, to ensure that motorists are not put through this pain again," Ms Aitchison said.
So what caused the debacle in the first place?
What is shotcrete?
Shotcrete is a mixture of concrete, water and lots of sand or silt.
It is shot through a hose with force created by a pump and pressurised air.
As it moves through the hose, chemicals are added to speed up its setting time and bonding strength.
The high velocity helps the mixture bond quickly and strongly with the surface it is sprayed onto.
Permanent shotcrete can last 50 to 100 years without additional maintenance, but is still perfectly serviceable indefinitely and can be replaced with a new mix when needed.
It is used in countries around the world including the US, UK and France.
What is it used for?
Shotcrete is commonly used in Australia to help line walls in mines, subways and tunnels such as the one that comprises part of the M4.
It is also used to construct retaining walls and secure excavations, especially on slopes like the ones on either side of a road cutting through what used to be a hill.
It can stick to hard surfaces like sandstone or rock, or can be sprayed onto mesh wiring fitted to a more fragile surface.
The mixture is used instead of normal concrete because it works well on overhead or vertical surfaces due to its quick setting times.
It does not usually need a mould, can be quicker to cover larger areas, and is helpful for repair work.
Standard concrete takes more time and resources to spread, level and set.
Is it reliable?
Geotechnical consultant and owner of Geotechnical Designs, Shaloo Puri, said shotcrete was "very reliable and very strong".
She said a set of "rare conditions" were needed for it to start failing and that those geological problems were "such that nobody can predict".
"Somebody has designed the wire mesh and shotcrete but [if] the load is coming on that more than what has been designed for, then it will definitely fail," Ms Puri said.
"There is so much traffic going over the top and there are lots of vibrations around, so it happens sometimes."
She said it was possible for the shotcrete to "crack" and "collapse" given the right circumstances and agreed with the decision to close the motorway.
What next?
Ms Puri said engineers had likely removed the patches of shotcrete damaged by either water or rock dislodgement.
For issues caused by water leakage, the area is usually drained, she said.
"If the rock itself is putting pressure on the shotcrete, they will put in a big rock bolt and tie that fragment with other rocks and they will put another layer of shotcrete on that damaged portion."
Transurban issued a statement on Friday morning confirming the tunnel has now fully reopened Westbound.
"We apologise for the delays motorists incurred yesterday," a spokesperson said.
"Safety is, and continues to be, our number one priority."
Motorists affected by the traffic gridlock have been offered a toll refund through the Linkt app or its 133 331 phone line.