A combination of natural and man-made factors, including the thin rock face left behind after excavation, could have led to the collapse of a whole quarry side in Mqabba, according to an independent architect.

While the rock in the area was characterised by a number of naturally occurring faults, architect Robert Musumeci told The Times that what seemed like debris behind the collapsed side may have exerted pressure on the "apparently thin rock face". Water seeping through the debris, he added, may have exacerbated the situation.

Mr Musumeci, who specialises in restoration architecture, had submitted a similar theory to explain the collapse of a Floriana bastion last year.

The 12-storey cliff face collapsed on February 28, pulling down the boundary wall between the quarry and the Mqabba primary school's playground.

The play area has a five-a-side artificial turf pitch that was constructed around three years ago on land previously leased to the quarry owner by the government.

The playground has since been closed off by the education authorities until an assessment of the area is carried out. Deep furrows in the soil on the school premises are still visible, raising doubts as to the stability of the rock beneath.

Mr Musumeci said friction between rocks on a fault line minimised movement but did not prevent the gradual build-up of strain on the inside of the fault.

"An over build-up of strain may be reached, which is suddenly released and sudden ruptures occur without warning," he said.

Quarry owner Ċensu Farrugia, known as Ta' Puzzu, had told The Sunday Times that water may have been the cause of the collapse.

He said no rock cutting had taken place for quite some time in the area that collapsed.

"The school used to leave a lot of water running and I think that is what caused it," Mr Farrugia said with reference to a tract of soil on school premises which separated the play area from the boundary wall alongside the quarry.

However, the chief executive of the Occupational Health and Safety Authority has said it is "still early in the day" to determine what led to the rock face collapse, insisting a geological survey had to be undertaken.

The authority, which was only informed about the incident five days after the collapse, has ordered the quarry owner not to use or approach the area where the collapse occurred.