A geological study has established that the quarry wall collapse which damaged the Mqabba primary school grounds was partly caused by the rubble and soil used as filling for an adjacent quarry and on which the school's football pitch was built.

The report, commissioned by the Education authorities and seen by The Times, says a number of factors caused the 12-storey cliff face to collapse on February 28, pulling down a retaining wall and damaging the school grounds.

It also points out that the "structural integrity" of the school building was not affected by the collapse because it was built on solid rock.

However, the conclusions are bad news for students, who will not be able to use the now damaged five-a-side pitch for some time because the ground below it is unstable and will remain so until the neighbouring quarry, still in use, is also filled with rubble.

The report does not pin the blame on anyone.

The artificial turf pitch was built about three years ago by the Foundation for Tomorrow's Schools in a field on the grounds of a reclaimed quarry. The field was surrounded by a deeper and much larger quarry on two sides and the steep rock face abutting onto the quarry was characterised by natural faults.

Quarries dug side by side are normally separated by a thin rock face. In this case, the filling material of the disused quarry, composed of rubble and soil, caused pressure on the thin skin of rock and contributed to the collapse.

The situation was further compounded by an underground room roofed over with arches and stone slabs, which was possibly used as a water cistern. Pressure from the filling material resting on the room's ceiling may have caused the room to cave in, the study reports.

"It seems that a roof span failure in the room together with progressive failure of rock beds caused migration of cavities causing a sudden collapse."

The report adds there were "no records indicating the presence" of this room situated on the boundary between the school grounds and the quarry.

On the stability of the ground below the five-a-side pitch, the report says that "the fill material, where the collapse occurred, is no longer retained and cracks are seen within it".

"For these reasons it is important that children are not allowed within the football pitch area until remedial measures have been taken to eliminate all safety concerns. The open space next to the buildings may continue to be used as there is no danger over there," the report says.

It adds that when the adjacent quarry is filled up, the pressure from the dumped material beneath the school grounds will be neutralised.

"Any possibility of slips taking place would then be eliminated. This will take quite some time and, hence, until that time, other measures should be taken," the report says.

The education authorities had taken immediate action to seal off the damaged grounds and a boundary wall was built to cordon off the play area. As a precaution, another yard adjacent to the wall was not being used.

However, the report suggests that a new football ground could be built in this yard, which was unaffected by the collapse. A spokesman for the Education Ministry confirmed that works on the new pitch would start in summer when children are out of school.

How quarries are separated and filled

Quarries that are exhausted or no longer economically viable to be worked are filled with inert material. They are then covered with top soil and the newly reclaimed land is used for agricultural purposes.

Adjacent quarries belonging to different owners would be separated by a relatively thin section of rock that would extend the whole depth of the shallower quarry.

It can therefore happen that the hole formed by one of the quarries gets filled up with dumped material while the adjacent quarry continues to be extended deeper.

The rock separating the said adjacent quarries would then act as a retaining wall. The pressure exerted by the retained material together with hydrostatic pressure that may build up may prove too much for the rock wall to withstand them and collapse results.

The presence of joints and fissures within the rock mass is another factor that may lead to rock failure.