What went wrong at the Naxxar collapse site?

Updated 9am

A report into last week’s partial collapse of a massive development in Naxxar has described the situation at the site as “very precarious, at various levels” and said it was the result of construction methods used.

The report by structural engineer and University of Malta professor Alex Torpiano attributes the partial collapse at the foundation of AC Group’s multi-storey development to the “severe imbalance” between the level of filling between supporting walls and the adjacent “excavated void”.

Torpiano was hired to address the collapse by the site's developer AC Group director Anton Camilleri, known as ‘tal-Franċiż’, and is not the architect for the project.

The collapse in the former quarry was “the result of the construction system adopted and the sequence of backfill material shifting,” the report says. It recommended the foundations be backfilled again as quickly as possible.

On Tuesday, remedial works were underway at the site where Times of Malta saw a truck unloading sand-like construction material.

The report describes parts of the foundational structure exposed by the collapse as “clearly unstable,” “very dangerous” and “liable to fail without warning”.

Camilleri told Times of Malta the remedial work would “not take long” but declined to comment further.

Outward pressure

The report explains that the site is a former hardstone quarry which had been exhausted and previously backfilled.

The foundations comprise large volumes of backfilled construction material up to 12 metres high, separated by 23cm wide radial fin walls – one of which appears to have buckled – supporting the double curved outer wall running along the base.

The outward pressure of the large volume of interior construction material against the outer wall was held only by the “frictional interface of the toothing (‘mursalli’)” between the fins and outer walls, the report explains.

Describing the toothing as “generally only one leaf [a single vertical layer of masonry] deep” with mortar “not likely to have provided any significant bond at the joints”, the report said it was “therefore not surprising that the external wall was pushed out by the pressure”.

Detail of the failed fin wall underlying the three garage levels.

Surmising that the collapse of the wall and contained material “would not have happened in a flash,” it said it was “reasonable to presume that uneven back-fill pressures were also created laterally, on the sides of the fin-walls themselves” - which the report said explained the buckled and damaged wall exposed by the collapse.

Further risks

Highlighting further risks to the site and the surrounding area, the report said the situation beyond the collapsed area towards nearby Triq is-Seneskalk, to the east of the development, “is presumably built in the same way, is back-filled in the same way, and is therefore in a similar state of pressure imbalance”.

It added that failure could “progress rapidly”, especially with the addition of water, and that stones above the area of collapse could fall without warning.

“Since the whole situation is so fragile, remedial interventions have to induce minimal vibrations – since these could trigger incipient collapses”, the report says. It emphasised that workers should not be underneath or in the vicinity of the precarious area of the site.

The project comprises apartment blocks rising to four floors plus receded floor and overlying garages. The collapse occurred in the region of apartment blocks B1/B2. Image: Alex Torpiano

Torpiano notes that while backfilling the foundations should negate the backfill imbalance, rebuilding the damaged fin wall will be a “difficult task” and rebuilding vertical supports for the outer wall will also need to be considered.

He stressed that the remedial works are only intended to stabilise the damage to the development.

“The ability of the whole repaired substructure to carry the levels of residential apartments envisaged, will need to be carefully considered at a later stage, since the proposed interventions do not necessarily enhance the load-bearing capacity of the substructure.”

The massive development will eventually see the construction of 129 garages topped with 35 residential units across six receded floors when completed.

Stop-work order

The backfilling seen Tuesday follows the Building and Construction Authority (BCA) earlier that day approving remedial works at the site between the hours of 5am and 8pm. 

The approval requires updated insurance documents to be provided within 48 hours and is on the condition that the Naxxar Local Council does not object to the activity. A warranted architect and civil engineer are required to be on site overseeing the works. AC Group is expected to regularly update the BCA on the progress of the work.

The Building and Construction Authority, replying to questions by Times of Malta on Wednesday, said that the only emergency works permitted are those outlined in the method statement prepared by Prof. Torpiano.

The Stop Works Notice shall remain in effect until the remedial works are completed and the necessary reports are submitted.

"The developer and contractor are required to adhere strictly to the conditions imposed by the BCA, including continuous supervision by a warranted architect, as well as the maintenance of a daily log and photographic records of the works carried out," the BCA said.