Malta’s leading geotechnical engineer has confirmed that Middle Globigerina limestone found in the Hamrun basin area can be “extremely dangerous” and “unforgiving”, meaning the necessary precautions must be taken before any excavation.
Fears over the Hamrun basin were first raised by geologist Peter Gatt who plotted how four collapses converge on the map shown above.
“This doesn’t mean one cannot build on it or excavate within. It just means that whoever is doing this needs to understand how the material responds, so that the necessary precautions can be taken,” said Adrian Mifsud, who lectures at University of Malta and co-authored the Quintano report into the tragic collapse that killed Miriam Pace.
“Today we are digging deeper than ever, with loads higher than ever, and therefore we can’t simply continue adopting methodologies that could have been acceptable for shallow excavations in relatively strong materials. We need to respect the fact that the ground is a very variable and sometimes ‘ill-behaved’ material, which will not respect a ‘one-size fits all’ approach that we often adopt for the sake of expediency and convenience.”
Mifsud was asked to react to geologist Peter Gatt’s report published today by Lovin Malta which points to a geological connection with four collapses within a three-year period. The report was commissioned by Matthew Montebello whose family fears their home in Fleur-de-Lys, Birkirkara, will be next to fall due to works happening next door. Excavation works have been halted by the Building and Construction Agency (BCA) until the site’s rubble can be cleared and a geotechnical report can be done.
“Middle Globigerina is indeed variable but also predictable, due to the known weathering transformation that it exhibits. Although it is a rock, it contains a minimal amount of clay that makes it very different from the ‘Franka’ (lower Globigerina) we use as a building stone,” Mifsud explained.
“Whilst at depth it can be as strong as Franka, towards the surface it is transformed with time to a progressively weaker material that is akin to soil at the very top. Most older buildings are typically founded on these weaker layers – not necessarily the soil at the top, but the weaker and fractured layers immediately below (referred to as ‘turbazz’, in site terminology).”
“Excavation immediately next door reduces lateral confinement to this weak material, which then undergoes a drastic reduction in strength, immediately below the foundations of the neighbouring building. This is why it can be extremely dangerous – other rocks have the same problem, but this particular material is much more unforgiving in this respect.”
Mifsud said deep excavations have been carried out in this stratum in a completely safe manner and with negligible movement, especially if underpinning techniques and post-tensioned anchoring systems are implemented, as commonly done in other countries.
“Such excavations need to be modelled and designed, and not simply left to the whims of whoever sits in driving seat of the excavator. If the developer and his architect adopt this approach, and if the neighbours co-operate, and allow the underpinning and anchoring of their foundations (and not insist on maintaining the 2’6” of rock, which is completely useless in such situations) then it’s is a win-win situation for all concerned parties,” he said, referring to the Birkirkara case.
Matthew and Ramona Montebello have been battling against a development of some 28 garages three floors below ground level, adjacent to their home. They fear they will be the next victims of Malta’s collapse crisis due to poor workmanship, conflicts of interest and lax enforcement, among other issues.
The method statement for the development next door already had to be changed three times due to their complaints.
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