Healing Stone… by infection

Roderick Micallef has a long family history within the construction industry. He coupled this passion with a fascination with science when reading for an undergraduate degree in Biology and Chemistry (University of Malta). To satisfy both loves, he studied the chemical makeup and physical characteristics of Malta’s Globigerina Limestone.

Micallef (supervised by Dr Daniel Vella and Prof. Alfred Vella) evaluated how fire or heat chemically change limestone. Stone heated between 150˚C and 450˚C developed a red colour. Yellow coloured iron (III) minerals such as goethite (FeOOH) had been dehydrated to red coloured hematite (Fe2O3). If the stone was heated above 450˚C it calcified leading to a white colour. This colour change can help a forensic fire investigator quickly figure out the temperature a stone was exposed to in a fire — an essential clue on the fire’s nature.

While conducting this research, Micallef came across an Italian study that had concluded that different strains of heterotrophic bacteria can consolidate concrete and stone. Locally, Dr Gabrielle Zammit had shown that this process was happening on ancient limestone surfaces (Zammit et al., 2011). These bacteria have the potential to act as bio-consolidants and Micallef wanted to study if they could be used to reinforce the natural properties of local limestone and protect against weathering. 

Such a study is crucial in a day and age where the impact of man on our natural environment is becoming central to scientific research. The routine application of conventional chemical consolidants to stone poses an environmental threat through the release of both soluble salt by-products and peeled shallow hard crusts caused by incomplete binding of stone particles. Natural bio-consolidation could prove to be an efficient solution for local application and is especially important since Globigerina Limestone is our only natural resource. 

This research is part of an Master of Science in Cross-Disciplinary Science at the Faculty of Science of the University of Malta, supervised by microbiologist Dr Gabrielle Zammit, and chemists Dr Daniel Vella and Prof. Emmanuel Sinagra. The research project is funded by the Master it! Scholarship scheme, which is part-funded by the EU’s European Social Fund under Operational Programme II — Cohesion Policy 2007–2013.

Xemxija and Earthquakes

On February 22, 2011, a magnitude 6.3 earthquake struck the city of Christchurch, New Zealand, killing 181 people and causing widespread destruction. Curiously, this damage was not evenly distributed, even for areas right next to each other. This phenomenon is called the site effect and depends on the underlying geology.

Malta, unlike New Zealand, is not typically associated with earthquakes. The islands lack a seismic building code and many structures could be damaged with moderate shaking. Malta’s past records list several earthquakes that have damaged buildings and even caused some to collapse. Apart from not being reinforced, buildings have been built on less stable ground, which increases risk.

 

Setting up the Micromed Tromino, the instrument used to perform recordings of ambient noise measurements
Setting up the Micromed Tromino, the instrument used to perform recordings of ambient noise measurements

Sharon Pace (supervised by Dr Pauline Galea) investigated this effect in one test area — Xemxija, in the north of Malta. She studied how sites in Xemxija would respond to the energy from an earthquake by using a portable seismograph to record ambient noise (caused by sea waves, vehicular traffic, and other anthropogenic sources) at over 100 points across the village (pictured). The ground’s surface can be considered a vibrating platform, which can be shaken both by ambient noise as well as stronger waves from earthquakes. The ground may “resonate” at particular frequencies, or not at all, depending on the kind of rock or soil layers making up the top 30 to 50 metres. Analysis of ambient noise shows if such resonance phenomena exist, how they are related to the local geology, and how this would translate into actual earthquake shaking.

 

Resonant peak frequency distribution patterns around the Xemxija area
Resonant peak frequency distribution patterns around the Xemxija area

At Xemxija, the study confirmed that the presence of clay (whether at the surface or buried) does amplify the grounds motion at certain frequencies.  The results match previous studies in other areas, but this research went further by constructing geological models that can determine the ground’s underlying structure .

Taken together, the survey shows areas in Xemxija that might need extra support to survive future earthquakes and prevent deaths. Xemxija is not the only area with soft clay geology, the urbanised area of Mellieħa and historic citadel Mdina are built on top of similar structures. Considering the importance of these areas means that more studies are needed to better understand the structure of Maltese buildings and how they would respond to earthquakes.

 

This research was performed as part of a Masters of Science in Physics at the Faculty of Science. It was partially funded by the Strategic Educational Pathways Scholarship (Malta). This scholarship is part-financed by the European Union — European Social Fund (ESF) under Operational Programme II — Cohesion Policy 2007 – 2013, “Empowering People for More Jobs and a Better Quality of Life”.