News on Sunday wants to know the reasons behind the cracks and breakup of the recently built Terre Rouge\Verdun link road at a cost of more than a billion rupees and which caused much concern to the general public. In this context, Dr Bhanooduth Lalljee, who holds a PhD in Soil Science, explains the causes. He was previously the Director of Consultancy and Contract Research at the University of Mauritius. He has more than 30 years of experience in teaching, research and consultancy in soil and environmental issues. He is currently the Head of the Department of Agriculture and Food Science at the Faculty of Agriculture, University of Mauritius.
We have seen a number of press reports these days about collapses and cracks in roads and buildings. What are the reasons?
It is important to note that most of these infrastructural damages are occurring on mountain slopes and in valleys. The explanation behind such events is quite technical and complex. I will try to simplify them as much as possible. In those regions, where the newly built roads are opening up, the top soils are of a very specific nature. They are known as Smectite clays. Smectite clays have very peculiar properties – one unit of this clay consists of a natural silica sheet between two natural alumina sheets. Such a structure attracts and retains water, which then causes the clay material to expand. There are various types of Smectite clays, some of which can expand up to a certain limit only (technically called Vermiculite clays), while others can undergo almost unlimited expansion (technically called Montmorillonite clays). Both these groups of clay exist in several regions of Mauritius, like in Vallee des Pretres, Vallee Pitot, Baie du Cap, Quatre Soeurs, Deux Freres, Champ de Mars, Montagne Longue, Valton….The soils are also known as Dark Magnesium Clays (DMC). They have a very large surface area and are highly plastic, for example, one gram of such a clay may occupy 800 m2! In addition, these clays have a very high charge, are very sticky when wet, and have an immense capacity to hold water. Anyone who has been to the Champs de Mars on a rainy day must have experienced the stickiness of the soil. That is why, when they are dry, they are shrunken and cracked, but when they get wet, they expand hugely, generating massive forces that can crack the most solid foundation of bridges, roads, buildings and other infrastructures.
The economic losses can be phenomenal, for instance in the US, the infrastructural damage caused due to these expansive clays has been estimated to be much higher that caused by natural calamities such as earthquakes, hurricanes, and tornadoes. Such soils are very good and fertile for agriculture and crop production, but notoriously difficult for infrastructural and civil engineering works. Construction on such soils is very challenging and requires considerable expertise and inputs from experts in different fields, such as civil engineers, environmentalists, soil scientists, drainage engineers…
Road engineers must have studied the site and advised policy makers accordingly?Various options and combinations of options exist for construction in these areas, for example, one alternative is to remove the expansive top soil completely and refill it with rocks. However, this requires huge investments. Another option is to construct on pillars that go down to the hard bedrock or to a different type of clay that is non-expansive. A third option is to change the properties of these expansive clays by mixing it with some other material, such as crushed limestone. The calcium in the limestone acts as bridges in the clay crystals and thereby reduces its expansive properties. In Mauritius, we can also use the ash generated from the burning of coal and bagasse in our power plants for this purpose. Apart from using the right construction material, draining and removing the excess water from the soil is very important to prevent the clay from expanding and causing cracks in infrastructure. Hence, it is critical to build proper drains, both lateral and vertical, around these structures. Regular maintenance of these drains is also important. Moreover, the functioning of the drainage systems should be regularly monitored in order to detect any anomaly in the road surface that will be indicative of problems in the underlying soil, which then needs prompt intervention and correction. The engineers may be wise after this event. Proper soil analysis must have been undertaken before embarking on major undertaking and the soil scientist should be taken on board and his/her opinion duly considered.
Following your explanation given on the soil, should the government abandon the present Terre Rouge\Verdun road project?
Too much money has already been spent and it will not be wise now to bring major deviation. However remedial measures may be brought, as explained above. More important, there should be close and regular monitoring of the road structure.