Come to think of it – Cementing bacteria

Delta 30-08-2007

‘If the technique developed by Leon van Paassen works well, houses will no longer sag and trains can rumble over rail embankments without a problem.’

Traditionally, when soils need to be stabilized, this is done by various methods, such as injecting cement into the soil – a process called grouting. Such methods are elaborate and expensive, however. For his PhD project, Leon van Paassen investigated the possibilities of flushing sandy soils with solutions of bacteria, ureum and calcium chloride. The bacteria transform the added chemicals into calcium carbonate, which functions as a type of natural cement between the grains of sand. The result: sandstone.

The principle underlying this process, known as biogrouting, had already been demonstrated in laboratories. Van Paassen conducted additional lab research in order to determine, among other things, how much calcium chloride needs to be added to arrive at a certain soil strength, and to test the mechanical properties of the resulting sandstone. Another important aspect of his research was to try and make the process work in larger scale soil experiments. “This turned out quite well”, Van Paassen says. “We conducted a successful experiment with a 5-meter long soil column. After that, we found a contractor who was willing to carry out some additional testing, and we then conducted a successful experiment in which we reinforced the sand in a sandbox measuring 8 x 5 x 2.5 meters.”

Van Paassen also made calculations based on practical applications for this technique, such as, for instance, stabilizing the soil beneath railroad tracks. “It’s a common problem that railroad tracks situated on top of weak soil deform”, Van Paassen explains. “There is for example a stretch of track between Gouda and Woerden with weak soil underneath it, topped with a layer of sand. Trains can only travel over this stretch of track at limited speeds, because otherwise the load of the passing train would deform the soil too much and thereby risk a derailment. I calculated that if you reinforce the layer of sand underneath these tracks using biogrouting, the train could travel over this stretch of track at speeds of up to 20 percent faster.”

There is however a ‘but’: the cost. The raw materials for this type of biogrouting are rather expensive. Plus, when calcium carbonate forms, ammonium chloride is formed as a by product, and this compound needs to be flushed out of the soil after the process is complete, which is also costly. “At the end of my project I looked into alternatives for the current bacterial process”, Van Paassen adds. “It is for instance also possible to use denitrifying bacteria, which leads to less harmful byproducts.” A Dutch company, Deltares, is now continuing the development of this process and trying to find a suitable client and location for a pilot project. Van Paassen is partially involved in this through his research. The rest of his time he spends teaching at TU Delft, where he now holds the position of lecturer.

Na jarenlang discussiëren hebben de vakbonden en het college van bestuur nu dan toch afspraken gemaakt over de besteding van de 1,1 miljoen euro die de afgelopen jaren niet is uitgegeven aan gratis ADSL voor medewerkers thuis. Dit geld wordt de komende vijf jaar verdeeld en toegevoegd aan het IKA-pakket (Individueel Keuzemodel Arbeidsvoorwaarden). Dit betekent dat elke medewerker vanaf dit jaar vijf jaar lang 220 euro extra kan inzetten voor bijvoorbeeld extra verlof, pensioen of reiskosten. Met ingang van dit jaar komen er twee nieuwe doelen bij IKA: bedrijfsfitness en – eenmaal in drie kalenderjaren – een onbelaste vergoeding bij de aanschaf van een fiets. Daarvoor is al netto 228 euro beschikbaar en vanaf nu dus ook deze 220 euro. Wie geen gebruikmaakt van dit extra geld, is het kwijt. Bij de ondertekening vorige week spraken de bonden van ‘een goede stap’ en het college over een ‘heuglijk moment’.