Geotechnical laboratories: our ground freezing facility in Bochum

What defines a ground freezing lab?

Peter: Technically speaking, a ground-freezing laboratory is a geot­ech­ni­cal facility. What makes it so special though is the operating temperature we work in – it gets as low as minus 25 degrees Celsius. Our Bochum lab features several walk-in climatic/cold chambers for us to conduct various inves­ti­ga­tions. Temper­a­tures in those cold chambers usually are at -10 or -20 degrees Celsius, while the climate chambers provide +5 degrees Celsius.

What sort of inves­ti­ga­tions do you conduct in a ground-freezing lab?

Peter: It‘s basically strength behaviour that is being analyzed in that kind of lab. As with decreasing temperature or in frozen state physical char­ac­ter­is­tics of different soils vary according to their degree of saturation which also is decisive for the inves­ti­ga­tion results. Therefore, you need water within the soil itself as rock doesn’t freeze. So the water freezes, the soil’s strenght increases and water perme­abil­ity decreases, and this way even sand can reach a stability comparable to that of concrete. We receive soil samples from all around the globe at our ground-freezing facility, i.e. from Australia, Singapore, Canada, the US, Russia and Belgium. We actually inves­ti­gated the soil underneath the Leaning Tower of Pisa as well. When taking a soil sample, metal or plastic sleeves are rammed into the ground. These sleeves then get sent to our lab in Bochum where they get frozen at -10 or -20 degrees Celsius in our climate chambers. Any construc­tion progress is dependent on these results, e.g. calculating the frost body.

Can you name any example for the need of ground-freezing?

Peter: Ground-freezing is partic­u­larly used in tunneling. In order to freeze the ground, so-called freezing lances are driven into the ground which then get flushed with saline solution. These are frozen at -35 degrees Celsius and thereby freezing the ground around them. Another option would be using liquid nitrogen, providing an even greater stability, allowing for -196 degrees Celsius. Greater stability is required for example in case of existing buildings above the tunnel. So ground-freezing allows for high stability of the ground whitout the need for concrete – providing the capability to withstand great compression caused by overlying buildings. For instance, we evaluate the required thickness of the frost body to withstand the existing compression.

Is there any special training/qual­i­fi­ca­tion that is needed to work in the ground-freezing lab?

Peter: There is no single special training or qual­i­fi­ca­tion applicable to work in our ground-freezing lab. Our staff consists of qualified Chemical-Technical Assistants, Physical-Technical Assitants as well as qualified Construc­tion Material and/or Raw Material Examiners.

What sort of tests do you conduct in the ground-freezing lab?

Peter: In case of a ground-freezong assignment, we perform various investigations of the soil. In addition to special tests, we also evaluate the stability or deformation, e.g. by uniaxial and triaxial compression tests with the soil sample being clamped into a test compactor. This test compactor then pushes the soil sample against a traverse and crushes it, thereby assessing the soil’s stability. Furthermore, we also conduct so-called creep tests with the sample also clamped in and then exposed to a static load, remaining under this load for several days. These tests determine occuring deformation caused by load. As mentioned above we also do special tests, such as frost heave tests or freezing pressure tests.

However, those frost heave tests are not done at temperatures below freezing point but at +5 degrees Celsius. The soil sample is put onto a stand which is warmed to +5 degrees Celsius. The sample is then covered by a cap the size of the sample with the sample then being cooled. This way, the soil sample gets frozen top to bottom like for example tarmac on the streets would - with the top being frozen and the base not frozen causing so-called frost lenses in the middle layer. These frost lenses then expand causing for the top layer to heave or burst. So in our lab we measure the degree of heaving. If the ground is unable to heave as it does in this simulation what happens is called freezing or frost pressure. So, the test simulates the ground’s expansion caused by freezing  and it’s behaviour when pushed against something. Out in the field this could possibly be groundworks, a baseplate, or tunnel facing. This test is vital for the construction progress as frost damage can cause major destruction.

What makes the ground-freezing lab so unique?

Peter: Ground-freezing is our USP (Unique Selling Proposition). We have been specialized in ground-freezing for over 30 years now, evaluating samples from all over the world. With us, our clients get the one-stop solution with our expertise based on longtime experience.