Geotechnical Laboratory: Ground Freezing Laboratory in Bochum

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 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.

It‘s basically strength behaviour that is being analysed 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.

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.

There is no specific training path for working in our ground freezing lab. Our team brings together qualified chemical and physical laboratory technicians and specialists in construction materials and raw materials testing.  

In case of a ground freezing assignment, we carry out various tests on 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. We also conduct so-called creep tests. During these tests, the soil sample is likewise clamped into the test compactor and then exposed to a static load. The sample remains under this load for several days. These tests allow us to measure how the soil deforms over time when subjected to continuous loading. In addition to these tests, we perform special tests, known as frost heave tests and frost pressure tests. 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. In our lab, we measure the degree of heaving. If the ground is unable to heave, as it does in this simulation, this is referred to as freezing or frost pressure. The test therefore simulates hoe soil expands when it freezes and how it behaves when it is confined or pressed against a structure. 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.

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.