The ancient Greeks already knew that plants protect slopes against landslides and erosion. How strong this protection effect is, however, still cannot be expressed by reliable figures today. Common research methods meet their limits – in the field as well as in the laboratory. WSL and SLF have therefore developed a new device, a so called shear-apparatus. The scientists’ goal: To quantify protection effects of plants in a way that allows engineers to appropriately consider them in safety calculations for areas prone to landslides.

Bringing nature into the laboratory

The new shear-apparatus (Fig. 1) combines the advantages of common field and laboratory methods: easily controllable and repeatable experiments on the one hand, and large soil samples that appropriately represent natural conditions on the other hand.

Fig. 1: During experiments in the shear-apparatus, a well-defined normal force is applied to the samples. During the so called “shearing process”, the two frames of the sample container are moved parallel to each other – either the upper frame against the lower, fixed frame or the other way round. The force and the resistance of the samples are recorded with high resolution. The maximum stability of the sample is defined as the shear force that is necessary to cause the sample to fail.

To investigate the stability of planted and unplanted soil samples, the scientists apply well-defined forces – with the samples not only in horizontal position, but also inclined up to an angle of 45°. This allows, for the first time, to investigate soil in the laboratory under conditions that are similar to real slopes. Another advantage of the new shear-apparatus are the big sample containers: With their dimensions of 50 x 50 x 40 cm, they surpass common sample sizes by far (Fig. 2). These containers allow scientists to use natural soil material with rocks up to 10 cm. Furthermore, they offer enough space for plants to develop large root systems – not only for single species but also for compositions of various species in due number. To make sure that the plants develop similarly as on hillsides, the sample containers are kept inclined during the whole period of plant growth. Results from such experiments substantially support the selection of suitable plant species for stabilizing slopes prone to landslides. Furthermore, scientists are planning experiments with micro-organisms such as mycorrhizal fungi, to unravel the mystery of their impact on soil stability.

Fig. 2: The sample containers that are tested in the shear-apparatus consist of two frames lying over each other.

Shear-apparatus in the rain simulator

Landslides are often triggered by heavy rainfall. To study the impact of rainfall on soil stability, the shear-apparatus is waterproof. It is thus possible to put the whole apparatus under a rain simulator, with the sample containers inclined. Depending on the soil material and plants, a simulated thundershower may cause the soil to slip without applying any additional force – just like in the real world. If the sample resists the rainfall, the scientists subsequently conduct a shear experiment to measure the remaining stability of the sample.

Snow and ice can be tested too

The new shear-apparatus is a milestone of soil stability research at WSL and SLF. The scientists expect that the results will be of great benefit for practice: If the protective effects of plants can be calculated, engineers will be able to target the use of plants more effectively in slope stabilisation – either to support or even replace technical constructions. Furthermore, the new shear-apparatus is suitable for experiments with snow and ice too. This opens new perspectives to avalanche and permafrost research.