Digging a test trench gives valuable information about active layer thickness and properties of near-surface permafrost soils. For technical reasons, the maximum trench depth in alpine terrain mostly does not exceed 4-5 metres. It is, however, ideal to sample soils for additional laboratory investigations (block samples). Grain size distributions and ice contents can be estimated within a trench. Because exposed ice thaws rapidly, mapping has to be carried out immediately after the excavation. It is also important to record even fine ice lenses because the layering contains valuable information on the frost susceptibility of the active layer. However, it is only a shallow investigation and because the soil properties generally change significantly with depth, further investigations are required that give information on deeper ground sections. Not only is this necessary for deep foundations, but also for shallow ones since thermal disturbances at the surface affect the ground at greater depth. If thaw sensitive soils exist at greater depth, long-term thermal changes may cause thaw settlements or instabilities here.
Was this article helpful?