## 7 Soil-Atmosphere Interaction Research Methods  
The UK's historical geotechnical infrastructure experiences several changes between saturated and unsaturated states annually, which impact behaviour of the soil. The issue can be conceptualised using a framework of unsaturated variables set out by [PERSON] [63], which are useful to consider when designing an investigation into the interaction of geostatures and weather-related geohazards. The flow diagram in Figure 9 outlines their relationship to the Factor of Safety (FOS). The square boxes represent 'fixed' values while the circular boxes are 'uncertain' variables. This approach demonstrates the considerable quantity of complex interactions between unsaturated soil variables required for assessing slope stability. It is notoriously difficult to test these complex soil-atmosphere interactions in the laboratory in a truly representative manner. Furthermore, the interaction of these factors with the soil fabric provides further complexity which is difficult to capture and measure. Therefore, it is generally considered that full-scale embankment testing is a more representative approach. This was shown by [PERSON], et al. [64], where laboratory permeability tests were a magnitude lower than the in situ equivalent. This was a result of the absence of macroscale structures (desiccation cracks and plant roots) in laboratory samples. It also resulted from both sampling time (a three-year gap between laboratory tests and in situ testing was recorded) and differences in testing depth (laboratory samples were retrieved from 2 m depth below crest, whereas in situ tests were taken at 0.3 m and 1.4 m). The Bionics test embankment in northeast England was constructed to circumvent this problem and allow for field monitoring of soil behaviour on a full-scale embankment [46; 65]. The testing regime included permeability testing, pore water pressure monitoring, and Electrical Resistivity Tomography (ERT) [65; 66; 67]. The results were able to demonstrate the heterogeneity of pore water response within the embankment owing to prevailing wind direction, vegetation type, capping type (surface permeability), and compaction conditions [64]. This approach allowed for the investigation of both micro and macro structures that element testing alone would not allow. Despite this, monitoring was limited temporally, as to consider seasonal variation (soil-atmosphere interaction) and temporal changes in soil fabric, years of data collection are required. Furthermore, the number of monitoring points in terms of depth and spatial distribution, as well as the occurrence of uncontrollable variables such as pooling water on the slope surface, provide additional limitations [64]. Moreover, in the wider context of the transport network, the spatial variability of historical geostatures (including their construction materials and techniques) produces an additional layer of complexity which is difficult to evaluate. These  
Figure 9: Unsaturated soil variables effect on transport infrastructure slope stability (modified after [PERSON] [63]). limitations make it challenging to come to absolute conclusions owing to variability at all scales, especially for prolonged conditions. The limitations of full-scale testing are partially mitigated using physical modelling techniques. A prime example was reported by [PERSON], et al. [68] with regards to seasonal climatic cycling of hydraulic stresses on a centrifuge model and limit-equilibrium back analysis. This approach allowed the problem to be scaled down in its entirety, including temporally, to account for numerous seasonal variations. Their testing reported the 'ratcheting' effect of seasonal volumetric changes, measured using vector displacements propagating from the toe of the embankment. This has a significant advantage, as negative pore water pressures could easily be measured using a small number of high-capacity tensiometers placed in the slope face.