## 4 Discussion
For example, Bath, Kings Lynn, and Peterborough show minimal deformation over Lithomorphic soils whilst Grantham and Bristol show significant deformation profiles. As a major soil group, these soils are mostly very shallow and consist of seven sub-groups [27]. Therefore, analysis of the major soil group in this case may not fully represent the area's soil characteristics and thus the resultant infrastructure movement being observed. Further analysis of more detailed soil classes is required to fully characterise the interactions between Lithomorphic Soil types and infrastructure movement. Despite its low sample size of just 113 PS points, infrastructure underlain by Peat soils showed subsidence rate of \(-7.5\) mm year\({}^{-1}\). Peterborough is the only site to have included analysis of Peat soils, but the findings of this remain important due to the high potential impact subsidence can have on the railway network operation and safety. Rates of deformation observed in this study are slightly less than that found by the assessment in [26] of the East Anglia Fenland wastage, in which it is noted that mean average wastage (over open fenland) can vary between 19 mm year\({}^{-1}\) and 127 mm year\({}^{-1}\) for thin and thick deposits of Peat soils respectively [26]. However, [PERSON] et al. [26; 28] makes no direct mention of the impact of infrastructure on peat wastage. The railway observed within this study is situated on a raised, highly-engineered embankment just to the west of the Holme fen nature reserve. The impact of the railway embankment compressing the peat may lead to compaction of the Peat soil, which is suggestive as to the cause of rates of subsidence observed within this study. A validation of the PSI output was undertaken over the disused Meldon Quarry in Dartmoor National Park (2014) (Figure 5). This test site was selected for its highly stable bedrock and coherence. No vertical movement was expected during this observation period, apart from some deposition from the weathering of the quarry walls. Figure 5 shows minimal movement from the 1579 PS points on the quarry. However, in between March and May 2016 uplift and subsidence of approximately \(\pm\)1.5 mm is observed. As this movement is consistent across the inter-quartile range of the points, and more sudden and extreme than expected, it is suggested that this movement is due to issues of the data, data cleansing, or PSI technique. This level of uncertainty must be taken into consideration when reporting these results. Given the scale of the movements considered in this study and their seasonal patterns, observations of the trend--rather than individual movement--are suggested as being appropriate. Obtaining levelling or GPS data during this study's observation period for a number of locations in all study sites would improve the validation of the results. The rates of deformation in this study are aligned to previous investigations which have used PSI to monitor earth surface movement in the UK (2014). Apart from (2020; 25), there is limited work reporting investigation of the relationships between major soil group type and infrastructure movement using PSI, particularly in the UK, so limited direct comparisons can be made. Several papers have investigated infrastructure deformation using the PSI technique (3; 8; 14), but these studies use PSI to assess the structural condition of infrastructure assets in areas of known continual ground deformation. These studies note the importance of regional deformation phenomena, such as ground-water abstraction, legacy mining works or seismic activities as key considerations to make when investigating infrastructure deformation at a network level. The rates of deformation from these studies (3; 8; 14) are highly variable, but often much higher than those reported in this study (up to \(-\)73.3 mm year\({}^{-1}\)) in the case of (3). [PERSON] et al. (2015) used PSInSAR to monitor the impacts of soil movement upon building damage in Rotterdam, Netherlands.