# Drone SAR Imaging for Monitoring an Active Landslide Adjacent to the M25 at Flint Hall Farm
## 2 Study Area  
Flint Hall Farm is situated immediately east of the M25 Junction 6 in Godstone, Surrey, UK (grid reference TQ3652) (Figure 1). The south-facing slope was cut between 1976 and 1979 for the construction of the M25, with road overbridges at the east and west of the site. The resultant slope has angles between 5\({}^{\circ}\) and 16\({}^{\circ}\), with a maximum height of 25 m [1; 25]. Figure 2 presents a 1 m Light Detection and Ranging (LiDAR) Digital Terrain Model (DTM). The closely spaced elevation contour lines show that the site is situated in an area of steep elevation change. The approximately 900 \(\times\) 130 m site has a sloping, uneven and hummocky ground surface, and it is currently utilised as pasture for cattle.  
In the walkover and geomorphological mapping report by Atkins (2018), the site is divided into three zones of geotechnical risk, based on the geomorphology, proximity to the M25 and the presence of instrumentation and stabilisation measures, amongst other factors. The zones were subjected to a semi-quantitative risk assessment by Atkins to assess the likelihood and impact of further ground movement hazards affecting the safety and operation of the M25, based on a review of all available ground monitoring data and geotechnical timelines. The Flint Hall Farm Zone, Midslope Zone and Rooks Nest Farm Zone are presented in Figure 3; the geotechnical hazard risk ratings for the sub-zones are the product of the hazard likelihood and impact and are listed in Table 1, with their association to the three landslide systems in Figure 4.  
Figure 1: Flint Hall Farm study area (hatched red pattern), with annotated M25, Godstone, and regional UK overview map.  
Figure 2: Flint Hall Farm study area (hatched red pattern), with 1 m LiDAR Composite DTM for elevation and labelled contour lines.  
Given the geotechnical hazard and associated risks of landslide encroachment onto the M25, the site has been monitored on a sub-monthly basis since 2003 by a conglomerate of firms, including National Highways (formerly the Highways Agency and later Highways England), AECOM, Arup, Atkins, Connect Plus Services (CPS), and Geotechnical Observations. Field surveys and in situ instrumentation data from inclinometers, piezoelectric sensors, Shape Accel Array (SAA) sensors, and siphon well pressure sensors inform the production of geotechnical reports available through the National Highways Geotechnical and Drainage Management Service (GDMS). Monitoring the site through remote sensing, in this case InSAR, will provide millimetric rates of surface deformation for the entire site at a sub-daily temporal resolution if required.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Zone**} & \multicolumn{2}{c}{**Sub-Zone**} & \multirow{2}{*}{**Risk Rating**} & \multirow{2}{*}{**Landslide System in Figure 4**} \\ \cline{2-3} \cline{5-5}  & **Number** & & & \\ \hline \multirow{4}{*}{Flint Hall Farm} & 1 & Landslide & Low risk, with medium & \\ \cline{2-3}  & 2 & Flower Lane cutting & risk of creep failures & Flint Hall Farm Landslide \\ \cline{2-3}  & 3 & South & Low risk & Flint Hall Farm South Landslide \\ \hline \multirow{2}{*}{Midslope} & 1 & North & Medium to high risk & \\ \cline{2-3}  & 2 & South & Low risk & N/A \\ \hline \multirow{4}{*}{Rooks Nest Farm} & 1 & Landslide North & Medium risk & \\ \cline{2-3}  & 2 & Landslide South & & \\ \cline{2-3}  & 3 & Northeast & Low risk, with medium & \\ \cline{2-3}  & 4 & Southeast & risk of creep failures & N/A \\ \hline \hline \end{tabular}
\end{table}
Table 1: Zones and sub-zones at the Flint Hall Farm site, with geotechnical hazard risk ratings. Adapted from [25].  
Figure 3: Zones and sub-zones at the Flint Hall Farm site, including the Flint Hall Farm Zone, Zones 1–3 (red); the Midslope Zone, Zones 1–2 (blue); and, the Rooks Nest Farm Zone, Zones 1–4 (yellow) [25]. The zone colour shading is more transparent than the legend colours for surface feature visibility.