# Drone SAR Imaging for Monitoring an Active Landslide Adjacent to the M25 at Flint Hall Farm
## 2 Study Area
### Landslide Mechanisms  
Gault Clay is susceptible to slope instability at gradients as shallow as 7\({}^{\circ}\)[28]. Former perigglacial processes such as soliflution created shear surfaces at the upper 6 m region of the clay [26; 28; 29], with shallow historical landslides in this region, mostly 3-5 m deep [30]. Landslides and historic landslip plane reactivation present a significant geotechnical hazard to the M25 and other National Highways assets that are situated in this formation.  
One active and twelve relict landslides are clustered between Junction 6 of the M25 and Titsey, which encompasses Flint Hall Farm. These were identified through desk-based geomorphological mapping using LiDAR and aerial photography [26]. The active landslide is defined as exhibiting recent movement that is expected to continue; inclinometer data  
Figure 5: Simplified geological map of the study area, with Flint Hall Farm (red circle), and a geological cross-section for line A–A\({}^{\prime}\) (green circle). Adapted from [26].  
indicates creep at a rate of 10 mm over six months. Geomorphological evidence for this landslide mechanism includes shallow depressions and indistinct toe lobes, which produce hummocky ground [26].  
The Gault Clay is also susceptible to slide-type failures, caused by either the artificial destabilisation of the slope through cuttings for infrastructure, or the acceleration of creep movement [26]. Instrumentation was installed at Flint Hall Farm specifically, in conjunction with the aforementioned sub-monthly site visits, following the landslide that occurred on 19 December 2000. Excessive winter rainfall caused the failure of an 80 \(\times\) 200 m section of the cut slope, which mobilised over 90,000 m\({}^{3}\) of material; some of which encroached the M25 anti-clockwise carriage [1] (Figure 6). The landslide was a wedge-shaped failure, with a basal shear plane 10 m bgl [1] (Figure 7). A trial pit shows the highly polished and moist contact between the head deposits and the Gault Clay, which enabled independent movement of the two formations [1]. Geomorphological evidence for this landslide mechanism included a distinct backscatter and well-defined toe lobe with a hummocky main body; however, these features are no longer visible due to remediation works, including cut-off and counterfort drains and piles [1; 26].  
Figure 6: Schematic of the Flint Hall Farm landslide, which occurred on 19 December 2000 [1].  
Figure 7: Geological cross-section schematic of the Flint Hall Farm landslide, which occurred on 19 December 2000 [1].  
As shown in Figure 4, three landslides are present at the Flint Hall Farm study area: the Rooks Nest Farm Landslide, Flint Hall Farm Landslide and Flint Hall Farm South Landslide. The Rooks Nest Farm Landslide is a multi-rotational and translational movement which was reactivated by the M25 construction when the embankment increased loading at the lower sections. As stated, the Flint Hall Farm Landslide is a wedge failure within the Gault Clay, activated by high groundwater levels and the M25 construction. The Flint Hall Farm South Landslide is a dormant creep movement, with occasional reactivation during wet conditions [25].