## 6 Steps towards Criteria Fulfilment
### Automated Feature Detection and Evaluation
#### 6.1.2 Common Defects in Masonry Railway Tunnels
With time, large sections of rings can debond, causing arcs of brickwork to fall away as slabs. Such 'delamination' events can deform rails or damage the railbed presenting a derailment risk. Depending on size, debonding can significantly weaken substantial volumes of surrounding masonry. * **Railbed Faults** (Figure 12i)--Subgrade layers of ballast facilitate even distribution of rail-traffic weight, ensuring rails remains level on uneven ground to prevents derailments. Displacement induced by ballast fouling [75, 76, 77] and frost heave [238, 241, 242]  
Figure 12: Common defects in masonry railway tunnels: (**a**) Haunch Deformation; (**b**) Cracks; (**c**) Water Ingress; (**d**) Perished Mortar; (**e**) Missing/Loose Brickwork; (**f**) Spalling; (**g**) Ring Separation and Debonded Wall-Section; (**h**) Frost Heave [238]. can damage rails and offsets train weight distribution, increasing in-tunnel derailment risk and subsequent likelihood of major network disruptions. * **Drainage Faults--**Tunnels contain integrated pipework and catchment pits to safely remove excess water and silt. Flooding may occur if pipes become blocked, rupture due to freezing and expansion, or become overwhelmed by intense weather events [243]. Improvement works can also fail. For example, 18 bolts supporting a water catchmentray in Balcombe Tunnel (2011) decoupled due to resin failure [244]. Sag reduced tunnel clearance from 0.87 m to 0.3 m posing a dangerous obstruction to rail-traffic. Upon reflection, we first note recent studies focus primarily on (and achieve) detection of three main defects: (i) surface-visible cracks via CRP; (ii) voids via Acoustics, Thermography or GPR; (iii) water ingress via Thermography and GPR. By extension, with suitable modification to enable multi-directional scanning, similar hardware could reasonably detect (i) open joints and perished mortar, spalling, missing brickwork; (ii) ring separation and debonding and (iii) drainage faults in future applications. Secondly, we observe that currently cross-sectional deformation can only be directly imaged by laser methods as they boast 3D point cloud data capture. We note that traditional visual inspection also detects deformation, but notetaking is inherently far less accurate than imaging. Although no encountered literature directly utilised laser methods to detect railbed faults, we may reasonably assume cross-sectional imaging would also provide usable insight with relative ease. Finally, we report that no data acquisition method is single-handedly able to detect each identified common RTSSI defect. This agrees with our findings in Section 3. Discounting Thermography since it cannot directly measure target depth, we remark that GPR has capability to detect the widest range of defect types (five of nine). Note that direct arrival waveforms are liable to mask small surface level defects, hence are not included in this statistic. To improve this ratio, we believe future research into multi-sensory data acquisition systems presents high potential impact. A unit amalgamating RGB-CRP (surface imaging), LiDAR (cross-sectional profiling) and 360 GPR (subsurface imaging) could feasibly detect all defects listed.