## 6 Steps towards Criteria Fulfilment
### Automated Feature Detection and Evaluation
#### 6.1.2 Common Defects in Masonry Railway Tunnels  
Defects constitute any imperfections in the material or form of a structure. Indicative of degradation and increased failure likelihood under strain. Swift detection in an enclosed tunnel environment is critical. We provide a brief overview of causes and hallmarks for common defects in masonry railway tunnels, then reflect on the efficacy of their detection in modern surveys, highlighting any opportunities for future research:  
* **Arch Barrel and Cross Sectional Deformations** (Figure 12a)--Shifting tunnel landmark induces changeable tensile and compressive forces within arch barrels. This can trigger sidewall bulging and buckling; haunch distortion; tunnel floor bowing or side-offset of the crown. * **Cracks and Fracturing** (Figure 12b)--Localised shear forces and vibrations from rolling stock can split and displace masonry. Damage ranges from hairline cracks lacking obvious signs of displacement, to large open fractures exhibiting significant displacement. * **Water Ingress** (Figure 12c)--Rain infiltrates tunnels through shafts and groundwater propagates through dissolved subsurface joints and fissures (karsts) [236]. Leading of mortar begins as water percolates between masonry before flowing down the sidewall. Lubrication of masonry joints leads to movement of the structure, resulting in lining deformation. Owing to joint length, significant quantities of ingress can accumulate, scouring supports and flooding tunnels without proper drainage or saturated catchpits, as ingress often carries dissolved ochre (acquired during subsurface percolation) which forms crystalised limonite deposits [197] that block drains. Out-flow down sidewalls forms noticeable white streaks emanating from the region of breech, therefore can be used as indicators of ingress source. However, establishing subsurface mortar leaching extent is reliant on NDI methods for RTSSI. * **Open Joints and Perished Mortar** (Figure 12d)--Characterised by the deterioration and eventual absence of mortar between brickwork. As brick tunnels can date back over 150 years, mortar naturally begins to deteriorate from reactions with moisture in the bricks, air and subsurface [227; 231; 237]. This process is accelerated by wash from nearby ingress, vibrations induced by both rolling stock and air-pressure waves from passing locomotives. * **Loose and Missing Brickwork** (Figure 12e)--Early onset of spalling and failed patching repairs can result in loosened or missing brickwork, indicated by brickwork rubble on the railbed. Individually, gaps pose no substantial loss of structural rigidity, but can provide opportunity for larger defect growth if not addressed quickly. Furthermore, if present in tunnel haunch or crown, falling rubble can damage rolling stock, or cause serious injury to ground crews working below. **Spalling** (Figure 12f)--Perishing masonry on the tunnel intrados in vicinity of the haunch or crown can be dislodged by gravity, leaving the next layer of brickwork exposed. Repetition gradually creates rough-profiled recessed quadrants. Bricks in newly exposed layers lack tarnishing from exposure to soot and locomotive exhaust fumes, thus a second hallmark is more vibrant brickwork colouration. * **Ring Separation and Debonding** (Figure 12g)--Arch barrels contain multiple layers of concentric brickwork rings. Literature encountered discussed brick tunnels ranging from 3-15 layers [237, 239, 240], evidencing significant possible contextual variation. Ingers, deterioration of intrados mortar and poor quality workmanship can all cause neighbouring rings to separate within the wall. Gravity pulls innermost layers downward, causing debonding from layers behind forming slit voids (subsurface hairline fractures). Over time, large voids begin to grow. If near-surface, separating rings may briefly cause visible cracking of intrados mortar, allowing detection. However, separations deeper than a single ring are completely invisible to an in-tunnel observer.