Complex sentence: These tunnels have a relatively gentle gradient of 1 in 90 (1.11%) while the western approaches are steeply graded at 1 in 42 (2.5%), while the frequent gaps between the tunnels provide good natural ventilation.
Split into simpler sentences: These tunnels have a relatively gentle gradient of 1 in 90 (1.11%). However, the frequent gaps between the tunnels provide good natural ventilation.