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Table 1 Relationships among risk levels, effects, and safety control measures

From: Safety risk assessment of heritage buildings in metro construction based on SPA theory: a case study in Zhengzhou, China

Level

Risk

Effects

Safety control measures

I

Safe

Basically intact, no effect

Routine inspections, no essential reinforcement measures required

II

Low risk

Slight impact: building’s outdoor floor has a few slight cracks (width < 0.05 mm), without non-uniform settlement; non-structural components have slight cracks (width < 0.05 mm); the main structure of the building has no visible cracks, lateral displacement occurs at the top of the structure (< H/700); the maximum surface settlements (Smax < 5 mm); angular distortion (βmax < 1/1000)

To monitor surface subsidence and building displacement, no essential reinforcement measures required

III

Medium risk

Medium impact: building outdoor floor has some cracks (0.05–0.30 mm), without non-uniform settlement; non-structural components have some cracks (0.05–0.30 mm); the main structure of the building has a few slight cracks (width < 0.05 mm); lateral displacement (H/700~H/600); Smax (5–25 mm); βmax(1/1000–1/400)

Take grouting, mixing, rotary jet grouting and other general soil reinforcement measures before construction; strengthen monitoring of surface subsidence and building displacement; conduct trend analysis weekly for building damage caused by tunneling

IV

High risk

Large impact: building outdoor floor has many cracks (0.30–0.50 mm), with slight non-uniform settlement; non-structural components have many cracks (0.3–0.5 mm); the main structure of the building has many cracks (0.05–0.25 mm), lateral displacement (H/600~H/500); Smax (25–37.5 mm); βmax(1/400–1/100)

Take setting isolation piles, grouting, mixing, rotary jet grouting and other soil reinforcement measures before construction; strengthen monitoring and feedback of surface subsidence and building displacement; conduct field tests to guide the adjustment of parameters during construction; conduct trend analysis weekly for building damage caused by tunneling

V

Extreme risk

Severe impact: building outdoor floor has many cracks (width > 0.50 mm), with significant non-uniform settlement; non-structural components have many cracks (width > 0.5 mm); the main structure of the building has many cracks (width > 0.25 mm), lateral displacement (> H/500); Smax(> 37.5 mm); βmax(> 1/100)

Conduct field tests, soil and structural reinforcement analyses; optimize the construction plan according to parametric analysis and expert opinions; strengthen monitoring and feedback of surface subsidence and building displacement; invite experts to analyze weekly the trend for building damage caused by tunneling, and make a contingency plan; after construction, repeat safety assessment