Table 2 Safety risk evaluation index system of heritage buildings

Safety risk of heritage buildings

Heritage building factors (\(b_{1}\))

Importance degree \({\text{c}}_{1}\) (score)

Subjective index. Heritage buildings are of great value and are precious material and spiritual wealth left by ancestors [34]. The more important they are, the greater the loss after the disaster, and the higher the risk level

Geometric character \({\text{ c}}_{2}\) (score)

Subjective index. Geometric character refers to the length, width and height of a building. The higher the building, the greater the L/W, the smaller the L/H, or the greater the H/W, the worse the overall stability of the building, and the higher the risk level [35]

Structure type \({\text{c}}_{3}\) (score)

Subjective index. Different structure types have different ability to resist external deformation. Their safety is mainly based on the structural components bearing capacity, such as reinforced concrete > frame/shelf > concrete > brick/stone wood > wood structure, reinforced concrete > plain concrete > brick/rubble stone foundation [30]

Deterioration degree \({\text{c}}_{4}\) (score)

Subjective index. Most of the heritage buildings have a long history, and the structural components gradually suffer from aging and damage, which cannot reach the original carrying capacity. The higher the degradation degree [43], the higher the risk level is

Metro construction factors (\(b_{2}\))

Cover depth \({\text{ c}}_{5} { }\) (m)

Objective index. The deeper cover depth, the smaller the surface settlement is, and the lower the risk level is. There is an approximate linear relationship between the cover depth and surface settlement [20]. Generally, the cover depth of metro tunnel is above 6 m and below 30 m, but there are other conditions (Chandpole Gate in Jaipur 4.5 m, Huaihai Middle Road station of Shanghai 33 m). The value range is set as 0 m-40 m

Horizontal distance \({\text{ c}}_{6}\)

(m)

Objective index. When the metro interval structure side passage the existing structure, the monitoring range is within 30 m of the metro structure outer edge [44]. For heritage buildings, this distance can be appropriately increased. The value range is set as 0 m-40 m

Tunnel diameter \({\text{ c}}_{7}\) (m)

Objective index. The tunnel diameter is approximately equal to the diameter of the shield machine. At present, the shield tunneling machine used in the tunnel construction has a maximum size of Bertha from the United States with a diameter of 17.45 m, used in The SR99 tunnel of Seattle, and the minimum size is Ruicheng from Jiangsu, China with a diameter of 2.1 m, used in the sewage pipeline project of Nanjing. The typical metro tunnel is 6.2 m/6.8 m, but there are also 12.26 m (Dalian Metro Line 5) and 15.2 m (Wuhan Metro line 7). Considering all the existing dimensions, the value range is set as 0 m-20 m

Advancing speed \({\text{ c}}_{8}\) (mm/min)

Objective index. The greater the advancing speed is, the greater the disturbance range to the soil, and the higher the risk level will be. Therefore, reducing the advancing speed is very important for the stability of the earth pressure around the shield [38]

Soil factors (\(b_{3}\))

Friction angle \({\text{ c}}_{9}\) (°)

Objective index. The friction angle increases, the surface settlement decreases, and the risk level decreases [40]. The interval value is found in the relevant literature [14]

Compression modulus \({\text{ c}}_{10}\) (MPa)

Objective index. The compression modulus increases, the surface settlement decreases, and the risk level decreases [40]. interval value is found in the relevant literature [14]

Poisson's ratio \({\text{ c}}_{11}\)

Objective index. The Poisson's ratio increases, the surface settlement decreases, and the risk level decreases [20]. interval value is found in the relevant literature [14]

Soil loss ratio \({\text{ c}}_{12}\) (%)

Objective index. The soil loss rate increases, the surface settlement increases, and the risk level increases [17]. Wei studied the value and distribution of soil loss rate caused by shield tunnel construction, which was 0.20%–3.01%, and 95.77% of the data was distributed in the range of 0.20%–2.0% [41]

Management factors (\(b_{4}\))

Monitoring measurements \({\text{ c}}_{13}\) (score)

Subjective index. The Chinese code GB 50911–2013 specifies in detail the monitoring technical specifications for urban rail transit projects, including monitoring methods, monitoring technology, monitoring frequency and the arrangement of measuring points, etc. [45]

Management system \({\text{ c}}_{14}\) (score)

Subjective index. A sound production safety management system includes the management system, management personnel, division of duties, and management level. Perfect system, complete personnel, clear division of responsibility, high management level made strong guarantees for production safety

Contingency plan \({\text{ c}}_{15}\)

(score)

Subjective index. The improvement of the contingency plan can locate the hidden danger and weak link in time, and prepare for the previously unpredicted danger, which has the value of early warning and emergency

Monitoring engineer \({\text{ c}}_{16}\) (score)

Subjective index. Engineers with high professional level and rich engineering experience are helpful to the reliability of monitoring results. According to the feedback of monitoring results, the safety status and development trend of engineering structures and surrounding environment can be analyzed and predicted