Study on dynamic safety distance of multi-storey buildings on the top of loess slope

In view of the frequent occurrence of strong earthquakes and the actual characteristics of buildings built on the top of slopes in the loess region of Western China, the dynamic response and stability law of loess slope under the building load on the top of the slope are discussed by using the three-dimensional finite element numerical calculation method. Taking the slope safety factor, acceleration amplification coefficient, building permanent displacement and slope sliding range as evaluation indexes, the dynamic safety of buildings on the top of the slope under earthquake is analyzed, and the recommended value of dynamic safety distance between top building and slope shoulder is put forward. The results show that: firstly, affected by the amplification effect of slope elevation, the dynamic response of buildings on the top of slope is stronger than that of buildings under slope. At the same time, affected by the amplification effect of the free surface of the slope, the dynamic response amplitude of the building is negatively correlated with the distance from the slope shoulder. Secondly, the horizontal acceleration ratio between the top building and the under building is the largest at the height of 3–6 m, indicating that the multi-storey building with pile raft foundation is most prone to damage at this height. Thirdly, the dynamic safety distance of multi-storey buildings with pile raft foundation located on the top of loess slope with slope height H ≤ 30 m and slope gradient 25° ≤ α ≤ 70° can be 24 m.

At present, the commonly used methods for slope stability analysis when considering earthquake action are (a) quasi-static method 11 , (b) dynamic strength reduction method 12 , (c) Newmark method [13][14][15] , (d) "Pseudo-static approach" + "Strength reduction method". The essence of quasi-static method is limit equilibrium method. The essence of dynamic strength reduction method is strength reduction method, which needs to be combined with finite element numerical method to calculate the safety factor of slope. The Newmark method takes the yield acceleration as the evaluation index. The yield acceleration refers to the vibration acceleration when the safety factor of the sliding block is 1, which is applicable to the deposit slope. In this paper, the method "d" is used to calculate the safety factor of the slope. This method is essentially a quasi-static method, and the strength reduction method is used to specifically solve the safety factor of the slope. Because the strength reduction method can only analyze the static field, the seismic action is simplified into a horizontal static force, and the gravity field corresponding to the peak strength of ground motion is added in the horizontal direction. This method is to treat the dynamic problem as a static problem, The advantage is that it can comprehensively use the advantages of strength reduction method and quasi-static method, does not need to assume the sliding surface in advance, and has high calculation efficiency. Table 1 summarizes the main principles, advantages and disadvantages of the above four analysis methods.
There are two main indexes for evaluating the stability of soil slope under earthquake: seismic permanent deformation and stability safety factor 16,17 . Critical acceleration, critical displacement and plastic strain zone penetration rate [18][19][20] are often used as specific evaluation indexes in numerical analysis to evaluate slope stability. Due to the topographic reasons of the slope, the seismic wave elevation amplification effect from bottom to top and the free surface amplification effect from inside to outside are produced. Therefore, the dynamic amplification factor is also an important evaluation index to judge the dynamic stability of the slope and the building on the top of slope. When the slope has permanent deformation but no sliding failure and the safety factor is still within the safe range, the buildings on the slope may be in the critical state of instability due to the settlement and inclination. Therefore, due to the existence of buildings on the slope, more strict requirements should be put forward for the deformation of the slope. When studying the slope and buildings on the slope as a system, the stability evaluation of the slope should take into account the deformation safety of the building.Due to the existence of buildings on the top of slope, more strict requirements are put forward for the deformation of the slope. Therefore, when the buildings and the slope are studied as a system, the stability evaluation of the slope should take into account the deformation safety of the buildings. In fact, taking the Wenchuan earthquake as the node, the seismic fortification capacity and level of Chinese buildings have been greatly improved. By summarizing the damages of buildings on the top of slope caused by strong earthquakes and landslides [21][22][23] , The damage of buildings is mainly deformation damage and overall collapse with the slope. Therefore, the permanent deformation of the building, the potential sliding range of the slope and the dynamic amplification factor of the building should be important indicators to evaluate the safety of the buildings on the slope. www.nature.com/scientificreports/ In view of the above analysis, based on the previous research on the dynamic response law of high-steep loess slope under building load 24,25 , the distribution characteristics of slope buildings in loess area are further investigated and counted, and three-dimensional models considering the influencing factors of slope height, slope gradient and the distance between top buildings and slope shoulder are established. The safety factor, acceleration and displacement response law of loess slope under earthquake are discussed. This manuscript attempts to take the acceleration and displacement attenuation law of slope top, the permanent displacement of the building, the slope safety factor and the slope sliding range as the evaluation indexes to evaluate the safety of multi-storey buildings on the top of slope. The research results can provide a scientific basis for the site selection of buildings on the top of slope in loess area, and provide a new criterion for the instability of soil slope under building load and seismic load.

Distribution characteristics of buildings on top of slopes in loess region of China
The research group focused on the investigation and statistics of rural houses and urban buildings built on slopes in three key provinces in the loess region of Western China, Gansu Province, Shanxi Province and Shaanxi Province. According to the investigation and statistical analysis results, the buildings on the top of slope can be divided into two categories: ① The buildings are close to the slope shoulder without avoidance distance. Most of the buildings are single-layer or two-layer agricultural houses. The building foundation is mainly rubble foundation or expanded foundation, and the building structure is mainly soil wood structure, brick wood structure and brick concrete structure. ② The avoidance distance of buildings is random, most of which are within 10 m. The buildings are mainly multi-storey masonry, and the building foundation structure is mainly pile raft foundation. The gradient and height of the building slope meet the following characteristics: the gradient range of slope: α = 25°-70°, the vertical height of single grade slope from slope toe to slope shoulder: H = 15 m ~ 60 m, and most single-stage slopes are concentrated in the range of 15 m ~ 30 m. Figure 2 shows some typical buildings on the top of loess slope in the west of China.

Model settings
Based on the above survey results and combined with the research results of site amplification effect 26 : the amplification effect of loess site is more obvious when the thickness of soil layer is 15 m < H < 30 m. Therefore, three-dimensional numerical models considering the influencing factors of slope height, slope gradient and the distance between top buildings and slope shoulder are established by using Midas GTS software. The specific geometric dimensions of the slope model are shown in Table 2. In Table 2, the distance between building and slope shoulder is from 5 to 35 m, the reason is: Firstly, based on the research results of "1. Distribution characteristics of buildings on top of slopes in loss region of China", the slope distance of buildings in loess area is within 10 m, so 5 m and 10 m are set. Secondly, in order to explore the attenuation relationship between the dynamic response of buildings and the distance between top building and slope shoulder, two cases of 20 m and 35 m are set up. Due to the lack of construction land in the loess area, if the distance is too larger, it will no longer meet the original intention of building on the top of the slope. Therefore, set the distance between building and slope shoulder as 5 m, 10 m, 20 m and 35 m.
The building size on the slope is long * wide * height = 30 m * 12 m * 24 m, simulating an 8-storey building of 24 m, the building foundation adopts pile foundation, and the foundation spacing is 4 m × 5 m, 12 m long pile, the section size is 1 m × 1 m. As combined with the code for design of concrete structures (Chinese GB50010-2010), the model material parameters are shown in Table 3. Figure 3 shows the geometry of the numerical model. In this study, the distance between the building on the top of the slope and the slope shoulder is defined as d, the height of the slope is represented by H. In this study, the distance of d is set with four variables: d = 5 m, d = 10 m, d = 20 m and d = 35 m, and the height of the slope is set with two variables: H = 15 m and H = 30 m.
In this manuscript, the nonlinear time history analysis is carried out. The boundary of the models are viscoelastic boundary, which is used to simulate the infinite region in seismic analysis. The seismic wave adopts the Table 1. Basic principle, advantages and disadvantages of slope dynamic stability analysis method.

Method name
Pseudo-static approach Dynamic strength reduction method Newmark "Pseudo-static approach" + "Strength reduction method" Basic principles Based on the limit equilibrium method, the seismic action is simplified as the seismic inertial force acting on the sliding block The seismic history is divided into equal length steps, and the safety factor of the slope in each step is calculated When the effective acceleration caused by earthquake is greater than the yield acceleration, the sliding body will slide Based on the strength reduction method, the seismic action is equivalent to a horizontal gravity field

Disadvantages
The sliding surface needs to be assumed in advance, which can not reflect the dynamic response between the slope and other structures The calculation time is too long to converge The yield acceleration is not easy to determine, and this method is only applicable to the slope whose strength does not decrease significantly in earthquake Compared with the dynamic finite element strength reduction method, the safety factor calculated by this method is an approximate solution,

Advantages
The concept is clear, the calculation workload is small, and the parameters are easy to determine The variation curve of safety factor with ground motion time history can be obtained Considering the deformation safety of slope, it is suitable for deposit slope or rock slope Combining the advantages of strength reduction method and quasi-static method, the calculation time is shorter than that of dynamic strength reduction method, and there is no need to assume the sliding surface in advance www.nature.com/scientificreports/ far-field wave collected by Wenchuan earthquake in Tangyu Town, Shaanxi Province of China. The seismic peak acceleration is 0.12 g, and its Fourier spectrum is bimodal. The predominant frequency of the first main peak is 0.83 Hz, and the predominant frequency of the second main peak is 2.87 Hz. The seismic wave time history and Fourier spectrum are shown in Fig. 4. Filtering and baseline correction are carried out before seismic wave input, and the filtering frequency band is 0.1-25 Hz.

Influence analysis of elevation amplification effect on buildings
In order to analyze the influence of the elevation amplification effect on earthquake resistant behavior of building, the relative variation law of the horizontal peak acceleration ratio of the top building and the under building along the building height is summarized as shown in      Fig. 7. The most likely reason is that the natural vibration period of the middle part of the building is close to the superior frequency of the input seismic wave, resulting in the resonance failure of the middle layer.
It can also be seen from Fig. 5 that the amplification effect is negatively correlated with the distance of d, that is, the smaller of the d, the higher the value of A top /A under will. But at H = 30 m, α = 45°, when the distance of the d is 10 m, the value of A top /A under is greater than d is 5 m. In order to analyze the reason, the horizontal displacement nephogram of H = 30 m, α = 45°models is compared and analyzed, as shown in Fig. 8. Figure 8 shows the most dangerous sliding surface of the slope with a black dotted line according to the distribution and change law of the displacement nephogram.
As can be seen from Fig. 8, When d = 5 m, only part of the top building is located within the most dangerous sliding area of the slope, while when d = 10 m, all parts of the building are located within the most dangerous sliding area of the slope. When d = 20 m and 35 m, with the increase of the distance of d, the buildings gradually move away from the most dangerous sliding surface of the slope. Therefore, the model with height H = 30 m and the gradient α = 45°, when the distance between top building and slope shoulder is 5 m, the top building will have the highest risk of sliding failure with the slope.Therefore, in Fig. 5e, the acceleration amplification effect of the curve d = 10 m is the most obvious. so when H = 30 m, α = 45°,d = 10 m, the site amplification effect is the most obvious.
The displacement 5 s after the end of earthquake is defined as the permanent displacement. The variation of the permanent displacement of the top floor of the building with the distance of d is analyzed, as shown in Fig. 8. Figure 9 shows that the change of the distance between the top building and slope shoulder has an important impact on the permanent displacement of the building. The overall change trend is that the permanent

Influence of distance between top building and slope shoulder on dynamic response law of slope
The acceleration amplification coefficient is defined as the ratio of the horizontal acceleration of each measuring point on the slope top to the horizontal acceleration of the slope toe. Along the central axis of the slope, extract the acceleration of each measuring point with different distance from the slope shoulder on the top slope, and analyze the variation law of the acceleration amplification coefficient with the distance between each measuring point and the slope shoulder, as shown in Fig. 10. The abscissa of Distance is the horizontal distance between the measuring point on the top of slope and the slope shoulder.
According to the analysis of Fig. 10, firstly, according to the horizontal coordinate value of the intersection of the line with acceleration amplification coefficient is1 and other acceleration amplification coefficient curves, the minimum dynamic safety distance of the building on the top of slope can be taken as 20-24.3 m. Secondly, under the influence of soil -structure interaction, the acceleration of soil near the building foundation increases sharply, and this sudden increase effect is more obvious with the increase of slope gradient and height. Thirdly, the acceleration amplification coefficient at the top of the slope is reduced to about 0.4 times at the right boundary, which is due to the essence of the viscoelastic boundary, that is, damping plays an absorbing role at the boundary. www.nature.com/scientificreports/ Try to expand the boundary length by 100 m, and the acceleration near the boundary is still small. Therefore, the specific value of acceleration at the boundary is not discussed here, and only the overall change trend is discussed. Similarly, the displacement curve of each measuring point on the top of slope with the distance from the slope shoulder is statistically analyzed, as shown in Fig. 11. The displacement amplification coefficient is defined as the ratio of the horizontal displacement of each measuring point on the slope top to the horizontal displacement of the slope toe.
As can be seen from Fig. 11 the variation law of displacement is similar to that of acceleration: with the increase of the distance, the displacement amplification coefficient gradually decreases, and the displacement amplification coefficient suddenly increases at the junction of building foundation and soil. The smaller the distance, the more obvious the displacement amplification factor of the soil near the building foundation increases. The magnification of displacement is less than that of acceleration.
In the above analysis, it is concluded that the amplification effect of acceleration and displacement increases significantly at the junction of building foundation and soil. Further statistical analysis is carried out on 24 models with different slope height, gradient and distance of d. It is found that the influence range of soil amplification effect near the foundation caused by "soil-structure interaction" is within 3 m-6 m, in order to avoid the influence of this interaction between buildings, the distance between multi-storey frame buildings shall be greater than 12 m.
The variation law of acceleration amplification coefficient at the joint of building foundation and slope soil with the distance of d is statistically analyzed, as shown in Fig. 12.
According to the analysis of Fig. 12, the acceleration amplification factor of the soil at the building foundation gradually decreases with the decrease of the distance of d, which shows that the building dynamic amplification characteristics caused by the amplification effect of the free surface of the slope decrease with the increase of the distance of d. The scatter plots in Fig. 12

Influence of the distance between building and slope shoulder on slope safety
The safety factor of the slope is calculated by the combination of strength reduction method and quasi-static method. The variation curve of the safety factor of each slope model with the distance of d is shown in Fig. 13.
As shown in Fig. 13, firstly, when H = 15 m, the safety factor of the slope increases gradually with the increase of the distance of d. when the d increases to 20 m, the safety factor tends to not change again. When H = 30 m, similar to the slope with H = 15 m, when d increases to 20 m, the safety factor basically does not change. The www.nature.com/scientificreports/ difference is that the safety factor of slope is less affected by the change of distance of d. Secondly, with the increase of slope gradient, the safety factor of slope decreases linearly, and the slope models with two different heights are in line with this change characteristic. According to the isoline change trend of the maximum shear strain nephogram, the most dangerous sliding surface of each slope model is summarized as shown in Fig. 14.
As can be seen from Fig. 14: ① The most dangerous sliding surface of loess slope is related to the distance of d. The smaller the distance of d, the larger the instability range of the slope. ② α = 25°, i.e. when the slope gradient is gentle, the sliding surface of the model with the building distance d = 20 m tends to be consistent with the sliding surface without the building model, which indicates that the slope effect basically disappears and the buildings on the slope top tend to be safe after the building distance d > 20 m. ③ When α = 45°, H = 15 m, the most dangerous sliding surface of the pure slope is 15 m away from the slope shoulder, while the most dangerous sliding surface of the d = 20 m model is 18 m away from the slope shoulder, indicating that the building on the slope top will not slide with the sliding failure of the slope after the building distance d > 20 m. When α = 45°, H = 30 m, the most dangerous sliding surface of the slope model without building is 20 m away from the slope shoulder, and the most dangerous sliding surface of the d = 20 model is close to the pure slope model, indicating

Dynamic safety distance of multi-storey frame structure on the top of loess slope
Based on the impact analysis of elevation amplification effect on top buildings, the impact analysis of building distance of d on slope dynamic response law and the impact analysis of building distance of d on slope safety, respectively, Taking the attenuation law of the acceleration amplification factor of the soil near the building foundation as the index, and according to the change of the safety factor and sliding range of the slope, the dynamic safety distances d 1 , d 2 and d 3 of multi-storey frame building of pile raft foundation on slope are summarized in Table 3.
Considering the influence of the slope distance of the top building on the slope stability, taking the slope safety factor as the evaluation index, the dynamic safety distance of the multi-storey frame building with pile raft foundation on the top of slope is defined as d 1 . Considering the relationship between the permanent deformation of the top building and the distance of d, taking the displacement of the building 5 s after the earthquake as the evaluation index, the minimum safe distance between the multi-storey frame building with pile raft foundation   www.nature.com/scientificreports/ on the top of slope is defined as d 2. Considering that the "soil structure" interaction is affected by the amplification effect of free surface of seismic wave, taking the acceleration amplification coefficient of soil near the building foundation on the top of slope as the evaluation index, the minimum safe distance between the multi-storey frame building with pile raft foundation on the top of slope is defined as d 3. Considering the position relationship between the sliding surface of the slope and the top building, taking the sliding range of the slope as the evaluation index, the minimum safe distance between the multi-storey frame building with pile raft foundation on the top of slope is defined as d 4. Finally, the dynamic safety distances d 1 , d 2 , d 3 and d 4 of the multi-storey frame building with pile raft foundation on the slope are summarized, as shown in Table 4. When the stability of the slope and the permanent deformation safety of the top buildings are comprehensively considered, the dynamic safety distance of the buildings on the top of slope can be 20 m. Considering the effect of free surface amplification of seismic wave on the "soil structure" interaction of soil near the building foundation, the dynamic safety distance of the buildings on the top of slope can be 24 m. When the slope stability is low and the landslide risk is high, the dynamic safety distance of buildings on the top of slope should be d 4 from the perspective of avoiding common sliding of buildings with the slope. Influencing factors α and H are fitted with the dependent variable d 3 to obtain the relationship as shown in (2), R 2 = 0.70.

Discussion
The distance between the top building and slope shoulder proposed in this paper is based on the conclusion obtained by considering four evaluation indexes: the sliding range of the slope, the safety factor of the slope, the permanent deformation of the building and the acceleration amplification factor. Among the four evaluation indexes, the distance obtained by taking the sliding range of the slope as the evaluation index is the smallest, and the distance obtained by taking the permanent deformation of the building as the evaluation index is the largest. According to different fortification requirements, when the buildings on the slope top are required not to produce serious deformation and damage under the action of earthquake, the slope safety distance of the buildings on the   www.nature.com/scientificreports/ top of slope should be given with the permanent displacement of the buildings as the evaluation index. When the construction land is very limited, starting from the fortification concept of "no collapse in a large earthquake", the dynamic safety distance can be determined according to the most dangerous sliding range of the slope.

Conclusion
This manuscript discusses the dynamic stability of loess slope under the building load on the top of the slope, discusses the dynamic response law of buildings on the top of slope under the amplification effect of elevation and free surface, and puts forward the dynamic safety distance of multi-storey buildings on the top of slope. The main conclusions are as follows.
(1) When the building is located on the top of the slope, the damage degree of "soil structure" interaction to the building and slope is negatively correlated with the distance between the building and slope shoulder. (2) Taking the slope safety factor, acceleration amplification factor, building permanent displacement and slope sliding range as evaluation indexes, the slope safety of buildings on the top of loess slope under seismic load  www.nature.com/scientificreports/ is analyzed, and the dynamic safety distance value of multi-storey buildings with pile raft foundation on is put forward. Without considering the influence of slope height and gradient, the dynamic safety distance of multi-storey buildings with pile raft foundation located on the top of loess slope with slope height H ≤ 30 m and slope gradient 25° ≤ α ≤ 70°can be 24 m. (3) The ratio of the peak value of the horizontal acceleration of the building on the top of the slope to the building under the slope is used to reflect the influence of the elevation amplification effect of the seismic wave on the top building. The results show that the horizontal acceleration ratio between the top building and the under building is the largest at the height of 3-6 m, indicating that the buildings are most prone to damage at this height. This manuscript explains the research conclusion from the corresponding relationship between the natural vibration period of slope and the predominant frequency of seismic wave at the height of 3-6 m of top building. (4) The dynamic safety distance of building on the top of slope proposed in this manuscript is based on the physical and mechanical parameters of Q 3 loess slope, the slope gradient range is from 25°to 70° and the height range is from 15 to 30 m. For buildings on the top of slope, it is applicable to buildings with no more than 8 floors. Since the seismic intensity input in the calculation is 0.12 g, the dynamic safety distance of the building on the top of slope proposed in this manuscript is applicable to the loess area of China with the basic fortification intensity of 7 degrees. The sliding range of the slope, the amplification and attenuation rules of seismic waves and the permanent displacement of the building under the action of higher intensity earthquakes need to be further discussed and analyzed.