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In urban areas, it is common to construct underground structure nearby existing buildings. To investigate the impact of excavation construction on the adjacent existing buildings and surrounding soil, eight parallel scale model tests that considered the process of cut and cover construction are carried out with two kinds of the diameter of support piles, two kinds of the adjacent structure, and two kinds of the relative horizontal distance from the excavation in the laboratory. And, the variations of horizontal and vertical displacements of adjacent buildings and the soil pressure surrounding the excavation and the foundations of existing buildings with different parameters are presented and discussed. Then, the experimental data and the results of eight prototype finite element models are compared and analyzed.

Urbanization rates continue to climb steadily throughout China. Nowadays, in the quest to maintain rapid economic growth, Chinese cities are confronted with many pressing problems which greatly threaten the urban living environment, such as overpopulation, land resource shortage, worsening urban traffic congestion, and environmental degradation. In the process of urban development, underground space, as an important component of the urban land space resource, has been increasingly utilized to effectively solve urban problems like municipal transportation, disaster prevention, environmental protection, and land scarcity [

Open cuts and underground excavations are gradually increasing in frequency because of the development and upgrade of infrastructures and the construction of new buildings. In urban areas, there are many situations, where basement construction or underground facilities such as underground civil air defense project are proposed to be constructed adjacent to existing buildings. In this case, it is important to evaluate the influence of excavation construction on surrounding soil and adjacent buildings and to estimate settlement of the ground for proper designing of the underground structure. The construction of the underground structure often causes ground settlement ranging from regional subsidence to small-scale collapses leading to many problems, such as building damage and structural failure [

Similarity theorem is a kind of principle studying similar phenomenon and similarity principles. It is the basic of the model test and feasible to apply this theorem in the new model test design. The theory of the scale model similitude explains the relationship between the scale model and the behavior of the corresponding prototype. Rocha [

In this research, some new aspects and mechanism of the influence due to underground excavation are illustrated using scale model tests and finite element analysis. The objective of this study is the impact of underground excavation on adjacent existing buildings and surrounding soil in the cases of different influence parameters, such as the size of support piles, the load and layer of adjacent existing buildings, and the relative horizontal distance from the excavation. The process of excavation construction is simulated by the way of the indoor scale model test based on the similarity principle. This paper compares and analyzes the difference in changes of the horizontal and vertical displacements of the existing buildings and the soil pressure near the foundation of the existing building models according to eight parallel tests of different influence parameters and numerical simulate analysis.

The foundation of the similarity theorem is the three similarity principles, the science of the conditions under which physical phenomena are similar. It is related to dimensional analysis and provides the basis for physical modeling. Similarity theory establishes similarity criteria for different physical phenomena and studies the properties of the phenomena by means of these criteria. Physical similarity is a generalization of the elementary and intuitively obvious concept of geometric similarity. In physics, a calculation is said to be from first principles, if it starts directly at the level of established laws of physics and does not make assumptions such as empirical model and fitting parameters. Two geometrical objects are called similar if they both have the same shape, or one has the same shape as the mirror image of the other. More precisely, one can be obtained from the other by uniformly scaling (enlarging or reducing), possibly with additional translation, rotation and reflection. This means that object can be rescaled, repositioned, or reflected, so as to coincide precisely with the other object. If two objects are similar, each is congruent to the result of a particular uniform scaling of the other. A modern and novel perspective of similarity is to consider geometrical objects similar if one appears congruent to the other when zoomed in or out at some level.

In geometry, if corresponding geometrical elements of two figures or solids are in proportion, the figures are said to be similar. Based on the first principle, the size of the prototype is reduced (or enlarged) by a certain proportion to make the model, and the similarity constants of parameters are as follows:

The concrete test box.

Considering the most unfavorable situation in the underground excavation construction, the soil used in this scale test is fine sand, the density of the sand is found to be 1.56 kg/m^{3}, the void ratios is 0.43, and the moisture content is 3.43%. The specific gravity of the soil particles was measured by the pycnometer method. Three tests were carried out producing an average value of specific gravity of 2.63. Furthermore, the optimum moisture content is 10.1%, and the maximum dry density is 1.875 kg/m^{3} by five compaction tests. The particle size distribution was determined using the dry sieving method, and the results are presented in Figures

The dry sieving test.

Grain size distribution of the sand.

The triaxial compression test.

The sand was evenly poured into the test box three times to ensure the uniformity of the sand density and 72 hours after each pouring. As shown in Figures ^{−5} kPa, China) were buried horizontally in the sand after filling, which were used to measure the change of soil pressure. And, the data of sensors were collected by static strain collection instrument (Donghua DH3816, China). Furthermore, the soil pressure sensors were calibrated before being buried by the liquid water pressure test, to determine the rate

Soil pressure sensor.

Static strain collection instrument.

As Figure

The position plan of soil pressure sensors.

Horizontal and vertical displacements were measured during scale tests. Twelve dial indicators (range 0–10 mm, accuracy 0.01 mm, China) were installed by magnetic bases, respectively, to monitor the settlement response of the adjacent building models during excavation construction simulation, as seen in Figure

Dial indicators for measure displacement.

Table

Influence parameters of each parallel test.

A load | B load | |||
---|---|---|---|---|

_{D} = 0.08 m, |
_{D} = 0.12 m, |
_{D} = 0.08 m, |
_{D} = 0.12 m, | |

_{1} = 0.4 m, _{2} = 0.1 m |
I | II | III | IV |

_{1} = 0.8 m, _{2} = 0.4 m |
V | VI | VII | VIII |

Existing building models with load blocks.

The cut-and-cover method is a traditional construction method of the underground structure: firstly, the cover soil is excavated, and then the support structure (contiguous piles) is constructed on both side of the excavation. Secondly, the soil is excavated while the temporary support structure is strengthened. Thirdly, the bottom plate, side walls columns, and roof of the underground structure are poured in turn. Finally, the underground excavation construction is completed after the backfilling of cover soil. Each parallel scale model test simulated the construction process of the underground excavation used cut-and-cover method. The main test steps are as follows: the initial state, excavation of cover soil, construction of the support structure, excavation of soil, construction of the underground structure, and backfill of cover soil, as shown in Figure

Existing building models with load blocks: (a) support structure and excavation; (b) the underground structure; (c) backfill of cover soil.

The test data of soil pressure sensors and dial indicators were collected at the time of two hours, six hours, ten hours, and twenty hours after each test step. Figure

The renderings of the scale test model.

In the process of underground structures construction, the adjacent buildings are affected by the soil excavation unloading, support piles squeezing soil, and so on, which could cause horizontal tilt and vertical settlement. The horizontal and vertical displacements of the existing frame structure model were measured by dial indicators, which were installed at the top and bottom of the adjacent structure models for the horizontal displacement and at the nearest and farthest from the excavation for the vertical displacement. During each parallel scale model test, the steps of dial indicators data acquisition are demonstrated in Table

The steps of dial indicators data acquisition.

Step | Data acquisition |
---|---|

1 | The initial state |

2 | Excavation of cover soil |

3 | Construction of the support structure after 2 hours |

4 | Construction of the support structure after 6 hours |

5 | Construction of the support structure after 10 hours |

6 | Construction of the support structure after 20 hours |

7 | Excavation of soil after 2 hours |

8 | Excavation of soil after 6 hours |

9 | Excavation of soil after 10 hours |

10 | Excavation of soil after 20 hours |

11 | Construction of the underground structure after 2 hours |

12 | Construction of the underground structure after 6 hours |

13 | Construction of the underground structure after 10 hours |

14 | Construction of the underground structure after 20 hours |

15 | Backfill of cover soil after 2 hours |

16 | Backfill of cover soil after 6 hours |

17 | Backfill of cover soil after 10 hours |

18 | Backfill of cover soil after 20 hours |

The contrast variations of horizontal displacement measured from each parallel test on the top and bottom of the adjacent buildings are shown in Figure

Comparison of horizontal displacement of existing buildings: (a) the top of the left building; (b) the bottom of the left building; (c) the top of the right building; (d) the bottom of the right building.

Additionally, the amplitude of change and value of the horizontal displacement are relatively larger in the model test, in which both the number of structural layers and the load are less, but the diameter of support pile is larger. The horizontal displacement of the left adjacent structure model is not obvious because the relative horizontal distance from the excavation is farther, and the value is within a very small range. Moreover, it should be mentioned that the peak value of the horizontal displacement on the bottom of the frame structure model exists in the test II, with the largest diameter of support piles and the least relative horizontal distance, and the impact is evident considerably as the support piles construction. However, the magnitude and the peak value of the horizontal displacement of the right frame structure are obviously larger than that of the left frame structure.

The vertical displacement variations of the adjacent frame structure model from each parallel scale model test are shown in Figure

Comparison of vertical displacement of existing buildings: (a) left building farthest from excavation; (b) left building nearest from excavation; (c) right building farthest from excavation; (d) right building nearest from excavation.

To investigate the impact of excavation construction on adjacent buildings, the horizontal displacements measured from each scale model test at the top and bottom of the left and right frame models in the scale model tests are subtracted to obtain the relative values as shown in Figure

Relative horizontal displacement of adjacent buildings: (a) relative horizontal displacement of the left building; (b) relative horizontal displacement of the right building.

In the same way, the relative vertical displacement is obtained by subtracting the vertical displacements of the nearest and farthest distances from the excavation in each scale model test, as shown in Figure

Relative vertical displacement of adjacent buildings: (a) relative vertical displacement of the left building; (b) relative vertical displacement of the right building.

Excavation-induced ground movements and their impact on the adjacent existing buildings is a source of significant concern for the underground structure. This paper analyses the change of soil pressure surrounding the excavation which could reflect the disturbance condition of the Earth by excavation construction. During each parallel scale model test, the steps of soil pressure sensors data acquisition are demonstrated in Table

The steps of soil pressure sensors data acquisition.

Step | Data acquisition |
---|---|

1 | The initial state |

2 | Excavation of cover soil |

3 | Construction of the support structure |

4 | Construction of the support structure after 2 hours |

5 | Construction of the support structure after 6 hours |

6 | Construction of the support structure after 10 hours |

7 | Construction of the support structure after 20 hours |

8 | Excavation of soil |

9 | Excavation of soil after 2 hours |

10 | Excavation of soil after 6 hours |

11 | Excavation of soil after 10 hours |

12 | Excavation of soil after 20 hours |

13 | Construction of the underground structure |

14 | Construction of the underground structure after 2 hours |

15 | Construction of the underground structure after 6 hours |

16 | Construction of the underground structure after 10 hours |

17 | Construction of the underground structure after 20 hours |

18 | Backfill of cover soil |

19 | Backfill of cover soil after 2 hours |

20 | Backfill of cover soil after 6 hours |

21 | Backfill of cover soil after 10 hours |

22 | Backfill of cover soil after 20 hours |

The soil pressure sensor nos. 112, 118, and 122 were embedded under the middle of excavation every 0.1 m directly, which was measured from each scale model test and are shown in Figure

Comparison of soil pressure below the excavation: (a) 112; (b) 118; (c) 122.

To investigate the effect of relative vertical distance on the soil below the underground structure, the numerical relationship between the relative distance from the bottom of excavation and the soil pressure from each scale model test is fitted in Figure

Fitting curve of soil pressure under the excavation.

Figure

Comparison of soil pressure below the excavation in the horizontal direction. (a) 107. (b) 110. (c) 114.

In each scale model test, the soil pressure sensors are buried 0.1 m below the foundation of the existing frame structure model on the left and right sides. Figures

Comparison of soil pressure below the left building foundation. (a) 101. (b) 102. (c) 106.

Comparison of soil pressure below the right building foundation. (a) 111. (b) 115. (c) 120.

In conclusion, the soil pressure displays different changes from each parallel scale model test because of different size of support piles, the load and layer of adjacent existing buildings, and the relative horizontal distance during excavation construction. The size of support piles is obvious comparatively to the disturbance of the surrounding soil, and the soil pressure under the excavation is affected by the unloading of the excavated soil and the loading of the underground structure. Furthermore, the peak value of the fluctuation is greater than the soil pressure under the adjacent building foundation. Additionally, due to the discreteness of the sand used in the model test and the influence of the similarity principle of the scale model, there are certain differences in the data results of some soil pressure sensors, but it does not affect the overall trend of soil pressure around underground structure during excavation construction. The measured values of the soil pressure sensors in each of the scale model test could reflect the changes of surrounding soil caused by the influence parameters in approaching construction of underground structure.

In order to compare the influence of the scale model test and the prototype model on the construction of the underground engineering, eight different finite element models of the prototype excavation tests are carried out with the finite element software MIDAS/GTS based on the geometric similarity principle of the first similarity theorem, as shown in Figure

Finite element models of tests. (a) Test I. (b) Test III. (c) Test V. (d) Test VII.

The dimensions of the model were selected so that the effect of boundary conditions on the results of the numerical analysis was negligible. Except for the ground surface, boundary conditions were totally fixed; only the top surface boundary was free. Eight-node isoperimetric hexahedral elements were used to represent the soil mass. Four-node shell elements with reduced integration points were used to model the plates of the surrounding buildings and the underground structure. Two-node line elements were used to simulate the beams, columns, foundations, and support structures. An elastoplastic constitutive model is needed for the elastoplastic stress and deformation analysis of the soil. The numbers of nodes and elements of prototype models are about 31620∼34340 and 29195∼32369, respectively. The material parameters of the soil are same as each scale model test, the concrete floor of adjacent buildings is C25 (the elastic modulus is 2.8 × 10^{4} MPa), and the underground structure and other concrete structures such as the column, beam, and foundation of adjacent buildings and contiguous piles are C30 (the elastic modulus is 3.0 × 10^{4} MPa).

In this paper, the Drucker–Prager elastoplastic constitutive model was used for natural soil in the numerical analysis. Surrounding buildings, underground structure, and support structures were assumed to have a linear-elastic behavior. Key construction conditions of the scale model test are simulated, such as the initial state, excavation of cover soil, construction of support structure, excavation of soil, construction of underground structure, and backfill of cover soil.

Comparison of horizontal displacement of existing buildings between the experimental and numerical simulation results is shown in Figure

Comparison of horizontal displacement of existing buildings between experimental and numerical results: (a) the top of left building; (b) the bottom of left building; (c) the top of right building; (d) the bottom of right building.

As shown in Figure

Comparison of vertical displacement of existing buildings between experimental and numerical results: (a) left building farthest from excavation; (b) left building nearest from excavation; (c) right building farthest from excavation; (d) right building nearest from excavation.

From the above comparative analysis, we can see that the changes of the data curve measured by the eight parallel scale model tests can reflect qualitatively the influence of underground excavation construction on the adjacent buildings and the soil surrounding the excavation with different influence parameters, such as the disturbance of the soil by construction of the support piles, the settlement of the existing structure close to the excavation, and other phenomena.

Although the indoor scale model tests satisfy the geometric similarity based on the first similarity theorem, the experimental results have a certain degree of dispersion and do not fulfill the strict proportional relationship with the numerical calculation results due to the poor cohesion of the sand soil in the model box and the stiffness of the frame model itself. The influence zone induced by the underground excavation is mainly concentrated at the bottom and both sides of the excavation. The finite element results of the prototype excavation construction show that the soil in the 7.5 m area directly under the excavation is disturbed to varying degrees, and the maximum vertical displacement value of soil reaches to 11 mm. The disturbance of the soil on both sides of the underground excavation is mainly concentrated within 1.5 m from the sidewall of the underground structure due to supporting effect of contiguous piles. Furthermore, the influence of excavation construction decreases with the increase of the relative distance to the excavation, and the change trend is consistent with the results of the indoor scale tests.

This paper presents the impacts of underground excavation on adjacent existing buildings and surrounding soil through eight parallel scale model experiments and numerical simulation when considering different key influence parameters, which are the size of support piles, the load and layer of adjacent existing buildings, and the relative horizontal distance from the excavation, respectively. The main findings from this work are summarized as follows.

The experimental data show that the horizontal and vertical displacement variations of the existing structures are small, and the fluctuations are within a relatively small subrange. Besides, the displacement variation of the adjacent building models related to the stiffness of the structure from the scale model.

The obtained soil pressure from experiments reflected the pressure changes of surrounding soil in approaching construction of underground structure. The results show that the soil pressure has different changes at each parallel scale model test with different key influence parameters. The disturbance of the surrounding soil related to the size of support piles, and the soil pressure under the excavation is affected by the unloading of the excavated soil and the loading of the underground structure; also the peak value of the fluctuation is greater than the soil under the adjacent building foundation.

Moreover, not only the measured data but also the prototype finite element models can qualitatively reflect the influence of underground excavation construction on the adjacent buildings and the soil surrounding the excavation with these key influence parameters.

This work can be extended in future to explore the displacement and the soil pressure when consider other influence parameters such as the depth and width of the excavation and the soil parameters. We believe that apart from looking for the relationship with influence parameters, future research should look for the disturbance of soil as underground structure construction.

The data used to support the findings of this study have been deposited at

The authors declare that they have no conflicts of interest.

This work was supported by the National Natural Science Foundation of China (grant no. 51108135).

The content of the file named “MATLAB-data” is the processing program of Figures