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Surface settlements caused by twin tunneling are a major interest in underground space engineering. Most prevailing studies have investigated the surface settlements observed over twin tunnels constructed in clay. Yet, less attention is given to the twin tunnels excavated in silty sand, which is the focus of this study. A comprehensive field monitoring scheme was designed in the Hangzhou metro line six project to measure the surface settlements during twin tunneling by adopting earth pressure balance shields. The influence of four shield operational parameters, namely, face pressure, tail grouting pressure, penetration rate, and pitching angle on the surface settlements due to twin tunneling are explored. In general, the total settlement profiles observed over twin tunnels follow an inverted bimodal shape. The post-settlement observed over the 1st tunnel due to the 2nd tunnel excavation is non-trivial when the twin tunnels are spaced closely. Low face pressure and tail grouting pressure can induce excessive peak surface settlements during twin tunneling in silty sand.

This study integrates different machine learning (ML) methods and 5-fold cross-validation (CV) method to estimate the ground maximal surface settlement (MSS) induced by tunneling. We further investigate the applicability of artificial intelligent (AI) based prediction through a comparative study of two tunnelling datasets with different sizes and features. Four different ML approaches, including support vector machine (SVM), random forest (RF), back-propagation neural network (BPNN), and deep neural network (DNN), are utilized. Two techniques, i.e. particle swarm optimization (PSO) and grid search (GS) methods, are adopted for hyperparameter optimization. To assess the reliability and efficiency of the predictions, three performance evaluation indicators, including the mean absolute error (MAE), root mean square error (RMSE), and Pearson correlation coefficient (R), are calculated. Our results indicate that proposed models can accurately and efficiently predict the settlement, while the RF model outperforms the other three methods on both datasets. The difference in model performance on two datasets (Datasets A and B) reveals the importance of data quality and quantity. Sensitivity analysis indicates that Dataset A is more significantly affected by geological conditions, while geometric characteristics play a more dominant role on Dataset B.

Estimating surface settlement induced by excavation construction is an indispensable task in tunneling, particularly for earth pressure balance (EPB) shield machines. In this study, predictive models for assessing surface settlement caused by EPB tunneling were established based on extreme gradient boosting (XGBoost), artificial neural network, support vector machine, and multivariate adaptive regression spline. Datasets from three tunnel construction projects in Singapore were used, with main input parameters of cover depth, advance rate, earth pressure, mean standard penetration test (SPT) value above crown level, mean tunnel SPT value, mean moisture content, mean soil elastic modulus, and grout pressure. The performances of these soft computing models were evaluated by comparing predicted deformation with measured values. Results demonstrate the acceptable accuracy of the model in predicting ground settlement, while XGBoost demonstrates a slightly higher accuracy. In addition, the ensemble method of XGBoost is more computationally efficient and can be used as a reliable alternative in solving multivariate nonlinear geo-engineering problems.

The excavation of a foundation pit will break the original stress balance and affect the surface settlement of the natural foundation of adjacent buildings, so deformation control is very important. In this paper, numerical simulation results of the additional stress of a natural foundation caused by a foundation pit excavation were compared with experimental results to verify the accuracy of the numerical model of a foundation pit excavation and obtain a reliable ratio of building width to the average particle size of the sand that was not affected by the particle size effect. Then, the distribution and variation law of the surface settlement of a natural foundation caused by excavation was studied by a model test, and the relationship between the deformation of a natural foundation and the depth of a foundation pit excavation and the additional load of building was obtained.

In contrast to straight tunnels, the mechanisms of displacement of surrounding soil induced by shield excavation of small radius curved tunnels are more complex. Based on field monitoring data of surface settlement and horizontal displacement of a small radius curved shield tunnel in a section of Zhengzhou Metro Line 3, a numerical model using three-dimensional a finite element method is established to evaluate factors of the displacement of surrounding soil. The results verify the validity of numerical simulation by comparison with field monitoring data and the influence of unbalanced additional thrust at tail jacks, curvature radius of a tunnel and tail grouting pressure on surface settlement and horizontal displacement of surrounding soil. Maximum surface settlement and horizontal displacement of surrounding soil at the outer side and inner side of curved tunnel axes are positively related to thrust ratio, while negatively related to curvature radius and grouting pressure. The ultimate objective of this study is to ascertain factors of displacement of surrounding soil induced by small radius shield excavation and provide a theoretical basis and technical support for the design and construction of similar tunnel.

International audience; Increasing tunnel excavations in dense urban areas faces the issue of accurately predicting tunnelling-induced surface settlement. The quality of those predictions closely depends on the choice of soil constitutive model. In this context, some widely used soil models in engineering practice not only fail to reproduce some characteristics of soil behaviour on the constitutive level but also lead to shallower settlement. This article studies the influence of the choice of the soil model on tunnelling-induced displacements through a case study. Three different soil models are used to simulate soil behaviour, namely the hardening soil model, the modified cam clay model and a proposed enhanced critical state model incorporating deviatoric and volumetric hardening. The proposed model is first validated on different triaxial tests and its features are especially highlighted for overconsolidated soils. This model is then compared to the other two models on tunnelling simulations. The results show great predicted settlement sensitivity to the choice of soil model. The proposed model engenders deeper settlement and, consequently, enables a more conservative design. Furthermore, parametric studies demonstrate significant disparities in the predicted displacements by the different soil models depending on the initial conditions and the stress relaxation ratio.

Displacements induced by mineral dissolution and subsurface volume contraction affect overlying soils. In this study, we examine the consequences of mass loss or volume contraction at shallow depths using a discrete element method. The goal of the study is to identify particle-scale and global effects as a function of the relative depth of a dissolving inclusion, initial soil density, and granular interlocking. There are successive arch formation and collapse events, and a porosity front propagates upwards as grains slide down to refill the space. Grains around and within the refilled cavity are loosely packed and have small contact forces that are sufficient to avert the buckling of granular arches that form around the dissolving zone. Denser packings and interlocking combine to exacerbate rotational frustration and lead to more pronounced force chains along granular arches, looser fill, and reduced surface settlement. In fact, surface settlement vanishes, and the sediment hides the localized dissolution when deep inclusions z/D ≥ 5 dissolve within dense sediments. While scaling relations limit the extrapolation of these numerical results to tunneling and mining applications, macroscale trends observed in the field resemble results gathered in this study.

To address the problem of dissolution damage during the long-term service of concrete hydraulic buildings, the concrete dissolution damage degree D and contact dissolution loss model with time were proposed by combining the results of indoor concrete dissolution tests. A mathematical model of the evolution of concrete parameters with time was developed. In the context of an underground concrete culvert project, a numerical analysis model of concrete dissolution deterioration was established, according to which the effects of different damage locations on the stress and displacement of concrete culverts were analyzed. The results show that the settlement value of the surface and the soil pore pressure value increase more when the bottom of the culvert is damaged by dissolution, and the settlement of the surface from the bottom damage is about 1.07~1.16 times of that from the top damage. When the damage position deviates from the center line of the culvert, the maximum point of vertical displacement in the transverse settlement trough of the ground surface deviates from the centerline of the culvert by about 3~5 m in the horizontal direction. At the bottom of the culvert, the concrete permeability coefficient reaches 6.957 × 10−8 m/s~8.354 × 10−8 m/s in about 13~16 years after the occurrence of dissolution damage, and local leakage occurs, causing a significant increase in the pore pressure of the soil near the leakage location. The overall settlement and uneven settlement resulting from the dual action of dissolution deterioration of concrete and infiltration pressure can have a serious impact on the upper channel buildings.

Pipe-roof Pre-construction Method (PPM) is regarded as a safer method to construct underground space, especially suitable for the construction sites with dense surface buildings, underground pipelines and complicated geological conditions. Xinleyizhi Station of Shenyang Metro constructed by PPM. In order to ensure safety in construction, the whole construction process was closely monitored. In this paper, monitoring results of surface settlement in PPM is analyzed. According to the monitoring results, the most serious settlement occurred in pipes jacking, which was the first and the most crucial step in PPM. The settlement reasons in each step are discussed, and controlling methods of surface settlement in each step are elaborated. Through close monitoring and timely control, the construction of Xinleyizhi Station completed smoothly. Because of the obvious advantages of PPM, the method will be used more widely in construction of shallow buried excavation under complicated surrounding and geological conditions.

The increase in transportation in large cities has made it necessary to construct twin tunnels at shallow depths. As far as the parallel excavation of mechanized twin tunnels is concerned, most of the cases reported in previous studies have focused on the interactions between two horizontally driven tunnels. However, less work has been devoted to the interactions between tunnels stacked over each other. The numerical investigation performed in this study has made it possible to evaluate the influence of the construction process on two stacked tunnels, using the FLAC 3D finite difference element software. The structural forces induced in each of the stacked tunnels and the displacements in the surrounding ground have been highlighted. The results of the numerical analysis indicate that new tunnel construction can have a great impact on an existing tunnel. The greatest impacts are observed when the upper tunnel is excavated first. The excavation of the upper tunnel generally leads to greater surface settlements than when the lower tunnel is excavated first. This study also shows that the normal forces induced in the lower tunnel are always greater than those developed in the upper tunnel. The normal displacements and the bending moments induced in the lower tunnel are usually smaller than those in the upper tunnel.

The Greater Cairo metro-Line 3, the major project of underground structure in Cairo city, Egypt, is currently under constructed. Ground movement is expected during the construction with tunneling boring machine as Cairo metro tunnel passes through sand soil. In the present study, finite element model is used to model tunnel system performance based on the case study. An elasto-plastic constitutive model is adopted to represent the soil behavior surrounding the tunnel. The effects are expressed in terms of surface displacement and soil stress change caused by tunneling. The subsoil stresses undergo three phases of change. At these phases, the loading steps of the tunnel construction are predicted using the 2-D finite element analysis. Ground movement and construction influence are obtained by the numerical model. A comparison is made between the computed tunnel performance and the observed behavior. The comparison reveals a good agreement between the calculated and the observed values.