Back Analysis Method Research Articles

Application of numerical back analysis for determination of soil mass specifications during tunnel construction

Geotechnical parameters play key roles in constructing and developing civil engineering projects, especially underground structures. One of the conventional methods to evaluate the tunnels’ behavior and safety is applying a back-analysis method to assess the displacement amount during the construction of different parts of the tunnel. Applying this method and using displacement values obtained by tunnel monitoring, the estimated values of geotechnical parameters may converge to their real values. The present study was conducted to improve the estimation accuracy of geotechnical parameters for the tunnel route of Karaj (located 36 km west of Tehran, Iran) subway using the alternative univariate back analysis in FLAC3D software and modifying tunnel maintenance requirements, accordingly. The results showed that the use of geotechnical parameters extracted using back analysis lead to a more accurate design of the tunnel. Overall, through the applied back analysis, it is possible to reduce the shotcrete thickness of some subway stations up to 43% and obtain an optimum design.

Deformation and failure mechanism of surrounding rocks in crossed-roadway and its support strategy

Stability control for soft and broken surrounding rock of crossed-roadway is one of the important contents of underground engineering support. Take the transformer substation in Hanzhuang coal mine in Shandong Province, China as a case study, deformation and failure mechanism of surrounding rock are analyzed by field monitoring and borehole imaging method. A finite element model under original support scheme is established by ABAQUS, and the rheological parameters of surrounding rocks are determined by back analysis method according to the field monitoring deformation data. Then, a partitioned anchor–grouting reinforcement technology is proposed and its supporting effect is studied by numerical simulation. Field application and deformation monitoring results show that the new support scheme can meet the requirement of stability control for soft and broken surrounding rock of crossed-roadways and has guiding significance for the similar projects.

Mechanism of Secondary Lining Cracking and its Simulation for the Dugongling Tunnel

A large amount of lining cracks, pavement uplift and cable trench overturning occurred during the operation of the Dugongling tunnel, which finally led to the closure of the whole line. The study on the lining cracking mechanism is very important to ensure that the tunnel opens again early. First, the actual cracks of the lining were investigated in the field. According to the geological data, the expansion and softening of the surrounding rock is likely to be the main cause of lining cracking. To verify this inference, a corresponding research program was drawn up. Then, representative sections were selected and the back-analysis method was used to simulate the cracking characteristics of the lining. Based on the simulation results, the cracking mechanisms were studied. The research showed that the increased water content caused the expansion and softening of the surrounding rock, increased the uneven loading on the lining structure, and, finally, led to lining cracking and pavement uplift. Based on the cracking mechanism, progressive enhancement schemes were proposed for each section. The research results of this paper can provide a reference for the design and construction of this project and similar projects.

Predicting of land surface uplift caused by the flooding of underground coal mines – A case study

The liquidation of underground mines as in flooding of mining excavations and goafs often leads to movements of the rock mass and land surface as a result of pressure changes in the flooded zones. The changes resulting from the rising water table caused by the changes in the stress and strain state as well as the physical and mechanical properties of rock layers can lead to damage to surface construction objects and environmental changes, such as chemical pollution of the surface water. This is why the ability to predict the movements of rock masses generated as a result of mine closure by flooding serves a key function in relation to the protection of the land surface and buildings present thereon. Currently, the world literature presenting methods of forecasting ground surface uplift during mine flooding is very modest and is limited to three analytical methods: Pöttgens', Fenk's and Sroka's method. However, the numerical methods used in this subject area are limited only to modeling the phenomenon using the back analysis method and are not used for forecasting. The author's methodology based on the finite element method presented in the article allows on, after calibrating the 3D numerical model using surface subsidence measurements, as well as geological and mining data, to simulate the flooding process by changing the density of the medium according to classical soil and rock mechanics principles and then obtain prognostic values for ground surface uplift. This methodology has been used in the example of a German underground coal mine, which has been flooded in 1999. The authors have built a 3D numerical model and then performed simulations. The results of calculations were compared with the results of measurements obtaining compliance at the level of 5%.

Parameter inversion and location determination of evolutionary weak layer for open-pit mine slope

In light of the complex and dynamic mechanical properties of evolving weak strata in open-pit mines, and the consequent difficulty of determining their mechanical parameters, this study uses the ultimate balance theory, along with the back analysis method combined with monitoring data on field displacement, to carry out parameter inversion using the FLAC3D numerical simulation software. The edge slope of a working pit of the Weijiamao open-pit mine was used as research object to this end. As the results obtained by the constitutive model were consistent with the field monitoring data, the evolving weak strata in the slope and the position of the landslide in the mine could be obtained. The landslide was directed northeast. The mechanism of the edge slope of the working pit was identified as unloading shear failure, and the feasibility of the method of parameter inversion was verified. The internal friction angle ϕ and cohesion C of evolving weak strata in the slope of the open-pit mine were also obtained, where this compensated for the deficiency of laboratory tests and enabled the transformation from qualitative to quantitative analysis. This can provide a reliable basis for the safe operation of open-pit mines.

IDE-MLSSVR-Based Back Analysis Method for Multiple Mechanical Parameters of Concrete Dams

AbstractA back analysis method based on multioutput least-squares support vector regression machine (MLSSVR) and improved differential evolution algorithm (IDE) is proposed to estimate multiple mec...

Automatic back analysis of mechanical parameters using block discrete element method and PSO algorithm

This article presents a novel automatic PSO-based back analysis method for mechanical parameters of block discrete element method. Incorporating particle swarm optimization (PSO) algorithm and recursive algorithm, a parameter back-analysis program is developed. Two cases, a laboratory test of columnar jointed rock mass and a typical landslide, are implemented for validation. The results show that the proposed method is feasible and accurate. As an automatic solution for parameter back analysis, this work has broad application prospects in geotechnical engineering.

Analytical Model and Back-Analysis for Pile-Soil System Behavior under Axial Loading

The interaction mechanism between piles and soils is very complicated. The load transfer function is generally nonlinear and is affected by factors such as pile side roughness, soil characteristics, section depth, and displacement. Therefore, it is difficult to solve the pile-soil system based on load transfer function. This paper presents a new method to study the soil-pile interaction problem with respect to axial loads. First, the shapes of the axial force-displacement curves at different depths and the displacement distribution curves along pile axis at different pile-top displacements were analyzed. A simple exponential function was taken as relationship model to express the relationship curves between two distribution functions of axial force and displacement along pile shaft obtained by using the geometric drawing method. Second, a new analytical model of the pile-soil system was established based on the basic differential equations for pile-soil load transfer theory and the relationship model and was used to derive the mathematical expressions on the distribution functions of the axial force, the lateral friction, and the displacement along pile shaft and the load transfer function of pile-side. We wrote the MATLAB program for the analytical model to analyze the influence laws of the parametersuandmon the pile-soil system characteristics. Third, the back-analysis method and steps of the pile-soil system characteristics were proposed according to the analytical model. The back-analysis results were in good agreement with the experimental results for the examples. The analysis model provides an effective way for the accurate design of piles under axial loading.

Back analysis of tensile test drilled shafts and barrettes

ABSTRACT This paper presents a method to interpret the tensile test results for a concrete pile when the concrete is stressed beyond its tensile limit, which results in anomalies in the strain gage data that is collected during active load testing. Two drilled shafts and two barrettes are studied using the proposed method. Any pile loads that are computed using strain data and which are greater than the specific applied load are discarded because these results are physically impossible. A Spline function is then used to fit the remaining valid data for pile load along the depth. Using the fitted Spline function and assuming that the behavior at the pile/soil interface can be modeled as a hyperbolic model, a back-analysis method is used to determine the parameters for the model. The software, SHAFT is used to compare the results for the back analysis and the measured gage data. It is found that the back-analysis method that is used in this study gives a reasonable interpretation of the pile loading test results, such as the t-z curves, using the pile force and a depth fitting function.

Image‐based back analysis for tension estimation of suspension bridge hanger cables

In suspension bridges, hanger cables are the main load-supporting members. The tension of the hanger cables of a suspension bridge is a very important parameter for assessing the integrity and safety of the bridge. In general, indirect methods are used to measure the tension of the hanger cables of a suspension bridge in use. A representative indirect method is the vibration method, which extracts modal frequencies from the cables' responses and then measures the cable tension using the cables' geometric conditions and the modal frequencies. In this study, ambient vibration tests were conducted on a suspension bridge in use to verify the validity of the image-based back analysis method, which can estimate the tension of remote hanger cables using the modal frequencies as a parameter. The tension estimated through back analysis, which was conducted to minimize the difference between the modal frequencies calculated using finite element analysis of the hanger cables and the measured modal frequencies, was compared with that measured using the vibration method. It was confirmed that reliable tension estimation is possible even with low-order modal frequencies when the image-based back analysis method is used.

Tunnel Back Analysis Based on Differential Evolution Using Stress and Displacement

The stability of tunnels has mainly been evaluated based on displacement. Because displacement due to the excavation process is significant, back analysis of the structure and ground can be performed easily. Recently, the length of a segment‐lined tunnel driven by the mechanized tunneling method is increasing. Because the internal displacement of a segment‐lined tunnel is trivial, it is difficult to analyze the stability of segment‐lined tunnels using the conventional method. This paper proposes a back analysis method using stress and displacement information for a segment‐lined tunnel. A differential evolution algorithm was adopted for tunnel back analysis. Back analysis based on the differential evolution algorithm using stress and displacement was established and performed using the finite difference code, FLAC3D, and built‐in FISH language. Detailed flowcharts of back analysis based on DEA using both monitored displacement stresses were also suggested. As a preliminary study, the target variables of the back analysis adopted in this study were the elastic modulus, cohesion, and friction angle of the ground. The back analysis based on the monitored displacement is useful when the displacement is significant due to excavation. However, the conventional displacement‐based back analysis is unsuitable for a segment‐lined tunnel after construction because of its trivial internal displacement since the average error is greater than 32% and the evolutionary calculation is finalized due to the maximum iteration criteria. The average error obtained from the proposed back analysis algorithm using both stress and displacement ranged within approximately 6–8%. This also confirms that the proposed back analysis algorithm is suitable for a segment‐lined tunnel.

Two Improvements to Gardner's Method of Measuring the Hydraulic Conductivity of Non‐saturated Media: Accounting for Impedance Effects and Non‐constant Imposed Suction Increment

AbstractGardner's (1956, https://doi.org/10.2136/sssaj1956.03615995002000030006x) transient method of measuring the hydraulic conductivity function of unsaturated media has been largely used, together with the improved graphical method proposed by Kunze and Kirkham (1962, https://doi.org/10.2136/sssaj1962.03615995002600050006x) to account for the impedance effect resulting from using a low hydraulic conductivity ceramic disk in porous plate testing. These methods are nowadays seldom used, since they have been replaced by numerical back analysis and methods for parameter optimization. Based on tests carried out on a specific device allowing to determine the water retention and transport properties of water in a coarse granular media at low suctions (up to 50 kPa), it was found necessary to account (i) for impedance effects and (ii) for the effects of nonconstant suction increments, as is often the case when using the hanging column technique. A new method is proposed to account for impedance effects, based on an analytical solution of the equations governing water transfer. The validity of this method is tested by considering experimental data from three distinct materials: a coarse green roof volcanic substrate, poorly graded sand, and undisturbed silty clay. Compared to the graphical method Kunze and Kirkham's method, it is less operator‐dependent and hence more objective. It is also simpler than numerical back analysis methods, since it does not require any use of numerical code or parameter optimization algorithm, providing a more direct and reliable access to the hydraulic conductivity. An analytical solution is also proposed to solve the problem resulting from the application of a nonconstant suction increment.

Analysis of rainfall-triggered landslide in Baleagung Village, Magelang Regency, Central Java

The majority of Magelang Regency is considered as intermediate and high susceptible areas against landslide. As happened in Baleagung Village in January 2018, a landslide has damaged local residency buildings. The landslide occurred because of high-intensity rainfall on the day before. Meanwhile, cracks formed in several areas that were indicated by the slope around the village were still actively moving. Therefore, mitigation is urgently needed to minimize the risk of further landslide movement. This study aimed to analyze the mechanism of ground movement at Baleagung Village which was affected by rainfall. This research was conducted by using back-analysis method to determine the initial groundwater level conditions and its effect on the slope stability. Soil parameters were obtained by laboratory testing and permeability tests in the field. Subsequently, rainfall data was collected from Himawari-8 satellite and used as the data input parameter for numerical analysis using SEEP/W and SLOPE/W. The results obtained from back-analysis showed that rainfall infiltration into the soil affected the degradation of safety factor value that was caused by the increase of pore-water pressure and the decrease of effective stress in a certain period. The degradation of safety factor was caused by the increasing of soil saturation degree when water infiltrated into the ground.

Reliability back analysis of landslide shear strength parameters based on a general nonlinear failure criterion

The shear strength parameters of a sliding surface are important parameters for the stability evaluation and engineering design of a landslide. To consider the randomness, uncertainty and the nonlinear characteristics of the geotechnical, the reliability back analysis method of determining shear strength parameters of a landslide based on nonlinear Mohr-Coulomb failure criteria is proposed in this study. By analyzing a slope with linear failure surface, the feasibility and validity of the proposed method are discussed in an engineering case. The proposed method provides a new choice and reference for the determination of the shear strength parameters of the sliding surface of a landslide.

Fully coupled analysis of consolidation by prefabricated vertical drains with applications of constant strain rate tests: Case studies and an open-source program

The paper proposes a new approach to use measured data of the constant strain rate test (CRST) for analysis of consolidation by prefabricated vertical drains (PVDs). Each PVD has an influence zone that idealised as a unit cell. Consolidation behaviour of a unit cell is studied with an axisymmetric finite element (FE) model based on Biot's theory. From a CRST data, ASTM-D4186 or the back-analysis method is used to obtain stress-dependent parameters for the model. An open-source FE software named CONAXIS was developed for these purposes. Data from two projects in Mekong Delta Vietnam were used in this study. In the first project, nine CRSTs with various depths from a borehole were conducted. Two tests were chosen to be simulated using the proposed approach implemented in CONAXIS and the soft soil model in PLAXIS for validation and comparison purposes. Comparing to the laboratory data, CONAXIS gave more accurate results than PLAXIS. Then CONAXIS was used to calculate the settlement of the ground surface during the construction process with different scenarios. For the second project, six CRSTs from three boreholes were used to set up the model in CONAXIS. Modelled results of both projects showed good agreements with field monitoring data.

A nonlinear elasto‐viscoplastic model for clayed rock and its application to stability analysis of nuclear waste repository

AbstractThe creep behavior is one of the most important topics on the stability analysis of rock mass in underground engineering. In order to characterize the creep deformation of clayey rock more accurately, this study proposes a nonlinear elasto‐viscoplastic (EVP) model based on a new yield surface and a modified Mohr‐Coulomb (MC) flow potential. First, the finite element (FE) formulation for the constitutive creep behavior is derived and a user subroutine to define the material's mechanical behavior (UMAT) is implemented in the commercial finite element package ABAQUS. The numerical simulation is validated against the triaxial creep tests of a cylinder, and the results show that the proposed model can effectively predict the creep responses of clayey rock and thus compensate for the deficiency of the creep model in ABAQUS. Then, the proposed model is used to study the long‐term stability of clayey rock formation surrounding an underground experimental site. Based on the results from triaxial creep tests and in situ monitoring of the lining deformation, the time‐dependent material creep parameters are obtained for rock by using the back analysis method. The predicted lining deformation is in good agreement with the field data with most of the relative errors less than 3.5%. The creep zone in the surrounding rock demonstrates that the horizontal scope is larger than the vertical one. The proposed model is proven to provide accurate prediction on the creep characteristics of clayey rock, showing a potential to assess the long‐term stability and reliability of large‐scale underground engineering structures.

A novel displacement back analysis method considering the displacement loss for underground rock mass engineering

Most back analysis methods for geotechnical engineering are based on the measured displacement. However, before the monitoring sections are assembled, the displacement—termed the displacement loss—has already been induced; this displacement is difficult to determine, and thus, it is not considered in the back analysis. In the present study, a novel displacement back analysis method considering the displacement loss is developed, that can obtain not only the reasonable mechanical parameters of rock masses but also the displacement loss. To reduce the computational cost of back analysis, a new hybrid optimization algorithm based on the Gaussian process (GP) and particle swarm optimization (PSO) is presented. The GP is used as an inexpensive fitness evaluation surrogate to predict the global optimum solution and accelerate the local search of PSO, which is employed to determine the best mechanical parameters for the model. Combined with FLAC3D numerical analysis, a novel back analysis method called GP-PSO-FLAC3D is proposed. The results of a case study demonstrate that this method can effectively predict more reasonable mechanical parameters and displacement loss using the monitored displacement. An engineering application in the auxiliary tunnel of the Jinping II hydropower station indicates that the elastic deformation of the surrounding rock increases rapidly after excavation, especially for deep tunnels, thereby resulting in a large displacement loss. The back analysis results for the main powerhouse of the Taian pumped storage power station indicate that the displacement loss also exists in engineering processes involving ordinary geostress conditions. Therefore, the displacement loss of a surrounding rock mass cannot be ignored in the stability evaluation or back analysis of underground engineering, especially for deep underground rock engineering.

Subsea tunnel site selection evaluation: A new evaluation model

The foundation of subsea tunnel construction is that the expected benefit of the tunnel is greater than the construction cost. The location of subsea tunnels is the most fundamental factor determining the size of both tunnel revenue and cost. At present, the evaluation model for the site selection of subsea tunnels is mainly based on specific engineering projects, and the universal evaluation model for the subsea tunnels site selection is deficient. In this paper, nine parameters including lithology, geologic structure, rock strength, environmental conditions, economic benefit, construction cost, Minimum leakage method, Coal cutting experience under water method, and Norway experience method are selected as evaluation indexes for the site selection of subsea tunnels. The weight of each index is determined by weight back analysis. An evaluation model for subsea tunnel site selection is established based on weight back analysis method and efficacy coefficient method, and evaluation criteria are divided into four grades: excellent, good, medium, and bad. The evaluation model of subsea tunnel site selection is applied to the Qingdao Jiaozhou Bay subsea tunnel, the Xiamen Xiangan subsea tunnel, and the Seikan subsea tunnel. Results show that the site selection of the Qingdao Jiaozhou Bay subsea tunnel is grade II, the site selection of the Xiamen Xiangan subsea tunnel is grade III and the site selection of the Seikan subsea tunnel is grade IV. This new selection process provides a new method for subsea tunnel site selection and evaluation and also provides some reference for site selection of other types of structures.

Full-scale creep test and back-analysis of the long-term settlement of heavy-loaded shallow foundations on a high rockfill embankment

The long-term settlement of heavy-loaded shallow foundations on a high rockfill embankment was studied using a full-scale field creep test, in which four concrete platforms were constructed and loaded on the top of a 24-m-high rockfill embankment. The test programme was introduced, and the long-term deformation of the high rockfill embankment was analyzed based on the monitoring data. Then three-dimensional numerical simulation was performed, using a rheological model for the rockfill. When the model parameters were derived from the laboratory creep tests, the numerical simulation seriously underestimated the long-term settlement compared with field measurements. The model parameters were further back-determined using a two-stage back-analysis method. Reasonable agreement was found between the predicted results and the field measurements, not only for the surface creep settlement but also for the sub-surface creep settlement.

Joint back‐analysis for dynamic material parameters of concrete dam based on time‐frequency domain information

It is significant to obtain dynamic material parameters for checking seismic safety of the concrete dam. At present, it is common to optimize dynamic material parameters based on modal parameters of the concrete dam, which are often identified by seismic response sequences. In this work, except for the frequency-domain index, time-domain index is also extracted from the seismic response sequence and contributes to joint back-analysis for dynamic material parameters. Due to different physical meanings and dimensions of two objective functions, a fast nondominated ranking genetic algorithm with elitist strategy is introduced to conduct the multiobjective optimization, and then, the Pareto optimal frontier of dynamic parameters of the concrete dam material is obtained. The seismic response sequence of a realistic concrete dam in China is analyzed using the proposed back-analysis method. It shows that joint back-analysis for dynamic material parameters of concrete dam material based on the time-frequency domain has strong ability to identify parameters and good stability, which indicates good application prospect in practical engineering.