FLAC Research Articles

Probabilistic tunnel face stability analysis: A comparison between LEM and LAM

It is a key issue in the tunnel design to evaluate the stability of the excavation face. Two efficient analytical models in the context of the limit equilibrium method (LEM) and the limit analysis method (LAM) are used to carry out the deterministic calculations of the safety factor. The safety factor obtained by these two models agrees well with that provided by the numerical modelling by FLAC 3D, but consuming less time. A simple probabilistic approach based on the Mote-Carlo Simulation technique which can quickly calculate the probability distribution of the safety factor was used to perform the probabilistic analysis on the tunnel face stability. Both the cumulative probabilistic distribution and the probability density function in terms of the safety factor were obtained. The obtained results show the effectiveness of this probabilistic approach in the tunnel design.

Incorporating ground motion effects into Sasaki and Tamura prediction equations of liquefaction-induced uplift of underground structures

In metropolitan areas, the quantity and density of the underground structure increase rapidly in recent years. Even though most damage incidents of the underground structure were minor, there were still few incidents causing a great loss in lives and economy. Therefore, the safety evaluation of the underground structure becomes an important issue in the disaster prevention plan. Liquefaction induced uplift is one important factor damaging the underground structure. In order to perform a preliminary evaluation on the safety of the underground structure, simplified prediction equations were introduced to provide a first order estimation of the liquefaction induced uplift. From previous studies, the input motion is a major factor affecting the magnitude of the uplift. However, effects of the input motion were not studied and included in these equations in an appropriate and rational manner. In this article, a numerical simulation approach (FLAC program with UBCSAND model) is adopted to study effects of the input motion on the uplift. Numerical results show that the uplift and the Arias Intensity (Ia) are closely related. A simple modification procedure to include the input motion effects in the Sasaki and Tamura prediction equation is proposed in this article for engineering practices.

A case study on asymmetric deformation mechanism of the reserved roadway under mining influences and its control techniques

The double-lane arrangement model is frequently used in underground coal mines because it is beneficial to improve the mining efficiency of the working face. When the double-lane arrangement is used, the service time of the reserved roadway increases by twice, which causes several difficulties for the maintenance of the roadway. Given the severe non-uniform deformation of the reserved roadway in the Buertai Coal Mine, the stress distribution law in the mining area, the failure characteristics of roadway and the control effect of support resistance (SR) were systematically studied through on-site monitoring, FLAC 3D numerical simulation, mechanical model analysis. The research shows that the deformation and failure of the reserved roadway mainly manifested as asymmetrical roof sag and floor heave in the region behind the working face, and the roof dripping phenomenon occurred in the severe roof sag area. After the coal is mined out, the stress adjustment around goaf will happen to some extent. For example, the magnitude, direction, and confining pressure ratio of the principal stress at different positions will change. Under the influence of high-stress rotation, the plastic zone of the weak surrounding rock is expanded asymmetrically, which finally leads to the asymmetric failure of roadway. The existing roadway support has a limited effect on the control of the stress field and plastic zone, i.e., the anchor cable reinforcement cannot fully control the roadway deformation under given conditions. Based on obtained results, using roadway grouting and advanced hydraulic support during the secondary mining of the panel 22205 is proposed to ensure roadway safety. This study provides a reference for the stability control of roadway with similar geological conditions.

Associations of maternal quitting, reducing, and continuing smoking during pregnancy with longitudinal fetal growth: Findings from Mendelian randomization and parental negative control studies.

Maternal smoking during pregnancy is an established risk factor for low infant birth weight, but evidence on critical exposure windows and timing of fetal growth restriction is limited. Here we investigate the associations of maternal quitting, reducing, and continuing smoking during pregnancy with longitudinal fetal growth by triangulating evidence from 3 analytical approaches to strengthen causal inference. We analysed data from 8,621 European liveborn singletons in 2 population-based pregnancy cohorts (the Generation R Study, the Netherlands 2002-2006 [n = 4,682]) and the Born in Bradford study, United Kingdom 2007-2010 [n = 3,939]) with fetal ultrasound and birth anthropometric measures, parental smoking during pregnancy, and maternal genetic data. Associations with trajectories of estimated fetal weight (EFW) and individual fetal parameters (head circumference, femur length [FL], and abdominal circumference [AC]) from 12-16 to 40 weeks' gestation were analysed using multilevel fractional polynomial models. We compared results from (1) confounder-adjusted multivariable analyses, (2) a Mendelian randomization (MR) analysis using maternal rs1051730 genotype as an instrument for smoking quantity and ease of quitting, and (3) a negative control analysis comparing maternal and mother's partner's smoking associations. In multivariable analyses, women who continued smoking during pregnancy had a smaller fetal size than non-smokers from early gestation (16-20 weeks) through to birth (p-value for each parameter < 0.001). Fetal size reductions in continuing smokers followed a dose-dependent pattern (compared to non-smokers, difference in mean EFW [95% CI] at 40 weeks' gestation was -144 g [-182 to -106], -215 g [-248 to -182], and -290 g [-334 to -247] for light, moderate, and heavy smoking, respectively). Overall, fetal size reductions were most pronounced for FL. The fetal growth trajectory in women who quit smoking in early pregnancy was similar to that of non-smokers, except for a shorter FL and greater AC around 36-40 weeks' gestation. In MR analyses, each genetically determined 1-cigarette-per-day increase was associated with a smaller EFW from 20 weeks' gestation to birth in smokers (p = 0.01, difference in mean EFW at 40 weeks = -45 g [95% CI -81 to -10]) and a greater EFW from 32 weeks' gestation onwards in non-smokers (p = 0.03, difference in mean EFW at 40 weeks = 26 g [95% CI 5 to 47]). There was no evidence that partner smoking was associated with fetal growth. Study limitations include measurement error due to maternal self-report of smoking and the modest sample size for MR analyses resulting in unconfounded estimates being less precise. The apparent positive association of the genetic instrument with fetal growth in non-smokers suggests that genetic pleiotropy may have masked a stronger association in smokers. A consistent linear dose-dependent association of maternal smoking with fetal growth was observed from the early second trimester onwards, while no major growth deficit was found in women who quit smoking early in pregnancy except for a shorter FL during late gestation. These findings reinforce the importance of smoking cessation advice in preconception and antenatal care and show that smoking reduction can lower the risk of impaired fetal growth in women who struggle to quit.

Development and application of a structured clay model in FLAC

Development and application of a structured clay model in FLAC

Considerations of rock dilation on modeling failure and deformation of hard rocks—a case study of the mine-by test tunnel in Canada

For the compressive stress-induced failure of tunnels at depth, rock fracturing process is often closely associated with the generation of surface parallel fractures in the initial stage, and shear failure is likely to occur in the final process during the formation of shear bands, breakouts or V-shaped notches close to the excavation boundaries. However, the perfectly elastoplastic, strain-softening and elasto-brittle-plastic models cannot reasonably describe the brittle failure of hard rock tunnels under high in-situ stress conditions. These approaches often underestimate the depth of failure and overestimate the lateral extent of failure near the excavation. Based on a practical case of the mine-by test tunnel at an underground research laboratory (URL) in Canada, the influence of rock mass dilation on the depth and extent of failure and deformation is investigated using a calibrated cohesion weakening and frictional strengthening (CWFS) model. It can be found that, when modeling brittle failure of rock masses, the calibrated CWFS model with a constant dilation angle can capture the depth and extent of stress-induced brittle failure in hard rocks at a low confinement if the stress path is correctly represented, as demonstrated by the failure shape observed in the tunnel. However, using a constant dilation angle cannot simulate the nonlinear deformation behavior near the excavation boundary accurately because the dependence of rock mass dilation on confinement and plastic shear strain is not considered. It is illustrated from the numerical simulations that the proposed plastic shear strain and confinement-dependent dilation angle model in combination with the calibrated CWFS model implemented in FLAC can reasonably reveal both rock mass failure and displacement distribution in vicinity of the excavation simultaneously. The simulation results are in good agreement with the field observations and displacement measurement data.

ASSESSMENT OF STRESS DISTRIBUTION IN DARAB'S ROODBAL EMBANKMEND DAM USING COMMERCIAL CODE FLAC

Abstract. Evaluation of performance and behavior of a dam is viable by monitoring of dam using instrumentation. Stress and deformation analysis during end of construction is one of the important parameters in analysis, design, and stability of an embankment dam. In this study, a comparison between various models in stress and deformation analysis of Roodbal dam during end of construction using finite element method in FLAC software was established. The results showed that Mohr-Coulomb model has good accuracy in analysis of vertical settlement of dam core. It was also known that vertical displacement of Darab's Roodbal dam is slow ascending which is slower in upstream than downstream shell. The overall stress in dam body is also slow ascending that shifted from lower level near the core to middle lower level of core during first impoundment through the end of first 10-year utilization period. The analysis over whole body showed that after completion of construction and then initial impoundment of dam pore water pressure in the core of dam is 370 kPa which was the same in middle level of core. According to analysis conducted over whole body of dam, displacement reached 96 cm after completion of construction which was also the same for one-third middle level of core.

Effects of Adding Lime to Improve Levee Soil Layer Using Numerical Method (Case Study: Part VI of Bijar – Zanjan Four-Lane)

Abstract. Soil stabilization is defined as improvement of its physical and engineering properties to provide a series of pre-determined objectives. Some soil are poor due to unfavorable technical specifications or consisting clay or silt to work in road construction. In this study, the effects of lime application in soil stabilization have been investigated in levee soil layers of part 6 of Bijar – Zanjan four-lane in Kordestan province. In the next stage, remodeling of this issue in the implementation of reinforcing layers and the results of two of these cases were compared. Finite difference FLAC.2D software was used to model soil profiles. Comprehensive results were obtained examining the role of factors such as soil type, the percentage of lime, water content, particle of reinforcing layers, energy caused by density etc.

Evaluation of Active Earth Pressure behind Retaining Walls with Lateral Movements under Cyclic Loading

To have a more clear judgment about distribution and point of application of active earth pressure behind retaining walls, simultaneous studies on both physical and numerical models are required. In this study the behavior of backfill soil under static and cyclic surcharges has been evaluated using the FLAC software while the wall has some lateral movements. To verify the numerical model the results have been compared with those measured behind the same physical model developed in soil mechanics laboratory of Amirkabir University of Technology. After that an extensive parametric studies have been carried out to investigate the influences of different factors such as amplitude, position and type of surcharges as well as backfill soil properties and the amount of the wall movements. The results of these studies have been explained and discussed in different section of the present paper.

Dynamic Analysis of Hunchback Concrete Block Quay Wall and Recommendation for Optimum Geometrical Parameters

Concrete block quay wall, is consisted of solid or hollow concrete blocks sitting on each other to form the wall section. In this type of wall geotechnical condition, design calculation and ability of contractor determine the blocks dimensions. In these walls resistance against applied loads is provided by wall weight. To reduce active soil pressure and to increase wall stability in seismic regions, hunchback section is formed by placing a protruding knapsack to landside. In this formation according to backward tendency of blocks, wall stability will be increased. For knapsack section, special considerations such as trimming the blocks, more accurate setting blocks and consequently increasing construction costs should be considered. “Inclination angle” and “ratio of inclination height to entire wall height” play an important role in geometrical design of hunchback section. Selecting proper amount of these two helps decreasing soil dynamic pressure behind the wall and consequently reduces the concrete volume required construction. In this article, in order to evaluate the proper amount of “inclination angle” and “ratio of inclination height to entire wall height”, applying numerical modeling, FLAC 2D is used for modeling different hunchback block quay walls. Comparing the results, the proper amounts of the aforementioned parameters have been recommended. The results of modeling are categorized in below sections. - Soil dynamic pressure behind the wall, - Displacement of upper block - Concrete volume required for construction of unit berth length. The results show that a negative inclination angle of 50-55 degree which starts from middle of the wall height is the optimum formation to reduce the soil pressure and the concrete volume per berth length unit.