Landfill side slope lining system performance: A comparison of field measurements and numerical modelling analyses

Low permeability engineered landfill barriers often consist of a combination of geosynthetics and mineral layers. Even though numerical modelling software is applied during the landfill design process, a lack of data about mechanical performance of landfill barriers is available to validate and calibrate those models. Instrumentation has been installed on a landfill site to monitor multilayer landfill lining system physical performance. The lining system comprises of a compacted clay layer overlaid by high density polyethylene geomembrane, geotextile and sand. Data recorded on the site includes: geosynthetic displacements (extensometers), strains (fibre optics, Demec strain gauges, extensometers) and stresses imposed on the liner (pressure cells). In addition, temperature readings were collected by a logger installed at the surface of the geomembrane, at the clay surface using pressure cell thermistors and air temperature using a thermometer. This paper presents readings collected throughout a period of three years and compares this measured performance with the corresponding numerical modelling of the lining system for stages during construction. Numerical modelling predictions of lining system behaviour during construction are comparable with the measurements when the geosynthetics are covered soon after placement, however, where the geosynthetics are left exposed to the sun for an extended period of time, in situ behaviour of the geosynthetics cannot be replicated by the numerical analysis. This study highlights the significant influence of the effect of temperature on geosynthetics displacements. A simple thermal analysis of the exposed geosynthetics is used to support the explanation for observed behaviour.     

Optimized back analysis method for stress determination based on identification of local stress measurements and its application

This paper presents an optimization method, including the identification of local stress measurements and a two-stage back analysis, to determine in situ stress under complex conditions. Firstly, the macro-regional distribution characteristics of in situ stress can be interpreted by analyzing regional geological conditions, topography, as well as rock mass failure phenomena. Then, a stereographic projection method (SPM) is used to determine the distribution features of stress tensor on an arbitrary plane. On this basis, some representative measured data, which can reflect the distribution characteristics of in situ stress in the study area can be identified. After that, a first-stage back analysis by finite difference method (FDM) is used to calculate the field stresses from the selected in situ measured data. The obtained boundary stresses are taken to consist of a constant term, a term that varies linearly with depth, and a hyperbolic term. Further, a second-stage back analysis by discrete element method (DEM) is carried out to determine the local field stresses which are significantly affected by discontinuities and excavations. The FDM back analysis results can serve as the initial input stress conditions for DEM analysis, and the optimum stress conditions for DEM analysis will be reasonably obtained by using the uniform design method through the second-stage back analysis. To show the feasibility of the optimized method, it is applied to the Wudongde Hydropower Station to determine in situ stress. Based on local stress measurements by borehole stress relief method using hollow inclusion strain gauges, some representative measured data are selected by SPM. The unknown state of stress in the station is estimated through the first-stage back analysis by FDM. For some local key areas, encountered discontinuities or excavations in the vicinity, the local field stresses are calculated through the second-stage back analysis by DEM. Finally, the results are compared with those elicited from the borehole stress relief method. It is shown that the calculated results by the first-stage back analysis agree well with the measured ones on the whole, but the discrepancy is large in the vicinity of discontinuities as well as excavation disturbance. However, the calculated results by the second-stage back analysis roughly coincide with the measured data with lower allowable error.

     

Analysis of stress measurements using a numerical model methodology

A method is described that enables the boundary conditions of numerical models to be calibrated to individual or groups of stress measurements. The method was developed to interpret stress measurements made in mines where it is not possible to locate the measurement points far enough away from excavations to obtain a direct measurement of the pre-mining stress field. It can also be used to analyze measurements that are influenced by surface topography. The stress field at any point is assumed to be comprised of gravitational and tectonic components. The tectonic component is assumed to act entirely in the horizontal plane in the far-field and at the model boundary. Unit normal and shear tractions are applied to the model boundaries and the response is computed at the location of the measurement points in the model. An optimization procedure is used to compute the proportions of each unit response tensor that is required, in addition to the gravitational stress, to reproduce the measured stress at the measurement point in the model. Scaling of the measured stress tensor can be included in the optimization to account for incorrect rock modulus scaling of the measured strains. The method is demonstrated using a synthetic set of stresses from a numerical model in which the measurements are influenced by a nearby excavation and topography.     

High-frequency field measurements and time-dependent computational modelling for wind turbine siting

Field measurements by cup anemometer at a sampling frequency of 1 Hz and computational modelling were used for characterisation of the wind conditions at a region of complex orography. Because the wind rose displayed low wind occurrences and high turbulence intensity for specific wind directions, spectral analysis of computational data was performed which showed the existence of preferred time scales for some wind directions, in agreement with spectral analysis of field data. Empirical orthogonal functions and frame by frame analysis of the time-dependent computational data showed that these time scales were related with a pair of vortices in the lee of a mountain. We concluded on the appropriateness of the methodology being used, which enabled the determination of these structures without reverting to the use of more complex and time consuming techniques. This study, initiated as part of a wind resource evaluation and possible installation of a wind farm, also proves the need for more detailed determination of the wind conditions, compared with the standard practice in wind turbine siting.     

Designing waste rock barriers by advanced numerical modelling

Design of waste rock barriers forming safety berms for haul trucks requires knowledge of complex interactions which cannot readily be tested by physical means. An advanced numerical model based on non-smooth multi-domain mechanics is presented together with model calibration using limited full-scale experimental data. Waste rock is represented by spherical particles with rolling resistance, and an ultra-class haul truck is represented by a rigid multibody system interconnected with mechanical joints. The model components are first calibrated and then the calibrated model is used for simulating various collision scenarios with different approach conditions and safety berm geometries. Numerical predictions indicate that the width of the berm is most critical for efficiently stopping a runaway truck. The model can also predict if a certain berm geometry is capable of stopping a runaway truck. Results are summarised in a series of diagrams intended for use as design guidelines by practitioners and engineers.     

地铁车站混凝土温度场应力数值模拟分析

以某地铁车站主体结构为研究对象,根据热传导理论及有限元方法,采用ADINA有限元分析软件,仿真模拟了典型的地铁车站结构承受温度荷载下的应力变化情况,分析了结构在不同荷载条件下的应力变化规律,结合结构允许强度峰值,提出了防止结构开裂的施工预防措施。     

Ventilation and radon transport in Dutch dwellings: computer modelling and field measurements

In 1995 and 1996 radon concentrations and effective air flows were measured in approximately 1500 Dutch dwellings built between 1985 and 1993. The goal of this investigation was to describe the trend in the average radon concentration by supplementing the first survey on dwellings built up to 1984 and to quantify the contributions of the most important sources of radon. In the living room of new dwellings the average radon concentration was 28 Bq m(-3), which is 50% higher than in dwellings built before 1970. Measurements of effective air flows showed the most important source of radon in the living room of new dwellings to be the building materials, with an average contribution of 70%. The other 30% comprised outside air and air from the crawl space in equal quantities. The long-term increase in the indoor radon concentration is mainly due to improvements in insulation since 1970, resulting in a fourfold decrease in infiltration through the building shell. Model calculations, supplementing the field measurements, confirmed the dominant effect of increasing airtightness of dwellings compared to effects of the observed trend in the use of building materials.     

Lessons learned from field measurements in tunnelling

Over the past two decades, various numerical methods of analysis have become popular in the field of geotechnical engineering. However, the accuracy of numerical analyses varies considerably, primarily because of the uncertainties involved in modelling ocmplex geological formations with the complex geomechanical characteristics of soils and rocks. Field measurements carried out during construction can be used to overcome this difficulty. The author reviews ways of using measurement results to improve numerical analyses, including the determination of a “hazard warning level” for each measurement item prior to the start of construction, the use of back analysis. The importance of choosing a proper model is also discussed.     

Numerical modelling of industrial indoor environments: A comparison between different turbulence models and supply systems supported by field measurements

Ventilation, heating and cooling systems in industrial premises are important issues as they are related to both energy cost and indoor climate management and the health of the premises’ occupants. The present paper has two aims: (1) to evaluate the performance of Computational Fluid Dynamics (CFD) for planning new or renovating existing industrial ventilation systems, and (2) to evaluate the performance of two different supply principles in a contaminant-intensive process with temperature and density stratification.     

Dynamic analysis of a bridge repaired by CFRP: Experimental and numerical modelling

A significant number of existing reinforced-concrete bridges all over the world require maintenance and repair. Hence, the need for a rapid evaluation procedure for the diagnosis of existing bridges. This paper presents the application of a dynamic analysis methodology for structural evaluation of reinforced-concrete bridges. The methodology is based on the application of ambient vibrations non-destructive testing method and the identification of the structure total response using finite element method. A case study of a three span reinforced concrete bridge in a strong seismic activity area in the north of Algeria is analysed. The ambient vibration testing was carried out on the bridge, before and after its repair by the application of carbon fibre composites. The tests were conducted using an acquisition system made up of four accelerometers with three components placed at specific locations on the bridge. The finite element model gave comparable results to the experimental ambient vibrations tests. The modal parameters of the bridge before and after repair were identified by this in situ testing. The application of composite material to strengthen the structure increases the transverse rigidity of the structure and thus its modal frequency.     

Laboratory and field verification of a new approach to stress measurements using a dilatometer tool

A new method is proposed for in-situ stress measurements. The stresses are estimated from the borehole pressure required to open axial fractures emanating from the borehole, similar to hydrofracture stress measurements. However, the borehole is pressurized by inflation of a dilatometer, such as a packer and a sleeve, and the borehole pressure to reopen the fractures is detected from circumferential deformation of the borehole. The fractures are induced by hydraulic fracturing or sleeve fracturing in advance. To demonstrate this method, we developed a new dilatometer tool. The tool consists essentially of a packer around which deformation transducers sensitive to circumferential strain are affixed at 10°–30° intervals. The packer is inflated to touch the borehole wall, and then the packer pressure is increased step-wise and the resultant strains recorded. Initially, the circumferential strain developed around the borehole is radially symmetric. However, this becomes strongly asymmetric once the fractures begin to open. Thus, the records of strain at different locations around the borehole indicate both the reopening pressure and the orientation of fractures. In the case of a vertical borehole, the detected reopening pressure allows us to estimate the maximum horizontal stress on condition that the minimum horizontal stress is estimated using other approaches.     

新疆沥青路面温度场的数值模拟研究

基于传热学理论,利用ANSYS有限元软件建立路面结构温度场分析的二维有限元模型,通过热稳态计算出路面基层温度场,再进行热瞬态分析,并通过回归方程预测了沥青路面的开裂温度。     

Stress determination from hydraulic fracturing tests: the system stiffness approach

Hydraulic fracturing tests with mud is the most inexpensive method for determining the minor in situ horizontal stress in deep petroleum wells.This paper presents a unified model of pump-in/shut-in and pump-in/flowback tests. The key element of the model is the system stiffness resulting from the stiffness of the fluid volume pressurised, and the stiffness due to the fracture.It is shown that the change in system stiffness at fracture closure is observed in both kinds of tests. The model is used directly in the design and interpretation of the flowback tests, where direct measurement of pressure versus flowback volume, and hence system stiffness is possible. However, the model also predicts the time development of pressure, and suggests methods for the analysis of the pressure versus time curve for both flowback and shut-in tests.We argue that the character of the pressure versus time curve in a shut-in test with mud may be significantly influenced by the low permeability of the mud cake which is formed on the face of permeable formations. This invalidates the traditional interpretation procedures, making new analysis methods outlined in the paper necessary for a successful interpretation of such tests.The model calculations are supported by extensive field data from offshore Norway.     

Sandbox modelling of interactions of landslide deposits with terrace sediments aided by field observation

Bulletin of Engineering Geology and the Environment - The Loess Plateau can be considered as a landslide-prone area in northwest China. The genera consensus about the interaction between landslide...     

Stress determination in soils from variation in level of infrared radiation during unloading

A procedure for and results of model experiments conducted on insulated soil specimens in connection with the development of a method of measuring stresses in soil masses in accordance with an “unloading” scheme using infrared radiometry are discussed. The principal feasibility of the complete unloading of a volume of a slightly cohesive soil without its “instantaneous” failure with simultaneous thermoradiation measurements is indicated. A linear relationship is established between the change in the intensity of the radiation flux during unloading and the contact stress on the boundary of the specimen, which simulates the level of stresses in the mass. The value of the coefficient linking the indicated values can be used to interpret IR measurements conducted in other tests of the type of soil in question, including in-situ tests. The study was conducted with support from the Russian Fund for Fundamental Research (Project. No. 93-05-09246) Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 1, pp. 2–6 January–February, 1995.     

大体积混凝土温度应力场有限元分析与开裂确定

本文利用实测大体积混凝土表层的温度发展规律曲线进行混凝土匹配养护,测试不同龄期混凝土静弹性模量和劈裂抗拉强度。利用实测温度校正的ANSYS数值分析的大体积混凝土温度场和混凝土静弹性模量等作为模拟参数,进行ANSYS数值分析大体积混凝土温度应力场。实验结果表明,混凝土在匹配养护条件下,静弹性模量早期发展很快,之后增长速率逐渐减小,3d可达28d的80.4%,同样,混凝土的3d劈裂抗拉强度(fts)可达84.5%。模拟结果显示:承台表面混凝土温度应力S1的最大值3d时达最大,为4.46MPa;3d时fts/S1值最小为1.12,此后逐渐增大,说明3d是混凝土最易开裂的时刻。对于底面全约束混凝土,温度应力最大值均集中在承台底部四周,这些部位为最易开裂部位。在混凝土表面养护条件良好的情况下,当fts/S1值大于1.12时,大体积混凝土表面不会开裂。     

Back analysis of coalbed strength properties from field measurements of wellbore cavitation and methane production

Direct measurement of field representative strength and permeability characteristics of coalbed methane is difficult due to the fact that coal has a cleated structure. These cleats or discontinuities vary in orientation and size throughout the coal mass. The cleats' network largely controls the strength of the coal and also provides the passage for fluid flow. Permeability of the cleats varies with the confining stress and shear stress level. Laboratory test results are greatly influenced by the size and orientation of the cleats, and the magnitude and direction of applied stresses. To obtain the large-scale hydraulic and strength properties of coal, afield problem involving wellbore cavitation and a concomitant increase in gas production was numerically analysed. Calibration of the numerical model against the field observations which included a 10 fold enlargement of the wellbore radius and a four fold increase in gas production indicated that the apparent cohesion of coal is approximately 100 kPa and not 2–5 MPa, as was previously thought. The validity of this finding was later confirmed by performing a set of laboratory direct shear tests at low effective stress levels. The findings from this study have significant implications in the design of wellbores for production of coalbed methane.     

Inverse modelling of thermal conductivity from temperature measurements at the Prototype Repository, Äspö HRL

The Prototype Repository, at the Äspö HRL (Hard Rock Laboratory), is a demonstration project for the deposition of spent nuclear fuel, and provides a full-scale reference for testing predictive models relating to a spent nuclear fuel repository, both its individual components as well as the complete system. The final layout involves six deposition holes, four in an inner section and two in an outer, each fitted with an electrically heated canister. The access tunnel is backfilled with a mixture of bentonite and crushed rock. In 2001, the inner section was completed and monitoring of the heating process started. Temperature measurements in the rock mass are performed at 37 different points.In this paper, the measured thermal response in the surrounding rock is analysed by inverse modelling of the thermal conductivity of the rock mass. A three-dimensional finite difference model of the prototype repository (canisters, buffers, tunnel, etc.) is used to calculate the transient temperature increase due to the heat generation in the canisters. The value of a homogeneous rock thermal conductivity is chosen to obtain the best fit with measured data for each of the 37 temperature sensor points. The evaluation period for the fitting procedure is varied in order to study sensitivity to different time-scales.Measurements of thermal properties have been conducted within the prototype repository prior to the full-scale test. The thermal properties were predicted based on both field and laboratory measurements. These predictions are verified by comparison with thermal conductivity values calculated through inverse modelling.     

Assessment of the behavior of an active earth-slide by means of calibration between numerical analysis and field monitoring

The assessment of hazards associated with active landslides and the related risk management takes advantage nowadays of using the integration of information arising from field monitoring data, including both displacement data, at ground surface and at depth, and pore pressure measurements well distributed throughout the landslide area, along with the results of numerical models. This paper provides an example of the application of this methodological approach to a case study represented by an active sector of the large Montaguto earthslide, located in the Italian Southern Apennines, which has shown in recent years a continuous slow movement, despite the draining interventions executed in 2011 and the general stability of the other portions of the earthslide. The near real-time topographic monitoring network installed in 2010 shows the presence of different kinematic sectors within the same landslide body, characterised by different velocities and evolution trends. After the proper emergency phase occurred in 2010, a specific area has still shown in 2011 and 2012 clear signs of activity, with acceleration stages generally recorded in the Spring. In order to explore the factors that presumably control the activity of this landslide sector, a two-dimensional finite element model has been developed by using PLAXIS-2D code. Based on the available geological information, pore water pressure measurements and soil geotechnical properties, the numerical results indicate the role of geometry of the landslide mass in sector E as a factor promoting the instability of this specific area. The numerical results are in good agreement with the displacement field measured throughout the landslide channel and confirm that numerical modelling can represent a reliable support for the interpretation of the landslide failure mechanism and the corresponding evolution, when calibrated against the in situ landslide behaviour reconstructed through a monitoring system.