Stress field determination from local stress measurements by numerical modelling

A back analysis using a three-dimensional (3D) boundary element method (BEM) is used to calculate the far-field stress state from local stresses measured in situ. The far-field stresses are decomposed into tectonic and gravitational components and account for the influence of localized faulting and topography. Therefore, the far-field stresses are taken to consist of a constant term, a term that varies linearly with depth, and a hyperbolic term, with one of the principal stresses being vertical. A BEM for inhomogeneous bodies is introduced to calculate elastic gravitational stresses, which is necessary for determination of the far-field stresses.An application to the stress field determination for the Mizunami underground research laboratory (MIU) is carried out. Based upon the local stresses generally measured by conventional hydraulic fracturing (HF), the unknown stress state at MIU is estimated and compared with the measurements carried out recently by the improved HF method with flow rate measurements at the position of straddle packer. After calculating the far-field stress state by BEM back analysis, 3D-finite difference methods (FDM) forward analysis was carried to calculate the in situ stresses at certain locations. The 3D FDM results roughly coincide with the measured results.     

Parameter identification of a Round-Robin test box model using a deterministic and probabilistic methodology

In this paper, two different methodologies are applied to the parameter estimation problem of a computational model of a Round-Robin test box. The numerical model is developed using TRNSYS. A global sensitivity analysis is carried out to determine the most important parameters that should be considered in the subsequent calibration procedure. Using the Bayesian (probabilistic) approach, the posterior distribution of the unknown input parameters is estimated via simulation techniques. Using the deterministic approach, executed in GenOpt, the calibration is performed by the minimization of an objective function that measures the differences between model predictions and real measured data. Parameter estimation results obtained with both methodologies are then compared and discussed. A reduction of the Coefficient of Variation of the Root Mean Square Error (CV (RMSE)) after calibration over 40% with both methods has been obtained, being the CV (RMSE) for calibration and validation periods on average 3.21% and 2.40%, respectively.     

Modelling floor heating systems using a validated two-dimensional ground-coupled numerical model

This paper presents a two-dimensional simulation model of the heat losses and temperatures in a slab on grade floor with floor heating which is able to dynamically model the floor heating system. The aim of this work is to be able to model, in detail, the influence from the floor construction and foundation on the performance of the floor heating system. The ground-coupled floor heating model is validated against measurements from a single-family house. The simulation model is coupled to a whole-building energy simulation model with inclusion of heat losses and heat supply to the room above the floor. This model can be used to design energy efficient houses with floor heating focusing on the heat loss through the floor construction and foundation. It is found that it is important to model the dynamics of the floor heating system to find the correct heat loss to the ground, and further, that the foundation has a large impact on the energy consumption of buildings heated by floor heating. Consequently, this detail should be in focus when designing houses with floor heating.     

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.     

Experimental measurements and numerical modelling of a green roof

Green roof utilisation has been known since ancient times both in hot and cold climates. Nowadays, it has been reconsidered at issue of energy saving and pollution reduction. In this paper, some measurement sessions on a green roof installed by the Vicenza Hospital are described. A data logging system with temperature, humidity, rainfall, radiation, etc. sensors surveyed both the parameters related to the green roof and to the rooms underneath. The aim is to evaluate the passive cooling, stressing the evapotranspiration role in summer time. Furthermore, the enhanced insulating properties have been tested during winter time. A predictive numerical model has been developed in a building simulation software (TRNSYS) to calculate thermal and energy performances of a building with a green roof, varying the meteorological dataset for a specific geographic zone.     

Assessment of Tc-99 monitoring within the western Irish Sea using a numerical model

Water circulation patterns and associated material transport within a highly dynamic system such as the Irish Sea are complex phenomena. Although Tc-99 monitoring programme undertaken by the Radiological Protection Institute of Ireland provides a good insight to the material distribution on the east coast of Ireland, transport patterns within the Irish Sea have not been fully explored. In this study a validated transport model was used to hindcast transport of Tc-99 discharged from the Sellafield plant to determine extents of Tc-99 migration within the Irish Sea and reassess transit times to east coast of Ireland. Transit times are also estimated within a context of changes in meteorological conditions and fluctuations in discharges. Additionally, seasonal and inter-annual circulation patterns were examined.Relationships between discharge times and timing of far field concentrations are highly variable and are dependant on sea dynamics controlling the accumulation and removal of Tc-99 mass. Transport towards the Irish east coast, and consequently transit times, vary intra- and inter-annually, and depend on the prevailing hydrodynamic conditions resulting from meteorological conditions. The transit times from Sellafield to Balbriggan fall within the wide range of 30-240 days; with summer releases resulting in the shortest transit times. The model also indicated a strong relationship between summer concentration peaks on the east coast of Ireland and the strength of the Western Irish Gyre. Sudden increases of Tc-99 concentrations at Balbriggan coincide with peak of sea surface temperatures when the gyre is strongest and when advection is fastest.The adequacy of the current radionuclide monitoring programme within the western Irish Sea is evaluated, and recommendations are made for the development of a more optimised monitoring programme.     

Simulation of a domestic ground source heat pump system using a three-dimensional numerical borehole heat exchanger model

Common approaches to the simulation of borehole heat exchangers (BHEs) assume heat transfer in circulating fluid and grout to be in a quasi-steady state and ignore fluctuations in fluid temperature due to transport of the fluid around the loop. However, in domestic ground source heat pump (GSHP) systems, the heat pump and circulating pumps switch on and off during a given hour; therefore, the effect of the thermal mass of the circulating fluid and the dynamics of fluid transport through the loop has important implications for system design. This may also be important in commercial systems that are used intermittently. This article presents transient simulation of a domestic GSHP system with a single BHE using a dynamic three-dimensional (3D) numerical BHE model. The results show that delayed response associated with the transit of fluid along the pipe loop is of some significance in moderating swings in temperature during heat pump operation. In addition, when 3D effects are considered, a lower heat transfer rate is predicted during steady operations. These effects could be important when considering heat exchanger design and system control. The results will be used to develop refined two-dimensional models.     

Searching for critical slip surfaces of slopes using stress fields by numerical manifold method

This study first reviews the numerical manifold method (NMM) which possesses some advantages over the traditional limit equilibrium methods (LEMs) in calculating the factors of safety (F_s) of the slopes. Then, with regard to a trial slip surface (TSS), associated stress fields reproduced by NMM as well as the enhanced limit equilibrium method are combined to compute F_s. In order to search for the potential critical slip surface (CSS), the MAX-MIN ant colony optimization algorithm (MMACOA), one of the best performing algorithms for some optimization problems, is adopted. Procedures to obtain F_s in conjunction with the potential CSS are described. Finally, the proposed numerical model and traditional methods are compared with stability analysis of three typical slopes. The numerical results show that F_s and CSSs of the slopes can be accurately calculated with the proposed model.     

Silo-quake—measurements, a numerical approach and a way for its suppression

Silo-quake can be observed in granular bodies during silo emptying in the form of dynamic effects as pulsations or shocks. Measurements were carried out in full-scale aluminium silos containing potato powder and polymer granulates, and in a model perspex silo containing different fills to investigate this phenomenon. Theoretically, dynamic effects in silos were analysed with a finite element method developed in the frame of a polar (Cosserat) continuum using an elastoplastic and a hypoplastic approach. The onset of a dynamic silo flow with controlled and free outlet velocity in a plane strain model silo was simulated. A reliable, practical method to significantly reduce dynamic effects and to suppress resonance effects in mass flow during silo emptying was proposed. It was verified with experiments and FE-calculations.     

Evaluation of the Alberta air infiltration model using measurements and inter-model comparisons

The Alberta air infiltration model (AIM-2) is a simplified single-zone model for predicting building air infiltration rates with a number of salient features. This paper presents an empirical study of this model using measured data from 16 detached houses in Ottawa, Canada. A single-fan depressurization test was first conducted for each house to determine its leakage characteristics. Then, the tracer gas concentration decay technique was employed to measure air infiltration rates under a wide range of weather conditions. The AIM-2 model was used to predict air infiltration rates for each of the 16 houses for the measured weather conditions. These model predictions were then compared with the air infiltration rates determined with the tracer gas tests. Additionally, the predictions of the AIM-2 model were compared with those of another model, the Lawrence Berkeley Laboratory (LBL) model. The AIM-2 model tended to underestimate air infiltration rates but performed better than the LBL model. On average, the AIM-2 model has an error of 19% while the LBL model has an error of 25%. The AIM-2 model requires an estimation of the house's leakage distribution, which may have contributed to some of this disagreement. An attempt was made to use genetic algorithms to reduce the uncertainty caused by estimating these model inputs.     

三维应力空间下改进状态变量的上下负荷面模型及其数值实现

为准确描述结构性土、超固结土的复杂弹塑性力学行为,对上下负荷面模型中的超固结状态变量R与结构性状态变量R*的演化规律进行改进,运用Sheng建议的破坏准则把模型的应用范围拓宽到三维应力空间,建立了三维应力空间下改进状态变量的上下负荷面模型。详细介绍了该模型隐式应力更新算法的数值实现过程,编制了对应的接口子程序,实现了对有限元软件的二次开发,并通过一系列数值模拟验证了模型的合理性、程序的精度与稳定性。最后应用本文模型模拟Boom黏土和Fujinomori黏土并与试验数据对比,结果表明,模型能够准确描述结构性土、超固结土的力学特征。     

Experimental validation of a numerical model for the transport of firebrands

The paper deals with the experimental validation of a numerical model for the transport and combustion of cylindrical and disk-shaped firebrands. The model solves the conservation equations of brand mass, kinetic and angular momentum, and volume. Validation consists in predicting the mass and spatial distributions of glowing firebrands that were produced from the experimental generator developed by Manzello and coworkers [S.L. Manzello, J.R. Shields, T.G. Cleary, A. Maranghides, W.E. Mell, J.C. Yang, Y. Hayashi, D. Nii, T. Kurita, On the development and characterization of a firebrand generator, Fire Saf. J. 43 (2008) 258–268]. Ten thousand firebrands per run are released with initial conditions that are randomly generated according to probability distribution functions deduced from experimental mass and spatial distributions under no-wind conditions. Whatever the wind conditions considered, numerical results are found to be in good agreement with experimental data.     

Enhancing rock stress understanding through numerical analysis

Numerical analysis provides a useful tool to enhance the understanding of rock stress. This paper presents several applications of numerical analysis to evaluate the influence of different factors, such as topology, excavation, loading history and geologic structure, on the state of stress in rock. The discussion focuses on the numerical technique known as the explicit, dynamic solution scheme, and describes how this scheme is well suited to simulate these factors. Recent advances with the explicit solution method are also presented, including the correlation of this method to acoustic and microseismic data to determine stress state information. Future development involves the extension of three-dimensional explicit solution models to simulate large-scale regions of a rock mass. An approach to this development is outlined.     

Stability assessment of a slope under a transformer substation using numerical modelling

This paper presents an analysis of the deformation mechanism and stability assessment of a slope in Yunnan Province, China. Field investigations indicated that the deformation of the slope was caused by the combined effects of the unfavorable topographical, geological and hydrogeological conditions and the placement of man-made fill. The stability of the slope was assessed by 3D numerical modeling which showed that the factor of safety of the slope was 1.1 in the natural state but reduced to 1.03 after fill was placed. Pile reinforcement was undertaken, which raised the factor of safety to 1.27.     

Ventilation measurements at model scale in a turbulent flow

Ventilation rates have been measured in a model building in a wind tunnel. Two types of opening, circular holes and model windows, have been tested under two wind conditions. One wind condition was selected to give the maximum flow through the model, while for the other condition the ventilation was due mainly to turbulent pressure fluctuations. The different characteristics of the two types of opening are illustrated. Comparisons are made between the measurements and theoretical predictions. The use of wind tunnels for ventilation studies is discussed.     

Improved bridge evaluation through finite element model updating using static and dynamic measurements

The potential of combining finite element (FE) analysis with on-site measurement through finite element model updating is indisputable. However, simplified initial models and too few measurements can lead to updated model parameters which conceal inaccurate modelling assumptions rather than improve estimates of the actual structural parameters. Therefore, the methodology proposed aims primarily to eliminate inaccurate modelling simplification by means of manual model refinements before parameters are estimated by non-linear optimization. In addition, multi-response objective functions are introduced, which allow combing different types of measurements to obtain a solid basis for parameter estimation. The proposed methodology was applied to one of the world’s largest single-arch bridges, the new Svinesund Bridge, and disclosed a need to use a non-linear model in order to estimate the structural parameters more accurately. The resultant model could reproduce the measurements with significantly improved accuracy without assigning unrealistic values to model parameters.     

A methodology to estimate benzene concentrations in a town through a traffic model

In this work an approach is presented that describes the changes of benzene concentration in the air, in relation with the special traffic characteristics of a road. The dominant input parameters of the model are traffic density and the vehicle's type distribution according to seven main categories characterized by different emission factors. The dispersion approach used is a semi-empirical relationship that apart from emission rates requires also wind speed and the direction, as well as the geometrical characteristics of the road. The methodology was validated for Ioannina, a Greek medium sized town with special traffic and geographic characteristics presenting high atmospheric pollution values. It is found that the benzene concentrations estimated by the methodology are in a very good agreement with the measurements.     

Reduced linear state model of hollow blocks walls,validation using hot box measurements

It is not obvious to accurately determine the heat loss under dynamic conditions for building envelopes made of hollow blocks using classical one-dimensional heat flow computations. Consequently, complex three-dimensional heat transfers analyses are necessary to correctly assess their thermal behaviour. This latter approach is characterised by linear state models with high-order matrixes. Therefore, this method is not very practical since it requires cautious numerical implementation and intensive computation time. One way to obtain important decrease of the computation time with no significant losses of precision is to use model size reduction techniques. This study presents such an approach based on Moore’s balanced method for two kinds of small-size concrete hollow blocks. The low-order models obtained after reduction for these two hollow blocks configurations are five-order state models (their complete state models had 680 modes and 973 modes, respectively). To estimate the accuracy and the efficiency of each reduction, the results are compared to those issued from the original complete models. The confrontations show that the proposed reduced models provide excellent prediction of hollow blocks thermal behaviour for excitation periods higher than 4 min. Moreover, the numerical results were very satisfactory comparing to experimental data obtained by means of classical calibrated hot box measurements. Finally, it must be noted that the approach developed in this study can be extrapolated to all kinds of heterogeneous walls. This can lead to simple “model libraries” within building simulation codes, based on “tabulated values” according to data issued by small matrixes set for each type of hollow blocks envelopes.     

TBM隧道在不同围岩条件下的数值模拟分析

通过利用ANSYS有限元分析软件,对TBM法隧道施工的过程进行数值模拟分析,结合隧道的工程地质情况,通过分析其应力应变的状态,提出了ANSYS模拟TBM在不同围岩条件下开挖隧道时围岩应力变化的可行性,得出了TBM隧道在不同围岩条件下的一些基本特征。     

Hydraulic fracturing stress measurements in Hong Kong

Since 1990, a total of 149 hydrofrac stress measurements to about 200 m depth were conducted in 18 boreholes as part of several geotechnical site investigation programs in the Hong Kong area. The in situ tests were carried out by using the wireline hydrofrac technique to move the straddle packer tool within the 76 mm or 101 mm diameter boreholes. Although the tests were performed both in fractured and unfractured crystalline rocks and the boreholes are located in areas of pronounced topographic relief, the results yield a consistent orientation of the maximum horizontal stress of N 108°±28°. Above 150 m depth, the vertical stress S_v due to the weight of the overburden with given rock density is the minimum principal stress, while the few deeper data available suggest that the minimum horizontal principal stress is the least principal stress. The derived stress magnitudes can be reasonably summarized by the following normalized stress-ratios:

where z is the depth in meters and S_h and S_H are the minimum and maximum horizontal principal stresses. Due to the considerable scatter of the stress data at shallow depth above 100 m, it is suggested that further detailed in situ stress measurements be undertaken in areas where large-scale underground excavations are planned.