Dry granular masses impacting on rigid obstacles: numerical analysis and theoretical modelling

Acta Geotechnica - The assessment of the time evolution of the impact force exerted by dry flowing masses on rigid obstacles is mandatory for the dynamic design of sheltering structures and the...     

Mechanical behaviour of geo-encased sand columns: small scale experimental tests and numerical modelling

This paper presents experimental monotonic and cyclic test results on small scale sand columns reinforced by means of geosynthetics. The influence of the stiffness of the georeinforcement on the system mechanical response is discussed as well as that of the previous loading history. The experimental test results have also been numerically simulated by employing a 3D finite difference commercial code. A parametric study concerning the georeinforcement stiffness and the soil-column interface friction angle is presented. The results prove the influence of the encasing geosynthetic on the overall stiffness of the system, as well as of the stiffening effect induced by a possible preload applied on the column. The effect of shear stresses at column-soil interface is also observed to play a non negligible role, which is expected to become even more evident for real scale geo-encased columns. Numerical approaches based on standard constitutive models are able to capture the behaviour of the system under virgin load, but they totally fail in reproducing the unload-reloading response.     

On numerical computation of impedance functions for rigid soil-structure interfaces embedded in heterogeneous half-spaces

We present fully-discrete procedures for computing the impedance functions of rigid massless soil-structure interfaces that are embedded in arbitrarily heterogeneous half-spaces. The finite element method (FEM) is used for obtaining the wave responses of (visco-)elastic half-spaces truncated by Perfectly Matched Layers (PMLs), which provide the wave absorbing boundary conditions. The devised FEM-PML approach is verified in both time and frequency domains by using various benchmark solutions. Requirements on the prescribed input excitations for obtaining accurate impedances in the time domain as well as the relative computational cost of time- and frequency domain solutions are investigated. Accuracy of the implemented PMLs in extracting the impedance functions is also examined in comparison to Lysmer–Kuhlemeyer dashpots; and it was found that this simplified boundary treatment is generally inadequate. The utility of the proposed method is demonstrated by extracting the impedance matrix of rectangular and circular voids embedded in a linearly stiffening half-space. Impedance functions for such complex soil-structure systems are shown to be highly coupled and frequency-dependent due to wave reflections and interference caused by the soil heterogeneity and interface geometry. Fully discrete approaches, such as those proposed herein, are necessary to adequately capture these effects.     

Performance-based design optimization of embankments resting on soft soil improved with T-shaped and conventional DCM columns

Acta Geotechnica - This paper presents a parametric study of the optimization design for T-shaped deep cement mixing (TDM) and conventional deep cement mixing (DCM) columns improved soft soil for...     

Design of tabular excavations in foliated rock: an integrated numerical modelling approach

Summary This paper presents a rock mechanics design methodology applicable to steeply dipping orebodies typical of many underground hardrock mines. The first stage in the design process is the characterization of the rock mass using bothin situ and laboratory data. The effects of anisotropy on rock mass behaviour are discussed with reference to laboratory and field observations. The second stage involves the use of a number of selected numerical modelling techniques to investigate ground response in the near-field rock mass surrounding the mining excavations. This study shows that the use of several numerical methods in conjunction, allowing for the advantages of each method to be maximized, provides a more comprehensive analysis of the different facets of stope design. This approach differs from those in the literature which seek to compare the different numerical methods in order to select just one method best suited for a problem. The design methodology employed emphasizes the importance of developing an understanding of ground deformation mechanisms as opposed to predicting absolute behaviour.     

斜坡含软夹层地基路堤离心模型试验与数值模拟

为了研究斜坡含软弱夹层地基路堤边坡的稳定性及破坏机制,采用离心模型试验和数值分析方法进行了研究。研究表明：斜坡均质地基路堤边坡仍是呈现整体圆弧滑动破坏;而对于含软弱夹层地基路基边坡来说,由于软弱夹层的影响其稳定性有了明显的降低,并有利于张拉裂缝的产生和发展,路堤边坡不再是整体圆弧滑动破坏,而是部分土体沿着软弱夹层发生滑动破坏。     

富水砂层斜井冻结壁力学特性及温度场研究

研究富水砂层斜井冻结壁力学特性及温度场分布规律,可为解决斜井冻结法凿井穿越富水砂层技术难点提供可靠依据。以陕北某斜井冻结法凿井工程为研究背景,通过室内物理力学试验、现场实测及有限元数值模拟相结合的方法,研究了冻结状态下砂土的热物理及力学特性,分析了斜井冻结法凿井期间冻结壁受力机制,深入分析斜井冻融过程中冻结压力变化规律及原因,对测温孔与冻结壁径向温度实测与数值模拟结果进行了对比分析。研究结果表明,随着温度降低,冻结砂土导热系数呈现出先增大后减小的趋势,且冻结温度对冻结砂土的内摩擦角影响较大;冻结温度、井筒埋深和地下水对冻结压力的影响较大;混凝土水化热对冻结壁的影响范围约为460~475 mm。研究结果可为富水砂层地区斜井冻结法凿井的优化设计和安全稳定性研究提供依据。     

The eccentric loading of a rigid circular foundation embedded in an isotropic elastic medium

This paper examines the problem of the eccentric loading of a rigid circular disc-shaped foundation embedded in bonded contact with an istropic elastic medium of infinite extent. The solution of this problem is achieved by using a Hankel integral transform technique which reduces the problem to two sets of dual integral equations. These systems of dual integral equations represent the equations which govern the axisymmetric and asymmetric states of deformation induced by the loaded foundation. Explicit results are derived both for the displacement and rotation of the circular foundation and for the contact stress at the interface.     

The axial loading of a rigid circular anchor plate embedded in an elastic half-space

The present paper examines the axisymmetric problem related to the loading of a rigid circular anchor plate which is embedded in bonded contact with an isotropic elastic half-space. A Hankel transform development of the governing equations is used to reduce the associated mixed boundary value problem to a set of coupled Fredholm integral equations of the second kind. These equatons are solved in a numerical fashion to generate results of engineering interest. In particular, the results indicate the influence of the depth of embedment on the axial stiffness of the rigid anchor plate.     

Fluid flow in extensional environments; numerical modelling with an application to Hamersley iron ores

The mechanical feasibility of focusing both surface- and basinal-derived fluids towards sites of iron ore genesis during Proterozoic deformation in the Hamersley Province is tested here by computer simulation. Finite difference modelling of porous media flow during extensional deformation of a mountain range shows that surface fluids are drawn towards areas of failure and focus into the centre of the mountain. The addition of permeable structures such as a normal fault provides focused fluid pathways in which mechanical and geological conditions are particularly conducive to both upward and downward flow. Upward flow from the base of the fault within the model overall is favoured by low permeability basement materials and supra-hydrostatic pore pressures. Downward migration of fluids becomes more prominent as extension progresses and upward fluid flow from the base diminishes. The introduction of sedimentary layering into the models allows lateral fluid flow, such that sites of potential fluid mixing may then occur within permeable iron formation units close to the fault zone. Allowing parts of the stratigraphy to become more permeable as a function of high fluid flux simulates permeability enhancement by silica dissolution as a mechanism for iron ore genesis. The involvement of both basinal and surficial fluids in the genesis of the ore deposits is supported by the mechanical models and in addition provides an explanation for a progression from relatively reduced to oxidised conditions at the Mt Tom Price deposit (and possibly other large deposits) with time.     

Performance and prediction of long-term settlement in road embankments constructed with recycled construction and demolition waste

The settlement of highway embankments can directly affect the structural safety of road pavement. Comprehensive assessment of the long-term settlement characteristics of highway embankments is important in highway safety management. The present study conducted long-term settlement monitoring in an embankment of the Xi’an-Xianyang Highway in Shaanxi Province, China, that was constructed with recycled construction and demolition waste. Variations in sectional settlement, cumulative settlement, differential settlement, and settlement rates were made in a barricade section and an arc-shaped protective slope section. The maximum settlement observed in the barricade section was 51.75 mm, with the embankment exhibiting “basin-shaped” settlement. The maximum settlement in the arc-shaped protective slope section was 42.22 mm, and the embankment exhibited “W-shaped” settlement. Furthermore, there were two stages of great increase in the settlement rate. Based on the results, control standards for the sectional settlement of embankments constructed with recycled construction and demolition waste are proposed. Finally, a model for predicting the long-term settlement in road embankments constructed with recycled construction and demolition waste is established based on a Weibull distribution probability density function.

     

高速公路下伏浅层老采空区强夯置换试验研究

采空区是指地下矿产被采出后留下的“空洞区”。迄今对浅层老采空区上建高速公路一类重大工程的研究较少,研究浅层老采空区上建设高速公路的工程地质适应性、质量控制及施工工艺具有重要意义。结合常吉高速公路工程,从设计处理要求、治理范围、治理方案对比优选、强夯置换现场试验分析、施工组织工艺设计等几个方面,对浅层老采空塌陷区强夯置换治理技术进行了研究,研究结果表明,强夯置换治理此类工程效果显著。     

Applications of numerical modelling in underground mining and construction

Numerical modelling has been used to investigate a variety of problems in underground mining and tunnelling: subsidence induced by longwall coal mining; stresses generated when an open stope is filled cemented backfill and the stability of exposures created during subsequent mining of adjacent stopes; the interaction of two tunnels; and the effects of under-mining a pre-existing tunnel and shaft. In each case, results from nonlinear stress analyses can be used to guide the design of excavations and rock support mechanisms.     

Recent developments in centrifuge modelling of tectonic processes: equipment,model construction techniques and rheology of model materials

The Experimental Tectonics Laboratory at Queen's University is equipped with a large-capacity centrifuge that is capable of subjecting tectonic models measuring 127 × 76 mm in plan and up to 51 mm in depth to accelerations as high as 20,000 g. This high capacity greatly extends the range of potential model materials and permits the use of relatively stiff and/or brittle substances.A number of new techniques of model construction have been devised, that permit internal and surface strain patterns and kinematic evolution to be monitored in detail. One particularly useful technique, which will find application in non-centrifuged experiments as well, allows the preparation of highly uniform anisotropic multilayers composed of alternating layers of Plasticine and silicone putty, with individual layer thicknesses as low as 20 μm and with controllable ratio between thicknesses of the relatively competent and incompetent units. Examples of models constructed using these new techniques are illustrated.One particular type of the commonly used model material, silicone putty, has been subjected to a series of rheological test. The results indicate that at strain rates in the range 10^?6-10^?3s^?1 (applicable to the centrifuge experiments) the silicone putty exhibits power-law rheology with n = 7 ± 2. At higher strain rates the material appears to tend towards linear behaviour.Available rheological data and dimensional analysis using standard scaling laws and appropriate model ratios suggest that the microlaminated Plasticine-silicone putty multilayer is a suitable analogue, in centrifuged experiments, for interbedded sequences of indurate limestone and incompetent shale. The excellent degree of dynamic similitude attained is demonstrated by the realistic form of fold and fault structures developed in models constructed of this material.     

桩间土拱效应离心模型试验及数值模拟研究

抗滑桩的设计理念是采用非连续结构,利用土体自身强度形成的拱效应来达到支挡的目的。通过离心模型试验,对抗滑桩加固滑坡形成土拱效应的现象进行研究,再现了桩间土拱形成的现象及其破坏模式。根据试验现象提出两种典型的土拱破坏模式,即拱顶发生破坏和拱脚发生破坏,并提出改善拱脚受力,防止土拱拱脚发生破坏的工程措施。基于离心模型试验,采用二维颗粒流方法对桩间水平土拱的应力分布和破坏过程进行数值模拟分析,提出土拱破坏的判别标准为抗滑桩所承担的荷载发生急剧变化,此时土拱传递荷载的功能部分失效,并研究了桩间土拱承载力与桩间净距的关系。随着桩间净距的增大,土拱承载力变化的趋势并非线性的减小,当桩间净距增大到一定程度后,土拱承载力会急剧减小,由此可合理确定桩间距的范围。     

Hierarchical multiscale numerical modelling of internal erosion with discrete and finite elements

This paper presents a coupled finite and discrete-element model (FEM and DEM) to simulate internal erosion. The model is based on ICY, an interface between COMSOL, an FEM engine, and YADE, a DEM code. With this model, smaller DEM subdomains are generated to simulate particle displacements at the grain scale. Particles in these small subdomains are subjected to buoyancy, gravity, drag and contact forces for short time steps (0.1 s). The DEM subdomains provide the macroscale (continuum) model with a particle flux distribution. Through a mass conservation equation, the flux distribution allows changes in porosity, hydraulic conductivity and hydraulic gradient to be evaluated for the same time steps at a larger, continuum scale. The updated hydraulic gradients from the continuum model provide the DEM subdomains with updated hydrodynamic forces based on a coarse-grid method. The number of particles in the DEM subdomains is also updated based on the new porosity distribution. The hierarchical multiscale model (HMM) was validated with the simulation of suffusion. Results for the proposed HMM algorithm are consistent with results based on a DEM model incorporating the full sample and simulation duration. The proposed HMM algorithm could enable the modelling of internal erosion for soil volumes that are too large to be modelled with a single DEM subdomain.

     

Torsional stiffness of non-uniform and hollow rigid piers embedded in isotropic elastic media

This paper examines the torsional response of a rigid pier type foundation, with a non-uniform or hollow cross-section which is embedded in bonded contact with a layered elastic half space. The tractions which act at the axisymmetric boundary surface between the pier and the surrounding elastic medium are represented by discretized regions of uniform traction. The compatibility of deformation at the boundary is used to determine the interface stress distribution. The torque-rotation response for the, rigid pier foundation is obtained for different choices of the pier geometry and shear modulus mistnatch between the layer and the underlying half space.     

Field and numerical study of the lateral response of rigid piles in sand

Acta Geotechnica - Large diameter, short monopiles are the preferred foundation type for offshore wind turbines. These piles demonstrate a rigid response with significant rotation of the pile base...     

Meltwater pulses in the northern North Atlantic: retrodiction and forecast by numerical modelling

Changes in sea surface salinity, especially by sudden meltwater pulses, are the most effective process to modify the circulation in the Greenland–Iceland–Norwegian (GIN) seas. With “Sensitivity and Circulation of the Northern North Atlantic” (SCINNA), a three-dimensional ocean general circulation model, several experiments addressing the possible effects of meltwater inputs of different intensities were carried out. The experiments used (a) the last glacial maximum (LGM) reconstruction based on oxygen isotopes data from sediment cores and (b) the modern conditions of the GIN seas for their initial states. Meltwater inputs from Europe as recorded during the last deglaciation succeeding the LGM change the circulation pattern drastically. These pulses can push the high-salinity inflow from the northeast Atlantic away from Europe over to the southern coast of Iceland, thus allowing the low-salinity meltwater to spread all over the GIN seas. As a result, the deepwater formation in this region can be turned off and the circulation system shifts from the normal cyclonal-antiestuarine into an anticyclonal-estuarine mode. On the contrary, meltwater pulses originating from Greenland due to global warming mainly intensify the East Greenland Current without altering the overall circulation and temperature/salinity patterns significantly because they chiefly enhance the salinity minimum off the Greenland coast.     

Field measurements in river embankments: validation and management with spatial database and webGIS

This study focuses on the development of a system with a spatial database and a webGIS able to store, validate and display the data to assist the decision makers in managing early warning systems for river embankment failure. In order to obtain precise results, it is essential to have a tool with ability of managing a large number of data for checking their reliability and for locating them in the space. In this paper, special emphasis was given in the development of procedures to assess the reliability of the measures. For this purpose, the database includes all the information needed to describe the instrument performance, such as the sand pack size and casing diameter of open-standpipe piezometers for evaluating their time lag, and the calibration curves of transducers with the possibility of their updating. The position of the non-functioning instruments is identified through the analysis of the electrical signal and spatial displays, while the analyses of the redundancy and coherence of measures is used for detecting doubtful data. Database and webGIS were applied to the monitoring data of an embankment of the Adige River in Northern Italy. The database and webGIS system has proved to be a suitable and effective tool for the management and validation of real-time data and periodical field measurements.