Centrifuge modelling of the ground reaction curve of fibre reinforced soil

The phenomenon known as the 'arching effect' occurs when a portion of granular mass yields relative to an adjacent stationary region. The movement is resisted by shearing stresses which act to reduce the pressure on the yielding support and increase the pressure on the adjacent stationary supporting zones. Arching is widely observed in both natural and man-made structures such as piled embankments, tunnels, and above mine works and sinkholes. One method of increasing soil shear strength and its resistance to deformation is through the use of randomly distributed discrete fibres. The degree of improvement has been shown to be directly related to the fibre content in the soil, the fibre aspect ratio, orientation and mechanical properties. In this research the arching effect is recreated in a geotechnical centrifuge model using a 'trapdoor' apparatus within a plane strain container and the effect of fibre reinforcement on results is examined. Both the trapdoor and an adjacent support were instrumented to measure the force (and derived pressure) distribution. Soil and trapdoor displacements were determined using digital image analysis. The influence of fibre content is examined whilst maintaining constant fibre length, applied compactive effort, and soil height

Liquid limit of selected postglacial soils from west-central Poland

The aim of this paper is to establish a correlation between the liquid limit value obtained in the Casagrande apparatus (wL/cup) and in the cone penetrometer (wL/cone) for typical glacial sediments from west-central Poland and compare them with the correlation equations published in the literature. The following correlation was formulated: WL/cone = 0.91wL/cup+ 1.98. The analysis presented in this paper proves that deriving a generalized and universal relationship wL/cup/wL/cone for all cohesive soils, regardless of their origin, is very challenging and may not be reliable for use in the engineering practice. It is verified that, in the range of low values of liquid limit, the cone penetrometer gives higher values of wL than those obtained from the Casagrande apparatus. However, for the Polish postglacial soils analysed in this paper, the cone penetrometer underestimates the results in relation to the Casagrande apparatus above wL = 22%. For the purpose of this study, wL = 22% was defined as "the point of equivalent results" (wL/cone = wL/cup)

Modelling surface subsidence during underground coal gasification

Underground Coal Gasification (UCG) is an alternative method of extracting energy from coal whereby the coal is burnt within an in situ UCG reactor. The method has been established for almost a century, but it has not been widely used despite its advantages, which include the circumvention of underground human presence and the possibility to work with low quality coal that is deep underground. One of the main difficulties associated with the implementation of UCG on an industrial scale is the prediction of surface subsidence, which is required to assess potential damage to surface infrastructure, UCG equipment, and boreholes. This work considers the numerical modelling of surface subsidence during UCG. For this, the finite difference numerical modelling software FLAC3D by Itasca is utilized. Historically, this tool has been used for modelling surface settlement caused by traditional coal mining activities. The mechanism of surface subsidence during conventional coal mining and UCG are almost identical; however, the UCG reactor has some distinguishing features, for example, thermal strains and the resulting altered mechanical properties of the soil-rock. In this work, firstly, a thermal analysis is run to impose the thermal fields. Secondly, the engineering properties relationship with temperature is implemented in the model. Finally, model results are compared with field observations and discussed. The conclusion is drawn that updating the mechanical properties, i.e. elastic stiffness, friction angle and cohesion, in correlation with the elevated temperatures improves the surface subsidence predictions

Numerical shakedown and non-shakedown responses of a Tresca half-space to a three-dimensional moving load

Flexible pavements may fail due to excessive rutting as a result of accumulative plastic deformation; otherwise, if the load is small enough, pavements may deform plastically in the first number of load cycles and then reach a stable state which is termed as ‘shakedown’. Recently some lower-bound and upper-bound solutions have been developed to directly determine the load limit (i.e. shakedown limit) below which an elastic-plastic half space can shake down. However, the actual responses of an elasticplastic half-space subjected to repeated moving loads were not well revealed. In the present study, repeated moving surface loads are applied to a three-dimensional finite element model established in ABAQUS to research on the development of stresses and strains in a Tresca half-space. Also, a numerical shakedown limit can be determined according to the yield condition of structure under a static load following a number of load passes. It is found the development of residual stresses induced by plastic strains plays a key role in helping the half-space to reach the shakedown state. Good agreements are also observed between numerical and theoretical solutions for both shakedown limit and residual stress fields

Centrifuge Modelling of the Collapse of Shaft Linings

The collapse of abandoned and often hidden mine shafts is a serious problem in the UK and many parts of Europe. The collapse of these shafts is often related to the failure of the shaft lining. Understanding the mechanisms of ground movements around deforming/collapsing mine shafts is, therefore, important in the assessment of mine shaft location as well as lining condition. This paper presents an experimental study of the mechanisms of soil failure around a deforming shaft lining. Geotechnical centrifuge modelling of reduced-scale buried mine shafts was tested to determine the magnitude and pattern of ground deformations that occurred during loss of internal support pressure. An axis-symmetric centrifuge container was used along with half-cylindrical model shafts. These allowed for the acquisition of digital images of the sub-surface soil and mine shafts which enabled the measurement of soil and shaft deformation using image analysis techniques. The results from two model shaft tests are presented. The first test involved the loss of internal support along the entire shaft length, whereas the second test studied the effect of a discrete weakened zone within the lining

Case-based reasoning approach to estimating the strength of sustainable concrete

Continuing from previous studies of sustainable concrete containing environmentally friendly materials and existing modeling approach to predicting concrete properties, this study developed an estimation methodology to predicting the strength of sustainable concrete using an advanced case-based reasoning approach. It was conducted in two steps: (i) establishment of a case database and (ii) development of an advanced case-based reasoning model. Through the experimental studies, a total of 144 observations for concrete compressive strength and tensile strength were established to develop the estimation model. As a result, the prediction accuracy of the A-CBR model (i.e., 95.214% for compressive strength and 92.448% for tensile strength) performed superior to other conventional methodologies (e.g., basic case-based reasoning and artificial neural network models). The developed methodology provides an alternative approach in predicting concrete properties and could be further extended to the future research area in durability of sustainable concrete

An Experimental Evaluation of the Weathering Effects on Mine Shaft Lining Materials

Many shaft collapses are related to the deterioration and failure of the masonry shaft lining materials. In modern mine shaft, concrete is widely used to provide support. To analyse shafts stability, the properties of the lining need to be well defined. The behaviour of masonry and concrete can be considerably affected by long-term exposure to harsh mine water. This paper presents a study which focuses on the weathering effects of mine water on lining materials (brick, mortar, and concrete). To reproduce the weathering process, samples were placed into solutions of potable water, artificial mine water, and a more aggressive mine-water solution for just less than one year. Four phases of laboratory tests were conducted throughout the time period to assess the degradation of mechanical properties of the lining materials. Particular attention is given to the degradation of material strength and stiffness. Results indicate that the harsh acidic mine water has pronounced detrimental effects on the strength and stiffness of mortar. The weathering process is shown to have the most significant effect on the stiffness of concrete and mortar. It is also shown that the use of mass loss as an index for evaluation of mechanical properties may not be appropriate