Pattern formation and preservation in aeolian systems

Aeolian sediment transport forms natural patterns common on Earth and other planetary bodies. The self-organization of sand in transport results in dune fields with dune morphologies determined by wind regime. Patterning in dune fields is known to arise from the autogenic process of dune interactions, but the evolution of dune patterns over time remains poorly constrained. In this work dune fields were parameterized in terms of dune interactions to quantify dune-field pattern stability. Interactions are fundamental to dune-field development, but studies of interactions have focused on their surface expression, and how interactions are expressed in the ancient record has yet to be documented. This problem is addressed with five examples of interaction-generated stratigraphy identified in well-known Jurassic aeolian sandstones using criteria based on recent near-surface interpretations of interaction stratigraphy form White Sands Dune Field, New Mexico. Interactions control the autogenic development of dune fields, but allogenic factors including basin subsidence, water table rise, and sediment supply largely control the accumulation and preservation of aeolian strata. In a case study of a section of the Jurassic Entrada Sandstone, this work addresses the interplay between allogenic and autogenic controls on what is actually preserved in the rock record, and demonstrates how long stretches of time can be collapsed into surfaces between geologic units that represent relatively short-lived events. The competition between allogenic and autogenic influences on aeolian pattern formation is not unique to Earth, and Mars also hosts patterned landscapes thought to be generated by aeolian sediment transport. Such landscapes include intra-crater layered mounds such as Aeolis Mons in Gale crater, the landing site of the Mars Science Laboratory rover. Competing hypotheses about whether these mounds formed by aeolian erosion of crater-filling deposits, or by aeolian deposition were addressed with wind tunnel and large eddy simulation experiments. The results are compatible with an erosional origin of the mounds. Additional analyses of wind-formed landscapes within and around Gale crater further supported the wind-erosion hypothesis of the central mound.Geological Science

The reliability of rock mass classification systems as underground excavation support design tools

This thesis examines the reliability of rock mass classification systems available for underground excavation support design. These methods are sometimes preferred to rational methods of support design particularly if detailed information required for the latter mentioned methods is lacking. The classification approach requires no analysis of any specific failure mechanisms or the forces required to stabilise unstable rocks, yet, the support measures thus designed are considered to deal with all possible failure mechanisms in a rock mass.Amongst the several rock mass classification methods developed for application in underground excavation engineering, two have stood out. These are known as rock mass rating (RMR) and tunnelling quality index (Q), introduced by Bieniawski (1973) and Barton et al. (1974), respectively. Over the years, the two methods have been revised and updated so as to improve their reliability as support design tools, yet the two methods are know to have limitations and their reliability has long been a subject of considerable debate. Nevertheless, attempts to assess their reliability in a systematic manner have been limited. Further, some practitioners in the field of rock engineering continue to use these methods as the sole methods of support design for underground rock excavations. The objective of thesis, therefore, is to contribute to a better understanding of the reliability of the two classification methods.This study considered that the reliability of the RMR and Q methods can be assessed by comparing their support predictions with those derived by other applicable methods and also with the actual support installed. Such an assessment can best be carried out during excavation of an underground opening because representative data can be collected by direct observation of the as-excavated ground conditions and monitoring the performance of the support installed. In this context, the geotechnical data obtained during the construction of several case tunnels were reviewed and the two classification methods were applied. The effectiveness of their support predictions was then evaluated against the potential failures that can be predicted by some of the applicable rational methods. Since the rock masses intersected in the case tunnels are jointed, mostly the structurally controlled failure modes were analysed. The support measures predicted by the two methods were compared with each other and with the actual support installed in the case tunnels. Further, the RMR and Q vales assigned to the case tunnels were correlated to observe any relationship between the two.The study showed that the RMR and Q predicted support measures are not always compatible. In some circumstances, the two methods can either overestimate or under estimate support requirements

Assessing load transfer mechanism in CMC-supported embankments adopting Timoshenko beam theory

© The authors and ICE Publishing: All rights reserved, 2015. Controlled modulus columns (CMC) supported embankments are increasingly being used for construction of major highway embankments on expansive soils particularly near waterways or coastal regions. CMC is a faster, sustainable and economical ground improvement technology that stiffens the poor soil and transmits the load from the traffic to a lower bearing stratum. The key influencing elements of the load transfer mechanism include embankment fill, load transfer platform (LTP), CMC and the underlying soils. Use of LTP enhances the load distribution mechanism in the CMC improved soft ground and minimises the post construction settlement of the ground. In this paper, reinforced Timoshenko beam theory is introduced to simulate the LTP with one layer of geosynthetics resting on CMC improved soft soil. A parametric study is conducted to investigate the importance of the height of the embankment on the maximum settlement of the LTP, tension developed in the geosynthetics and stress concentration ratio (the ratio of the stresses acting on CMC and soft soils) for the CMC supported embankments. Special attention is given to the stiffness of soft soil and shear stiffness of the geosynthetic layer. It has been observed that height of the embankment, the stiffness of the soft soil and the shear stiffness of the geosynthetics significantly influence the maximum settlement of the LTP and the stress concentration ratio

Finite element modelling of transportation tunnels

The aim of this thesis is to determine the ground deformation and stress distribution around highway tunnels at various stages of excavation and for several support conditions using finite element modelling techniques. When ground is excavated and material removed the subsequent redistribution of stress in the remaining surrounding material needs to be treated by one of three methods. These are the gravity difference method, the stress reversal technique and the relaxation approach. The first two methods were chosen for the simulation of excavation in this study. The tunnel data are in the form of the dimensions of the tunnel, heights of the rock layers, details of the shotcrete lining and tunnel support systems. A pre-processing program was written to transform this information into a finite element mesh in a format suitable for use by PAFEC-FE software. This enables different tunnel models and meshes to be produced with minimum error and time. The lack of adequate post-processing facilities available in PAFEC-FE dictated that computer programs needed to be written in order to reformat the textual output files and process the mesh stress and displacement outputs for graphical display using UNIRAS. In this way repeated use could be made of PAFEC-FE without time-consuming and error-prone manual retrieval of data. The tunnels were modelled at various stages of excavation and with such support provided at those stages as to allow the computed displacements to be compared with measurements made on highway tunnels in Turkey. The stresses generated in the tunnel supports and surrounding ground were also calculated to enable the possibility of damage or failure of the support structure or ground to be assessed and the selection of an optimal support system. Insertion of a support system into the model has a marginal effect on the development of rock strength around an excavation boundary