Analysis of Excavation-induced Deformation with Different Soil Models
Abstract: The monitored case of deep excavation works located in the centre of Warsaw is described and back analysed as a boundary value problem with the finite element method. The excavation was carried out in over-consolidated clayey layers under the support of braced diaphragm walls. Accurate simulation of such soil-structure interaction problems requires advanced soil constitutive models – especially for the pre-failure range of small strains. On the other hand, such models should not be very complex and their material parameters should be relatively simple to obtain from laboratory and in situ surveys. By means of a case study, this paper examines several simple elasto-plastic constitutive models for the simulation of the behaviour of soil layers. The influence of such characteristics of the soil behaviour, as anisotropy and non-linearity of stiffness, is studied in the paper. The discussion concerns the problems related to the application of the models for a practical example. The comparison is focused on the obtained and measured displacements of the neighbouring building and diaphragm walls.
Investigations of threshold effects in soil deformations
Some of the methods used in deformation analyses are briefly reviewed. It is observed that often there is a large difference between data quoted from laboratory tests and that obtained from the back analysis of either field tests or back analysed structures. Some of the factors which affect soil stiffness are reviewed. The major factors affecting soil stiffness are identified as the stress history of samples, the current stress state and the current stress path. The effect of sample disturbance is to reduce measured stiffnesses, opinion has in the past attributed the differences in stiffness to this factor. However this is not supported by the results of high quality laboratory tests in which field and laboratory data agree well. The importance of care when defining stiffness moduli and the need for the definition to be given at appropriate points on the stress-strain curve are emphasised. Some indication of the significance of recent stress history in the form of threshold effects is discussed, although this is based on limited evidence in the case of stress path effects. The soils to be tested and the equipment to be used for the soil test program are discussed. Particular emphasis is placed on the calibration of the apparatus and system compliance. Details of the tests conducted are briefly given with details of the method of preparation of soil and soil samples for testing. The results of these tests indicate that as the deviation of the current stress path from that immediately preceding increases so the stiffness on a given stress path increases. In addition it is observed that the strain increment ratio is non-unique. These effects are observed to be most important for high plasticity soils. Further tests indicate the independence of time and stress path threshold effects. The variation of these effects with stress history and overconsolidation ratio are investigated. It is found that these effects are most significant for soils which have been isotropically compressed and swelled to have an overconsolidation ratio of two. The results of these tests indicate some of the reasons for difficulties in tests to examine path dependence in earlier research. Finally some conclusions from this experimental work are drawn together and topics for further research suggested. The importance of considering the complete stress history of samples including recent stress paths and periods of rest are emphasised for inclusion in the stress path method.
Cleat mineralization of Upper Permian Baralaba/Rangal Coal Measures, Bowen Basin, Australia
Abstract Coals from the Permo-Triassic Bowen Basin have been investigated using a variety of complementary analytical techniques. Face cleat minerals in the studied samples are dominated by authigenic clays, notably pure illite or illite-chlorite mixtures. Butt cleats and joints are dominated by carbonates, mostly calcite with less abundant ferroan calcite, ankerite and siderite. K/Ar ages of cleat-fill illites and one fracture illite-smectite fill indicate three phases of illite formation during the Triassic: (1) an early phase with ages clustered around 244 Ma ago; (2) a second phase about 232 Ma ago; and (3) the latest phase about 219 Ma ago. Thermal modelling of selected boreholes from the study area and other geological data indicate that the first phase of illitization occurred when the coal seams were about 1000 m below the surface and at a temperature of between 70 and 80°C, at a time when the basin was rapidly subsiding. These early illites were stable and retained their 1M polytype even after being exposed to temperatures of 150–190°C during maximum burial in the southeastern Bowen Basin. The second phase of illitization (at 232±3) occurred at about the time of maximum burial. The latest phase took place between 223±3 and 212±3 Ma ago and occurred in a wider temperature range (between 170 and 100°C) during a rapid uplift in the Late Triassic. The Bowen Basin area experienced a second cycle of subsidence that commenced in the Early Jurassic with the formation of the Surat Basin, which overlies the Bowen Basin and forms part of the Great Artesian Basin system. Regional uplift and erosion in middle Cretaceous times terminated sediment accumulation in the Surat Basin. During the second cycle of burial widespread carbonate mineralization in butt-cleats and joints took place. Calcite fluid inclusions indicate that calcite was precipitated from meteoric water at about 80°C. The widespread carbonate mineralization in butt cleats and the near-absence of carbonates in the face cleats is here attributed to permeability anisotropy, caused by a change in the direction of lateral compressive stress during Jurassic-Cretaceous times relative to that during the Triassic Hunter-Bowen Orogeny.