For many projects involving drilled shafts, cores are required to be taken below the shaft base for visual identification of the underlying material. For example, the Texas Department of Transportation (TxDOT) requires a core length of at least 1.5 m (5 ft) or equal to the shaft diameter, whichever is greater, at the shaft base. Although the verification cores are to be extracted at the shaft base, The Department of Transportation of many states do not provide guidance to eliminate the effect of the verification core on the point bearing capacity. A recent study shows that the verification core hole is either filled with concrete in dry condition or with sand-gravel mixture in wet pour (Raibagkar, 2008). This finding is crucial because the point bearing capacity of drilled shafts with an empty hole at the base should be significantly lower than that of drilled shafts without verification core. Although the materials that fill in the verification core remove the risk of losing large point bearing capacity, the exposure of the core holes to air-drying may have an adverse effect on the point bearing capacity tipped in clay shales, especially when the basal material is susceptible to weathering. Therefore, the effect of the verification core on the point bearing capacity has been thoroughly investigated with emphasis on changes in the material properties of four clay shales (Del Rio Clay, Eagle Ford Shale, Taylor Marl, and Navarro Shale) in central Texas. The effect of verification core on the point bearing capacity of drilled shafts was investigated using finite element method (FEM) software, PLAXIS. The results from laboratory tests were converted to input material parameters for Mohr-Coulomb failure criterion, and the thickness of degraded zone around the core was interpreted from fullscale condition degradation tests. The load-displacement curves at the shaft base were created from PLAXIS analyses, and the point bearing capacities were obtained at 5%D and 10%D displacement from load-displacement curves. These capacities were used to calculate reduction factors that relate the point bearing capacity of the reference model (without a verification core) with that of the “core models” (with a verification core). The reduction factors are good indicators to determine if verification core had a positive or negative effect on the point bearing capacity. It was found that the reduction in point bearing capacity of “core models” is typically within 10% capacity of the reference model, and a maximum reduction of 14% was found for the Taylor Marl that was dried for 48 hours. / text

published_or_final_version / Civil Engineering / Master / Master of Philosophy

Thesis (Ph. D.)--University of Texas at Austin, 2008. / Vita. Includes bibliographical references.

Graduation date: 1991

The core-logger has been designed for the high-resolution radiometric analysis of soil cores using multiple detectors. This device allows for the automation of the measuring process and eliminates the need to dissect the cores. The design is aimed at measuring soil-cores with a 10cm radius and a length of 1m and allows for up to 4 detectors to be mounted on the measuring platform. Currently a combination of Bismuth-German.te (BGO) and C.esiwn-Iodide (CsI) detectors are used. The core logger required a good spatial resolution of - 1 cm. This has been difficult to obtain and has required extensive investigation. The shielding configurations were varied and the effect of background radiation was looked at in detail to determine an optimal construction. A secondary objective has been the complete measurement of a single core in 24 hours. This has also been difficult to achieve because the low activity of natural radiation in the core samples needs longer measuring times. The BGO detectors were used as a more efficient detector (than, e.g. CsI) which helped to reduce the required measuring time. Measured spectra have been analysed to determine the activity concentrations of the specific radionuclides of interest: 232Th, 238U, 40K and 137Cs. These activity profiles of the measured cores provide information that can then be used to radiometrically fingerprint the sample to determine soil characteristics such as grain size and mineral content. However, because the actual resolution of the system ( ~3cm) is greater than the typical core slice (~ 2cm), the radiometric information in a specific core-slice contains contributions from its adjacent slices. This folding or convolution of the measured spectra can be undone using a deconvolution method. which was examined and commented on. / Thesis (M.Sc.)-University of Natal, Durban, 2001.

Of great benefit, but not limited to seafloor mineral exploration, is a technique that fairly rapidly determines the composition of a drilled vibracore (in a time comparable to the time involved in obtaining the core). The rapid assessment is desired to predict whether a given region warrants further exploration by coring. A proposed monitoring system, based on neutron capture gamma ray analysis, consists of a container tank filled with water and tubular extensions that house a Cf-252 neutron source and a detector positioned within the tank. The core sample is passed through the system in stop and count steps. The net count rates, due to "signature" capture gamma rays from neutron capture in elements in the core sample, are proportional to the amount of the element responsible for emitting the capture gamma ray. The proposed system was modeled and simulated by the Monte Carlo method to predict the relationship between the response of the detector and the elemental concentrations within the sample. Accurate and detailed treatment of neutron transport and gamma ray production and attenuation within the system were employed not only to predict the relationship of the photopeak responses with respect to elemental concentrations, but also to permit investigation of the design parameters and structural material changes in the system. The developed Monte Carlo code utilizes a variety of variance reduction techniques, such as implicit absorption with Russian Roulette and deterministic production of the gamma rays of interest, along with a form of correlated sampling to predict simultaneously the responses over a range of interest of the elemental concentrations. The predicted results were compared with predictions obtained from a well established general purpose Monte Carlo code (MCNP). / Graduation date: 1990

The seabed is envisaged to meet the increased future demands for minerals from the rapidly growing industrialized societies of the world. Shipboard analysis of cores can significantly reduce the cost and time spent at the exploratory drilling stage by obviating the need to go back to land for analysis. It can further speed the exploration process by enabling a quick modification of the exploration plan based on the results of the shipboard analysis. A ²⁵²Cf-based analyzer utilizing the prompt gamma neutron activation analysis technique has been designed. The analyzer is a spherical iron shell with the source at its center. The seabed core is passed through a hollow composite tube which is positioned a short distance directly below the source and the resulting prompt gamma rays are collimated to a HPGe detector. The rest of the sphere is filled with paraffin. The gamma ray flux at the detector is converted into a count rate by using a semi-empirical detector response function. This count rate data are then used to determine the sensitivity and detection limits for the chosen elements (Mg, Al, Ti, Cr, Mn, Fe, Ni and Cu). Monte Carlo simulations using the Monte Carlo neutron photon coupled transport code, MCNP, were carried out for a parametric study of important variables influencing the design of the analyzer. These parameters included the moderator type, source to sample distance and sample porosity. MCNP was then used to model the analyzer and to generate the neutron flux profiles in the sample and the prompt gamma flux at the detector. Due to the non-availability of the prompt gamma data for most of the elements of economic interest in the ENDF/B-V cross section libraries associated with MCNP, the point kernel photon transport code ISOSHLD-II (modified for high energy gamma rays) was used to generate the gamma flux at the detector for specific elements. The ISOSHLD-II source term was calculated based on known gamma production data (thermal capture only) and the thermal neutron flux in the sample obtained from MCNP computations. The sensitivity and detection limits obtained from the isotopic source based analyzer were compared for the case of aluminum with values reported from reactor facilities. The results obtained indicate that the analyzer designed in this work could prove suitable for the on-line analysis of many elements of economic interest in seabed cores at the 1 weight percent level. / Graduation date: 1992

Thesis (Ph. D.)--Rice University, 1974. / Includes bibliographical references (leaves 112-125).

Thesis (M.S.)--Baylor University, 2009. / Superscript: 6 and 3 in "106m3". Includes bibliographical references (p. 170-175).

Thesis (M.S.)--University of Tennessee, Knoxville, 2009. / Title from title page screen (viewed on Mar. 11, 2010). Thesis advisor: Sally P. Horn. Vita. Includes bibliographical references.