An experimental investigation was conducted on the static liquefaction behaviour of very loose lightly cemented sands. Undrained and drained triaxial compression tests, one dimensional consolidation, high stress compression, and unconfined compression tests were performed on artificially prepared lightly cemented loose samples with cement-sand ratios of 2, 4 and 6%. Additional tests were also conducted on uncemented samples prepared at the same initial void ratio as the cemented samples. Besides the influence of degree of cementation, the effects of void ratio and confining pressure on the liquefaction potential of cemented sands were examined. The aim of this study is to make significant contribution to the understanding of static liquefaction failures in lightly cemented sands. It is shown that cementation could increase the initial stiffness and yield strength of cemented sands but its effect might decrease considerably after the peak strength because of destruction of the cementation bond. The response of cemented sands at lower cement contents was very similar to the response of loose sands and behaviour approached the response of medium to dense sands with increase in the degree of cementation. It is also shown that degree of cementation has a significant influence on liquefaction resistance. Even though the presence of small amounts of cementation did not prevent liquefaction failure, the liquefaction resistance of cemented sands generally increased for higher degrees of cementation. The consolidation, high stress compression and unconfined compression tests demonstrated the effect of cementation in increasing both the stiffness and strength of cemented sands. The unconfined compression strength increased approximately linearly with the increase in cement content. The rate of strength gain increased with an increase in the dry density of the compacted sample, indicating that the cementation was more for denser samples.

An experimental investigation was conducted on the static liquefaction behaviour of very loose lightly cemented sands. Undrained and drained triaxial compression tests, one dimensional consolidation, high stress compression, and unconfined compression tests were performed on artificially prepared lightly cemented loose samples with cement-sand ratios of 2, 4 and 6%. Additional tests were also conducted on uncemented samples prepared at the same initial void ratio as the cemented samples. Besides the influence of degree of cementation, the effects of void ratio and confining pressure on the liquefaction potential of cemented sands were examined. The aim of this study is to make significant contribution to the understanding of static liquefaction failures in lightly cemented sands. It is shown that cementation could increase the initial stiffness and yield strength of cemented sands but its effect might decrease considerably after the peak strength because of destruction of the cementation bond. The response of cemented sands at lower cement contents was very similar to the response of loose sands and behaviour approached the response of medium to dense sands with increase in the degree of cementation. It is also shown that degree of cementation has a significant influence on liquefaction resistance. Even though the presence of small amounts of cementation did not prevent liquefaction failure, the liquefaction resistance of cemented sands generally increased for higher degrees of cementation. The consolidation, high stress compression and unconfined compression tests demonstrated the effect of cementation in increasing both the stiffness and strength of cemented sands. The unconfined compression strength increased approximately linearly with the increase in cement content. The rate of strength gain increased with an increase in the dry density of the compacted sample, indicating that the cementation was more for denser samples.

An experimental investigation was conducted on the static liquefaction behaviour of very loose lightly cemented sands. Undrained and drained triaxial compression tests, one dimensional consolidation, high stress compression, and unconfined compression tests were performed on artificially prepared lightly cemented loose samples with cement-sand ratios of 2, 4 and 6%. Additional tests were also conducted on uncemented samples prepared at the same initial void ratio as the cemented samples. Besides the influence of degree of cementation, the effects of void ratio and confining pressure on the liquefaction potential of cemented sands were examined. The aim of this study is to make significant contribution to the understanding of static liquefaction failures in lightly cemented sands. It is shown that cementation could increase the initial stiffness and yield strength of cemented sands but its effect might decrease considerably after the peak strength because of destruction of the cementation bond. The response of cemented sands at lower cement contents was very similar to the response of loose sands and behaviour approached the response of medium to dense sands with increase in the degree of cementation. It is also shown that degree of cementation has a significant influence on liquefaction resistance. Even though the presence of small amounts of cementation did not prevent liquefaction failure, the liquefaction resistance of cemented sands generally increased for higher degrees of cementation. The consolidation, high stress compression and unconfined compression tests demonstrated the effect of cementation in increasing both the stiffness and strength of cemented sands. The unconfined compression strength increased approximately linearly with the increase in cement content. The rate of strength gain increased with an increase in the dry density of the compacted sample, indicating that the cementation was more for denser samples.

Cemented carbide hardmetals have been produced since 1910 and have gradually taken over from tool steels as the major material for high speed metal cutting and forming. One such forming operation is that of cold heading used in the production of nail and screw fasteners. This operation subjects a cemented carbide die to cyclic compressive and tensile stresses and may be considered as a fatigue process. Cold heading manufacturers have always been aware that some dies will produce millions of components and yet others will fail after a much smaller number and, in view of this, have reasoned that the surface finish of the die may be limiting factor in die life. This research has been carried out to investigate how the surface finish .affects die life and to find if there is any numerical correlation between surface finish and life. Commercially manufactured cemented carbides were subject to fatigue trials using standard miniature Wohler specimens. The grades of materials used were B, N and TT with a cobalt content of 6, 6 and 25% respectively. Grades N and TT were commercial grades with a grain size of 1.5-3μm, whereas B grade was an experimental grade with a grain size of 3.0-5.0μm. The fatigue tests were carried out on Wöhler rotating bending fatigue machines with stress levels on the specimens of between 700 and 1400 MNm². Initial testing was carried out with specimens in the as received ground condition. In order to demonstrate the effect of surface treatment on the fatigue performance of the test grades, the test specimens were subject to various mechanical and thermal treatments.

Student Number : 0413336G - MSc(Eng)dissertation - School of Chemical and Metallurgical Engineering - Faculty of Engineering and the Built Environment / This dissertation reports systematic work aimed at determining the plastic deformation mechanisms that led to strains at fracture as high as 4.7% in WC-Co alloys at 1000°C when subjected to 3-point bending tests. The three grades investigated have a Co content of 15wt% and WC grain sizes of 1.3, 0.35 and 0.3 μ m respectively and were received after they were tested in bending. Fractography, macrostructural and microstructural investigations were carried out in attempts to identify the mechanisms leading to the large strains. Techniques used included light microscopy, scanning electron microscopy (SEM), field emission scanning electron microscopy (FESEM), energy dispersive spectroscopy (EDS) and quantitative image analysis. Through comparisons of the results from the three grades at various temperatures, it was possible to establish that the large strain at 1000°C are mainly due to cracking and cobalt drift. During the fractographic investigations it was found that the grades which contained VC as a grain refiner exhibited steps on the WC grains and that fracture propagated preferentially along the stepped WC grain boundaries.

The growth of TiN on cemented carbide, deposited by chemical vapour deposition (CVD), was studied. Today TiN is used as a seeding layer between the cemented carbide and the following layer. Previous experiments have shown that the coverage is uneven on the cemented carbide surface showing pits with a different growth than the main part of the surface. These pits most likely occur on some of the WC surfaces. Therefore the growth and orientation relation between the two phases were examined. Cemented carbide specimens were deposited with TiN under two different pressures and with different deposition time to give a layer as thick as the seeding layer used in the production and one about ten times thicker to study growth after the whole surface had been covered. Two pre-treatments where used on the specimens; one which were polishing with diamond and the other where the specimens were boiled in acid to remove the binder phase and expose the WC surfaces. The specimens were studied using XRD and a SEM equipped with EDS and EBSD detectors. The pictures taken with SEM showed that initial growth occurred at grain boundaries and polishing scratches. It also showed that growth occurred on all surfaces, which were confirmed by EDS. Both processes showed about the same appearance in the thinner layers but very different appearance in the thicker. This was confirmed by XRD were the thinner layers showed about the same result while the thicker ones differ from one another. Thus further growth is dependent on the parameters of the CVD process and not the surface beneath. EBSD showed an orientation relation between TiN and WC crystals in both processes. The process at lower pressure gave much finer grains which were difficult to index with EBSD, giving results in only three points. The process at higher pressure gave coarser grains which were more easily indexed. The relations WC{0001}-TiN{110}, giving WC{101̅0}-TiN{100}, and WC{0001}-TiN{111}, giving WC{101̅0}-TiN{211}, could be seen in more than one point. These results are consistent with previous studies of the growth of TiC on WC. This comparison between TiC and TiN makes it possible to draw the conclusion that also Ti(C,N) should behave the same. It also shows an orientation relation that is consistent no matter what WC plane is on the surface. The poor growth and the pits depend on the growth orientation of TiN on each specific WC surface.

Micro seismic data and coring studies suggest that hydraulic fractures interact heavily with natural fractures creating complex fracture networks in naturally fractured reservoirs such as the Barnett shale, the Eagle Ford shale, and the Marcellus shale. However, since direct observations of subsurface hydraulic fracture geometries are incomplete or nonexistent, we look to properly scaled experimental research and computer modeling based on realistic assumptions to help us understand fracture intersection geometries. Most experimental analysis of this problem has focused on natural fractures with frictional interfaces. However, core observations from the Barnett and other shale plays suggest that natural fractures are largely cemented. To examine hydraulic fracture interactions with cemented natural fractures, we performed 9 hydraulic fracturing experiments in gypsum cement blocks that contained embedded planar glass, sandstone, and plaster discontinuities which acted as proxies for cemented natural fractures. There were three main fracture intersection geometries observed in our experimental program. 1) A hydraulic fracture is diverted into a different propagation path(s) along a natural fracture. 2) A taller hydraulic fracture bypasses a shorter natural fracture by propagating around it via height growth while also separating the weakly bonded interface between the natural fracture and the host rock. 3) A hydraulic fracture bypasses a natural fracture and also diverts down it to form separate fractures. The three main factors that seemed to have the strongest influence on fracture intersection geometry were the angle of intersection, the ratio of hydraulic fracture height to natural fracture height, and the differential stress. Our results show that bypass, separation of weakly bonded interfaces, diversion, and mixed mode propagation are likely in hydraulic fracture intersections with cemented natural fractures. The impact of this finding is that we need fully 3D computer models capable of accounting for bypass and mixed mode I-III fracture propagation in order to realistically simulate subsurface hydraulic fracture geometries. / text

Underground mining can be summarized as the removal of economically viable volumes of rock which creates underground voids. In order to optimize ore extraction, a material is used to backfill these openings prior to creating any adjacent openings. The use of cemented paste backfill (CPB), a mixture of mine tails, water, and cement binder, has gained prominence as it not only provides a material that has engineered strength and can be deployed rapidly, but also decreases the surface storage volume of the mine tails. There is limited knowledge about the behavior of the stresses within the CPB during the filling of an underground opening, particularly during the early curing ages of the hydrating CPB which is critical to the design of fill barricades. This thesis presents a design procedure which can be used to determine the in situ stresses within the CPB. Three methodologies were used in the development of this design procedure. The first was to develop a laboratory testing method that determined the time-dependent consolidation characteristics and strength parameters of the hydrating cemented paste material. The second was to collect several field-data sets. The third methodology was to numerically model the CPB using Itasca’s FLAC3D, which incorporated the underground void’s geometry, backfilling strategy, and time-dependent backfill parameters in order to determine the in situ stresses of the CBP. This simulation allowed for the prediction of both total and effective stress throughout the stope. The model and the laboratory results were used to model the stresses in several test stopes so that a comprehensive comparison could be made between the model and field instrumentation results. Four case studies were examined using a total of six different field instrumentation datasets. The results from these case studies showed that the modeling approach, given some model calibration, is capable of quantitatively representing the important geomechanical aspects of paste filling and curing.

Measurement of the strength development of Cemented Paste Backfill in laboratory cast cylinders does not replicate the in situ strengths of CPB in mine stopes. The mass of CPB in a filled stope is large and temperature rises due to the heat of hydration of the cementing materials, thus accelerating the gain in strength, relative to laboratory specimens stored at ambient temperature. The purpose of this study was to determine the impact on strength development when CPB test cylinders were subjected to a temperature profile mimicking that in a large mass, such as a mine stope. Also, maturity (the integral of time and temperature during hydration of the CPB) was compared to actual strengths, and the maturity – strength concept used in concrete technology was applied. It was found that the strength- maturity relationship was applicable to CPB once the base line or datum temperature was adjusted.

Underground mining can be summarized as the removal of economically viable volumes of rock which creates underground voids. In order to optimize ore extraction, a material is used to backfill these openings prior to creating any adjacent openings. The use of cemented paste backfill (CPB), a mixture of mine tails, water, and cement binder, has gained prominence as it not only provides a material that has engineered strength and can be deployed rapidly, but also decreases the surface storage volume of the mine tails. There is limited knowledge about the behavior of the stresses within the CPB during the filling of an underground opening, particularly during the early curing ages of the hydrating CPB which is critical to the design of fill barricades. This thesis presents a design procedure which can be used to determine the in situ stresses within the CPB. Three methodologies were used in the development of this design procedure. The first was to develop a laboratory testing method that determined the time-dependent consolidation characteristics and strength parameters of the hydrating cemented paste material. The second was to collect several field-data sets. The third methodology was to numerically model the CPB using Itasca’s FLAC3D, which incorporated the underground void’s geometry, backfilling strategy, and time-dependent backfill parameters in order to determine the in situ stresses of the CBP. This simulation allowed for the prediction of both total and effective stress throughout the stope. The model and the laboratory results were used to model the stresses in several test stopes so that a comprehensive comparison could be made between the model and field instrumentation results. Four case studies were examined using a total of six different field instrumentation datasets. The results from these case studies showed that the modeling approach, given some model calibration, is capable of quantitatively representing the important geomechanical aspects of paste filling and curing.