With increasing development in the oil and gas industry, exploration and production is continuously moving deeper off the continental shelf and onto the continental slopes. This increases the risk of submarine slope failures leading to damage of offshore structures. Thus there is a need to study and understand properties of offshore marine clays on slopes. This study was undertaken in order to understand better the characteristics of a sub-marine clay deposit taken from the Gulf of Mexico. This thesis presents the results of SHANSEP triaxial testing performed on undisturbed samples of Gulf of Mexico clay. Background information is given about the clay, the sampling program and the laboratory testing program. The GEOTAC Truepath automated stress path triaxial apparatus implemented for this research and the laboratory procedures used are described in detail. Data is summarized from the various types of tests run on the clay (CKoU compression and extension, CIU compression and extension tests, consolidations tests) and the stress history of the deposit is evaluated. The SHANSEP reconsolidation technique was used for a comprehensive program of Koconsolidated-undrained (CKoU) triaxial compression and extension tests at overconsolidation ratios (OCR) ranging from one to eight. Eighteen tests were run on jumbo piston core samples from one particular core. The consolidation phase of these SHANSEP tests provided most of the preconsolidation pressure values used to establish the stress history at the two test sites. These tests were used to estimate the in situ Ko and how it varies with OCR. The undrained shear phase of the tests provides detailed information on the values of S and m for use in the SHANSEP undrained strength equation, Su= 0vo = S(OCR)m, effective stress failure envelopes, etc.

The use of plastic concrete for cutoff walls in dams for remediation of seepage issues has become more widely used in the past 25 years, however, the in-situ material properties are still not well understood. The research presents a new testing procedure that combines two existing testing methods, triaxial shear and permeability testing. By developing this laboratory testing method, material properties of the cutoff wall backfill material can be more accurately examined and explained using changes in the permeability of the material to discern the ductility and stiffness.

Liquefaction is a phenomenon that occurs in loose saturated sand deposits that are subjected to earthquake loading. This phenomenon can cause massive displacements and significant destruction. Many methods for mitigating liquefaction have been proposed and investigated including compaction, drainage, and grouting. One such liquefaction mitigation technique involves the addition of bentonite fines to the pore spaces of a loose, saturated sand via permeation of an engineered clay suspension. This method of soil improvement has provided the basis and motivation for this research. Also, the effect of plastic and non-plastic fines on the static and cyclic response of sands is somewhat contradictory throughout the literature. Thus, the primary objective of this study was to characterize the affect of an engineered bentonite pore fluid on the undrained monotonic response of loose, saturated Ottawa sand in order to determine its feasibility for use as an effective method for liquefaction mitigation. The permeation of engineered bentonite suspensions is proposed as a passive site remediation technique. Thus, the suspensions were delivered to loose Ottawa sand specimens in the laboratory by permeation in a newly designed three-way split mold. This split mold was used to create easily tested specimens that would have an initial soil fabric similar to that expected after permeation in the field. The bentonite suspensions were treated with sodium pyrophosphate to reduce the initial yield stress and viscosity in order to allow for permeation. Three different bentonite suspensions were utilized throughout this study each having different properties and delivering slightly different amounts of bentonite to the loose, saturated sand. The affect of this engineered pore fluid on the undrained shear response of loose, saturated Ottawa sand was compared to the undrained shear response of clean sand and dry-mixed sand and bentonite. The specimen preparation method (dry-mixed or permeated) was shown to have a significant effect on the response of the sand specimens. While the dry-mixed specimens produced larger and more sustained positive pore water pressures than the clean sand (resulting in an increased tendency to flow), the permeated specimens showed a marked decrease in the generation of excess pore water pressures, displayed a more dilative response, and thus resulted in a soil structure that was less likely to flow. Finally, the results of tests on specimens permeated with engineered bentonite suspensions show that there is little to no change in the effective friction angle at critical state. A method for effectively testing permeated soil specimens was developed in this study. This method has laid the framework for further investigations into the use of engineered bentonite suspensions for liquefaction mitigation by permeation grouting. / text

In 2010 and 2011 Christchurch, New Zealand experienced a series of earthquakes that caused extensive damage across the city, but primarily to the Central Business District (CBD) and eastern suburbs. A major feature of the observed damage was extensive and severe soil liquefaction and associated ground damage, affecting buildings and infrastructure. The behaviour of soil during earthquake loading is a complex phenomena that can be most comprehensively analysed through advanced numerical simulations to aid engineers in the design of important buildings and critical facilities. These numerical simulations are highly dependent on the capabilities of the constitutive soil model to replicate the salient features of sand behaviour during cyclic loading, including liquefaction and cyclic mobility, such as the Stress-Density model. For robust analyses advanced soil models require extensive testing to derive engineering parameters under varying loading conditions for calibration. Prior to this research project little testing on Christchurch sands had been completed, and none from natural samples containing important features such as fabric and structure of the sand that may be influenced by the unique stress-history of the deposit. This research programme is focussed on the characterisation of Christchurch sands, as typically found in the CBD, to facilitate advanced soil modelling in both res earch and engineering practice - to simulate earthquake loading on proposed foundation design solutions including expensive ground improvement treatments. This has involved the use of a new Gel Push (GP) sampler to obtain undisturbed samples from below the ground-water table. Due to the variable nature of fluvial deposition, samples with a wide range of soil gradations, and accordingly soil index properties, were obtained from the sampling sites. The quality of the samples is comprehensively examined using available data from the ground investigation and laboratory testing. A meta-quality assessment was considered whereby a each method of evaluation contributed to the final quality index assigned to the specimen. The sampling sites were characterised with available geotechnical field-based test data, primarily the Cone Penetrometer Test (CPT), supported by borehole sampling and shear-wave velocity testing. This characterisation provides a geo- logical context to the sampling sites and samples obtained for element testing. It also facilitated the evaluation of sample quality. The sampling sites were evaluated for liquefaction hazard using the industry standard empirical procedures, and showed good correlation to observations made following the 22 February 2011 earthquake. However, the empirical method over-predicted liquefaction occurrence during the preceding 4 September 2010 event, and under-predicted for the subsequent 13 June 2011 event. The reasons for these discrepancies are discussed. The response of the GP samples to monotonic and cyclic loading was measured in the laboratory through triaxial testing at the University of Canterbury geomechanics laboratory. The undisturbed samples were compared to reconstituted specimens formed in the lab in an attempt to quantify the effect of fabric and structure in the Christchurch sands. Further testing of moist tamped re- constituted specimens (MT) was conducted to define important state parameters and state-dependent properties including the Critical State Line (CSL), and the stress-strain curve for varying state index. To account for the wide-ranging soil gradations, selected representative specimens were used to define four distinct CSL. The input parameters for the Stress-Density Model (S-D) were derived from a suite of tests performed on each representative soil, and with reference to available GP sample data. The results of testing were scrutinised by comparing the data against expected trends. The influence of fabric and structure of the GP samples was observed to result in similar cyclic strength curves at 5 % Double Amplitude (DA) strain criteria, however on close inspection of the test data, clear differences emerged. The natural samples exhibited higher compressibility during initial loading cycles, but thereafter typically exhibited steady growth of plastic strain and excess pore water pressure towards and beyond the strain criteria and initial liquefaction, and no flow was observed. By contrast the reconstituted specimens exhibited a stiffer response during initial loading cycles, but exponential growth in strains and associated excess pore water pressure beyond phase-transformation, and particularly after initial liquefaction where large strains were mobilised in subsequent cycles. These behavioural differences were not well characterised by the cyclic strength curve at 5 % DA strain level, which showed a similar strength for both GP samples and MT specimens. A preliminary calibration of the S-D model for a range of soil gradations is derived from the suite of laboratory test data. Issues encountered include the influence of natural structure on the peak-strength–state index relationship, resulting in much higher peak strengths than typically observed for sands in the literature. For the S-D model this resulted in excessive stiffness to be modelled during cyclic mobility, when the state index becomes large momentarily, causing strain development to halt. This behaviour prevented modelling the observed re- sponse of silty sands to large strains, synonymous with “liquefaction”. Efforts to reduce this effect within the current formulation are proposed as well as future research to address this issue.

The aim of the research is to investigate the effect of increased axle loading on saturated and unsaturated railway foundation materials for heavy haul applications. The research methodology comprises of a literature review to identify the lacuna in the scientific knowledge, finite element modelling for characterisation of railway cyclic loading, development of a cyclic triaxial apparatus for laboratory testing and experimental work, followed by analysis, interpretation and discussion of results and lastly the formulation of conclusions and recommendations. The axle loading of interest start with a base load of 20 tonnes per axle for general freight followed by increased axle loading of 26, 30, 32.5 and 40 tonnes per axle for heavy haul. The test materials used in the experimental work are representative of the subballast and subgrade layers in a railway substructure. As a reproduction of the climatic conditions in the field and the loading from passing trains, experimental testing was carried out on saturated samples under undrained conditions and unsaturated samples under constant water content. Unsaturated samples were prepared to matric suctions of 50, 100 and 225 kPa via axis translation. Monotonic and cyclic tests were conducted to investigate the behaviour of railway foundation materials. Critical state theory for saturated and unsaturated soils is used as a method of analysis in establishing the failure criterion and the failure envelope. Various parameters, such as stress states, strains, resilient modulus, pore water pressure and matric suction are also utilised in investigating trends and behaviours. Based on the monotonic test results, the shear strength of unsaturated samples was found to be greater than that of saturated samples, attributed mainly to strain hardening caused by the unsaturated soil conditions, with the presence of a peak deviator stress when plotted on the stress-strain graph. However, unsaturated samples were also found to be prone to load-collapse during monotonic shear, even when the water content and confining stress remained constant, which resulted in brittle behaviour with the sudden rupture and formation of multiple bifurcation shear bands and slip planes. Based on the cyclic tests on saturated materials, it was discovered that increased axle loading can result in phase-transition in soil behaviour, based on the stress states in the soil relative to the critical state line plotted in the effective stress space. Stress states below the critical state line resulted in a no-phase transition with dilation behaviour. Stress states on the critical state line resulted in a single-phase transition from dilation to contraction. Stress states above the critical state line resulted in a double-phase transition from dilation to contraction behaviour and then strain-softening. It is therefore concluded that increased axle loading can only be sustained by materials which presented dilation and no phase-transition in soil behaviour, which occurred at axle loading of 20 and 26 tonnes per axle for the subballast and subgrade materials. Based on the cyclic tests on unsaturated materials, it was established that increased axle loading did not cause material failure for all load axle cases and materials. The stress states of all tests plotted well below the failure envelope in the net stress space, which is indicative of resilient and elastic behaviour. Increased axle loading instead resulted in decreased permanent strain, until the critical level of repeated deviator stress of 32.5 tonnes per axle was found, where the permanent strain increased. It is therefore concluded that, as a result of the increased shear strength from the strain hardening property of unsaturated materials, an increased axle loading of 32.5 tonnes per axle can be safely sustained by the tested materials provided the matric suction in the soil is greater than 50 kPa. / Thesis (PhD)--University of Pretoria, 2020. / Civil Engineering / PhD / Unrestricted

<p>This thesis investigates the effects of sample preparation methods, which has substantial influence on the internal structure or fabric of the sample, and interparticle locking on the behaviour of sand through experimental study. Extensive laboratory tests were conducted on two sands (namely, Ottawa sand and crushed limestone) with distinct particle shape and surface texture, using a Bishop-type triaxial testing system.</p><p>A total of eight sample preparation methods were used to fabricate specimens with different initial fabric, with specimens being fabricated using water pluviation, moist tamping, and moist rodding. The experimental data reveal that sample preparation methods have significant effect on both deformation characteristics and shear strength of sand, in addition to the density and the effective confining pressure applied to the specimens. More specifically, water pluviation and moist tamping tend to yield specimens of high anisotropy and large dilation, which in turn results in higher friction angle in conventional triaxial compression. The effect of sample preparation methods was also observed from undrained tests on saturated sand.</p><p>Laboratory tests on crushed limestone consisting of angular particles demonstrate that strong interparticle locking may develop owing to particle angularity. The shear resistance of sand with angular particles has contributions from interparticle friction, dilatation and interparticle locking. Moreover, interparticle locking, which largely exists at the peak shear resistance of sand but vanishes with dilation at large deformation, exists under both low and high stress levels investigated in this study. A conceptual model was proposed to take into account the energy consumption associated with breaking interparticle locking during deformation when estimating the dilatancy and strength of granular soils.</P><P>The behaviour of sand along proportional strain paths was also investigated, with the focus being placed on strain softening and material instability in the context of Hill's second order work. Depending on the strain path or the deformation history, a dilatant sand displaying hardening and stable behaviour under isochronic (undrained) conditions, which is often used as a reference in soil mechanics, may succumb to unstable flow type behaviour along dilative strain paths. More specifically, when the imposed rate of dilation exceeds the inherent rate of dilation of the material, a dense sand specimen will have flow failure similar to that of a saturated loose specimen subjected to undrained compression. On the other hand, a loose sand may not have a flow failure when it is forced to have contractive volume change along imposed strain paths.</P> / Thesis / Master of Science (MSc)

Traditionally, flexible pavement design relies on past experience and semi-empirical methods developed through a combination of element testing and modelling. Element testing in this area especially, has centred on the performance of clean sands. This is in conflict with actual practice where a wide range of fines and soil gradations could be present in a real-world project. This research investigates the characteristics of natural sands and examines the influence of these marginal materials in pavement design using element testing and controlled modelling of an actual flexible pavement system. The element tests concentrated on separate, natural soils sourced from Kazakhstan which had similar mineralogy, but varying amounts of fines. One of the key parameters examined was equivalent void ratio and its efficiency to account for the behaviour change in granular materials which comes from increased fines content. Starting with monotonic triaxial results combined with strength-dilatancy methods it was shown that prediction of shear strength in a silty-sand could be improved by 13%. Incorporating this finding into repeat load triaxial tests, the transitions between elastic, plastic, and ratcheting failure behaviours (i.e. shakedown boundaries), commonly used to help predict the lifespan of a flexible pavement, were examined. It was seen that cycling a silty-sand, the stress path and yield surface could change depending on the fines content. The Cambridge Airfield Pavement Tester (APT) was designed and constructed to measure permanent subgrade deformation resulting from various surface loads. The number of input variables required to design flexible pavements is one of the most frequently stated problems in the field; variation of aircraft types, environmental conditions, and materials makes mechanistic design of the soil foundation problematic. Accordingly physical pavement modelling continues to be the only experimental method that allows input parameters and material characteristics to be examined simultaneously. Digital image correlation (DIC) was incorporated into the system; the first time this technology has been used in flexible pavement research. A Null Pressure System was also installed to measure soil stress distributions. It was observed that the critical failure mechanisms for thin and thick surficial layers are different, resulting in changes in the rates of surface rutting. Finally, by combining element and APT results, knowledge of the causal relationships between subsurface deformation and failure mechanisms in flexible pavement were advanced. In-situ soils, which are frequently incorporated into subgrade designs, were found to have a substantial role in the serviceability of the pavement. Correlations between element tests and APT results highlighted the complicated loading and boundary conditions present in a pavement.

The liquefaction resistance of undisturbed soil samples collected from a selected location in Carbondale, Southern Illinois was evaluated by conducting cyclic triaxial tests. Index property tests were carried out on the sample for identification and classification of the soil. Cyclic triaxial tests were conducted on undisturbed sample after saturation, undisturbed sample at natural water content and remolded samples prepared by compaction in the lab. The results were used to evaluate the effect of saturation and remolding on liquefaction resistance of the local soil. Effect of effective confining pressure on dynamic properties of soil (Young's Modulus and Damping ratio) was also studied. Forty five stress controlled cyclic triaxial tests were performed. Three different values of initial effective confining pressure (5 psi, 10 psi and 15 psi) were used and cyclic stress ratio was varied from 0.1 to 0.5 in order to apply different cyclic shear stresses. The results show that the cyclic load resistance of soil decreases as a result of remolding. Saturated undisturbed samples show increase in resistance to liquefaction with increase in initial confining pressure. Remolded samples were prepared by compaction in the lab keeping unit weight and water content equal to that of undisturbed samples. Remolded samples show increase in liquefaction resistance with increase in confining pressure. Undisturbed samples at natural water content show increase in resistance to develop axial strain with increase in confining pressure. Both the rate of excess pressure development and axial strain development increase significantly as a result of remolding. While investigating the effect of saturation of undisturbed samples on liquefaction resistance of soil, interesting observations were made. The excess pressure buildup rate was faster in case of saturated undisturbed samples compared to that in samples with natural water content. On the other hand, rate of strain development was significantly high in case of sample with natural water content compared to that in saturated sample. Also, results obtained from cyclic triaxial tests on saturated undisturbed samples were compared with results obtained from similar tests on Ottawa Sand (Lama 2014) sample. The comparison shows that the saturated undisturbed soil samples of the selected local soil have very high resistance to liquefaction both in terms of initial liquefaction and development of 2.5% and 5% axial strain. Modulus of Elasticity and damping ratio were studied as important dynamic properties of soil. Young's Modulus was observed to decrease significantly at higher strain levels for all three types of samples. Young's modulus increased with increase in effective confining pressure, the effect of confining pressure being large at low strain level and almost insignificant at higher strain level. Damping ratio was highest in undisturbed sample at natural water content and smallest in remolded sample and damping ratio for saturated undisturbed sample falls in between. The damping ratio did not show any definite correlation with strain and confining pressure at lower strain level. But, for strain higher than 1% double amplitude axial strain, damping ratio significantly decreases with increase in strain. Damping ratio increases with increase in confining pressure as observed at high strain for all samples.

The strain rate dependent behaviour of fine grained soils is an important aspect of geotechnical engineering. During dynamic or rapid events such as earthquakes and rapid pile testing, a fine grained soil will display significantly different behaviour than may be observed over the long life span of a structure. There is currently little understanding of the factors which influence the behaviour of fine grained soils during dynamic events (extremely high strain rates), making their response difficult to predict. This research investigates the behaviour of fine grained soils subjected to a wide range of constant strain rates in monotonic triaxial compression testing. Each test is conducted under drained conditions to observe the behaviour of soils as they transition from a drained response at lower strain rates, through to an undrained or viscous response at higher strain rate tests. Where the response of soils is drained or partially drained, higher strain rate tests measure a decrease in strength. The point of transition from partially drained to undrained behaviour corresponds to the lowest strain rate dependent strength. Further tests at higher strain rates measure consistently greater strength. The strain rate dependence of three fine grained soils is investigated, enabling a comparison of strain rate effects with soil index properties. The influence of initial state on the strain rate dependence of these Kaolin based model soils is also evaluated. The drained to partially drained response of the soils to strain rate increase is controlled by the coefficient of consolidation. Tests at high strain rates show the undrained or viscous strain rate effect on strength is related to liquidity index. Local strain instrumentation allowed comparison of strain rate effects on small strain stiffness. At higher strain rate the soils display increasingly linear behaviour. At non-linear elastic strains, liquidity index appears to control the magnitude of the strain rate effects on stiffness.

Although silt and clay mixtures were mostly considered to be resistant to cyclic loading due to cohesional components of their shear strength, ground failure case histories compiled from fine grained soil profiles after recent earthquakes (e.g. 1994 Northridge, 1999 Adapazari, 1999 Chi-Chi) revealed that the responses of low plasticity silt and clay mixtures are also critical under cyclic loading. Consequently, understanding the cyclic response of these soils has become a recent challenge in geotechnical earthquake engineering practice. While most of the current attention focuses on the assessment of liquefaction susceptibility of fine-grained soils, it is believed that cyclic strain and strength assessments of silt and clay mixtures need to be also studied as part of complementary critical research components. Inspired by these gaps, a comprehensive laboratory testing program was designed. As part of the laboratory testing program 64 stress-controlled cyclic triaxial tests, 59 static strain-controlled consolidated undrained triaxial tests, 17 oedometer, 196 soil classification tests including sieve analyses, hydrometer, and consistency tests were performed. Additionally 116 cyclic triaxial test results were compiled from available literature. Based on this data probability-based semi-empirical models were developed to assess liquefaction susceptibility and cyclic-induced shear strength loss, cyclically-induced maximum shear, post-cyclic volumetric and residual shear strains of silt and clay mixtures. Performance comparisons of the proposed model alternatives were studied, and it is shown that the proposed models follow an unbiased trend and produce superior predictions of the observed laboratory test response. Superiority of the proposed alternative models was proven by relatively smaller model errors (residuals).