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Dissertation, RWTH Aachen University, 2019; Dissertation, RWTH Aachen University, 2019, The quantitative understanding of grain-scale deformation mechanisms in clay is important in hydrocarbon and water exploration as well as in the evaluation of clay formations as potential host rock for long-term underground repositories for high-level radioactive waste. Analyses on the nano-scale allow understanding the underlying microphysical processes and therefore form the basis for microphysics based constitutive laws for transport and deformation, which can be confidently extrapolated to conditions outside those in experiments. A microphysical understanding of the transport and deformation properties is important, because the properties of Boom Clay are known to be complex: deformation is anisotropic, after a certain amount of strain deformation tends to localize, and the transport properties are also expected to be dependent on deformation. In addition, one of the favourable properties of Boom Clay is that fractures have the tendency to self-seal, but a microphysical understanding of this process is so far not available. This study examines the development of microstructure during triaxial tests on Boom Clay freshly collected at HADES level with σ1 applied parallel and perpendicular to the bedding, to various total axial strains. We used a range of methods integrated in a multi-scale analysis. First, the samples were saturated and deformed in consolidated-undrained (CU) triaxial tests starting at 2.2 MPa effective stress, combined with in-situ micro computed tomography (μ-CT). The μ-CT data with a resolution of 13.5 μm/pixel were analysed by 3D digital image correlation (DIC) to compute the incremental displacement fields, and the evolution of the strain field in the sample. Deformed samples were slowly dried, sectioned, and the microstructure studied by optical and scanning electron microscopic (SEM) imaging with resolutions down to a few nanometres. The stress-strain curves our experiments are in good agreement with previous studies by Coll (2005); Sultan et al. (2010); Deng et al. (2011b); Bésuelle et al. (2014). The orientations of the shear zones (SZ) with respect to the shortening direction are 40 to 45°, in reasonable agreement with published values of friction angles. The behaviour is slightly anisotropic. The initial pore water pressure increase Δu of samples shortened perpendicular to the bedding (S⊥B) is higher than the values measured in samples deformed with stress parallel to the bedding (S ‖ B), which is in agreement with what is expected from microstructure. DIC analysis shows that the evolution of strain is also different in S⊥B and S ‖ B samples, although most samples localize the strain at about 2% axial shortening. From this point on, more and more of the axial shortening is taken up by movements along the SZ and the distributed strain in the sample decreases. Non-localized strain in samples S⊥B is highest (up to 3 %) in cone shaped zones close to the top and bottom of the samples. In samples S ‖ B, strain is more homogeneously distributed prior to strain localization, which occurs at slightly higher axial strain than in S⊥B. Because of the prominence of the evolving SZ, the shear strength at high shear strains of S⊥B and S ‖ B is similar. Based on the DIC maps, we selected representative regions with different styles of deformation for imaging at high resolution to understand the evolution of microstructure and porosity. We defined four different structural domains:1. OSZ: shear strain < 3 %, non-localized deformation and a microstructure comparable to undeformed samples;2. OSZ-HS: higher shear strain than in OSZ (≥ 3 %), but still non-localized deformation, in S⊥B: Microstructure comparable to undeformed samples, in S ‖ B: Microstructure characterized by numerous micro-kinks and -folds; in both, S⊥B and S k B, OSZ-HS are present in significant parts of the samples more so in S ‖ B;3.OSZ-TZ: S k B at both boundaries of the SZ, shear strain comparable to OSZ-HS (≥ 3 %), non-localized deformation with a strongly altered microstructure characterized by micro-kinks;4.ISZ: SZ with a shear strain between 5 and 50, strongly localized deformation with a shape preferred orientation (SPO) of elongated grains, reduction of porosity and pores parallel to the shearing direction. Evolved SZ in S ‖ B always have a kink-zone surrounding them. The thickness of SZ varies between 20 and 200 μm, increasing with increasing shear strain. The internal structure of ISZ is similar in S ‖ B and S⊥B. Microstructures show evidence for frictional/granular deformation mechanisms (grain rotation, grain sliding, pore collapse and reorientation, mica grain bending) and no evidences for cataclastic processes. This is in agreement with microstructure which contains about 20 vol.% silt-size hard grains (calcite, feldspar and quartz) embedded in a highly porous clay matrix. It is also generally observed that quartz and feldspar grains in SZ are much smaller than in the bulk of Boom Clay. This is not because they were fragmented to smaller pieces, but because the SZ develop in regions where are no large grains. This study provides a microstructural basis for the construction of a microphysics-based model for the deformation of Boom Clay, which forms the basis for a microphysics based constitutive law, which can be extrapolated to conditions outside those used in our experiments.

Oil wells subjected to cyclic steam injection often present cement sheath failures due to thermal loads. Currently, different Portland based systems are employed to mitigate this risk. We developed and tested geopolymer systems that meet oil well cementing requirements and achieve high mechanical performance. A metakaolin-potassium-based formulation, containing microsilica, mineral fiber and retarder, was adjusted to the desired rheology and thickening time. Triaxial compression and indirect tensile tests were performed and the properties were used in finite element simulations of the life of a model onshore well. Compared to the Portland based system currently in use, the geopolymer has superior uniaxial compressive strength and lower stiffness. Tensile strength increased significantly with the addition of mineral fibers and became higher than that of the Portland system. Numerical simulations show that the geopolymer can withstand steam injection temperatures comparable to a conventional system, remaining in the elastic region. These results show that geopolymers are a viable alternative well cementing system for this application, with significant improvement in mechanical performance.

This paper presents measurements, and an interpretation of these measurements, based on the use of bender-element probes for Boštanj silty sand. The samples were prepared at different initial void ratios and isotropically compressed up to 5 MPa. The bender-element technique was used to determine the dynamic shear modulus ($G_0$) of the soils at very small strains. The multiple bender-element probes were shot at different excitation frequencies in order to increase the reliability of the measurements. The $G_0$ stiffness was determined by using three different techniques: a) the first-time arrival, b) the phase-change method and c) the cross-correlation method. The systematic differences observed between the $G_0$ values, calculated using the three techniques, are discussed. The variation of $G_0$ in the log$G_0$ - log$p'$ plane was evaluated for the Boštanj silty sand and compared with other sands. V članku je predstavljeno merjenje in interpretacija meritev z bender elementi na meljastem pesku iz Boštanja. Vzorci so bili pripravljeni pri različnih začetnih količnikih por in obremenjeni v izotropni kompresiji do 5MPa. Tehniko merjenja z bender elementi smo uporabili za določitev dinamičnega strižnega modula ($G_0$) materiala pri zelo majhnih deformacijah. Zanesljivost meritev smo povečali z uporabo skupine signalov z različnimi frekvencami vzbujanja. Togost $G_0$ je bila določena z uporabo treh različnih pristopov, in sicer: a) časovnega prihoda vala, b) spremembe faze in c) navzkrižne korelacije. V članku so pojasnjene razlike med vrednostmi $G_0$ glede na uporabo določenega pristopa, ki so se sistematično pojavljale. Prikazano je spreminjanje togosti $G_0$ meljastega peska iz Boštanja v ravnini log$G_0$ - log$p'$ in primerjave z drugimi peski.

This article focusses on mineralogical and physical characteristics of snail soil and their influence on parameter values of geomechanical characteristics.Snail soil, which got its name from fossil remains, is a typical layer observed in the Ljubljana marsh. It is distincltly porous, saturated and in a liquid consistency state. Snail soil was investigated for mineralogical and physical characteristics in the Laboratory of Soil Mechanics, Faculty of Civil Engineering of the University in Maribor. Mineral and chemical composition, visual appearance, specific surface and grain property were determined. Physical characyteristics show that snail soil is saturated in nature, highly porous and almost liquid. Geomechanical characteristics were investigated for their interdependency on physical characteristics. A series of triaxial tests were performed on snail soil samples of different porosity, density and water content. Cylindrical samples of the height of 100 mm and the diameter of 50 mm were tested using three-axial testing apparatus. The results of the tests show that interdependency exists between geomechanical characteristics and porosity. These relationships can be expressed as functions of density, porosity or water content. It is evident from the results that changes of the coefficient of permeability, the coefficient of consolidation, and the coefficient of volume compressibility are nonlinear with respect to changes in porosity. Changes of mechanical parameters, such as Young`s modulus, Poisson`s ratio andfriction angle are indistinct and almost linear at lower changes of porosity. Prispevek obravnava mineraloške in fizikalne lastnosti polžarice in njihov vpliv na vrednosti parametrov geomehanskih lastnosti. Polžarica, ime je dobila po vsebovanih fosilih, je tipični sloj Ljubljanskega barja. Je izrazito porozna, zasičena in se nahaja v židkem konsistentnem stanju. V Laboratoriju za mehaniko tal (LMT), Fakultete za gradbeništvo, Univerze v Mariboru je bila izvedena raziskava polžarice s preiskavo mineraloških in fizikalnih lastnosti. Za raziskavo je bil odvzet niz vzorcev na jugozahodni lokaciji Ljubljanskega barja. V Centru za elektronsko mikroskopijo Fakultete za strojništvo, Univerze v Mariboru sta bila raziskana kemična sestava in optični izgled. Kemična sestava je bila določena z visokoločljivim vrstičnim elektronskim mikroskopom SIRION. Optični izgled treh vzorcev polžarice je bil preiskan z okoljskim vrstičnim elektronskim mikroskopom QUANTA 200 3D. Mineralna sestava in zrnatost polžarice sta bila določena na Geološkem zavodu Slovenije. Mineralna sestava je bila določena z rentgenskim difraktometrom. Zrnatost polžarice je bila določena s pomočjo laserskega analizatorja Fritsch particle sizer analysette 22. Specifična površina je bila določena na Kemijskem inštitutu Slovenije, s pettočkovno BET metodo, za meritve je bil uporabljen avtomatski plinski adsorbcijski analizator TriStar 3000, proizvajalca Micromeritics Instrument Corporation, Norcross, ZDA. Ostale fizikalne lastnosti polžarice so bile določene v LMT. Raziskane fizikalne lastnosti kažejo, da je polžarica v naravi zasičena, zelo porozna in skoraj tekoča meljna zemljina. Preiskava stisljivosti kaže, da je polžarica izrazito stisljiva, volumenske spremembe so velike že ob malih spremembah napetosti. Parametri trdnosti imajo značilno nizke vrednosti, ki se ujemajo s predhodnimi raziskavami. Raziskava geomehanskih lastnosti je bila izvedena s poudarkom na sovisnosti fizikalnih lastnosti. Izveden je bil niz triosnih preizkusov polžarice različne gostote oz. vlažnosti in poroznosti. Preizkusi so bili izvedeni v triosnih aparatih na valjastih preizkušancih višine 100 mm in premera 50 mm. Rezultati raziskave kažejo na sovisnost geomehanskih lastnosti in poroznosti. Odnose lahko izrazimo kot funkcije gostote oz. poroznosti ali vlažnosti. Iz rezultatov je razvidno, da so spremembe vodoprepustnosti, konsolidacije in stisljivosti nelinearno odvisne od sprememb poroznosti. Spremembe mehanskih parametrov kot so elastični modul, Poissonov količnik in strižni kot, so pri manjših spremembah poroznosti neizrazite in skoraj linearne.

Pond ash, being a form of coal ash, consists mainly of silt-size glassy particles with a significant percentage containing internal occluded voids. By analysing the evolution of particle size distribution before and after triaxial containing internal occluded voids. By analysing the evolution of particle size distribution before and after triaxial testing at normal stress range using a Laser Particle Size Analyser, specific gravity and SEM photography, it wasfound that these hollow particles may breakup as a result of shearing. This type of particle disintegration is different from localised grain crushing or shearing off of asperities at high contact stress points. The stress-strain responses measured in an extensive programme of triaxial testing were synthesised. It was established that a unique and consistent critical state line was achieved, irrespective of initial state, drainage conditions and stress histories.Furthermore, the overall pattern of stress-strain responses was related to the location of initial state relative to the critical state line. This supports the use of critical state soil mechanics framework in synthesising and modelling the stress-strain behaviour of pond ash. Refereed/Peer-reviewed

International audience; Non-homogeneity of the pore pressure field in a specimen is an issue for characterization of the thermo-poromechanical behaviour of low-permeability geomaterials, as in the case of the Callovo-Oxfordian claystone (k < 10−20 m2), a possible host rock for deep radioactive waste disposal in France. In tests with drained boundary conditions, excess pore pressure can result in significant errors in the measurement of material parameters. Analytical solutions are presented for the change in time of the pore pressure field in a specimen submitted to various loading paths and different rates. The pore pressure field in mechanical and thermal undrained tests is simulated with a 1D finite difference model taking into account the dead volume of the drainage system of the triaxial cell connected to the specimen. These solutions provide a simple and efficient tool for the estimation of the conditions that must hold for reliable determination of material parameters and for optimization of various test conditions to minimize the experimental duration, while keeping the measurement errors at an acceptable level.

The monotonic behavior of pond ash was studied experimentally and with a particular focus on evaluating the applicability of critical state soil mechanics framework in synthesizing and modeling the observed mechanical behavior. Specimens were reconstituted with a paste deposition technique that closely mimics the wet disposal of dominantly fine coal ash in ash ponds. This technique can produce segregation-free specimens over a wide range of as-formed density states. The test results indicated that the observed behavior was similar to granular geomaterials, except that only nonflow behavior could be observed in undrained shearing. More importantly, a unique and consistent critical state line (CSL) in triaxial compression was obtained in both q-p' and e-log(p') planes irrespective of its drainage conditions, initial density states, and stress histories. The CSL in the e-log(p') plane was used to predict behavior patterns. It was found that the state parameter, as defined relative to the CSL in e-log(p')plane, correlates with important modeling parameters. Thus, this study unambiguously supports the applicability of critical state soil mechanics framework in synthesizing and modeling the behavior of pond ash.Introduction Refereed/Peer-reviewed

Two model soils, one made from sand with plastic fines and one made from sand with non-plastic fines, were tested in a triaxial apparatus to examine the basic mechanics of transitional soils, that is, soils whose initial density has a very strong influence on the behaviour. The sand with non-plastic fines defined parallel critical state lines and state boundary surfaces at the medium strain levels that can be reached in triaxial testing, the locations of which depended on the initial specific volume of the soil when it was created. Overconsolidating the soil, or shearing it a second time at an increased stress level, was not found to change this behaviour. There was also no significant effect of the initial specific volume on the soil stiffness, which was predominantly a function of the current stress level for normally consolidated states. A variety of different sample preparation methods was used, but the methods did not have any clear influence on the critical state volumes. Although the testing for the sand with non-plastic fines was more limited in extent, the patterns of behaviour seemed to be similar. The relationships between the initial specific volume and the critical state line intercept, Γ, gave gradients of 0.52 and 0.97 for the sands with plastic and non-plastic fines, respectively, showing significant differences in volume remaining after isotropic compression and monotonic compressive shearing.

Landslides are widespread along coasts worldwide. Understanding initiation, type, and areas affected by such landslides, is thus one of the primary concerns for coastal communities and infrastructure projects, like harbor constructions and residential settlements. Important short-term, high-energy impacts that may trigger a landslide are earthquakes and heavy rainfall events. Weak zones within the depositional succession comprising the slope are another important factor contributing not only to landslide initiation, but also to post-failure landsliding. Assessing the landslide hazard in coastal regions therefore requires a good understanding of the complex interrelations between the various high-energy external impacts and the internal mechanical characteristics of the slope material. Some of the most damaging landslides in the past occurred in soil with high sensitivity, a measure of the post-failure strength loss in the failure zone during landsliding. Such soil exhibits very low shear strength after failure, predisposing highly mobile landslides with long runout distance and dimensions difficult to predict. High sensitivities have been described for post-glacial sediments in Norway, as well as for altered tephra in New Zealand. In both regions, it is of common interest to better understand the weakening and mobilization processes in sensitive slope forming soil subjected to cyclic loading such as earthquake shaking. Furthermore, the processes that lead to high sensitivities in altered tephra are still poorly understood. This doctoral thesis aims to broaden the understanding of failure mechanisms and mobilization processes in landslides at the interface between land and water. Two landslides were investigated, that occurred in sensitive soil and affected society, economy, and natural environment in coastal regions: (1) The coastal submarine Orkdalsfjord landslide in postglacial silt, Norway, and (2) the coastal subaerial Omokoroa flow slide in altered tephra, New Zealand. The vulnerability of the Orkdalsfjord landslide to cyclic loading was studied by using in situ vibratory cone penetration tests and laboratory cyclic triaxial testing. Very coarse silt layers, interbedded in the post-glacial silt unit overlying the failure surface, is more vulnerable to cyclic loading compared to surrounding finer silts. Accordingly, the very coarse silt layers may have contributed to the weakening and mobilization of the Orkdalsfjord landslide in case cyclic loading occurred during landsliding. The cyclic loading behavior of altered tephra from the Omokoroa flow slide was analyzed by monotonic and cyclic triaxial testing. The altered tephra experiences brittle failure and has higher friction coefficients than normally consolidated clay, being similar to granular soil. Comparing the cyclic shear strength of altered tephra with that of marine clays shows that for altered tephra the number of loading cycles to shear failure depends more strongly on the level of shear stress applied and that altered tephra is more resistant to small cyclic loading but fails within a narrower range of shear stresses. The development of high sensitivities in altered tephra was analyzed by scanning electron microscopy and laboratory vane shear measurements along a drill core comprising the intact tephra succession of the Omokoroa flow slide. The secondary clay mineral halloysite dominates the Pahoia Tephra, a sequence that was involved in the Omokoroa flow slide. The halloysite particle morphologies are highly variable with depth. While tubular morphologies are prevalent in the upper tephra successions, the lower Pahoia Tephra sequence is dominated by spheroidal halloysite. This change in halloysite morphology coincides with an increase in sensitivity with depth. Therefore, spheroidal halloysite is likely the key in the development of sensitivity in altered tephra from New Zealand and potentially elsewhere in regions of similar volcanic origin. In the failure surface of the Omokoroa flow slide, a new open-sided spheroidal halloysite particle shape in the form of a mushroom capsa is recognized for the first time that governs the development of high mobility in the failure surface during landsliding. Based on a new a attraction-detachmenta model, it is suggested that the rearrangement in the halloysite texture during the failure process reduces the attractions between the particles at nanoscale dimensions and thus predisposes flow sliding.

Laboratory testing of soil conditioning for EPB tunnelling is a common procedure to assess the suitability of the conditioning itself in different tunnel projects, but normally the stress influence is not taken into account. This work describes the behaviour of two different conditioned masses in different stress conditions through a large diameter triaxial cell, which allows to reproduce different stress scenarios. The results obtained show the influence of the stress on the shear strength of the conditioned material and the different attitude of behaving as a fluid testing a granular material and a more heterogeneous one. Also the two material behaved differently as the presence of a more relevant fine part in the conditioned mass allows the material to maintain its fluidity also at higher pressures.

A series of bender element tests and drained and undrained monotonic triaxial compression and extension tests were performed on air-pluviated samples of Hostun sand. Samples were prepared to different initial void ratios, consolidated under various isotropic and anisotropic stress states, and sheared using different stress paths and a wide range of deformations to characterize the sand’s stress-strain response. The results suggest that the sand’s small-strain behavior essentially depends on the current void ratio and mean effective stress. Within the medium to large strain range, a state-parameter approach in conjunction with the critical-state framework can successfully predict the distinctive states of the sand’s monotonic response, namely the phase-transformation, peak-stress-ratio, and critical states. Furthermore, the data are used to examine a stress-dilatancy relationship often incorporated in constitutive models. The characterization presented herein aims at assisting the efficient calibration of numerical models and provides insight into this sand’s behavior, thus supporting the interpretation of results of physical modeling involving this sand. This paper highlights the importance of characterizing sand’s behavior over the full strain range and shows that accurate predictions of the critical state and small-strain stiffness are crucial to assess other aspects of the sand’s behavior.

This paper studies the behavior of a number of gravelly modeled materials by conducting large-scale triaxial testing. Undrained monotonic compression tests were carried out on the high compacted gravelly soil specimens with different fine content and stress levels. It was observed that high compacted gravelly soil specimens show dilative behavior at failure in a wide range of fine content. The effective stress paths of the materials having less than 10% and more than 22% of material finer than 0.2 mm are completely different. Variations of the CU material parameters with respect to the confining pressure, fine content, shape, strength and particle size were investigated. The CU material results were compared with those in CD conditions. The value of internal effective friction angle in CD conditions is slightly higher than those of the CU conditions. Moreover, where the volume strain in CD conditions is minimum, the excess pore water pressure in CU conditions decreases to zero.

This article investigates the development of volumetric strain nonuniformities in sand specimens subjected to drained cyclic triaxial compression loading. The assessment is performed by comparing volumetric strain determinations using an external volume gauge and local axial and radial strain measurements mounted on the center of the specimen. The experimental investigation has been performed for both frictional and enlarged lubricated ends on sand specimens of different densities and fabricated using both moist tamping and dry deposition techniques. It will be shown that considerable discrepancies between the global and local volumetric determination arise even in specimens tested with enlarged lubricated ends, as a result of different volumetric tendencies (contraction or dilation) of the center and the boundaries of the specimens. These discrepancies are more pronounced for dense specimens cycled at high average stress ratios and amplitudes. The influence of three different assumptions employed to account for the specimen’s deformed profile (namely the right cylinder, parabolic, and sinusoidal profile) on the local volumetric determinations will be also assessed. Some recommendations for the need for local volumetric measurements will be attempted.

The experimental investigation of clayey rocks which are typical for cap rock formations above CO 2 storage reservoirs is described and results are given. One series of claystone samples is gathered from the injection well of the Ketzin pilot test site in Germany. Another series comes from the same Keuper formation in a near-surface outcrop in the Southwest of Germany. The near-surface condition allowed drilling of samples with a standard size of 100 mm diameter and 200 mm height as well as large samples with a diameter of 550 mm and a height of 1200 mm. A special triaxial testing procedure is applied both on standard and large size samples which allows determination of the strength, stiffness and viscosity behavior of the rock in one path. A multi-stage technique (stepwise variation of the confining pressure) gives the strength behavior of each single sample. Stepwise varied deformation rates, over two orders of magnitude, lead to steps in the stress–strain-curve from which the viscosity index can be calculated. The viscosity index can be directly used in the Norton’s constitutive relations for a viscoplastic simulation. The comparison of results from standard and large samples shows that for clayey rocks a scale effect is negligible. Strength, stiffness and viscosity are not dependent from the sample size. The observed failure modes of the samples show that the transition from cataclastic to non-cataclastic behaviour occurs in the range of the applied levels of pressure and deformation rates even at room temperature. This transition limit is very important for the judgment of the sealing capacity of a cap rock. If numerical simulations result in deformation rates which, for the given pressure and temperature conditions in a cap rock formation during the CO 2 injection, stay sub-critical for cataclastic behaviour, the formation of new cracks can be excluded.

The monotonic behaviour of pond ash was comprehensively investigated under the critical state soil mechanics (CSSM) framework using an extensive triaxial testing programme. In order to investigate the influence of specimen reconstitution, two techniques were employed in the reconstitution of pond ash. One was a newly developed paste deposition (PD) technique aimed at simulating the wet disposal of pond ash in ash ponds, and the other the moist tamping (MT) technique which has been widely used for the reconstitution of sand and sand with fine. The particle size distribution of the pond ash was analysed by a laser particle analyser before and after triaxial testing to investigate particle breakage. Also, the tested pond ash was reused for triaxial testing, aiming to investigate the influence of particle breakage on its behaviour. Test results indicated that the pond ash generally exhibited similar monotonic behaviour to that of sand and other granular geo-materials, and the CSSM framework is applicable to predicting the behaviour of pond ash, irrespective of reconstitution technique and drainage condition. However, the behaviour of pond ash was also found to be highly dependent on the reconstitution technique. Specifically, the critical state line (CSL) of the MT pond ash in the e-logp' plane was located statistically higher than that of the PD pond ash, whilst the MT pond ash exhibited a higher strength and more dilative/less contractive behaviour than its PD counterpart. Moreover, both the PD and MT pond ash underwent particle breakage during shearing, and irrespective of reconstitution technique, drainage condition and stress history, the amount of breakage, as quantified by the increase in specific surface area, exhibited a unique correlation with the total input energy per unit mass (E), in which the effects of stress and strain could be combined. Although particle breakage introduced bias into the measurement of the void ratio, the bias was so small that the interpretation of the test results was not affected. Furthermore, the CSL of the reused pond ash in the e-logp' plane was located distinctly below that of the original pond ash, and under a given E, the increase in the specific surface area that newly occurred on the reused pond ash was lower than that on the original pond ash.

International audience; The aim of this paper is to investigate the effect of soil grading on the stress-strain and the critical-state behavior of granular materials from idealized spheres to natural soils as well as from discrete to continuous modeling. The three-dimensional discrete-element method has been applied to study the mechanical behavior of idealized granular materials. The results confirm that the critical-state line (CSL) shifts down-ward as grading broadens with an increase of the coefficient of uniformity Cu. An exponential relationship between the critical-state parameters and the coefficient of uniformity Cu can then be established. Furthermore, experimental investigations on an artificial material (glass beads) and a natural material (Hostun sand) were carried out. The experimental results confirm the grading dependency of the critical-state behavior, and a similar relationship between CSL and Cu, for both glass beads and natural sand, extending previous literature results for high Cu values up to 20. Moreover, the experimental results show that the CSL tends to stabilize when the coefficient of uniformity Cu becomes higher than 10. Based on all test results, a simple elastoplastic model accounting for the influence of the grain size distribution (GSD) on the mechanical behavior of granular materials has been developed within the framework of critical-state soil mechanics. Comparisons between experimental results and simulations show that the model can accurately reproduce the mechanical behavior of granular materials with different GSDs.

中国華清池(驪山)地すべりの発生機構の一つとして, 岩盤のクリープ運動が考えられている。有力なすべり面と考えられている断層を挟む2種の片麻岩(上盤をLGNA, 下盤をLGNBと呼ぶ)が採取され, 室温で封圧40MPaまでの, 圧縮強度と残留強度が測定された。両者の強度およびその封圧依存性は等しいことが判明した。LGNAは異方的な結状構造が発達し, これに沿った巨視的なクラックが形成されている。一方, LGNBは巨視的には等方的である。これらの結果から, LGNAの方が空隙率が大きいことを考慮すると, 2種の岩盤境界の間隙水圧が大きくなる可能性があり, この境界のすべりが予期されると考えられる。, Two gneisses from the Lishan landslide slope were studied experimentally, for understanding the mechanism of Lishan landslide.These two rocks are situated faced to a possible potential sliding surface, upper bedrock LGNA and lower one LGNB.LGNA has an extensive anisotropic structure of mylonitized foliation, while LGNB is isotropic.The compressive strength and the residual strength of these rocks were measured at room temperature under a confining pressure up to 40 Mpa.Both rocks have the same compressive and residual strengths and the same pressure dependencies.The compressive strength for the LGNA sample with foliation oriented at 30°to the compressive stress direction is close to the residual strength.LGNA has higher porosity thanLGNB.These results suggest that a high pore pressure could be possibly formed in the boundary between these bedrocks and the boundary could form a potential sliding surface.

Triaxial tests are widely used to determine the behavior and strength characteristics of soils without due attention to the differences in specimen size. Several drained and undrained monotonic triaxial compression shear tests are performed on three different specimen sizes of the same sand to investigate the influence of specimen size and scale effect on the shear behavior. The test results indicate that the behavior of loose sand is strongly influenced by the specimen size, with larger specimens exhibiting a stiffer behavior during isotropic compression, and mobilizing smaller shear strengths and effective friction angles. Triaxial testing also involves many sources of errors that could significantly affect shear strength parameters if not corrected. Extensive errors are investigated and it is found that negligence in making corrections accounting for these errors will result in an overestimation as much as 42% and 15 degrees in the critical shear strength and critical state friction angle, respectively. Furthermore, the measured critical state parameters and shear strengths are employed to compare the static and seismic slope stability of an earth embankment dam, calibrate a critical state soil constitutive model, study the soil behavior under shallow foundations, and evaluate liquefaction triggering and failure of retaining structures. The results show that all of these analyses are significantly affected by the strength parameters of the same soil determined from different specimen sizes. While using small size samples for determining shear strength parameters might result in un-conservative design, the choice of a large sample size is consequently a more accurate representation of soil strength conditions and field deformations.

A modified contour diagram is created for the Frederikshavn Sand in the undrained case for a relative density of ID = 80 %. It can be used to estimate the number of cycles to failure for a given combination of pore pressure, average and cyclic load ratio. The diagram is based on a series of undrained cyclic triaxial tests, which have been performed at the Geotechnical Laboratory at Aalborg University. In order to ensure offshore conditions, the tests were fully saturated and performed with a relative density of 80 %. During cyclic loading, special attention was given to the development of pore pressure and deformation. A modified contour diagram is created for the Frederikshavn Sand in the undrained case for a relative density of ID = 80 %. It can be used to estimate the number of cycles to failure for a given combination of pore pressure, average and cyclic load ratio. The diagram is based on a series of undrained cyclic triaxial tests, which have been performed at the Geotechnical Laboratory at Aalborg University. In order to ensure offshore conditions, the tests were fully saturated and performed with a relative density of 80 %. During cyclic loading, special attention was given to the development of pore pressure and deformation.

International audience; To provide a complete local monitoring of the state of an unsaturated soil sample during triaxial testing, a local water content measurement device was adapted to a triaxial device comprising the measurement of local displacements (Hall effect transducers) and suction (high-capacity transducer). Water content was locally monitored by means of a resistivity probe. The water content/resistivity calibration curves of an intact, natural, unsaturated loess from northern France extracted by block sampling at two depths (1 and 3.3 m) were carefully determined, showing good accuracy and repeatability. The validity of two models giving the resistivity of unsaturated soils with respect to their water content was examined. The first triaxial tests carried out with this device in the range of in situ stresses gave satisfactory results, but with some effects of the applied cell stress on the water content measurements. Some preliminary behaviour characteristics of the natural unsaturated loess, a material rarely tested up to now in the literature, were evidenced. Also, the yield stresses appeared significantly higher than the in situ stresses, confirming the combined effect of partial saturation and bonding.

The paper presents the results of laboratory tests carried out on undisturbed specimens of a gravelly soil sampled by in situ freezing. Because of the heterogeneity of the natural deposit, the grain size distribution of the specimens ranged from uniform sand with a very low percentage of gravel to well-graded sandy gravel, and could be divided into three different families (respectively, named A, B and C). The testing programme consisted of drained and undrained monotonic triaxial tests and undrained cyclic triaxial tests, carried out on both isotropically and anisotropically ( K 0 ) consolidated specimens. Because of the coarse gradations of the soils, a large triaxial cell was used. The monotonic and cyclic tests showed that the two finer materials (soil A and B) behave in a similar way, confirming that the fabric matrix dominates the overall mechanical behavior when coarser particles are few and floating. The results also confirmed that the coarsest material (soil C) has a lower tendency to liquefy. The tendency to undergo liquefaction or cyclic mobility during undrained cyclic tests was referred to initial soil conditions expressed through the state parameter ψ , which proved once more to be a parameter very useful to this aim.

Fibre reinforced soils have been investigated for several decades and different models have been suggested to estimate their improved shear strength. Micro and macro mechanical characteristics of both fibres and soils affect the shear strength of such composite materials (e.g. relative sizes of fibres and soil grains, fibres aspect ratio, stress state, mechanical properties of the fibres), even though no model is available to explicitly take all of them into account. The aim of this work is to establish a new expression for the shear strength of the reinforced material, able to consider the main characteristics of the soil and the fibres as well as the effect of fibre to grains relative dimensions. Data from triaxial tests carried out on fibre reinforced soils with distinct grain size distributions (from clayey sands to sandy gravels) and from previous experimental works were considered and have been analysed successfully within the proposed framework.

The triaxial test is a most widely used laboratory method for determining the soil shear strength. It is assumed that a soil sample deforms uniformly during triaxial testing. But one often faces a case when the sample in the triaxial apparatus deforms on the contrary. The non-uniformity can be caused by the end restraining effect, the sample height influence factor, the insufficient drainage, the membrane effect and the sample self-weight factor etc. An analysis of known investigations lead to the following tools that could be employed for reducing an inaccuracy related to the non-uniform stress-strain distribution per soil sample during triaxial testing: reducing the sample height/diameter ratio from 2 to 1, eliminating the friction between the sample ends and the plates. Having not eliminated the above - mentioned influence, factors during the testing procedure the angle of internal friction φ and the cohesion c for the sample of φ ≠ 0 are determined larger than the actual ones. The method for determining the angle of internal friction φ and the cohesion c, when testing the soil sample of height/diameter H/D = 1 is proposed., Lietuviška santrauka. Triašio slėgio aparatas šiuo metu yra plačiai naudojamas pasaulyje nustatant grunto kerpamojo stiprumo savybių rodiklius. Imama, kad bandinys bandymo metu deformuojasi vienodai. Tačiau dažniausiai bandinys deformuojasi nevienodai dėl grunto bandinio galų horizontaliųjų poslinkių suvaržymo, bandinio aukščio, nepakankamo drenavimo, guminės membranos poveikio, jo savojo svorio ir kt. Apžvelgtoje literatūroje triašio slėgio aparato grunto bandinyje vienodiems įtempimams gauti siūloma sumažinti bandinio aukščio ir skersmens H/D santykį nuo 2 iki 1 ir eliminuoti bandinio ir štampų trintį. Tokiu atveju gruntams, kurių vidinės trinties kampas φ ≠ 0, gaunamos didesnės vidinės trinties kampo φ ir sankibos c reikšmės. Pasiūlytas metodas apskaičiuoti grunto vidinės trinties kampo φ ir sankibos c reikšmėms, kai bandinio aukščio ir skersmens santykis H/D = 1.

2005

Tez (Yüksek Lisans) -- İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü, 2001, Thesis (M.Sc.) -- İstanbul Technical University, Institute of Science and Technology, 2001, Modern zemin mekaniğinin büyük bir kısmı laboratuarda hazırlanmış veya yoğrulmuş zeminlerin özelliklerinin incelendiği ayrıntılı çalışmaların sonuçlarının değerlendirilmesiyle gelişmiştir. 1960'lı yıllarda Cambridge Üniversitesi Zemin Grubu tarafından formüle edilen Kritik Durum Zemin Mekaniği anlayışı daha önce ayrı ayrı ele elınan bir takım temel bulguları tek bir çerçevede açıklamaktaki başarısı nedeniyle önem kazanmıştır. Böylece, bu anlayış zeminlerin davranışlarının modellenmesinde basit ve güvenilir bir teori oluşturmuş, mühendislik problemlerinin çözümünde giderek daha çok uygulanmaya başlanmıştır. Kritik Durum Teorisi, zeminin kayma direncinin gerilme geçmişinden ve drenaj koşullarından bağımsız olarak yalnız boşluk oranı ve efektif gerilmenin bir fonksiyonu olarak ifade edilebileceği varsayımına dayanır. Gerilme uzayında bu fonksiyon Kritik Durum Eğrisi verilen bir eğri ile temsil edilir. Bu teoriye uyan ideal bir zemininin davranışını tarifleyen ilk matematiksel model Cam Kili (Cam Clay) adı verilen zemin modelidir. Bu model Kritik Durum Parametreleri olarak da adlandırılan şu beş parametre ile tanımlıdır: Spesifik hacim-ortalama efektif gerilme düzleminde normal sıkışma eğrisinin eğimi; , spesifik hacim-ortalama efektif gerilme düzleminde kabarma eğrisinin eğimi; , deviatör gerilme-ortalama efektif gerilme düzleminde Kritik Durum Eğrisi'nin eğimi; , spesifik hacim-ortalama efektif gerilme düzleminde Kritik Durum Eğrisi'nin ve normal sıkışma eğrisinin yerlerini belirlemek için p'=1 kN/m2' ye karşı gelen spesifik hacim değerileri, sırasıyla, ve . Bu çalışmada Hamdi Peynircioğlu Laboratuarında belli başlı uluslararası zemin mekaniği laboratuarlarında kullanılan Speswhite Kaolin kilinden hazırlanmış suya doygun numuneler üzerinde bir seri üç eksenli basınç ve ödometre deneyleri yapılmıştır. Elde edilen deney sonuçlarından, incelenen kilin Kritik Durum Parametreleri belirlenmiştir. Belirlenen bu değerler, aynı kil ile çalışan çeşitli araştırmacıların yayınladıkları değerlerle karşılaştırılmış ve sonuçların uyum içerisinde olduğu görülmüştür. Böylece Hamdi Peynircioğlu Laboratuarında oluşturulan deney düzenin ve izlenen deneysel yöntemin uygunluğu kanıtlanmıştır., Much of modern soil mechanics has developed from the results of comprehensive studies of the properties of remoulded and reconstituted natural or artificial soils. By far the most successful conceptual framework proposed for understanding the behaviour of soils is that known under the general name of Critical State Theory. The theory was developed originally in Cambridge University based on laboratory testing of reconstituted artificial clays (i.e. Spestone kaolin). Although it has been modified and changed and some more sophisticated variations have been proposed, over the last three decades the classical theory has been widely taught and increasingly applied to the solution of engineering problems. The critical state framework, which was formulated by the Cambridge soil mechanics group under the leadership of Roscoe, derives its success from its ability of unifying the 'consolidation' and 'shearing', the 'drained' and 'undrained' behaviour of soils. In this way, the theory provides a logical and simple framework for mathematical modelling of soils. The very first mathematical model developed based on the concepts of Critical State Theory is known as Cam clay. Cam clay model requires five parameters for its full description. These are: the slope of the normal compression lines in specific volume-mean effective stress space, ; the slope of the swelling line in specific volume-mean effective stress space, ; the slope of the critical state line in deviator stress-mean effective stress space, and ; value of specific volume at p'= 1 kN/m2 on the Critical State Line and on the normal compression line, respectively. The work presented here describes the triaxial and odemeter tests conducted on reconstituted saturated samples of Speswhite Kaolin and the determination of critical state parameters from the results of these tests. The parameters obtained are compared to those published by various researchers who worked on the same soil and are found to be compatible. In this way, the accuracy of the apparatus used as well as the adequacy of the experimental procedure applied has been confirmed., Yüksek Lisans, M.Sc.

Obrađuje problem određivanja površine uzorka tijekom smicanja u troosnom posmiku pri različitim deformacijama, kod tvrdih glina, daje preporuke o korekciji površine i njenom utjecaju na određivanje parametara čvrstoće glina.