Search

Recently, the topological flat bands and spin Hall effect have been experimentally observed in the AB-stacked MoTe$_2$/WSe$_2$ heterostructures. In this work, we systematically study the Berry curvature effects in moir\'{e} transition metal dichalcogenide (TMD) heterobilayers. We point out that the moir\'{e} potential of the remote conduction bands would induce a sizable periodic pseudo-magnetic field (PMF) on the valence band. This periodic PMF creates net Berry curvature flux in each valley of the moir\'{e} Brillouin zone. The combination of the effect of the Berry curvature and the spin-valley locking can induce the spin Hall effect being observed in the experiment. Interestingly, the valley-contrasting Berry curvature distribution generated by the PMF can be probed through shift currents, which are DC currents induced by linearly polarized lights through nonlinear responses. Our work shed lights on the novel quantum phenomena induced by Berry curvatures in moir\'e TMD heterobilayers., Comment: 10 pages, 7 figures

Recent studies have shown that moir\'{e} flat bands in a twisted bilayer graphene(TBG) can acquire nontrivial Berry curvatures when aligned with hexagonal boron nitride substrate [1, 2], which can be manifested as a correlated Chern insulator near the 3/4 filling [3, 4]. In this work, we show that the large Berry curvatures in the moir\'{e} bands lead to strong nonlinear Hall(NLH) effect in a strained TBG with general filling factors. Under a weak uniaxial strain $\sim 0.1\%$, the Berry curvature dipole which characterizes the nonlinear Hall response can be as large as $\sim$ 200{\AA}, exceeding the values of all previously known nonlinear Hall materials [5-14] by two orders of magnitude. The dependence of the giant NLH effect as a function of electric gating, strain and twist angle is further investigated systematically. Importantly, we point out that the giant NLH effect appears generically for twist angle near the magic angle due to the strong susceptibility of nearly flat moir\'{e} bands to symmetry breaking induced by strains. Our results establish TBG as a practical platform for tunable NLH effect and novel transport phenomena driven by nontrivial Berry phases., Comment: 5 pages, 3 figures. Comments are welcome

In recent years, it has been shown that Berry curvature monopoles and dipoles play essential roles in the anomalous Hall effect and the nonlinear Hall effect respectively. In this work, we demonstrate that Berry curvature multipoles (the higher moments of Berry curvatures at the Fermi energy) can induce higher-order nonlinear anomalous Hall (NLAH) effect. Specifically, an AC Hall voltage perpendicular to the current direction emerges, where the frequency is an integer multiple of the frequency of the applied current. Importantly, by analyzing the symmetry properties of all the 3D and 2D magnetic point groups, we note that the quadrupole, hexapole and even higher Berry curvature moments can cause the leading-order frequency multiplication in certain materials. To provide concrete examples, we point out that the third-order NLAH voltage can be the leading-order Hall response in certain antiferromagnets due to Berry curvature quadrupoles, and the fourth-order NLAH voltage can be the leading response in the surface states of topological insulators induced by Berry curvature hexapoles. Our results are established by symmetry analysis, effective Hamiltonian and first-principles calculations. Other materials which support the higher-order NLAH effect are further proposed, including 2D antiferromagnets and ferromagnets, Weyl semimetals and twisted bilayer graphene near the quantum anomalous Hall phase., 17 pages, 5 figures

Recently, it has been pointed out that the twisting of bilayer WSe$_2$ would generate topologically non-trivial flat bands near the Fermi energy. In this work, we show that twisted bilayer WSe$_2$ (tWSe$_2$) with uniaxial strain exhibits a large nonlinear Hall (NLH) response due to the non-trivial Berry curvatures of the flat bands. Moreover, the NLH effect is greatly enhanced near the topological phase transition point which can be tuned by a vertical displacement field. Importantly, the nonlinear Hall signal changes sign across the topological phase transition point and provides a way to identify the topological phase transition and probe the topological properties of the flat bands. The strong enhancement and high tunability of the NLH effect near the topological phase transition point renders tWSe$_2$ and related moire materials new platforms for rectification and second harmonic generations., 6 pages, 3 figures, plus supplementary. Comments are welcome

In solids with an odd number of electrons per unit cell, band theory requires that they are metals, but strong interaction can turn them into insulators, called Mott insulators. In this case, the electrons form local moments that, in turn, form an antiferromagnetic ground state. In 1973, P. W. Anderson proposed that, in certain cases, quantum fluctuations may prevent magnetic order and result in a paramagnetic ground state called a quantum spin liquid. After many years of searching, a few examples have been discovered in the past several years. We point out that a well-studied material, TaS2, may be a spin liquid candidate. We propose further experiments that probe the exotic properties of this new state of matter.

The significance of studying soil behaviour in a constant shear stress path to understand rain-induced slope failures and debris flows has long been recognized. Studies with constant shear tests have, however, been limited, and some past results from undisturbed soils appear to show stress path–dependent volume change behaviour. The present study systematically investigates the behaviour of recompacted residual soils in a constant shear stress path using a comprehensive experimental program. It is shown that the results of this test program and previously published data can be interpreted using the concepts of critical-state soil mechanics.

Results of field monitoring of moisture content and matric suction at the crest and berm of a large cut slope in completely decomposed granite in Hong Kong are reported. Field moisture content and matric suction clearly show that the volumetric water content increases as a result of rainfall infiltration and decreases as a result of evaporation. Correspondingly, the matric suction decreases due to rainfall infiltration and increases due to evaporation. Soil–water characteristic curves at different depths at the crest and near the berm of the cut slope are determined from the field data. Unlike soil–water characteristic curves determined in the laboratory by testing small specimens, the field measured soil–water characteristic curves show negligible hysteresis. Engineering implications of the field soil–water characteristic curves are discussed.

A full-scale field experiment has been conducted in an instrumented saprolite slope in Hong Kong. Soil moisture probes, tensiometers, piezometers, inclinometers, earth pressure cells, and a rain gauge were installed in the slope before and during excavation of a cut. This paper presents the results from soil moisture probes, tensiometers, and piezometers to reveal the surface infiltration process. The soil moisture and matric suction results show that the maximum "wetting front" during the wet season was limited to the top 3 m of soil, and a transient perched water table could develop in the soil during a very heavy rainstorm. A new method based on the variation of volumetric water content in the soil is proposed to analyze rainfall infiltration. The results indicate the following phenomena: (i) rainfall infiltration was around 70% of the total rainfall, (ii) the infiltrated water mainly increased the soil moisture content of the soil at shallow depths, and (iii) the surface vertical infiltration had a very limited effect on the permanent groundwater table at depth. The wetting-front analysis shows that the unsaturated wetting band theory can be used to assess the movement of the wetting front in the unsaturated saprolite soil.Key words: rainfall infiltration, moisture content, matric suction, wetting front, field instrumentation.

On 13 July 2003, a landslide with a volume of approximately 20 × 106 m3 occurred on the left bank of the Qinggan River, Zigui County, Hubei Province, China. As a result, 14 people died and 10 people are missing. A landslide dam was formed, blocking the Qinggan River. A channelized diversion was constructed for prevention of upstream flooding and damage downstream caused by dam breaching. The landslide was a typical translational rockslide in weathered shale and sandstone, a block of which slipped down along the bedding plane. It is estimated that the landslide was caused by the combined effect of the following factors: (i) a bedding plane between incompetent weathered shale and competent sandstone that daylights at or is shallowly buried at the bottom of the Qinggan River; (ii) a thin layer of clayey soil acting as the slip plane that is likely a preexisting shear surface; (iii) excavation of the shale as raw material for brick fabricating on the slope; and (iv) a combination of prolonged rainfall and reservoir impounding.Key words: landslide, landslide dam, rainfall, impounding, Three Gorges area.

Slope failures in fill slopes formed by loosely compacted, completely decomposed granite in Hong Kong occur commonly during intense tropical rainstorms. The stress path greatly influences the shear strength of the soil mass, and is therefore crucial to the identification of slope-failure mechanisms. The soil mass in this case is largely unsaturated. In situ hydrologic response to rainstorms indicates that soil suction is reduced by rainfall infiltration, which often becomes the triggering factor in initiating slope instability. The constant dead-load tests on unsaturated, loosely compacted, completely decomposed granite appropriately simulate the field stress path of rainfall-induced fill-slope failure by reducing suction. The tests indicate that matric suction contributes to the dilative or contractive behavior of the unsaturated soils. The anisotropically consolidated undrained triaxial tests demonstrate the consistently contractive behavior of the specimens. On this basis, we delineate the in situ stress conditions leading to the initiation of rainfall-induced fill-slope failure, and the stress paths of the transformation from local failures to flowage. Based on a systematic study of fill-slope case records in Hong Kong, implications of such mechanisms on fill-slope stability are given.

The technique of soil nailing is seldom used in stabilizing loose fill slopes because there is a lack of understanding of the interaction behaviour of nails in loose fills. A large-scale laboratory apparatus has been built to study the soil–nail interaction in loose fill materials. Pullout tests were performed in a displacement-rate-controlled manner on steel bars embedded in loose, completely decomposed granitic soils. The load–displacement curves have distinct peak values followed by a sharp decrease in the pullout force. The test results also show that the normal stress acting on the nail changes because of the volume-change tendency and arching effect of the soil being sheared around the nail. The post-peak decrease in the pullout force is mainly due to the reduction in the normal stress caused by the arching effect of soil around the nail. The conventional method of analysis tends to give a low interface friction angle and high interface adhesion. The correct interface parameters can be determined by taking the changes in the normal stress acting on the nail into account.Key words: arching effect, interface friction angle, laboratory test, loose fill, pullout resistance, soil–nail interaction.

The characterization of weathered rocks by the use of drilling parameters is presented. Drilling parameters obtained from the drilling process monitoring system during the installation of soil nails into weathered rocks are used in this investigation. A new drilling index Pi, simplified from the concept of the specific energy, is used in an attempt to differentiate weathered rocks. With Pi or the penetration rate, boundaries of weathered rocks between (1) soil and rock ranges, and (2) grade II and grade III rocks in accordance with the six-fold rock material decomposition grade are identified. For weathered rocks in the soil range, quantitative characterization has been established in this study by using the drilling parameters. An empirical equation has been established for estimating the standard penetration resistance N value from the drilling parameter Pi, which is used in turn to determine the Mohr–Coulomb friction angle based on the work of Schmertmann. Although this approach tends to yield a slightly larger Mohr–Coulomb friction angle than that from laboratory tests, results obtained in this attempt reveal that estimation of the Mohr–Coulomb friction angle from drilling parameters is attainable with reasonable accuracy.Key words : drilling, standard penetration, percussion index, penetration rate.

The Terzaghi superposition assumption has been widely used to determine the bearing capacity of shallow footings. Although this assumption always errs on the safe side, a rigorous procedure to calculate the bearing capacity is still of engineering value. This paper presents such a procedure that is free from errors as a result of the superposition assumption. It demonstrates that the ultimate bearing capacity can be precisely expressed by the Terzaghi equation, except that the bearing capacity factor Nγis dependent upon the surcharge ratio. A recently developed numerical method, i.e., the critical slip field method, is used to calculate the modification coefficient for modifying Nγ. It is found that this modification coefficient increases with the surcharge ratio at small values of surcharge ratio and then remains constant for large values of surcharge ratio. However, the errors invoked by the superposition assumption do not exceed 10%. On the basis of numerical calculations, a simple closed-form expression of the modification coefficient is proposed that yields the theoretically rigorous ultimate bearing capacity. In the later part of the paper, errors in bearing capacity calculations owing to the use of conventional procedures are analyzed. It is concluded that the continued use of conventional procedures is justified, but the inherent errors should not be neglected in assessing the performance of shallow foundations.Key words: shallow foundation, strip footing, ultimate bearing capacity, critical slip field.

We propose to study the lattice-symmetry protection of Majorana zero bound modes in topological crystalline superconductors (SCs). With an induced $s$-wave superconductivity in the $(001)$-surface of the topological crystalline insulator Pb$_{1-x}$Sn$_x$Te, which has a C$_4$ rotational symmetry, we show a new class of 2D topological SC with four Majorana modes obtained in each vortex core, while only two of them are protected by the cyclic symmetry. Furthermore, applying an in-plane external field can break the four-fold symmetry and lifts the Majorana modes to finite energy states in general. Surprisingly, we show that even the C$_4$ symmetry is broken, two Majorana modes are restored exactly one time whenever the in-plane field varies $\pi/2$, i.e. $1/4$-cycle in the direction. This novel phenomenon has a profound connection to the four-fold cyclic symmetry of the original crystalline SC and uniquely demonstrates the lattice-symmetry protection of the Majorana modes. We further generalize these results to the system with generic C$_{2N}$ symmetry, and show that the symmetry class of the topological crystalline SC can be demonstrated by the $2N$ times of restoration of two Majorana modes when the external symmetry-breaking field varies one cycle in direction., Comment: 4+pages, 2 figures. Figures and discussions have been updated

A laboratory study has been conducted to investigate the dynamic behaviour of Champlain Sea clay from two locations in the Ottawa River valley region. The test program includes cyclic triaxial tests and resonant-column tests. The soil samples were consolidated at a range of pressures before the dynamic shear in order to cover the dynamic behaviour under both the overconsolidated and the normally consolidated states. An energy concept is introduced to interpret the test results. Mathematical relationships have been established for describing the various aspects of dynamic behaviour. These aspects include excess pore pressure, dynamic strength, dynamic shear modulus, and plastic strain. The study shows that the energy concept provides a promising way to analyze dynamic soil behaviour. Key words : energy, dynamic behaviour, clay, cyclic test, earthquake, excess pore pressure, shear modulus, strength.

The earthquake that hit the San Francisco area on October 17, 1989, is reviewed with respect to damage to buildings, transportation facilities, and services. The San Francisco experience underlines that soil conditions and inadequate structural integrity are the two most important factors in the seismic risk to a building and its inhabitants. This earthquake is used as a model for the damage prediction in the Greater Vancouver area from a "design earthquake" that is implied in the National Building Code of Canada. In comparable housing density the expected damage would be somewhat greater than that observed in the San Francisco region in October 1989 because of differences in amplitude of ground motions and building design standards. This study is seen as a first step in the detailed assessment of damage potentials for the Vancouver region, or other similar metropolitan areas. Potential shortcomings in the 1985 National Building Code of Canada were identified in the seismic requirements for non-engineered buildings (Part 9) concerning lateral bracing, beam splice ties over supports, and anchorage and reinforcing of chimneys. Key words: earthquake damage, building code, damage prediction.

An energy method for assessing liquefaction potential of granular soils was developed based on laboratory tests and observational data obtained in past major earthquakes. Cyclic triaxial and cyclic simple shear tests were conducted and the results show that a unique relation exists between the dissipated energy during cyclic load and the excess pore pressure that eventually led to liquefaction failure. This unique relation has been combined with an energy attenuation equation to develop a criterion for defining the liquefaction potential of a site. Parameters for the criterion were evaluated from 136 sites involved in 13 major earthquakes over the world. A comparison was made between the energy method and the commonly used stress method. Then energy method was found to be simpler to apply and more reliable., Reprint from: Canadian Geotechnical Journal, v. 27, no. 3, 1990, p. 320-329.

Piezocone penetration tests were carried out at five sites in marine clay with overconsolidation ratios ranging from 1·3 to 4·8 A simple model for the behaviour of these clays indicates that the effective vertical yield stress mobilized along the cone axis during penetration is equal to the difference between the actual induced total vertical stress in the soil and the total porewater pressure. The actual induced total vertical stress can he derived from piezocone test data. Theoretically the ratio of the vertical yield stress inferred from piezocone results and the preconsolidation pressure should correlate with the overconsolidation ratio. This hypothesis is supported by the field and laboratory investigations presented in this study. Des essais de pénétration avec un piezocone ont été effecté sur cinq sites d'argile marine ayant des rapports de surconsolidation variant de 1,3 à 4,8. Un modèle simple de comportement de ces argiles indique que la contrainte effective verticale à l'état limite mobilisée durant la pénétration est égale à la différence entre la contrainte totale verticale et la pression interstitielle générées dans le sol. La contrainte totale verticale peut être déterminée à partir des réultats des essais de piezocone. En théorie, le rapport de la contrainte verticale à l'état limite, calculée à partir des résultats du piezocone, et de la pression de surconsolidation devrait être une fonction du rapport du surconsolidation. Cette hypothése est vérifiée par les essais in situ et de laboratoire présentés dans cet article.

This paper reports the observation and analysis of the rate and magnitude of settlement under the test embankment at Canadian Forces Station Gloucester. The embankment has been in existence for 7 years and, because of extensive instrumentation, a sufficiently complete record has emerged. An advanced finite element method has been used for the computation of the induced stresses in the foundation soil. Both block samples and 5 in. (12.7 cm) diameter Osterberg samples were recovered at various depths from the site. An experimental program, including the use of the 6 in. (15.2 cm) Rowe cell, has been carried out and an analysis of the test results based on the Gibson and Lo theory. The test results have also been used in the estimation of the field performance. From the present study it is found that: the coefficient of consolidation and the primary and secondary compressibility can be adequately determined from samples of size 4.5 in. (11.3 cm) diameter by 2 in. (5.1 cm) high or larger; the secondary compression contributes significantly to the total settlement for the soil considered; and the Gibson and Lo theory predicts fairly accurately both the time rate and magnitude of settlement in the field., Reprinted from: Canadian Geotechnical Journal, v. 13, no. 4, p. 339-354

With the advent of piezocones — penetrometers measuring both the mechanical resistance and induced pore pressures near the tip during penetration into the soil — a new interpretation of penetrometer test data is possible. This paper presents a review of available interpretation methods for obtaining the undrained shear strength of soft soils, and introduces a new interpretation taking into account measured pore pressures. The undrained shear strength is considered to be solely related to the ultimate cavity expansion pressure, which is one of the components of the tip resistance. The other component is calculated assuming that effective friction is developed at the cone–soil interface.Parametric studies on the parameters required for strength determination based on the proposed method are also presented. Special self-boring pressuremeter tests to obtain relevant values of soil rigidity index, which is a key parameter for cavity expansion modelling, are described. Tests were conducted at three sites having the characteristics of soft sensitive clay, stiff sensitive clay, and clayey silt. The operational undrained strength mobilized during the cone insertion derived from the proposed model is equal to or lower than the CK0U triaxial strength, which depends on soil brittleness. The proposed approach yields results consistent with known soil behaviour at all three sites. Key words: piezocone penetrometer, pressuremeter, excess pore-water pressure, undrained shear strength, rigidity index, in situ tests, laboratory test.

This Paper proposes a system of defining the major and minor principal stress increments in the Skempton's (1954) pore pressure equation. Using this system, a unified trend emerges for use in analysing published data that are seemingly conflicting. The system is also shown to give a consistent interpretation, both theoretically and experimentally, to triaxial stress states involving principal stress rotation. The practical implications resulting from the study of this system are examined. Cette étude propose un système pour la détermination des augmentations de contraintes principales majeure et mineure, dans l'équation de pression interstitielle de Skempton (1954). En utilisant ce système, une tentative d'unification en découle pour être utilisée dans l'analyse des données publiées qui sont apparement contradictoires. Le système paraît aussi donner une interprétation satisfaisante théoriquement aussi bien qu'expérimentalement, aux états de contraintes triaxiales impliquant la rotation de la contrainte principale. Les implications pratiques résultant de l'étude de ce système, sont examinées.

The Cambridge self-boring pressuremeter and the Norwegian Geotechnical Institute field vane apparatus have been used at three sites of different subsoils. Two of the sites, South Gloucester and National Research Council of Canada grounds, are located in Leda clay in the Ottawa area. The third site features a highly plastic lacustrine clay in northwestern Quebec. At this site tests were conducted beside and under a fill that had undergone 1 m of surface settlement. At all three sites, the pressuremeter yields higher strengths than the field vane apparatus.Conventional and special laboratory tests were conducted on large diameter tube or block samples from South Gloucester. It is found that the conventional triaxial compressive strength is intermediate between the pressuremeter and field vane strengths. In accounting for the strength discrepancy, factors such as anisotropy, stress path, and disturbance have been studied in the light of the results of the special tests.

A laboratory testing program for predicting undrained excess pore pressures beneath embankments constructed on sensitive clays is reported. A study of the pore pressure response, based on Henkel's equation applied to triaxial and plane strain tests performed on marine clay from South Gloucester, Ontario, provides a basis for comparing behaviour under conditions of axisymmetry and plane strain. Comparison shows that (1) pore pressure is smaller in plane strain than in axisymmetric conditions; (2) a below-elastic response prevails at the prepeak stage of the stress–strain curve; and (3) there is an elastic response at the peak.Finite element and modified elastic methods are developed for estimating undrained excess pore-water pressures from laboratory test results. The estimated values are used to evaluate the effect of consolidation under the center of an embankment during construction. The procedure is illustrated with case records.

This paper examines the influence of the cutting shoe size of a pressuremeter apparatus in the light of soil behaviour after some unloading and loading. An oversized cutting shoe creates a gap between the borehole and the pressuremeter probe, causing a stress release with a probable consequence of overestimating the shear strength of the soil. An undersized cutting shoe imposes a certain load to the surrounding soil prior to the pressuremeter test, thus introducing errors in measuring the stress–strain relationship of the soil.To gain a quantitative idea of such an influence, Cambridge self-boring pressuremeter tests were carried out on Leda clay at two sites in the Ottawa region. It was found that in the case of an oversized cutting shoe, both the modulus and shear strength were overestimated by about 30 and 80% respectively. In the case of an undersized cutting shoe, the shear resistance was reduced at small strains. In both cases, however, the stress–strain relationship beyond a moderate strain (5%) showed little dependence on the cutting shoe size.