Undrained residual strength, s(ur), often termed remolded or postcyclic strength, is a critical input into embankment dam numerical deformation analyses. There are multiple methods available to assess s(ur) for fine-grained soils, each with advantages and disadvantages. Field tests, such as the vane shear test and the cone penetration test, can provide reliable in situ measurements of s(ur). In the laboratory, s(ur) can be estimated by measuring the shear stress mobilized at high strains in monotonic tests such as direct simple shear or triaxial shear. s(ur) is also frequently determined from postcyclic monotonic testing; however, the postcyclic stress-strain curves can be difficult to interpret because of high excess pore water pressure existing at the start of monotonic shear due to the sample being previously subjected to cyclic loading. Such analyses often have a significant amount of uncertainty. The work described here presents two new methods developed to quantify s(ur) through lab testing, namely, analysis of stress paths from postcyclic monotonic tests and iterative strain-controlled cyclic loading. This paper introduces the new approaches and presents results from testing performed on five fine-grained soils from the foundations of embankment dams. Values of s(ur) from the new approaches are compared with those from VST and monotonic and postcyclic monotonic direct simple shear testing. The paper details the new approaches and presents results and conclusions from five fine-grained soils from various sites across the western United States.

Soil-pile interaction damping plays a crucial role in reducing wind turbine loads and fatigue damage in monopile foundations, thus aiding in the optimized design of offshore wind structures and lowering construction and installation costs. Investigating the damping properties at the element level is essential for studying monopole-soil damping. Given the widespread distribution of silty clay in China's seas, it is vital to conduct targeted studies on its damping characteristics. The damping ratio across the entire strain range is measured using a combination of resonant column and cyclic simple shear tests, with the results compared to predictions from widely used empirical models. The results indicate that the damping ratio-strain curve for silty clay remains S-shaped, with similar properties observed between overconsolidated and normally consolidated silty clay. While empirical models accurately predict the damping ratio at low strain levels, they tend to overestimate it at medium-to-high strain levels. This discrepancy should be considered when using empirical models in the absence of experimental data for engineering applications. The results in this study are significant for offshore wind earthquake engineering and structural optimization.

In order to estimate accumulated excess pore pressures in the soil around a cyclically loaded (offshore) foundation structure, cyclic laboratory tests are required. In practice, the cyclic direct simple shear (DSS) test is often used. From numerous undrained tests (or alternatively tests under constant-volume condition) under varying stress conditions, contour diagrams can be derived, which characterize the soil's behavior under arbitrary cyclic loading conditions. Such contour diagrams can then be used as input for finite element models predicting the load-bearing behavior of foundation structures under undrained or partially drained cyclic loading. The paper deals with the general behavior of a poorly graded medium sand in cyclic DSS tests under undrained loading conditions. The main objective of the research was to investigate and parametrize the soil's behavior and to identify possible effects of sample preparation. Numerous tests with varying cyclic stress ratios (CSR) and mean stress ratios (MSR) have been conducted. Also the relative density of the sand was varied. A new set of equations for a relatively easy handable mathematical description of the resulting contour plots was developed and parametrized. In the original tests, the sand was poured into the testing frame and carefully compacted to the desired relative density by tamping. In offshore practice, a preconditioning of a soil sample is usually realised by cyclic preshearing with a certain CSR-value or additionally by preconsolidation under drained conditions. By that, a more realistic initial state of the soil shall be achieved. In order to investigate the effect of such a preconditioning on the resulting contour diagrams, additional tests were conducted in which preshearing and preconsolidation was applied and the results were compared to the test results without any preconditioning. The results clearly show a significant effect of preshearing and an even more pronounced effect of preconsolidation for the considered poorly graded medium sand.

The direct simple shear (DSS) test is one of the most popular testing techniques for measuring the shear strength of soils and mine waste tailings. However, uncertainties remain regarding the suitable sample diameter and whether a DSS sample should be saturated or can be tested without flushing with water. Various designs and configurations of shearing caps are also incorporated in different DSS equipment with little information on their performance and comparison soil shearing behavior with different caps. This study examines the monotonic shearing behavior, static liquefaction and instability, and post-liquefaction strength of a coarse oil tailings sand in extensive series of monotonic DSS tests on two different specimen diameters of 50 mm and 70 mm. Moist-tamped samples are reconstituted with and without flushing with water and sheared using top and bottom caps with concentric wedges and projecting pins. These are examined across a wide range of consolidation vertical stress, and for three different stress paths corresponding to undrained (CV), drained (CVS), and constant-shear unloading (CSU) shearing paths. Static liquefaction and instability were triggered in the CV and the CSU tests at the emergences of undrained strength reduction and volumetric collapse, respectively. The results show little effects of sample flushing and diameter on the static liquefaction triggering and post-liquefaction shear strengths of the tailings sand. The effect of sample diameter was primarily observed on the one-dimensional compressibility and volumetric strain of samples. The smaller diameter specimens underwent smaller volume changes during one-dimensional compression and drained shearing compared with the larger D = 70 specimens.

The direct simple shear (DSS) test serves as a vital method in geotechnics, allowing the measurement of peak and post-liquefaction shear strengths, along with the critical state friction angle of soils. Additionally, the simple shearing mode applied in a DSS test is the predominant failure mode in many geotechnical engineering problems. Although the DSS test is widely used to determine soil strength, a significant challenge with the DSS device is the non-uniformity of stress and strain distributions at the specimen boundaries. This non-uniformity depends on not only the specimen size but also the size of soil particles. The influence of specimen size on boundary effects is typically evaluated using the ratio of specimen diameter (D) to height (H). The median particle diameter (D50), as an indicator of a soil's particle size, could be another influential factor affecting the non-uniformities of stress and strain on specimen boundaries in a DSS test. Through three-dimensional discrete element method (DEM) simulations, this research explores these factors. Specimens were generated with a particle size distribution (PSD) scaled from a coarse sand sample. Laboratory monotonic DSS testing results on the coarse sand were employed to calibrate the DEM model and ascertain the modeling parameters. Boundary displacements were regulated to maintain a constant-volume condition which represents undrained shearing behavior. Various specimen diameters were simulated with identical void ratios to investigate the influence of D/H on stress path, peak and post-peak shear strengths, and critical state behavior. DEM simulations allowed the generation of several particle size distributions through different scaling factors applied to the sand gradation to determine the combined effect D50 and D/H. Limiting D/H and D50/D ratios are subsequently proposed to mitigate specimen boundary effects.

Studying the shear rheological properties of clay is crucial for evaluating slope stability and preventing excessive displacement of roadbeds and retaining walls. In this study, a series of direct simple shear tests were conducted by a novel apparatus to investigate the shear rheological behavior of clay in western China. Test results reveal that both the shear strain-time curve and shear stress-strain curve can be well described by power functions, and the power of shear strain-time curve is independent of the shear stress level. Based on this finding, an empirical shear rheological equation under constant shear stress is built. By assuming the shear stress-strain curves as a series of parallel lines in a double logarithmic coordinate axis, shear equivalent timelines are proposed based on Yin Graham's equivalent timeline theory. The shear equivalent time is then introduced into the proposed empirical shear rheological equation, thereby an equivalent timeline shear rheological model considering the effect of consolidation pressure under varying shear stresses is derived. The shear rheological strains predicted by the model are shown to agree well with test data before clay failure.

Energy dissipation can macroscopically synthesize the evolutions in the microstructure of the marine clay during cyclic loading. Hence an energy-based method was employed to investigate the failure criterion and cyclic resistance of marine clay. A series of constant-volume cyclic direct simple shear tests was conducted on undisturbed saturated marine clay from the Yangtze Estuary considering the effects of the plasticity index (IP) and cyclic stress ratio (CSR). The results indicated that a threshold CSR (CSRth) exhibiting a power function relationship with IP exists in marine clay, which divides the cyclic response into non-failure and failure states. For failed specimens, the development of energy dissipation per cycle (Wi) with the number of cycles (N) exhibited an inflection point owing to the onset of serious damage to the soil structure. In this regard, the energy-based failure criterion was proposed by considering the inflection point as the failure point. Consequently, a model was proposed to quantify the relationships between failure energy dissipation per cycle (Wf) [or failure accumulative energy dissipation (Waf)], initial vertical effective stress, IP, and the number of cycles to failure (Nf,E). An evaluation model capturing the correlation among CSR, IP, and Nf,E was then established to predict the cyclic resistance, and its applicability was verified. Compared with the strain-based cyclic failure criterion, the energybased failure criterion provides a more robust and rational approach. Finally, a failure double-amplitude shear strain (gamma DA,f) evaluation method applicable to marine clay in different seas was presented for use in practical geotechnical engineering.

The amount of energy dissipated in the soil during cyclic loading controls the amount of pore pressure generated under that loading. Because of this, the normalized dissipated energy per unit volume is the basis for both pore pressure generation models and energy-based liquefaction analyses. The pattern of energy dissipation in the soil in load-controlled cyclic triaxial and load-controlled cyclic direct simple shear tests and displacement-controlled cyclic triaxial and displacement-controlled cyclic direct simple shear tests is quite different. As a result, the pattern of pore pressure generation associated with load-controlled tests is markedly different from that in displacement-controlled tests. Pore pressure generation patterns for each of the four test types were proposed based upon the manner in which the load was applied during the test and the soil's response to that loading. The results of four tests, two load controlled and two displacement controlled, were then used to verify these patterns. Pore pressure generation rates in load-controlled and displacement-controlled tests are different when plotted against their cycle ratios. Conversely, the tests produce nearly identical patterns when plotted against energy dissipation ratio. This occurs because of the relationship between energy dissipation ratio and pore pressure generation is independent of the loading pattern.

The dynamic stability of offshore foundations in the marine environment is one of the major technical challenges. Millions of loading cycles from waves and winds act on the foundations and seabed in their entire service life. The mixing of adjacent soil layers due to naturally occurring sediment or the construction process raises more difficulties in the design. Previous studies have focused on the dynamic constitutive relations of idealized clean sand or clay under cyclic loading, while the cyclic characteristics of sand-clay mixtures are still unclear. In this study, the dynamic responses of sand-clay mixtures under a large number of shear cycles were investigated through 25 constant-volume cyclic direct simple shear tests on sand-clay mixtures with sand contents of 0 %, 25 %, 50 %, and 75 %. The results indicate that high sand contents and high cyclic stress ratios accelerate the reduction of effective stress in sand-clay mixtures, resulting in a more rapid increase in shear strain. As the sand content increases from 0 % to 75 %, the normalized shear modulus decreases by 35.9 % at a shear strain of 1 %. In contrast, the normalized dynamic shear strength is reduced from between 0.490 and 0.525 to between 0.181 and 0.325 after over 5000 loading cycles.

Based on the modified simple direct shear device which can directly measure the interface pore pressure and interface shear displacement, a series of interface shear tests and corresponding pure clay shear tests were conducted at an undrained state in constant normal load (CNL) boundary conditions or equivalent undrained state in constant volume (CV) boundary conditions. The clay-structure interfaces, consisting of seabed clay and Speswhite kaolin clay with overconsolidation ratios (OCR) of 1 and 3, were tested at three shear rates, respec-tively V1 = 0.0002 mm/s, V2 = 0.001 mm/s, and V3 = 0.01 mm/s. The results demonstrated that the shear strength of the clay-structure interface is lower than that of pure clay, and this difference is more pronounced under CV boundary conditions. In CNL condition, though the pure clay strength decreases with increasing shear rate at OCR = 1 and increases with increasing shear rate at OCR = 3, the shear rate effect on clay-structure interface strength is not obvious. In CV condition, the strength of the interface with the normally consolidated (NC) and over consolidated (OC) clay increases approximately linearly with the shear rate on the semi -logarithmic scale. the shear rate parameter p is used to describe the growth rate of pure clay or clay-structure interface shear strength with a tenfold increase in shear rate. As for normally consolidated clay, in CV condi-tion, the corresponding shear rate parameter satisfies that p (with R1 roughness)> p (pure clay)> p (with R2 roughness). The rate parameter corresponding to NC seabed clay is significantly higher than the rate parameter corresponding to NC Speswhite kaolin clay. For OC clay, the shear rate parameter for interface strength is higher than that for pure clay, meeting with the relationship that p (with R1 roughness)> p (with R2 roughness)> p (pure clay).