Moist Tamping Research Articles

Practical considerations for numerical simulation of large-diameter monopiles for offshore wind turbines in overconsolidated sand: A case study of the VIBRO project in Cuxhaven, Germany

Monopiles are typically the preferred foundation for offshore wind turbines, which are becoming increasingly large. While 3D finite element modelling can aid the design process of monopile foundations, achieving reliable results requires precise constitutive soil models and input parameters derived from laboratory and field data, as well as careful consideration of soil sample preparation method. This study explores various soil preparation methods, including moist tamping, dry tamping, and air pluviation, along with two sample ratios (1 and 2) during triaxial tests. It uses an advanced soil model, the DeltaSand model, and a practical industry model, the Hardening Soil model with small strain (HSsmall). To evaluate model performance, the Vibro project performed in Cuxhaven, Germany, is used as a case study. Simulations using the DeltaSand model show good agreement with field data. The effects of soil preparation methods and sample ratios are more pronounced in HSsmall model simulations, in which better results are obtained when the model is calibrated using dry tamping data and a sample ratio of one.

Adaptations to a triaxial equipment for testing of mine tailings

Mine tailings are usually materials with particle sizes in the range of silts, frequently exhibiting very low consistency. This low-consistency condition poses challenges when trying to reconstitute specimens for triaxial testing. The first part of this work presents a description of the main geotechnical properties of mine tailings used in the tests and the difficulties associated with triaxial testing. Next, some developments, in terms of equipment and testing procedure, are presented: the design of a compaction mould, the specimen preparation using moist tamping and the lubricated end platens. Finally, the process of dismantling the specimen using freezing is presented and discussed. The final part of the paper presents some results and summarizes the challenges associated with adapting the equipment and implementing the testing procedures.

Quality of reconstituted tailings samples based on their mechanical response

With a growing global population and greater environmental awareness modern societies inevitably need more mineral resources. Thus, an increase in mining activity is expected through the reactivation of old mines and/or the licensing of new explorations. Increased mining operations will lead to the production of higher volumes of tailings that often endanger people and the environment. To suitably assess the behaviour of tailings, it will be necessary to carry out a complete and exhaustive laboratory characterisation in order to determine the necessary parameters for the correct sizing of the tailings disposal structures. In this sense, for a successful laboratory work, it is necessary to obtain good quality intact or remoulded samples for testing and comparison. However, good quality intact samples of tailings are rarely available. Therefore, reconstituted samples assume an important role. Currently the most commonly used reconstitution methods for mine tailings are the moist tamping and slurry methods. This paper aims to discuss the potential of the most commonly used sample reconstitution methods in mine tailings. Some mechanical characterisation studies of tailings are also presented, namely when subjected to monotonic and cyclic stresses, with the aid of triaxial and oedometer equipment. Complementary tests with unusual equipment, such as Hollow Cylinder Apparatus (HCA) and bender elements, are also presented. Finally, and in view of the research carried out, the main limitations in laboratory tests carried out on these materials are identified.

Effect of initial fabric from sample preparation on the mechanical behaviour of a carbonate sand from the South China Sea

Mega reclamation projects have been undertaken on the coral reefs. However, research on the geotechnical properties of the hydraulically filled carbonate sand is limited, and the impact of the initial fabrics from different densification methods on the mechanical behaviour of carbonate sand remains unclear. In this work, a systematic experimental investigation on the effect of initial fabric induced by sample preparation method on the mechanical behaviour of carbonate sand has been performed. Four sample preparation methods, including moist tamping, vibration, dry pluviation and water pluviation were adopted to reconstitute samples with distinct fabrics. It is found that, for a single preparation method, the particle breakage as well as the peak friction angle of carbonate sand is affected by the drainage condition significantly, and both the critical state friction angle (φ'cs) and the compressibility (λ) of carbonate sand are strongly stress dependent. For samples prepared by different methods, an obvious divergence of critical state points can be seen in the specific volume and mean effective stress plane at low stress levels. The dry pluviation samples show the lowest φ'cs and highest λ, due to the highest anisotropy. In contrast, moist tamping samples give the highest φ'cs and lowest λ, due to the most isotropic fabric.

Experimental Investigation on the Post-liquefaction Behavior of Sands in Simple Shear Conditions

Abstract Experimental evidence shows that earthquake induced liquefaction can occur more than once in sandy soils. Moreover, despite an increase in soil density caused by the dissipation of the excess pore pressure induced by earthquakes, the liquefaction resistance of soils that have experienced liquefaction may be lower than that of virgin soils. This paper offers insight into this topic starting from the analysis of the undrained monotonic behavior of post-liquefied sands by means of tests performed with a simple shear cell equipped with flexible boundaries, which maintains a constant diameter to guarantee the “K0-condition.” The control system of cyclic, reconsolidation, and monotonic phases is described in detail. The experimental results show that neither the relative density, effective confining stress, cyclic stress ratio, nor the direction of shear strain play important roles in the monotonic behavior of post-liquefied soils. Moreover, the comparison between the monotonic response of virgin and post-liquefied soils (prepared by moist tamping technique) shows that it is not affected by the stress–strain history experienced by soils. It can be explained through a microstructural interpretation. According to which, the initial soil fabric generated with the moist tamping method and that formed during liquefaction remain almost unchanged because of the rotation of principal stress directions occurring during simple shear tests. A further confirmation is given by the results of tests performed on specimens prepared by air pluviation method.

Effects of seepage flow on liquefaction resistance of uniform sand and gap-graded soil under undrained cyclic torsional shear

Internal erosion is the transportation of soil particles from within or beneath geotechnical structures, caused by seepage flow, that impacts the subsequent mechanical and hydraulic behaviour of the soil. However, it is difficult to predict the liquefaction resistance of eroded soil due to several factors related to the soil fabric. The present study investigates the impact of seepage flow on the undrained cyclic behaviour of two types of soil: uniform sand and gap-graded soil with a fines content of 20%, using a novel erosion hollow cylindrical torsion shear apparatus. From the results for the uniform sand, the soil fabric formed by moist tamping (MT) leads to higher liquefaction resistance than that formed by air-pluviation (AP). However, after applying seepage flow, the liquefaction resistance of the eroded MT specimens becomes even lower than that of the non-eroded AP specimen. Therefore, the liquefaction resistance of soil is expected to decrease due to the rearrangement of the initially stable coarse particles during seepage flow. On the other hand, the liquefaction resistance of the gap-graded soil tends to increase after the removal of fines as the number of stable contacts between the coarse particles is increased. Under these test conditions, the latter effect is found to be greater for the given gradation, leading to a slight increase in the liquefaction resistance of the tested gap-graded soil after internal erosion. Furthermore, the intergranular void ratio and small-strain shear modulus are seen to be well correlated with the liquefaction resistance of the tested soil.

A Critical Assessment of the Effect of Initial Fabric on Key Small-Strain Design Parameters of Slurry-Deposited Silts and Sands

Whereas moist-tamped specimens of silts and sands are most used in engineering practice to characterize tailings, offshore sediments, and fluvial/alluvial deposits, design parameters derived from moist-tamping data sets can be significantly different from those obtained from slurry or underwater deposition. This study shows that moist-tamped silty and sandy specimens may exhibit phase transformation at stress ratios that are 25% to 50% lower than those observed for slurry-deposited specimens. Conversely, the small-strain stiffness of the moist-tamped specimens tested can be 50% higher than those from slurry deposition. With tailings dams’ performance receiving increased worldwide attention due to recent dam failures in several parts of the world, this study provides new, specific, and concerning insights about the crucial impact that the selection of moist tamping can have on design parameters. More realistic and rigorous laboratory testing procedures involving tailings remain a key requirement for engineering assessments of tailings behavior. A novel slurry-deposition setup is presented that allows underwater reconstitution of silts, sands, and their mixtures, yielding high-quality uniform specimens. Systematic uniformity checks, which are mandatory to avoid segregation of silty materials, are described. A detailed analysis of typical errors affecting initial void ratio evaluation is also presented to ensure that comparisons between different methods are done with the highest degree of confidence possible.

Integrated effects of inherent and induced anisotropy on reliquefaction resistance of Toyoura sand with different strain histories

Recent earthquakes in New Zealand and Japan showed that pre-shaking histories significantly affected the reliquefaction resistance of soils. In this study, a series of experimental tests was conducted to elucidate the coupled effects of inherent and induced anisotropy on reliquefaction resistance of Toyoura sand, which have not been studied before. Accordingly, loose and dense Toyoura sands were prepared with two different methods: dry deposition and moist tamping. The specimens were sheared cyclically using a hollow cylinder torsional shear apparatus under various cyclic stress ratios up to different residual shear strains (γres) and reconsolidated at different states. The experimental results were assessed from various perspectives, including stress–strain relationships, failure mechanisms, liquefaction/reliquefaction resistance, excess pore water pressure (EPWP) generation and compressibility in conjunction with micromechanical interpretations. It was shown that fabric evolution affects the reliquefaction characteristics of Toyoura sand substantially. Interestingly, a unique correlation exists between EPWP and shear strain accumulation for all tests. An energy-based model was developed to uniquely correlate the dissipated energy with the cyclic resistance based only on residual shear strains; this model shows great promise to develop a unified, energy-based criterion for quantifying the liquefaction/reliquefaction resistance of soils with different fabrics.

Evaluation of structural formation of granular materials using anisotropy of magnetic susceptibility

The structure of granular materials is one of the most important influential factors in mechanical behaviors. A few inevitable limitations exist in current optical measuring techniques, such as structure disturbance, operational complexity, and high cost. This study attempts to take a new application of the anisotropy of magnetic susceptibility (AMS) technique to investigate the structure of granular materials. To illustrate the feasibility of the AMS in detecting the structure, each of five different kinds of sand, including beach sand, river sand, aeolian sand, and calcareous sand, was reconstituted by moist tamping method and air pluviation method, respectively. Based on the structural features obtained from the optical and the AMS techniques, the validity and applicability of the AMS technique on granular materials with different magnetic mineral compositions were discussed systematically. It is found that the AMS technique is a feasible and efficient method to investigate the structure of granular materials both in laboratory tests and field sampling tests.

Mechanics of an Iron Ore Tailings Exhibiting Transitional Behaviour

Laboratory critical state line (CSL) testing used in engineering practice to analyse the stability and liquefaction susceptibility of tailings explicitly assumes a unique CSL for a particular tailings. However, there is evidence of non-unique CSLs in some soils, a behaviour commonly referred to as “transitional behaviour”. There is currently limited consensus or understanding of its applicability, what factors contribute to its occurrence, and whether there is a fundamental divergence from critical state theory or an artefact of the strain limitations of laboratory element tests. In this research, normally consolidated samples of an iron ore tailings prepared using the moist tamping and slurry deposition methods were sheared to critical state in triaxial compression tests. The results of these tests showed that a different CSL was apparent for the two specimen preparation methods used. This further emphasises the need to re-evaluate the effects of fabric on the uniqueness of the CSL, which may be a consequence of different sample preparation methods typically used to prepare laboratory element tests.

Liquefaction Characteristics of Sandy Soil Distributed in Wind Power Farms, Soc Trang Province, Vietnam

Wind power has been widely utilized and has become a safe and renewable energy source in many countries. In Vietnam, many wind power farms have been built in coastal provinces such as Soc Trang, Bac Lieu, Ninh Thuan, Binh Thuan, Ca Mau, Vung Tau, and the Central Highlands. For onshore wind power farms, the dynamic loads generated from wind and the operation of the turbine will act on soil ground and affect the long-term strength and liquefaction potential of soil, especially for sandy soil. Thus, it is necessary to investigate the potential for liquefaction of the sandy soil layer in wind power farms. In this study, the liquefaction potential of sand distributed in onshore wind power farms in Soc Trang province, Vietnam was evaluated using the undrained cyclic triaxial test. The sand specimens were remolded using dry pluviation and moist tamping methods to simulate the field density. The analysis of test results showed that the very fine sand with a loose state distributed at the depth of less than or equal to 11.5 m could be liquefied under the testing conditions. While the fine sand specimens with a dense to very dense state distributed deeper than 30 m were not liquefied under testing conditions. However, for these specimens, the pore water ratio could be increased by 11.3 to 18.6% and this increase can lead to a decrease in soil strength.

Influence of plastic fines content on the liquefaction susceptibility of sands: cyclic loading

The paper presents an experimental study on the effect of plastic fines content on the undrained behavior and liquefaction susceptibility of sand-fines mixtures under cyclic loading. The results of undrained cyclic triaxial tests conducted on mixtures of Hostun sand with varying amounts (0–20%) and types (kaolin and calcigel bentonite) of plastic fines are presented. The specimens were prepared with different initial densities using the moist tamping method, consolidated at the same isotropic effective stress of 100 kPa and subjected to different deviatoric stress amplitudes. From the experimental observations, it was found that sand-clay mixtures with 10% or 20% clay content showed a lower cyclic liquefaction resistance than pure sand. Furthermore, the reduction in the cyclic stress ratio resulting in liquefaction after twenty cycles was found larger for sand-kaolin mixtures than for the sand-calcigel ones. Possible explanations are provided.

Small-strain shear stiffness anisotropy of a saturated clayey loess

The mechanical behaviour of clayey loess is strongly affected by the soil structure, but although anisotropy has been identified in loess by some, the anisotropy of small-strain behaviour is rarely reported. This paper presents an experimental study on the inherent and stress-induced small-strain shear stiffness anisotropy of a clayey loess from China. Both undisturbed and reconstituted specimens were tested with bender elements under isotropic compression and shearing conditions. Under an isotropic stress state, an inherent anisotropy was found for undisturbed specimens, while the reconstituted specimens prepared by moist tamping behaved isotropically. During shearing, the ratio of horizontal to vertical shear moduli of the undisturbed specimens decreased due to both an increase of stress anisotropy and the destruction of the intact structure. In contrast, the stiffness ratio of the reconstituted specimens only decreased due to stress anisotropy and it became more anisotropic at the critical state. This study reveals the influence of intact structure and inherent anisotropy on the behaviour of loess soils, which cannot be reproduced by compaction, thus highlighting the importance of characterising the undisturbed loess.

Influence of specimen nonuniformity and end restraint conditions on drained triaxial compression test results in sand

Drained triaxial compression tests on sand are common in geotechnical engineering due to their application in determining strength and critical state properties, and calibration of constitutive models. However, questions around reliability of test results due to strain localization and specimen nonuniformity have sparked debate around the adequacy of various specimen preparation techniques and configuration of top cap and bottom pedestal, among other variables. A rigorously calibrated and validated three-dimensional finite element model using an advanced constitutive model, NorSand, was used to address some of these concerns. The results generally confirmed that common specimen preparation techniques, moist tamping, and (air or water) pluviation, if done properly, can reasonably estimate the properties of a perfectly uniform specimen with the same density. Unsurprisingly, it was also confirmed that the post-peak response of dense specimens is by far the most sensitive part of sand behaviour characterized by these tests. It was suggested that for two-to-one height-to-diameter ratio specimens, end platen lubrication does not necessarily improve test results and can cause concentration of localized deformations near the platens. A fixed cap appeared to enforce a better boundary condition than a free-to-rotate one by preventing cap rotation driven by the formation of a single failure plane.

Shear strength of fibre reinforced cemented Toyoura sand

<abstract> <p>A series of consolidated drained and undrained tests are conducted on unreinforced, fibre, cement, and fibre reinforced cemented Toyoura sand specimens with varying relative densities. Three different types of materials e.g., Toyoura sand, polyvinyl alcohol (PVA) fibres, and ordinary Portland cement (OPC) are employed in this study. Specimens in dimensions of 50 mm in diameter and height of 100 mm are prepared in a polyvinyl chloride (PVC) mold to a target dry density value, $ {\rho }_{d} $ = 1.40 g/cm<sup>3</sup> (Dr = 20%) and $ {\rho }_{d} $ = 1.489 g/cm<sup>3</sup> (Dr = 60%) of Toyoura sand using under-compaction moist tamping technique. Fibre reinforced cemented Toyoura sand samples were prepared with 10% moisture content by weight of sand-fibre-cement mixtures. The results on density variation shows that due to a better contact between sand-fibre interaction or sand-cement-fibre bonding and interaction for the denser specimens, a greater increase in shear strength is observed. However, the general effectiveness of fibre and cement additives alone and when mixed together also enhances the strength of unreinforced specimens for loose conditions based on the variation of fibre and cement contents. The results and findings in the current study can be used for the construction of economical and sustainable geotechnical infrastructures.</p> </abstract>

Consolidated drained behaviour of PVA fibre reinforced cemented Toyoura Sand

ABSTRACT In this study, a series of consolidated drained (CID) compression tests were performed On unreinforced and reinforced cemented Toyoura sand specimens. Specimens were cured for 3, 7, 14, 28, 56 days, with cement contents of 0–3% (dry weight), and different Polyvinyl alcohol (PVA) fibre contents of 0, 0.5, 1, 2, and 3% (weight ratio). Samples were prepared to a target dry density value (e.g. = 1.489 g/cm3) of Toyoura sand using under-compaction moist tamping technique with 10% water content by dry weight of soil. Triaxial compression tests (CID) were carried out at a confining pressure of 100 kPa. The monotonic stress–strain, stiffness, volumetric change behaviour, dilatancy angle, peak strength, and residual strength and brittleness index of the samples were investigated in greater detail. Results show that peak strength increases due to increase in curing duration by approximately 50–700%, and 0–300% in residual strength at 30% axial strain, compared to Toyoura sand. For reinforced cemented specimens there is an increase in volumetric strain with increasing curing duration or in other words, the rate of dilation increases with the increase in curing period. The dilatancy behaviour increases due to the addition of fibres and cement, this dilative behaviour has been attributed to cemented particles forming highly interlocked clusters. Brittleness of the cemented specimens reinforced with 0.5%-1% fibres have been supressed significantly compared to the higher concentrations of fibre (2–3%). Hence, it shows that there might be a threshold fibre concentration range (0.5%-1%), where fibres are considered to be the most effective and beyond this concentration, fibre causes further adverse effects.

The initial fabric of undisturbed and reconstituted fluvial sand

High-quality undisturbed samples of fluvial sand were obtained from the field using the ground freezing technique. In the laboratory, the in situ void ratio of these high-quality undisturbed frozen samples was replicated using four different reconstitution methods: dry deposition, moist tamping, water sedimentation (by spooning) and slurry deposition. The initial fabric of the specimens was evaluated using X-ray micro-computed tomography and advanced image analysis. Initial fabric features were assessed in terms of the particle orientation, anisotropy, void ratio distribution and particle sizes within the specimens. Fabric analysis results suggest that none of the laboratory reconstitution techniques used captures the true three-dimensional initial fabric of undisturbed fluvial sand. However, the slurry deposition method managed to reproduce the inherent particle orientation, anisotropy and the variations of void ratio and particle sizes of the undisturbed fluvial sand. This observation explains why previous rigorous studies on the macro-mechanical behaviour of sands deposited under water have systematically demonstrated that slurry deposition is the most suitable method to reconstitute in the laboratory natural sands deposited under water. This has major implications for geotechnical testing and analyses of liquefaction of sands deposited under water such as fluvial, offshore and tailings sands.

Consolidated drained (CID) behavior of fibre reinforced cemented Toyoura sand in triaxial loading conditions

The effects of fibre (0–3 %) and cement (0–3 %) additives, on Toyoura sand were examined under consolidated drained compression and extension loading conditions. All samples were prepared to a target dry density value (e.g., {rho }_{d}= 1.489 g/cm3) of Toyoura sand using under-compaction moist tamping technique. In compression, the unreinforced specimens exhibited a behavior of medium dense sand and reached a peak deviator stress (qp) at approximately 4 % axial strain ({epsilon }_{a}) for the varying mean effective stresses, pʹ (i.e. 50–400 kPa). The peak drained strength increases in fibre reinforced cemented specimens were found to be up to 132 % (lower effective stresses) and 243 % (higher effective stresses), while, the drained strength increases at critical state for the fibre reinforced cemented specimens were found to be up to 105 % (lower effective stresses) and 245 % (higher effective stresses). Overall, the fibre and cement additives increased the stiffness, peak and strength at critical state of pure Toyoura sand but were found to be least effective in extension loading. Moreover, the stress ratio, peak and critical state stress ratios increase with the addition of fibres and cement. The secant modulus shows limited increases for the fibre reinforced specimens. However, a significant improvement in the secant modulus is observed for the fibre reinforced cemented specimens. For both unreinforced and reinforced specimens there is a decrease in volumetric strain with greater effective stresses or in other words, the rate of dilation decreases with increases in effective stresses. The fibre and cement additives also increased the strength parameters (frictional angle, cohesion), dilatancy angle, slope of the critical state line, and decreased the state parameter of pure Toyoura sand.

Small-strain shear modulus of calcareous sand under anisotropic consolidation

In this study, the small-strain shear modulus of a calcareous sand was investigated by conducting bender element tests on both horizontal and vertical planes. The effects of sample preparation method, stress path and stress history on the developing of void ratio, the parameters in the modified Hardin Equation, and the stiffness anisotropy were examined. The test results show that the moist tamping samples have the least void ratio variation among the five samples. The void ratio recovery in effective horizontal stress [Formula: see text] = 100 kPa tests is higher than that in the effective vertical stress [Formula: see text] = 100 kPa tests. The samples prepared in the dry state have lower stiffness than those prepared in the moisture state, which is not influenced by the anisotropic stress state. The stiffness anisotropy induced by the sample preparation method is significant under anisotropic consolidation. In [Formula: see text] = 100 kPa tests, the stiffness ratios at the end of the unloading stage are lower than the initial values at the loading stage, which is not found in the [Formula: see text] = 100 kPa tests, meaning that the stress history and stress path could affect the stiffness anisotropy and eliminate the impact of fabric anisotropy.

Influences of initial anisotropy and principal stress rotation on the undrained monotonic behavior of a loose silica sand

Triaxial compression and extension tests have been conducted under different initial anisotropy conditions to investigate the undrained response of a crushed silica sand. The loose to medium specimens were prepared using the moist tamping method. Five stress paths with different stress ratios (q/p′) were employed to prepare anisotropically consolidated specimens. Several specimens were consolidated under a specific condition in which a stress rotation occurred under undrained monotonic shearing similar to a reversed cyclic shear stress loading during an earthquake. The effects of initial induced anisotropy at consolidation on the onset of liquefaction, phase transformation, and critical state are investigated within the framework of Anisotropic Critical State Soil Mechanics (ACSSM). In addition, fabric evolution during shearing towards the critical state is evaluated using bidirectional bender element tests. The results illustrate the fact that there is a unique anisotropic critical state representing anisotropic fabric, irrespective of initial anisotropy, and the states of stress. Similar to the critical state line, the phase transformation line has the same loci for different initial anisotropies.