Key geomechanical properties of the historically liquefiable TP-Lisbon sand

The intrinsic properties of sands have a strong influence on the behaviour of these soils. Therefore, particle shape and granular packing can provide relevant insights into the mechanical properties of granular geomaterials. This study presents the characterisation of the key mechanical geomechanical properties controlling the behaviour of an alluvial fine sand, which composes the liquefiable layer of the geological profile at ‘Praça do Comercio’ square, located in the downtown of Lisbon (Portugal). For this purpose, a comprehensive experimental plan was conducted in the laboratory using accurate testing procedures. The experimental plan addressed: (i) the evaluation of the particle shape from a large number of particles using a computational geometry algorithm and statistical procedures; (ii) the estimation of the minimum and maximum void ratio through two standard methods; (iii) the assessment of the stress–strain behaviour by triaxial tests using lubricated end platens and an embedded connection piston, and void ratio measurements using end-of-test soil freezing; and (iv) the stress-dependency assessment of seismic wave velocities using bender elements. Results are interpreted within the critical state soil mechanics framework, highlighting the effects of the particle shape and granular package on the behaviour of this natural sand. In addition, critical state and small-strain parameters are compared and thoroughly discussed against selected data of other sands. Results provide experimental evidence about the influence of the particle shape and granular packing on the key geomechanical properties of TP-Lisbon sand.     

Strength and dilatancy behaviors of deep sands in Shanghai with a focus on grain size and shape effect

With rapid development of infrastructures like tunnels and open excavations in Shanghai, investigations on deeper soils have become critically important. Most of the existing laboratory works were focused on the clayey strata up to Layer 6 in Shanghai, i.e. at depth of up to 40 m. In this paper, Layers 7, 9, and 11, which were mostly formed of sandy soils at depth of up to 150 m, were experimentally investigated with respect to physico-mechanical behaviors. The stress–strain behaviors were analyzed by the consolidated drained/undrained (CD/CU) triaxial tests under monotonic loading. One-dimensional (1D) oedometer tests were performed to investigate the consolidation properties of the sandy soils. Specimens were prepared at three different relative densities for each layer. Also, the micro-images and particle size analyzers were used to analyze the shape and size of the sand grains. The influences of grain size, density, and angularity on the stress–strain behaviors and compressibility were also studied. Compared to the other layers, Layer 11 had the smallest mean grain size (D₅₀), highest compressibility, and lowest shear strength. In contrast, Layer 9 had the largest mean grain size, lowest compressibility, and highest shear strength. Layer 7 was of intermediate mean grain size, exhibiting more compressibility and less shear strength than that of Layer 9. Also, the critical state parameters and maximum dilatancy rate of different layers were discussed.     

Role of particle characteristics in the compression behaviour of gap-graded sands

The compression in gap-graded mixtures of sands with combined mineralogy has been investigated in recent research, focusing on the key factors that might imply the occurrence of convergent or non-convergent paths in compression (i.e., transitional or non-transitional behaviour). From previous work, the mineralogy of a matrix composed of larger grains seems to determine the possibility of the occurrence of transitional behaviour. Hence, if there is a strong and stiff matrix made of quartz sand particles, which are either larger than or at least of similar size to the other component, then non-convergent compression paths (i.e., transitional behaviour) are likely to occur. As a further confirmation of this hypothesis, this technical note presents the results of oedometer tests on the same range of mixtures of a quartz sand and a carbonate sand as was used in previously published work, but with the larger component being of the stiffer and stronger quartz sand. In agreement with the hypothesis, transitional behaviour occurred.     

考虑可逆与不可逆剪胀的粗粒土动本构模型

在边界面塑性理论的框架内,建立了一个新的粗粒土动本构模型。在模型中,剪胀体应变由一个可逆的体应变分量和一个不可逆的体应变分量构成。前者取决于当前剪应力的大小和方向,后者主要取决于剪切作用的历史,它们分别服从不同的剪胀规律。建立了描述可逆剪胀与不可逆剪胀体应变的数学表达式,探讨了可逆和不可逆剪胀体应变分量对应力应变关系的影响,并通过数值模拟与试验结果的对比对模型进行了初步验证。     

Stability of solid waste sea fills: shear strength determination and sensitivity analysis

Little is known about the geotechnical properties of waste materials, either in their initial state or after various levels of decomposition. Such information is critical when assessing the stability of waste fills during placement, and at the time of closure and rehabilitation. Recent efforts to address these particular concerns remain in developmental stages, and practical problems persist with regards to in‐situ or laboratory scale testing. In addition, the complexity of the problems is increased due to the variability in composition, placement conditions and levels of decay of the various constituents of the waste fills. This paper describes laboratory experiments that were conducted to establish relevant shear strength parameters for municipal solid waste. The shear strength testing was accomplished by using a large direct shear apparatus specifically designed and developed for the purpose. Results from the laboratory‐testing program are then applied in conducting geotechnical stability analyses at a major sea fill along the Mediterranean shore. A parametric sensitivity analysis affecting stability is also presented.     

Laboratory evaluation of governing mechanism of frictionally connected MSEW face and implications on design

This paper presents a laboratory evaluation of purely frictionally connected geotextile and concrete facing block of Mechanically Stabilized Earth Wall (MSEW) systems. The study focuses on investigating the governing failure mechanism along the wall face, as determined from the pullout of reinforcement in between the facing blocks (herein referred as pullout mechanism) and sliding of the blocks over the geotextile, where the reinforcement stays stationery (herein referred as direct shear mechanism). A total of seventy-two tests were performed to investigate the effect of laboratory specimen size, difference in geotextile reinforcement, and repeatability of the test results. Overall, the results showed that at lower normal loads, sliding of the blocks over the geotextile reinforcement along the wall face is more likely to occur before the pullout of the geotextile in between the blocks. At higher normal loads, this order is reversed and pullout of the geotextile appears to occur first. The test results also indicated that the size of the specimen tested in the laboratory frictional connection evaluation has an effect on the measured connection strength.     

Aging effects on liquefaction resistance of sand estimated from laboratory investigation

Liquefaction resistance is known to increase concomitantly with the increase in time after construction or sedimentation. Nevertheless, the mechanisms of its aging effect on liquefaction have not been completely elucidated. To clarify the mechanisms of aging in sandy soils, the liquefaction resistance (CRR), initial and secant shear moduli (G₀ and G_sec), and laboratory penetration resistance of long-term consolidated sand specimens were examined using cyclic undrained triaxial tests, local small strain (LSS) tests equipped with bender elements (BEs), and penetration index tests, respectively. Based on the existing reports, the CRR was inferred from G₀, G_sec, and the laboratory penetration resistance. In the case of Toyoura sand of D_r = 40%, the CRR increased by about 14% with a 360-day consolidated specimen in the cyclic undrained triaxial tests. However, increases in the CRR evaluated from G₀ and the laboratory penetration resistance were nothing and only 2%, respectively. G_sec started to degrade at greater shear strain in the long-term consolidated specimens. An increase in the CRR, evaluated from the G_sec of 0.01% shear strain, had a much better agreement with that obtained from the cyclic undrained triaxial tests.     

Failure of a soft cohesive soil subjected to combined static and cyclic loading

This paper describes the type of failure that a soft cohesive soil can exhibit when acted upon by combined static and cyclic loading. The conclusions are based on the results of a comprehensive experimental research in which, in addition to identification and classification testing, 15 monotonic simple shear tests and 138 cyclic simple shear tests were carried out in which, prior to the cyclic shear stresses, different levels of monotonic shear stresses were applied. Laboratory tests were performed on undisturbed samples taken from the southern area of the port of Barcelona, Spain. In general, the results thus obtained indicate that the undrained shear strength for a given number of cycles is clearly affected by the initial shear stress, as it is explained in this paper.     

On Control Tests of Piles under Static Loads and Interpretation of Their Results

The need for field control tests of piles during construction and reconstruction as an addition to effective regulatory documents on static pile testing is substantiated. Proposals are cited for the refinement and correspondence of positions of GOST 5686-94 and Construction Rule and Regulation 2.02.03-85 as applies to control tests of piles.     

水泥改良土力学检测指标的对比分析

通过对几种力学检测指标试验方法的比较,提出了静态变形模量的检测结果较K30和Evd具有明显优点。对试验段几种力学检测指标的试验结果进行了对比分析,提出了合适的检测时间,并提出了降低无侧限抗压强度检测频率的建议,对水泥改良土的现场施工具有指导意义。     

直接剪切试验方法及应用分析

抗剪强度是土的重要力学性质指标之一,它为工程勘察设计提供地基评价和计算承载力的重要参数。直接剪切试验是测定土抗剪强度最常用的方法,在简要阐述土的直接剪切试验方法的基础上,结合多年的实际操作经验,对不同的直剪方法及不同类型试样的特点进行了分析对比,并总结出值得注意的一些操作要点。     

Case study of a driven pile foundation in diatomaceous soil. II: Pile installation,dynamic analysis,and pore pressure generation

This paper presents the results from a case study highlighting the difficulties of pile driving in diatomaceous soils.In the companion(first)paper to this article,results of an extensive laboratory and in situ testing program were presented while the results from pile driving and further analysis of field observations were presented herein.Unexpected high pile rebound(HPR)was observed during driving of a closed-end pipe pile,with refusal occurring at a depth of less than 5 m.Subsequent open-ended piles were thus driven.Piezometer and case pile wave analysis program(CAPWAP)data were collected during driving of both closed-and open-end piles.Piezometer data indicated that negative pore water pressures(PWPs)were generated while the closed-ended pile exhibited high rebound.Results from in situ tests indicated change in material stiffness and strong dilative tendencies near the depth of refusal.A hypothesis for observed behavior was proposed that considers the soil beneath the pile as a medium with an effectively infinite bulk modulus.     

Experimental studies of the shear behaviour and strength of lipped channel beams with web openings

Cold-formed steel members are increasingly used as primary structural elements in the building industries around the world due to the availability of thin and high strength steels and advanced cold-forming technologies. Cold-formed lipped channel beams (LCB) are commonly used as flexural members such as floor joists and bearers. However, their shear capacities are determined based on conservative design rules. Current practice in flooring systems is to include openings in the web element of floor joists or bearers so that building services can be located within them. Shear behaviour of LCBs with web openings is more complicated while their shear strengths are considerably reduced by the presence of web openings. However, limited research has been undertaken on the shear behaviour and strength of LCBs with web openings. Hence a detailed experimental study involving 40 shear tests was undertaken to investigate the shear behaviour and strength of LCBs with web openings. Simply supported test specimens of LCBs with aspect ratios of 1.0 and 1.5 were loaded at mid-span until failure. This paper presents the details of this experimental study and the results of their shear capacities and behavioural characteristics. Experimental results showed that the current design rules in cold-formed steel structures design codes are very conservative for the shear design of LCBs with web openings. Improved design equations have been proposed for the shear strength of LCBs with web openings based on the experimental results from this study.     

Disk shaped piezo-ceramic transducer for P and S wave measurement in a laboratory soil specimen

Piezo-ceramic elements are customized as transducers for the measurement of mechanical properties of materials in the engineering field. This was made possible with the industrial production of piezo-ceramic elements in various shapes and sizes. This paper describes the development of a single flat disk shaped piezo-ceramic transducer for measuring both compression and shear wave on an identical soil specimen. Procedures for interpreting results, initial verifications of its performance and applications are also presented. Two types of piezo-ceramic elements, one for measuring P waves and the other for S waves, were placed together in a metal housing, which worked as a wave measuring transducer installed in a triaxial apparatus. Three kinds of granular geo-materials, fine, medium-coarse and coarse sands, were tested. Small strain shear stiffness, G_max, of the tested sands was evaluated by various techniques, including proposed disk transducer method, trigger and accelerometer method and traxial small strain cyclic loading. Shear moduli obtained from all the techniques fell in a similar range within allowable scatters and it was confirmed that the disk transducer was one of applicable wave measurement technique for laboratory soil specimens.     

Investigation of unsaturated frozen soil behavior: Phase transformation state,post-peak strength,and dilatancy

In this study, a database of triaxial compressive tests on unsaturated frozen soils is compiled to investigate the mechanical behavior that has not been considered in previous studies. The results for the stress-strain volume changes are presented first. Then, the physical mechanisms that might control the deformation of unsaturated frozen soils, namely, volumetric compression and frictional sliding, are used to interpret the changes in volume and deviator stress during the tests. The relationship between the compression rate due to the mean stress and the dilation rate due to shearing determines the changes in sample volume and shear stress with an increasing axial strain. The test results indicate that confining pressure and temperature significantly affect the phase transformation state, the post-peak strength, and the maximum dilation ratio. A higher post-peak strength ratio is observed in tests performed at higher confining pressure or under a higher temperature. As the confining pressure is increased, the shear stress at the phase transformation state initially increases and then decreases or stabilizes. However, the maximum dilation ratio decreases considerably and tends to reach zero after a certain confining pressure is reached. Both the deviator stress at the phase transformation state and the maximum dilation ratio are higher under a lower temperature given constant confining pressure. Ice cementation and pressure melting are attributed to the specific features of frozen soils compared to those of unfrozen soils. This paper provides new insights into the mechanical behavior of frozen soils.     

Design method for geotextile tubes considering strain - Formulation and verification by laboratory tests using photogrammetry

Geotextile tubes are usually made of at least one layer of high-strength woven geotextiles. However, if only highly elastic materials are used, experience shows -comprehensibly - that the designed geometry and stresses, derived from standard formulations or special computer software, both using small strain membrane theory, essentially differ from reality. Therefore, an extended formulation, considering large strains, was developed within the scope of a study on the structural and hydraulic design of geosynthetic dewatering tubes at the University of Rostock. The solution was evaluated using a sensitivity analysis and verified in laboratory experiments, where small-scale tubes were surveyed using a low-budget, high-precision photogrammetric measurement technique. The new design method proved to produce good results. It should be used if the construction is indeed a long tube and if the limit strain of the geotextile tubes exceeds 10% strain. Finally, the results were compared with the existing methods for the structural design of geosynthetic applications, adding to the discussion about attenuation factors in geotextile tube dewatering projects. For short-term applications without refilling of the partly dewatered tubes, the product of attenuation and safety factors will usually not exceed a value of two.     

Linking inherent anisotropy with liquefaction phenomena of granular materials by means of DEM analysis

The liquefaction phenomena of sands have been studied by many researchers to date. Laboratory element tests have revealed key factors that govern liquefaction phenomena, such as relative density, particle size distribution, and grain shape. However, challenges remain in quantifying inherent anisotropy and in evaluating its impact on liquefaction phenomena. This contribution explores the effect of inherent anisotropy on the mechanical response of granular materials using the discrete element method. Samples composed of spherical particles are prepared which have approximately the same void ratio and mean coordination number (CN), but varying degrees of inherent anisotropy in terms of contact normals. Their mechanical responses are compared under drained and undrained triaxial monotonic loading as well as under undrained cyclic loading. The simulation results reveal that cyclic instability followed by liquefaction can be observed for loose samples having a large degree of inherent anisotropy. Since a sample having initial anisotropy tends to deform more in its weaker direction, leading to lower liquefaction resistance, a sample having an isotropic fabric potentially exhibits the greatest liquefaction resistance. Moreover, the effective stress path during undrained cyclic loading is found to follow the instability and failure lines observed for static liquefaction under undrained monotonic loading. From a micromechanical perspective, the recovery of effective stress during liquefaction can be observed when a threshold CN develops along with the evolving induced anisotropy. Realising that the conventional index of the anisotropic degree (a) is not effective when the CN drops to almost zero during cyclic liquefaction, this contribution proposes an alternative index, effective anisotropy (a×CN), with which the evolution of induced anisotropy can be tracked effectively, and common upper and lower bounds can be defined for both undrained monotonic and cyclic loading tests.     

Study on the shear strength of soil–rock mixture by large scale direct shear test

Soil–rock mixtures (S–RM) which formed in the quaternary period are a type of extremely inhomogeneous and loose geomaterial with a certain percentage of rock blocks. They are composed of rock blocks with various sizes and high strength, fine grained soil and pores. The meso-failure mechanism and macro-physical and mechanical characteristics of S–RM are largely controlled by its rock block proportion and the granular distribution. As we know, when the rock blocks in the S–RM are larger, it is difficult to take an in-situ sample for an on-site test. In addition, it is difficult to obtain the granular distribution of rock blocks in S–RM by traditional sieving tests. This paper uses a new method called digital image processing (DIP) in which the rock blocks in S–RM samples are separated from the soil matrix, and the proportion and distribution of the rock blocks is obtained quantitatively. The results are used for the sample preparation of the large scale direct shear tests which provide a new method for the test study of S–RM. According to the results of large scale direct shear tests the rock block size proportion controls the deformation and fracture mechanism of the S–RM. The shape of the shear stress vs horizontal displacement curve and the vertical displacement vs horizontal displacement curve of the S–RM samples are different from that of general “soil” and “rock”. With the increment of the rock block proportion the shear band of the S–RM increases. When the rock block proportion lies in the range of 25–70%, the increment of the internal friction angle linearly increases with the increment of the rock block proportion. The cohesion of the S–RM decreases compared with that of the soil. When the rock block proportion is larger than 30%, however, there is only a little decrease in the cohesion with the increment of the rock block proportion.     

Clay soil settlement: In-situ experimentation and analytical approach

On the basis of the existing relationship between soil structure and water content, and in order to avoid the multiple factors influencing the study of soil behavior in the laboratory, a full-scale in-situ testing was performed in Ouarzazate (Morocco) to quantify the soil vertical displacements according to the environmental conditions.The study presented in this paper is devoted to analyzing the load-settlement relationship of active clay soil during the drying process. A time-dependent model is presented to quantify the soil settlement amplitude according to the hydraulic and mechanical states of the soil mass.     

Shear properties of cemented rockfills

Application of cemented rockfilling to underground mining could not be separated from the corresponding backfill's shear strength properties. The shear of cemented rockfill (CRF)-rock wall and the shear interaction occurring within CRFs both have some disadvantageous failure chances. In this study, we tried to investigate the complete shear properties of CRFs using direct shear and triaxial tests of cemented granite rockfill. Large-scale triaxial testing was held to accommodate the large CRF sample. Direct shear testing on the prepared flat and smooth surfaces was assessed with brief conversions and their corrections were used to approximate the shear strength envelopes of CRF joint interfaces. Two types of CRFs with the same aggregate size and distribution but different unconfined compressive strengths (UCSs) due to different mixture designs indicated insignificant differences between their basic friction angles, and also their asperity inclination angles. Nevertheless, investigation between direct shear test and triaxial test showed that the specimen with higher UCS tended to have a slightly lower friction angle but a higher cohesion than the other one.