Shearing response of geotextile-reinforced calcareous soils using monotonic triaxial tests

Abstract

Different types of geosynthetic reinforcements have been widely used for improving shear strength of soil materials in recent decades. In this study, two different calcareous soils reinforced by geotextile layers were evaluated by monotonic compression triaxial tests. The calcareous soils were obtained from Hormuz Island and Boushehr Port located in the north coast of the Persian Gulf. Different reconstituted samples were prepared at dense conditions with and without geotextile layers and consolidated under different effective confining pressures ranging from 100 to 600?kPa. Different layers of reinforcements including one, two and three geotextile layers were placed in the reinforced samples and sheared under drained condition. The results show that confining pressure has a significant effect on the efficiency of reinforcements. Evidently, increasing the reinforcement layers decreased the particle breakage of the calcareous soils used in this study.     

Undrained Monotonic Response and Instability of Medium-Dense Sandy Soil

A series of undrained monotonic triaxial compression tests were performed on natural, medium-dense (relative density (RD) = 50%) Chlef sand containing 0.5% of non-plastic fines, under different confining pressures of 50 kPa, 100 kPa, and 200 kPa. This article focuses on distinctive states of the monotonic undrained response of sands, namely the critical state, the phase transformation state, the quasi-steady state, and the state of undrained instability (onset of flow liquefaction). Specimens were prepared using dry funnel pluviation and wet deposition to investigate the effect of the initial sand fabric on these states. The present data suggest that the initial fabric of the sand appears to have a significant effect on the undrained behavior in the early stages of shearing, with its influence vanishing at large strains. Wet deposition specimens demonstrate considerably larger undrained instability state strength than their dry funnel pluviation counterparts, and a unique critical state locus is reached by both dry funnel pluviation and wet deposition.     

低围压水平下砂的排水行为(英文)

采用干样沉降法制备样品 ,选用静三轴仪对松砂和密砂样在 10 k Pa至 10 0 k Pa的低围压下的排水行为进行了实验研究 ,并对实验结果作了橡皮膜校正 ,在围压为 10 k Pa的密砂实验中 ,轴向应力的校正值可达到 12 %。实验研究表明低围压下松砂的剪胀性非常明显 ,且存在剪应力极值 ;围压越低 ,松砂和密砂的剪应力极值越小 ,达到最大剪应力所对应的剪应变越小。同时还表明低围压范围内相对密度不同 ,砂的应力应变特性不同 ;此外 ,低围压下砂的内摩擦角高于高围压下的值 ,从而在工程上应重视低围压水平下砂土工程参数的合理选取     

Experimental study on the unconfined compressive strength of carbon fiber reinforced clay soil

To study the effect and mechanism of carbon fiber reinforced clay, a series of unconfined compression tests for clay reinforced with carbon fiber have been performed under the conditions of controlled water content and dry density. The carbon fiber is mixed into soil with the quality percentage of 0.01, 0.02, 0.03, 0.05, 0.1, 0.15, 0.25, 0.35, and 0.5%, then a certain quality of water was added in the soil to achieve the optimum soil water content. Ten groups of samples were tested by the unconfined compression experiment. The results showed that the incorporation of carbon fiber elements can effectively improve the unconfined compressive strength and brittle failure mode of soil. The soil is strengthened at the beginning and then weakened with the increased incorporation of carbon fiber, the effect is especially significant when the mix percentage becomes 0.1%. The interaction at the interface between carbon fiber surface and soil matrix is analyzed by using scanning electron microscopy (SEM). It is found that the enhancement mechanisms of carbon fiber reinforced soil are one-dimensional reinforcement of a single carbon fiber thread and three-dimensional reinforcement caused by fiber network respectively.     

Reinforcement of Soft Foundation with Geotextile and Observation for Sea Dike Project of Zhapu Port

1 .IntroductionTheZhapuPortissituatedonthenorthbankoftheHangzhouBay ,nearZhapuTowninPinghuCity ,Zhejiangprovince .Asthefirststepofconstruction ,reclamationwasaccomplishedinthehinter landoftheharbor.Theborderdikeofafulllengthof 1 2 71mislocatedinthetidalregionoftheHangzhouBay .ThepreliminarydesignofthisprojectwascompletedinJune 1 986 ,anditsconstructionbeganinJuly 1 986 ,andwascompletedinMarch 1 991 .TheseadikewasbuiltontheQ4marinedepositofsaturatedsoftclaywithathicknessofabout2 7mandgeo…     

Experimental study on the cyclic behavior of loose calcareous sand under linear stress paths

Abstract

Hollow cylinder torsional shear tests on loose isotropically and anisotropically consolidated calcareous sand were conducted to investigate the cyclic behavior under three different linear stress paths, including horizontal line, oblique line, and vertical line stress paths, in a coordinate system of the normal stress difference and the horizontal shear stress. The dominant strain components of the isotropically consolidated specimens are affected by the stress paths. With increasing consolidation stress ratio, axial strain gradually becomes the dominant strain component under the three different stress paths. The cyclic strength of the isotropically consolidated specimens under the three different stress paths are almost the same, while for the anisotropically consolidated specimens, the cyclic strengths are strongly affected by the stress paths. These results indicate that conventional cyclic triaxial tests may overestimate cyclic strength in some cases. Irrespective of the stress paths and cyclic stress ratios, the terminal residual excess pore pressure ratio decreases with increasing consolidation stress ratio. Moreover, an empirical equation is proposed to describe the relationship between the normalized shear work and the normalized residual excess pore pressure ratio. The comparative study reveals that the relationship proposed for silica sand is not suitable for the dynamic analyses of calcareous sand.     

Cementation of sand due to salt precipitation in drying process

Salt in soil can cause cementation effect and increase the shear strength and stiffness of soil during the drying process. In this paper, an experimental study is presented to explore the strength, stiffness, and particle level structure of a salt-cemented sand at the dry state. Unconfined compression tests were carried out on sands with various amounts of precipitated dry salt. Scanning electron microscopic and elemental analysis were also conducted. The study shows that the strength and stiffness of sand can increase significantly with salt content. The strength versus salt content curve displays a convex shape, instead of a concave shape as found in cemented or biocemented sands. This implies that the effect of dry salt on soil strength is strong at low levels of salt content. The microscopic and elemental analysis evidences that salt tends to precipitate at particle contacts and form bridges between particles at relatively low salt content. The results presented in the paper may explain why even small amount of salt can lead to an overestimation of soil strength.     

珠江口横门大气氮、磷干湿沉降的初步研究

通过对2006年12月至2007年11月中山横门的大气沉降采样分析, 初步探讨了珠江口大气氮、磷干湿沉降的特征。结果表明, 观测期间铵态氮(NH4+-N)、硝态氮(NO3--N)、总氮(TN)、总磷(TP)降雨量加权平均浓度分别为0.82、0.52、2.14、0.039mg.L-1, 干湿总沉降通量分别为1.584、1.142、4.295和0.055g.m-2.a-1。NH4+-N、NO3--N和TN干、湿沉降通量相当, 而TP以湿沉降为主。TN大气沉降通量春、夏、秋三季相当, 均明显高于冬季, TP则以夏季最大, 秋季次之, 而冬季最小。     

Research on Flexural Behavior of Coral Aggregate Reinforced Concrete Beams

Through the flexural behavior test of coral aggregate reinforced concrete beams (CARCB) and ordinary Portland reinforced concrete beams (OPRCB), and based on the parameters of concrete types, concrete strength grades and reinforcement ratios, the crack development, failure mode, midspan deflection and flexural capacity were studied, the relationships of bending moment-midspan deflection, load-longitudinal tensile reinforcement strain, load-maximum crack width were established, and a calculation model for the flexural capacity of CARCB was suggested. The results showed that with the increase in the reinforcement ratio and concrete strength grade, the crack bending moment (M_cr) and ultimate bending moment (M_u) of CARCB gradually increased. The characteristics of CARCB and OPRCB are basically the same. Furthermore, through increasing the concrete strength grade and reinforcement ratio, M_cr/M_u could be increased to delay the cracking of CARCB. As the load increased, crack width (w) would also increase. At the beginning of the loading, w increased slowly. And then it increased rapidly when the load reached to the ultimate load, which then led to beam failure. Meanwhile, with a comprehensive consideration of the effects of steel corrosion on the loss of steel section and the decrease of steel yield strength, a more reasonable calculation model for the flexural capacity of CARCB was proposed.     

Shear Behavior of Sand–Silt Mixtures: A Laboratory Investigation of Coastal Silty Sand Soils of Mostaganem

In recent years, interest in understanding the mechanisms of mechanical instability of porous media, leading to catastrophic failure has been continuously revised to include, new porous media parameters generating the phenomenon of liquefaction under static or dynamic loadings. Results from experimental test programs have concluded too many physical concepts based on material intrinsic properties, initial states, and other characteristics. Despite the great progress on the subject, these concepts do not allow a unified treatment of such porous media. The assessment of critical shear strength of sandy soils as porous media under undrained conditions is a major challenge in stability analysis. Such strength serves to evaluate the occurrence of flow deformation under liquefaction phenomena. The determination of the critical undrained strength is essentially fundamental for the design of soil structures such as earth dams, bridge supports, building foundations as well as soil densification process to avoid catastrophic failure due to soil instability manifested by failure or large displacement such as settlement. In this work, experimental program on reconstituted loose and medium dense specimens of terrigenous silica sands with different specified fine contents was carried out to analyze its mechanical behavior under undrained conditions. The present article is an attempt to experimentally describe mechanical behavior and theoretically justify such response of loose and medium dense sand by means of critical state soil parameters.     

Uplift of symmetrical anchor plates by using grid-fixed reinforced reinforcement in cohesionless soil

Uplift response of symmetrical anchor plates with and without grid fixed reinforced （GFR） reinforcement was evaluated in model tests and numerical simulations by Plaxis. Many variations of reinforcement layers were used to reinforce the sandy soil over symmetrical anchor plates. In the current research, different factors such as relative density of sand, embedment ratios, and various GFR parameters including size, number of layers, and the proximity of the layer to the symmetrical anchor plate were investigated in a scale model. The failure mechanism and the associated rupture surface were observed and evaluated. GFR, a tied up system made of fiber reinforcement polymer （FRP） strips and end balls, was connected to the geosynthetic material and anchored into the soil. Test results showed that using GFR reinforcement significantly improved the uplift capacity of anchor plates. It was found that the inclusion of one layer of GFR, which rested directly on the top of the anchor plate, was more effective in enhancing the anchor capacity itself than other methods. It was found that by including GFR the uplift response was improved by 29%. Multi layers of GFR proved more effective in enhancing the uplift capacity than a single GFR reinforcement. This is due to the additional anchorage provided by the GFR at each level of reinforcement. In general, the results show that the uplift capacity of symmetrical anchor plates in loose and dense sand can be significantly increased by the inclusion of GFR. It was also observed that the inclusion of GFR reduced the requirement for a large L/D ratio to achieve the required uplift capacity. The laboratory and numerical analysis results are found to be in agreement in terms of breakout factor and failure mechanism pattern.     

Shear Strength of Jumunjin Sand according to Relative Density

The properties of sands pertaining to shear strength have been studied extensively by plane strain and triaxial tests for relative densities above 40%. Unfortunately, properties of sand have not been comprehensively studied for relative density below 40%. An experimental investigation of the shear strength of Jumunjin sand was carried out using a plane strain, triaxial and direct shear apparatus according to relative density ranges from 0 to 100%. Because of the complexity of the shear tests on coarse materials, correlations were developed that would be useful for practical purposes. This article presents a comparison of strength properties' results and their correlation with relative density.     

Experimental study on strength characteristics and microscopic mechanism of marine soft clays

Abstract

The effect of microstructure on shear strength of saturated marine clays was investigated by conducting a series of consolidated-drained (CD), consolidated-undrained (CU) triaxial shear tests and mercury intrusion porosimetry (MIP) tests on undisturbed and reconstitute specimens. The valuable findings from the experimental study are follows: (1) The shear strength of undisturbed specimens is lower than that of corresponding reconstituted specimens due to larger void ratio at the same confining pressure. However, undisturbed specimens have higher strength than reconstituted specimens when their void ratios are the same. (2) The main reason for the lower shear strength of reconstituted specimens with the same void ratio as undisturbed specimens is that more volume of inter-aggregate pores exists in the reconstituted specimens according to the MIP test results. And the difference in shear strength between undisturbed and reconstituted specimens is mainly caused by the difference in soil fabric. (3) The shear test results dealt with a reference void ratio, as a fabric index, show that there is a unique linear relation between strength and void ratio at failure to the reference void ratio. Moreover, the linear relation is suitable for other marine clays from the literature. Therefore, the reference void ratio can be used as a soil fabric index to normalize the strength characteristics of marine soft clays.     

Effect of strain-dependent dynamic properties of backfill and foundation soil on the external stability of geosynthetic reinforced waterfront retaining structure subjected to harmonic motion

A simplified method to determine the minimum length of reinforcement required for the external stability of waterfront reinforced soil structures under seismic conditions is presented. In the present analysis, strain-dependent dynamic properties (shear modulus and damping ratio) are used. The results obtained from the present method are well compared with the results of pseudo-static method of analysis. For the set of input parameters, the estimated minimum length of reinforcement required against sliding failure is nearly 27–29% higher for an input normalized frequency of 1.06 and is nearly 22–25% lower for another input normalized frequency of 1.94 when compared with the results of pseudo-static approach. This can be attributed to the mode change behaviour of the waterfront structure. In addition, the effect of foundation type on the external stability of waterfront reinforced soil structures has also been presented and it is found that the foundation type has a significant effect on the same. For the given set of input parameters, the length of minimum reinforcement required for a slope and vertical wall having a flexible foundation are about 26–28% and 32–38% larger than that of a slope and vertical wall with rigid foundation, respectively.     

金属离子种类对珊瑚砂微生物固化影响研究

选用巴斯德芽孢杆菌,对微生物诱导钙离子、镁离子以及铁离子形成碳酸盐固化松散珊瑚砂颗粒的效果进行对比研究。试验结果表明,在相同试验方法、相同注入次数的条件下,微生物诱导形成的碳酸钙和碳酸镁能将珊瑚砂松散颗粒固化成为整体,加入铁离子的试样因为反应过程的原因没有成功。通过渗透性、干密度变化、无侧限抗压强度及微观结构等方面进行对比分析,钙离子试样相对于镁离子试样,具有固化过程平稳、干密度增量大、强度高、微观颗粒包裹更好的优点。综合试验结果得出,钙离子是现阶段微生物固化珊瑚砂较为理想的金属离子。本试验首次探讨了金属离子种类对珊瑚砂微生物固化效果的影响,为微生物固化技术提供了一定理论基础,对该技术的进一步应用具有一定借鉴意义。     

Correlating mass physical properties with ALOS reflectance spectra for intertidal sediments from the Ba Lat Estuary (northern Vietnam): an exploratory laboratory study

Characterization of the sediment composition of tidal flats and monitoring of their spatiotemporal changes has become an important part of the sustainable management of coastal environments. To accurately classify sediments through remote sensing, a comprehensive understanding of sediment reflectance spectra is indispensable. The present laboratory-based study explores the performance of the high spatial resolution (10?×?10 m) Advanced Land Observing Satellite (ALOS) launched in 2006. Relationships between reflectance spectra (bands 1 to 4) and four typical mass physical properties were investigated under wet and dry experimental conditions for intertidal sediments sampled near the Ba Lat Estuary in northern Vietnam. Reflectance in the near-infrared region corresponding to ALOS band 4 (0.76–0.89 μm) was found (1) to have a strong negative correlation with sand content (dry wt%) under both wet and dry conditions (linear correlation coefficient r?=?–0.7859 and –0.8094, respectively), (2) to increase with decreasing relative water content (%) in a given sediment type (r?=?–0.7748 to –0.9367 for mud, sandy mud, muddy sand, and sand), (3) to have a positive correlation with organic matter content (r?=?0.7610 and 0.6460 under wet and dry conditions for contents >0.20 dry wt%), and (4) to be insignificantly correlated with mineral composition assessed in terms of contents (wt%) of quartz, clay minerals, and mica group minerals. Positive relationships between reflectance and water content for the pooled data of all sediment types (r?=?0.6395) or organic matter content contrast with previous findings, and can be attributed to close interrelationships between these properties and the predominance of sand content as controlling factor of reflectance. This study clarifies that ALOS band 4 provides the most useful imagery for intertidal monitoring because its reflectance, as simulated using the laboratory data, shows the strongest correlation with sand content. In a next step, these experimental findings should be verified by identifying the reflectance relationships at satellite image scales, and also considering the effects of other tidal flat features on reflectance, such as microtopography and biological surface characteristics.     

San Diego trough geotechnical test area

Abstract

The San Diego Trough Geotechnical Test Area, located about 24 km southwest of San Diego in a water depth of about 1.2 km, lies near the base of the Coronado Escarpment directly north of the Coronado Fan. A new bathymetric map delineates a shallow basin in the soft, highly plastic, clayey silts flooring the Test Area. Measurements of shear strength by vane and static cone pene‐trometer, and bulk density by nuclear densitometer, were made in place from the submersible Deep Quest. Sixteen short (< 1.6 m) gravity cores were collected from ships.

The geotechnical properties show little areal variation and generally change uniformly with depth within the 55 km² Test Area. Silt is the predominant grain size, averaging about 62%. In‐place bulk density shows little change with increasing depth, values range from 1.23 to 1.26 Mg/m³; laboratory density values increase with depth, ranging from 1.30 to 1.52 Mg/m³ between the surface and a depth of about 1.1 m. The difference between the in place and laboratory values may indicate sampling densification of the cored sediment. Water content in the cores decreases uniformly within the range of 249 to 43% dry weight. Shear strength increases linearly with depth. The laboratory shear strength values are lower than the in place values, which range from 4 kPa at the surface to about 29 kPa at a depth of 3.27 m. Predictor equations relate Atterberg limits, bulk density, water content, and laboratory and in place shear strength to depth. Sedimentation‐compression e log p curves have an equivalent compression index of 1.5 to nearly 2. Excluding rurbidite layers and sampling disturbance effects, all cores indicate a uniform depositional environment in the surface to 1.6 m of sediment sampled. The geotechnical properties indicate that the sediments in the west central and southwest parts of the Test Area exhibit vertical heterogeneity due to thin silt‐sand layers, presumably of turbidity current origin, that originated from the Coronado Canyon.     

Experimental Research on Shear Behavior of Reinforced Concrete Composite Beams Under Uniformly Distributed Load

An experimental program was carried out to study the shear behavior of the reinforced con-crete composite beam(RCCB)subjected to two-phase uniformly distributed load.A total of 12 reinforcedconcrete composite beams were tested:10 of them were the RCCB subjected to two-phase uniformly dis-tributed load,the other 2 were the comparative reinforced concrete beams cast at the same time as theRCCB subjected to one-phase uniformly distributed load.The interface of precast unit and recast concretewas natural and rough.The test range of the main composite factors:the ratio of precast section depth tocomposite section depth was from 0.35 to 0.65,the ratio of first-phase load moment to precast section ulti-mate bearing moment was from 0.25 to 0.65.Based on the test results,the stresses of the longitudinal rein-forcements and stirrups,the load-bearing properties of the interface,the crack state and the failure charac-teristics of the RCCB under uniformly distributed load are discussed.The effects of the stirrups,the     

Analytical Crack Model for 2-D Reinforced Concrete Structures

A two-dimensional smeared crack model for reinforced concrete members is presented. Special emphasis is placed on the bond between concrete and reinforcement as the main factor influencing tension stiffening in cracked reinforced concrete. With the derived tangential stress-strain equations for concrete in the direction perpendicular to the cracks, the constitutive relationship for cracked reinforced concrete is established. Experimental specimens have been analyzed with the analytical model, and the analytical and experimental results are found to be in good agreement.     

Laboratory study of shear strength response of Chlef natural sand: Effect of saturation

Abstract

The study of the unsaturated soils is a very complex field to which several researches in laboratory and on site are directed these last years. An experimental study aims to quantify the influence of the Skempton coefficient B characterizing the degree of saturation on the behavior of the granular sand to the liquefaction resistance of sand. The study is based on undrained triaxial tests performed on natural Chlef sand carried out at an initial relative density Dr = 50% under a confining pressure of 50, 100 and 200?kPa for Skempton coefficient B?=?10 to 92%. The results of the tests show that an increase of confining pressure leads to an increase of the shear stress of the samples. For the lower value of the degree of saturation results of our findings indicate an increase of the resistance to liquefaction; our results are in good agreement with others observed in several research projects conducted with other sands. The increase of Skempton coefficient B induces an increase of pore water pressure and a decrease of the shear strength. The pore pressure (u) increases with an exponential manner with the increase of the degree of saturation; while the residual shear strength (Rs) decreases with logarithmic manner with the increase of degree of saturation (B).