Dilatancy and Shear Strength of Sand at Low Confining Pressures

Sand dilates with shearing at a rate that increases with increasing relative density (DR) and decreases with increasing effective confining stress (σc′). The peak friction angle of a sand depends on its critical-state friction angle and on dilatancy. In this paper, we develop a simple correlation between peak friction angle, critical-state friction angle, and dilatancy based on triaxial compression and plane-strain compression test data for sand for a range of confining pressures from very low levels to approximately 196 kPa.     

Direct Shear Tests on JSC-1A Lunar Regolith Simulant

A series of direct shear tests were conducted on the JSC-1A lunar regolith simulant in a 101.6-mm- (4-in.-) diameter container. The direct shear test provides a unique mode of failure that aids the development of excavation tools for the Moon. Relative density and normal load were varied to study the strength behavior of such granular material at peak and critical state conditions. The values of the internal friction angle ranged from 30 to 70°. A relationship between the internal friction angle of the direct shear and the published triaxial compression test results is presented. Additionally, the measured dilatancy angle is related to the difference in peak and critical state stress friction angles.     

3D Shape Characterization and Image-Based DEM Simulation of the Lunar Soil Simulant FJS-1

This paper describes a procedure used to characterize the three-dimensional (3D) grain shape of lunar soil and undertake simulations of lunar soil by image-based discrete element method (DEM). Given that detailed 3D grain-shape information is unavailable for real lunar soil, a simulant material, FJS-1, is used in this study. We use the high-resolution micro X-ray CT system at SPring-8, a synchrotron radiation facility in Japan, to visualize precise 3D images of the granular assembly of FJS-1. A newly developed image-analysis procedure is then applied to identify individual grains. Using the obtained grain-shape data, a sufficient number of FJS-1 grains are directly modeled for DEM simulation using an efficient modeling scheme. A series of particle flow simulations are then performed with the modeled grains. The resulting slope angles are in good agreement with experimental results. We discuss the effect on the slope angle of grain parameters such as contact stiffness, restitution coefficient, and interparticle friction.     

Computation of Cavity Expansion Pressure and Penetration Resistance in Sands

A cavity expansion-based theory for calculation of cone penetration resistance qc in sand is presented. The theory includes a completely new analysis to obtain cone resistance from cavity limit pressure. In order to more clearly link the proposed theory with the classical cavity expansion theories, which were based on linear elastic, perfectly plastic soil response, linear equivalent values of Young's modulus, Poisson’s ratio and friction and dilatancy angles are given in charts as a function of relative density, stress state, and critical-state friction angle. These linear-equivalent values may be used in the classical theories to obtain very good estimates of cavity pressure. A much simpler way to estimate qc—based on direct reading from charts in terms of relative density, stress state, and critical-state friction angle—is also proposed. Finally, a single equation obtained by regression of qc on relative density and stress state for a range of values of critical-state friction angle is also proposed. Examples illustrate the different ways of calculating cone resistance and interpreting cone penetration test results.     

Shear Strength Parameters of Municipal Solid Waste with Leachate Recirculation

The objective of this study was to characterize relative changes in waste shear strength parameters during waste decomposition. Twelve direct shear tests (100?mm diameter by 50?mm thickness) were performed on waste specimens ranging from fresh to well-decomposed residential refuse. In addition, nine direct shear tests were performed on selected waste components including fresh paper, partially decomposed refuse, and plastics. Results indicate that the friction angle of refuse decreased with decomposition. As refuse decomposed, the plastic content increased, which contributed to a decrease in friction angle as the friction angle of plastics is 18–19° as compared to 33° for fresh shredded waste. The extent of refuse decomposition was characterized by the cellulose plus hemicellulose to lignin ratio [(C+H)/L]. The measured friction angle decreased from 32 to 24° as (C+H)/L decreased from 1.29 to 0.25. The shearing pattern for decomposed refuse showed a peak, followed by residual, which was then followed by a steady increase in shear stresses with displacement; the final rate of increase was similar to that observed in fresh paper specimens. Results from this work were comparable to data from previous reports, though it is important to characterize the extent of solids decomposition for a valid comparison with published studies.     

不同层理页岩常规三轴压缩力学特性离散元模拟

页岩作为页岩气储层,在沉积过程中形成部分弱面,在力学特性上表现出各向异性特征。所以,使用离散元软件从微细观层面探讨深部页岩力学各向异性特征具有重要实践意义。基于页岩室内常规三轴压缩试验结果,采用离元程序PFC2D对常规三轴压缩下不同层理倾角页岩进行了颗粒流模拟研究,分析了层理倾角及围压对页岩力学特性的影响规律。结果表明:（1）页岩峰值强度与黏聚力随层理倾角的增加整体呈“U”形变化,但峰值强度在不同围压下的变化趋势有所区别;而内摩擦角随层理倾角的增大呈非线性变化。（2）层理倾角对页岩周围颗粒的位移方向及大小的影响随着层理面与轴向应力的夹角的增大而减小。（3）同一层理倾角试样最终破坏时的微裂纹总数随着围压的升高有所增加;同一围压下,试样最终破坏时的微裂纹数目,随着层理倾角的增加呈现先减少后增多的趋势。（4）同一层理倾角页岩的脆性随围压的增长整体呈下降趋势;低围压情况下,页岩脆性随层理倾角的增加呈两端大中间小的变化规律。      

Use of State-Dependent Strength in Estimating End Bearing Capacity of Piles in Sand

The pressure and density dependence of the shear strength of sand poses a tricky problem in pile foundation design. In this study, a correlation is suggested to link the effective friction angle of sand with its initial confining pressure and relative density, and a simple approach incorporating this correlation is presented for predicting pile end bearing capacity. Assessment of the approach against pile load tests shows reasonably good agreement between predictions and measurements. It is also shown that the effect of the state-dependent strength is particularly important in cases where long piles are installed in dense sand deposits and the use of critical state friction angle will produce a conservative prediction in such cases.     

Shear Strength of Municipal Solid Waste

A comprehensive large-scale laboratory testing program using direct shear (DS), triaxial (TX), and simple shear tests was performed on municipal solid waste (MSW) retrieved from a landfill in the San Francisco Bay area to develop insights about and a framework for interpretation of the shear strength of MSW. Stability analyses of MSW landfills require characterization of the shear strength of MSW. Although MSW is variable and a difficult material to test, its shear strength can be evaluated rationally to develop reasonable estimates. The effects of waste composition, fibrous particle orientation, confining stress, rate of loading, stress path, stress-strain compatibility, and unit weight on the shear strength of MSW were evaluated in the testing program described herein. The results of this testing program indicate that the DS test is appropriate to evaluate the shear strength of MSW along its weakest orientation (i.e., on a plane parallel to the preferred orientation of the larger fibrous particles within MSW). These laboratory results and the results of more than 100 large-scale laboratory tests from other studies indicate that the DS static shear strength of MSW is best characterized by a cohesion of 15?kPa and a friction angle of 36° at normal stress of 1?atm with the friction angle decreasing by 5° for every log cycle increase in normal stress. Other shearing modes that engage the fibrous materials within MSW (e.g., TX) produce higher friction angles. The dynamic shear strength of MSW can be estimated conservatively to be 20% greater than its static strength. These recommendations are based on tests of MSW with a moisture content below its field capacity; therefore, cyclic degradation due to pore pressure generation has not been considered in its development.     

Laboratory Evaluation of Crushed Glass–Dredged Material Blends

A comprehensive laboratory evaluation of blending 9.5?mm (3/8?in.) minus curbside-collected crushed glass (CG) with dredged material (DM) was conducted to evaluate their potential for beneficial use as fill materials for urban applications. Tests were performed on 100% CG (USCS classification SP) and 100% DM (OH) specimens and 20/80, 40/60, 50/50, 60/40, and 80/20 CG–DM blends (dry weight percent CG content reported first). The addition of 20% CG resulted in a 10–20 point (33–67%) reduction in wopt while increasing the dry density by approximately 1–3?kN/m3 for standard and modified levels of compaction, respectively. Simultaneously, the compressibility of the DM was reduced by approximately 50% and the hydraulic conductivity was reduced by ? order of magnitude. The addition of 20% CG significantly decreased the moisture content and significantly improved the workability of the 100% DM, where workability refers to the ease of handling, transport, placement, and compaction of the CG–DM blends (compared to 100% DM). CIū triaxial strength testing indicated effective friction angles of 34 and 37° for 100% DM and CG compacted to a minimum of 95% relative compaction by ASTM D1557, respectively. A peak effective friction angle of 39° occurred for the 60/40 and 80/20 CG–DM blends which were also 1 and 3 orders of magnitude more permeable than 100% DM, respectively. Related increases in cv resulted in decreased times required for consolidation. The range of properties obtainable by the CG–DM blends offers a versatility that allows for the design of fills that can be potentially optimized to meet multiple design parameters (e.g. strength, settlement, drainage, or higher CG or DM content).     

Engineering Properties of Lunar Soil Simulant JSC-1A

This study was carried out to assess the tensile and shear strength in lunar soil, and to examine the variation as a function of density and confinement. Geotechnical engineering properties of the lunar soil simulant designated Johnson Space Center Number One-A lunar soil simulant (JSC-1A) have been investigated experimentally. To better understand these soil properties, a variety of conventional and unconventional experiments were conducted on JSC-1A to determine its grain-size distribution, cohesion, friction angle, dilatancy angle, tensile strength, and appropriate low strain elastic constants. These experiments were conducted on JSC-1A at a variety of densities prepared through tamping densification to quantify the response of the soil over a range of conditions. To simulate lunar conditions, the samples were prepared at medium to very high relative densities. Grain-size distribution, shear strength, tensile strength, dilatancy angles, and elasticity modulus of the JSC-1A were compared with lunar soil and other simulants.     

Particulate flow analysis in inclined pipes and transfer chutes using tomography imaging,discrete element simulations and continuum modeling approaches

In particulate material transfer systems,traditional shear test based steady state analysis can provide some insight into the strength of the bulk material and subsequent resistive frictional forces during flow.For fast flowing transfer points,dynamic flow conditions dominate and additional modelling techniques are required to improve design guidance.The research presented shows the evolution of a design solution which utilises two distinct processes;a continuum method and a discrete element method(DEM). Initially,the internal structure of dense granular flow,down vertical and inclined pipes was investigated using a twin sensor,12 electrode electrical capacitance tomography device.Subsequently,DEM simulations were conducted using the commercial software,PFC3D.Initially,two particle types and their flow behaviours were analysed:plastic pellets and sand.The pipe angle was varied between 0°and 45°to the vertical.For both the plastic pellets and the sand,good qualitative agreement was found with the spatial particle concentration analysis.Generally,the flow had a dense particle region at its core with the particle concentration reducing away from this core.As expected,at 0°, the core was centrally located within the pipe for both the plastic pellets and sand.At pipe angles 5°or greater,the dense core of particles was located on or near the pipe wall.Average flow velocity analysis was also conducted using the results of wall friction test analysis.The velocity comparisons also showed good agreement between the ECT image analysis and the DEM simulations. Subsequently,the DEM method was used to analyse a complex transfer system(or chute) with the continuum method providing comparative flow analysis with the DEM flow analysis.     

基于UDEC-GBM的矿物晶粒解理特征对硬岩石破坏过程的影响

基于块体离散单元数值模拟方法（UDEC-GBM）,以钾长石矿物颗粒为例,详细研究了矿物晶粒解理倾角、解理倾角围压效应及解理间距对硬质岩石力学性质、微观开裂过程及机理的影响,并探讨了解理特征在工程实际中可能带来的影响。数值研究结果表明:（1）晶粒解理具有明显倾角效应,当解理倾角由0°增加到90°时,岩石的弹性模量、单轴压缩强度及峰后脆延特征都会发生变化,穿晶总裂纹数受影响明显,主要体现在钾长石张拉穿晶裂纹显著增加,钾长石剪切裂纹数量在60°增加到最大值后减少,石英穿晶张拉裂纹数量也有明显变化,总体而言不断增加,而沿晶裂纹数量呈减少趋势,整个开裂过程仍以张拉沿晶主导;（2）晶粒解理倾角效应受围压影响,围压会导致沿晶裂纹和穿晶裂纹数量和二者比值发生变化,但不同倾角下围压对沿晶裂纹和穿晶裂纹数量和比值变化影响不一样;（3）当解理间距由2 mm增加到4 mm时,穿晶裂纹数量有增加趋势,而沿晶裂纹数量减少,总剪切和张拉裂纹数量比值不变,对岩石微观张拉、剪切破坏机制无明显影响。此外,具有解理结构的矿物晶粒含量较高且矿物晶粒本身性质对岩石性质及响应影响显著时,解理特征对板裂、岩爆等破坏的影响应给予重视。      

Field and Laboratory Testing of St. Peter Sandstone

The purpose of this project was to evaluate mechanical properties of St. Peter sandstone by in situ testing, and to compare the field data with laboratory results. Direct shear tests were conducted to evaluate the strength-dilatancy behavior, and thin-section microscopy was used to help explain the significant friction angles associated with the material. St. Peter sandstone is nearly cohesionless, but it possesses a friction angle of 57–63° at low confinement. The large angle of internal friction at failure may be due to locking of sand particles or to postdepositional quartz overgrowths. Tests on pulverized densely packed sand and loosely packed sand were conducted in the same manner as the intact specimens and yielded friction angles of about 42 and 34°. Pressuremeter tests were performed in situ and the results were interpreted using an elasto-plastic analysis in terms of total stresses. By appropriate consideration of system stiffness, Young’s modulus was found to be about 0.5 GPa, slightly lower than the laboratory value, although unload-reload cycles were not attempted. Assuming associative behavior, the friction angle was estimated to be at least 56°.     

含陡倾角软弱结构面的花岗岩力学特性及声发射特征

利用MTS 815.03岩石试验机对试件进行三轴压缩试验,采用DISP声发射测试系统进行声发射数据收集,并对含陡倾角软弱结构面的岩体试件和岩石试件进行试验对比.两种试件随围压的增加强度逐渐增加,破坏由脆性向延性转变;岩体试件总是沿结构面滑移破坏,岩石试件为剪切破坏.随着围压的增加,两种试件的弹性模量、变形模量、峰值应变和峰值强度增加;岩体试件弹性模量、变形模量值和峰值强度低于岩石试件,而峰值应变高于岩石试件.岩体试件内摩擦角小于岩石试件,而黏聚力大于岩石试件.随围压的增加,两种试件在峰值应力阶段声发射事件远高于其他阶段,而岩体试件声发射事件集聚量远远高于岩石试件.研究结果表明软弱结构面的存在降低了岩体的力学性质,因此在高放废物地质处置库选址时结构面发育特征是需要考虑的关键因素.      

NAO-1: Lunar Highland Soil Simulant Developed in China

A new lunar highland soil simulant, NAO-1, has been created in National Astronomical Observatories (NAO), Chinese Academy of Sciences. This simulant was produced by gabbro, which includes large quantity of feldspar (An>90). The simulant’s chemical composition, mineralogy, particle-size distribution, density, angle of internal friction, and cohesion have been analyzed and results demonstrated that most characteristics of NAO-1 are similar with lunar highland soil samples. NAO-1 will benefit the scientific and engineering research of lunar soil.     

Shear Band Formation in Plane Strain Experiments of Sand

A series of biaxial (plane strain) experiments were conducted on three sands under low (15 kPa) and high (100 kPa) confining pressure conditions to investigate the effects of specimen density, confining pressure, and sand grain size and shape on the constitutive and stability behavior of granular materials. The three sands used in the experiments were fine-, medium-, and coarse-grained uniform silica sands with rounded, subangular, and angular grains, respectively. Specimen deformation was readily monitored and analyzed with the help of a grid pattern imprinted on the latex membrane. The overall stress-strain behavior is strongly dependent on the specimen density, confining pressure, sand grain texture, and the resulting failure mode(s). That became evident in different degrees of softening responses at various axial strains. The relationship between the constitutive behavior and the specimens' modes of instability is presented. The failure in all specimens was characterized by two distinct and opposite shear bands. It was found that the measured dilatancy angles increase as the sand grains' angularities and sizes increase. The measured shear band inclination angles are also presented and compared with classical Coulomb and Roscoe solutions.     

不同影响因素下路用黄河泥沙动剪切模量和阻尼比试验及理论模型研究

沿黄河高速公路建设过程中,黄河泥沙作为路基填料的可行性已经得到验证和重视,然而目前有关黄河泥沙作为路基填料的动力特性的研究较少.本文利用英国GDS动态三轴试验系统,对取自黄河中下游郑州段的泥沙进行应力控制的动三轴试验,探究了围压、相对密实度和试验频率对黄河泥沙动剪应力–动剪应变关系、动剪切模量G和阻尼比D的影响,绘制了动剪应力–动剪应变关系骨干曲线和滞回曲线.结果表明,黄河泥沙的动剪切模量、阻尼比与剪应变关系可以用Hardin双曲线模型描述,围压对G和D的影响较大、试验频率对G和D的影响较小.综合与其他土体的动力特性对比表明,黄河泥沙动剪切模量折减曲线规律以及阻尼比D曲线规律和其他土体相符,其动力特性更接近于粉土和砂土,但与其他土体并不完全一致,具有一定的特殊性.最后,本文考虑了围压、相对密实度的影响,并结合现有经验公式,建立可以较好描述黄河泥沙最大动剪切模量G_max与围压、孔隙比关系的经验公式,同时建立了动剪切模量比G/G_max和D的数学模型,拟合结果显示,建立的模型能较好地描述黄河泥沙的G/G_max和D随剪应变的变化...     

Predicted Loads in Steel Reinforced Soil Walls Using the AASHTO Simplified Method

The paper investigates the accuracy of the AASHTO simplified method by using load measurements reported in a large database of full-scale instrumented walls for bar mat, welded wire, and steel strip soil reinforced walls. The accuracy of the AASHTO simplified method is quantified by computing the mean and coefficient of variation of the ratio (bias) of measured loads under operational conditions to predicted loads. The paper shows that for steel strip walls, the AASHTO simplified method is reasonably accurate for granular backfill soils with friction angles less than 45°. For bar mat walls, the method is demonstrated to be slightly conservative. The simplified method underpredicts reinforcement loads at shallow overburden depths for steel strip walls with backfill friction angles greater than 45° due to compaction-related effects. It is concluded that these compaction-induced loads near the wall top do not contribute to internal instability due to pullout.     

Postpeak Strength of Interfaces in a Stress-Dilatancy Framework

Laboratory sand-steel interface tests, using a range of sand sizes on a wide range of surface roughnesses, have been conducted using a direct shear apparatus modified to enable reliable measurements of both friction and dilation. The paper looks at the minimum interface strength after peak, termed here the postpeak strength, and assesses its dependence on roughness, density, and stress level. Its upper limit is the large displacement direct shear friction angle, related to but not equal to the critical state friction angle. When data are normalized by this value, they show linear dependence on the logarithm of relative roughness in the intermediate zone between smooth and rough. Once the roughness dependence of the postpeak strength has been allowed for, dilatant interfaces are found to follow classical stress–dilatancy relationships. It appears that there is no fundamental difference in the responses of sand-on-steel or sand-on-sand interfaces.