砂岩Ⅰ型断裂韧度及其与强度参数的相关性研究

岩石的破坏和断裂是密切相关的,岩石强度准则的材料参数和断裂理论的断裂韧度是存在特定关系的,基于此,设计了紧凑的试验方案,对干燥和饱水状态下的砂岩试样进行了3点弯曲断裂韧度试验和抗压、抗拉强度试验,试验结果表明,饱水后,砂岩的Ⅰ型断裂韧度软化效应明显,饱水试样和干燥试样的 比值与砂岩抗压强度的软化系数相近,具有相似的软化效应。同时,从理论上分析了岩石I型断裂韧度与抗拉强度之间的关系,并结合大量的试验数据进行了验证,相关结论为以往的岩石I型断裂韧度与抗拉强度之间的数据统计拟合公式提供了理论基础。最后,统计分析了砂岩Ⅰ型断裂韧度与抗压强度、黏聚力、内摩擦角等强度参数之间的关系。研究成果对于把握砂岩Ⅰ型断裂韧度及其与强度参数的相关性具有较大的参考价值,相关的分析结论、试验方法和试验结果也可以为其他类型岩石的研究提供有益的参考。     

非饱和红粘土和膨胀土抗剪强度的比较研究

红粘土是对环境湿热变化敏感的塑性粘土,具有一般膨胀土吸水膨胀失水收缩的特性。与普通粘性土相比,红粘土与膨胀土的强度特性更为复杂。它既是土体抵抗剪切破坏能力的表征,也是计算路堑、渠道、路堤、土坝等斜坡稳定性以及支挡构筑物土压力的重要参数。通过试验研究讨论了红粘土与膨胀土的强度特性以及与一般粘性土的差别及其各种影响因素,并探讨了非饱和红粘土与膨胀土的抗剪强度指标与含水量之间的相关关系。试验结果表明,红粘土与一般膨胀土的吸水膨胀规律完全相同。其试验结果可为红粘土与膨胀土地区工程设计与建设提供参考依据。     

层理面特性对砂岩断裂力学行为的影响研究

层理弱面对层状岩石的力学特性影响较显著，为了研究层理面特性对岩石断裂力学特性的影响，开展了具有不同层理方向的砂岩试样三点弯试验，探讨了砂岩断裂韧度及断裂模式的各向异性。之后基于有限元中的黏聚单元建立了数值模型，采用数值模拟方法研究了层理面强度对各层理角度试样断裂力学行为的影响规律。结果表明：层理方向影响下砂岩的断裂韧度及模式存在各向异性；同一层理方向试样的断裂韧度随层理面强度的增大而增大，且试样的层理面与加载方向夹角越小，断裂韧度受层理面强度变化影响越明显；试样的断裂模式不仅与层理面强度有关，还受层理倾角的控制，层理面与加载方向夹角θ = 0o试样断裂模式基本不受层理面强度影响，θ = 30o试样主要沿层理面张拉或剪切破坏，且沿层理面的破裂长度随层理面强度的降低逐渐增大；层理面强度较大时，θ = 45o试样主要沿层理面张拉破坏，θ = 60o～90o试样主要以贯穿层理的张拉破坏为主；层理面强度较小时，θ = 45o～90o试样均以沿层理面的剪切破坏为主，其中θ = 45o试样沿层理剪切长度最大。另外，通过数值模拟结果分析了层理面强度及方向对试样的起裂角及裂纹扩展路径产生的影响。该研究成果可作为层状岩石断裂力学理论的有益补充。     

胀缩性土抗拉强度试验研究

抗拉强度是黏性土重要的力学指标之一。自行研制了简易土工拉伸仪,并通过试验确定了合适的制样方法。在此基础上,对具有胀缩性的武鸣红黏土和百色膨胀土分别进行试验,探讨了其抗拉强度与干密度、含水率、干湿循环次数等影响因素的关系,研究结果表明：两种土的抗拉强度-含水率曲线在饱和度接近66%时出现峰值,该峰值对应含水率接近最优含水率,峰值两侧呈指数关系变化,抗拉强度随含水率的变化规律受土中水的形态影响。抗拉强度随干密度的增加而线性递增,增加幅度显著。抗拉强度随干、湿循环次数增加而衰减,1～2次循环时,强度衰减幅度最大,但3次循环后,趋向于一稳定值,稳定值为初始值的20%左右,土体干、湿循环后抗拉强度降低是微结构劣化的结果。     

Determination of Rock Fracture Toughness <Emphasis Type="Italic">K</Emphasis><Subscript>IIC</Subscript> and its Relationship with Tensile Strength

Hydraulic fracture propagation is greatly influenced by mode-II fracture toughness since this is one of the factors which determine whether a fracture diverts. Direct measurement of rock fracture toughness is constrained by high cost, limited number of available cores and long turn around time. Therefore, to overcome these constrains, it is necessary to develop an effective mode-II fracture toughness test which can be used in a prediction analysis for deep rock formations. Consequently, a mode-II fracture toughness test system was designed for rocks using the straight-notched Brazilian disc (SNBD) test methodology. In the experiment, this system was used to test 20 rock samples from the WG oilfield. This enabled a fracture toughness prediction model to be established, based on an analysis of the test data. H341 acoustic, density and gamma-ray logging data were used to predict horizontal stresses and rock tensile strength. When combined with the mode-II fracture toughness prediction model, continuous values were predicted, which were successfully confirmed by field fracturing practices. It was confirmed, therefore, that this successful method met the need of providing continuous fracture toughness data during field fracturing operations.     

Discrete element model for quasi‐brittle rupture under tensile and compressive loading

In the absence of initial cracks, the material behavior is limited by its strength, usually defined in homogeneous conditions (of stress and strain). Beyond this limit, in quasi‐brittle case, cracks may propagate and the material behavior tends to be well described by fracture mechanics. Discrete element approaches show consistent results dealing with this transition during rupture. However, the calibration of the parameters of the numerical models (i.e., stiffness, strength, and toughness) may be quite complex and sometimes only approximative. Based on a brittle rupture criterion, we analyze the biaxial response of uncracked samples. Thus, tensile and compressive strengths are analytically identified and become direct parameters of our discrete model. Furthermore, a physically reliable crack initiation (and subsequent propagation) is shown to be induced during rupture and verified by the simulation of three‐point bending and diametral compression tests. Copyright © 2016 John Wiley & Sons, Ltd.     

A simple model to predict the effect of volume fraction,diameter, and length of fibres on strength of fibre reinforced brittle matrix composites

A simplified model is presented to predict the strength variations of brittle matrix composites, reinforced by steel fibres, with the variations of fibre parameters—length, diameter and volume fraction. This model predicts that its tensile and flexural strength increase non‐linearly with the fibre volume fraction. It also predicts that similar non‐linear behaviour should be observed with the reduction of the fibre diameter when other parameters are kept constant. The experimental results support both these theoretical predictions. It is also explained why an increase in the fibre length does not always significantly increase the fracture toughness. The objective of this paper is not to explain and understand in great detail the science of all phenomena responsible for the strength increase of fibre reinforced brittle matrix composites, but to provide a simple engineering explanation as to why its strength increases with the fibre addition, and how this increase can be quantitatively related to the variations in fibre parameters—fibre volume fraction, fibre length and diameter. These simplifying steps are needed to provide a tool that the practicing engineers can use to predict the brittle matrix strength variation with the fibre parameters. In the area of geomechanics, the results presented here can be used to assess and predict the behaviour of fibre‐reinforced earth. Copyright © 2000 John Wiley & Sons, Ltd.     

Experimental Study of Tunis Soft Soil Improved by Deep Mixing Column

Tunis soft clay belongs to the category of problematic soils because of its weak strength characteristics, high compressibility and high content of organic matter (22%). Deep mixing column (DM) presents a well-developed technique to improve the properties of Tunis soft soils by increasing its bearing capacity and reducing settlements. This paper describes a laboratory experimental program performed to assess the effectiveness in improving the shear strength of Tunis soft clay. The two mains objectives of this research are to evaluate the effect of reinforcement by DM on the behaviour of the Tunis soft clay and to investigate parameters of influence on its strength characteristics. Five parameters are studied from the performed tests: (1) curing time: CT = 7, 14, 21 and 28 days; (2) injection pressure: IP = 50, 100 and 150 kPa; (3) water–cement ratio w/C = 0.7, 1 and 1.2; (4) rotation rate V = 15, 30 and 40 rpm and (5) consolidation pressure: б₃ = 100, 150 and 200 kPa.     

基于水压致裂法的岩石抗拉强度研究

为了揭示水压致裂法和巴西劈裂法测量岩石抗拉强度的关系,开展了理论和现场试验研究。基于经典的水压致裂法理论,推导了不同围压下钻孔破裂压力和抗拉强度。利用断裂力学理论建立了水压致裂法和巴西劈裂法测得抗拉强度的关系。利用预制切槽方法模拟天然裂纹,对水力裂缝的起裂压力进行了研究。结果表明：围压为最大主应力等于3倍最小主应力测得的抗拉强度大于围压为0测得的抗拉强度;水压致裂法和巴西劈裂法测量抗拉强度关系与应力场、裂纹长度、断裂韧度3个变量有关;通过在晋城矿区王台铺矿的预制切槽试验,运用断裂力学建立的抗拉强度计算式更为符合现场实际。研究结果可为坚硬难垮落顶板预制切槽的水力压裂设计提供参考。     

Compressibility behaviour of lime-stabilized clay

Lime stabilization reactions result in the formation of inter-particle cementation bonds that improves strength and reduce compressibility of clay soils. Though several studies in the past have focused on the impact of lime stabilization on the strength behaviour, few studies have examined the impact of lime stabilization on the compressibility behaviour of clay soils. The present study examines the compressibility behaviour of lime-stabilized soils in light of the frameworks proposed for saturated cemented clays. Cementation bonds formed during the lime stabilization reactions imparted yield stress in the range of 3900–5200 kPa to the artificially cemented specimens. The compression behaviour of these artificially cemented specimens in the pre-yield stress and post-yield stress regions conformed to the framework proposed for saturated cemented soils.     

非饱和红黏土的强度特性研究

红黏土是一种富含胶结作用物质的塑性黏土,与普通黏性土相比,红黏土的强度特性更为复杂。通过直剪仪对不同含水率和不同含丙三醇溶液的重塑红黏土进行了抗剪强度试验,发现红黏土的强度、黏聚力随着溶液含量的增加出现了“双峰”值现象,且内摩擦角基本保持不变,用丙三醇溶液替代水溶液配制的土样其强度有了明显的降低。结合核磁共振仪对红黏土试样的孔径分布研究情况,基于Bishop提出非饱和土的有效应力原理,探讨了非饱和红黏土的强度随含水率变化出现“双峰”现象的影响机制,认为红黏土本身孔径大小分布特征起了非常重要的作用。     

冻土的单轴抗压、抗拉强度特性试验研究

在寒区工程建筑物设计中,冻土的抗压、抗拉强度是两个重要的力学指标.在负温条件下,对粉质黏土、黄土和砂土进行单轴抗压和劈裂抗拉试验,研究冻土破坏时的破坏形态、破坏机理、应力-应变曲线和拉应力与径向位移关系曲线的形式,分析单轴抗压强度和劈裂抗拉强度的差异以及这两种强度随土质特性和温度的变化规律.试验结果表明：单轴载荷作用下试样破坏后呈鼓状,且表现为应变软化型塑性破坏特征;劈裂作用下产生沿直径向试样两侧延伸的裂缝,不同土质破坏后裂缝扩展的宽度和深度不同;冻土的抗压强度与抗拉强度均与负温存在很好的线性相关性,随温度的降低而增大;在相同温度条件下,冻土的抗压强度大于其抗拉强度;对于同一种冻土,其抗压强度的温度效应比抗拉强度的温度效应显著.本试验分析结果可为寒区工程的实际应用提供参考.     

Experimental studies on the effects of fracture on the P and S wave velocity propagation in sedimentary rock (“Calcarenite del Salento”)

Experimental studies were carried out in laboratory in order to investigate the effects of fracture on compressional (P) wave and shear (S) wave velocity propagation and therefore the relations between seismic properties and rock mass parameters. The discontinuity index, I_d, fracture density parameter C, linear fracture parameter Γ and the rock quality designation (RQD) were used to describe the rock mass parameters. These parameters are analyzed and then related to the seismic properties. Four vertical aligned fractures were created on an intact calcarenite block, 0.6 m long, 0.4 m thick and 0.4 m width, by sawing. The measures were carried out in four different blocks of cacarenite, having the same physical properties, and in four different phases: in first block the fractures were filled with air; in the second block the fractures were filled with “terra rossa”; in third block the fractures were filled with wet “terra rossa” and in the fourth block the fractures were filled with clay. The test results were statistically analysed using the method of least squares regression and polynomial relationships with high correlation coefficient were found between the fractured rock parameters and P-wave, S-wave velocities and V_p/V_s ratio. The investigations suggest that the P-wave and S-wave velocities decrease with increasing the fracture parameters, while the V_p/V_s ratio increases with decreasing the fracture parameters.

Furthermore the results of the experimental studies were applied on the seismic refraction tomography data acquired in a great measurements campaign undertaken in the Adriatic salentina coast (south Italy) in order to monitor the coastal erosion.

The geophysical results, using the polynomial relationships between the fractured rock parameters and P-wave velocity, are in good agreement with the geomorphological and geological results.     

Experimental and Numerical Studies on Determination of Indirect Tensile Strength of Rocks

Indirect tension tests using Brisbane tuff Brazilian disc specimens under standard Brazilian jaws and various loading arcs were performed. The standard Brazilian indirect tensile tests caused catastrophic, crushing failure of the disc specimens, rather than the expected tensile splitting failure initiated by a central crack. This led to an investigation of the fracturing of Brazilian disc specimens and the existing indirect tensile Brazilian test using steel loading arcs with different angles. The results showed that the ultimate failure load increased with increasing loading arc angles. With no international standard for determining indirect tensile strength of rocks under diametral load, numerical modelling and analytical solutions were undertaken. Numerical simulations using RFPA^2D software were conducted with a heterogeneous material model. The results predicted tensile stress in the discs and visually reproduced the progressive fracture process. It was concluded that standard Brazilian jaws cause catastrophic, crushing failure of the disc specimens instead of producing a central splitting crack. The experimental and numerical results showed that 20° and 30° loading arcs result in diametral splitting fractures starting at the disc centre. Moreover, intrinsic material properties (e.g. fracture toughness) may be used to propose the best loading configuration to determine the indirect tensile strength of rocks. Here, by using numerical outcomes and empirical relationships between fracture toughness and tensile strength, the best loading geometry to obtain the most accurate indirect tensile strength of rocks was the 2α?=?30° loading arc.     

Study on the swelling behaviour of blended clay–sand soils

Soils containing expansive clays undergo swelling that can be both detrimental and beneficial in various applications. In the Arabian Gulf coastal region, natural heterogeneous soils containing clay and sand (tills, shales, and clayey sands) support most of the civil infrastructure systems. Likewise, mixes of clay and sand are used for local earthwork construction such as roads and landfills. A clear understanding of the swelling behaviour of such soils is pivotal at the outset of all construction projects. The main objective of this paper was to understand the evolution of swelling with increasing clay content in local soils. A theoretical framework for clay–sand soils was developed using phase relationships. Laboratory investigations comprised of mineralogical composition and geotechnical index properties of the clay and sand and consistency limits, swelling potential, and morphology of clay–sand mixes. Results indicated that soil consistency of mixes of a local expansive clay and an engineered sand depends on the weighted average of the constituents. Mixes with 10% clay through 40% clay capture the transition from a sand-like behaviour to a clay-like behaviour. Influenced by the initial conditions and soil matrix, the swelling potential of the investigated mixes correlated well with soil plasticity (SP(%) = 0.16 (I _p)^1.188). The parameters sand void ratio and clay–water ratio were found to better explain the behaviour of blended clay–sand soils.     

原状冻土非线性断裂韧度测试与修正

在以往原状冻土现场试验方法的基础上,针对原状冻土的非线性断裂力学特性进行了Ⅰ型和Ⅱ型断裂力学试验。在不同深度的冻胀量、冻深与时间变化规律下严格控制试样制作及试验时的温度,利用动态数据采集系统测出试样加力点处位移与力的关系曲线,并计算出相应的非线性参数。提出了利用非线性修正因子计算原状冻土断裂韧度的方法,并算出Ⅰ型、Ⅱ型直裂纹加卸载试样的非线性断裂韧度,将其与岩石断裂力学中的塑性修正因子方法的计算结果进行了对比,二者基本一致,说明该方法可以用来计算、修正原状冻土非线性断裂韧度,从而为冻土非线性理论的研究拓宽了思路。     

Ⅰ型断裂韧度模拟方法及细观影响因素研究

既有离散元参数敏感性分析大多集中在压缩试验及巴西劈裂试验,对I型断裂韧度 试验细观影响因素及3D破裂过程系统分析的报道较少。采用三维平节理模型（FJM3D）研究微观结构参数及黏结细观参数对不同切槽形状的I型断裂韧度试验的影响。微观结构参数包括晶粒平均半径的平方根 、模型分辨率Ψ和最大/最小晶粒直径 。黏结细观参数包括平均配位数CN、S类型单元比例 、黏结抗拉强度 、黏结内聚力 、摩擦系数 和摩擦角 。参数敏感性分析结果表明, 与 、CN及 正相关,与 、 负相关,而与 、 、 和 无明显的线性关系,此外为获得较低的 波动水平,给出了参数 和 的建议范围。根据参数敏感性分析结果,校核匹配了Kowloon花岗岩直切槽半圆盘（SCB）和人字形切槽半圆盘（CCNSCB）试样的宏观力学性质。从细观角度直观、深入分析不同切槽形状I型断裂韧度试验破裂机制,得出SCB试验曲线的峰前和峰后行为与室内试验更为吻合。     

Effect of Fibercast and Fibermesh inclusion on the direct shear and linear shrinkage response of clay

Inclusion of synthetic fibers is becoming a routine task in soil reinforcement. The ability of synthetic fibers in controlling the shrinkage cracks in concrete is the main drive to consider its benefits in clay and other soil materials. The polypropylene fibers are nonbiodegradable and can perform well even in aggressive chemical exposure conditions. The direct shear testing is a popular geotechnical approach to assess the shearing strength for a range of soils. This study is aimed at investigating the effect of fiber inclusion on the direct shear response of semi-arid clay soils. This research is conducted using two different types of polypropylene fibers, viz., Fibercast and Fibermesh, having different surface properties on the shear strength envelope and parameters (angle of internal friction and cohesion). The aspect lengths were varied as 6 and 12 mm, and the dosages were varied as 0.2, 0.4, and 0.6 % by weight of the soil. The results were viewed in relation to the fiber type, size, and dose. The soil response and shear resistance measured in consolidated undrained direct shear test is presented for the targeted doses, and the results revealed useful insight compared to unreinforced. The Fibermesh material proved to be the more appropriate fiber additive to typical semi-arid clay soils. The data provides helpful guide for the design geotechnical engineers.     

“Lag” — a geochemical sampling medium for arid regions

Desert land surfaces are commonly characterized by a veneer or pavement of siliceous and/or ferruginous stony material. This material can be shown in most cases to be predominantly bedrock-derived, despite often severe modification by prolonged weathering, and can therefore be selectively sampled and analyzed as an indicator of bedrock geochemistry in geochemical surveys.Most pavements probably owe their existence to the interaction of several dispersion mechanisms. However, a common factor in all cases is the concentration of coarse particles at the surface as a result of the selective removal by erosion of fine dilutant material. Hence, the preferred name when used in the geochemical context is “lag”.In lag sampling, particles in the range 2.0–6.0 mm are screened on site from the unconsolidated surface material. Material in this size range has been found to be quite uniformly distributed over a wide range of arid region environments, including areas where residual soils are severely diluted by transported alluvial and aeolian materials.Data from exploration programmes for Au, Cu-Pb-Zn-Ba, and Ni allow comparison of results for lag sampling with those for alternative sample media in a variety of arid region environments. Analysis of lag samples for Au, Cu and As clearly indicates the presence of bedrock Au mineralization in the Paterson and Eastern Goldfields Provinces of Western Australia. In these areas both lags and soils exhibit good anomaly contrast, but lags show more extensive lateral dispersion, leading to advantages in reconnaissance exploration.Strong anomalies for Ni and Cu are developed in lags, compared with subdued response in fine-fraction soils over a Ni sulphide occurrence in the Eastern Goldfields which has been subjected to deep lateritic weathering.Lag geochemistry also clearly reflects sub-economic base metal and barite occurrences in the McArthur Basin, N.T., in spite of the dilution of surface soils by sands probably related to a Mesozoic marine incursion. Orientation sampling over a Pb-Zn prospect in the Pine Creek Geosyncline has demonstrated optimum response in lag samples compared with various size fractions of the associated lithosols.Variable dilution of lag samples by coarse quartz sand can be a problem in areas with substantial transported overburden. A simple procedure to ‘correct’ trace-element values using regression analysis based on the Fe content of samples is described as a means of reducing ‘noise’ resulting from such matrix variations.     

黏土的张拉-剪切耦合强度研究

现有的土体强度研究主要集中在土体受到压缩剪切作用,而针对单轴拉伸、拉剪耦合或多向拉伸作用的研究较少。当前土的联合强度理论仍建立在土体受压缩剪切作用的基础上,不适用于分析土的抗拉强度和拉剪耦合强度。针对饱和黏土中的拉剪联合作用,建立了饱和黏土的张拉-剪切耦合强度模型,给出了饱和黏土张拉-剪切耦合强度公式以及多向拉伸作用下的强度公式;基于已有的饱和黏土试验数据以及所做的饱和黏土室内试验数据对所建立的强度模型预测能力进行了验证,并与其他联合强度模型进行对比分析,检验了强度模型的适用性和准确性;强度模型全面、合理地描述了饱和黏土的压剪强度、张拉-剪切耦合强度及多向拉伸强度特征,为饱和黏土张拉-剪切耦合问题的解决奠定了理论基础。