Investigation of the hydro-mechanical behaviour of compacted expansive clay

The hydro-mechanical behaviour of compacted expansive Romainville clay was investigated. The soil was air-dried, crushed, and passed through a 2 mm sieve before being statically compacted to a dry density of 1.35 Mg/m³. The mechanical behaviour was investigated by tests in oedometer with controlled suction using the vapor equilibrium technique (suction s = 0, 9, 39, and 110 MPa). The vertical stress was applied in the range of 0–800 kPa. The experimental results are shown as follows: 1) wetting-induced swelling was higher at lower vertical stresses; 2) the vertical stress under which no swelling occurred during water flooding was estimated at 60 kPa, which can be considered as the swelling pressure of the soil tested; 3) the soil compressibility (changes of volume upon stress increases) was strongly influenced by the soil suction: the lower the suction, the higher the compressibility. The hydraulic behaviour was investigated using a large-scale inltration chamber (800 mm × 1000 mm in section and 1000 mm high). The large size of the soil column allowed burying the volumetric water content sensors (ThetaProbe) without signicantly affecting the water transfer and the soil swelling during inltration. The soil suction was monitored along the soil height (every 100 mm) using various relative humidity sensors and psychrometers. In the inltration test, water was kept on the soil surface and changes in suction and volumetric water content were monitored for 338 d. The wetting front has reached the bottom of the soil column at the end of the test. The data from the simultaneous monitoring of suction and water content were used to determine the water retention curve and the unsaturated hydraulic conductivity using the instantaneous prole method. It has been observed that the soil water retention curve depends on the soil depth; that is to be related to the soil depth-dependent swelling. The unsaturated hydraulic conductivity was found to be quite low, comprised between 3 × 10⁻¹¹ m/s (at saturated state) and 10⁻¹⁴ m/s (at about 100 MPa suction).     

Investigation of the hydro-mechanical behaviour of compacted expansive clay

The hydro-mechanical behaviour of compacted expansive Romainville clay was investigated. The soil was air-dried, crushed, and passed through a 2 mm sieve before being statically compacted to a dry density of 1.35 Mg/m³. The mechanical behaviour was investigated by tests in oedometer with controlled suction using the vapor equilibrium technique (suction s = 0, 9, 39, and 110 MPa). The vertical stress was applied in the range of 0–800 kPa. The experimental results are shown as follows: 1) wetting-induced swelling was higher at lower vertical stresses; 2) the vertical stress under which no swelling occurred during water flooding was estimated at 60 kPa, which can be considered as the swelling pressure of the soil tested; 3) the soil compressibility (changes of volume upon stress increases) was strongly influenced by the soil suction: the lower the suction, the higher the compressibility. The hydraulic behaviour was investigated using a large-scale infiltration chamber (800 mm × 1000 mm in section and 1000 mm high). The large size of the soil column allowed burying the volumetric water content sensors (ThetaProbe) without significantly affecting the water transfer and the soil swelling during infiltration. The soil suction was monitored along the soil height (every 100 mm) using various relative humidity sensors and psychrometers. In the infiltration test, water was kept on the soil surface and changes in suction and volumetric water content were monitored for 338 d. The wetting front has reached the bottom of the soil column at the end of the test. The data from the simultaneous monitoring of suction and water content were used to determine the water retention curve and the unsaturated hydraulic conductivity using the instantaneous profile method. It has been observed that the soil water retention curve depends on the soil depth; that is to be related to the soil depth-dependent swelling. The unsaturated hydraulic conductivity was found to be quite low, comprised between 3 × 10⁻¹¹ m/s (at saturated state) and 10⁻¹⁴ m/s (at about 100 MPa suction).     

等向应力下原状黄土的压缩及增湿变形特性研究

 用非饱和土三轴剪切渗透仪,在等向应力条件下对原状黄土进行增湿–加载,加载–增湿及加载–增湿–加载3个系列加载增湿路径试验,分析吸力对压缩变形和加载屈服特性的影响,探讨增湿时应力对变形及屈服特性的影响,通过对单线法及双线法的试验结果对比,确认原状黄土的增湿体积变形与加载增湿路径有关,加载及增湿屈服线不具有唯一性,进而提出等向应力条件下原状黄土的弹塑性体变模型。研究结果表明：吸力及应力分别对屈服前压缩及增湿变形特性指标几乎没有影响,而对屈服后的指标皆有明显的影响;单线法与双线法确定的增湿体积变形皆随应力的增大而先增大后减小,峰值点处应力与吸力丧失程度及加载增湿路径无关,且近似等于天然状态土样的初始屈服应力;增湿变形与加载增湿路径有关,单线法的值比双线法确定的值小,差值随增湿程度增大而减小,增湿至饱和时2种方法确定的湿陷变形近似相等。对于相同的塑性体应变,吸力减小屈服线位于加载屈服线之下方,二者随塑性体应变的增大而耦合联动扩大。提出的弹塑性体变模型可以较好地预测不同吸力下的压缩变形,比采用唯一加载湿陷屈服线的模型更好地预测不同应力下增湿变形。     

南阳膨胀土变形与强度特性的三轴试验研究

对南阳膨胀土做了4种应力路径的三轴试验，即控制吸力的各向同性压缩试验、控制净平均应力的收缩试验、控制净平均应力的膨胀试验、同时控制净室压力和吸力的排水剪切试验，结果表明：膨胀土各向同性压缩屈服特性仍可用一般非饱和土的LC屈服线描述；不同围压下的收缩路径的屈服吸力随围压增大而减小，且屈服后的收缩指数接近各向同性压缩屈服后的压缩指数；非饱和状态下的内摩擦角近似等于饱和状态下的有效内摩擦角，总粘聚力随吸力增加而线性增大；在各向同性压缩试验和三轴剪切试验过程中，土样含水量随净平均应力的变化都近似为线性关系。     

OPHELIE回填混合物的热-水-力耦合特征

在以比利时高放废物处置参考设计方案的OPHELIE地面大型试验的框架为背景下，进行了一套完整的固结试验项目以获取回填材料的热-水-力反应特征。缓冲材料是预先经高压压实的土块，其主要成分为60%的FoCa泥岩，35%的砂以及5%的石墨（干燥重量），除了对温度和吸力控制设备的简要描述之外，还将阐述有代表性的试验结果，包括持水特性，不同应力，不同吸力，不同温度下的压缩性，膨胀性能，以及在吸湿，吸热条件下的塌陷现象。     

Experimental research on water retention and gas permeability of compacted bentonite/sand mixtures

Highly compacted bentonite-based materials are often considered as buffer or sealing materials for deep high-level radioactive waste repositories. In situ, the initial state of bentonite-based materials is only partially saturated, which has a very high suction that will promote water absorption from the host rock. In addition, a gradient of water saturation will be formed between the external part and the central part of the compacted bentonite blocks. In this paper, water retention tests, under both constant-volume and free-swelling conditions, were performed to investigate the suction behavior of a compacted bentonite/sand mixture. In order to investigate the sealing ability of the partially saturated bentonite/sand mixture, gas permeability tests were also carried out under the in situ confining stress. It was found that the confining conditions have a limited effect on the water retention capacity of the compacted bentonite/sand mixture at lower levels of relative humidity (RH), while this influence is significant at higher RH levels. The results of gas permeability tests show that gas permeability is very sensitive to the water content and the confining pressure. When the sample (stable at RH=98%) was subjected to a in situ confining pressure (7–8 MPa), the gas permeability was very low (1.83×10^–14 m/s) which indicates that gas tightness can be obtained even though the sample is not fully saturated.     

Visko‐elastisches Verhalten von Holz‐Klebstoff‐Verbindungen unter zyklischer Zug‐Druck‐Belastung

Visco‐elastic behavior of bonded wood under cyclic tensile and compression loading In the present contribution, a test method is used to simulate the static, cyclic loading of adhesive joints due to swelling and shrinking of the wood and to demonstrate the plastic deformation in the low load range. For tensile shear specimens prepared from beech wood and bonded with three different adhesives (MUF, PRF, PUR), the elastic behavior under cyclic tensile and compression loading was investigated and the loss and storage of energy was determined. All tested adhesives showed viscose parts even at a very low load level of 3 MPa. At a load level of 7 MPa, the PRF joints revealed a more elastic behavior than the other. The increased loss energy determined for the PUR bonding indicates a softening of the adhesive joint.     

侧限状态下高压实高庙子膨润土非饱和渗透性的试验研究

高放废弃物深地质处置库中,由高压实膨润土形成的人工屏障,起着阻障围岩中地下水渗入内库并引起核素迁移以及库内高放废物的辐射扩散的作用。因此,有必要研究侧限状态下高压实膨润土的非饱和渗透性能。采用瞬时截面法试验研究了侧限状态下高压实高庙子膨润土的非饱和渗透特性。结果表明:侧限状态下,干密度为1.7g/cm3的高压实高庙子膨润土的非饱和渗透系数,数值为1.13×10-13～8.41×10-15m/s,且与土中吸力呈非单一增减关系。当土中吸力约为68MPa时,非饱和渗透系数最小;吸力大于68MPa时,其非饱和渗透系数随着吸力的增加而增加;而当吸力小于68MPa时,非饱和渗透系数随着土中吸力的增加而减小。     

The effects of technological voids on the hydro-mechanical behaviour of compacted bentonite–sand mixture

Compacted bentonite-based materials are often used as buffer materials in radioactive waste disposal. A good understanding of their hydro-mechanical behaviour is essential to ensure disposal safety. In this study, a mixture of MX80 bentonite and sand was characterised in the laboratory in terms of water retention property, swelling pressure, compressibility and hydraulic conductivity. The effects of the technological voids or the voids inside the soil were investigated. The technological voids are referred to as the macro-pores related to different interfaces involving the buffer material, whereas the voids inside the soil are referred to as common macro-pores within the compacted bentonite/sand mixture. The results obtained show that at high suction, the amount of water absorbed in the soil depends solely on suction, whereas at low suction it depends on both suction and the bentonite void ratio. There is a unique relationship between the swelling pressure and the bentonite void ratio, regardless of the sample nature (homogeneous or not) and the sand fraction. However, at the same bentonite void ratio, a higher hydraulic conductivity was obtained on the samples with technological voids. The effect of sand fraction was evidenced in the mechanical yield behaviour: at the same bentonite void ratio, the bentonite–sand mixture yielded at a higher pre-consolidation stress.     

干密度对非饱和黄土基质吸力的影响

为了研究起始湿度、天然密度以及竖向荷载对吸力特性的影响,对西安原状黄土进行不同起始湿度、密度条件下的常含水量压缩试验,提出无应力作用下同时反映初始含水率和天然干密度的基质吸力表达式。研究结果表明:天然干密度、初始含水率、应力相关的干密度对基质吸力的影响程度都与试样的起始湿度有关,起始湿度小于试样塑限含水量时,影响都比较大,起始湿度大于试样塑限含水量时影响都很小。净竖向荷载对吸力特性的影响与起始湿度、天然密度有关,净竖向荷载小于等于400 kPa时,对吸力的变化影响很小,可不考虑净竖向荷载影响,可用未受荷载作用时的SWCC曲线来描述原状试样的吸力特征;净竖向荷载大于400 kPa时,对吸力的变化影响较为显著。视起始湿度和天然密度的不同,净竖向应力相关的干密度增大引起基质吸力非单调增减或单调下降,天然干密度的增大引起基质吸力单调上升。     

干湿循环过程中膨胀土的胀缩变形特征

为了了解干湿循环过程中膨胀土的胀缩变形特征,分别开展了两组干湿循环试验。在控制吸力干湿循环试验中,吸力控制范围为 0.4 ～ 262 MPa ,采用了两种吸力控制方法,分别为渗析法（吸力＜ 4 MPa ）和蒸汽平衡法（吸力＞ 4 MPa ）,当每一级吸力达到平衡时,测量试样对应的含水率和体积;在常规干湿循环试验中, 采取了两种干缩路径,分别为全干燥和部分干燥,并测量试样在每次干湿循环过程中的轴向变形及循环结束后的含水率。结果表明： 在脱湿和吸湿过程中,试样孔隙比随吸力变化可分 3 个典型阶段：大幅变化阶段（ 0.4 ～ 9 MPa ）、过渡阶段（ 9 ～ 82 MPa ）和平缓阶段（ 82 ～ 262 MPa ）; 当吸力大于 113 MPa 时,试样的胀缩变形基本可逆,而当吸力小于 113 MPa 时,试样的胀缩变形 表现出明显的不可逆性,且不可逆程度随吸力的减小而增加。试样在常规干湿循环过程中的胀缩变形随循环次数的增加逐渐趋于稳定;胀缩特征受干缩路径的影响非常明显,全干缩路径中测得的膨胀率高于部分干缩路径,膨胀速率随干湿循环次数的增加而增加;试样在干湿循环过程中的膨胀率大小在一定程度上取决于吸湿能力。     

高放废物处置库中COx黏土岩回填材料压缩特性研究

 高放废物处置库中回填材料需要预先压缩到一定密实度,其压缩特性对处置库的设计和安全性有重要影响。目前,粉碎后的Callovo-Oxfodian(COx)黏土岩(法国)被考虑为可选的回填材料之一。通过开展一维压缩试验,研究2种不同粉碎工艺下获得的粗/细COx土样的压缩特性,结果表明：压缩曲线受粒度成分的影响非常明显,为获得同等压实度,细粒土所需的压实功能较粗粒土高,除此之外,细粒土的压缩指数也高于粗粒土,表现出较强的压缩性;随着土样压实密度的增加,粗/细土样的压缩曲线逐渐靠拢,粒度成分对压实功能影响逐渐减弱;土样卸荷时回弹指数随干密度的增加而增加,受粒度成分的影响不明显;高压实(rd = 2.0 g/cm3)粗粒土样在7 MPa的轴向应力下饱和时,体积发生明显的塌陷现象,饱和后土样的压缩指数小于饱和前,而回弹指数则较饱和前高。     

圆饼状高压实膨润土膨胀力各向异性特征研究

针对作为我国缓冲/回填材料的高压实高庙子膨润土，采用自主研制的膨胀力-渗透性一体化测试仪器开展膨胀力试验，分别测定圆饼状试样的轴向膨胀力和径向膨胀力发展规律，研究试样初始干密度、初始吸力和尺寸效应对其膨胀力各向异性特征的影响。试验结果显示，轴向膨胀力和径向膨胀力均随着试样初始干密度的增加呈指数增大，膨胀力各向异性特征愈加显著。轴向膨胀力和径向膨胀力均随着试样初始吸力的增加而呈减小趋势；相同初始吸力条件下，径向膨胀力均大于轴向膨胀力，但是膨胀力各向异性系数随着初始吸力增大而不断减小。试样尺寸对膨胀力有显著影响，增加试样高度，轴向膨胀力不断增加，径向膨胀力增加到某一数值后不再随高度增加，膨胀力各向异性系数随着试样高度的增加逐渐减小。最后，基于膨润土膨胀机理、压实效应和尺寸效应，分析了膨胀力各向异性特征的形成机理。     

干湿循环作用下非饱和重塑黏土变形特性研究

为了研究了基质吸力与干湿循环对非饱和重塑黏土压缩变形特性的影响,利用最新研制的全自动非饱和土固结仪,在控制基质吸力的条件下进行非饱和土压缩试验。研究结果表明,吸力越大,土体的压缩性越小,屈服应力越大,含水率减小的幅度也越小。试样经历干湿循环后,土体的压缩性增大,屈服应力减小,土体弹性变形的应力范围也减小。非饱和土的压缩变形以孔隙气体的压缩为主,土体中孔隙水的排出处于次要地位。干湿循环对非饱和土变形特性有重要影响,考虑其影响对于实际工程应用是十分必要的。     

Elastic-plastic collapse of non-uniform cylindrical shells subjected to uniform external pressure

The objective of this paper is to derive analytical solutions for the elastic buckling and plastic collapse pressures of a cylindrical shell with reduced thickness over part of its circumference. The section of reduced thickness is used to represent a corroded region in a pipe. The proposed solutions are extensions of Timoshenko's solutions for the elastic-plastic collapse of a linear elastic, perfectly plastic cylindrical shell subjected to uniform external pressure. A modified interaction formula for the fully plastic membrane forces and bending moments in the non-uniform cylinder has been proposed for plastic collapse. A parametric study shows that the elastic buckling pressure decreases smoothly with corrosion angle when the corrosion depth is less than 0.5t. When the corrosion depth is greater than 0.5t, the elastic buckling strength first decreases very rapidly with corrosion angle. Furthermore, the elastic buckling pressure decreases uniformly with corrosion depth when the corrosion angles are greater than 30°, while the elastic buckling strength decreases more rapidly at higher corrosion depths when corrosion angles are less than 30°. Another parametric study on a steel pipe shows that the initial and fully plastic yield pressures both decrease monotonically with corrosion depth for a given corrosion angle and imperfection.     

热–力耦合作用下黏性土体积变形特性试验研究

采用温控三轴仪,对饱和黏性土在热–力耦合作用下的体积变形特性进行了研究。试验中共考虑了温度循环后的应力加卸载试验、升高或降低不同温度后的应力加卸载试验3种方案。结果表明,黏性土温度体积变形的大小与围压无关。温度循环会造成黏性土塑性体积变形持续发展,但单周塑性应变增量随温度循环周数的增加而减小,体现了温度历史的影响。将土样继续加载后,温度的历史效应将被覆盖,后续温度循环中产生的塑性体积应变量值及发展规律与初始循环下的类似。不同温度下土体的压缩指数和回弹指数基本不变。同一塑性体积应变下,屈服应力随温度的增加而减小。温度变化引起的塑性体积应变也会造成屈服应力提高,可近似采用与力学塑性体积应变硬化一致的规律进行描述。     

非饱和原状黄土的变形及屈服特性试验研究

对不同吸力非饱和原状黄土进行了等应力比三轴压缩及常规三轴剪切路径下的试验,研究了偏应力比q/p(q及p分别为净平均应力及偏应力),吸力及净围压对不同应力路径下变形及屈服特性的影响。研究结果表明:在等应力比三轴压缩及常规三轴剪切过程中,孔隙比e与净平均应力p关系(e-lnp)皆位于二条相交的直线上,吸力、偏应力比及净围压对弹性指数的影响很小,对屈服应力及压缩指数的影响较大。等应力比三轴压缩路径下,偏应力比一定时,屈服应力及压缩指数皆随吸力的增大而增大;吸力一定时,随偏应力比的增大而先增大后减小。常规三轴剪切路径下,吸力一定时,屈服应力及压缩指数皆随净围压的增大而增大;净围压一定时,随吸力的增大,屈服应力增大,压缩指数减小。以塑性体应变作为硬化参数时,q-p平面上的初始及后继屈服面皆为对称于饱和土K₀线的倾斜椭圆,吸力及应力增大时,屈服面没有旋转,而是产生等向扩大。     

Behavior and characteristics of compacted expansive unsaturated bentonite-sand mixture

Limited studies dealt with the expansive unsaturated soils in the case of large-scale model close to the field conditions and therefore,there is much more room for improvement.In this study,expansive(bentonite-sand(B-S) mixture) and non-expansive(kaolin) soils were tested in different water contents and dry unit weights chosen from the compaction curve to examine the effect of water content change on soil properties(swelling pressure,expansion indices,shear strength(soil cohesion) and soil suction)for the small soil samples.Large-scale model was also used to show the effect of water content change on different relations(swelling and suction with elapsed time).The study reveals that the initial soil conditions(water content and dry unit weight) affect the soil cohesion,suction and swelling,where all these parameters slightly decrease with the increase in soil water content especially on the wet side of optimum water content.The settlement of each soil at failure increases with the increase in soil degrees of saturation since the matric suction reduces the soil ability to deform.The settlement observed in B-S mixture is higher than that in kaolin due to the effect of higher swelling observed in B-S mixture and the huge amount of water absorbed which transformed the soil to highly compressible soil.The matric suction seems to decrease with elapsed time from top to bottom of tensiometers due to the effect of water flowing from top of the specimen.The tensiometer reading at first of the saturation process is lower than that at later period of saturation(for soil sample B-S3,the tensiometer #1 took 3 d to drop from 93 kPa to 80 kPa at early stage,while the same tensiometer took 2 d to drop from 60 kPa to 20 kPa).     

Numerical analysis on mechanical behavior of steel-concrete composite beams under fire

This paper investigated the fire behavior of steel-concrete composite beams (SCB) and partially encased steel-concrete composite beams (PEB) through numerical analysis. The numerical models established by the software ABAQUS were verified against experimental results. Parametric studies were performed to study the influences of load ratio, strength of concrete and steel, width of concrete slab, size of steel beam, fire protection layer, and degree of shear connection on the fire behavior of SCB and PEB. The analysis results show that the deformation stages of SCB and PEB under fire both go through four stages: elastic, elastic–plastic, plastic small deformation, and plastic large deformation. The web of SCB experiences a tension–compression–tension process under fire, and the bottom flange of PEB may even change from tension to compression at a lower load ratio. The failure mode of PEB, whether the concrete is crushed, depends on the load ratio. When SCB fails, the concrete is crushed and only the bottom flange of the steel beam yields. Under various parameters, the fire resistance of SCB is about 22 min, while the fire resistance of PEB is 82–93 min under a load of 0.4. When the load ratio increases from 0.2 to 0.6, the fire resistance of SCB decreases by 8 min, while that of PEB decreases by 110 min. To meet class I fire resistance rating under a normal service load ratio of 0.4, additional measures for PEB are still required, and at least 15 mm of fire protection layer is required for the steel beam of SCB. Finally, considering the temperature internal fore, a coefficient related to the fire time was introduced to modify the formula of ultimate flexural capacity of SCB and PEB, which showed good accuracy.     

On the Yielding and Plastic Compression of Sand

This paper presents an analysis of the yielding and plastic hardening of uniformly-graded samples of a silica sand subjected to one-dimensional normal compression. Single grains of silica sand have been compressed diametrically between flat platens to measure indirectly tensile strength. Approximately 30 grains were tested for each of the following nominal particle sizes: 0.5 mm, 1 mm and 2 mm diameter. It was found that the data could be described by the Weibull statistics of brittle ceramics, and the Weibull modulus could be taken to be about 3.1. Uniform aggregates of the same sand were then compacted to maximum density and subjected to one-dimensional compression. The initial particle size distributions were 0.3-0.6 mm, 0.6-1.18 mm and 1.18-2 mm, and aggregates were subjected to stresses of up to 100 MPa. All particles were initially of similar shape, and hence the initial voids ratios of the aggregates at maximum density were approximately equal. The yield stress was defined to be the point of maximum curvature on a plot of voids ratio against the logarithm of effective stress, and found to increase with decreasing particle size, and to be approximately proportional to the tensile strength of the constituent grains. However, the plastic compressibility index was found to be approximately constant and independent of the initial grading, and a fractal distribution of particle sizes appeared to evolve under increasing stress. There is evidence to suggest that the aggregates evolve towards a fractal dimension of 2.5 under high stresses.