A creep model for ultra-deep salt rock considering thermal-mechanical damage under triaxial stress conditions

To investigate the specific creep behavior of ultra-deep buried salt during oil and gas exploitation,a set of triaxial creep experiments was conducted at elevated temperatures with constant axial pressure and unloading confining pressure conditions.Experimental results show that the salt sample deforms more significantly with the increase of applied temperature and deviatoric loading.The accelerated creep phase is not occurring until the applied temperature reaches 130℃,and higher temperature is beneficial to the occurrence of accelerated creep.To describe the specific creep behavior,a novel three-dimensional(3D)creep constitutive model is developed that incorporates the thermal and mechanical variables into mechanical elements.Subsequently,the standard particle swarm optimization(SPSO)method is adopted to fit the experimental data,and the sensibility of key model parameters is analyzed to further illustrate the model function.As a result,the model can accurately predict the creep behavior of salt under the coupled thermo-mechanical effect in deep-buried condition.Based on the research results,the creep mechanical behavior of wellbore shrinkage is predicted in deep drilling projects crossing salt layer,which has practical implications for deep rock mechanics problems.

Determination of Elastic and Creep Properties of Thin-film Systems from indentation Experiments

Based on the detailed computer simulation of the indentation testing on the thin-film systems, the present paper explores the detailed procedure of determining elastic properties (elastic modulusE^(f) and Poisson ratio v(f)) and creep parameters (CCREEP^(f) and nCREEP^(f)) for a simple Norton law (ε=CCREEP^(f)σ^n CREE^(f), where e is creep strain rate, and a is the stress) material for a thin film coated on a creep substrate, whose elastic properties(E^(s) and v^(s)) and creep properties (CCREEP^(s) and nCREEP^(s)) of the substrate are known, from indentation elastic and creep testing,respectively. The influences of the thickness of the thin-film and the size of the indenter on the indentation behavior have been discussed. It is shown that the boundary between the thin film and the substrate has great influence on the indentation creep behavior. The relative sizes of indentation systems are chosen so that the behavior of the indentation on the film is influenced by the substrate. The two elastic parameters E^(f) and v^(f) of the film are coupled on the influence of the elastic behavior of indentation. With the two different size indenters, the two elastic parameters E^(f) and v^(f) of the film can be uniquely determined by the indentation experimental slopes of depth to applied net section stress results. The procedure of determining of the two Norton law parameters CCREEP^(f) and nCREEP^(f) includes the following steps by the steady indentation rate d. The first step to calculate the creep indentation rate on certain loads of the two different sizes of indenters on a set of assumed values of CCREEP^(f) and nCREEP^(f)Then to build relationship between the creep indentation rate and the assumed CCREEP^(f) and nCREEP^(f) With the experimental creep indentation rate to intersect two sets of which have the same values of d. The last step is to build the CCREEP^(f) and nCREEP^(f)curves from the intersection points for the two indenters. These two curves CCREEP^(f) and nCREEP^(f)

The slope creep law for a soft rock in an open-pit mine in the Gobi region of Xinjiang, China

The lithology of the strata in the Gobi region of the Xinjiang autonomous region of China is mainly composed of mudstone,silty mudstone,and other soft rocks.Because of the low strength of the rock mass and the serious effects of physical weathering in this area,the slope stability in open-pit mines is poor,and creep deformation and instability can readily occur.Taking the Dananhu No.2 open-pit mine as a typical example,the creep test of a mudstone sample under different stress levels was studied.Then,based on a bottom friction experiment and a FLAC3D numerical simulation,the deformation and failure processes of the slope were analyzed.The stress–displacement curve and the displacement–time curve for the monitoring points were plotted to obtain the relationship between the stress and displacement for the slope of the soft rock.The results showed that the long-term strength of the mudstone was between 8.0 and 8.8 MPa,and that stable creep occurred when the slope was under low stress.The potential failure mode for this type of slope is that the front edge creeps along the weak layer and then a crack is formed at the trailing edge of the slope.When the crack penetrates the weak layer,cutting bedding and bedding sliding occur.The deformation process of the stable creep slope includes an initial deformation stage,an initial creep stage,a constant velocity creep stage and a deceleration creep stage.

高温作用后的北山花岗岩时效变形特征研究

以甘肃高放废物重点预选场址的北山花岗岩为研究对象,开展不同热损伤和围压条件下的三轴分级蠕变试验,分析热力耦合作用下的北山花岗岩蠕变变形机理。试验结果表明,在单轴条件下,北山花岗岩的微裂纹密度随热处理温度升高而逐渐增加,进而导致轴向峰值应变持续增大。150℃和300℃温度荷载造成的轻微热开裂引发了裂纹尖端钝化,从而增强了热损伤花岗岩的长期强度;当热处理温度达到600℃后,石英相变产生的差异性热膨胀导致北山花岗岩的热裂纹密度显著增加,进而造成长期强度急剧降低。相比之下,在5 MPa和25 MPa围压的三轴蠕变试验中,静水压力能够愈合较低温度荷载造成的热开裂,使150℃和300℃热损伤花岗岩的轴向峰值变形和长期强度恢复至无损岩石水平。然而,上述静水压力未能完全愈合600℃热处理温度造成的热开裂,热损伤花岗岩的残余微裂纹导致轴向峰值应变增大,并造成了裂纹尖端的钝化效应,最终导致北山花岗岩的长期强度增大。

Triaxial mechanical creep behavior of sandstone

Based on laboratory results of time-dependent mechanical behavior tests,we investigated short-term and mechanical creep behavior of sandstone,observed in conventional triaxial compression experiments at room temperature,using a servo-controlled rheology testing machine.Given our short-term experimental test results,we confirmed deviatoric creep stress levels of sandstone.Multiple deviatoric stress levels were applied in steps to each sample.Each deviatoric stress level before the final failed deviatoric stress was maintained for 48 h or longer.Time-dependent variations of axial strains of sandstone samples are discussed and evaluated.During the creep tests,complete tertiary creep curves of sandstone were observed under failed deviatoric stress levels with different confining pressures.Slices of coal in sandstone samples can lead to distinct tertiary creep deformation failure.

Aging Coefficient, Creep Coefficient and Extrapolating Aging Coefficient from Short Term Test for Sealed Concrete

Aging coefficient and creep coefficient are important parameters for creep analysis of any structure. With the aim to obtain those parameters, an experimental investigation is carried out on sealed concrete. Altogether ten specimens were tested, out of which four were for creep, two for shrinkage and remaining four for relaxation test. A year of relaxation test and two years of creep test results were analyzed to compute aging coefficient and creep coefficient. From regression analysis of observed and calculated aging coefficient using formula of Bazant and Kim, modification for aging coefficient is performed and extrapolation equations are generated. Instead of ACI model, B3 model has been used to obtain compliance function as the parameters of B3 model seems more relevant to creep mechanism of concrete compared to that of ACI model.

Modification of B3 Model for Sealed Concrete with Additives Based on Experimental Observations

Thirteen specimens were tested out of which nine were for creep of sealed concrete and four for shrinkage test,for a period of 700 d under controlled temperature condition.The experimental results for creep and shrinkage were compared with creep and shrinkage computation model B3 and distinct discrepancies between observed and calculated creep and shrinkage strains were observed.Based on regression analysis,modification on B3 model has been formulated which will be applicable at least for concrete of characteristics strength of C40 and C50 with additives.Besides,on the basis of observation on identical specimens with varied stress strength ratio,a function is generated which accounts effect of stress strength ratio on creep.Finally,Civil Engineering community is suggested not to follow the creep prediction models without correction at least for modern concrete,as they do not account the effect of additives on its compliance function.

加载龄期对机制砂自密实混凝土徐变性能影响

【目的】机制砂代替天然砂配制自密实混凝土已成为工程建设的发展趋势,徐变是混凝土的固有特性,为研究加载龄期对机制砂自密实混凝土徐变性能的影响,【方法】对7 d、14 d、28 d三种不同加载龄期的混凝土试件进行了压缩徐变试验,探究了徐变随加载龄期的发展规律,并通过LINK3D非线性软件对不同加载龄期和石粉含量的混凝土徐变进行数值模拟,分析了加载龄期和石粉含量对机制砂自密实混凝土徐变的影响。【结果】结果表明:加载龄期增大会减小机制砂自密实混凝土的徐变,持荷250 d时,14 d、28 d加载龄期的徐变系数是7 d龄期的82.5%、71.5%;LINK3D可以较好地模拟混凝土的徐变试验,当石粉含量为5%时,填充效应的正作用使徐变变形减小,石粉含量大于10%时,随石粉含量增加徐变增大;模拟得到的不同加载龄期的单位徐变比在普通混凝土的范围内。【结论】机制砂中石粉含量宜在10%以内;当徐变对结构性能影响较大时,可增大养护龄期。

预应力混凝土梁上拱度试验研究

结合武汉绕城高速公路的桥梁施工工程 ,选取 30 m预应力混凝土 T梁作为研究对象 ,通过开展对梁内应变的观测和梁上拱度的长期测量观测的试验研究 ,探讨了混凝土收缩、徐变和温度等因素对预应力混凝土梁上拱度的影响以及预应力混凝土梁上拱度随时间的变化规律 ,为预应力混凝土梁桥的设计和施工提供了参考依据 .

深部灰岩单轴蠕变特性试验研究

采用MTS815液压伺服岩石试验系统和DISP-2声发射监测系统,对重庆市某煤矿深部灰岩进行了单轴蠕变压缩实验,对单轴蠕变实验过程中伴随的声发射特征进行了研究,认为岩石的声发射特征能很好的表征岩石整个蠕变变形过程,包括岩石微观结构如晶格缺陷的扩散、孔隙裂隙的张合、粒间协调变形及微观破裂的产生、扩散贯通等岩石蠕变特征。同时对实验曲线进行了不同的模型参数的反演,认为用对数函数型及指数函数型模型均能很好的表征该类灰岩蠕变变形性能,且二类模型中对数模型为最佳模型。

盐岩蠕变特性的试验研究

盐岩蠕变特性会因矿物组成成分、加载应力水平的不同而异。通过对钙芒硝盐岩及氯化钠盐岩在不同载荷作用下多于100 d的蠕变试验研究发现:(1)在相同载荷7.0 MPa作用下,不同矿物成分盐岩的蠕变特性不同,钙芒硝盐岩的蠕变速率仅为氯化钠盐岩的3.67%,分别为8.72×10-6和2.38×10-4 d-1,二者有2个数量级之差;(2)在不同载荷作用下,盐岩的蠕变特性不同。在7.0 MPa载荷作用下,盐岩试件的蠕变速率为2.38×10-4/d;而在4.0 MPa载荷作用下,试件的蠕变速率为3.77×10-5 d-1,仅为前者的15.87%,表现出盐岩蠕变明显的应力效应特征;(3)在7.0和12.0 MPa两种不同载荷作用下,钙芒硝盐岩的蠕变率分别为8.72×10-6 d-1和1.12×10-5 d-1,后者为前者的1.28倍,与变形量比例相一致。最后,通过分析建立盐岩瞬态蠕变和稳态蠕变的耦合本构方程,该方程拟合曲线与试验结果曲线吻合较好。所获试验结果及本构方程可对层状盐岩矿床内建造油气储库的稳定性分析提供一定的参考依据。

充填黄土压缩蠕变特性与湿陷性试验研究

利用美国MTS815岩石力学伺服试验系统，测得充填黄土在0～7．5MPa范围内的σ-ε曲线、压实度-应力曲线以及4种不同应力条件下的蠕变曲线；并根据不同应力条件下黄土的蠕变特征，回归了ησ-σ曲线及其方程。用双线法湿陷试验得到了黄土5种不同埋深条件下的湿陷系数筑与δs荷P的关系曲线，并对压实过程中黄土的湿陷机理进行了分析，总结了黄土的疋峰值δs埋深的关系曲线，得到了其回归方程。结合曹村煤矿的采矿地质条件，计算出了充填黄土的最大下沉量和最终压缩系数，为工程应用提供了依据。

新老混凝土组合梁混凝土收缩徐变试验研究

进行了预应力作用下新老混凝土组合梁混凝土收缩徐变的试验研究,包括组合梁的挠度-时间曲线、混凝土应变-时间曲线及预应力损失的测定.试验结果表明,由于混凝土的收缩徐变,在预应力作用下组合梁的挠度、混凝土应变、新老混凝土界面间的应变差、截面曲率随时间的增长明显增加.虽然界面间的混凝土应变差值在逐渐增加,但是滑移值较小,界面间的粘结可靠.预应力前期的损失较大,随着时间的增长,损失逐渐减小.采用规范给定的混凝土收缩徐变模型分析了预应力损失.在试验研究的基础上采用随时间变化的换算弹性模量建立了新老混凝土收缩徐变的有限单元法,为了验证建立的有限单元法的正确性,对试验梁进行了分析.计算结果表明,建立的有限单元法与试验值吻合较好.

侧向约束条件下砂岩的压缩蠕变特性研究

近年来,岩石蠕变而引起的隧道工程及大型地下硐室的长期稳定性问题受到国内外学者的广泛关注,蠕变试验的研究也取得了极大的进展。模拟巷道围岩的受力状态,通过试验研究了砂岩在单轴压缩及侧向约束条件下的蠕变特性,对比分析了2种试验条件下的蠕变变形规律:侧向约束下将发生加速蠕变的应力阀值和极限应变有较大提高,蠕变速率得到减缓。在试验数据的基础上,分析了岩石粘弹性变形随应力水平不同和时间发展的变化规律,为数值分析和理论计算提供了试验依据。

缓黏结预应力混凝土梁长期受力性能试验研究

通过4根预应力混凝土梁在均布荷载作用下的长期受力性能试验,对其挠度和应变进行约400d的测试,并对缓黏结预应力混凝土梁与普通预应力混凝土梁的受力性能进行分析比较。考虑混凝土收缩徐变效应的影响,对6根圆形徐变柱进行约700d的长期徐变测试。试验结果表明:缓黏结预应力混凝土梁与普通预应力混凝土梁的挠度、应变实测数据变化规律基本一致。在试验全过程中,缓黏结预应力混凝土梁较普通预应力混凝土梁有较大的挠度、应变实测值,在持荷378d时,矩形、T形缓黏结预应力混凝土梁的挠度实测数据分别较普通预应力混凝土梁的挠度偏大9%、5%。卸载稳定后,矩形、T形缓黏结预应力混凝土梁的挠度实测数据分别较普通预应力混凝土梁的挠度偏大10%、4%,表明缓黏结预应力筋与混凝土之间具有足够的黏结力,具有有黏结预应力混凝土梁的受力特性。

二次预应力梁的徐变和承载力试验研究

在中小跨径的预应力梁式桥中,经常出现张拉过程中直梁反拱过大,弯梁横移,刚构桥立柱开裂等问题,这些都是由于预应力附加内力过大引起的。为了解决此类问题,提出二次预应力新型桥梁结构的概念。通过对这种结构与常规预应力结构的比较分析和10 m跨径的二次预应力梁和常规预应力梁的对比试验,证明二次预应力技术在低高度梁中的应用,可以大大改善低高度梁在施工和长期使用过程中的受力和变形性能。同时推导了二次预应力结构任意时刻的徐变内力计算公式,并验证了二次预应力结构的承载能力。指出二次预应力技术在中小跨径预应力梁式桥中有着广阔的应用前景。

单板层积梁弯曲蠕变特性

在自制的弯曲蠕变试验台上对桦木和椴木单板层积梁进行弯曲蠕变试验,蠕变测试采用简支梁中部集中加载的方式。选用的加载应力水平分别为层积梁弯曲极限强度的40%、50%和60%,用百分表记录蠕变数值。实验周期为40～60d。试验结果表明,单板层积梁弯曲蠕变特性受树种、加载应力水平的影响较大,与树种的材性有较大关系:桦木单板层积梁的抗蠕变性能要明显好于椴木单板层积梁;同时,相同树种不同应力状态下,蠕变曲线为非线性的,蠕变过程由于加载应力水平不同而呈不同的规律。蠕变变形量和曲线上的跳跃点随着应力水平的增加有增大的趋势。试验测试过程中,60%应力状态下,椴木单板层积梁在连续加载60d时突然断裂。

自然环境混凝土徐变试验和预测模型研究

为准确预测实际工程结构混凝土徐变的发展规律,在反映恒温、恒湿条件下混凝土徐变性能的基准徐变系数基础上,引入温度、湿度徐变系数,建立了预测实际环境温、湿度条件下混凝土徐变的组合徐变模型.借鉴徐变计算理论,提出了由环境温度变化引起的混凝土附加徐变的实用计算方法.研究结果表明:自然环境中随时间变化的温、湿度导致现行徐变模型的预测结果与实际的徐变变形存在显著差异,其引起的混凝土附加徐变随季节更替而产生周期性增减交替变化;组合徐变模型给出的结果与试验结果最大相对偏差为6%,与试验结果最为接近的现行徐变模型相比,减小了7%.

二次预应力超薄梁徐变效应试验研究

预应力混凝土超薄结构应用于多层立交中将产生巨大的经济和景观效益,但在超薄梁上施加预应力将导致反拱过大和徐变长期反拱不断发展。针对为解决此问题提出的二次预应力新型结构,推导了二次预应力结构任意时刻徐变和徐变内力的计算公式;并且通过与常规预应力梁的对比试验研究,证明了二次预应力能大大改善预应力混凝土梁的变形性能。二次预应力超薄结构在控制变形中的优势,使其有着广阔的应用前景。

煤岩流变模型与地表二次沉陷研究

通过流变实验研究发现，煤岩流变具有低流变系数和高蠕变门槛值特征，并符合对数型经验公式和修正的伯格斯理论模型。作者以河北峰峰矿区为例，求出了各级应力水平下煤岩流变经验公式的典型参数集，并结合峰峰二矿工业广场建筑物下条带开采地表沉陷实例进行了分析。