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  力学学报  2016, Vol. 48 Issue (4): 936-943  DOI: 10.6052/0459-1879-15-367
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页岩气专题论文

引用本文 [复制中英文]

韩铁林, 师俊平, 陈蕴生, 李伟红. 轴、侧向同卸荷下砂岩力学特性影响的试验研究[J]. 力学学报, 2016, 48(4): 936-943. DOI: 10.6052/0459-1879-15-367.
[复制中文]
Han Tielin , Shi Junping , Chen Yunsheng , Li Weihong . EXPERIMENTAL STUDY ON MECHANICS CHARACTERISTICS OF SANDSTONE UNDER AXIAL UNLOADING AND RADIAL UNLOADING PATH[J]. Chinese Journal of Ship Research, 2016, 48(4): 936-943. DOI: 10.6052/0459-1879-15-367.
[复制英文]

基金项目

国家自然科学基金(11302167) 和陕西省教育厅自然科学专项(2013JK0609) 资助项目.

通讯作者

韩铁林, 博士生, 主要研究方向:岩石损伤与断裂力学.E-mail:s3050210133@163.com

文章历史

2015-10-03 收稿
2016-03-21 录用
2016-03-25网络版发表
轴、侧向同卸荷下砂岩力学特性影响的试验研究
韩铁林1,2, 师俊平2, 陈蕴生1,2, 李伟红1,2     
1. 西安理工大学岩土工程研究所, 西安 710048 ;
2. 西安理工大学土木建筑工程学院, 西安 710048
摘要: 本文以实际岩体工程为背景,利用WDT-1500 仪器开展了轴向、侧向同时卸荷条件下砂岩的三轴试验. 结果表明:轴、侧向同卸荷这种卸荷路径下,砂岩试样破坏时并没有出现应力峰值,为了定义试样的破坏强度,将最大与最小主应力差随最小主应力的变化关系曲线上应力跌落的拐点处的应力值定义为破坏强度. 砂岩变形初始段发生应力跌落和轴向应变回弹,破坏前无明显的弹性和屈服阶段;试验的过程中,砂岩的侧向变形明显大于轴向变形,其体积应变一直处于膨胀状态;相对于砂岩的常规三轴试验结果,试样破坏时的强度在轴向、侧向同时卸荷条件下有所降低. 初始轴压和初始围压对试样的力学特征有十分显著的影响,但围压的卸荷速率却并不显著. 砂岩的破坏特征主要是以张-拉为主的混合张剪的破坏.
关键词: 岩石力学    砂岩    卸荷    应力路径    变形特性    破坏特征    
0 引言

岩体卸荷是岩体工程施工中不可避免的问题,岩体卸荷引起其破坏特征及其破坏机制与加载过程中有着显著的差异[1],许多学者开展了相关的试验与理论研究[2-39].Jaeger [2]提出有必要研究和讨论岩石的强度和加载路径间的关系;陈禺页等[3]开展了不同的加卸荷条件下岩石的实验研究,获得:岩石的强度与其应力路径间有一定的关系.昊玉山[4]通过对凝灰岩进行侧压恒定轴压加载和侧压卸荷轴压加载这种应力路径下的力学特性的试验研究,得到:采用与工程实际相一致的应力路径来确定岩石的力学特征是必要的.李天斌等[5]对玄武岩的变形破坏特征开展卸荷三轴压缩的试验研究.尤明庆等[6-7]对砂岩和大理岩这两种岩石分别进行了不同应力路径下强度特征和变形特征的实验研究.韩铁林等[8]开展了不同的加卸荷应力路径下岩样的实验研究.结果表明:不同的卸荷条件下,初始轴压与初始围压对其力学特征有十分显著的影响,但围压的卸荷速率对力学特性的影响却并不显著. 轴、侧向同卸荷这种卸荷路径下岩石力学特性等方面的研究却鲜见报道.因此,有必要进一步对轴、侧向同卸荷这种卸荷路径下岩石力学特征等开展试验和理论研究.

本文进行了轴、侧向同卸荷这种卸荷路径下砂岩力学特性的试验研究,并在试验的基础上,分析了这种卸荷路径对试样力学特性等方面的影响.

1 试验材料与方法 1.1 试验仪器及试样制备

利用 WDT-1500仪器对岩样的力学特征展开研究,轴压的范围为0$\sim $1 500 kN,围压为0$\sim$80 MPa,轴向应变、侧向应变的测量范围分别为0$\sim $10%和0$\sim $5%.

本实验以砂岩为研究对象,严格按照规范要求将砂岩试件加工成径高比为1:2的圆柱体试样,直径为5 cm.砂岩试样平均密度2.54 g/cm$^{3}$,含水率7.24%$\sim $9.68%.

1.2 试验方案

先加载围压,再轴向加载直至达到表 1中的值后,按照表 1中的实验方案,同时卸除轴压和围压,直至岩石发生失稳破坏.

表 1 砂岩在轴、侧同卸荷条件下的试验方案 Table 1 Test project of sandstone specimen under different axial unloading and radial unloading path

主要研究:(1) 30 MPa的初始围压下,初始轴压(140 MPa,150 MPa和160 MPa)的影响;(2)65 MPa的初始轴压下,初始围压(12 MPa ,15 MPa和20 MPa)的影响;(3)12 MPa的初始围压和65 MPa的初始轴压下,围压卸荷速率(0.5,0.8,1.0 MPa s-1)的影响.试验方案详见表 1.

2 试验结果分析 2.1 变形特性

图 1为轴、侧向同卸荷下砂岩$({\sigma _1} - {\sigma _3}) - {\varepsilon _1}$曲线.由图 1可知,试验初期,有应力的衰减与回弹现象发生,其他阶段的应力应变关系曲线呈线性关系,破坏过程比较平稳,无明显的应力降现象.

初始轴压的影响:由图 1(a)可知,不同初始轴压下砂岩试样$({\sigma _1} - {\sigma _3}) - {\varepsilon _1}$关系曲线基本一致,主要区别体现在初始阶段,砂岩试样的应力发生了跌落;随着实验的进行,初始轴压分别为150 MPa, 160 MPa时试样的$({\sigma _1} - {\sigma _3}) - {\varepsilon _1}$关系曲线呈现不同程度的回弹现象.

初始围压的影响:由图 1(b)可知,初始轴压和围压卸荷速率相同时,试样的$({\sigma _1} - {\sigma _3}) - {\varepsilon _1}$关系曲线呈现出应力跌落和回弹现象.

围压卸荷速率的影响:由图 1(c)可知,围压的卸荷速率对砂岩试样变形特性的影响不明显,这可能与本实验所选的围压卸荷速率的范围有关.

图 1 轴、侧向同卸荷下砂岩的$({\sigma _1} - {\sigma _3}) - {\varepsilon _1}$曲线 Fig. 1 $({\sigma _1} - {\sigma _3}) - {\varepsilon _1}$ curves of sandstone under axial unloading and radial unloading

图 2为轴、侧向同卸荷下砂岩的$\varepsilon _3$-$\varepsilon _1 $曲线(与图 1对应). 由图 2可知:砂岩的 $\varepsilon _3$-$\varepsilon_1 $曲线在起始段呈现明显的回弹现象,这主要是由于卸荷初始阶段,侧向应变迅速增大,加之,其轴向应变有所降低(由图 1中可知),导致其$(\sigma _1 - \sigma _3)$-$\varepsilon _{1} $关系曲线呈现回弹现象;随着试验的进一步进行,其侧向应变的增加速率有所减小,试样的轴向应变和侧向应变可近视地认为是线性关系,其斜率逐渐减小,砂岩的侧向膨胀现象却一直非常明显.

图 2 轴、侧向同卸荷下砂岩的$\varepsilon _3$-$\varepsilon _1 $曲线 Fig. 2 $\varepsilon _3$-$\varepsilon _1 $ curves of sandstone under axial unloading and radial unloading

图 3为轴、侧向同卸荷下砂岩试样的$(\sigma _1 - \sigma _3 )$-$\varepsilon _{v} $曲线(与图 1图 2对应).从图中可以看出: 在轴、侧向同卸荷条件下,砂岩试样的体积应变始终处于膨胀状态.

图 3 轴、侧向同卸荷下砂岩的$(\sigma _1 - \sigma _3 )$-$\varepsilon_{v} $曲线 Fig. 3 $(\sigma _1 - \sigma _3 )$-$\varepsilon _{v} $ curves of sandstone under axial unloading and radial unloading

初始轴压的影响:由图 3(a)可知,不同的初始轴压下,岩样的$(\sigma _1-\sigma _3 )$-$\varepsilon _v$关系曲线基本一致,体积应变受初始围压的影响并不显著. 由砂岩试样$\sigma _3 $-$\varepsilon _v$关系曲线可以看出,$\sigma _3$$ 30 \sim 15$ MPa 范围内,试样的体积应变膨胀速率较小,而${\sigma _3} \le 15$ MPa 时,体积应变迅速膨胀,即$ \sigma _3 = 15$ MPa 为试样体积应变的拐点,这表明围压对砂岩体积应变的作用明显.

初始围压的影响:由图 3(b)可知,在卸荷初期,试样的$(\sigma _1 - \sigma _3 )$-$\varepsilon _v$关系曲线呈现应力跌落与回弹现象,应力跌落段,初始围压对体积应变的影响并不明显;而在应力回弹阶段,初始围压越大回弹现象越明显,同时,试样的体积应变衰减越大.

围压卸荷速率的影响:由图 3(c)可知,相同的初始轴压和初始围压下,本实验范围内的围压卸荷速率对试样$(\sigma _1 -\sigma _3 )$-$\varepsilon _v $关系的影响并不显著.

与文献[8]中的实验结果对比可知,在轴、侧向同卸荷下砂岩的变形特征与定围升轴(常规三轴试验)、定轴卸围和卸围升轴下的相比存在差异:在轴、侧向同卸荷下砂岩的$(\sigma_1 - \sigma _3 )$-$\varepsilon _{1}$关系曲线并未出现明显的弹性阶段和屈服阶段,但在实验初期,试样呈现出明显的应力跌落和回弹现象;砂岩的体积在整个试验过程中一直处于膨胀状态;而卸围升轴下在屈服阶段体积就开始膨胀,定围升轴下砂岩在破坏阶段体积才开始膨胀.

2.2 强度特性

图 4为轴、侧向同卸荷下砂岩$(\sigma _1 - \sigma _3 )$-$\sigma _3 $关系曲线.轴向、侧向双向同时卸荷是一种比较特殊的应力路径,而在这种卸荷条件下,砂岩试样破坏时并没有出现应力峰值,仅仅在破坏点出现明显的应力跌落,为了定义试样的破坏强度,本文将$(\sigma _1 - \sigma _3 )$-$\sigma _3$关系曲线上应力跌落的拐点定义为破坏点,其应力值为破坏强度值.

图 4 轴、侧向同卸荷下砂岩的$(\sigma _1 - \sigma _3 )$-$\sigma _3 $曲线 Fig. 4 $(\sigma _1 - \sigma _3 )$-$\sigma _3 $curves of sandstone under axial unloading and radial unloading

$(\sigma _1 - \sigma _3 )$-$\sigma _3$关系曲线上应力跌落的拐点定义为破坏点,其应力值为破坏强度值,这主要是由于此时砂岩试样已经破坏,已不能再继续承受荷载.从图 4也可以看出,该拐点相当于常规三轴压缩试验峰值点,经过该拐点以后出现明显的应力跌落,试样进入破坏阶段.

通过对表 2和文献[8]比较得出,轴、侧向同卸下试样的破坏强度均小于常规三轴试验(定围升轴)下的.

表 2 轴、侧向同时卸荷条件下砂岩的试验结果 Table 2 Test results of sandstone sample under axial unloading and radial unloading

图 5中可以看出:岩样破坏时的强度与初始的围压、轴压和围压卸荷速率间的关系比较显著:

图 5 轴、侧向同卸荷下砂岩的破坏强度 Fig. 5 Peak strength of sandstone under axial unloading and radial unloading

岩样破坏时的强度随初始的围压或初始的轴压或围压的卸荷速率的增加而逐渐增大,与文献 [2-8]的研究结果相一致.

2.3 破坏机制分析

不同加卸荷下,岩体的破坏特征有所差异.从宏观的角度出发,常规三轴实验条件下岩石以剪切破坏为主,而在轴、侧向同卸荷下呈现出以张裂为主的张剪破坏.

图 6为轴、侧向同卸荷下岩样的破坏特征,由图可知,轴、侧向同卸荷下,岩样的破坏特征以张裂为主的张剪混合形式. 与文献[8]中定围升轴(常规三轴压缩试验)、卸围升轴以及定轴卸围相比,其张性破坏更加明显. 卸围升轴条件下,岩石的破坏时呈现较少的张拉裂纹,破坏形式为以剪切为主的张剪混合破坏模式;定轴卸围,轴、侧向同卸荷下,试样的张拉裂纹明显增多,破坏模式为以张裂为主的张剪混合破坏,而轴、侧向同卸荷下砂岩破坏的张拉特征更加明显.

图 6 轴、侧向同卸荷下砂岩试样的破坏特征(从左到右依次为A1,A2,A3,A4,A5,A6,A7) Fig. 6 Failure forms of sandstone under axial unloading and radial unloading (from left to right A1, A2,A3,A4,A5,A6,A7)
3 结论

(1) 轴、侧向同时卸荷条件下,初始围压和初始轴压对岩样力学特征的影响相对于围压卸荷速率较大,围压卸荷速率对岩样的影响并不明显,这与实验所选择的围压卸荷速率有关.

(2) 轴、侧向同卸荷这种卸荷路径下,砂岩试样破坏时并没有不出现应力峰值,而只是在破坏点出现明显的应力跌落,为了定义试样的破坏强度,本文将$(\sigma_1 - \sigma _3 ) $-$\sigma _3 $关系曲线上应力跌落的拐点定义为破坏点,其应力值为破坏强度值.

(3) 轴、侧向同卸下岩样的破坏强度相对于常规三轴试验下的有所降低,轴、侧向同卸荷下,岩样破坏时的强度随着初始围压、初始轴压及围压卸荷速率的增大而增大.初始轴压和初始围压对试样有十分显著的影响,但围压的卸荷速率却并不显著,其中初始轴压的影响最大.

(4) 常规三轴压缩试验下,岩样的破坏特征由张拉坡坏(无围压)向张剪破坏(较低围压)转变,再向剪切破坏(高围压下)转变. 而轴、侧向同时斜荷试样则呈现出以张-拉为主的混合张剪破坏.

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EXPERIMENTAL STUDY ON MECHANICS CHARACTERISTICS OF SANDSTONE UNDER AXIAL UNLOADING AND RADIAL UNLOADING PATH
Han Tielin1,2, Shi Junping2, Chen Yunsheng1,2, Li Weihong1,2     
1. Institute of Rock and Soil Mechanics, Xi'an University of Technology, Xi'an 710048, China ;
2. School of Civil Engineering and Architecture, Xi'an University of Technology, Xi'an 710048, China
Abstract: This article take the actual rock mass project as a background, triaxial compression for sandstone specimen under axial unloading and radial unloading—this path is realized on WDT-1500 reactive material testing machine. The test results show that the failure of sandstone specimen don't appear peak stress under axial unloading and radial unloading— this path, to define the inflection point of stress drop of (The maximum principal stress -minimum principal stress) the minimum principal stress curves of sandstone for failure strength. The stress drop and the axial strain of resilience of sandstone specimens were happened under axial unloading and radial unloading, which had no obvious elasticity and yield step before rock specimens' failure. The lateral deformation is larger than the axial deformation in the process of test, and volumetric strain of the sandstone specimen is always in a state of expansion. The strength of sandstone is reduced relative to triaxial compression. the deformation property and strength property of sandstone under this path are mainly influenced by initial axial pressure and initial radial pressure, but the influence of the unloading speed of radial pressure is not clear. The failure characteristics of samples often present mixed zhang-shear failure under axial unloading and radial unloading.
Key words: rock mechanics    standstone    unloading    stress path    deformation properties    failure feature