钙质胶结西域砾岩浸水力学特性劣化机制研究

Study on the Degradation Mechanism of the Mechanical Properties of Calcareous Cemented Xiyu Conglomerate Inundated with Water

摘要

西域砾岩广泛分布于新疆维吾尔自治区天山南北，针对其遇水易软化，物理力学性质变化影响库岸边坡变形与稳定的问题，以新疆维吾尔自治区天山南麓某工程钙质胶结西域砾岩为研究对象，通过不同浸水时间后单轴、三轴压缩试验，研究钙质胶结西域砾岩的应力‒应变曲线特征及变形破坏模式，并结合扫描电子显微镜（SEM），分析不同浸水时间后岩石微观结构变化特征，阐明钙质胶结西域砾岩力学特性劣化机制。结果表明：浸水作用对钙质胶结西域砾岩的力学性能影响显著，相较于干燥状态，浸水7、14、28 d后，单轴抗压强度下降了45.5%、65.6%、71.3%，浸水7 d岩样劣化效应尤为明显，浸水14 d后强度及变形参数劣化趋势逐渐缓慢；围压越大，劣化趋势越明显；随着浸水时间增长，钙质胶结西域砾岩的破坏模式由脆性向延性破坏特征转化，破坏时在砾石颗粒边缘位置处产生了绕砾发展裂纹和砾石颗粒劈裂破碎特征；浸水作用下钙质胶结西域砾岩的微观结构变化特征明显，岩样内部黏土矿物吸水膨胀及胶结物溶解破坏，使岩石结构由致密孔隙胶结结构逐渐向松散多孔结构发展，从而导致宏观上强度及变形参数的劣化。研究结果可为西域砾岩地区库岸边坡变形与稳定分析提供理论依据。

Abstract

Objective Calcareous cemented Xiyu conglomerate readily softens upon contact with water, and its physical and mechanical properties change significantly after immersion, directly affecting the deformation behavior and stability of reservoir bank slopes. Based on X‒ray diffraction mineral composition analysis, uniaxial and triaxial compression tests are conducted to investigate the stress-strain characteristics and damage modes of calcareous cemented Xiyu conglomerate after different immersion durations. Combined with SEM microstructural analyses, these tests further clarify the mechanisms responsible for the deterioration of the mechanical properties of calcareous cemented Xiyu conglomerate. Methods Considering the natural storage conditions of calcareous cemented Xiyu conglomerate, this study simulated the inundation process of calcareous cemented Xiyu conglomerate bank slopes and conducted uniaxial and triaxial compression tests under different inundation durations (dry, 7, 14, and 28 days) and confining pressures (0, 0.5, 1.0, and 2.0 MPa). Stress‒strain curves were plotted based on the experimental data to analyze the influence of inundation time on the macroscopic mechanical parameters and failure modes of the calcareous cemented Xiyu conglomerate. Combined with SEM analysis, the macroscopic physical and mechanical properties of the calcareous cemented Xiyu conglomerate were examined to investigate the deterioration process and mechanism from a microscopic perspective under water immersion. Results and Discussions The main mineral compositions and their relative contents were obtained by analyzing the X‒ray diffraction mineral analysis spectra of the calcareous cemented Xiyu conglomerate samples. The macroscopic mechanical properties of the calcareous cemented Xiyu conglomerate changed significantly under uniaxial and triaxial compression tests, and the stress‒strain curves exhibited obvious mechanical property transition points, namely: the compaction stage, linear elastic stage, plastic deformation stage, failure stage, and residual stage. With the increase in water immersion time, the compaction section of the stress‒strain curve gradually increased, the slope of the elastic deformation section gradually decreased, the peak compressive strength progressively deteriorated, and the axial strain corresponding to the peak stress gradually increased, resulting in a strain-softening phenomenon. The strength and deformation parameters of the calcareous cemented Xiyu conglomerate deteriorated to different degrees, and the peak compressive strength decreased by 26.2% to 45.5%, 38.1% to 65.6%, and 55.8% to 71.3%, respectively, under different confining pressures after 7, 14, and 28 days of immersion. The elastic modulus and deformation modulus exhibited the same deterioration pattern as the peak compressive strength. The elastic modulus decreased by 0.1% to 42.5%, 42.6% to 58.0%, and 66.1% to 75.9%, respectively, whereas the deformation modulus decreased by 10.0% to 48.4%, 44.8% to 60.8%, and 66.9% to 76.4%, respectively. Overall, the deterioration rate was faster before 7 d of water immersion and gradually slowed after 14 days of immersion. In addition, the greater the confining pressure was, the more pronounced the deterioration trend became. After 7, 14, and 28 days of water immersion, the cohesion decreased by 13.9%, 40.0%, and 47.0%, respectively, while the internal friction angle decreased by 6.2%, 9.1%, and 20.4%, respectively. The deterioration of cohesion occurred more rapidly before 14 d of water immersion, and the deterioration tendency slowed and stabilized after 14 d of immersion, whereas the internal friction angle exhibited a nearly linear deterioration trend over time. In the dry state, the calcareous cemented Xiyu conglomerate exhibited a dense cemented structure. When the confining pressure was low, tensile splitting failure occurred. With the gradual increase in confining pressure, the failure mode transitioned from tensile failure to a tensile-shear mixed failure mode. Shear failure developed along the main rupture surface, and cracks developed around the gravel particles, accompanied by gravel particle splitting at their locations. With increasing water immersion time, the dissolution and expansion of mineral components weakened the cementing effect of the cemented structure, the specimens gradually became softer, and the calcareous cemented Xiyu conglomerate transformed from brittle to ductile failure characteristics. In the dry state, the particles of the calcareous cemented Xiyu conglomerate were connected through cementation, tiny primary cracks were present, and the microstructure was relatively dense. After water immersion, the mineral components of the cement gradually dissolved and expanded, resulting in the enlargement of tiny pores. The microcracks and pores within the specimens gradually developed, the cement between mineral particles gradually loosened, and the microstructure of the calcareous cemented Xiyu conglomerate gradually evolved from a dense pore structure to a loose porous structure, which manifested as macroscopic strength degradation and deformation. The microstructure of the calcareous cemented Xiyu conglomerate gradually developed from a dense pore-cemented structure to a loose porous structure, and the deterioration of the strength and deformation parameters was manifested at the macroscopic level. Conclusions Under submergence conditions, the dissolution and degradation of the calcium-cemented matrix in Xiyu conglomerate, together with the expansion of clay minerals caused by water absorption, lead to the gradual transformation of its microstructure from a dense cemented pore structure to a loose porous structure. This transformation is macroscopically manifested by the deterioration of strength and deformation characteristics, as well as changes in the damage mode. The results of this study provide a valuable reference for the deformation and stability analysis of bank slopes in Xiyu conglomerate regions.

本刊网刊

西域砾岩分布于天山南北坡山前地带、塔里木等山间盆地中，主要由山前冲洪积碎屑岩沉积而成^[1‒2]。在长期风化、重力等作用下，西域砾岩边坡多高陡、直立，其抗河流掏蚀能力差，在浸水作用下，其物理力学性质发生显著变化^[3]，尤其在河水冲蚀作用下，胶结成分不断流失，易形成倒坡，引起库岸边坡产生不均匀变形，使得水库蓄水后边坡变形失稳破坏^[4‒5]，造成一系列工程安全稳定问题，引起工程人员的普遍关注。

近年来，国内外研究学者针对水岩作用对力学特性的影响，开展了大量理论及试验研究。研究表明，随着含水率^[6]、饱和度^[7]、干湿循环次数增加^[8‒9]，岩石的抗压强度、弹性模量和抗拉强度等物理力学参数均发生不同程度的降低^[10]。同时，邓华峰^[11]、郭晓萍^[12]等从细微观角度探究了岩石的变形破坏特征及水岩劣化机理。Zhao^[13]、Li^[14]等研究发现，水的存在使岩石的摩擦系数、弹性模量、黏滞系数等参数均不同程度地降低。金成等^[15]研究发现，浸水对泥页岩的黏聚力较摩擦角的影响显著，随着浸水时间的延长，泥页岩由脆性张拉劈裂破坏逐渐转变为拉剪混合破坏。周翠英等^[16]以华南地区的红色砂岩及黑色炭质泥岩等不同类型的典型软岩为研究对象，进行了不同含水条件下的软岩力学性质测试，认为软岩所含黏土矿物遇水发生反应是导致其软化的重要原因^[17‒18]。谢小帅^[19]、Yang^[20]等结合X射线衍射试验及电镜扫描，探究了不同含水状态下软岩的微观结构特征及其遇水软化机制，认为软岩遇水后微观结构上的变化导致其力学性能的改变^[21]。随着干湿循环次数的增加，不同类型软岩的抗压强度、黏聚力、内摩擦角均逐渐减小^[22‒23]，其中：前6次干湿循环作用导致的劣化效应尤为明显^[24]。干湿循环作用导致胶结物的溶蚀及颗粒接触强度降低^[25]，微裂隙的扩展与连通，使水向岩石内部更深区域渗透^[26]。

目前，关于西域砾岩的研究大多聚焦于成因、种类特征等方面，在干燥状态下，钙质胶结西域砾岩具有一定的强度和抵抗变形能力。然而，针对其在浸水条件下的软化现象及其对岩体物理力学性质的影响，仍需开展进一步研究。本文以新疆维吾尔自治区库车某工程库岸边坡钙质胶结西域砾岩为研究对象，考虑不同浸水时间变化，进行单轴、三轴压缩试验，分析浸水时间对其宏观力学参数及破坏模式的影响规律，结合电子显微镜（SEM）扫描，从微观角度分析了浸水作用下钙质胶结西域砾岩的宏观物理力学特性劣化过程及机理。研究成果可为西域砾岩地区库岸边坡防灾减灾提供参考依据。

1 试验方案设计

1.1 试样制备

本文试验岩样取自新疆维吾尔自治区库车某工程钙质胶结西域砾岩，采用钻孔取芯的方式采集样品，为准确测试岩样的物理力学参数，保证其表面受力均匀，参考以往研究者对砾岩力学试验测试的处理方法，试验选用石膏进行试样端部表面填充，在不改变其力学特性的前提下，确保岩样的完整性。经过切割、打磨制备成标准试样，严格控制试样高度、直径的偏差绝对值\le0.3 mm，试件端面的不平整度绝对值\le0.05 m^[27]。剔除具有明显裂纹及存在缺陷的试样，选取密度相对集中的岩样开展试验，从而保证试样的均匀性以及试验结果的离散性。钙质胶结西域砾岩试样如图1所示。

钙质胶结西域砾岩试样的X射线衍射矿物分析谱如图2所示。

利用X射线衍射图谱，结合粉末衍射卡片对岩石矿物进行定性分析，鉴定出各个物相衍射强度及各组分，进行定量或半定量分析。XRD结果表明，试样主要矿物成分及相对含量如下：石英（42.7%）、方解石（23.3%）、钠长石（14.6%）、绿泥石（3.4%）、高岭石（5.7%）、水云母（9.6%）和赤铁矿（0.1%）。

1.2 试验方案

考虑钙质胶结西域砾岩的天然赋存条件，模拟库岸边坡长期浸水作用过程，将试样按照不同浸水时间分为4组。首先，对未浸水（干燥）试样进行单轴、三轴压缩试验，试验围压取值分别为0、0.5、1.0、2.0 MPa；然后，在水中浸泡7、14、28 d后进行试验，根据试验数据绘制应力‒应变曲线，并进行SEM电镜扫描，探究浸水作用对其微观结构特征的影响。

2 试验结果分析

2.1 浸水作用下应力‒应变曲线特征分析

在浸水作用下，钙质胶结西域砾岩宏观力学特性发生了显著变化。图3为不同浸水时间下的应力‒应变曲线。

由图3可知，三轴压缩试验下钙质胶结西域砾岩的应力‒应变曲线与单轴压缩试验曲线形态基本一致，均具有明显的力学特性转换点，应力‒应变曲线可简化为5个阶段，即压密变形阶段、弹性变形阶段、塑性变形阶段、破坏阶段和残余阶段。随着浸水时间增加，应力‒应变曲线压密段逐渐增长，弹性变形段的斜率逐渐减小，其峰值抗压强度逐渐降低劣化，达到峰值应力所对应的轴向应变值逐渐增大，产生应变软化现象，由脆性特征逐渐向延性转化。

为分析浸水作用对钙质胶结西域砾岩力学特性的影响，根据应力‒应变曲线，对其抗压强度、抗剪强度、弹性模量、变形模量等参数劣化规律进行分析。

2.2 浸水作用下强度特征分析

2.2.1 浸水时间对峰值强度的影响

不同浸水作用下钙质胶结西域砾岩的抗压强度及轴向峰值应变如表1所示。

为了更直观地分析浸水作用下钙质胶结西域砾岩强度劣化规律，根据试验数据作出其抗压强度劣化规律曲线，如图4所示，轴向峰值应变随浸水时间变化规律曲线如图5所示。

根据表1和图4可知，在浸水作用下，钙质胶结西域砾岩试样的峰值强度劣化显著。浸水7、14、28 d后，在不同围压下，其峰值强度分别下降了26.2%～45.5%、38.1%～65.6%、55.8%～71.3%。随着浸水时间的增加，试样的峰值强度均减小，且围压越大，岩样峰值强度降低得越快，劣化趋势越明显。峰值强度在浸水7 d前劣化速度较快，浸水14 d后劣化速度逐渐减缓。

由表1和图5可知，随着浸水时间增加，钙质胶结西域砾岩试样的峰值应力对应的轴向应变值均发生不同程度的变化。当围压较小时，轴向应变值几乎保持不变；而当围压较大时，岩样峰值应变出现增长趋势。

2.2.2 浸水时间对岩石抗剪强度参数的影响

根据摩尔‒库仑准则计算出不同浸水时间下钙质胶结西域砾岩强度参数值（c、φ）。根据式（1）、（2）分别计算内摩擦角φ和黏聚力c。

\phi =\mathrm{a}\mathrm{r}\mathrm{c}\mathrm{s}\mathrm{i}\mathrm{n}\frac{m-\mathrm{1}}{m+\mathrm{1}}（1）

c=b\frac{\mathrm{1}-\mathrm{s}\mathrm{i}\mathrm{n}\phi }{\mathrm{2}\mathrm{c}\mathrm{o}\mathrm{s}\phi }（2）

式（1）、（2）中，m为图6中的拟合直线斜率，b为拟合直线与纵坐标轴的截距。

图6为不同浸水时间后三轴压缩试验获得的散点图和线性拟合结果。

将拟合直线的斜率和截距代入式（1）、（2）中，得到不同浸水时间后的强度参数，如表3所示。

图7为不同浸水时间下，钙质胶结西域砾岩黏聚力及内摩擦角劣化规律曲线。

根据表3和图7可知，在浸水作用下，钙质胶结西域砾岩的黏聚力和内摩擦角均发生不同程度的劣化。在浸水7、14、28 d后，其黏聚力分别下降了13.9%、40.0%、47.0%，内摩擦角分别下降了6.2%、9.1%、20.4%。试样黏聚力在浸水14 d前劣化速度较快，浸水14 d以后劣化趋势变缓并逐渐趋于稳定，与其抗压强度的劣化趋势总体基本一致。试样内摩擦角在浸水过程中随时间变化呈线性劣化趋势。因此，在浸水作用下，钙质胶结西域砾岩黏聚力较内摩擦角劣化度大，浸水时间对黏聚力的影响更加显著。

2.3 浸水作用对钙质胶结西域砾岩变形特征分析

不同浸水时间下，钙质胶结西域砾岩弹性模量及变形模量劣化规律曲线如图8所示。

由图8可知，钙质胶结西域砾岩弹性模量和变形模量均随浸水时间的增加而降低，同一浸水时间下，试样弹性模量和变形模量均随围压的增大而增大。浸水7、14、28 d后，在不同围压下，钙质胶结西域砾岩弹性模量分别下降了0.1%～42.5%、42.6%～58.0%、66.1%～75.9%；变形模量分别下降了10.0%～48.4%、44.8%～60.8%、66.9%～76.4%。因此，钙质胶结西域砾岩在浸水前7 d劣化速度较快，14 d后劣化速度逐渐减缓，其劣化规律与强度劣化特征规律一致。

2.4 浸水作用下钙质胶结西域砾岩破坏特征分析

为研究不同围压、不同浸水时间下钙质胶结西域砾岩的破坏特征，进行三轴压缩试验，部分试样破坏图如图9所示。

由图9可知，钙质胶结西域砾岩试样在三轴加载条件下破坏模式以张拉劈裂和张剪破坏为主，在砾石颗粒位置处均产生了绕砾发展裂纹和砾石颗粒劈裂破坏现象，如图10所示。图10中，红色实线表示绕砾发展裂纹，红色虚线表示砾石劈裂裂纹。岩样在荷载作用下，当砾石颗粒承受的弯矩增大至其极限载荷时，砾石将发生劈裂或剪切破坏；砾石颗粒的强度高于胶结物与砾石的胶结强度时，在结合面处易形成应力集中，导致微裂纹发展到砾石颗粒附近时，沿颗粒胶结面发展破坏，形成绕砾裂纹。

干燥状态下，岩样胶结结构致密，围压较低时，钙质胶结西域砾岩呈张拉劈裂破坏，岩样表面产生大量轴向张拉裂纹，未贯穿整个试样；随着围压逐渐增大，其破坏模式由张拉破坏逐渐向拉剪破坏过渡，沿主破裂面产生剪切破坏。随着浸水时间增加，矿物成分的溶解、膨胀削弱了胶结物结构的胶结作用，导致其黏聚力降低，试样逐渐变软，由脆性张拉破坏逐渐转变为拉剪为主的混合破坏；浸水28 d后，试样破坏程度增大，产生大量沿轴向及径向分布的绕砾发展裂纹，导致岩样破碎程度较高。因此，随着浸水时间的增长，钙质胶结西域砾岩由脆性向延性破坏特征转化。

2.5 浸水作用下钙质胶结西域砾岩微观结构特征分析

为研究钙质胶结西域砾岩在浸水作用过程中微观结构的变化特征，针对不同浸水时间后的岩样，选取经过力学试验后的试样破坏面内部胶结物块体进行扫描电镜试验，统一取放大倍数为1 000倍的照片进行对比分析，岩样的SEM照片，如图11所示。

观察图11中钙质胶结西域砾岩微观结构SEM照片可知，在浸水过程中，岩样的微观结构特征发生了显著变化具体如下。

1）在干燥状态下，试样矿物颗粒表面平整，颗粒互相胶结连接，存在原生微小孔隙和裂纹，岩体结构相对密实。在宏观上，岩样的抗压强度及弹性模量均较高。

2）浸水7 d之后，岩样胶结物表面模糊，矿物颗粒呈碎屑状，亲水黏土矿物膨胀破坏明显，矿物颗粒间微孔隙发育，部分大颗粒边界出现明显裂纹。

3）浸水14 d之后，试样胶结物进一步溶解，矿物颗粒之间的孔隙、裂纹更加发育，其结构逐渐松散多孔。

4）浸水28 d之后，矿物颗粒间的胶结较弱，大颗粒被分解成小颗粒状，存在部分大孔隙，微裂隙、孔隙不均匀地分布于颗粒间，试样整体逐渐向块状和鳞片状松散结构转变。

综合上述微观结构变化过程可知，在浸水过程中，由于胶结物矿物成分的溶蚀、膨胀作用，钙质胶结西域砾岩试样的微观裂纹、孔隙逐渐发育，矿物颗粒之间的胶结逐渐松散，岩样由致密状态逐渐向不规则的疏松多孔结构发展，进而导致了其宏观强度变形的劣化特征。

3 钙质胶结西域砾岩浸水劣化机理分析

岩石的水理性质与其矿物成分和微观结构特征密切相关^[28‒29]，第2节试验结果分析表明，在浸水作用下，钙质胶结西域砾岩物理力学性质劣化趋势显著，分析其物理力学特性劣化机理如下。

钙质胶结西域砾岩岩样矿物成分主要包括石英、钠长石、方解石和绿泥石、水云母、高岭石等黏土矿物，其中，石英、长石等为块状鳞片状粗粒微结构，黏土矿物颗粒充填在粗颗粒间，并与粗颗粒形成胶结连接，岩样原生孔隙发育，结构性较差。在浸水过程中，长石、方解石和黏土矿物易发生水化反应，导致微观结构发生改变，尤其是亲水黏土矿物吸水易膨胀，由于黏土矿物成分的膨胀系数差异，胶结结构不能同时发生体积膨胀，这种膨胀作用所产生的应力分布不均匀，试样内部易出现拉应力，产生大量微孔隙，造成岩样内部的孔隙被水充填，使微小孔隙不断扩张，并且在水压力作用下，在原生裂纹处易产生应力集中现象，加速微裂隙扩张，使试样孔隙率增加，更有利于形成水分子的渗透通道，进而增加了黏土矿物与水发生物理化学反应的几率，胶结物不断溶解，导致岩样胶结结构逐渐发生破坏。

由图11中SEM图像可知，随浸水时间增加，岩样的微观结构发生了显著变化。干燥状态下，钙质胶结西域砾岩颗粒互相胶结连接，存在微小原生裂隙，微观结构较为致密；试样浸水后，岩石内的胶结物及矿物颗粒不断与水发生物理化学反应，所以在浸水7 d时，试样的强度、变形值劣化速率较快；在浸水14 d后，黏土矿物充分溶解、膨胀，浸水溶液中离子浓度趋于稳定，岩样浸水劣化逐渐缓慢。随着岩样微观裂纹的增多，矿物颗粒之间由点‒面接触转变为点‒点接触，导致颗粒间的胶结结构逐渐破坏，岩样结构逐渐向块状和鳞片状疏松多孔结构转变。

基于上述分析可知，在浸水过程中，钙质胶结西域砾岩矿物成分发生了物理化学反应，导致矿物颗粒间胶结作用减弱，从而导致岩样内部结构发生软化、崩解破坏。宏观上表现为力学性能改变，压密阶段变形增大，其抗压强度、抗剪强度和弹性模量逐渐降低，破坏模式由脆性向延性特征转化。

4 结 论

基于钙质胶结西域砾岩单轴、三轴压缩试验，研究了浸水时间对其强度、变形及破坏形式的影响，主要得到如下结论：

1）在浸水作用过程中，钙质胶结西域砾岩试样的抗压强度、抗剪强度、弹性模量、变形模量均出现明显的劣化趋势，浸水前7 d劣化速率较快，浸水14 d后，其强度变形参数劣化趋势逐渐缓慢。

2）钙质胶结西域砾岩破坏形式由张拉破坏逐渐向张剪破坏转变。随浸水时间增加，岩样破坏时贯通裂纹逐渐减少，次生裂纹逐渐增加，在砾石颗粒位置处均产生了绕砾发展裂纹和砾石颗粒劈裂破碎现象。

3）在浸水作用下，岩样胶结物溶解以及黏土矿物的吸水膨胀作用，导致矿物颗粒间的胶结趋于松散，微观孔隙逐渐发育，微观结构由相对致密逐渐转变为微裂隙发育的松散多孔结构，试样逐渐软化，宏观上则表现为抗压强度、抗剪强度及变形特征的劣化趋势和破坏模式的转变。

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