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青藏高原东北部黄土区柠条锦鸡儿根系的锚固效应

刘亚斌, 梁燊, 石川, 王舒, 胡夏嵩, 李国荣, 朱海丽

刘亚斌,梁燊,石川,等. 青藏高原东北部黄土区柠条锦鸡儿根系的锚固效应[J]. 中国地质灾害与防治学报,2023,34(5): 107-116. DOI: 10.16031/j.cnki.issn.1003-8035.202208024
引用本文: 刘亚斌,梁燊,石川,等. 青藏高原东北部黄土区柠条锦鸡儿根系的锚固效应[J]. 中国地质灾害与防治学报,2023,34(5): 107-116. DOI: 10.16031/j.cnki.issn.1003-8035.202208024
LIU Yabin,LIANG Shen,SHI Chuan,et al. The root anchorage effect of shrub species Caragana Korshinskii Kom. in the loess area of northeastern Qinghai–Tibet Plateau[J]. The Chinese Journal of Geological Hazard and Control,2023,34(5): 107-116. DOI: 10.16031/j.cnki.issn.1003-8035.202208024
Citation: LIU Yabin,LIANG Shen,SHI Chuan,et al. The root anchorage effect of shrub species Caragana Korshinskii Kom. in the loess area of northeastern Qinghai–Tibet Plateau[J]. The Chinese Journal of Geological Hazard and Control,2023,34(5): 107-116. DOI: 10.16031/j.cnki.issn.1003-8035.202208024

青藏高原东北部黄土区柠条锦鸡儿根系的锚固效应

基金项目: 国家自然科学基金项目(42002283);第二次青藏高原综合科学考察研究项目(2019QZKK0905)
详细信息
    作者简介:

    刘亚斌(1987-),男,青海西宁人,副教授,主要从事生态工程地质等方面的研究工作。E-mail:liuyabincug@163.com

    通讯作者:

    梁 燊(1999-),男,青海湟源人,硕士研究生,从事生态工程地质研究工作。E-mail:ls111630@163.com

  • 中图分类号: P643.22

The root anchorage effect of shrub species Caragana Korshinskii Kom. in the loess area of northeastern Qinghai–Tibet Plateau

  • 摘要: 柠条锦鸡儿是青藏高原东北部黄土区主要的护坡和水土保持灌木,然而对该灌木根系锚固作用机理及其固土护坡效应方面仍缺乏系统性认识。鉴于此,该项研究在阐明柠条锦鸡儿根系锚固机理的基础上,提出了根系对黄土斜(边)坡浅层土体稳定性贡献计算模型。在此基础上,以研究区内生长期为11 a的柠条锦鸡儿为主要研究对象,通过现场根系挖掘试验、原位拉拔试验和理论分析,明确了假定滑动面条件下根系锚固力取值,并进一步定量评价了柠条锦鸡儿根系对黄土浅层滑坡稳定性的增强作用。结果表明:由于根系在地表没有“锚头”结构,故在确定滑动面几何特征的情况下,柠条锦鸡儿根系所能提供的实际锚固力大小取滑动面以下锚固段根系最大抗拔出力和滑动面以上根系锚固反力之间的最小值较为合理;生长期为11 a的单株柠条锦鸡儿根系锚固于最大厚度为2 m的圆弧形滑动面不同条块上时,潜在滑动面稳定性系数增幅为0.018%~0.427%,当单株根系锚固力作用于潜在滑动面中上部条块时,潜在滑动面稳定性系数相对高于根系锚固力作用于最顶部和下部条块;当4株柠条锦鸡儿根系以2块条块的间距(约3 m)作用于潜在滑动面时,潜在滑动面稳定性系数可提高1.035%~1.111%,显著高于(P<0.05,ANOVA)单株根系作用时的稳定性系数。试验株根系锚固作用能够提高降雨入渗条件下黄土斜(边)坡浅层土体稳定性,但是作用效果有限。
    Abstract: Shrub species Caragana korshinskii Kom. dominates slope protection and soil and water conservation in the loess area of the northeastern Qinghai–Tibet Plateau. However, the root anchoring mechanism and the effects of soil consolidation and slope protection of this shrub species remain unclear. This study aimed to elucidate the anchoring mechanism of roots of the C. korshinskii roots and establish a calculation model to evaluate their contribution to the stability of shallow loess slopes. C. korshinskii plants with an 11-year growth period were selected as the study subject. The anchoring force of C. korshinskii roots was determined through in-situ excavation tests, in-situ root pullout tests, and theoretical analysis, along with their impact on the stability of shallow loess soil slopes. The results showed that, due to the absence of a “bolt head” structure on the root surface, it was reasonable to consider the anchoring force provided by the roots as the minimum value between the maximum pullout resistance of the roots below the sliding surface and the anchoring reaction force of the roots above the sliding surface, based on the geometric characteristics of the sliding surface. When the roots of an 11-year-old C. korshinskii roots were anchored on different sliding blocks of a shallow landslide with a maximum thickness of 2 meters, the stability coefficient of the potential sliding surface increased by 0.020% to 0.408%. When the roots of a single plant were anchored in the middle and upper parts of the potential sliding surface, the stability coefficient of the potential sliding surface was relatively higher than when the plant roots were anchored at the top and bottom positions. Moreover, when four C. korshinskii roots were anchored to the shallow landslide with a row spacing of two sliding blocks (approximately 3 m), the stability coefficient of the potential sliding surface increased by 1.035% to 1.111%, which was significantly higher than when a single C. korshinskii root was anchored (P<0.05, ANOVA). The anchorage effect of the root systems could enhance the stability of shallow soil on loess slopes under rainfall infiltration conditions, but the effectiveness was limited.
  • 流滑型黄土滑坡是黄土地区沿沟道或斜坡发生远程滑动和形成长条状堆积的特殊类型滑坡,一般具有明显的滑源区、流通区和堆积区。流滑型黄土滑坡多为高速远程滑坡,发生过程犹如“蛟龙出沟”,常造成意想不到的严重灾害。如1978年7月12日,天水市麦积区刘家湾沟脑因暴雨诱发体积约2.50×106 m3的黄土滑坡,前部滑体以平均6 m/s的速度沿沟道滑动1550 m,压埋沟口陇海铁路150 m,中断行车15 d[1]。2013年7月21日天水市马跑泉镇大沟发生体积约29.7×104 m3的流滑型滑坡,滑距达850 m,摧毁9户村民137间房屋及村委会[24]

    长期以来,高速远程滑坡运动机理一直是滑坡研究的热点,学者们提出了众多机理和假说。1932年HEIM[5]对瑞士Elm滑坡研究后提出了颗粒流理论,并建立了高速远程滑坡运动的“雪橇”模型;KENT[6]对美国Madison峡谷滑坡研究后提出了空气润滑理论。基于不排水环剪试验,SASSA等[7]提出了“滑动面液化”的高速滑坡机制;汪发武[8]也提出了土颗粒破碎导致超孔隙水压力的高速滑坡形成机制;HUTCHINSON等[9]提出了流滑性滑坡的不排水荷载效应;殷跃平[10]研究了汶川8级地震触发的高速远程滑坡滑动中的抛掷、碰撞、铲刮和气垫效应;刘传正[11]分析了滑坡规模、不同岩性和状态的滑道对高速远程滑坡的影响。胡广韬等[12]按照滑坡启动加速度、滑速两方面特征进行了组合分类和研究。段钊等[13]对陕西泾阳南塬流滑型黄土滑坡的滑动特征和液化机理进行了研究,统计出其滑距为坡高的4倍,属典型的高速远程滑坡。彭建兵等[2]、张帆宇等[3]、翟张辉等[4]对天水大沟滑坡泥石流的运动过程和速度进行了模拟计算。王玉峰等[14]将目前的研究成果总结为摩擦生热减阻、滑带液化减阻、动力破碎减阻、底部裹挟减阻、剪切振动减阻和动量传递减阻等六大滑动机理类型。

    2013年7月22日7时45分,甘肃发生岷县漳县Ms6.6级地震。在距震中4 km处的岷县梅川镇永光村触发了2处黄土滑坡,其中永光1#滑坡体积约23×104 m3,造成12人遇难。滑坡前后缘高差175 m,滑坡总长度1030 m,二者之比值为0.17,小于0.33,属远程滑坡。作为流滑型黄土滑坡研究的典型事例,许多学者开展了成因和滑动特征的研究[1517]。但在永光1#滑坡的滑动过程、历时长短和滑速快慢等方面有不同认识。本文通过现场调查、影像对比和滑动过程观察资料的综合分析,探讨了其滑动过程特征、不同部位的滑速及变化情况,并简要分析了滑动机理。以期为此类滑坡的进一步研究和数值模拟提供参考。

    岷县地处青藏高原东北边缘阶梯地形带,是甘南高原、陇西黄土高原和陇南山地的过渡地带,海拔2040~3872 m,切割深度500~1000 m。南部为迭山山系,北部为西秦岭山地,中部为相对低缓的洮河谷地。

    本区属西秦岭构造带的北支西伸部分,地质构造总体展布方向为NWW向(图1)。其中,临潭—岷县—宕昌活动断裂带全新世以来活动强烈,地震频发。历史上区内曾发生过3次Ms6.0级以上强震[18],2003年以来岷县发生Ms5.0级以上中强地震4次。地震动峰值加速度0.15 g

    图  1  “7•22”岷县漳县地震烈度与构造背景图
    Figure  1.  Seismic intensity and structural background map of the July 22 earthquake in Minxian-Zhangxian earthquake

    岷县地区高寒阴湿,年均降水量560.8 mm,年最大降水量709.3 mm,5—9月降水量占全年的78%以上,随海拔的升高降水量增加明显。日最大降水量94 mm,1 h最大降水量69.2 mm。

    永光1#滑坡位于岷县北部西秦岭南麓黄土覆盖的斜坡区,斜坡坡向南,平均坡度11.5°。斜坡上部为南北长700 m、东西宽约500 m的缓坡,冲沟不甚发育,坡面较为完整;斜坡下部冲沟较发育,切割深度20~150 m,地形较为破碎。斜坡主要由上更新统马兰黄土和古近系泥岩组成,马兰黄土一般厚度5~20 m。黄土底部地下水较丰富,含水层厚度2~3 m,隔水层为泥岩[17]。永光1#滑坡位于黄土斜坡中下部近南北向小型冲沟的沟脑部位,坡脚曾有泉水分布。

    永光1#滑坡位于岷县梅川镇永光村四社北部黄土斜坡的中下部。滑坡总面积4.2×104 m2,平均厚度5 m,总体积23×104 m3,滑体主要由马兰黄土组成,局部夹少量泥岩碎块,为中型黄土滑坡。“7•22”岷县漳县地震触发其剧烈滑动,大量堆积在下部平台村庄区,破坏8户村民房屋,造成12人死亡。前缘约6×104 m3滑体滑入前部近南北向较宽缓的小型冲沟,再转向进入主沟道远程流滑并沿沟底不断堆积,其前舌直达沟口乡村公路小桥处。滑坡总长度1030 m,滑坡后缘高程约2702 m,滑舌处高程约2527 m,前后缘高差175 m,滑坡前后缘高差与滑坡总长度之比(H/L)为0.17,小于0.33,属远程滑坡。

    永光1#滑坡平面形态复杂,总体呈不规则的“L型”(图2图3)。根据滑坡形态、堆积特征和滑动环境,将滑坡分为滑源区—平台堆积区和流通区—沟道堆积区两大部分(图4)。

    图  2  永光滑坡正射影像图
    Figure  2.  Orthophoto image map of Yongguang landslide
    图  3  永光滑坡平面图
    Figure  3.  Plan view of the Yongguang landslide
    图  4  永光1#滑坡工程地质剖面图
    Figure  4.  Engineering geological cross-section of the Yongguang 1# landslide

    滑源区—平台堆积区为滑坡上部(图5)。平面形态近似矩形,南北长290 m,东西宽75~110 m,面积约2.76×104 m2,平均厚度6 m,堆积体积约17×104 m3,主滑方向210°。

    图  5  永光1#滑坡滑源区—平台堆积区
    Figure  5.  Sliding source area of Yongguang 1# landslide:platform accumulation area

    滑坡后缘高程2702 m,剪出口高程约2645 m,高差57 m,平均坡度13°。上部斜坡较陡,坡度18°左右;下部较缓,坡度约6°,为南北宽度约180 m的平台,是永光四社村民院落区。地震首先触发上部黄土陡坡段失稳滑动,为永光1#滑坡的滑源区,主滑动面位于饱和黄土与泥岩接触面附近[17]。滑坡在前部较宽缓的平台区大量堆积。由图2图3可看出,滑动过程中,滑坡向东侧地形较低的村民院落区扩散和堆积,形成长约150 m、宽20~40 m、高3~5 m的鼓丘,展布方向与滑动方向近于一致,压埋了村庄。滑坡西侧中前部为长约130 m、宽50 m、深3~5 m,向南部冲沟敞开的洼地,洼地内横向拉张裂缝发育。鼓丘与洼地之间因差异滑动形成的剪切带明显。

    前缘中下部有部分坡体保留了滑坡前的原地形,宽约15 m、高约12 m的可见范围土体结构和地表植被完整,说明未发生滑动。其顶部有6~8 m厚的滑坡堆积,地面局部反翘。

    永光1#滑坡后壁平面形态呈圈椅状弧形,高10~15 m,坡度约55°,由马兰黄土组成,其上擦痕较为清晰。

    另外,永光1#滑坡西北部还发生一处相对独立的次级黄土滑坡。滑源区前部宽30 m,后部宽55 m,长60 m,面积2800 m2,体积约1.2×104 m3。宽20~30 m,长约190 m的滑体呈长条状叠加堆积在已滑的滑坡体上,滑动方向由160°向南偏转为210°。从滑坡叠加堆积次序分析,该次级黄土滑坡发生的时间稍晚。

    由上可见,永光1#滑坡的滑源区—平台堆积区滑坡堆积量较大,为该滑坡的主体部分,主滑面位于黄土、泥岩界面处,属地震诱发的黄土—泥岩接触面滑坡。此类滑坡在岷县地区发育较广泛。

    为永光1#滑坡前缘滑体沿沟道发生特殊流滑型远程滑动的部分,以南部正对的小型宽浅冲沟作为流通区,呈流滑状向南滑动,并转向东南方向进入狭窄的主沟道继续发生远程流滑,沿主沟道不断堆积形成长条状滑体(图6)。

    图  6  流通区—沟道堆积区及堰塞湖
    Figure  6.  The sliding body flowing along the channel and barrier lake

    流通区沟道地形较宽缓顺直,沟道坡度大。塑流状滑体沿流通区快速向下游滑移,在流通区滑坡堆积物较少,厚度2 m左右。

    滑体主要沿主沟道堆积,长590 m,宽度10~35 m,面积1.5×104 m2,平均厚度4 m,体积约6×104 m3,占永光1#滑坡总体积的1/4。长宽比达24,呈典型的长条状,平面形态及滑动方向主要受沟道控制。主沟道平均坡度6.5°,最陡段9.3°,最缓段5.5°,在流通区与主沟交汇处、沟道较缓地段滑坡堆积厚度较大,最厚处6 m左右。滑坡前舌位于主沟沟口小桥处,厚度2 m左右,表面散布破坏民房的木材,小桥受滑坡推挤而拱起、开裂(图7)。滑坡在主沟道滑动过程中,未出现因刮铲两岸沟坡形成的滑塌和弯道外侧的明显超高。

    图  7  滑坡前舌及挤压变形的小桥
    Figure  7.  Small bridge at the front lobe of the landslide with extrusion deformation

    同时,在流通区与主沟汇合处有部分滑体向上游滑动约40 m,堵塞沟道,形成小型堰塞湖。

    根据滑坡形态特征、滑坡前后遥感影像、地质环境条件和观察到的部分滑动过程资料分析认为:永光1#滑坡在滑源区—平台堆积区和流通区—沟道堆积区经历了2次加速—减速过程,滑动过程复杂多变、特征差异明显。

    在强烈地震、滑坡灾害叠加的情景下,永光1#滑坡最初的滑动情况难以知晓。根据树木、滑坡迹象调查对比分析,滑源区—平台堆积区的滑距多为50~100 m,主滑方向210°。滑坡后部及东侧滑距50 m左右,滑速较低,东侧形成鼓丘,压埋村民院落。西侧中前部滑动相对强烈,最大滑距约130 m,估计最大滑速3~4 m/s,部分滑体滑入前部沟道,形成向南敞开的长条状洼地。受强烈地震作用,永光1#滑坡失稳后快速滑动,但加速阶段持续时间短;在孔隙水压力作用下,后期减速阶段滑速较低而持续时间较长。

    在平台南部与流通区上部衔接处中部有未滑动坡体,顶部滑坡堆积较厚、地面反翘;前缘部分滑体从其东西两侧分两股滑入流通区。据此分析认为,滑坡前缘一带的滑速较低且部分已发生制动,部分滑体以很低速度持续滑入下部冲沟。否则,前缘中部临空条件好、强度低的未滑动土质坡体很难抵抗上部快速滑坡的推力而保留下来,也将有更多滑体来不及制动而滑入前部流通区。

    为滑坡南部小型冲沟,高差40 m,水平长度约150 m,沟道坡度14.5°,相对较陡。冲沟呈上部宽浅、下部窄深的漏斗状,方向195°。漏斗口正对滑坡前缘,成为永光1#滑坡前缘部分滑体进入冲沟继续滑动的通道,是该滑坡滑动的重要加速段。滑体分两股从未滑动坡体两侧进入流通区,以西侧通道为主,又开始加速滑动。到达沟底时部分滑体堆积,并在对岸斜坡区形成垂直高度约4 m的逆冲超高(图89)。

    图  8  流通区下段与主沟道处滑坡堆积
    Figure  8.  Landslide deposits in the lower section of the runout zone and the accumulation of sliding body in the main channel
    图  9  流通区滑速计算剖面简图
    Figure  9.  Simplified profile for calculating slide velocity in the runout zone

    永光1#滑坡前缘部分滑体从流通区上部到对岸逆冲停滑处的高差Δh为34 m,水平滑距Lmax为190 m,根据常用的架空坡理论公式[5]

    $$ {\mathrm{tan}}{\varphi _{\rm{r}}} = \frac{{\Delta h}}{{{L_{\max }}}} = 34/190 = 0.179 $$ (1)

    得出流通区滑带土的滑动摩擦角$ \varphi_r $为10.1°。

    滑坡到达流通区底部时滑速最大。高差取38 m,滑距取180 m,按式(2)[19]求得最大滑速 :

    $$ v = \sqrt {2g\left( {\left. {\Delta h - L \cdot {\mathrm{tan}}{\varphi_r}} \right)} \right.} = 10.6\;{\text{m/s}} $$ (2)

    另外,滑体从流通区底部急剧转弯75°进入主沟道时,弯道外侧超高(d)为4 m,转弯半径(r)为23 m,滑体宽度(b)为16 m。按照滑坡弯道处滑速计算式(3)[20]

    $$ v=(gdr/b{)}^{0.5}=(9.8\times 4\times 23/16{)}^{0.5}=7.5\;{\rm{m/s}} $$ (3)

    用以上两种方法求得永光1#滑坡前缘部分到达流通区底部时的滑速为7.5~10.6 m/s。实际上,本滑坡为黏滞性较高的流滑型黄土滑坡,上述两种方法计算的滑速均可能大于实际滑速。式(2)也未考虑滑动过程中滑体变形等耗能,计算滑速更大。

    图8可知,滑体在流通区下部与主沟交汇处受到对面斜坡区的迎头阻拦,发生逆冲爬高、大角度转弯和部分滑体突然制动堆积,滑坡运动状态发生了显著变化。但爬高堆积物边界较为规整,并未出现抛出、边缘溅起散落等现象,与本文分析和计算结果基本相符。而文献[15]认为永光1#滑坡在流通区底部的滑速达25 m/s。

    受地形条件控制,从流通区进入主沟道的大部分滑体转向75°,滑向下游主沟道。由于滑坡滑入主沟时的滑速较高,且大量堆积使沟道坡降加大,滑坡在主沟道中起初的滑速也较快,估计为4~5 m/s,否则在支沟与主沟交汇处将有更多的滑坡堆积。滑坡沿主沟下滑200 m后再转向为160°滑至沟口小桥处,滑速沿途逐渐减缓并堆积。

    地震当天几位地质灾害应急调查的专业人员在赶往永光1#滑坡灾害点的途中,中午12时观察到滑坡前舌距小桥还有80 m左右(图7),下午3时返回时滑坡前舌到达了小桥处。地震当天航拍的图2中也显示滑坡前舌还未到达小桥处。这为分析永光1#滑坡在主沟道中的滑动状态和过程提供了现场依据。从地震诱发滑坡发生到下午3时,滑坡在主沟道的滑动时间总体经历了约7 h,滑距590 m,平均滑速0.023 m/s。其中,前4 h的平均滑速为0.035 m/s,后3 h的平均滑速为0.0075m/s。可见,滑坡在主沟道中处于较为缓慢的流滑状态。而文献[15]模拟计算的永光1#滑坡的整体滑动过程仅为120 s,平均滑速8.3 m/s,差异甚大。其原因可能与对永光1#滑坡的实际滑动过程与特征调查分析不够,以及模型计算参数取值有关。

    综合分析永光1#滑坡上述滑动过程和滑速变化情况,建立滑动过程的地质概念模型(图10),显示出该滑坡经历了2次加速—减速的滑动过程。较客观地反映了复杂的滑动过程,为认识永光1#滑坡的滑动过程和机理,进一步进行数值模拟提供了基础资料。

    图  10  永光1#滑坡滑速分析图
    Figure  10.  Analysis of the sliding speed of the Yongguang 1# landslide

    永光1#滑坡在前期降水入渗和地震的耦合作用下发生,其远程滑动主要与地震、高含水率滑带土孔隙水压力升高、液化和特殊沟道地形有关。

    2013年岷县降水量高于年平均值,其中7月份高出当月平均值100 mm(图11)。降水增加坡体自重的同时,也有部分入渗补给地下水,对斜坡稳定性产生不利影响[21]

    图  11  2013年岷县降水量曲线
    Figure  11.  The precipitation curve for Minxian County in 2013

    永光1#滑坡距“7•22”岷县漳县Ms6.6级地震震中仅4 km,地震场地效应明显。据甘肃省地震局王谦模拟[22],该黄土斜坡地表加速度PGA达0.291~ 0.355 g。黄土孔隙发育且底部地下水较丰富,强烈地震使黄土结构破坏并产生体积剪缩,产生孔隙水压力,导致斜坡稳定性急剧降低,发生失稳滑动。结合滑源区地质条件进行动三轴试验,振动作用下饱和滑体产生的动孔隙水压力情况如图12[22]。动应力(σd)为20 kPa时,最大动孔隙水压力比(Ud/σ0')为0.53;动应力为 25 kPa时,试样破坏时动孔隙水压力比为0.60;动应力为30 kPa时,试样破坏时的动孔隙水压力比为0.41;动应力为35 kPa时,试样破坏时的动孔隙水压力比仅为0.31。随着循环振次的增长动孔隙水压力增长速率总体加快。

    图  12  滑体土动三轴试验曲线图
    Figure  12.  Dynamic triaxial test curve of the sliding soil

    滑动过程中饱和滑带土孔隙水压力进一步升高或发生液化,摩阻力大幅降低,饱和黄土滑带的孔隙水压力消散缓慢,持速效应明显。另外,滑源区后期发生的小型黄土滑坡叠加在滑坡洼地区,对中前部滑体产生推挤作用,也进一步加剧了其滑动。使永光1#滑坡在宽缓的平台区滑距达130 m,滑速3~4 m/s。也为滑坡前缘部分进入沟道继续滑动创造了条件,提供了动力。

    永光1#滑坡滑源区的滑动面受黄土与泥岩接触面控制,属黄土—泥岩接触面滑坡。甘肃黄土地区由降水诱发的此类滑坡的滑速较低[2324],滑距一般为数十米[2526]。“7•22”岷县漳县Ms6.6级地震加速了滑源区黄土斜坡的失稳和快速滑动。相对区内一般黄土—泥岩接触面滑坡,在降水和地震的耦合作用下,地震触发的永光1#滑坡在平台区的滑速和滑距均明显较大。

    滑坡前部流通区高差40 m、坡度14.5°,上部漏斗状开口正对滑坡前缘,延伸方向与滑动方向基本一致,顺直通畅,有利于滑坡的滑入和快速滑动。在较陡流通区的滑动过程中,高含水率滑带土因快速滑动进一步产生孔隙水压力并部分液化,滑坡再次加速滑动,在下部与主沟交汇处的滑速达7.5~10.6 m/s,是永光1#滑坡滑速最大的部位,也为滑坡沿主沟道继续滑动提供了动力。

    高含水率滑带土中产生的孔隙水压力,在永光1#滑坡的滑动过程中始终起到重要减阻作用。据试验,滑体的含水率普遍在25%~30%,洛阳铲取出岩芯呈泥状,敲击振动时液化严重(图13)。

    图  13  饱和滑体振动液化现象
    Figure  13.  Vibration-induced liquefaction in the saturated sliding mass

    滑坡沿主沟道滑动过程中孔隙水压力的作用更加明显。主沟道沟底平均坡度6.5°,最大为9.3°。而滑体在流通区滑动时,受孔隙水压力作用的影响,滑带土滑动摩擦角为10.1°。按照此滑动摩擦角,滑坡不会沿主沟道滑动,且滑体沿主沟道滑动时主地震的作用过程也已结束。所以,滑坡沿较缓主沟道的滑动存在更高的孔隙水压力。

    7月21日8时至7月22日8时地震发生时,永光村周围雨量站记录的降水量为11~24 mm,使地表土体处于高含水状态[17]。沟底主要由吸水性和膨胀性强、渗透性弱的古近系软弱泥岩组成,地震前期的大量降水和沟道洪水的浸泡,使沟道泥岩表层非常湿滑,摩阻力很低。当滑坡借助在流通区形成的较高滑速进入表部高含水的主沟道时,强烈的剪切和挤压使沟底浸泡软化的泥岩发生剪缩,在滑带土中进一步产生较高孔隙水压力,甚至发生部分液化,摩擦力大幅降低(图14),使其沿主沟道缓慢滑动。主震后频繁发生的余震(包括90 min后的Ms5.6级强余震)也有助于滑带土的液化和滑坡继续滑动。

    图  14  主沟道中的饱和滑带与液化现象
    Figure  14.  Saturated sliding zone and liquefaction phenomenon in the main channel

    降雨入渗条件下,圈闭的沟底地形、滑带土的低渗透性,易在滑带产生孔隙水压力且消散非常缓慢,托浮滑体缓慢向下滑动,持速效应更加明显(图15)。因此,在圈闭沟底中渗透性差且高含水率的滑道和滑体的有利组合产生孔隙水压力,降低有效应力和摩阻力,为滑坡的滑动提供了湿润的滑道,沿主沟道缓慢而持续地发生远程滑动。

    图  15  沟道中滑带孔隙水压力作用模式图
    Figure  15.  Schematic diagram of pore water pressure in sliding zone of the channel

    滑坡在沟道中缓慢滑动,说明其重力产生的下滑力略大于滑带土摩阻力,接近平衡状态,由于滑动状态的滑带土黏聚力近似于零,滑动摩擦角略小于沟道坡度。考虑滑带土孔隙水压力作用时,受力状态简化为:

    $$ W\sin\alpha > (W\cos\alpha-P\mathit{\mathrm{_W}})\mathit{\mathrm{tan\mathrm{ }}}\varphi $$ (4)

    式中:W——单位面积滑带上部滑体重量/kPa;

    Pw—单位面积滑带土孔隙水压力/kPa;

    α——滑道倾角/(°);

    $ \varphi $——滑带土摩擦角/(°)。

    由于Pw/Wcosα=ruru为孔隙水压力比。代入式(4)整理后得:

    $$ r_{\rm{u}}>1-\mathrm{tan}\alpha/\mathrm{tan}\varphi $$ (5)

    根据资料,α取6.5°,$ \varphi $取20°[15]。按照式(5)计算,主沟道段滑带土中的ru的平均值大于0.68。可见,孔隙水压力是永光1#滑坡在主沟道缓慢滑动的重要因素。

    随着滑坡在流通区和主沟道中累计740 m的远程滑动,该滑坡由最初的黄土—泥岩接触面滑坡转化为流滑型黄土滑坡,也是永光1#滑坡的重要滑动特征之一。

    综上所述,永光1#滑坡总滑距870 m,滑动总历时7 h,属远程非高速滑坡。近年来,黄土高原区此类滑坡时有发生,需引起重视和深入研究。

    (1)“7•22”岷县漳县Ms6.6级地震触发的永光1#滑坡总体积23×104 m3,滑坡主要由马兰黄土组成,平面形态呈不规则的“L型”。大部分滑体堆积在前部平台区,前缘6×104 m3滑体沿下部沟道远程滑动并不断堆积。可分为滑源区—平台堆积区和流通区—沟道堆积区两部分。

    (2)永光1#滑坡最大滑距870 m,前后缘高差与长度比为0.17,属远程滑坡;经历了2次加速—减速的复杂滑动过程,不同部位滑动特征差异较大;滑源区—平台堆积区滑距50~130 m,最大滑速3~4 m/s。流通区—沟道堆积区滑坡沿顺直较陡的流通区加速滑动150 m,最大滑速7.5~10.6 m/s;主沟道段滑距590 m,滑动持续时间7 h,平均滑速0.023 m/s。属远程非高速滑坡。

    (3)滑源区底部饱和黄土广泛分布,地震前连续降水和沟道洪水对沟底古近系泥岩的浸泡和软化,高含水率滑带土在地震和滑动过程中产生孔隙水压力及部分发生液化。圈闭的沟底地形和滑带土的低渗透性,产生更高孔隙水压力且消散非常缓慢,使全滑程摩擦力大幅降低,持速效应明显,是永光1#滑坡远程滑动的主要原因。

    (4)在滑源区—平台堆积区滑坡类型为地震和前期降水耦合作用触发的黄土—泥岩接触面滑坡;前缘部分滑体沿沟道发生远程流滑,形成特殊的长条状流通区—沟道堆积区,最终转化为流滑型黄土滑坡。

    致谢:地震当天到达滑坡现场的何文贵、王世宇、张永军教授级高工、苏永奇博士提供了宝贵照片和滑坡滑动情况的资料,在此一并致谢!

  • 图  1   柠条锦鸡儿根系锚固黄土斜坡浅层土体及其锚固机理示意图

    Figure  1.   Schematic diagram of the shallow soil body of the C. korshinskii roots anchoring loess and its anchoring mechanism

    图  2   假定滑动面以上根系原位挖掘试验

    Figure  2.   In-situ excavation test of root system above the hypothetical sliding surface

    图  3   简化边坡模型

    注:图中红色线段为限定的滑动面剪入口和剪出口的区域。

    Figure  3.   Simplified slope model

    表  1   试验株地上植株和根系形态学指标统计

    Table  1   Statistical analysis of morphological indexes of above-ground plants and root system above the hypothetical sliding surface of the testing plants

    植株及根系指标 树龄/a 冠幅/m 株高/m 总根数/根 根幅/m 一级侧根/根 二级侧根/根 三级侧根/根 主根平均根径/ m 总根长/m 根表面积/m2
    取值 11 1.76 2.20 27 1.99 20 3 3 0.027 34 0.87
    下载: 导出CSV

    表  2   试验株根周土体物理力学特性

    Table  2   Physical and mechanical properties of root-soil interface of the testing plants

    物理力学性质 天然密度/(g·cm−2 天然含水量/% 黏聚力/kPa 内摩擦角/(°)
    取值 1.40±0.08 7.56±1.21 15.83±7.97 13.56±4.25
    下载: 导出CSV

    表  3   试验株锚固段根系原位拉拔试验结果

    Table  3   In-situ pullout test results of root system in anchoring section of testing plants

    序号抗拔出力/kN拉拔端根径/m断裂方式
    10.2060.007根皮和木质部同时拉断
    21.2430.020
    30.5190.005
    41.6660.012
    51.1680.011
    下载: 导出CSV

    表  4   试验株生长于不同条块时锚固反力计算参数与结果

    Table  4   Calculation parameters and results of anchoring reaction force of testing plants growing on different blocks

    条块号 根-土间黏聚力/kPa 根-土间摩擦角/(°) 单根平均根径/m 根系垂直埋置深度/m 静止土压力系数 土体重度/(kN·m−3 锚固反力/kN
    1 8.31 12.51 0.040 0.461 0.6 17.7 0.513
    2 8.31 12.51 0.033 1.204 0.6 17.7 1.207
    3 8.31 12.51 0.029 1.655 0.6 17.7 1.546
    4 8.31 12.51 0.027 1.912 0.6 17.7 1.709
    5 8.31 12.51 0.027 2.000 0.6 17.7 1.805
    6 8.31 12.51 0.027 2.000 0.6 17.7 1.805
    7 8.31 12.51 0.027 1.930 0.6 17.7 1.729
    8 8.31 12.51 0.029 1.748 0.6 17.7 1.650
    9 8.31 12.51 0.033 1.485 0.6 17.7 1.529
    10 8.31 12.51 0.033 1.147 0.6 17.7 1.141
    11 8.31 12.51 0.036 0.737 0.6 17.7 0.762
    12 8.31 12.51 0.040 0.257 0.6 17.7 0.278
      注:表中单根平均根径取对应条块滑动面深度范围内主根平均根径;根系垂直埋置深度取值为对应条块滑动面深度,即滑动面中间点至地表的距离。
    下载: 导出CSV

    表  5   柠条锦鸡儿根系锚固作用下边坡潜在滑动面稳定性系数计算结果

    Table  5   Calculation results of stability coefficient of potential sliding surface in the C. korshinskii roots system anchored slope

    锚固力作用
    条块号
    土条
    自重
    /kPa
    土条
    宽度
    /m
    圆弧破坏面切线与
    水平面的夹角/(°)
    根系锚固段轴线与
    该处破坏面切线
    之间的夹角/(°)
    根系水平
    间距/m
    圆弧破坏面
    处土体内
    摩擦角/(°)
    圆弧破坏面处
    土体黏聚力
    /kPa
    根系锚
    固力/kN
    稳定性
    系数
    增幅/%
    1 7.374 0.903 57.966 32.034 2 12.510 8.310 0.513 1.3391 0.156
    2 19.252 0.903 50.216 39.784 2 12.510 8.310 1.207 1.3417 0.349
    3 26.465 0.903 43.601 46.399 2 12.510 8.310 1.546 1.3426 0.417
    4 30.577 0.903 37.659 52.341 2 12.510 8.310 1.709 1.3427 0.427
    5 29.967 0.839 32.352 57.648 2 12.510 8.310 1.805 1.3425 0.414
    6 29.967 0.839 27.531 62.469 2 12.510 8.310 1.805 1.3420 0.378
    7 30.434 0.891 22.776 67.224 2 12.510 8.310 1.729 1.3413 0.324
    8 27.560 0.891 18.043 71.957 2 12.510 8.310 1.650 1.3406 0.272
    9 23.414 0.891 13.435 76.565 2 12.510 8.310 1.529 1.3399 0.217
    10 18.082 0.891 8.914 81.086 2 12.510 8.310 0.513 1.3388 0.134
    11 11.619 0.891 4.449 85.551 2 12.510 8.310 1.207 1.3379 0.071
    12 4.055 0.891 0.010 89.990 2 12.510 8.310 1.546 1.3372 0.018
    A - - - - - - - - 1.3510 1.047
    B - - - - - - - - 1.3519 1.111
    C - - - - - - - - 1.3508 1.035
    对照 - - - - - - - - 1.3370 -
      注:该项研究根系所能提供的锚固力取值为试验株根系单株作用在各条块时对应的锚固反力,并假设锚固力方向铅垂向下,故灌木根系锚固段轴线与该处破坏面切线之间的夹角为滑块滑动面切线与铅垂方向的夹角;灌木根系的水平间距取值由野外实际测量数据确定。
    下载: 导出CSV
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  • 收稿日期:  2022-08-14
  • 修回日期:  2022-10-13
  • 网络出版日期:  2023-07-26
  • 刊出日期:  2023-10-30

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