地震作用下泥石流源区砾石土体强度的衰减实验
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摘要
泥石流源区广泛分布着砾石土,从野外观察分析和理论计算可知,泥石流源区产流是由于土体强度降低的结果。经已往研究发现,地震作用产生的振动经常导致泥石流的发生,世界上许多地震活动带,同时也是泥石流活动带,这种分布的关联性使我们有理由推断地震的作用会使泥石流源区的砾石土体强度降低,导致土源大量激发,促进泥石流的暴发。基于这样的分析,利用土动三轴剪力仪,取泥石流源区的砾石土作为实验对象,以砾石土的含水量,土体的深度(围压),土体的破坏标准和地震的震级(振动周次)为变量,取土体的粘聚力c、有效粘聚力c、土体的摩擦角j 和有效摩擦角j 作为土体强度衡量的指标进行实验。实验结果表明在地震作用下,土体的c值和c值,随含水量的增加先是增加而后降低,其最大值时饱和度达84%左右。随着振动强度的增加,土体的粘聚力c、有效粘聚力c 逐渐减小,高含水土体的c值在足够的振动强度条件下可以降为0。随着含水量的增加,j 和j 值逐渐降低,但j 值仅当饱和土体在强烈振动作用下会降低到0。一般地,土体的内摩擦角j,总是随着振动周次的增加而减低,但减低的幅度较小;而有效内摩擦角j 值的变化相对较复杂,在低地震强度(振动周次20周以下)的作用下,其值随着振动周次(震级)的增加有较小的降低。而在振动周次达到20
There is widely distributed gravelly soil in triggering area of debris flow. From the observation,analysis and calculation,it is found that triggering of debris flow results from soil strength reduction. In fact,debris flow areas are often located in earthquake belt. The distribution relation between debris flow and earthquake makes us to suggest that earthquake lead to the strength reduction of gravelly soil and make triggering of debris flow. On the basis of the suggestion,triaxial shear equipment is used to make testing study for the gravelly soil in the triggering area of debris flow. The variables are water content,soil depth or confining pressure,the destroy criterion and earthquake intensity or vibration times. The cohesion and friction are chosen to measure the strength of soil. As a result,it is found that the cohesion increases first and then drops with the increment of water content,and the max value appears when the saturation degree gets up to 84%. With the increment of vibration times the cohesion decreases. For the sample with high water content,the cohesion can drop to zero with enough vibration times. As usual the friction angle decreases with increment of water content and can drop to zero only for the nearly saturated sample. The friction angle always decreases slightly with increment of vibration time. The friction angle changes with vibration times unstably. When the vibration times are lower than 20 and earthquake intensity is under 7.5 grade,the friction angle decreases slightly with the increment of vibration times,but when the vibration times are higher than 20,the friction angle of low saturated sample increases. For the high saturated sample when the vibration times arrive 50 times,the friction angle drops to zero. For the nearly saturated sample with saturated degree of 92%,the friction angle increases or decreases unstably,which is dependent on the other factors. In conclusion,the tests show that earthquake may reduce the soil strength and under some special cases it may leads soil to be liquefied. Therefore earthquake makes an important role in triggering of debris flow.
There is widely distributed gravelly soil in triggering area of debris flow. From the observation,analysis and calculation,it is found that triggering of debris flow results from soil strength reduction. In fact,debris flow areas are often located in earthquake belt. The distribution relation between debris flow and earthquake makes us to suggest that earthquake lead to the strength reduction of gravelly soil and make triggering of debris flow. On the basis of the suggestion,triaxial shear equipment is used to make testing study for the gravelly soil in the triggering area of debris flow. The variables are water content,soil depth or confining pressure,the destroy criterion and earthquake intensity or vibration times. The cohesion and friction are chosen to measure the strength of soil. As a result,it is found that the cohesion increases first and then drops with the increment of water content,and the max value appears when the saturation degree gets up to 84%. With the increment of vibration times the cohesion decreases. For the sample with high water content,the cohesion can drop to zero with enough vibration times. As usual the friction angle decreases with increment of water content and can drop to zero only for the nearly saturated sample. The friction angle always decreases slightly with increment of vibration time. The friction angle changes with vibration times unstably. When the vibration times are lower than 20 and earthquake intensity is under 7.5 grade,the friction angle decreases slightly with the increment of vibration times,but when the vibration times are higher than 20,the friction angle of low saturated sample increases. For the high saturated sample when the vibration times arrive 50 times,the friction angle drops to zero. For the nearly saturated sample with saturated degree of 92%,the friction angle increases or decreases unstably,which is dependent on the other factors. In conclusion,the tests show that earthquake may reduce the soil strength and under some special cases it may leads soil to be liquefied. Therefore earthquake makes an important role in triggering of debris flow.
引文
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19WongR T,SeedH B,HanC K.Cyclic loading liquefaction of gravelly soils[J].Journal of theGeotechnicalEngineeringDivision,ASCE,1975,101(GT6):571～583
20SeedH B,IdrissI M.Simplified procedure for evaluating soil liquefaction potential[J].Journal of theSoilMechanics andFoundationsDivision,ASCE,1971,97(SM9):1249～1273
2IversonR M.Slope instability from groundwater seepage discussion[J].ASCE Journal ofHydraulicEngineering,1997,123(10):929～930
3CarlaG.Experimental evidence from the triggering of debris flow along a granular slope[J].Phys.Chem.Earth(B),2000,25(4):387～390
4马东涛,石玉成.试论地震在泥石流形成中的作用[J].西北地震学报,1996,18(4):38～42
5李树德,任秀生,岳升阳等.地震与泥石流活动[J].水土保持研究,2001,8(2):26～27
6吴积善.云南蒋家沟泥石流观测与研究[M].北京:科学出版社,1990,62～70
7杜榕桓.云南小江泥石流[M].重庆:科学技术文献出版社重庆分社,1987,103～113
8王裕宜,费祥俊.粘性泥石流颗粒悬浮机理研究[J].中国科学(E辑),1999,29(4):372～377
9唐邦兴.中国泥石流[M].北京:商务印书馆,2000,192～212
10IversonR M,LahusenR G.Dynamic pore-pressure fluctuations in rapidly shearing granular materials[J].Science,1989,246:796～799
11刘颖,谢君斐.砂土地震液化[M].北京:地震出版社,1984,1～101
12IshiharaK,KosekiJ.Cyclic shear strength of fines-containing sands[A].In:Earthquake andGeotechnicalEngineeringJapaneseSoc.[C].[s. l.]:[s. n.],1989,101～106
13PrakashS,SandovalJ A.Liquefaction of low plasticity silts[J].SoilDynamics andEarthquakeEngineering,1992,5(1):2～53
14DaiFuchu,LeeC F.WangSijing.Analysis of rainstorm-induced slide-debris flows on natural terrain ofLantauIsland,HongKong[J].EngineeringGeology,1999,51(3):279～290
15汪闻韶.土的动力强度和液化特征[M].北京:中国电力出版社.1997,1～169
16刘令瑶,李桂芬,丙东屏.密云水库白河主坝保护层地震破坏及砂砾料振动液化特性[A].见:水利水电科学研究院科学研究论文集第8集[C].[s. l.]:[s. n.],1982,181～194
17汪闻韶,常亚屏,左秀泓.饱和砂砾料在振动和往返加荷下的液化特性[A].见:水利水电科学研究院科学研究论文集第23集[C].[s. l.]:[s. n.],1986,195～203
18BanerjeeN G,SeedH B,ChanC K.Cyclic beharvior of dense coarse-grained materials in relation to dams[R].ReportNo.UCB/EERC-79/13,1979
19WongR T,SeedH B,HanC K.Cyclic loading liquefaction of gravelly soils[J].Journal of theGeotechnicalEngineeringDivision,ASCE,1975,101(GT6):571～583
20SeedH B,IdrissI M.Simplified procedure for evaluating soil liquefaction potential[J].Journal of theSoilMechanics andFoundationsDivision,ASCE,1971,97(SM9):1249～1273
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