冲击荷载下花岗岩残积土的滞回曲线特征与损伤定量评价

为评价冲击荷载下花岗岩残积土的损伤发展规律,基于不同振幅(A=100~400 kPa)、频率(f=3~15 Hz)和围压(σ'3=50~500 kPa)下室内循环冲击试验得到滞回曲线的形态特征,提出4个反映试样在冲击荷载下的能量消耗、损伤程度、刚度衰减和塑性应变发展特性的定量结构损伤参数:累积耗散能量E N、累积损伤度d N、刚度退化度δN和残余塑性应变εN。借助各参数的演化规律和相互关系实现了冲击损伤的定量评价,并提出试样的损伤与破坏机理。结果表明,高振幅(A=400 kPa)、低频及超高频(f=3或15 Hz)的冲击荷载作用下E N增长迅速,试样的d N较大。相同频率和围压下高振幅试样的d N为低振幅试样(A=200 kPa)的9.5倍,同等条件下高频和超高频试样的d N也比中等频率(f=10 Hz)试样的d N高出24%。更高的损伤度引起试样刚度衰减更加严重,发生破坏试样的δN普遍超过0.65。这进一步导致试样εN快速发展,最终产生破坏。高围压(σ'3=500 kPa)下试样能量耗散慢,d N较小,δN仅为低围压(σ'3=50 kPa)的13%,因此抵抗冲击变形的能力也增强。根据试验结果指出,冲击荷载下试样的变形与破坏实质上是冲击能量耗散引起的土体结构损伤,从而导致的刚度衰减进而产生宏观塑形变形累积的综合体现。工程中应尽可能避免采用高振幅与低频率及超高频率荷载冲击土体,必要时可通过挤密加固土体有效防范冲击荷载的危害。研究有助于深化冲击破坏机理的理解,为我国花岗岩残积土地层的施工与设计提供技术指导。

Hysteretic curve features and damage quantitative evaluation of granite residual soil under impact load

In order to evaluate damage development law of granite residual soil under impact load,laboratory cyclic impact tests based on different amplitudes,frequencies and confining pressures were conducted to obtain morphological features of hysteretic curves.4 quantitative structure damage parameters including accumulative dissipated energy EN,cumulative damage degree dN,stiffness degradation degree δN and residual plastic strain εN were proposed to reflect specimens’energy dissipation,damage degree,stiffness degradation and development characteristics of plastic strain under impact load.The quantitative evaluation of impact damage was realized by means of evolution laws and relations of parameters,and damage and failure mechanisms of specimens were proposed.The results showed that EN increases rapidly under impact load with high amplitude(A=400 kPa),low frequency or ultra-high one(f=3 or 15 Hz),specimens’dN is larger;under the same frequency and confining pressure,dN of high amplitude specimen is 9.5 times of that of low amplitude one(A=200 kPa),and dN of high frequency and ultra-high frequency specimen is 24% higher than that of medium frequency one(f=10 Hz);the higher the damage degree,the more serious the specimen stiffness attenuation;δN of failure specimen is generally more than 0.65,this further leads to the rapid development of specimen εN,and finally leads to failure;under high confining pressure(σ'3=500 kPa),energy dissipation is slow and dN is smaller,δN is only 13% of that of low confining pressure(σ'3=50 kPa),so the ability to resist impact deformation is also enhanced.According to test results,it was pointed out that deformation and failure of specimen under impact load is essentially damage of soil structure caused by dissipation of impact energy,the caused stiffness attenuation generates the comprehensive embodiment of macro plastic deformation accumulation;high amplitude,low frequency and ultra-high frequency impact load should be avoided as far as possible in the project;if necessary,compaction and reinforcement of soil can be used to effectively prevent damage of impact load;the study is helpful to deepen the understanding of impact failure mechanism and provide a technical guidance for the construction and design of granite residual soil layer in China.