爆破冲击荷载下花岗岩残积土的力学响应试验研究

Experimental study on mechanical characteristics of granite residual soil under blast loading

地铁隧道盾构施工前,通常需要对风化层中的孤石进行爆破处理,爆破荷载不可避免地对周围土体产生扰动.为了调查爆破冲击荷载对花岗岩残积土力学行为的影响,开展了不同冲击速率、频率、振动峰值应力及有效围压作用下花岗岩残积土的冲击动力试验,分析冲击荷载引起的超静孔隙水压力和变形的发展规律与破坏模式.结果表明:高速冲击引起的颗粒重排会增大土的内摩擦角,但强度增大的同时也会破坏原生结构强度,土体产生更多裂缝;振动峰值应力与频率对残积土的影响均存在着临界值,振动峰值应力与频率超过临界值时,强度软化且变形量增大;低围压下残积土可能会出现剪胀与剪切带破坏;提出的3种爆破冲击破坏类型中,低频高振动峰值应力的冲击条件下易出现严重的冲击破坏,频率3 Hz、振动峰值应力400 kPa是厦门花岗岩残积土介于破坏型与亚稳定型的临界影响值,因此,工程实践中应关注低频、高振动峰值应力的爆破冲击对周围土体带来的危害.研究可为花岗岩风化层中爆破法处理孤石提供施工参数,也可为考虑爆破振动影响的土层的力学参数确定与评价提供技术支持.

Blasting is commonly adopted prior to subway tunnel construction to deal with the boulders in weathered granite formations,which inevitably exercise disturbance on surrounding soils.In order to investigate the influence of blast loading on the mechanical behavior of granite residual soil,a series of dynamic impact tests under various impact rates,frequencies,blast peak stresses and effective confining pressures were carried out.And the development of excess pore water pressure,axial deformation and failure mode were analyzed.The results indicated that the rearrangement of soil particles caused by impact loading will increase effective internal friction angle.However,the original structural strength of soil will be damaged and more cracks emerged at the same time.The critical values for blast peak stress and frequency were suggested.When blast peak stress and frequency exceed the critical value,the strength of soil softens and the deformation increases.Under low confining pressure,dilatancy and shear band failure were observed.Furthermore,three failure modes of granite residual soil under blasting impact loading were proposed.Severe impact damage will be caused under low frequency or high blast peak stress impact conditions.The frequency of 3 Hz and the blast peak stress of 400 k Pa are the critical conditions that determine whether the soil is metastable or seriously destroyed.More attentions should be paid to the hazards caused by blasting impact with the low frequency and high blast peak stress in granite residual soil in Xiamen.This research can provide parameters for the boulder treatment in soil formations disturbed by blasting.