冲击荷载作用下花岗岩残积土的动力损伤与破坏机理

为调查冲击荷载作用下花岗岩残积土的力学行为,开展了不同冲击频率(3～15 Hz)和振幅(100～400 kPa)影响的循环冲击试验,分析冲击荷载引起的超静孔隙水压力和变形的发展规律。结果表明:振幅和频率的影响均存在临界值,振幅与频率超过临界值时,土体损伤强烈会引起强度迅速衰减。低频与超高频冲击下更易产生较高孔压,从而导致有效应力降低进而引起强度下降。根据冲击应力与应变的滞回曲线的形态特征提出了花岗岩残积土冲击动力损伤的3个定量评价参数,并据此提出了3种冲击破坏类型与辨识方法,指出冲击能量耗散引起的结构损伤及塑形变形累积是花岗岩残积土产生冲击破坏的根本原因,其影响程度取决于土的原始结构强度与微观裂隙发育程度,也与冲击模式和应力水平导致的裂隙扩展规律和塑性累积变形大小有关。工程实践中应查明土体在冲击荷载下的临界振幅与临界频率,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。研究有助于了解冲击荷载的作用规律和土体力学响应,为中国花岗岩风化地层的施工与设计提供科学理论指导。

Structural damage and dynamic failure mechanism of granite residual soils under impact loading

A series of impact tests with various frequencies and amplitudes are performed to study the influences of impact loading on the mechanical behaviors of granite residual soils, particularly for the development of deformation and excess pore water pressure. Similar critical values for amplitude and frequency are observed. Once the amplitude and frequency exceed their critical values, the soil is damaged severely and its strength decreases. Under the impact loading with low frequency or ultra-high frequency, higher pore water pressure is generated, resulting in a decrease of the effective stress and strength. Three quantitative parameters of the morphological features of the hysteresis curve are proposed to evaluate the dynamic damage of granite residual soils. Furthermore, three modes of impact damage and their characteristics are suggested. The structural damage caused by impact energy dissipation and accumulation of plastic deformation is proved to be the dominant cause of soil failure. The influence degree of impact loading on the granite residual soils depends on the natural structure strength and the amount of micro-cracks of the soil, as well as the propagation of cracks and plasticity strain induced by impact loading. Based on test results, it is suggested that the critical value for amplitude and frequency of the soils be ascertained before construction and impact loading with high-amplitude and low or ultra-high frequency be avoided. This study can enhance the understanding of the mechanical response of soils under impact loading and provide technical guidance for construction.