Numerical stability assessment for slopes and boulders subjected to blasting

Abstract

The use of blasting for rock excavation is frequently employed in the construction of underground tunnels and site formation works. In some urban areas, such as Hong Kong, blasting operations are often conducted in close proximity to buildings and geotechnical features such as soil slopes and boulders. The control of blasting works is usually carried out by site monitoring with an allowable ground vibration defined in terms of peak particle velocity (PPV). In Hong Kong, a pseudo-static method is commonly used to assess the stability of geotechnical features that could be affected by blasting. This method applies an equivalent static blasting force to the feature under consideration within a limit equilibrium analysis. However, it does not consider the dynamic nature of transient blasting waves nor the frequency content that affects the soil dynamic responses. In order to have a better understanding of the dynamic effects of blasting on the stability of soil slopes and boulders, this paper presents a pilot study examining the effects of blasting on slopes and boulder fall based on non-linear, dynamic numerical analyses. Blasting time histories recorded from different sites are used as bedrock input beneath slopes within the numerical analyses. The soil slope displacements and eccentricity of boulder from the numerical analyses are calculated based on a range of blasting records with PPV from 25 mm/s to 100 mm/s. The key findings indicate that the empirical formula for predicting displacements of slopes subject to earthquakes would be overly conservative for application of blasting motion due to different frequency content.