Rock discontinuities such as joints, faults or bedding planes play a dominant role in the whole conduct of rock mass. Joint geometry is usually described by the primary characteristics such as joint shape, planarity, size, persistence, location, spacing, orientation, aperture and roughness. Rock masses featuring joints have generally less strength, tends to deform and are highly anisotropic unlike intact rocks. Seismic response of rock mass is also affected by existence of rock joints. P-waves also arises on detonation of explosive column in a borehole. P-waves encounter these joints resulting in partial reflection and transmission. P-wave velocity (Vp) has been ascertained to be in close relation with rock properties by researchers. Various factors including rock type, pore water, grain-size and shape, porosity, density, anisotropy, temperature and confining pressure along with rockmass properties such as weathering, bedding planes and joint belongings (gouge material, water, dip and strike) influence the P-wave velocity. Several methods have been used to determine the P-wave velocity in the field as well as in the laboratory. In the present study, an application of Artificial Neural Network (ANN) is introduced for the prediction of P-wave velocity of model jointed rocks. Different joint orientation angles and different roughness coefficients as per Barton's standard of jointed rocks and their uniaxial compressive strength has been considered as input parameters. A one-layered network has been trained with the feed-forward back propagation algorithm. On investigating, it has been concluded that artificial neural network is indeed efficacious in predicting the P-wave velocity of jointed rocks. The coefficients of correlation values within the predicted and ascertained values are found out to be high and encouraging.

1. Introduction

During blasting in rocks, the stress waves come into existence on detonating the explosive column in borehole. At the moment of initiation, the detonation pressure which is quite high on the borehole wall actuates the shock wave in the neighboring rockmass and then gradually decreases propagating at the same velocity as of longitudinal wave. Gas pressure loading is responsible for triggering of cracks in the borehole which straightaway travels after longitudinal wave. Extension and propagation of cracks is caused by entering of the same longitudinal waves into the cracks. So, a deep insight of longitudinal waves velocity i.e. P-wave velocity is a pre-requisite for proper utilization of explosive energy, rockbolt reinforcement and grouting.

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