Vibratory energy transmitted through propagating waves generated by machine foundations can be detrimental to adjacent structures or sensitive equipment operating nearby. Most of the vibratory energy affecting structures nearby is carried by surface waves that propagate in a zone close to the ground surface. Thus, for machine foundations vibrating at moderate to high frequencies, vibration amplitudes in the vicinity of the foundation can be reduced significantly by installing an annular in-filled trench wall around it. An in-filled trench wave barrier reduces the ground motion by interception, scattering, and diffraction of the surface waves. This approach of isolating a vibrating machine foundation from its surroundings is known as active isolation. This paper presents a numerical investigation on active isolation of machine foundation by in-filled trench wave barriers in a three-dimensional viscoelastic half-space. A rigorous boundary element method (BEM) algorithm for dynamic analysis of three-dimensional solids has been used to study the effects of various geometric and material parameters on the performance of the in-filled trench barriers. Important dimensionless parameters have been identified and design guidelines are presented.