The diffusion of turbulent submerged flows is studied to understand the core mechanism of jet grouting and to enhance the efficiency of treatments. Because of the particularly high speeds, experimental measurement is very difficult, and theoretical relationships found at lower velocities are misleading. The velocity distribution and its relationship with the injection parameters are then obtained, combining experiments, numerical calculations and theory. A numerical finite-volume model simulating the turbulent propagation of submerged jets is initially calibrated using a set of experimental data reported in the literature. This model is used to perform a parametric study, varying the injection parameters within the typical range of jet grouting and evaluating their influence on the velocity distribution. The results of numerical calculations are finally interpreted with a mathematical function relating the velocity distribution to the nozzle diameter, inlet velocity and fluid composition. The effect of insulating the injected fluid with a coaxial air jet is also investigated, to clarify the principles of double- and triple-fluid jet grouting systems.