In this paper, a simple expression to predict the average diameter of columns created with single, double and triple fluid jet grouting is proposed considering both the energy of the eroding jet and the resistance of the soil. The injection system (single, double or triple) and the composition of the injected fluids are taken into account on the basis of theoretical analysis of the turbulent diffusion of submerged jets. Instead of separately considering the energy of the air jet shrouding the eroding fluid, a simple parameter is introduced which represents the beneficial effect of air in reducing the energetic dissipation on the jet external surface. In this way, a new expression of the specific kinetic energy of the jet at any distance from the nozzle is proposed, which is a relevant step forward if compared with that at the nozzle or at the pump previously proposed, as it takes into account both the system characteristics and the composition of the eroding jet (either grout or water). In the expression proposed to predict the diameter of jet grouting columns, soil resistance is considered by way of the results of the standard penetration test and cone penetration test, in terms of N-SPT and q(c) respectively. The expression has been calibrated on a number of experimental data, and in most cases proved its efficiency in predicting columns' diameter with a scatter of less than +/- 20%. The field trials results highlight that jet grouting is more effective in coarse-grained soils, as clearly shown by an efficiency parameter proposed by the authors. Design charts are drawn to quickly estimate the average diameter D-a for different soils, injection systems and input energies.
