The characteristic strength of jet-grouted material

In spite of being one of the most popular ground improvement techniques, jet grouting is often viewed with suspicion, its properties underestimated and its role downgraded to provisional or subsidiary, mostly because of the lack of reliable methods to manage uncertainty. The random composition and scattered properties of the material observed at the small scale of laboratory tests lead designers to assume arbitrarily unjustified over-conservative strength. A methodology is herein introduced to quantify uncertainty, characterising the different factors of variability in the jet-grouted material and simulating their effects on representative structural elements. An experimental database representative of a broad range of situations, consisting of laboratory tests on samples cored from jet-grouted columns and sonic tomography scans performed on large blocks, is first examined to characterise the mechanical response of the jet-grouted material and its variability. Probabilistic and autocorrelation functions are then introduced to simulate the stochastic and spatially correlated variability and virtually reproduce realistic populations of samples. The virtual simulation of uniaxial compression tests on these samples with a three-dimensional finite-element method code leads to the statistical distribution of the strength being obtained and the mean and variance being computed. Repeating systematically this process for the variable parameters of the original functions and for different shapes and dimensions of the samples leads to general formulas expressing the statistical distribution of the uniaxial compressive strength. The inferred relations are finally combined to calculate correcting factors giving the characteristic strength of prismatic jet-grouted elements with variable dimensions and slenderness, starting from the distribution observed at the laboratory scale.