Permeation grouting of fine grained pyroclastic soils

Abstract A research activity was recently started at the University of Napoli Federico II on the unusual application of permeation grouting to low permeability soils and rocks. The project is aimed to find a remedial mean to some of the geotechnical hazards which involve the subsoil of Napoli, mostly made in its upper part of collapsible unsaturated pyroclastic silty sands (pozzolana). This paper refers on the results obtained by treating pozzolana with sodium silicate solutions made of water with a rather little percentage of silicates and of an inorganic additive. Laboratory injection tests and numerical simulations of the permeation process were first carried out, showing that the permeation times for the adopted grouts and soil are reasonable. Notwithstanding the low permeability of pozzolana, in fact, the typical unsaturated site conditions help in making permeation grouting possible if very low viscosity grouts are adopted. To fit this severe viscosity constraint, very diluted solutions have to be used. Because of the well known chemical activity of pozzolana grains, grouting can be effective even with such diluted grouts. Chemical tests were carried out in laboratory on both grouts and grouted materials: the influence of the percentage of additive on grout gelification time and the evolution of the reticulation process within the grout were carefully analysed, as well as the chemical and physical interaction between pozzolana and grout. Mechanical laboratory tests were carried out on specimens of grouted pozzolana to quantify the effect of grouting. The results clearly indicate that the very diluted silicate grouts adopted are effective in this case, improving mechanical properties of pozzolana not only from a physical point of view, that is by building up a polymeric net within the voids, but also with a direct chemical interaction among the grout and the soil grains. The mechanical properties of grouted pozzolana depend on curing time, grout content and surrounding moisture conditions. In the lowest moisture curing conditions, the largest mechanical strength are reached in no more than 30 days, while much longer curing times are needed in very humid curing conditions. All other factors being the same, the strength of the grouted material increases as the amount of retained grout increases. Whatever the grout content and curing moisture conditions, grouting increases the cohesive strength component. As a consequence, it helps in mitigating the hazard posed by the collapsible behaviour of the untreated pozzolana. The reduction of permeability was not a goal of this research, and so no specific analyses where carried out to check the reduction of such parameter upon treatment. However, the available SEM pictures indicate that some reduction must be expected, because both the porosity and the dimensions of the voids decrease upon grouting.