On the sand response of fracture grouting on a bell tower foundation

Fracture grouting is a rather recent technique as a soil reinforcement intervention. Careful and controlled creation of fractures can greatly improve strength and stiff- ness of the treated soils. However, the control of fractures generation is a difficult task, especially in sand where permeation processes and local inhomogeneities play an important role. Recent laboratory tests have enabled to better understand the governing factors of fracturing processes, but field experiences are still rare and the effect of the interventions can hardly be directly analysed. In this paper a fracture grouting intervention, carried out on the foundation soil of the Frari bell tower in Venice (Italy), is described. The field response of the sandy soil was observed and analysed mainly in relation to the results of the in situ-investigations carried out after the intervention. The comparison with the behaviour of the over- lying silty clay layer is also described. The intervention produced different types of soil-grout interactions in sand: a diffused uncemented varved structure, soft lenses of cement, bentonite and sand and local consistent grout lenses. On the contrary, diffused sub-vertical cement lenses were found in clay. The comparison between the results of in situ tests performed at different stages of the intervention show an overall improvement of soil mechanical characteristics. The intervention has proved to be less effective in sand than in clay, also due to the general lower treatment intensity.