Pre-stressing of soil and structures due to jet grouting

Jet grouting is a widely used technique for soil stabilisation, which provides support to geotechnical structures and buildings. One of the main problems related to this technique is excessive displacement and occasionally high pressures induced on structures in direct contact with the jetted area. This paper studies another possible problem caused by jet grouting: the excessive pre-stressing of retaining walls and soil in an excavation pit and the subsequent pressure release to the retaining structural elements, the steel struts, during and after excavation. In the example described here, the construction of a deep jet grouting slab, equivalent to a soil-embedded strut, induced stresses in the soil and in the diaphragm walls of a test shaft. The stresses were released to the steel struts during the subsequent excavation. These additional stresses could potentially exceed the design loads and, in the worst-case scenario, lead to failure. In this case study, the forces released to the struts after the excavation were 2·5 times higher than those predicted by considering only earth pressure without introducing the pre-stressing induced by the jet grouting. Thanks to application of the observational method and adequate risk management strategy, no failure occurred.