Simple method for the design of jet grouted umbrellas in tunneling

Tunnel excavation in cohesionless soils implies the use of a temporary supporting structure prior to lining installation. This temporary structure has to couple safety and economy, and can be conveniently realized using ground improvement techniques (for instance, by creating an arch of partially overlapped sub-horizontal jet grouted columns). The adoption of ground improvement techniques results into structures far from having a perfect shape because intrinsically affected by defects (in both geometrical and mechanical characteristics), and therefore their design may hide unforeseen risks. As a consequence, this is the typical case in which sophisticated numerical analyses may just give the illusion of being refined, if possible defects are not correctly taken into account. In this paper a simple yet rational analytical method for the design of non-closed tunnel supporting structure that may be of some help to this aim is presented. It is done with reference to a simple 2D scheme. In the first part of the paper, a design chart to design an optimal shape and minimum structural thickness of the cross section of the supporting structure is shown. In the second part, an iterative procedure to verify the stability or to design the minimum structural thickness of an existing supporting structure with a predefined shape is described. This method, coupled with the analysis of structural demand, allows in principle to plot design charts. This approach can easily take into account structural defects with a semi probabilistic approach and therefore with a chosen risk level, being of great help for the designer at least in a preliminary design stage. The proposed semi probabilistic procedure is applied to the case of a temporary supporting structure realized by partially overlapped sub horizontal jet grouted columns, intrinsically affected by defects in diameter and position. The variability of these geometrical parameters was considered based on the large experimental evidence collected in field trials by the authors and published elsewhere. CE Database Subject Headings: Arches, Jet Grouting, Limit Analysis, Probabilistic Methods, Structural Design, Tunnel Supports, Thrust line