Foundation Shaft Executed with Jet Grouting-Paper

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The paper examines jet-grouted shaft foundations, which support heavy bridge structures while stabilizing excavation walls and limiting groundwater inflow. A shaft comprises a circular wall of overlapping jet-grouted columns and a watertight bottom plug.<br /><br />The case study is the foundation of a 65 m-high pier of the Bormida River highway viaduct in Italy. The 12 m-diameter shaft was built through permeable alluvial deposits above fractured carbonate rock. Its wall consisted of two concentric rows of 85 columns, each nominally 0.6 m in diameter and 21 m long. An 8.5 m-thick bottom plug was formed from overlapping 0.8 m columns. Field trials established construction parameters, while core sampling and sonic tomography checked column properties and treatment continuity. Compressive-strength tests showed substantial variability, but construction confirmed the treatment’s effectiveness despite groundwater supplied by the river.<br /><br />The authors analyse wall behaviour during excavation. Overlapping columns create an arch effect that transfers lateral soil pressure into compression, while longitudinal elements resist bending. A discontinuity in the wall can dramatically increase bending moments; in the example analysed, interruption of horizontal continuity raised them by about 23 times.<br /><br />Three approaches are compared for estimating the wall’s limiting excavation depth: deterministic calculations using uniform properties, a semi-probabilistic method applying design values derived from statistical variability, and a probabilistic Monte Carlo method that generates variable column diameters, strengths, inclinations, and orientations. Both variability-based approaches predict lower safe excavation depths than the deterministic method. The Monte Carlo analysis gives the lowest limits because it captures irregular wall cross-sections and their higher stresses.<br /><br />The study concludes that probabilistic analysis most realistically represents the geometric and mechanical variability of jet-grouted columns and is therefore the most rational approach to design.

Keywords

jet-grouted shaft

bridge foundation

groundwater inflow

excavation support

overlapping columns

bottom plug

Bormida River viaduct

probabilistic analysis

Monte Carlo method

wall stability