Oriented Jet Grouting 'M' Supports For Underpinning Historical Foundations-Paper

Paper

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The paper presents an “M”-shaped jet-grouting method for underpinning historic masonry foundations where drilling through the foundation or excavating beneath it could cause damage. The method is intended to increase bearing capacity and stiffness while confining soil and rubble beneath the foundation, including where access is restricted.<br /><br />Small-diameter vertical and inclined jet-grout columns are installed from the foundation’s edges. Together, they form an enclosing structure around a wedge of soil, making the confined soil part of a composite foundation system. Installation is sequenced to maintain support: perimeter vertical columns are placed first, followed by overlapping inclined columns from opposite sides. Spacing between uncured columns is important because fresh grout temporarily has little shear strength; six column diameters is suggested as a conservative starting point, subject to site-specific assessment.<br /><br />The authors evaluated the approach using finite-element modelling of a 3 m-wide foundation. The model compared existing conditions, complete underpinning, and M-shaped underpinning. The estimated bearing capacity increased from 431 kPa in the existing condition to 1,015 kPa for both underpinning options. The M-shaped arrangement also produced settlement results equivalent to complete underpinning, with approximately 11 mm of additional settlement under increased loading.<br /><br />The paper emphasizes that successful implementation depends on adequate soil encapsulation, verified column overlap and alignment, test grouting, and monitoring of both structural movement and grouting operations. The authors conclude that M-shaped jet grouting can provide effective support without penetrating fragile historic foundations, particularly where granular soils are present.

Keywords

jet grouting

historic masonry foundations

underpinning

bearing capacity

foundation stiffness

inclined grout columns

soil encapsulation

finite-element modelling

settlement analysis

historic preservation