Post-Grouting Behaviour of Ground Anchors in Dense Cohesionless Soils: Field Observations and Theoretical Insights

Abstract

Post-grouting is a widely adopted technique in ground anchor systems intended to enhance grout–soil interaction through pressure-induced mechanisms. It is generally assumed to improve load transfer by inducing fissures in the initially set grout, expanding the grout body, and enlarging the fixed-length diameter. This study investigates grout behaviour under post-grouting pressure in dense cohesionless soils using data from 870 post-grouted anchors installed at nine construction sites in Budapest, Hungary, with similar ground conditions. Grouting volumes were statistically analysed, focusing on the correlation between grouting volumes and their relationship to soil properties particularly grain size distribution. Theoretical grout body diameters were estimated using expansion and infiltration models. Load–displacement test data were used to back calculate apparent free lengths and infer fixed lengths, which were compared with literature values. Two post-grouted test anchors were excavated. Both showed uniform diameters matching the original borehole, with no signs of grout body enlargement. However, large irregular grout masses were found in permeable zones, suggesting material migration toward weaker paths under pressure. These findings indicate that in medium dense to dense sands, post-grouting enhances performance primarily through local soil densification not geometric expansion, highlighting the need to revise traditional assumptions regarding grout behaviour in such conditions.