Analysis of the mitigating effect of post-grouting on the prestress loss of anchorage system

With the promotion and application of prestressed active support technology in tunnel engineering, issues related to prestress loss and support system durability have gradually gained attention. This study combined laboratory experiments with numerical simulations to elucidate the mechanism through which post-grouting can mitigate prestress loss when the shear stress at the anchorage interface decreases. The laboratory experiments focused on investigating the influence of grout length on the loss mitigation efficacy of the anchorage system. The axial strain of the anchor rod was measured using distributed fiber optics and strain gauges. The results indicated that a loss of axial force occurred in the grout segment; after the stress loss of the anchorage segment, the axial force loss decreased as the distance from the anchoring section increased. As the grout length increased, the axial force loss (Fc2) at the tail end of the grout segment decreased until the grouting length reached >= 50 cm, after which Fc2 no longer experienced a loss. Subsequently, numerical calculations were conducted using the FLAC 3D software to determine the effects of different grout lengths and shear moduli of the grout solid on the loss mitigation of the anchorage system. The results revealed that under the same shear modulus conditions, the required minimum grout length was consistent. Moreover, when the grout length was less than the minimum required length, increasing the grout length and the shear modulus of the grout solid could improve the loss-mitigating effect of post-grouting. However, increasing the grout length improved the loss-mitigating effects compared to increasing the shear modulus of the grout solid. For instance, when the grout length was 20 cm, and the shear modulus of the grout solid was 2 GPa, even if the shear modulus was increased to 18 GPa, the loss rate of Fc2 remained at 32.41 %, whereas increasing the grouting length to 60 cm resulted in a loss rate of Fc2 of 0%. Considering that the shear modulus of commonly used grout materials in engineering ranges from 2 to 10 GPa, there is a minimum grout length that ensures effective grouting outcomes. In Class II surrounding rock, when the initial prestress was 40 kN, the minimum grout length was 799-986 mm, 60 kN resulted in a minimum grout length of 975-1217 mm, and 80 kN resulted in a minimum grout length of 1151-1360 mm. When the initial prestress was 60 kN, the minimum grout length was 975-1217 mm for the Class II surrounding rock, 1133-1294 mm for Class III, and 1338-1364 mm for Class IV.