Strengthening of precast reinforced concrete metro tunnel lining to prevent settlement in weak surrounding soils

Purpose – to establish the effect of tunnel settlement on the opening of joints between segments and to substantiate a strengthening method that will prevent further settlement accompanied by joint opening to critical values. Methodology. The effect of tunnel settlement on the opening of joints between segments was determined by theoretical analysis of survey monitoring data. To determine the effect of ground loss beneath the tunnel and to substantiate a lining strengthening method that will prevent settlement, the stress-strain state of the tunnel and the surrounding soil mass was analysed by the finite element method (FEM) using the LIRA-SAPR software package. Findings. An equation was derived for estimating the joint opening width between the segments of a reinforced concrete segmental metro tunnel lining based on survey monitoring data. This equation makes it possible to assess the hazard of tunnel settlement caused by soil washout through opened joints. For the case considered, it was established that at a settlement of 25 mm the joint opening width reaches 0.6 mm. This exceeds the particle size of fine sands (0.2 mm), which caused their washout into the joints and the observed settlement. Such tunnel sections require continuous instrumental settlement monitoring and strengthening of the lining where settlements occur. A structural and technological solution is considered for strengthening the reinforced concrete lining with longitudinal concrete beams reinforced with steel or composite (FRP) bars, placed within the track-bed concrete and anchored to the invert segment of the lining. A finite element model was developed of the segmental reinforced concrete metro tunnel lining without tension ties between segments and of the surrounding soil mass, and the patterns of strain and stress distribution caused by the formation of a settlement trough beneath the lining were obtained. The analysis of the stress-strain state of the tunnel lining and the surrounding soil mass showed that, when the foundation soil is removed, tunnel settlements can reach hundreds of millimeters, and the following stresses develop in the lining: compressive – not more than 9.9 MPa, tensile – up to 2.52 MPa. Such compressive stresses are not critical, whereas the tensile stresses may cause cracking of the lining. The proposed strengthening reduces the settlement by a factor of 1.55, the compressive and tensile stresses in the lining by a factor of 1.08 … 1.13, and the internal forces in the lining by a factor of 1.18-2.83. Originality. For the first time, an equation was derived for estimating the joint opening width between the segments of a reinforced concrete segmental metro tunnel lining based on survey monitoring data. An original finite element model was developed of the segmental reinforced concrete metro tunnel lining without tension ties between segments and of the surrounding soil mass, and the patterns of strain and stress distribution caused by the formation of a settlement trough beneath the lining were obtained. Practical value lies in the possibility of assessing the hazard of tunnel settlement caused by soil washout through opened joints and preventing it by strengthening with longitudinal beams placed within the track-bed concrete and anchored to the invert segment of the lining.