A grouting technique is introduced as a measure to protect tunnels adjacent to a large-scale excavation. Field measurements indicated that this grouting technique reduced the maximum horizontal displacement in the tunnel from 9.2 to 4.6 mm. Based on this application and two field tests, the following recommendations are proposed: (1) the upper boundary of the grouted zone should be set at or below the tunnel; (2) grouting holes should be installed 3–4 m from the tunnel and at an interval of the length of a tunnel ring along the tunnel axis; (3) in a row of grouting holes, grouting should be conducted at an interval of six to eight holes in a day, and the grout volume per hole should be small, such as $0.5{\mathrm{m}}^3$ ; and (4) when using multiple rows of grouting holes, a far-row-first grouting scheme, in which the grouting sequence progresses toward the tunnel, should be adopted.

The response of existing tunnels to adjacent excavation at an oblique intersection angle is still unclear. This study investigated the deformation of an existing tunnel when new excavation was conducted obliquely, based on a case history from Tianjin, China. In this project, a cutoff wall was installed to protect the tunnel from excessive horizontal movement. Finite element analyses were then performed to investigate the optimization of the cutoff wall design. The monitoring results show that due to the oblique intersection, the tunnel rings near the narrow and wide boundaries of the excavation ultimately exhibited heave and subsidence after excavation. The tunnel also underwent significant torsional deformation and horizontal displacement toward the excavation. The maximum horizontal displacement occurred at the tunnel ring closest to the midpoint of the wide boundary of the excavation. The numerical analyses show that as the intersection angle between the cutoff wall and the excavation increased, the efficiency of the cutoff wall first decreased and then increased. The effectiveness of the cutoff wall parallel to the tunnel was up to 122% greater than that of the wall parallel to the excavation, justifying the installation of cutoff walls parallel to tunnels.

In deep excavations, pre-excavation dewatering (PED) usually causes notable diaphragm wall (D-wall) deformation and ground settlement. However, this problem has not attracted sufficient attention. In this paper, the mechanisms and control methods of settlement caused by PED in a deep excavation in Tianjin, China are analysed. A hydraulic connection between the soil inside and outside the excavation area due to aquifer leakage was observed during PED. Therefore, although the first level of struts had been installed before PED, the deformation of the D-wall and ground settlement reached 6·9 mm and 12·5 mm, approximately 29·0% and 73·5% of the maximum allowable values, respectively. The monitoring results demonstrated that leaky aquifers could reduce the deformation of the D-wall but increased the magnitude and extent of ground settlement. Based on the numerical simulations of D-wall deformation and groundwater change, in this case, the ground settlement within the area extending two times the depth of unloading (two times the depth of dewatering, 2H_d) from the excavation was found to be caused by both the deformation of the D-wall and the drawdown of the groundwater, whereas the noticeable settlement in the area beyond 2H_d was caused by only the drawdown of the groundwater. When hydraulic connections exist between the soil inside and outside the excavation area, artificial recharge should be used to control the noticeable settlement induced by groundwater drawdown.

Collapses of braced or tied-back excavations have occurred frequently. However, the influence of the failure of some retaining structure members on the overall safety performance of a retaining system has not been studied. Model tests of failures of retaining piles, struts or anchors were conducted in this study and the load-transfer mechanisms underlying these conditions were analysed. When failures or large deformations occurred in certain piles, the increasing ratios of the bending moments in adjacent piles were much larger in the braced retaining system than in the cantilever system and more easily triggered progressive failure. When the strut elevation was lower or the excavation depth was greater, the degree of influence and range of pile failures became larger. When certain struts or anchors failed, their loads transferred to a few adjacent struts or anchors, possibly leading to further strut or anchor failure. The influence mechanisms of strut or anchor failure on piles were different from those of pile failure. As the number of failed struts or anchors increases, the bending moments of the piles in the failure zone first decrease and then increase to very high values. Therefore, the progressive failure path extends from struts or anchors to piles and will lead to large-scale collapse.