The soil-structure interaction dominates the performance of buried pipes and culverts. Under extreme ground loading conditions, such as ground subsidence and faulting offset, the deformation and loading subjected by the buried structure might exceed the limit values, and lead to the deterioration of the sealing performance and the structural integrity. Researchers have gained insight into the soil-pipe interaction under idealized loading conditions, but a paucity of understanding still exists in the effects of extreme ground loading conditions on the buried pipes, which limits the assessment and maintenance of buried structures. This study focuses on the interaction between soil and rigid pipes/culverts under extreme loading conditions. With the combination of model tests, numerical simulations, and theoretical analysis, the joint kinematics and the response and serviceability of rigid pipes are discussed, and the corresponding theatrical analysis methods are developed. The ingress erosion mechanism of groundwater on filling materials is investigated, and the forming process of erosion voids associated with the hydraulic conditions is investigated. A modified design method for the induced trench installation is proposed, and its effectiveness under extreme loading conditions are studied. The main conclusions can be drawn as following:
(1) A full-scale laboratory model test is conducted to investigate the kinematics of bell-spigot joints and the bending behavior of the vitrified clay pipeline under odd subsidence configuration in which the abrupt location is directly under the joint. The variations of joint kinematics and barrel bending with the subsidence displacement are investigated, and the joint leaking and structural failure behaviors are analyzed. Results show that significant joint kinematics occurs during the subsidence process. The joint rotation and axial extension are increased proportional to the increment of the subsidence displacement. Leakage is detected at a rotation response much lower than the allowable value regulated by the ASTM standard. Under the tested condition, the pipeline is breaking by the spilt failure of the bell.
(2) Based on the equilibrium condition of barrel bending and joint shear loading, a simplified three-dimensional finite element model for the bell-spigot joint is proposed. The pipe-soil interaction under both odd and even configurations, i.e., the shear band passes through the joint and through the mid-span of a pipe barrel, respectively, are simulated. Results show that the pipe displacement is close to the single pipe rotation model. The worst joint kinematic condition occurs in the even configuration at the joint which is uplifted due to the rigid rotation of the pipe segment. Its kinematic response is more remarkable under a shallower burial depth. Based on the pipeline displacement and its rigid motion behavior, a kinematic model is developed, which can provide straightforward and safety assessment on the axial translation of bell-spigot joint.
(3) The erosion mechanism of ingress of groundwater through the pipe invert on the filling material is investigated using model tests. The mass flux of eroded sand and inflow speed of groundwater are analyzed. The forming process of erosion voids is discussed. Results show that under a rising head hydraulic boundary condition, the erosion process can be divided into three stages: the initial leakage, the main erosion and the metastable stage. In a constant head condition, it contains the main erosion and the metastable stage only. The height of initial visible voids is in accordance with the air-entry head of the sand around the pipe. When the size ratio between the aperture and the eroded particles reaches the critical condition, sand bridging will be formed, and the aperture will be blocked, which can reduce the relative permeability of the soil-pipe system. Based on the effect of groundwater and the seepage stability of the sand, an empirical method is proposed to estimate the size of maximum erosion void.
(4) Based on the pipe-soil interaction under vertical relative displacement condition, the theoretical analysis method for the joint shear load and barrel bending behavior of segmented rigid pipes connected by flexible joints under both odd and even configurations are established. The change of the ultimate soil resistance and the corresponding pipe-soil relative displacement with the burial depth is studied. Results show that in the odd configuration, the pipe response is dominated by the uplift soil resistance, which was loaded nearly on the entire span of the pipe barrel. The maximum bending appears nearly at the mid-span of the barrel. Whereas in the even configuration, the pipe response is mainly controlled by the downward soil resistance, which was exerted on the half-length of the pipe barrel located in the stationary zone. The maximum bending appears to be at about 3/8L along the barrel from the spigot end to the bell end.
(5) The variation of joint shear loading the barrel bending behavior with the subsidence displacement is theoretically analyzed. The validity of analysis solution is verified with the results of the full-scale test and the numerical simulation. The effects of the relative burial depth and the subsidence displacement are examined. Results show that with the augment of relative burial depth, the allowable subsidence displacement for both configurations are remarkably diminished. Under relatively small burial depth, the shear load in the even configuration reaches first to the ultimate sealing and failure loading. With the increment of burial depth, the joint in the odd configuration could reach to its leakage resistance and tensile strength first, and then cause the split failure of the bell.
(6) A modified limit equilibrium solution for vertical pressure on top of induced trench rigid pipes is proposed. In the modified method, the stress above the soil element is assumed to be distributed in a parabolic pattern. Based on the deformation compatibility of the pipe, the soft layer and the fill material, the effect of the geometry and the physical property of the soft layer on load reduction is calculated. Results show that the modified method can provide estimates comparable to the field measurements. The increment of the thickness of the soft layer is beneficial to improve the load reduction, but the increase of the relative stiffness between the soft layer and the surrounding soil would have an adverse effect. The simplified design charts are proposed, which can facilitate the use of the modified method.
(7) The effectiveness of the induced trench installation on rigid pipes with bell-spigot joints under extreme ground loadings are analyzed. Based on the loading behaviors of the pipeline in both odd and even configurations, two arrangements of the soft layer, i.e., the single layer above the pipe, and the double layers at the top and invert of the pipe, respectively, are introduced and validated using numerical simulations. Results show that under extreme loading conditions, no significant difference is observed for the joint kinematics in both the induced trench condition and the projection condition. However, in the odd configuration, the single soft layer can obviously reduce the barrel bending moment and the joint shear loading, whereas in the even configuration, only the double layers arrangement can remarkably increase the allowable subsidence displacement, and effectively protect the functionality and the structural integrity of bell-spigot jointed rigid pipes.