Resumen de Hydraulic fills liquefaction. Effect on quay stability
The starting point of this thesis is the failure of part of a quay wall under construction. The quay was divided in two phases, failure affected Phase 1 only. The failure and all the relevant associated information on quay design, construction and monitoring records are described in detail together with the results of a site investigation campaign carried out after the failure. Flow liquefaction of the hydraulic fill emplaced behind the quay wall was the main cause of the failure. The state of the hydraulic fill in the quay area is assessed in the light of the current understanding of the phenomenon and of a number of flow liquefaction criteria. In addition, an elastoplastic constitutive law is described that is capable to simulate the undrained brittleness behaviour that underlies the phenomenon of flow liquefaction. It is a critical-state model that incorporates the concept of state parameter. Subsequent finite element analyses are able to reproduce satisfactorily the behaviour of the quay during construction and the features and circumstances of the failure. Two possible triggering mechanisms are identified that can explain the failure: spontaneous liquefaction, or liquefaction of a limited zone caused by the concurrent construction of an embankment. A parametric study verifies the robustness of the simulation and the dependence of stability on the degree of brittleness of the hydraulic fill. The same type of analysis of quay Phase 2, not involved in the failure, revealed that the margin of safety was small if the hydraulic fill liquefied. Soil improvement measures were implemented and an extensive monitoring system was installed. Construction of quay Phase 2 was completed applying the observational method that involved a continuous check of the monitoring data and its comparison with numerical simulation results. An additional site investigation confirmed that the soil improvement measures had succeeded in removing both the flow and the cyclic liquefaction potential of the hydraulic fill, according to currently accepted criteria. The Phase 1 quay involved in the failure was reconstructed with mainly terrestrial fill with no flow potential liquefaction, as confirmed by the corresponding site investigation.
Based on the experience gathered in the case and on the research carried out, an operational scheme dealing with quays constructed with hydraulic fills susceptible to flow liquefaction is proposed. It consists of a protocol to evaluate liquefaction potential of hydraulic fills, the use of a constitutive law capable of simulating flow liquefaction, the implementation of soil improvement measures to reduce flow liquefaction potential and the employment of a monitoring system to control the quay wall behaviour during construction and to provide data for comparison with numerical analyses results.
