A series of finite element analyses, conducted on the basis of modified triaxial tests incorporating radial drainage, were carried out to investigate the lateral deformation and stress state characteristics of prefabricated vertical drain (PVD) unit cells under vacuum preloading. The analyses revealed that the inward horizontal strain of the unit cell increases approximately linearly with the vacuum pressure (Pv) but decreases non-linearly with an increase in the initial vertical effective stress (sigma ' v0). The variations in the effective stress ratio, corresponding to the median excess pore water pressure during vacuum preloading of the PVD unit cell, were elucidated in relation to the Pv and sigma ' v0 using the simulation data. Relationships were established between the normalized horizontal strain and normalized effective stress ratio, as well as between the normalized stress ratio and a composite index parameter that quantitatively captures the effects of vacuum pressure, initial effective stress, and subsoil consolidation characteristics. These relationships facilitate the prediction of lateral deformation in PVD-improved grounds subjected to vacuum preloading, utilizing fundamental preloading conditions and soil properties. Finally, the proposed methodology was applied to analyze two field case histories, and its validity was confirmed by the close correspondence between the predicted and measured lateral deformation.

The recent construction of an underground mass rapid transit (MRT) station in Singapore involved 21 m deep excavations within underconsolidated marine clay. The lateral earth support system comprised 1 m thick diaphragm walls socketed into the underlying Old Alluvium and 4 levels of preloaded cross-lot struts. Deep soil mixing (DSM) and jet grouting piles (JGP) were used to improve up to 15 m thickness of the marine clay formation. Field monitoring data showed that these ground improvement processes caused large outward deflections of the diaphragm wall panels at some locations prior to the excavation and may have caused yielding within the wall panels. In this paper, the impacts of these prior wall deformations on the subsequent performance of the excavation support system are investigated. The measured performance at two indicative cross sections is compared with results from simplified 2D finite element analyses. The analyses simulate the effects of ground improvement through prescribed boundary pressures and represent the yielding of the diaphragm wall panels through zones of reduced bending stiffness. We show that large outward wall deflections and curvature observed during jet grouting at one contribute to higher inward wall movements and strut loads measured during excavation, while smaller movements (and curvature) prior to excavation at a second similar cross cause negligible change in the performance of the temporary earth retaining system. The results highlight (1) the importance of controlling ground movements associated with ground modification processes such as jet grouting, (2) the uncertainties in estimating mechanical properties for the improved soil mass, and (3) the need to improve the representation of non-linear, flexural properties (M-kappa) of reinforced concrete diaphragm panels.

To date, few models are available in the literature to consider the creep behavior of geosynthetics when predicting the lateral deformation (delta) of geosynthetics-reinforced soil (GRS) retaining walls. In this study, a general hyperbolic creep model was first introduced to describe the long-term deformation of geosynthetics, which is a function of elapsed time and two empirical parameters a and b. The conventional creep tests with three different tensile loads (P-r) were conducted on two uniaxial geogrids to determine their creep behavior, as well as the a-P-r and b-P-r relationships. The test results show that increasing P-r accelerates the development of creep deformation for both geogrids. Meanwhile, a and b respectively show exponential and negatively linear relationships with P-r, which were confirmed by abundant experimental data available in other studies. Based on the above creep model and relationships, an accurate and reliable analytical model was then proposed for predicting the time-dependent delta of GRS walls with modular block facing, which was further validated using a relevant numerical investigation from the previous literature. Performance evaluation and comparison of the proposed model with six available prediction models were performed. Then a parametric study was carried out to evaluate the effects of wall height, vertical spacing of geogrids, unit weight and internal friction angle of backfills, and factor of safety against pullout on delta at the end of construction and 5 years afterwards. The findings show that the creep effect not only promotes delta but also raises the elevation of the maximum delta along the wall height. Finally, the limitations and application prospects of the proposed model were discussed and analyzed.