Physical and numerical modelling study of Hong Kong marine deposits improved by reinforcements and vertical drains

Abstract

Ground improvement techniques with deep cement mixing soil (DCM) columns, prefabricated vertical drains (PVDs), or even stone columns have been adopted in a recent reclamation project of Hong Kong International Airport over a seabed consisting of mainly Hong Kong marine deposits (HKMD). In the main reclamation area, a load transfer platform (LTP) with geosynthetic reinforcement between the overlaid reclamation fills and the underlaid DCM column-improved HKMD has been designed based on the current design guideline of geosynthetic-reinforced column-supported (GRCS) embankments. However, the consolidation behaviour of soft soils with creep and the load transfer mechanism between reinforcements and soft soils in loading and unloading conditions are neither well covered by the current design guidelines nor fully understood. This study aims to investigate and deepen the understanding of the consolidation behaviour and load transfer mechanism of HKMD improved by PVDs and DCM columns with the consideration of the creep behaviour of HKMD by means of physical and numerical modelling approaches. The findings are of practical significance for the current design guidelines used in Hong Kong and settlement prediction of reclamation projects. Two small-scale physical model tests were designed and carried out in this study. The first one (Model Test 1) was a physical model test that involved singular and double-layer HKMD with PVDs and DCM columns at different stages. The second one (Model Test 2) was a physical model test of the geotextile-reinforced sand fill layer supported by DCM column-improved HKMD. The results of Model Test 1 demonstrate the consolidation behaviour and creep settlements of singular and double-layer HKMD improved by PVDs and DCM columns and reveal the load transfer mechanism between DCM columns and the surrounding soil. The performance of a self-designed effective stress cell for direct measurement of effective stress in soft soil is verified by the experimental results. In addition, a new simplified method that can consider the influence of area replacement ratio on the creep of HKMD improved by DCM columns or PVDs is proposed based on Hypothesis B method and verified by the experimental data. According to the results of a parametric study using the new simplified method, both primary consolidation and creep settlements of DCM column-improved soft soils can be significantly reduced by increasing the area replacement ratio when the area replacement ratio is smaller than 25%. The results of Model Test 2 reveal the load transfer mechanisms among geotextile, DCM columns, and HKMD before and after the yielding of DCM columns. Before the yielding of DCM columns, soil arching effect plays an important role in the loading transfer from HKMD to DCM columns. After the yielding of DCM columns, a reverse loading transfer from DCM columns to HKMD is found. Moreover, the pile efficacy and stress reduction ratio obtained from experimental results are compared with those calculated based on the current design guidelines, showing that the approaches recommended in current Dutch and American guidelines lead to a better prediction. A three-dimensional (3D) finite element (FE) model was built and calibrated by the experimental data of Model Test 2. The simulation results show that the soil arches developed in the geotextile-reinforced sand fill layer are triangular or bell-shaped, rather than the ideally semi-circular shape assumed in most design guidelines. To conduct a systematic parametric study, the 3D FE model was well converted into an axisymmetric model for the sake of high computing efficiency. Based on a project that adopts geotextile-reinforced LTP over DCM column-improved HKMD, the influence of creep on post-construction settlements and long-term loading transfer has been investigated by numerical modelling. The results of numerical simulations show that the loading transfer from HKMD to DCM columns brings HKMD into an over-consolidated state with a smaller creep strain rate. It is also found that the influence of creep on post-construction settlements and long-term loading transfer can be largely reduced when the area replacement ratio is larger than 30%. Research work presented in this thesis has made contributions to (i) development and verification of a new simplified method for settlement calculation of soft soils improved by PVDs or DCM columns, (ii) understanding the load transfer mechanisms among geosynthetic reinforcement, DCM columns, and soft soils, (iii) evaluation of the current design guidelines of determining soil arching effect, and (iv) prediction of long-term settlements and loading transfer of DCM column-supported soft soils with the consideration of creep.