CONSOLIDATION INTEGRATED BUOYANCY EQUATION FOR SOFT GROUND IMPROVED WITH LIGHTWEIGHT POLYURETHANE FOAM

ABSTRACT: Consolidation settlement occurs when a saturated soil is subjected to an increase in overburden pressure that causes a volume change in the soil. When a lightweight material is used as a ground improvement, the stress is reduced as the soft soil is partially removed and replaced by the lightweight material. In addition, the improved ground with lightweight material has a potential to uplift due to the buoyancy of lightweight material. The uplift force reduces the stress imposed on the underlying soil as it acts in the upward direction, thus further reducing the consolidation settlement. This study is executed to produce an alternative equation for consolidation settlement incorporating the buoyancy effect for lightweight polyurethane (PU) foam as a ground improvement method. A Rowe Cell consolidation laboratory test was conducted on untreated marine clay soil as well as on improved marine clay with different thicknesses of lightweight PU foam. Validation of the laboratory test results was done by finite element analysis, PLAXIS 2D. The thickness of PU foam governs the buoyancy and the hydrostatic pressure of water displaced by PU foam, which is incorporated in the alternative equation. The alternative consolidation settlement equation is applicable for ground improved with lightweight polyurethane foam and found to be more economical and practical as the buoyancy is taken into account in the equation. ABSTRAK: Mendapan pengukuhan berlaku apabila tanah tepu mengalami peningkatan tekanan beban yang menyebabkan perubahan isipadu tanah. Apabila bahan ringan digunakan sebagai penambahbaikan tanah, tekanan akan berkurang kerana sebahagian tanah lembut dikeluarkan dan diganti dengan bahan ringan. Selain itu, tanah yang diperbaiki dengan bahan ringan berpotensi untuk terangkat ke atas keranan daya apung bahan ringan. Daya angkat bahan ringan mengurangkan tekanan yang dikenakan ke atas tanah kerana daya bertindak ke arah atas, dan seterusnya megurangkan mendapan pengukuhan. Kajian ini dijalankan untuk menghasilkan persamaan alternatif bagi mendapan pengukuhan dan digabungkan dengan kesan daya apung untuk busa poliuretena ringan (PU) sebagai kaedah penambahbaikan tanah. Ujian makmal mendapan pengukuhan menggunakan peralatan Rowe Cell dilakukan pada tanah liat marin yang asal serta yang diperbaiki dengan ketebalan busa PU ringan yang berbeza. Pengesahan hasil ujian makmal dilakukan dengan analisis elemen terhingga, PLAXIS 2D. Ketebalan busa PU mempengaruhi daya apung dan tekanan hidrostatik bagi kedalaman air yang disesarkan oleh busa PU dan digabungkan dalam persamaan alternatif. Persamaan alternatif mendapan pengukuhan tersebut boleh digunapakai untuk pembaikan tanah menggunakan bahan ringan busa poliuretena dan didapati menjimatkan kos dan praktikal kerana keapungan diambilkira didalam persamaan tersebut.

Settlement Analysis of Sand Compaction Pile Composite Foundation in Transition Section of Immersed Tube Tunnel

The foundation of the immersed tube transition section of the Hong Kong-Zhu Hai-Macao Bridge Project is distributed with thick silty soil. The method of sand compaction pile + surcharge preloading is used to treat the soft soil ground. In order to determine the foundation consolidation settlement and subsequent residual settlement in the transition section of the immersed tube tunnel, a monitoring system using long wires combined with wireless transmission and long-distance data collection was developed to obtain the measured foundation settlements during the surcharge period. After comparing the measured value with the calculated value, the formula for the composite foundation of the sand compaction pile was revised to obtain a more reasonable residual settlement of the foundation, which could guide the design and construction of the immersed tube tunnel.

Assessment of Reclamation-induced Consolidation Settlement Considering Stratigraphic Uncertainty and Spatial Variability of Soil Properties

Consolidation analysis is a key task for reclamation design. Although consolidation is a long-term process, acceleration of consolidation is often preferred for speeding up the reclamations. Before proposing measures to accelerate consolidation and reclamation process, it is imperative to have an accurate prediction of consolidation settlement for fine-grained materials, which is greatly affected by spatial distribution of subsurface zones with different soil types (i.e., stratigraphic heterogeneities and uncertainty) and spatial variability of soil properties. In current practice, calculation of consolidation settlement often uses simplified stratigraphic boundaries and deterministic consolidation parameters without considering stratigraphic uncertainty or soil property spatial variability. The oversimplified practice might result in unconservative estimations of consolidation settlement and pose threats to safety and serviceability of constructed facilities on reclaimed lands. In this study, a stochastic framework is proposed for consolidation settlement assessment with explicit modeling of stratigraphic uncertainty and spatial variability of soil properties by machine learning and random field simulation from limited site investigation data. The proposed method effectively generates multiple realizations of geological cross-section and random field samples of geotechnical properties from limited measurements and offers valuable insights into spatial distribution of the estimated total primary consolidation settlement curves and angular distortion.

FACTORS AFFECTING THE THICKNESS OF REPLACEMENT LAYER ON MEDIUM CLAY

Soil replacement is a common technique that can be used to increase the soil bearing capacity and reduce the expected settlement. The thickness of replacement layer depends on many factors such as: the applied stress, original soil properties, material of replacement layer and the cost of foundation works. However, until now the practical thickness of replacement is usually selected based on soil experts’ experience. This study proposed an optimization model to assist geotechnical engineers in predicting the optimum thickness and material type of replacement layer that satisfy the main design requirements, i.e. bearing capacity, consolidation settlement and cost considerations at the same time. The Evolutionary solving method that uses a variety of genetic algorithm and local search methods was used to solve the research problem. Furthermore, the effect of the thickness and properties of clay layer and the depth of ground water table on determining optimum type and thickness of replacement soil were investigated. The study evaluated the relationship between the replacement layer thickness and the total direct cost of foundation works and found that, the notion of increasing replacement thickness to decrease cost limitlessly was not viable and an optimal thickness was usually achieved.

Numerical Simulation of Consolidation Characteristics of Diluted Debris Flow

The consolidation settlement of diluted debris flow is a complicated process of solid-liquid two-phase flow deposition. In this paper, the consolidation and settlement characteristics of uniform-graded and wide-graded debris flow materials in clear water and slurry are simulated by using this method. The results show that in the process of consolidation, sorting occurs in the top and middle position of uniform-graded materials at low concentration, while the middle particles are separated in reverse direction at high concentration. The middle particles of wide-graded material are hardly separated in the whole consolidation process. The velocity of dissipation of excess pore water pressure in the clean water is faster than that of slurry, and the dissipation time of wide-graded materials is longer than that of uniform-graded materials. The research results are helpful to reveal the mechanism of consolidation settlement of diluted debris flow in meso-scale.

PENGARUH KADAR AIR TERHADAP BESARNYA PENURUNAN PADA UJI KONSOLIDASI

Clay soil is a cohesive soil with low bearing capacity and low shear strength where the load on it will be limited. The addition of water content in clay soil will result in changes in soil volume that will affect the strength of the soil. The research aims to know how much water content affects the values of consolidation settlement. A consolidation test was carried out three times with variations of the original soil sample, 35% water content and 30% water content. The results obtained a settlement of 0,1864 cm for original water content, 0,1677 cm for 35% water content, and 0,1414 cm for 30% water content.