The time-dependent behaviour of soft and clayey soils treated with Deep Cement Mixing (DCM) columns is important for analyzing the long-term performance of civil engineering infrastructures. Previous studies on DCMinstalled composite soil (CS) have primarily focused on examining the soil strength and stiffness characteristics. The limited focus on the time-dependent settlement and stress-strain distribution of CS underscores the need for a more comprehensive understanding of this complex phenomenon. In this study, a lab-scale physical ground model is designed and developed to investigate the time-dependent settlement profile of the composite Montmorillonitic Clay soil (MMC). The settlement behaviour of the ground model is assessed using Creep Hypothesis B and the results are further validated with the Power Law Model. Additionally, a FEM-based numerical simulation is performed to examine the time-dependent settlement and the stress distribution between the column and surrounding clay soil at different depths. The results from the physical model test show that the time-dependent parameter of the ground model (i.e., DCM column installed in MMC) is proportionate to the loading rate until the failure of the DCM column is reached. However, the time-dependent parameter was found to be decreased by 59.04 % in the post-failure phase of the DCM column. This reduction indicates that the DCM column was the primary load-bearing component before its failure. The numerical study shows that the pore water pressure dissipation in the clay soil and DCM column interface was similar at various depths. The top and bottom sections of the DCM column possess higher stress levels, which demonstrates its susceptibility for failure in the DCM column.

Deep cement mixing (DCM) is a popular in situ soil stabilization method, while the investigation on long-term coupled consolidation and contaminant leaching behavior of cement-stabilized contaminated soil is limited. In this study, axisymmetric physical model tests were conducted to investigate the coupled behaviors of a composite ground, which consisted of a central column made of cement-stabilized arsenic-contaminated marine deposits and surrounding untreated marine deposits. The test results revealed the settlement development of composite ground and the mechanism of load transfer between the DCM column and surrounding soils with increasing loading. The presence of arsenic decreased the strength and stiffness of the DCM column through the reaction between arsenic and hydration and pozzolanic reaction products. With the increase of the water/cement ratio in the DCM column, the concentration level of arsenic in the draining-out water of the composite ground increased significantly, while that in the surrounding soil showed no obvious change, indicating that arsenic mainly migrated directly through the DCM column. A theoretical axisymmetric consolidation model coupling solute transport for composite ground was established and subsequently applied to analyze the test data. The numerical model accurately depicted the pore water pressure, settlement, and spatiotemporal distribution of arsenic concentration in the physical model.

The geosynthetic-reinforced and pile-supported (GRPS) embankment is a cost-effective and efficient method for addressing soft soil conditions. A noteworthy addition to this approach is the introduction of a precast concrete pile reinforced with cement-treated soil (PCCS), a novel composite pile formed by inserting a precast concrete (PC) pile into a deep cement mixing (DCM) column. This research aimed to delve into the bearing mechanism and deformation characteristics of a multielement composite foundation featuring PCCSs (long piles) and partially penetrated DCM columns (short columns) under embankment loads. Furthermore, the study performed a comparative analysis, juxtaposing calculated stress reduction values obtained from eight existing analytical methods with both field data and numerical results. The findings underscore the efficacy of the long-short pile composite foundation in effectively controlling settlements under embankment loads. Specifically, the maximum settlements recorded for the PCCS, the DCM column, and the surrounding soil within the piled-supported embankment system were 11.6, 19.0, and 54.0 mm, respectively. Under undrained end-of-construction conditions, the stable stress ratios of the PCCS-soil and the DCM column-soil were 10.7 and 4.5, respectively. As the loads transition from the surrounding soil to the piles, a major arch and a minor arch gradually form between the adjacent PCCSs and PCCS-DCM columns within the embankment filling. The outcomes of this investigation indicate that incorporating PCCSs and DCM columns as composite piles in GRPS embankments significantly enhances bearing capacity and curtails deformation under embankment loads.

Seawater cement slurry (SCS) is a commonly used binder in offshore deep cement mixing (DCM) construction. Seawater cement slurries are usually prepared before they are grouted into the seabed and mixed with marine clay. The aim of this study is to explore the feasibility of applying carbonation technology to fabricate SCS suitable for offshore DCM while achieving carbon sequestration and obtaining better mechanical properties for stabilised marine sediments. This study demonstrated that after appropriate carbonation, carbonized SCS can be used in DCM to replace conventional SCS. Short-term carbonation promotes cement dissolution and hydration rates under seawater conditions rich in magnesium, calcium, and other inorganic ions. The carbonates include calcite, vaterite and amorphous carbonates, which provide additional nucleation sites for the hydration of SCS, resulting in an increment for amorphous CS(A)H gel with a dense pore structure and binding interaction with soil particles. After carbonation with 20 % CO2 for 5 min (0.5 wt% of cement), the UCS and secant modulus of cement-soil mixtures by 15.7 % and 111 % at the age of 1 day, and by 6.82 % and 10 % at the age of 28 days when treating marine clay with 80 % moisture content at a dosage of 260 kg/m3. 3 .

The deep cement mixing (DCM) is used to improve the capacity and reduce the settlement of the soft ground by forming cemented clay columns. The investigation on the mechanical behaviour of the DCM samples is limited to either laboratory-prepared samples or in-situ samples under unconfined compression. In this study, a series of drained and undrained triaxial shearing tests was performed on the in-situ cored DCM samples with high cement content to assess their mechanical behaviours. It is found that the drainage condition affects significantly the stiffness, peak and residual strengths of the DCM samples, which is mainly due to the state of excess pore water pressure at different strain levels, i.e. being positive before the peak deviatoric stress and negative after the peak deviatoric stress, in the undrained tests. The slope of the failure envelope changes obviously with the confining pressures, being steeper at lower stress levels and flatter at higher stress levels. The strength parameters, effective cohesion and friction angle obtained from lower stress levels (c(0)' and phi(0)') are 400 kPa and 58 degrees, respectively, which are deemed to be true for design in most DCM applications where the in-situ stress levels are normally at lower values of 50-200 kPa. Additionally, the computed tomography (CT) scanning system was adopted to visualize the internal structures of DCM samples. It is found that the clay pockets existing inside the DCM samples due to uneven mixing affect markedly their stress-strain behaviour, which is one of the main reasons for the high variability of the DCM samples. (c) 2024 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).