Deep cement mixing (DCM) is an effective technique for improving soft soil grounds through injecting dry cement or cement slurry into the ground with continuous mixing and finally forming the cemented soils with improved mechanical properties. The DCM provides adequate bearing capacity and serves as the foundation for reclaimed lands and seawall in marine areas. Compared to traditional fully dredged land reclamation, DCM poses minimal disturbance to the marine ecological environment, and is recommended as an environmental-friendly land reclamation technique and broadly applied in coastal cities to solve the land shortage problem.

Before constructing DCM and land reclamation, site investigations, including boreholes and CPTu, must be performed to investigate the geological conditions. S...[ Read more ]

Deep cement mixing (DCM) is an effective technique for improving soft soil grounds through injecting dry cement or cement slurry into the ground with continuous mixing and finally forming the cemented soils with improved mechanical properties. The DCM provides adequate bearing capacity and serves as the foundation for reclaimed lands and seawall in marine areas. Compared to traditional fully dredged land reclamation, DCM poses minimal disturbance to the marine ecological environment, and is recommended as an environmental-friendly land reclamation technique and broadly applied in coastal cities to solve the land shortage problem.

Before constructing DCM and land reclamation, site investigations, including boreholes and CPTu, must be performed to investigate the geological conditions. Several CPTu-based soil behaviour classification systems (SBCs) have been developed for soil characterisation. When applying the SBCs to offshore sites, where the marine soils may be decomposed from rocks or mixed with artificial fills, the applicability of CPTu-based SBCs for marine soils are evaluated by comparing with composition-based SBCs from borehole records in the vicinity of each sounding. The interpretation of CPTu data is first performed to generate soil type variables comparable to borehole data, followed by a cross-validation study. The soil classification performance of each SBCs is quantified by the weighted kappa coefficient and the Kendall correlation coefficient between the soil types generated by the CPTu-based and composition-based SBCs. The classification accuracy for each soil type is also evaluated via the root mean squared error and the mean absolute error. The classified soil types from the CPTu data are associated with a median degree of consistency, indicating the need for combining CPTu-based and composition-based SBCs for marine soil classification.

The strength of the cement stabilised soils in the DCM cluster is an essential property for controlling the settlement and stability of the superstructure of the seawall and embankment on the DCM improved ground. Several factors may impact the strength of the DCM clusters. The complex components and specifications, including site investigation, construction design, equipment, procedure and quality assurance tests for offshore DCM construction in a test site at Pearl River estuary (PRE), are investigated, with the exploration of the principal chemical and physical mechanism of the cement stabilised marine soils. The potential factors impacting the unconfined compressive strength (UCS) from coring samples of the DCM cluster were also analysed, referring to site investigation records, construction records and quality test results. The key factors included construction materials and condition, construction procedure and curing conditions, including the recorded fluctuation of tidal level, original soil characterisation, volume fraction of injected water, volume fraction of injected cement slurry, injection rate of cement slurry, penetrating and mixing efficiency, curing age, and moisture content. Based on cementation mechanisms, the homogeneity and UCS of DCM were evaluated through the coefficients of variance, standardised regression coefficients from multivariate linear regression (MLR), indicator importance from out-of-bag error using random forest (RF).

The characteristics of spatial variation of the unconfined compressive strength (UCS) from DCM clusters in the marine area is investigated by establishing a large-scale spatial distribution model with kriging estimation and conditional Gaussian random field (RF). The spatial variability for the scale of fluctuation of UCS was evaluated by variogram model based on experimental UCS data, which further determined the auto-correlation model, followed by the RF modelling through sequential Gaussian simulation conditioned on the cored samples separately for both trend-detected and trend neutral layer. The generated spatial distribution and probabilistic assessment of UCS for the DCM construction site can be implemented as a tool for strength prediction and uncertainty quantification that demonstrates the spatial nature and heterogeneity of the geotechnical composite materials.

After the completion of DCM construction, quality assurance tests are performed at different curing ages, starting from 28 days, investigating the strength of the binder-soil mixtures and evaluating the construction quality of DCM. The quality tests include the in-situ cone penetration test (CPT) and the undrained compression test (UCT) for the full-length cored specimens of DCM clusters. Given that the DCM construction involves multiple stages, including penetration, bottom treatment, injection, mixing and lifting, the disturbance to the adjacent soils' original states is inevitable.The CPTu tests were conducted and investigated prior to and after DCM construction at the neighbouring location of DCM panels, aiming to evaluate the impacts of DCM construction on the properties of the adjacent natural soils. The direct measurements and the interpreted parameters from CPTu results, including soil state, strength, deformation, permeability, and consolidation, are compared separately for varied dominant marine soil types to evaluate the influence of the construction of DCM on adjacent marine soils, with the extent of the impacts quantified in statistical analysis.