Evaluation of the strength of cement-treated clay with a broad range of mix ratios and curing periods was conducted using unconfined compression tests (UCTs). The influence of cement content, total water content, and curing period on the unconfined compressive strength of cemented clay are investigated. It is found that, at constant total water content, higher cement content results in higher unconfined compressive strength, while the total water content has an opposite effect. A power function can be used to correlate the unconfined compressive strength with the cement content or the total water content. For a fixed mix ratio, the unconfined compressive strength of cement-stabilized clay increases with the curing period, the effect of which can be characterized by a semi-log formula. Also, a strength-prediction model that considers both mix ratios and curing periods for cement-admixed marine clay is developed and validated; the model can capture the effect of clay type by considering the plastic index of untreated soils. It is also proved that the proposed framework for strength development is also applicable for other cement types.

In this paper, the construction of a section of Nanjing Metro Line 2, a shallow underground tunnel constructed with an underground mining method, was analysed. Due to the existence of two aquifer soil layers near the tunnel, water and sand inrush occurred during the tunnelling process. This disrupted tunnelling project was covered until the artificial ground freezing technique was used to achieve a waterless environment. After the 40-day active freezing period, the ground temperature conditions satisfied the design requirements, thereby ensuring the construction schedule. The ground freezing technique and other supplementary measurements were introduced in this study, which could be useful for other similar projects.

When applying the artificial ground-freezing method in a shield-driven tunnel, the design of the freezing zone and freezing time is of practical interest. Therefore, the distribution and evolution of heat transfer in a frozen soil wall are key issues in the examination of the freezing effect. In this study, the horizontal ground-freezing method for soil reinforcement in shield-driven tunneling was considered. A cup-shaped freezing scheme was adopted at the launching shaft of the shield-driven tunnel. Three-dimensional (3D) finite-element analysis was used to examine the heat transfer within the cup-shaped frozen soil wall. By this method, the feasibility of the artificial ground-freezing method in the shield tunneling of Nanjing Subway Line 10 was demonstrated. Four types of soil were considered as the in situ soils surrounding the shield machine: sandy, cement-admixed sandy, soft clay, and cement-admixed soft clay. The simulation results indicate that the cement-admixed sandy soil was the most favorable material in terms of ground freezing; it had the highest decreasing rate of temperature in the process of ground freezing. In contrast, the soft clay was the most unfavorable material in terms of ground freezing. The results imply that the closure time of the cup-shaped frozen soil wall was sensitive to the thermal physical properties of soils in addition to the in situ water content.

The artificial ground freezing method can be used jointly with the deep cement mixing method during break-in and break-out processes of shield machines in a tunnel shaft. The frozen ground can fully cut off groundwater seepage, thus ensuring a watertight working platform. Cement-admixed soils can restrict frost heave and thaw-induced settlement because of the decreased permeability. Both methods can also enhance mechanical strength of the soil to enable construction to proceed. Two main sources of heterogeneity are likely to influence the freezing effect: spatial variability in in situ water content in natural soil and spatial variability in binder concentration in cement-admixed soils. Furthermore, positioning error when installing freeze pipes can also affect freezing efficiency. This study simulates in situ water content and binder concentration as Gaussian random fields, whereby variations in the thermophysical properties are estimated. Positioning error is also assessed by prescribing an incline angle in freeze pipes. The influences of those two sources of spatial variability as well as positioning error are examined with random finite-element analyses and statistical characteristics are estimated based on the results. Results are tabulated to offer practitioners a rule of thumb for estimating additional efforts needed in artificial ground freezing, accounting for variations in the thermophysical properties and positioning errors in installing freeze pipes.