# Dedometer Study Regarding the Consolidation Behavior of Nanjing Soft Clay
## 1 Introduction  
The consolidation of soft clay may result in different construction problems, such as ground settlement, building tilting and damage to underground utilities. Soft clay is commonly characterized with high compressibility, high natural water content and low shear strength. Foundations with soft clay may experience extreme settlement with time, which in turn may have negative impacts on the stability of buildings and their foundations. Moreover, Nanjing is located in the lower reaches of the Yangtze River and is part of the Yangtze River valley, which is located in a low mountainous hilly area [1]. In Nanjing, buildings are commonly constructed on the subgrade, which is not fully consolidated, and ground settlement continues after residents move in. As a result, it is crucial for design engineers to understand the consolidation behavior (including primary consolidation and secondary compression) of soft clay to assess the stability of designed facilities. In this study, the consolidation behaviors of soft clay in Nanjing were investigated using eodometer tests of different soil specimens taken in Pukou, in Nanjing. [PERSON] [2] proposed a rheological constitutive equation from both macro and micro perspectives, along with the quadratic time effect and the frame-by-frame theory, in a representation of the consolidation behavior of soft clay. [PERSON]'s one-dimensional consolidation theory suggests that deformations after the inflection point of the logarithmic time-deformation curve are primarily due to secondary compression deformation. Currently,there are two main hypotheses regarding the relationship between soil creep and primary consolidation and secondary compression. There are debates on the mechanism of consolidation, which consist of primary consolidation and secondary compression: (1) [PERSON] and [PERSON] [3] consider that secondary compression occurs after the pore water is drained, with a temporal precedence relationship to primary consolidation; (2) [PERSON] et al. [4] consider that primary consolidation and secondary compression occur simultaneously. [PERSON] et al. [5] suggested that, based on one-dimensional creep test results using Batiscan clay at different vertical stress levels, as shown in Figure 1, the relationship between creep strain and time can be summarized into three types. Type I corresponds to over-consolidated soil, where the vertical consolidation pressure is less than the pre-consolidation pressure, with no obvious intersection between primary consolidation and secondary compression. Type II and Type III both exhibit a more distinct intersection of primary consolidation and secondary compression settlement. Type II corresponds to normally consolidated soil in which the vertical consolidation pressure is close to the pre-consolidation pressure, and the slope of the secondary compression line is significantly greater than that of Type I. Type III corresponds to over-consolidated soil in which the vertical consolidation pressure is much greater than the pre-consolidation pressure, and the vertical deformation of the soil specimen decreases gradually with time in a clear inverted S-shape. The one-dimensional oedometer is one type of equipment commonly used to investigate the consolidation behavior of clay soil. As shown in Figure 2, experimental data of a long-duration consolidation creep test using an oedometer shows an inverted S-shape. From the results of the one-dimensional oedometer test, both the compression index \(C_{c}\) and the secondary compression ratio \(C_{a}\) can be determined. Figure 1: Illustration of different types of creeping behavior for Batiscan clay from [PERSON] et al. [5], where Type I corresponds to over-consolidated soil, Type II corresponds to normally consolidated soil, and Type III corresponds to over-consolidated soil. Figure 2: Division of primary consolidation and secondary compression.