# One-Dimensional Consolidation Properties of Soft Clay under Multi-Stage Loading
## 2 Multi-Stage Loading Consolidation Test  
The soil samples were collected from Cangzhou City, Hebei Province, China, near the Bohai Sea. These samples consisted of coastal sedimentary soft soil and were taken at a depth of 8 m below the groundwater level. The stratigraphic profile can be seen in Figure 1. The physical and mechanical properties of the soil samples are presented in Table 1. The high water content in the soil samples made them soft and plastic, which made the structure susceptible to damage [22; 23]. To mitigate the influence of soil structure, remolded soil samples were used. The initial consolidation state of the remolded soil sample was controlled with a dry density of \(\rho_{\mathrm{d}}=1.532\) g/cm\({}^{3}\). The preparation process involved air-drying the soil sample, passing it through a 0.5 mm sieve, weighing a subsample, mixing it in a mixer, and thoroughly incorporating airless water with a water content of 30.47%. A specimen with a diameter of 61.8 mm and a height of 20 mm was extracted from the mixed sample using a sample maker. The specimen was then placed in a vacuum saturation device (refer to Figure 1) and connected to the vacuum cylinder and extractor. The extractor was started and the vacuum pressure gauge was monitored until it reached a value close to atmospheric pressure. At this point, the pipe clamp was slightly opened to introduce clean water into the vacuum cylinder. Once the water submerged the saturator, the pumping was stopped and the pipe clamp was opened to allow air into the vacuum cylinder. The sample was left to stand for 12 h to ensure full saturation. Subsequently, the vacuum cylinder was opened and the specimen was removed using a ring knife. The total mass of the ring knife and sample was weighed to calculate the water content, and the saturation was determined using Equation (1). If the saturation was below 98%, further pumping saturation was performed. Once the sample saturation reached or exceeded 98%, the test could proceed. \[S_{r}=\frac{w_{sr}G_{s}}{e} \tag{1}\]  
The experiment adopted the new automatic high-voltage consolidation instrument (Figure 2). It has direct electrical conversion, an ultra-high speed, and a large flow, along with a fast reaction speed and load stability time of \(\leq\)1 s. Once the consolidation pressure is stable, the pressure error is less than or equal to 1.0%. Overall, this device effectively stabilizes the loading process while minimizing its impact on the test, thereby enhancing the accuracy and precision of experiments. It has a loading range of 0-3200 kPa and an air pressure control range of 0-0.9 Mpa. The whole process of the test is controlled using a computer, and data collection is completed automatically. The data collection accuracy is 0.001 mm. \begin{table}
\begin{tabular}{c c c c c c c c} \hline Natural unit weight (kN/m3) & Moisture content (\%) & Plastic limits (\%) & Liquidity limits (\%) & Plasticity index (\%) & Liquidity index (\%) & Clay content (\%) & Void ratio (\(e_{0}\)) \\ \hline
16.95 & 37.4 & 22.44 & 42.40 & 19.59 & 0.77 & 70.7 & 1.19 \\ \hline Permeability coefficient (cm/s) & Compression index (MPa\({}^{-1}\)) & Compression modulus (MPa) & Internal friction angle (\({}^{\circ}\)) & Cohesion (kPa) & Specific gravity (g/cm\({}^{3}\)) & Water saturation (\%) & \\ \hline
4.21 \(\times\) 10\({}^{-8}\) & 0.55 & 3.33 & 11.38 & 11.63 & 2.73 & 97.33 \\ \hline \end{tabular}
\end{table} 
Table 1: Physical and mechanical properties of soil specimen. Figure 1: Strata profile. Two groups of eight soil specimens were tested in total. In group A, four soil specimens were tested using the multi-stage loading compression consolidation test with a loading sequence of 50\(\rightarrow\)100\(\rightarrow\)150\(\rightarrow\)200 kPa. Each load level was maintained for 6 h, resulting in a total of 24 h of compression. In group B, four soil specimens were tested under single-stage loading.