## 3 The Indoor Tests on Loess Materials
### Test Procedure  
1. Standard consolidation test 1. In the consolidation container, the following were placed in order from bottom to top: large permeable stones, filter paper, a ring-knife containing the saturated soil sample (with the ring-knife facing downward), filter paper, and small permeable stones. Then, the guide ring was placed and covered with the pressure cap. The permeable stones and filter paper had been pre-soaked to moisten them. 2. Water was injected into the grooves around the ring-knife to ensure that the saturated sample remained unaffected by water evaporation during consolidation. During the test, the water in the grooves was replenished as needed. 3. The crossbeam used to apply vertical pressure was lowered, ensuring that the bolts at the bottom of the crossbeam were securely fitted into the concave holes on the upper part of the pressure cover. Then, the dial indicator was installed and adjusted to zero. 4. The vertical load was applied in five sequential stages: 50 kPa, 100 kPa, 200 kPa, 400 kPa, and 800 kPa. For each load level, the dial indicator reading was allowed to stabilize before the next load level was applied. 5. The readings of the dial indicator were recorded after each level of load had stabilized.  
Figure 2: Photos of instruments used in the tests.  
\begin{table}
\begin{tabular}{c c c c} \hline
**Soil Sample** & \begin{tabular}{c} **Dry Density** \\ \(\rho\)**s**/(**kg**\(\cdot\)**m\({}^{-3}\))** \\ \end{tabular} & \
\begin{tabular}{c} **Water Content** \\ _w/\%_ \\ \end{tabular} &
\begin{tabular}{c} **Void Ratio** \\ _/m_ \\ \end{tabular} \\ \hline Loess & 1.562 & 21 & 0.75 \\ \hline \end{tabular}
\end{table}
Table 3: The basic properties of the soil used in the indoor test.  
* Triaxial consolidation drained shear test
* Water-head saturation: The water head outside the pressure chamber was set to approximately 1 m higher than the top of the sample. Under the influence of gravitational potential energy, water gradually seeped upward from the bottom of the sample, while gas was expelled through the exhaust pipe at the top of the sample.
* Back-pressure saturation: Water-head saturation often fails to fully saturate the sample, making back-pressure saturation necessary. The back pressure was set to 100 kPa, the confining pressure to 110 kPa, and the test duration to 3 h.
* B-value detection: The back pressure and undrained conditions were kept unchanged, and the confining pressure was increased by 30 kPa. Then, the pore water pressure coefficient B was measured. When \(B=\Delta u/\Delta\sigma>95\%\), the sample was considered saturated.
* Consolidation: The back pressure was maintained at the level of the saturated back pressure, and the effective confining pressure was set to 100 kPa, meaning that the confining pressure was 100 kPa greater than the back pressure. The consolidation time was 1 to 2 days.
* Drained shear: The confining pressure was kept constant, and the strain rate was controlled with an equal strain rate of 0.008 mm/min to ensure that the pore water in the sample was fully drained. The test was stopped when the strain of the sample reached 20% or when the sample exhibited significant strain softening.
* Triaxial consolidation drained loading\(-\)unloading shear test The only difference between the triaxial consolidation drained loading\(-\)unloading shear test and the triaxial consolidation drained shear test was in step (e). Step (e): Loading\(-\)unloading shear: During the drained shear process, when the axial strain reached approximately 40% of the axial strain corresponding to the peak strength of the sample in the triaxial consolidated drained test, shearing was stopped and axial unloading was carried out to zero. After the unloading was complete, the drained shear process was repeated until the end of the test.