## 2 Materials and Methods  
Laboratory tests to determine undrained shear strength were performed in the Water Center Laboratory at the Warsaw University of Life Sciences (SGGW) using the torsional shear hollow cylinder apparatus (Figure 1). Tests were carried out on undisturbed (U) and reconstituted (R) soil samples of clay (CI(U)), sandy silty clay (ssaiCI(U) and ssaiCI(R)), and clayey sand (clSa(R)). The clay samples had an over-consolidation ratio (OCR) of 3.5 and a plasticity index \(I_{P}\) of 77.6%. The samples of sandy silty clay had an OCR of 2.7 and an \(I_{P}\) of 34.7%, whereas the samples of clayey sand had an OCR of 8.0 and an \(I_{P}\) of 10.5%. The index properties of the tested soils are presented in Table 1. The undrained shear strength was determined at angles of principal stress rotation, \(\alpha\), equal to \(0^{\circ}\), \(30^{\circ}\), \(45^{\circ}\), \(60^{\circ}\), and \(90^{\circ}\) for clay and clayey sand, and equal to \(0^{\circ}\), \(15^{\circ}\), \(30^{\circ}\), \(45^{\circ}\), \(60^{\circ}\), \(75^{\circ}\), and \(90^{\circ}\) for sandy silty clay. Undisturbed soil samples were taken directly from the excavation site at the Copernicus Science Center station of the II underground line in Warsaw, while reconstituted samples were prepared in the laboratory using material from undisturbed samples, following their failure. The TSHC tests were performed on soil samples in the shape of a hollow cylinder (Figure 2). Tests, both on undisturbed and reconstituted soils, were performed in six consecutive stages: flushing, saturation, consolidation, change of the intermediate principal stress parameter, \(b\), change of the angle of principal stress rotation, \(\alpha\), and finally, shearing in undrained conditions. Parameter \(b\) is defined as:  
\[b=\frac{(\sigma_{2}-\sigma_{3})}{(\sigma_{1}-\sigma_{3})}, \tag{2}\]  
where \(\sigma_{1}\) is the major principal stress, \(\sigma_{2}\) is the intermediate principal stress, and \(\sigma_{3}\) is the minor principal stress. Flushing was carried out to remove air and gases from soil samples and tubes having the largest dimensions. Saturation of soil samples was performed using the back pressure method. This stage lasted until the value of [PERSON]'s parameter, \(B\), exceeded 0.95. Next, anisotropic consolidation was performed. In the case of clay, the value of \(K_{\mathrm{o}}\) during the consolidation process was equal to 0.97, whereas for sandy silty clay, \(K_{\mathrm{o}}\) was equal to 0.83. After dissipation of excess pore water pressure, parameter \(b\) was changed to 0.5 (Equation (1)) [44]. In the next step, the value of angle \(\alpha\) was changed to the determined value in particular test. Finally, the process of sample shearing was carried out in the stress path, involving an increase in deviator stress, \(q\), and a constant value of total mean stress, \(p\). During the entire shearing process of the \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline _Soil type_[42] & \(w_{L}\) (\%) & \(w_{P}\) (\%) & \(I_{L}\) (\(\cdot\)) & \(I_{C}\) (\(\cdot\)) & \multicolumn{4}{c}{**Fraction**[43] **(\%)**} \\ \cline{5-8}  & & & & **Gr** & **Sa** & **Si** & **Cl** \\ \hline Cl(U) & 112.9 & 35.3 & \(-\)0.06 & 1.06 & 0 & 5 & 36 & 59 \\ \hline sasiCI(U) & 59.0 & 24.3 & 0.13 & 0.87 & 0 & 21 & 50 & 29 \\ \hline clSa(R) & 19.4 & 8.7 & 0.75 & 0.25 & 0 & 70 & 19 & 11 \\ \hline \multicolumn{8}{l}{Notes: \(w_{L}\)—liquid limit, \(w_{P}\)—plastic limit, \(l_{L}\)—liquidity index, \(l_{C}\)—consistency index. Gr—gravel, Sa—sand, Si—silt, Cl—clay, R—reconstituted, U—undisturbed.} \\ \end{tabular}
\end{table} 
Table 1: Index properties of the tested soils [40,41]. Figure 1: Torsional shear hollow cylinder apparatus (TSHCA) in the Water Center Laboratory at the Warsaw University of Life Sciences (SGGW). (1) TSHC apparatus cell, (2) pressure and volume controllers, (3) electronic measuring device, (4) computer. soil samples, values of parameter \(b\) and angle \(\alpha\) were kept constant.