## 2 Generation of Pore Pressure in Partially Saturated Soils
### Materials and Methods  
To examine the influence of saturation level on the liquefaction potential of soils, tests in a triaxial compression apparatus were carried out. The first series of tests, described in [37] (Series A), and subsequent additional tests (Series B) were performed on anthropogenic post-flotation tailings, denoted as OZM50, which are sharp-edged fine sand grains with a rough surface (Figure 4a). The third series (Series C) was carried out on a medium sand called Skarpa, which has rounded quarzitic grains with a smooth surface (Figure 4b) [54]. The basic index properties and strength parameters of both soils are presented in Table 1; grain size distribution curves are shown in Figure 5.  
Figure 4: Microphotographs of (**a**) OZM50 tailings and (**b**) Skarpa sand grains. In order to identify the initial state of particular samples of soils tested, critical state lines for both soils were independently determined based on standard triaxial tests (Figure 6): in drained and undrained tests for Skarpa sand [55] and undrained tests for OZM50 tailings [56]. \begin{table}
\begin{tabular}{c c c c c c c} \hline
**Soil** & \(\rho_{s}\) & \(d_{50}\) & \(d\)\(\ll\)0.075 mm & \(e_{min}\) & \(e_{max}\) & \(\phi^{{}^{\prime}}_{leose}\) \\
**Type** & **(g/cm\({}^{3}\))** & **(mm)** & **(\%)** & **(\(-\))** & **(\(-\))** & **(\({}^{\prime}\))** \\ \hline Skarpa & 2.650 & 0.420 & 0.25 & 0.432 & 0.677 & 34.8 \\ OZM50 & 2.675 & 0.157 & 5.8 & 0.643 & 1.026 & 33.0 \\ \hline \end{tabular} \(\rho_{s}\) is specific gravity, \(d_{50}\) is mean grain diameter, \(e_{min}\) and \(e_{max}\) are, respectively, minimum and maximum void ratios and \(\phi^{{}^{\prime}}_{leose}\) is an internal friction angle for soils in loose state.  
\end{table} 
Table 1: Basic physical characteristics and mechanical properties of the soils tested. Figure 5: Particle size distribution curves of tested soils. Figure 6: Critical state lines: **(a)** OZM50 tailings [56], **(b)** Skarpa sand, after [55]. The primary aim of the laboratory investigations was to empirically isolate the effect of saturation, expressed by the measured value of Skempton's parameter \(B\) on the response of soils during shearing in triaxial conditions. Therefore, in all tests performed, only the saturation level of samples was being changed in a controlled manner, whereas other parameters representing initial state conditions, for both soil types, were kept constant. Special attention was focused on the careful reconstitution of sandy samples in the triaxial apparatus, particularly by controlling void ratio from the very beginning of the specimens' installation as one of two parameters defining the initial state of non-cohesive soil. In order to fully control the value of the void ratio just prior to shearing, its changes during the first phases of the test (saturation, back pressure ramp, consolidation stages) were measured by local gauges making use of the Hall effect. A typical example of such changes is shown in Figure 7, representing the results obtained for test b1. All tested samples were in an initially contractive state. The samples were first subjected to isotropic consolidation by applying mean effective stress, \(p_{0}^{\prime}=400\) kPa and \(p_{0}^{\prime}=200\) kPa, for OZM50 tailings and Skarpa sand, respectively. All the tests were carried out in a strain-controlled mode at a constant strain rate, \(\dot{\epsilon}_{1}\cong 10\%/\)h, and a constant cell pressure, \(\sigma_{3}=\) const. The saturation level of the soil was determined by Skempton's parameter \(B\), measured during the test. In order to obtain assumed values of the parameter, back pressure was gradually increased in a controlled manner, which resulted in a decrease in the volume of air bubbles contained in soil voids, causing a reduction in pore air compressibility and thus a change in the compressibility of pore fluid as a mixture.