# Interface Direct Shear Tests on JEZ-1 Mars Regolith Simulant
## 4 Results
### Direct Shear Tests  
The direct shear tests were carried out on the dense specimens at different normal stresses (i.e., 25, 50, and 75 kPa), to find the cohesion and internal friction angle of JEZ-1. Figure 6a presents the shear stress responses of JEZ-1, direct shearing under a larger normal stress mobilizes larger shear stresses. The shear stress curves show a sharp increase then gradually go up to maximum finally to a constant volume state, due to the space between the upper (100 mm \(\times\) 100 mm) and the bottom (140 mm \(\times\) 100 mm) shear boxes, and the small holes on the bottom part of the shear box [15]. The JEZ-1 grains that go to the space and small holes mentioned above will result in a decrease on the sample density. Figure 6b presents the normal strain as a function of shear displacement. The curves of normal strain show contraction firstly then dilation.  
A simplified linear envelope is considered since the tests presented here were performed at neither quite low nor very high normal stresses, at which both the influence of dilation and particles crush were limited [7]. The linear-fitted envelopes are plotted in Figure 7. Peak and residual internal friction angles of the JEZ-1 are determined from the envelopes, as shown in Figure 7. JEZ-1 exhibits peak (\(\varphi_{\rm peak}\)) and residual (\(\varphi_{\rm cv}\)) internal friction angles of 40.6\({}^{\circ}\) and 39.7\({}^{\circ}\), respectively. In addition, the peak (\(\zeta_{\rm peak}\)) and residual (\(\zeta_{\rm cv}\)) cohesions are smaller than 4 kPa (see Figure 7).  
Figure 5: Void ratio as a function of the effective vertical stress.  
Table 4 presents the cohesion and internal friction angles derived from direct shear tests and compares these values with published results of other typical Martian simulants (JSC Mars-1 and MMS-1). JEZ-1 simulant is characterized with higher peak (3.1 kPa) and residual (3.2 kPa) cohesion than JSC Mars-1 (1.14 kPa and 0.87 kPa). However, MMS-1 has a peak cohesion of 15 kPa, which is considerably higher than the one of JEZ-1. The peak internal friction angle (\(\varphi_{\text{peak}}=40.6^{\circ}\)) of JEZ-1 is lower than JSC Mars-1 (48.8\({}^{\circ}\)) and MMS-1 (46.0\({}^{\circ}\)) with 8.2\({}^{\circ}\) and 5.4\({}^{\circ}\). While the \(\varphi_{\text{cv}}\) values between the three simulants are quite close (Table 4), with a slight difference range between 0.2\({}^{\circ}\) and 1.4\({}^{\circ}\).  
The cohesions of JEZ-1 are in agreement with the values obtained from in-situ tests from Spirit Rover (1\(\sim\)15 kPa), and Opportunity Rover (1\(\sim\)5 kPa), see [30; 31]. With regard to the internal friction angles, JEZ-1 has much higher values than the in-situ results (about 20\({}^{\circ}\)) from both Spirit and Opportunity Rovers [30; 31].  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multicolumn{1}{c}{**Name of Mars**} & \multirow{2}{*}{\(c_{\text{peak}}\) (\(\text{kPa}\))} & \multirow{2}{*}{\(c_{\text{cv}}\) (\(\text{kPa}\))} & \multirow{2}{*}{\(\varphi_{\text{peak}}\) (\({}^{\circ}\))} & \multirow{2}{*}{\(\varphi_{\text{cv}}\) (\({}^{\circ}\))} & \multirow{2}{*}{**Reference**} \\
**Simulant** & & & & & \\ \hline JEZ-1 & 3.1 & 3.2 & 40.6 & 39.7 & This paper \\ \hline JSC-Mars & 1.14 & 0.87 & 48.8 & 41.1 & [7] \\ \hline MMS-1 & 15 & - & 46.0 & 39.9 & [8] \\ \hline \hline \end{tabular}
\end{table}
Table 4: Cohesion and internal friction angle of Mars simulants.  
Figure 6: Direct shear test results of JEZ-1: (**a**) shear stress and (**b**) vertical strain as a function of horizontal displacement.  
Figure 7: Peak and residual failure envelopes from direct shear tests of JEZ-1.