# Effect of Water Content and Cementation on the Shear Characteristics of Remolded Fault Gouge
## 3 Results  
### Shear Stress-Displacement Curves of Fault Gouge  
Figure 4 presents the shear stress-displacement curves of fault gouge under different degrees of cementation (C) and water content (W). Careful inspection shows two fundamental curve shapes. The first rises to a peak and then drops steeply--a \"cliff-type\" profile that occurs mainly in the highly-cemented C3 series, particularly at W20-W30, and signals a sudden loss of load-carrying capacity characteristic of brittle failure. The second shape, typical of the low- to moderate-cemented C0 and C1 series, slopes gently into a plateau or only mild softening after the peak, indicating ductile deformation governed by ongoing particle rearrangement. Figure 3: Flow chart of fault gouge direct shear test. The coexistence of these two curve families demonstrates that when cementation becomes sufficiently strong, the fault gouge as a whole evolves from a loose soil to a weakly cemented rock, and its failure mode shifts from ductile to brittle. This cementation-controlled ductile-brittle transition is consistent with the findings of [PERSON] et al. [39], who reported that increasing the amount of bonding in granular materials ultimately converts ductile yielding to brittle failure. Water content acts cooperatively: higher moisture lowers the  
Figure 4: Shear stress–shear displacement curves of remolded fault gouge. effective normal stress along potential shear planes, so at the highest cementation level, the bonded skeleton collapses rapidly once peak strength is reached, producing the cliff-type drops seen in Figure 4m,n,o. In sum, abundant bonding together with pore water transforms fault gouge from a medium that can dissipate energy through plastic deformation into one that fractures abruptly upon reaching its peak strength. To facilitate a more intuitive analysis of the effects of water content and degree of cementation on the shear strength of fault gouge, the maximum shear stress under a normal pressure of 400 kPa is defined as the shear strength, and then the relationships between shear strength and water content as well as degree of cementation are plotted, as shown in Figure 5.  
Figure 5a shows that the shear strength of fault gouge initially increases and then decreases with increasing water content. As the degree of cementation increases, the magnitude of the strength increase becomes larger, while the magnitude of the subsequent decrease becomes smaller. Specifically, at a cementation degree of 0%, there is no initial increase in shear strength (0% increase), but a significant decrease of 71% is observed at higher water contents. At 1% cementation, the strength increases by 2% before decreasing by 49%. At 3% cementation, the strength increases by 52% and then decreases by only 10%. Figure 5b illustrates the variation in shear strength with the degree of cementation for fault gouge samples at different water contents. As the water content increases, the effect of increasing cementation degree on shear strength shifts from a decreasing to an increasing trend. The magnitude of the strength change progressively becomes larger with higher water content, showing percentage changes of \(-\)10%, +43%, +104%, +207%, and +330%, respectively. These observations indicate that the two variables exert opposite first-order influences on shear strength: rising water content tends to weaken the gouge, whereas increasing cementation systematically strengthens it. The interplay of these opposing effects gives rise to the nonlinear dependence of shear strength on both water content and cementation degree. Mechanistically, initial pore water can enhance bond mobilization and promote slight dilative hardening in a well-cemented matrix, but once the voids become saturated, lubrication along nascent shear planes lowers effective normal stress and accelerates bond breakage, causing strength to decline.