To reveal the heterogeneous cementation mechanism of grouted bodies in water-rich fault fracture zones and promote the transition of grouting design from empirical practice to quantitative approaches, a large-scale triaxial testing system (500 mm in diameter and 1 000 mm in height) is employed. Grouted samples are prepared using two grouting processes—full-section one-time grouting and layered segmented grouting—at grouting ratios of 0, 0.10, 0.15, and 0.20 under confining pressures of 400, 800, and 1 200 kPa, with reconstituted saturated soil-rock mixtures serving as the grouting medium. Subsequently, consolidated drained triaxial shear tests are conducted, and comparative analyses are performed based on stress-strain response, volumetric deformation characteristics, and failure patterns. The results indicate the following: (1) As the grouting ratio increases, the stress-strain behavior of the grouted body changes from strain hardening/softening to brittle failure. (2) Confining pressure substantially suppresses brittle behavior and promotes postpeak ductile strengthening. (3) Cohesion increases rapidly and nonlinearly with the grouting ratio, whereas the internal friction angle increases most markedly when the grouting ratio ranges from 0.10 to 0.15. (4) Grout selectively migrates and accumulates along preferential flow paths during grouting, forming a heterogeneous binary composite structure comprising an external structural shell and an internal grout-vein skeleton, which primarily causes pronounced brittleness and abnormal volumetric response at high grouting ratios. (5) Compared with one-time grouting, segmented grouting produces grouted bodies with higher uniformity and greater shear strength. These findings provide experimental evidence for selecting mechanical parameters and interpreting the heterogeneous cementation mechanism of grouted bodies in water-rich fault fracture zones.