Deformation-Control Performance of Pile Reinforcement for a Large Stilling Basin on a Heterogeneous Weak-Rock Foundation

Large stilling basins founded on heterogeneous weak-rock foundations may experience nonuniform deformation due to spatial variations in foundation stiffness. Although pile reinforcement is widely used for deformation control, its effectiveness may vary with local structure–foundation conditions. Overall and local deformation may also respond differently to pile parameters. This study conducted three-dimensional FLAC3D analyses of the Kala Hydropower Station stilling basin under the check-flood condition. Two representative transverse sections with different weak-rock distributions, pile embedment conditions, and structural configurations were compared under unreinforced and pile-reinforced conditions. The maximum resultant displacement (Umax) and maximum vertical joint offset (ΔUjoint) were used to characterize overall and local differential deformation, respectively. Under the 2.5 m × 2.5 m pile layout, Umax and ΔUjoint decreased by only 3.1% and 3.2% in Section 14–14, compared with 33.7% and 58.9% in Section 16–16. The two deformation measures also responded differently to pile spacing and pile length, and denser layouts or longer piles did not consistently reduce both overall and local deformation. Varying the thickness of competent rock overlying the weak rock produced only small changes in deformation, whereas increasing the weak-rock deformation modulus reduced both deformation measures, particularly in Section 16–16. These findings show that the effectiveness of pile reinforcement varies among structural locations with different foundation and embedment conditions. Pile layouts should therefore be evaluated for individual structural locations using both overall displacement and joint differential movement.