A 3-Dimensional numerical modeling (FLAC3D) was used to design Surge Tank (ST) (of THP) located in thinly foliated dark gray to black Slates and Phyllitic Slates. ST was oriented, located, designed and constructed as per pre-scribed Method Statement (MS). MS is prepared considering overburden > 250 m, weak & anisotropic rock mass, i.e., susceptible to sliding along foliation planes, and rock mass with weak layers bands, i.e., prone to failure. Due to very weak zone with shear bands encountered during excavation, rigorous exploration/testing was done via. Pilot tunnel and borehole scanning. ST was finally designed (with reduced dia. of 23.5 m) by shifting of 21 m southward based on the test result. Pilot hole drilling (dia. 0.25 m) and reaming (dia. 1.40 m) was conducted by RBM. Then shaft sinking method was implemented to excavate the shaft in 3 radial slices and 4 circumferential parts. ST has been excavated even by mechanical means in the weak rock mass. ST excavation (44,220 m3) was started on 01 Aug 2021 and finished on 21 Nov 2023 along with support measures consisting of pre-consolidation grouting, steel ribs/lattice girder, DCP anchor in addition to rock-bolts & shotcrete. The real-time monitoring data was used for continuous evaluation of underground stability, which enhanced the recursive and diligent design optimization in conjunction with a well-structured/professional MS thereby restrained stability. The integration of adaptive design, a well-defined construction strategy & MS along with continuous instrumentational monitoring and timely responses, has resulted the successful construction of ST.