The project site is situated in highly stressed rock formations located in upstate New York. The project location and client name are confidential. The objective of the study was to investigate the stability of an 87-year old surge chamber shaft embedded in a river gorge side slope with a slope height in excess of 150 feet. The shaft is concrete lined and approximately 22 feet in diameter and 75 feet high. Based on a visual inspection, the concrete shaft structure displayed extensive cracking with a crack pattern that appeared to be the result of high non-uniform stresses imposed by the surrounding rock formations with possible rock expansion and movement towards the gorge. The rock formations present at the site consist primarily of limestones, sandstones, and shales. An engineering geological investigation was designed to study the in situ rock mass characteristics, in situ stresses and ground behavior. The study included geological rock mapping (RMR & Q System), rock coring, geophysical investigation, dilatometer in situ testing, and laboratory rock testing. A numerical model of the rock gorge was built to simulate the rock stresses and behavior using two-dimensional Fast Lagrangian Analysis of Continua (FLAC) software. This paper presents the geological study and the analysis. The analysis results obtained were found to concur with initial field observations.
PROJECT SETTING & SITE GEOLOGY
URS Corporation (URS) was retained to undertake a detailed inspection, investigate existing ground conditions and evaluate the structural condition of underground structures associated with a power generating hydroelectric facility located on a river between two sets of falls in upper New York state. The underground structures comprised an intake tunnel and ancillary structures, including the intake shaft, the surge tube riser shaft (STRS), the tunnel transition area (TTA), and the three penstocks including the penstock transition zones (PTZs). The work included a geotechnical investigation and performance of numerical and empirical analyses to assess the structural stability of the aforementioned existing structures. This paper details the assessment of the structural stability of the STRS only.
The intake tunnel is located beneath a river extending from the upper falls (higher elevation) to the lower falls (lower elevation). The ground surface elevations in the area range from approximately elevation (EL) 450 feet at the top of the river gorge to EL 260 feet below the lower falls. The elevation of the river varies from EL 392 feet above the upper falls to EL 252 feet below the lower falls. Between the two falls, the river is at approximately EL 350 feet. The intake tunnel conveys water from the impoundment area (approx. EL 391 feet) located above the upper falls, through a series of three penstocks and turbines, to discharge points located at the base of the lower falls. The total elevation head of water between the impoundment area located above the upper falls and the base of lower falls is 139 feet.
