Studies on Stability Assessment of Large Caverns in Himalayan Region

Abstract

“Studies on Stability Assessment of Large Caverns in Himalayan Region” is a research study carried out to understand the behavior of large caverns in Himalayas, where rock mass is generally unfavorable for tunneling. Caverns at Tala hydroelectric project, Bhutan, situated in lower Himalayas were chosen for the study. The prime objective was to study the rock mass behavior at various stages of excavation of large caverns. Field studies were carried out to measure various parameters like, displacement, wall convergence, load on the rock bolts, load on steel ribs and strains in the rock bolt using magnetic ring multipoint borehole extensometers(MRMPBX), total station, reflective targets, load cells and instrumented bolts. 3D numerical modelling studies were carried out using continuum (FLAC-3D). Analysis of MRMPBX data in the crown of powerhouse cavern indicated significant influence of side slashing in the crown (accounted for 71-88% of total displacement) on the stability of the crown and super incumbent rock mass. There was clear indication of joint separation taking place even upto 10m above the crown, indicating the requirement of long rock bolts of minimum 12m length. Strains calculated in the crown at some places were more than 1%. Steel ribs installed in the crown also recorded more than 100t load. Benches within one time the width of cavern had maximum influence on the rate of load increase on the steel ribs. Upstream and downstream wall of powerhouse cavern converged by more than 200- 300mm in unit bay area and 135mm in service bay area. Calculated strains during the monitoring period were 0.92 to 1.74% in the unit bay area and upto 0.66% in the service bay area. Analysis of convergence data indicated that about 88% of convergence took place during excavation of caverns and about 12% was recorded during post excavation period. Average convergence rate during excavation of benches varied from 0.56mm/day to 0.67mm/day, whereas convergence rates during other miscellaneous excavations varied from 0.38 to 0.49mm/day. During the post excavation monitoring done for 3 years, convergence of 16-50mm was observed with average convergence rate of 0.021mm/day to 0.063mm/day. Predicted convergence from 3D continuum model compared well with the field monitored values within 15% at most of the places.3D modelling results indicated maximum strain of 3.34% on the upstream wall and 3.35% on downstream wall for the entire excavation period. On the upstream wall, strains of more than 2% were observed in unit bay area. In the service bay area, strains were restricted upto 2%. On downstream wall maximum strain was observed near the bus ducts. Higher strains were concentrated near the crown of bus ducts. On downstream wall, strains of less than 2% were observed in service bay area. In transformer hall also similar behaviour was observed. Results clearly indicated that when the height of excavation increases (w/h ratio decreases), unit bay area experiences higher strain. Rock bolts in the walls of powerhouse cavern recorded load upto 45t. Bolts on the upstream wall experienced loads of higher magnitude compared to the bolts on downstream wall. During post excavation, load on the bolts on downstream wall was greater (average 21.4%) than the load on bolts on upstream wall (average 8.7%). 3D modelling results showed higher displacements concentrated near the intersection of floor of the cross tunnels and downstream wall of the powerhouse cavern. In general, there was increase in principal stresses in the crown and reduction in principal stresses in the walls of cavern, which corroborates with high values of convergence recordings in the walls. Tensile stresses were observed in the rock mass, particularly in the floor of powerhouse cavern due to floor heave. Analysis of stress concentration factors in the pillar between powerhouse and transformer hall indicated factors due to maximum and intermediate principal stresses were almost equal (varied from 1.20 to 1.58). Normal stresses had higher influence on the stress concentration factors in the pillar. Stress concentration factors near the floor of transformer hall upstream wall was maximum and stress relaxation was to a lesser extent compared to other locations. Strength of the rock mass was estimated using 3D Hoek and Brown failure criterion. Caverns are found to be stable as the strength to stress ratio in the surrounding rock mass was greater than two except for few patches near the walls of the cavern. Tensile failures were noticed in the corners and at the floor of the powerhouse cavern. Monitoring in the cross tunnels indicated development of load in the range of 100-110t in the steel ribs and >35t load on the rock bolts.Changing the orientation of the cavern with respect to the maximum principal stress direction had maximum effect on the upstream wall of the powerhouse cavern with an increase of maximum displacement by 28.7% at an angle of 90o. Study also showed that maximum stress concentration factor in the pillar due to vertical stress does not change significantly with cavern orientation except in the pillar near the end walls. When the cavern is oriented parallel to maximum principal stress, maximum stress concentration factors due to vertical stress were the highest on the upstream wall of powerhouse. But at an angle of 90o, confinement provided by horizontal stresses decreased, that could cause instability problems in the wall. Confinement levels provided by horizontal stresses were greater on downstream wall of transformer hall than on the upstream wall of powerhouse cavern. Sensitivity analysis results clearly indicated that friction angle was the most sensitive parameter among the all considered parameters. Predicted convergence increased by an average of 76.2% when the friction angle was reduced by 20%, and decreased convergence by 33.9% when friction angle was increased by 20%. Thus, the friction angle has very considerable influence on the convergence in the model. Cohesion, elastic modulus and Poisson’s ratio were less sensitive in that order. In this research study, integrated approach involving field investigations during the excavation and post excavation period and 3D modelling followed by correlation between the results. Both the stages of investigations could explain the behaviour of the cavern and surrounding rock mass in a comprehensive manner under a Himalayan geological setup.