1 Introduction
2 Tunnel Hydraulics and Fluid Flow in the Rock Mass
2.1 Flow and Pressure Transients in a Hydropower Tunnel
2.2 Fluid Flow Through Rock Fractures
3 Brief on Roskrepp Hydropower Plant
3.1 Geological Setting of the Project
3.2 Joint Conditions at the Instrumentation Location
Joint set | Jf | Jfconductive | J1 | J2 |
---|---|---|---|---|
Strike | N 140°–160° E | N 150° E | N 80°–100° E | N 60°–75° E |
Dip | 75°–90° SW | 80° SW | 70°–85° SW | 20°–40° SE |
Persistence (m) | 3–10 | More than 10 m | 3–10 | 3–10 |
Joint wall weathering | Fresh (W1) | Slightly weathered (W2) | Fresh (W1) | Slightly weathered (W2) |
Joint roughness | Rough planar JRC 4-6 | Rough undulating JRC 14-18 | Rough planar JRC 4-6 | Smooth undulating JRC 10-14 |
Joint aperture (mm) | Tight (0.1–0.25 mm) | Partly open (0.25–1 mm) | Tight (0.1–0.25 mm) | Partly open (0.25–1 mm) |
Joint infilling condition | Clay | Washed out | Clay | Washed out |
Seepage | Damp but no dripping or following water present | Continuous flow | Wet with occasional drops of water | Continuous flow |
Typical spacing (m) | 1–2 m | More than 10 m | 1–2 m | More than 10 m |
4 Instrumentation Setup
4.1 Selection of Instrumentation Location
4.2 Instrumentation Layout and Details
Borehole | BH 1 | BH 2 | BH 3 | BH 4 | BH 5 |
---|---|---|---|---|---|
Trend/plunge | 255°/10° | 155°/10° | 260°/10° | 160°/10° | 80°/10° |
Location | Right wall | Left wall | Right wall | Left wall | Left wall |
Borehole length (L), m | 7 | 7 | 9 | 9 | 11 |
Depth of packer from tunnel wall (P), m | 2 | 2 | 4 | 4 | 2 |
4.3 Possible Error Sources and Remedies
5 Observed Behavior
5.1 Tunnel Filling and Dewatering
5.2 Pore Pressure Response
5.3 Response to Pressure Transients
5.4 Delayed Pore Pressure Response
6 Interpretation and Discussion
Borehole | Pore pressure response |
---|---|
BH1A | Responsive to transients In general, fast build-up of pressure. Pore pressure drops when tunnel pressure is static (days 8–11 and days 23–30), indicating possible seepage to the daylight area in the construction adit. Erratic behavior between days 63 and 166. Relatively fast pressure drop during dewatering. Pore pressure magnitude is high, almost equal to tunnel water pressure |
BH2B | Non-responsive to transients In general, slow build-up of pressure. Pore pressure increasing slowly even when tunnel pressure is static (days 44–46 and 72–76). Pore pressure magnitude is highest among all non-responsive boreholes |
BH3B | Non-responsive to transients In general, slow build-up of pressure. Pore pressure drops when tunnel pressure is rising between days 102 and 137, indicating possible seepage from the daylight area in the construction adit. Slowest pressure drop during dewatering. Pore pressure magnitude is second lowest |
BH4A | Responsive to transients In general, delayed pore pressure response observed during pressure transients in the tunnel. Fast build-up and drop of pressure during filling and dewatering. Pore pressure magnitude is high, almost equal to the tunnel water pressure in the tunnel |
BH5B | Non-responsive to transients In general, slow build-up of pressure. Pore pressure increasing slowly when tunnel pressure is static (days 72–76). Fast pressure drop during dewatering up to a certain level and then becomes slower. Pore pressure magnitude is lowest |
6.1 General Behavior
6.2 Pore Pressure Magnitude
6.3 Response to Pressure Transients
Water hammer | Mass oscillation | |
---|---|---|
Open joints | Water hammer travels deep into the rock mass but with reduced pore pressure amplitude (BH4). Higher effect is seen in shallow zones around the tunnel (BH1) | Mass oscillation travels into the rock mass. Pore pressure magnitude is not significantly affected by the length of flow path. But longer flow path causes delayed pore pressure response (described in Sect. 6.4) |
Tight joints | No effect | No effect |