Detailed engineering geological assessment of a shotcrete lined pressure tunnel in the Himalayan rock mass conditions: a case study from Nepal

Basnet, Chhatra Bahadur; Panthi, Krishna Kanta

doi:10.1007/s10064-019-01544-9

Cited by 5 publications

(24 citation statements)

References 24 publications

(46 reference statements)

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“…The characteristics of joints such as spacing, persistence, aperture and infilling conditions should be defined to understand both mechanical and hydraulic behavior of the joints. Basnet and Panthi (2020) studied joint details of the tunnel segment between T3 and T4 ( Fig. 3c).…”

Section: Geological Conditionsmentioning

confidence: 99%

“…The output from best-fit model was considered a best possible result for the NEW HRT alignment of UTHP. Basnet and Panthi (2020) used FLAC 3D model to analyze in-situ stress state of UTHP area within the model extent as shown in Fig. 9a.…”

Section: Stress State Analysismentioning

confidence: 99%

“…Mostly, the numerical modelling is carried out by considering rock mass as a homogeneous, isotropic and elastic material. However, while carrying out stress state analysis, it is also possible to include large scale discontinuities in the model as have been demonstrated by Basnet and Panthi (2018b,2019,2020. It is noted here that consideration of large-scale discontinuities may not fully represent the real situation since small scale discontinuities such as local weakness and shear zones or bands may also cause considerable spatial variation in the in-situ stress.…”

Section: Introductionmentioning

confidence: 99%

“…Since sprayed concrete is a permeable material, the shotcrete lined pressure tunnel is in principal considered to be similar as an unlined tunnel. Basnet and Panthi (2020) used Norwegian confinement criteria to study the feasibility of unlined or shotcrete lined pressure tunnel at UTHP. The results showed that the whole length of headrace tunnel alignment is safe against hydraulic jacking with a factor of safety exceeding 3.5.…”

Section: Introductionmentioning

confidence: 99%

“…The in-situ stress state analysis, on the other hand, showed some critical locations of the headrace tunnel, such as weakness zones and downstream stretch of the headrace tunnel where maximum hydrostatic water head will prevail during operation, the factor of safety was found to be less than the minimum required value of 1.3. The detail rock engineering assessment carried out by Basnet and Panthi (2020) indicated that the geological features consisting off local weakness zones, shear bands, unfavorable joints are critical features for unlined pressure tunnels. Therefore, the stress state analysis carried out for the UTHP was found not enough to address the behavior of geological features when exposed to full hydrostatic pressure during operation of the hydropower plant.…”

Section: Introductionmentioning

confidence: 99%

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Fluid Flow and Leakage Assessment Through an Unlined/Shotcrete Lined Pressure Tunnel: A Case from Nepal Himalaya

Panthi

Basnet²

2021

Rock Mech Rock Eng

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The use of unlined/shotcrete lined pressure tunnels and shafts are cost-effective solutions for a hydropower project and are being implemented worldwide. To implement this concept, the ground conditions at the area of concern should be favorable regarding minimum principal stress magnitude, which should be higher than hydrostatic water head acting on the tunnel periphery. In addition, the rock mass should be relatively unjointed or joints in the rock mass should be relatively tight. Among the most important issues in the design of unlined/shotcrete lined pressure tunnels is the extent of hydraulic jacking and water leakage out of the tunnel during operation. This manuscript first presents fluid flow and potential hydraulic jacking assessment of two selected locations of the headrace tunnel of Upper Tamakoshi Hydroelectric Project (UTHP) in Nepal using the UDEC. It is noted here that the 7960 m long headrace tunnel will experience a hydrostatic water head that will vary from 2.9 to 11.5 bars (0.29–1.15 MPa). The headrace tunnel is supported by sprayed concrete (shotcrete) in combination with systematic rock bolts in the tunnel walls and crown. The invert of the tunnel and few hundred meters downstream end (at surge shaft area) of the headrace tunnel is being concrete lined after the completion of all other works. The qualitative fluid flow assessment carried out using UDEC indicated considerable pressure built-up in the joint systems suggesting potential hydraulic jacking. This was especially the case at the downstream segment (downstream from chainage 7100 m) of the headrace tunnel. The manuscript further presents the quantitative results of water leakage estimation from the headrace tunnel carried out using Panthi (Panthi KK (2006) Analysis of engineering geological uncertainties related to tunnelling in Himalayan rock mass conditions. PhD Thesis, NTNU, Trondheim, Norway;Panthi, Note on estimating specific leakage using Panthi’s approach, NTNU, Trondheim, 2010;) approach. The leakage assessment carried out indicated an average specific leakage of about 2.5 l/min/m tunnel, which may result in over 210 l/s leakage from the headrace tunnel. The evaluation also indicated that the outer reach (860 m downstream segment) of the headrace tunnel after chainage 7100 m seems extremely vulnerable and over 80 l/s water leakage may occur only from this headrace tunnel segment during operation of the hydropower plant.

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Section: Geological Conditionsmentioning

confidence: 99%

Section: Stress State Analysismentioning

confidence: 99%