Location

Chicago, Illinois

Session Start Date

4-29-2013

Session End Date

5-4-2013

Abstract

The Nyamjang Chhu Hydro Electric Project (780 MW) located in District Tawang, Arunachal Pradesh, India, proposes construction of a barrage across Nyamjang Chhu River for diversion of water for power generation. The diversion barrage comprises of gated spillway and sluiceway, head regulator, surface desanders and a collection pool cum tunnel intake. Geotechnical investigations for assessment of subsoil strata at barrage location and its evaluation for foundation design included drilling of 13 no boreholes totaling 551m. In-Situ permeability and standard penetration tests (SPT) were conducted in the boreholes along with Dynamic Core Penetration Test (DCPT) conducted adjacent to boreholes. Disturbed and undisturbed soil samples were also collected for conducting laboratory tests. The laboratory testing included Mechanical Analysis, Atterberg’s Limits, Specific Gravity, In-Situ Density, Tri-axial Shear test under Consolidation Un-drained Condition (CU) and Consolidation test. Rock core samples were tested for Density, Sp. Gravity, Uniaxial Compressive Strength (UCS), Modulus of Elasticity and Poison’s Ratio. The field SPT values (NSPT) in 9 boreholes were observed in the range of 4 to 59 at different depth. In 2 boreholes, NSPT values were observed in the range of 41-72. In remaining 2 boreholes, SPT was not carried out as the foundation stratum comprises of boulders and rock. The soil stratum in Barrage area indicates presence of medium to fine & very fine sand of varying compactness. The 'NSPT' values observed in the boreholes were corrected for overburden pressure and dilantancy. Permeability tests indicate permeability value in the range of 10-2 to 10-3 cm/sec in upper 6 to 9 m depth in majority of the boreholes indicating presence of pervious sandy soil. A few permeability values were observed in the semi pervious range of 10-4 to 10-5 cm/sec. Dynamic Cone Penetration Test (DCPT) at 10 locations was conducted adjacent to boreholes to verify the NSPT test results. DCPT ‘N’ values in the range of 22 to 59 were observed with refusal depth varying from 5.10m to 12.30m. The DCPT ‘N’ values were converted to their corresponding NSPT values and the corrected SPT ‘N’ values and converted DCPT ‘N’ values are also compared. The project is located in earthquake prone area with MCE and DBE values of 0.288g and 0.197g for an earthquake magnitude of 7.7 (Mercalli Scale). In view of the significantly low SPT N values observed during field testing, analysis for liquefaction potential at barrage location was undertaken based on simplified approach suggested by Seed & Idriss (1982 & 1999) to evaluate liquefaction potential of sandy foundation strata. The approach involves computation of Cyclic Stress Ratio (CSR) τav/σ'v and comparing it with the Cyclic Resistance Ratio (CRR) given by Seed & Idriss corresponding to earthquake of same magnitude. The CSR value at various depths in each bore hole is also plotted against the critical boundary limit for liquefaction provided by Seed & Idriss. Factor of safety against liquefaction is then calculated as ratio of CRR to CSR with value less than and equal to 1 indicating liquefaction potential of sandy soil. Based on observed penetration resistance, foundation areas having ‘NSPT’ value lower than 40 are assessed to be liable for liquefaction and such areas are proposed to be treated by Vibro-compaction method to make the sand dense thereby eliminating the possibility of liquefaction due to development of large strains under high pore water pressure.

Department(s)

Civil, Architectural and Environmental Engineering

Appears In

International Conference on Case Histories in Geotechnical Engineering

Meeting Name

Seventh Conference

Publisher

Missouri University of Science and Technology

Publication Date

4-29-2013

Document Version

Final Version

Rights

© 2013 Missouri University of Science and Technology, All rights reserved.

Document Type

Article - Conference proceedings

File Type

text

Language

English

Share

 
COinS
 
Apr 29th, 12:00 AM May 4th, 12:00 AM

Geotechnical Investigation at Barrage for Foundation Design at Nyamjang Chhu Hydro Power Project, Tawang, Arunachal Pradesh, India — A Case Study

Chicago, Illinois

The Nyamjang Chhu Hydro Electric Project (780 MW) located in District Tawang, Arunachal Pradesh, India, proposes construction of a barrage across Nyamjang Chhu River for diversion of water for power generation. The diversion barrage comprises of gated spillway and sluiceway, head regulator, surface desanders and a collection pool cum tunnel intake. Geotechnical investigations for assessment of subsoil strata at barrage location and its evaluation for foundation design included drilling of 13 no boreholes totaling 551m. In-Situ permeability and standard penetration tests (SPT) were conducted in the boreholes along with Dynamic Core Penetration Test (DCPT) conducted adjacent to boreholes. Disturbed and undisturbed soil samples were also collected for conducting laboratory tests. The laboratory testing included Mechanical Analysis, Atterberg’s Limits, Specific Gravity, In-Situ Density, Tri-axial Shear test under Consolidation Un-drained Condition (CU) and Consolidation test. Rock core samples were tested for Density, Sp. Gravity, Uniaxial Compressive Strength (UCS), Modulus of Elasticity and Poison’s Ratio. The field SPT values (NSPT) in 9 boreholes were observed in the range of 4 to 59 at different depth. In 2 boreholes, NSPT values were observed in the range of 41-72. In remaining 2 boreholes, SPT was not carried out as the foundation stratum comprises of boulders and rock. The soil stratum in Barrage area indicates presence of medium to fine & very fine sand of varying compactness. The 'NSPT' values observed in the boreholes were corrected for overburden pressure and dilantancy. Permeability tests indicate permeability value in the range of 10-2 to 10-3 cm/sec in upper 6 to 9 m depth in majority of the boreholes indicating presence of pervious sandy soil. A few permeability values were observed in the semi pervious range of 10-4 to 10-5 cm/sec. Dynamic Cone Penetration Test (DCPT) at 10 locations was conducted adjacent to boreholes to verify the NSPT test results. DCPT ‘N’ values in the range of 22 to 59 were observed with refusal depth varying from 5.10m to 12.30m. The DCPT ‘N’ values were converted to their corresponding NSPT values and the corrected SPT ‘N’ values and converted DCPT ‘N’ values are also compared. The project is located in earthquake prone area with MCE and DBE values of 0.288g and 0.197g for an earthquake magnitude of 7.7 (Mercalli Scale). In view of the significantly low SPT N values observed during field testing, analysis for liquefaction potential at barrage location was undertaken based on simplified approach suggested by Seed & Idriss (1982 & 1999) to evaluate liquefaction potential of sandy foundation strata. The approach involves computation of Cyclic Stress Ratio (CSR) τav/σ'v and comparing it with the Cyclic Resistance Ratio (CRR) given by Seed & Idriss corresponding to earthquake of same magnitude. The CSR value at various depths in each bore hole is also plotted against the critical boundary limit for liquefaction provided by Seed & Idriss. Factor of safety against liquefaction is then calculated as ratio of CRR to CSR with value less than and equal to 1 indicating liquefaction potential of sandy soil. Based on observed penetration resistance, foundation areas having ‘NSPT’ value lower than 40 are assessed to be liable for liquefaction and such areas are proposed to be treated by Vibro-compaction method to make the sand dense thereby eliminating the possibility of liquefaction due to development of large strains under high pore water pressure.