Prediction of Shear Strength Parameters for Prototype Riverbed Rockfill Material Using Index Properties

Rockfill materials are being used widely in the construction of rockfill dam to trap river water because of their inherent flexibility, capacity to absorb large seismic energy and adoptability to various foundation conditions. Rockfill material is obtained from Noa Dehing dam site, Arunachal Pradesh. The maximum particle size of the material used in the dam is 600 mm. For testing, the size is scaled down to smaller sizes of 4.75, 10, 19, 25, 50 and 80 mm maximum particle size (d max ) using parallel gradation technique. Drained triaxial tests are carried out with a specimen size of 381 mm diameter and 813 mm height with varying confining pressure ( σ 3 ) from 0.2 to 0.8 MPa. All the d max are tested for 87% and 75% relative density (RD). The index properties of the rockfill materials viz. unconfined compressive strength (UCS) and uncompacted void content (UVC) are determined. Stress-strain-volume change behaviour of the modelled rockfill material is studied and presented. The shear strength parameter, angle of internal friction ( φ) is determined for all the d max of modelled rockfill materials tested with 87% and 75% RD. Strength law has been developed to determine the failure stresses using index properties of rockfill materials viz. UCS, UVC and RD and then φ-values are predicted for all the modelled rockfill materials satisfactorily. The φvalue of the prototype rockfill material is predicted using the proposed strength law. The predicted φ-value of prototype rockfill material is compared with the φ-value predicted by commonly used extrapolation technique (power law which requires laboratory triaxial test results) based on d max and found that φ-value match closely. Therefore, it is believed that the proposed method is more realistic, economical, can be used where large size triaxial testing facilities are not available and quick to determine φ-value using index properties.