Evaluation of SHANSEP parameters and initial-shear-stress induced strength anisotropy of a lagoonal deposit near Keʻehi Interchange, Honolulu, Hawaiʻi

Abstract

Four CKoUC, four CKoUE and four CKoUDSS tests were conducted to study the effects of initial-shear-stress-induced anisotropy on the strength of a Hawaiʻi lagoonal deposit classified as an elastic silt (MH). Undrained strength ratios were plotted versus OCR, and Stress History and Normalized Soil Engineering Properties (SHANSEP) parameters were determined. The SHANSEP parameters were then used to calculate undrained strengths based on the stress history of the lagoonal deposit. These values were then compared to undrained strengths from 11 unconsolidated undrained (UU) triaxial tests. If the undrained shear strength, Su is assumed to be half the maximum deviator stress (q) for the triaxial tests, the CKoUC strengths were largest, the CKoUE lowest and the CKoUDSS in between. If Su = qcosϕ' (where ϕ' = effective friction angle), the CKoUC and CKoUE curves shift downwards, resulting in the CKoUDSS curve almost coinciding with the CKoUC curve. The effective friction angles ϕ' were approximately 40o and 26o for the normally and overconsolidated samples, respectively. The SHANSEP parameter S for the Hawaiʻi lagoonal deposit was the largest compared to soils from other publications. An acid test to detect the presence of calcium carbonate in the lagoonal deposit was performed. Vigorous bubbling and effervescence indicated the presence of calcareous fines in the lagoonal deposit. This provides a possible explanation for the large normally consolidated φ' as well as the large SHANSEP parameter S. Using CKoUDSS SHANSEP parameters, calculated undrained shear strengths were within 10% on average of those from UU tests. In addition, the SHANSEP undrained strengths were slightly more conservative than the 11 UU strengths in general.