Effect of non-plastic fines on cone resistance in silty sands

PhD Student: Qiqi Huang

Publication Date: 2015

Advisor: Sabanayagam Thevanayagam

Abstract: Effect of non-plastic fines on the cone resistance and cyclic resistance of sands and silty sands remains an unresolved problem. This study focuses on: (a) model cone penetrometer experiment study on penetration resistance of sands and silty sands at 15 and 25% silt content, (b) numerical study on the effect of permeability and compressibility (representing the effect of silt content), diameter of cone (d  ) and penetration rate (v) on cone resistance in sands and silty sands, and (c) comparative analysis of the results from (a) and (b) of the effect of silt content on cone resistance through a non-dimensional parameter T (=vd , where v is the penetration rate, d is the cone diameter and cis the coefficient of consolidation) for all soils at the same equivalent inter-granular void ratio [(e )] or relative density [(D ) ].

In both experimental and numerical studies for saturated sands and silty sands, the normalized cone resistance (q) decreased with an increase in silt content, at the same [(D ) ], from 0 to 25%. However this influence of silt content on penetration resistance was absent for dry sands and silty sands at the same [(D ) ]. The difference of cone resistance in saturated sands and silty sands, is thought to be due to partial drainage occurring in saturated silty sands whereas the penetration process is thought to be nearly drained in saturated sands. In the case of dry soils, this pore pressure influence is absent and hence the same penetration resistance is observed in sands and silty sands. This indicates the important influence of pore pressures and its dissipation rates, depending on the silt content, on cone resistance.

Both experimental data and numerical results indicate that for the same (D ) , q decreases as T increases, which implies a decrease in cv (or increase in silt content), because of the penetration process transition from drained to partially drained or even undrained condition. In addition, the numerical analysis shows that v and d also influence q in silty sands. q decreases as v increase or d increases (T increases) in the partially drained condition. Since fines content is not the only factor affecting cone resistance a T dependent relationship between cyclic resistance ratio (CRR) and q for sands and silty sands was proposed. Further research is needed to evaluate and validate such a procedure using field data and physical model tests.