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PORE PRESSURE GENERATION CHARACTERISTICS OF SANDS & SILTY SANDS: A STRAIN APPROACH

GRADUATE RESEARCH ASSISTANT: Kenan Hazirbaba (graduated May 2005)

FUNDED BY: National Science Foundation CAREER Award

DESCRIPTION
This research investigates the pore pressure generation characteristics of sands and silty sands. Different from earlier studies, which placed an emphasis on characterization of liquefaction in terms of the induced shear stress required to cause liquefaction, this study adopted a strain approach because excess pore water pressure generation is controlled mainly by the level of induced shear strains. This approach was first proposed by Dobry et al. (1982). Multiple series of strain-controlled cyclic direct simple shear and cyclic triaxial tests were used to directly measure the excess pore water pressure generation of sands and silty sands at different strain levels. The soil specimens were tested under three different categories: (a) at a constant relative density, (b) at a constant sand skeleton void ratio, and (c) at a constant overall void ratio. The results from each of these groups were examined. In addition, laboratory measured pore water pressures of clean sands were compared to in situ (Rathje et al. 2005) measured values.

The findings from this study were used to develop insight into the behavior of silty sands under undrained cyclic loading conditions. In general, beneficial effects of the fines content up to 20% were observed in the form of a decrease in excess pore water pressure and an increase in the threshold strain (Figure 1). However, pore water pressure appears to increase when enough fines are present to create a sand skeleton void ratio greater than the maximum void ratio of the clean sand. The comparison between laboratory and in situ measurements indicated that larger pore water pressure was generated in situ as a result of the lower overburden pressure (Figure 2).

Figure 1 - Click on image to enlarge

Figure 2 - Click on image to enlarge

PUBLICATIONS
Hazirbaba, K. and Rathje, E.M. 2004. “A Comparison between In Situ and Laboratory Measurements of Pore Water Pressure Generation,” Paper No. 1220, 13th World Conference on Earthquake Engineering, Vancouver, Canada, August.

Kenan Hazirbaba (May 2005). "Pore Pressure Generation Characteristics of Sands & Silty Sands: A Strain Approach," PhD Disseration, Univ. of Texas at Austin