Estimates of effective stress beneath a modern West Antarctic ice stream from till preconsolidation and void ratio

Preconsolidation stress recorded in subglacial sediments provides important information about subglacial effective stresses. It is commonly used to re construct past effective stresses from sediments left after ice retreat. In this article, we use properties of sub-ice-stream till samples to estimate effective stresses beneath a modern West Antarctic ice stream. Two previou s estimates of sub-ice-stream effective stress were derived for the Upstrea m B (UpB) area of Ice Stream B from sheer wave velocities (50 +/- 40 kPa, B lankenship er al. 1987) and borehole water level measurements (63 +/- 24 kP a, Engelhardt & Kamb 1997). However, geotechnical tests per formed on sampl es of the UpB till have shown that if subjected to effective stress of 50-6 3 kPa this till would have significantly lower porosity (similar to0.32-0.3 5) and higher strength (similar to 22-28 kPa) than it apparently has in sit u (similar to0.4 and similar to2 kPa). We derive new estimates of sub-ice-s tream effective stress using: (1) Casagrande's construction applied to the results of six confined uniaxial tests, and (2) a combination of void-ratio data for 51 till samples and 3 experimentally constrained equations descri bing compressibility of the UpB till under normal consolidation. overconsol idation and in the critical state. Casagrande's method yields an upper boun d on effective stress of 25 kPa for four till samples and values of 13, and 4.4 kPa for two other samples. The void-ratio approach gives 11.7 +/- 2.6 (normal consolidation), 18.3 +/- 4.4 (overconsolidation) and 2.0 +/- 0.8 kP a (critical state). These new, lower estimates of effective stress are cons istent with the low till strength that has been independently measured and inferred from recent theoretical ice-stream models. Our interpretation of d ata on till void ratio in terms of sub-ice-stream effective stress means th at we can qualitatively evaluate the nature of the vertical distribution of this stress in the UpB till layer. We infer that in the sampled top 3 m of till the effective-stress distribution is non-hydrostatic, probably close to lithostatic. The results may be useful in future modeling of ice-stream behavior and may aid efforts to delineate paleo-ice streams based on their geologic record.