Strength and deformation characteristics of cement-mixed gravelly soil in multiple-step triaxial compression

When backfill is mixed with cement and compacted in the field, it is very difficult to obtain the multiple similar samples needed to determine a Mohr-Coulomb (M-C) failure envelope by a set of triaxial compression (TC) tests at different levels of effective confining pressure (sigma(h)'s), due to the inevitably large heterogeneity. The effects of the intermediate loading histories, with and without large stress-amplitude unload/reload cycles, on the stress-strain properties of compacted cement-mixed well-graded gravelly soil, particularly the peak strength, were evaluated in multiple-step loading (ML) drained TC tests using a single specimen. A set of single-step loading (SL) tests at different sigma(h)'s were also performed. Similar specimens, prepared in the laboratory, were used for the tests in this study. Although the M-C failure envelope can be quite accurately determined by relevant ML tests, the peak strength may be underestimated if (a) TC loading is ceased far before the peak stress state or (b) TC reloading is started after the peak stress state has been passed at previous loading steps. Cases (a) and (b) may take place in both ML tests, with increasing and decreasing sigma(h)', while Case (b) may take place in ML tests with decreasing sigma(h)'. The reloading stress-strain relation at the intermediate stage in ML tests may become very soft due to the additional damage caused by a negative irreversible shear strain increment occurring in the immediately preceding unloading process. This effect gradually decreases during reloading, while it totally disappears once large-scale yielding has started, with essentially no effect on the peak strength. It is of the first priority to perform ML tests by increasing sigma(h)'. It is recommended that ML tests be performed by decreasing sigma(h)' if possible, because the peak stress may be closely reached at some intermediate steps. An upper envelope of results from a pair of ML tests, with increasing and decreasing sigma(h)', could be more representative of the true M-C failure envelope. (C) 2012. The Japanese Geotechnical Society. Production and hosting by Elsevier B.V. All rights reserved.