Effect of laboratory compaction mode, density and suction on the tensile strength of a lime-treated silty soil

Facing the need to adopt a laboratory compaction method of natural or lime-treated soils which is repeatable and representative of real in-situ compaction conditions for dike or road constructions, the effects of compaction mode, dry density and suction on the tensile strength of natural and lime-treated silty soil compacted in laboratory have been investigated in a systematic way. Soil specimens were prepared from three different modes of compaction: the static kneading compaction, the standard Proctor compaction and a dynamic in-mold compaction. For kneading and Proctor compaction, small cylindrical samples were extracted at different locations in larger compacted specimen with a milling machine controlled by computer. On the contrary, the so-called “in-mold compaction” consists in compacting the soil in a mold with the final required dimensions. The small cylindrical samples were then submitted to various suctions from 0 to 2000 kPa during seven days. At the end, dry density of samples was measured with a 3-dimensionnal scanner and tensile strength was determined from indirect (Brazilian) tensile tests. The same investigation was also performed, with similar number of specimens, on untreated soil. In-mold compaction provides the best repeatability of obtained tensile strength (essentially because of the controlled and uniformly distributed dry density though the specimen) but is not representative of real compaction condition. Also, it is observed that the tensile strength of untreated soils is strongly affected by suction level and slightly by dry density. At the opposite, for lime-treated soil, little variations of dry density may have a significant impact on the tensile strength while suction plays a secondary role. This study reveals that, when compacted lime-treated soils are used as a bearing element (like in road subgrades or subbases), a particular attention must be paid on the quality of compaction process to avoid under-compacted zones that could lead to material weakness.