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Alkaline activation of fly ash was used to improve the mechanical performance of a silty sand, considering this new material as a replacement of soil-cement applications namely bases and subbases for transportation infrastructures. For that purpose, specimens were molded from mixtures of soil, fly ash and an alkaline activator made from sodium hydroxide and sodium silicate. Uniaxial compression tests showed that strength is highly increased by the addition of this new binder. The results described a high stiffness material, with an initial volume reduction followed by significant dilation. All specimens have clearly reached the respective yield surface during shearing, and peak strength Mohr-Coulomb parameters were defined for each mixture. The evolution of the microstructure during curing, responsible for the mechanical behavior detected in the previous tests, was observed by scanning electron microscopy. These results were compared to soil-cement data obtained previously with the same soil at similar compaction conditions. The main difference between both binders was the curing rate, with alkali activated specimens showing a more progressive and long-lasting strength increase. This was analyzed taking into account the chemical process responsible for the behavior of the mixtures.

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For the assessment of the quality of laboratory samples, a number of methods are available, though not universally applicable to any soils. This paper examines the issue of sampling quality and its assessment using comparisons between shear wave velocity measurements in situ and in recovered samples as the base, which is very useful in naturally structured granular soils, like residual soils. For this purpose, cross-hole and down-hole tests were performed in thê eld and bender elements measurements were made on triaxial samples collected from two experimental sites on residual soil from Porto granite. Various sampling techniques and tools were used, including block sampling and diŠerent tube samplers. The analysis of the results has led to a new classiˆcation of sampling quality and sample condition based on the comparison of normalised shear wave velocities in theˆeld and in the laboratory.

Recently, an innovative technique based on continuous modal identification of the first flexural resonant frequency of a simply supported composite beam has been proposed for continuous evaluation of the E modulus of concrete from the instant of casting (termed EMM-ARM: elasticity modulus measurement through ambient response method), and has been successfully compared with other existing techniques. In view of its potential applications for the laboratory and in situ characterisation of cement-stabilised soils, the extension of using EMM-ARM for these materials has been preliminarily tested with promising results, but the extension of the procedure has also resulted in the identification of new challenges. The purpose of this paper is to show the results and conclusions of an experimental programme devised to evaluate a sand-cement mixture from compaction to 28 days later by comparing several testing methodologies: EMM-ARM, unconfined cyclic compression tests, bender-extender elements, and ultrasonic contact probes. Attention is also given to the issue of obtaining in situ samples using the EMM-ARM technique through a specially designed mould.

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