Abstract The maximum shear modulus (G0) and the modulus degradation curve (G/G0 versus γ) are important information in the evaluation of the soil mechanical behavior, both for dynamic and static loads. Dynamic tests (resonant column and cyclic triaxial tests) are not routinely performed in geotechnical practice in Brazil, and the geotechnical literature on the dynamic behavior of unsaturated tropical soils is limited. This paper presents and discusses seismic dilatometer (SDMT), resonant column, and triaxial test with bender elements and internal instrumentation to determine G0 and the modulus degradation curve in an unsaturated tropical sandy soil profile. It was observed that G0 tends to increase non-linearly with soil suction and net stress (σ - ua). It was also observed that the in situ G0 values determined with the SDMT were higher than those from laboratory tests (bender elements and resonant column). The modulus degradation curves determined with resonant column were used to define the reference curve via SDMT for the studied site. Soil suction influence in shear modulus degradation curves determined with unsaturated triaxial compression tests with local instrumentation is also presented and discussed.

Abstract Maximum shear modulus (G0) has been used in various geotechnical jobs (e.g., seismic site assessment, machine vibration and pile driven). Laboratory and in situ determination of G0 is not a current practice in Brazil. G0 can be estimated from empirical correlations based on in situ tests like Standard Penetration Test (SPT) and Cone Penetration Test (CPT) in the preliminary design phase. Several empirical correlations to estimate G0 from SPT N value have been developed and are available in the literature. However, most of these correlations were established based on experience with well-behaved soils formed in temperate and glacial zones, which may not always be used for tropical soils. This paper assessed and discussed the applicability of some correlations for G0 estimative from SPT data in lateritic and saprolitic soils. The classical correlations for sedimentary soils underestimated G0 of tropical soils. After updating the database, the tropical soils correlations reasonably estimated G0 for the lateritic ones, which was not the case for the saprolitic soils. It was observed that differentiating the soils only as lateritic or saprolitic was not adequate for a good G0 estimate for the saprolitic sandy soils. It was found that only the lateritic soils correlation can be used with caution as a preliminary attempt to estimate G0 from SPT N value in soils with similar characteristics to the ones presented in this paper.

Abstract The slope stability is an important topic because it presents risks of socio-economic losses caused by eventual ruptures. It is necessary to identify the site profile, as well as obtaining soil strength parameters for the slope stability analysis. This paper presents and discusses the use of the Flat Dilatometer Test (DMT) in the geotechnical site characterization for slope stability analysis in an unsaturated tropical soil site. Six DMTs were performed to define the stratigraphical profile and estimate geotechnical parameters. Shear strength parameters were determined in the laboratory using saturated and unsaturated triaxial compression as well as soil water retention curves (SWRC) to support DMT data interpretation. A commercial software was used to perform the slope stability analysis a cut slope with 6.6 m height and a gradient around 55° to illustrate the application of DMT and triaxial test data. The DMT allowed the representative site profile to be identified, as well as estimating the design parameters that compared well to those interpreted from unsaturated triaxial test data for the in-situ soil suction. The DMT can be used as logging test in the preliminary characterization of studied site specially to define the stratigraphical profile, site variability, select the regions to collect disturbed and undisturbed soil samples and as the first attempt to estimate the geotechnical design parameters via correlations. It is important to emphasize that laboratory tests on undisturbed soil samples are essential in the slope stability analysis of unsaturated tropical soil profiles.

Abstract The seasonal variability of geotechnical parameters in the unsaturated zone is typically neglected in the design of geotechnical works. In most of the geotechnical projects the parameters are determined only for the saturated condition. Although it is known that this condition is the most critical to soil strength and deformability, this conservative approach may neglect a possible important contribution of the unsaturated condition, resulting in an increase in the cost of the geotechnical solution. This paper presents and discusses the site characterization of the active zone of an unsaturated sandy soil profile under different suction conditions. Laboratory tests with controlled suction (retention curves, triaxial compression with bender elements and oedometer tests) were carried out on undisturbed samples collected from 1.0 to 5.0 m depth. The results show that strength and deformability parameters are strongly affected by soil suction and are less influenced by confinement stress up to 5.0 m depth. All the investigated subsoil profile shows a collapsible behavior, more pronounced closer to the ground surface and under the effect of higher suction values. The findings highlight the importance of incorporating the suction influence in the site investigation, parameter determination, and geotechnical design for more economical, reliable, and environmentally sustainable solutions.

Resumen Este artículo presenta la caracterización geotécnica de un perfil de suelo tropical no saturado de un campo experimental localizado en la ciudad de Bauru, São Paulo, Brasil. Fueron ejecutadas varias campañas in-situ, que incluyen ensayos geofísicos (cross-hole, down-hole y up-hole con ensayos SPT) y ensayos mecánicos (SPT, CPT e SDMT). Según el sistema de clasificación MCT, los primeros 13 m del sitio de estudio corresponden a un colúvio de matriz arenosa con comportamiento laterítico (LA'), seguido por un suelo residual de comportamiento no laterítico (NA'). La relación obtenida entre los parámetros sísmicos ( Go ) y mecánicos ( qn, N60, ED, KD e MDMT ) permitió identificar la presencia de microestructuras y confirmar el comportamiento específico de este material. Los ensayos de CPT y SDMT fueron eficientes para definir detalladamente el perfil estratigráfico; no obstante, la estimativa de los parámetros geotécnicos no puede ser realizada basada únicamente en correlaciones convencionales definidas para suelos sedimentares.

Abstract This paper compiles site characterization data of an unsaturated tropical soil profile in an experimental research site in Bauru, São Paulo State, Brazil. Several in situ campaigns comprised geophysical borehole tests (cross-hole, down-hole and up-hole together with the SPT), as well as mechanical tests (SPT, CPT and SDMT) have been performed. According to the MCT Classification System for tropical soils, the first 13 m depth is a colluvium with lateritic soil behavior (LA’) over the residual soil with non-lateritic behavior (NA’). The ratio between seismic ( Go ) and mechanical ( qn, N60, ED, KD and MDMT ) parameters was useful to identify the presence of microstructure and to confirm unusual soil behavior for this tropical soil site. CPT and SDMT were efficient for detailed stratigraphic logging, however, the estimative of geotechnical parameters cannot be solely been done base on just conventional correlations defined for sedimentary soil.