Abstract

The fundamentals for predicting the mechanical behavior of soils by in situ tests have been developed for conventional soils (either clay or sand) based on the stress history. The behavior of unusual geomaterials, such as the tropical soils, is characterized by bonding and structure, anisotropy as well as by the unsaturated condition. The behavior of tropical soils cannot be correctly predicted by models and correlations developed by the Classical Soil Mechanics. This paper presents the fundamentals of the behavioral classifications used to interpret CPT and SDMT and discusses their applicability to tropical soils, especially those of pedogenetic evolution of sandstone. Laboratory and in situ tests (CPTu and SDMT) were carried out at two research sites in São Paulo state, Brazil, at different periods of the year to better understand the soil behavior and the seasonal effects. Classifications and correlations to estimate soil parameters from CPTu and SDMT are assessed. Interrelationships between elastic parameters of small (G0) and large to medium strain parameters (qc, ED, MDMT) determined by in situ tests are presented to identify the presence of microstructure and unusual soil behavior. Classification criteria based on these relationships to identify collapsible soils also is presented

Abstract

Characterization of the geomechanical behavior of the seabed along a high-pressure high-temperature (HPHT) pipeline route is important for understanding risks from geohazards and thermally induced displacements such as axial walking. Axial resistance to pipeline displacement, from the friction between seabed and the contact surface of the pipe, can be estimated through laboratory tests using interface shear box (ISB) and tilt table. Tests performed by NGI in several developements in Gulf of Mexico, South America, and West Africa revealed significant variation in shear resistance, potentially associated with the type of interface material and surface roughness characteristics. This paper illustrates the effect of clay activity and interface surface material on the residual undrained interface strength estimated from interface shear box tests. Two cohesive soil batch samples with varying activity were tested using two different interface plates (steel and silicon carbide sandpaper) of comparable surface roughness. Each soil batch-interface combination was tested under three different initial effective normal stresses and two different over consolidation ratios (OCRs). Undrained residual interface strength envelopes were developed for each soil batch-interface combination. The results from tests performed on steel interface showed an increase in residual undrained interface strength with plasticy and clay activity whereas a reverse trend was observed in the results of the tests performed using the sandpaper interface. This reinforces the importance of the choice of interface plate material (in addition to surface roughness) for PSI testing program to accurately capture the resistances offered by the seabed to pipe displacement.