Abstract

The Cone Penetration Test (CPTu) can be used to evaluate the soil behaviour and properties through correlations (empirical and/or analytical), and the results must be compared to laboratory tests to be validated. This paper presents the evaluation of the effective friction angle (φ’) of a stiff over-consolidated Brazilian clay based on CPTu data and laboratory tests characterization, such as index properties, and the comparison of the effective shear strength parameters obtained by triaxial tests. Based on CPTu data, the soil was classified using the Soil-Behaviour Type Classification System (SBTn) proposed by Robertson (2016), and properties such as over-consolidation ratio (OCR) and the effective friction angle were determined using well-known correlations such as Chen & Mayne (1996) and Ouyang & Mayne (2019). The shear response defined by CPTu was compared to the results of isotropic consolidated triaxial compression tests (CIUC). The results showed mainly a clay-like behaviour based on the CPTu data, in agreement with the laboratory characterization, which indicated high plasticity. Regarding the shear response, both the triaxial test and the CPTu data indicated dilative behaviour under shear. Furthermore, the effective friction angle obtained from the triaxial test and the CPTu data were similar depending on the OCR used in the equation, demonstrating the applicability of the adopted methodologies for the Brazilian stiff over-consolidated clay. Finally, based on CPTu data it is proposed a complement of the SBTn developed by Robertson (2016) with OCR isolines.

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.