Abstract

Assessing soil properties through geotechnical tests is a crucial activity to understand its behaviour. Field tests serve as a good approach to characterize the geotechnical behaviour of materials according to their in-situ condition. However, the interpretation of these tests often relies on empirical correlations, which can become complex when dealing with soils with notable heterogeneity. This paper objective is to determine the strength parameters and discuss the consolidation effects for a heterogeneous tropical soil deposit using data from field tests with different rates of penetration and laboratory tests. A layer identified as low-strength soil classified as silty-sand to silty-clay exhibited partial drainage during a standard-rate cone penetration. The approach proposed by Randolph and DeJong (2012) was used to determine the penetration rate necessary to mobilize an undrained behaviour of the material, since the cone penetration results shown that the material is mostly contractive and clay-like. CPTu tests were also conducted with penetration rates of 60 mm/s and 100 mm/s, to proper estimate the undrained shear strength (Su). These tests reinforced the presence of a preconsolidated upper layer, enabling the estimation for the overconsolidation ratio (OCR). For the normally consolidated portion of the residual soil, rapid tests that achieved a normalized velocity (V) associated with distinctly undrained penetration were used to derive the undrained shear strength (Su), with a cone factor (Nkt) estimated from standard-rate tests available. The geotechnical parameters estimated were then compared to the laboratory data results showing a convergence with the parameters estimated by the field tests.

Abstract

The Piezocone (CPTU) and the Marchetti Flat Plate Dilatometer (DMT) provide a number of empirical correlations for the purpose of soil identification. Each tool has defined a pore pressure parameter. For the piezocone, the parameter Bq ‘is based on the penetration pore pressures, the corrected tip resistance, the total overburden stress, and the existing hydrostatic pore pressure. This index has been used to characterize soil behavior rather than the actual soil type. For the dilatometer, the parameter Ud is based on the corrected pressures p0 and p2, calculated using the A and C readings and the existing hydrostatic pore pressure. This index can be combined with the material index ID to help with soil identification. Results from several test sites provide a promising direct correlation between Bq and UD, which can be used to enhance the identification of soil types by simply using the C-reading when performing DMT tests. Although the C-reading has been grossly ignored by users of the DMT, this new development will encourage measurement of this reading which provides similar trends to the pore pressures measured by the CPTU. The latest DMT equipment allows for the automatic measurement of the C-reading, which will assist in developing and enhancing existing correlations. This paper will present results at test sites located on opposite coasts of the USA. The trends of the Bq and UD indices are shown to be remarkably similar in all types of soils.

Abstract

This paper presents an interpretation of cyclic pressuremeter tests with pore pressure measurement performed at Larivot bridge site (French Guyana). In cohesive soils, test results are in accordance with the pressuremeter theory. The times at 50% dissipation correlate closely with those obtained from dissipation tests performed using a piezocone in the immediate vicinity.

Abstract

The research compares 2 CPTU profiles obtained with a same piezocone setup but having a different degree of saturation associated with the pore pressure measuring system. In the reference test, saturation was performed injecting 20cS silicon oil in the conduit connecting the porous stone to the pressure sensor and applying vacuum while submerged in oil for 15 minutes. The piezocone tip was then assembled with a saturated porous stone while submerged in oil. In the other test, the degree of saturation was purposely lowered by introducing air in the same conduct, whereas all other saturation steps were unchanged. The degree of saturation was compared quantitatively by measuring an analogue of the Skempton’s coefficient B, which is routinely used in laboratory testing to assess specimen saturation in a triaxial cell. The value associated with the saturation condition was measured employing a tool specifically designed for this purpose. The saturation procedures adopted were selected based on preliminary experimental activity in the laboratory, which provided target values of the pore pressure parameter corresponding to full or partial saturation. The CPTUs were performed at a test site presenting 10m clay unit followed by sand. The profiles measured were compared in terms of pore-pressure profiles, as well as the influence this had on corrected tip resistance, Soil Behaviour Type classification and mechanical properties. Additionally, a dissipation was performed for each test to compare consolidation parameters.