EXPERIMENTAL ANALYSIS OF FLAT RIGID PISTON DILATOMETER

Abstract

The Newcastle dilatometer (NDMT) is a distinct in-situ geotechnical investigation device developed at the University of Newcastle Upon Tyne, UK in 2001. The NDMT compatibility in medium-to-stiff cohesive and loose-to-medium cohesionless normally consolidated soil deposits has already been proved. The NDMT compatibility in overconsolidated cohesive and cohesionless soil deposits is yet to be established in-order to extend its application to larger variety of soils. Moreover, the stresses in soils due to pushing of the NDMT probe or expansion of the rigid piston are also to be investigated experimentally. These objectives were achieved in this research through an extensive field and laboratory investigation program comprising of the Newcastle Dilatometer Test (NDMT), Cone Penetration Test (CPT) and Standard Penetration Test (SPT). Test pits were excavated adjacent to the in-situ test locations for the collection of undisturbed (UD) soil samples. Laboratory tests were performed on undisturbed soil samples from the test pits which included; resonant column, triaxial compression, consolidation, and unconfined compression tests. The disturbed soil samples obtained from SPT spoon sampler were subjected to laboratory tests such as; natural moisture content, particles size analysis, Atterberg limits, specific gravity, maximum and minimum unit weight, direct shear, and permeability. The estimation of soil properties such as bulk unit weight, relative density, pre- consolidation pressure, overconsolidation ratio, coefficient of earth pressure at rest, undrained shear strength, friction angle, shear modulus, constrained modulus, and shear wave velocity were assessed from the NDMT data in comparison with laboratory, CPT and the SPT tests invariably. The horizontal stress index (K D ) was found to be the most comparable parameter from the NDMT in overconsolidated soils for deriving various correlations with laboratory and other in-situ test (CPT/SPT) parameters. The NDMT pressure penetration curves were analyzed likewise the pressuremeter test and it was observed that the soil properties especially the strength and stiffness are functions of the NDMT pressure penetration profile. The determination of strength and stiffness from the analysis of the final unloading portion of the NDMT curve is the exclusive finding of this iiiresearch, which can eliminate the need of inducing unload-reload loop in the pressure penetration profile of the NDMT. Correlations between soil stiffness (small strain and static shear modulus) and limit pressure obtained from the NDMT are also explored. This thesis also presents the development and test results of a calibration chamber assembly equipped with load cells, hydraulic pushing system and sample preparation system. The assembly was successfully used to determine stresses in soils due to pushing the NDMT probe and expansion of the NDMT piston. It was found that stresses during loading of the soil during testing and pushing of the NDMT remains quite uniform. It is concluded that the NDMT has a strong potential as a substitute in-situ test device. Further, the device has the capability; if not fully to replace the laboratory and other in- situ tests in geotechnical engineering. However, more research is required to enhance the capabilities of the NDMT in geotechnical engineering practice by strengthening the correlations and findings suggested in this study.