Processing advancements of free fall penetrometer data and their ground proving in regional studies in New Zealand

Abstract

In the last century, the necessity to understand geotechnical soil parameters has become of significance importance; not only for fundamental research questions but also for every day civil engineering purposes. While there are theoretical geotechnical soil mechanics as well as numerical assessments, most geotechnical investigations of the soils are based on empirical studies. The later is categorising the soil and correlating it to its physical properties. The cone penetration test has been recognised as an established testing techniques, developing over the last century. Developments have occurred in the technical department, including a transition from a mechanical to electrical cone penetration tests and the integration of side friction as well as pore pressure measurements. Additionally, the interpretational capabilities have been extended by the usage of controlled testing for example by the development of cone penetration calibration chamber tests. The numerous empirical studies in the cone penetration testing has enabled the correlation of the primary measurements of the cone penetration (cone resistance, sleeve friction, and pore pressure etc.) to other geotechnical properties like undrained shear strength, friction angle and shear wave velocity. Besides developments in the standard cone penetration tests, it has also become clear that the cone penetration test can be combined with many other data acquisition procedures. Common combinations include the seismic, magnetic, electric resistivity, video, and sampler cone penetration tests. Significant progress with the penetration testing was achieved in the 1970’s when the need for off-shore CPTu applications emerged. The deployment of standard marine cone penetration devices requires an off-shore sea rig to provide the necessary force for seafloor soil penetration. Research has demonstrated that the standard marine devices deform the uppermost deposits, whose physical properties are vital to aquatic related objectives. Such objectives are: (1) coastal protection projects, (2) sedimentary transport, (3) mining, (4) drilling foundations, (5) underwater pipelines as well as cables and tubes, (6) failure mechanism, (7) channel dredging, and (8) anchoring (other factors exist). Hence, dynamic penetration systems have been developed, which penetrate the aquatic soil with its own momentum via a free fall or semi-free fall mode. Evidently, the biggest advantage of such penetration campaigns is the enhanced deployment rate when compared with the standard cone penetration test resulting in financial benefits. In particular, the deployment via the pogo-style decreases acquisition time and thus research costs. Following the given list from above and considering the dangers of local sea level rise in the course of climate change, the most prominent objective will be the geotechnical investigation of coastal areas. Unfortunately, neither standard nor the free fall cone penetration systems are capable to test remotely in logistically challenging areas such as intertidal areas. Only the portable free fall cone penetration systems are able to overcome the restriction of the standard and free fall cone penetration lances. However, portable penetration lances have one big disadvantage: Most portable lances cannot measure the resistance forces directly. Consequently, many geotechnical analyses like standard soil behaviour type estimations are not possible with portable penetration lances. The total resistance estimated via the measurement of the penetrometers acceleration or deceleration. Additionally to the shortcomings in the measurements of the resistance forces, all free fall penetration systems show clear disadvantages in the pore pressure data acquisition. Firstly, the pore pressure data is yet uncorrected for penetration rate effects. Secondly, the dissipation test is often not applied in a free fall campaign because it enhances the data acquisition time especially when during deployments in low permeability soils.In this dissertation, five first author manuscripts, of which four have already been submitted or published, investigate the shortcomings mentioned above. Hence, this thesis aims to improve the processing procedures of free fall penetration tests to assess sediment (re-) mobilisation processes. Furthermore, this dissertation aims to identify geotechnical soil properties, which can be used as a proxy to monitor the remobilisation potential of sediments in coastal areas. For this purpose, a free fall calibration test has been designed to develop a processing technique, which 1) allows the estimation of cone resistance and sleeve friction from acceleration data, and 2) approaches the issues of the pore pressure rate corrections in a first attempt. The results from the free fall calibration tests have been utilised and verified in a local field study in the Firth of Thames, New Zealand. This includes first attempts to estimate the (re-)mobilisation potential of the local sediment by correlating the portable resistance forces to the geotechnical properties such as the coefficient of consolidation. In particular, the change in the consolidation potential was used to identify the interplay between the local floras (the mangrove forest) and the dewatering processes in the soil. Furthermore, a sophisticated and autonomous dissipation test model was developed to extrapolate short time dissipation tests in the field instantaneously. The autonomous model is termed ad hoc model. Its advantages lay up on its extrapolation potential and its autonomous variable analysis of the rigidity index and the coefficient of consolidation without any prior estimation of soil properties. The model was tested in the field on the peninsula Omokoroa at the Bramley Drive landslide, New Zealand. Four dissipation tests were carried out at depth of ~19, ~22, ~23, and ~26 m. The tests have been utilised and truncated to test the actual extrapolation potential and to identify the hydrogeological properties of the study area. The four different dissipation depths represent three different dominating halloysite morphologies (tubular, spheroidal and polyhedrons/plates) and have been chosen in order to study the influence of the halloysite morphology microstructure on the hydrogeological behaviour. Here, again, the coefficient of consolidation was used as a proxy to identify the sediment (re-)mobilization potential for the different designated depths.