Vibratory cone penetration test to investigate cyclic soil behavior in-situ

Climate change increased the need of using renewable energy as replacement for fossil fuel. This led to a fast growth of the offshore wind energy sector, especially in Germany where offshore wind energy turbines are built in great numbers in the North Sea. Piles are usually used as foundations for these turbines. In recent years, new projects planned to install these piles with the vibratory pile driving technique which installs piles with axial vibrations. One of the main challenges with vibratory pile driving is the choice of the vibrator that should have a sufficient weight and energy to drive the pile to the designed depth. The selection of the vibrator usually depends on predictions from drivability analyses that use parameters obtained from the conventional in-situ soil investigation methods such as cone penetration test CPT as an input for the analysis. CPT is a cone that is pushed into the ground at constant speed while sensors in the cone measure the cone resistance, sleeve friction, and pore water pressure. The static soil behavior obtained from these conventional CPTs differs from the cyclic soil behavior exhibited during vibratory pile driving. In this research, vibratory cone penetration test VCPT is introduced to improve the geotechnical in-situ methods for evaluating the cyclic soil behavior during vibratory pile driving. The new device controls displacement amplitudes in real time and it penetrates the ground while inducing vertical cyclic strains. The cyclic motion of VCPT resembles the motion of the piles during vibratory driving, therefore it could be used to investigate the cyclic soil behavior and degradation during vibratory pile driving. Major obstacles with any in-situ soil investigation method are the inherent variations in soil properties which could affect the results obtained from in-situ tests. Performing a number of tests in small spacings increases the statistical significance and reduces the effect of soil variations on test results. Still, there is no consensus on how to define the minimum spacing between testing zones and no consensus on the effect of disturbance caused by in situ tests. This doctoral thesis has divided the in-situ soil investigations into three studies: In the first study, static cone penetration tests were used to develop a procedure to characterize the in-situ properties of soil and to investigate the minimum spacing between tests without the effect of soil disturbance. A systematic grid of 33 CPTs was performed in the field by sequentially and successively refining the grid spacing between CPTs, starting with a spacing of 119 cone diameters down to a grid spacing of 7 cone diameters. It was found that the cone resistance is affected by previous CPT measurements below a spacing threshold of 24 cone diameters in medium-dense sands. Silt and clay layers showed no reduction in the cone resistance for our minimum grid spacing of 7 cone diameters. The study also showed that the spacing between the tests and natural variations in soil properties are deciding factors for the number of CPTs needed in the field for a sufficient statistical significance. In the second study, VCPT was used to investigate the cyclic behavior of the soil. The device proved to generate constant amplitudes until the final depth of penetration. Two sand layers reacted to the applied cyclic loads by showing high reduction in the resistance. It was also found that even a distance of 50 cm does not guarantee a good correlation between CPT and VCPT. Therefore, two or more pairs of CPT and VCPT should be conducted in order to minimize misinterpretations of the data which are caused by small scale geological structures such as the presence of inclined layers, cross bedding, or other heterogeneities. In the third study, nine static CPTs and VCPTs were performed in a systematic grid. The vibratory CPTs were performed at a constant frequency of 20 Hz and at three different amplitudes of 3, 5, and 7 mm. It was found that the degradation of soil resistance to vibratory cone penetration increased with increasing cyclic displacement amplitudes. This degradation was not accompanied by any increase in pore water pressure. Cyclic cone resistance-displacement hysteresis loops indicated the formation of a cavity between the cone and soil during the upward movement of the cone. Furthermore, a distinct difference between the loading and the unloading stiffness during the vibratory penetration was observed. The results demonstrated that there is no unique relation between static and vibratory cone resistance, therefore, current practices that estimate the degradation in soil resistance due to vibratory pile driving from the static cone resistance are probably not sufficient. The results obtained from the three studies showed that the current civil engineering approaches that assess the cyclic soil behavior during vibratory pile driving from static cone resistance are not sufficient because there is no unique relation between static and vibratory cone resistance. VCPT could be used to assess the cyclic soil behavior, however, several tests should be performed in order to have statistical significance to consider the effect of the variation in soil properties