An economical method, Sampling & Pull-Up Test (S&PUT), was proposed for investigation of the mechanical properties of the seabed for foundation design of bottom-mounted offshore wind turbine towers. After two kinds of prototype apparatus were manufactured, field experiments were carried out to investigate the feasibility of the proposed test method. The test results demonstrated that, for sounding performance, the PUTs were suitable, as the depth profiles of the wing resistances by PUTs and those of the cone resistances by CPTs were in harmony. Whereas the sampling performance was not satisfactory yet, as the sampling recovery ratios were found not greater than 80%.
An economical method for investigating the mechanical properties of the surface ground on the seabed is required for foundation design of bottom-mounted offshore wind turbine towers. The authors proposed a new sampling and pull-up test, denoted as a S&PUT for short, that can perform sampling and sounding simultaneously (Tani et al., 2020a). Using a hydraulic jack and a vibro- or a percussion hammer, a test device with a sounding probe for pull-up tests (PUTs) and cone penetration tests (CPTs) is statically or dynamically and continuously driven into the ground simultaneously with samplers equipped alongside rods. The purpose of this test method is to collect continuous samples not only of soft ground but also relatively hard ground, while conducting sounding tests to evaluate a depth profile of stiffnesses and/or strengths at the same location. Although limited to soft ground, CPTs can be performed, when the probe is driven statically at a constant rate. On the other hand, PUTs can be done even in harder ground, once the probe is installed dynamically at depths.
Fig. 1 compares the concept of S&PUTs and that of standard penetration tests (SPTs) which are most widely used for geotechnical investigations. The basic concept of these two test methods is identical in that, both sampling of disturbed soil samples and sounding for evaluation of the mechanical properties can be done at the same time in the same location. Thereby, similar results of geological columns that are the depth profiles of soil types and sounding measurements can be obtained effectively by both test methods. However, there are four-fold differences as below. The first is that, S&PUTs can be conducted much faster than SPTs. In SPTs, the works of driving the sampler while counting the number of blows, SPT-N values, for sounding, retrieval of the sampler for sampling, and drilling/boring to the next depth must be repeated every 1 meter in depth. Since installation and removal of a self-elevating platform (SEP) or a simple scaffolding to place the drilling machine are time consuming, it usually takes several days to a few weeks to do a SPT at each site. On the other hand, in S&PUTs, the works of driving/penetration and retrieval/pulling-up of the probe and the samplers are done only once. Since the operation of a pile driving barge to be used is simpler and quicker than that of a SEP, the time required to do a S&PUT at each site is estimated not longer than several hours or a day.
