New offshore geotechnical investigation method using Self Elevating Platform -Integrated survey of CPT and offshore reverse PS logging-

<p>　In offshore wind power projects, there are many cases where offshore CPT, which is easy to temporarily install and enable quick investigation, is carried out using Self Elevating Platform (SEP) as a method for measuring ground constants. However, in many cases, it is not possible to continue pushing to the required depth with CPT in the seafloor ground near Japan, and it is common to dig to a predetermined depth with a high-speed excavation method such as percussion boring and repeat CPT.</p><p>　In addition, in the offshore geotechnical investigation, it is important to understand the engineering base layer (Vs≧400 m/s). Generally, the acquisition of the S wave velocity is performed by a suspension PS logging (hereinafter referred to as susPS logging) using drilled boreholes, but problems such as being affected by the excavation diameter or the surface condition of the borehole wall have also been pointed out. This article shows the results of a new PS logging method using vibration during standard penetration test (SPT) in offshore boring and vibration during percussion drilling used in offshore CPT. This PS logging was named the submarine reverse PS logging because it is the reverse of the downhole PS logging performed on land (source position and receiving position are reversed). As a result of performing the offshore reverse PS logging in the same hole as the susPS logging or in the nearest hole and comparing the S wave velocities, it was found that the same results as the susPS logging can be obtained in the seafloor reverse PS logging. This method does not require a measurement sonde to be inserted into the borehole, and is effective for boreholes with collapse or large borehole diameters that are difficult to measure with susPS logging (usually the applicable diameter for susPS logging is approximately 200 mm or less).</p>