## 5 Methodology for Quality Control of Jet Columns  
The quality of jet columns was controlled by employing observational, mechanical, geophysical, and numerical methods. After some time, from the construction of the jet columns under the seabed, the divers visually observed the jet columns (Figure 10). However, as mentioned previously, some serious difficulties exist due to the offshore environment. For this reason, a geophysical measurement-based methodology was proposed. The main principles of the proposed methodology are summarized as follows:  
1. Before any jet grouting is performed, the entire area is measured using the seismic method while the site is in virgin conditions. The measurements obtained are evaluated together with the data obtained from the offshore boreholes. 2. Jet grout columns are constructed, and seismic measurements are obtained again. The seismic measurement results of both the virgin field and the field where the columns are constructed are compared. Figure 9: (**a–c**) The relationships between UCS, V\({}_{\text{P}}\), and g of the jet column samples. Each improved area is measured with the seismic method before the filling work continues, after which it is concluded that the improvement is successful. All geophysical measurements were performed by [PERSON] and [PERSON] [62]. Before jet grouting construction, the sea floor and the bottom structure were measured by the shallow seismic reflection method. For this reason, works were performed with the 10-3.5 kHz shallow seismic reflection (Stratabox subbottom profiles) system. Acoustic sub-bottom profiler methods produce sub-bottom images that scale horizontally in meters but vertically in milliseconds. During data collection, the speed of sound passing through the medium (water, silt, sand, etc.) is not constant. Therefore, the correct way to evaluate data is to calculate the time it takes for signals from the environment [62]. The numerical seismic data subject to this study were obtained using the \"Syqwest SubBottom Profiler\" system, which is a high-resolution Chirp source shallow seismic reflection system. A boat was used during the seismic measurements (Figure 11) and the boat speed was kept between 3-4 knots during the study. The seismic measurement lines before the jet column construction are shown in Figure 12. Consequently, the lithologic profiles were obtained and compared with the borehole logs. An example is given in Figure 13.  
After constructing the jet columns, the seismic measurements were performed again. An example is given in Figure 14. As can be seen from Figure 14, it is observed that the test jet columns, whose axis is indicated by green color, vary from the seabed down to approximately 5.5 m and their diameters vary between 70 and 90 cm. Figure 11: A view from seismic measurements [62]. Figure 10: (**a**) Measurement and (**b**) observation of jet columns underwater. In addition, the seismic measurements were checked by boreholes, and confirmed. A total of 6 boreholes were drilled to check the length and uniaxial compressive strength of the jet columns. Uniaxial compressive strength tests were applied on 45 core specimens. The minimum, maximum, and average uniaxial compressive strength values of the 45 core specimens were obtained as 1.23 MPa, 7.98 MPa, and 3.59 MPa. These values were consistent with the values obtained from the test columns. As can be seen in Figure 15, the core recovery values varied between 80% and 95%. Consequently, the methodology proposed in the present study provided good performance, and it contributed to the project schedule. Figure 12: Seismic measurement lines before jet column construction [62]. Figure 13: H2 seismic refraction section and comparison with DS-5 borehole log [62]. Finite element analyses were performed to evaluate the performance of the jet grouting application in the study site for static and pseudo-static conditions.