A Novel Approach for Assessing the Performance of Offshore Ground Improvement in Floating Storage and Regasification Unit (FSRU) Terminal Construction

Abstract

A floating storage and regasification unit (FSRU) terminal has been planned to be constructed in Saros Bay (Türkiye). This study presents the ground improvement method using jet grouting to prevent the liquefaction of marine sediments in the project area. An approach for performance assessment of jet column construction is also discussed. The study site has a liquefiable ground level with a thickness changing between 2 m and 8 m. Jet grout columns with an 80 cm diameter were constructed under the sea level, which varied between 4 m and 18 m for ground improvement. The main issue is controlling the quality and performance of these jet columns. Therefore, a practical quality control procedure containing observational, mechanical, and geophysical methods for offshore grouting operations was proposed. The factor of safety values against liquefaction varied between 0.04 and 0.29 for natural conditions, while the minimum factor of safety after jet column constructions was obtained as 1.01. The results of the numerical analyses showed that the constructed terminal has sufficient performance against liquefaction. Consequently, the results of these methods have demonstrated that the jet grout applications performed by following this procedure are a suitable and effective improvement method for offshore soils.

1. Introduction

Natural gas has an important role among energy sources from the point of being eco-friendly and cost effective. The liquefaction of natural gas makes it possible to transport larger quantities of gas [1]. Floating storage and regasification units (FSRU) have recently become a preferred strategic solution for transporting liquefied natural gas (LNG) between offshore terminals. As a result, the number of FSRU terminals built on marine sediments, which are unsuitable foundation soils, is increasing day by day in many parts of the world. Instabilities FSRU terminals can cause environmental pollution and economic consequences [2]. Therefore, the resilience of these terminals has crucial importance in terms of environmental safety.

Marine sediments are generally unfavorable foundation soils due to their susceptibility to liquefaction and low-bearing capacity. To increase the shear strength and prevent liquefaction, the marine sediments require improvement before construction. Some approaches have been developed to reduce the liquefaction problem. Deep soil mixing, jet grouting, dynamic compaction, lime stabilization, vibro-compaction, and ground freezing techniques can be employed to decrease the liquefaction potential [3]. Among the ground improvement techniques, jet grouting is one of the most commonly used. There are several previous studies about the application of the jet grout method and the evaluation of their performance [4,5,6,7,8,9,10,11,12]. The main subjects of the previous studies about jet grouting are the design of the columns [13,14,15,16,17], the mixture of grouting material [18,19,20], the injection methods [21,22,23,24], and the quality controls of the columns [25,26,27]. Farhangi et al. [28] have numerically demonstrated that using jet-grouted micro piles significantly reduces the risk of liquefaction in clean sands. However, most of these studies were conducted in a terrestrial environment. The application of jet grouting, particularly in the submerged sandy layers in ocean engineering, has emerged in the last decades [12].

Durgunoğlu et al. [29] discussed the improvement of the foundation ground that will settle on the seabed of a plaza planned to be built in Doha Port, Qatar by the jet grouting method. Durgunoğlu et al. [29] specified that non-destructive geophysical measurements are frequently used in controlling the quality of jet columns besides mechanical and observational control techniques. Lin et al. [30] evaluated the applicability of the surface wave method for identifying the jet grout columns produced in heterogeneous soils. The researchers found that the selection of lateral sampling range in surface wave testing has importance because no apparent difference was determined between the survey line along the reinforced and un-grouted ground when the spacing between the jet columns is less than the minimum wavelength. On such an occasion, Lin et al. [30] suggested that the total shear wave velocity of the jet-grouted ground should be measured with the multi-channel analysis of the surface wave (MASW) method, regardless of the location of the survey line. According to Lin et al. [28], the MASW method is a good tool for quantifying ground improvement. To estimate the diameter of the produced jet columns, Guerreros et al. [31] suggest a method based on the integration of standard cross-hole and downhole seismic measurements. Nevertheless, this approach is applicable in the terrestrial environment. In the literature, a quality control approach based on geophysical measurement before and after jet column construction in offshore environments has not been encountered. The quality control of jet grout columns is carried out conventionally by performing offshore drillings.

On 6 February 2023, a series of earthquakes struck Türkiye, with the largest earthquake measuring 7.8 on the Richter scale. The earthquakes caused widespread damage, including liquefaction of soil in some areas. Liquefaction is a phenomenon that occurs when soil loses its strength and behaves like a liquid. This can cause structures to sink or collapse. The earthquakes caused significant damage to the port of Iskenderun, located in the Hatay province of Türkiye. The containers at the port were overturned, and a serious fire broke out. The fire was brought under control after three days of effort. The February 2023 earthquakes have once again highlighted the need for engineering measures to prevent liquefaction, especially in offshore structures to be built in seismic regions.

The Petroleum Pipeline Corporation of Türkiye (BOTAS) plans to build an FSRU terminal in the Gulf of Saros (Figure 1). The project site is located near the Ganos Segment of the North Anatolian Fault Zone, one of the most seismic active regions on Earth. For this reason, a comprehensive geotechnical site investigation was carried out in the project area. The soil profile was identified by offshore drillings and the liquefaction potentials of marine sediments was determined. The foundation ground, which consists of liquefiable marine sediments, was improved by constructing jet grout columns on the seabed below the water level varying between 4 m and 18 m. The main purpose of the study is to investigate whether the jet columns are effectively constructed and to evaluate the quality of jet columns. Observational, mechanical, and geophysical methods were utilized to evaluate the quality of these columns. This study employs geophysical measurements in addition to mechanical boreholes and observations. The use of geophysical measurements for checking the jet grout performance is the novelty of the present study. The formation of jet columns was monitored step by step with geophysical measurements. Cores were then obtained from the constructed columns and their strength properties were evaluated. After jet grout improvement, the engineering fillings were constructed. The speed of quality control processes for assessment of the jet grout construction directly affects the overall construction time. Consequently, a fast procedure is proposed for the assessment of jet grout construction in offshore environments. This study is an interesting case study for offshore structures to be constructed in active seismic regions. The study also provides new geotechnical data for liquefaction from the Saros Gulf, Türkiye.

2. Characterization of the Study Area

The project area is situated in the Sazlıdere village on the northern shore of the Gulf of Saros in the Aegean Sea (Figure 1). The Gulf of Saros is 30 km wide and 55 km long with an east–west trending wedge-shaped morphology [32]. The depth of the Gulf of Saros increases towards the west. The seabed topography of the Gulf of Saros is asymmetrical, with the topography of the northern and southern parts differing from each other [33]. The slope of the shelf becomes steeper from where the water depth is deeper than 100 m. The coastline and the shoreline overlap, thereby the beach could not develop within the narrow shore in the project area. Rock slopes and shorelines lie following the end of the forest and vegetation cover (Figure 2). At a distance of 150 m from the shore, the water depth is 7 m and the slope of the seabed is less than 5%. Further from this section, the water depth at the end of the project site is 30 m and the slope of the seabed varies between 10% and 15%.

Figure 3 presents the geological map of the close vicinity of the project area. In the northern part of the Gulf of Saros, sedimentary and volcanic rocks aged from the middle Eocene to the Miocene are observed. Quaternary and Miocene sedimentary sequences are separated from each other by an angular unconformity in this region [34]. The main geologic unit in the project site is the Korudag Formation composed of the alternation of the upper Eocene-aged sandstone and claystone.

The evolution of the basin structure of the Gulf of Saros is mostly controlled by two tectonic regimes: the shearing activity of the North Anatolian Fault (NAF) System and the extension along the north–south direction of West Anatolia [32,35,36]. In this region, the NAF formed as a southward concave arc in the late–middle Miocene and divides into many northeast–southwest trending branches [37]. One of these branches, the Ganos Fault (GF), is the closest active fault to the project area (Figure 4). GF lies between the western part of the Marmara Sea and the North Aegean Trench. GF is defined as a dextral strike–slip fault on the land while it has an oblique component in the sea [38,39,40,41]. The most detrimental earthquake generated by the GF during the instrumental period was the Şarköy–Mürefte Earthquake (9 August 1912). A total of 216 people lost their lives in this earthquake with a moment magnitude (Mw) of 7.4. The largest horizontal peak ground acceleration (PGA) of the study area was determined from the interactive web application of Türkiye Earthquake Hazard Maps prepared by AFAD [42]. Accordingly, the peak ground acceleration for the rock environment obtained as 0.619 g and 1.051 g for the earthquake ground motion level with a probability of exceeding 50 years is 10% (recurrence period is 475 years) and 2% (recurrence period is 2475 years), respectively.

Figure 3. Geological map of the project area (modified after [43]).

Figure 3. Geological map of the project area (modified after [43]).

3. Geotechnical Site Investigations and Liquefaction Analysis

A comprehensive site investigation program was conducted by Sebat Co. [45] to determine the geotechnical characteristics of the foundation ground on which an FSRU terminal was planned to be constructed. A total of 10 offshore boreholes with depths ranging from 24 m to 49 m were drilled and several samples were collected for the laboratory testing. The location of the boreholes is given in Figure 5. The depth of the boreholes, the thickness of the lithologic units, and some characteristics of these units such as the Normalized Standard Penetration Test (SPT-N) and Rock Quality Designation (RQD) are summarized in

Table 1. Summary of boreholes and some properties of the lithologic units.

Borehole No.	Depth of Boreholes (m)	Depth of Seabed (m)	Lithologic Units (Depth from the Surface, m)	SPT-N30	RQD (%)
DS-1	24	4	Loose sand (4 m–5.8 m)	10	-
			Sandstone (5.8 m–24 m)	-	10–53
DS-2	24.8	4.8	Loose sand (4.8 m–7.5 m)	2–4	-
			Moderately dense sand (7.5 m–12 m)	12–16	-
			Sandstone (12 m–24.8 m)	-	13–100
DS-3	28.2	9.1	Loose sand (9.1 m–10 m)	2	-
			Moderately dense sand (10 m–18.3 m)	13–23	-
			Sandstone (18.3 m–28.2 m)	-	30–70
DS-4	28.2	8.1	Loose sand (8.1 m–11.5 m)	4–8	-
			Moderately dense sand (11.5 m–17.2 m)	12–21	-
			Sandstone (17.2 m–28.2 m)	-	7–40
DS-5	31.5	18.2	Moderately dense sand (17.2 m–26.4 m)	8–41	-
			Sandstone (26.4 m–31.5 m)	-	19
DS-6	49.0	29.0	Loose sand (30.0 m–34.2 m)	5–9	-
			Sandstone (34.2 m–49.0 m)	-	6–44
DS-7	47.5	27.6	Loose sand (29.2 m–33.5 m)	4	-
			Sandstone (33.5 m–47.6 m)	-	7–30
DS-8	43.0	22.9	Loose sand (24.2 m–26.0 m)	4	-
			Moderately dense sand (26.0 m–29.3 m)	10–11	-
			Sandstone (29.3 m–43.0 m)	-	10–33
DS-9	38.7	18.7	Moderately dense sand (19.5.0 m–27.3 m)	9–20	-
			Sandstone (27.3 m–38.7 m)	-	8–87
DS-10	32.0	12.0	Loose sand (12.0 m–13.5 m)	1	-
			Moderately dense sand (13.5 m–18.3 m)	9–10	-
			Sandstone (18.3 m–32.0 m)	-	9–20

. In the drillings of DS-5, DS-6, DS-7, DS-8, and DS-9, a mud layer with a thickness varying between 0.8 m and 1.6 m was observed. A loose sand level lies beneath the mud layer and its thickness ranges from 1.30 m to 4.30 m, and its SPT-N values were determined as 3–7. Following the loose sand, the moderately dense sand layer is situated with a thickness change between 1.8 m and 8.2 m. Considering the SPT-N values, the loose and moderately dense sand layers can be classified as ZE-type (loose sand and V_s30 < 180 m/s) and ZD-type (moderately dense sand and V_s30 = 180–360 m/s) soils, respectively, according to Türkiye Building Earthquake Regulation [46]. The normalized SPT value of this layer ranged between 8 and 41 with an average of 18. The jointed sandstone layer begins right after the sand layer and all drillings have been finished in this unit.

In the study, a total of 26 grain-sized distribution analyses were performed. Some of the grain-sized distribution curves and liquefiable soil boundaries defined by Tsuchida [47] are presented in Figure 6. The samples mainly consist of fine sand-sized grains with a small amount of silt, clay, and gravel. The samples presented in Figure 6 exhibited fines that were plotted partially outside the range of liquefiable soils. However, soils with similar grain-sized distributions have been interpreted to have liquefied in the past [48,49]. Wang [50] described the liquefiable soils as having a fines content (<0.005 mm) lower than 15% and a liquid limit lower than 35%. In the study area, the fines content of the seabed sediments is smaller than 10% and they are non-plastic. Consequently, the seabed sediments in this study are identified as liquefiable soils. The liquefaction phenomenon is a serious problem for the FSRU terminal due to the project site being located in a seismically active area and sandy layers being prone to liquefaction. Therefore, the factor of safety against liquefaction (FS_L), the liquefaction potential index, and the liquefaction severity index were calculated for the project site in this study. In liquefaction analysis, the peak ground acceleration for the study site is designated as 1.05 g (for a 2475-year return period). The FS_L for the project site was determined following the method suggested by Youd et al. [51]. FS_L is the ratio of the resistance force of the soil against liquefaction (cyclic resistance ratio, CRR) and the cyclic stress caused by an earthquake (cyclic stress ratio, CSR). The magnitude scaling factor was assumed as 1 considering the magnitude of the expected earthquake in the study area. The SPT blow counts were normalized by employing energy ratio correction (E_r = 60%) [52], overburden correction factor (C_N) [53], borehole diameter, rod length, and sampler correction factors [51]. Liquefaction analyses by using Equations (1)–(3) show that the FS_L of the project site varies between 0.04 and 0.29. A conventionally liquefaction-resistant site is considered to be one for which FS_L ≥ 1.2 for a 475-year ground motion using the NCEER procedure [54]. When considering the National Center for Earthquake Engineering Research procedure [55] and FS_L values obtained, the study area is not a liquefaction-resistant site. The input parameters of liquefaction analysis and the obtained results are given in

Table 2. Liquefaction analysis input parameters and the results for some boreholes.

Borehole No	Depth (m)	SPT-N	Soil Type	g (kN/m3)	Fine Grain Content	Corrected SPT-N	Corrected SPT-N Value for Fines Content (N1,60f)	Safety Condition against Liquefaction (tr/teq > 1.10)		Liquefaction Potential
										Liquefaction Potential Index		Liquefaction Severity Index
DS-2	0.4	2	SM	18	0	3.1	3.1	0.04	liq. expected.	10.93		11.38
	1.9	4	SM	18	2	6.1	6.1	0.06	liq. expected.	13.29		14.14
	3.4	12	SP-SM	18	2	20.8	20.8	0.15	liq. expected.	11.06		13.01
	4.9	16	SW-SM	18	1	28.2	28.2	0.24	liq. expected.	9.03		11.86
	6.4	15	SW-SM	18	1	23.1	23.1	0.17	liq. expected.	9.23		11.12
										LI = 54	Very High	Ls = 62	Moderate
DS-4	0.5	4	SW	18	0	6.1	6.1	0.05	liq. expected.	10.32		10.86
	1.7	4	SW	18	7	6.1	6.1	0.06	liq. expected.	11.99		12.76
	3.2	8	SW	18	0	13.9	13.9	0.10	liq. expected.	11.85		13.16
	4.7	12	SM	18	1	21.6	21.6	0.15	liq. expected.	10.23		12.03
	6.2	19	SM	18	1	29.7	29.7	0.29	liq. expected.	7.75		10.86
	7.7	21	SM	18	1	29.5	29.5	0.29	liq. expected.	6.72		9.42
	9.1	R	SM	18	1	_	_	_	_	_		_
										LI = 59	Very High	Ls = 69	High
DS-6	1.9	6	ML	18	59	9.2	9.2	0.11	liq. expected.	21.62		24.29
	3.2	5	ML	18	_	8.7	8.7	0.07	liq. expected.	11.36		12.21
	4.7	9	ML	18	53	16.2	24.4	0.18	liq. expected.	8.23		10.03
										LI = 41	Very High	Ls = 47	Moderate
DS-10	0.7	1	SM	18	_	1.5	1.5	0.04	liq. expected.	13.68		14.25
	2.2	10	SM	18	13	15.3	17.9	0.12	liq. expected.	12.24		13.91
	3.7	9	SM-SC	18	_	15.6	15.6	0.11	liq. expected.	11.38		12.79
	5.2	9	SM-SC	18	24	15.4	21.3	0.15	liq. expected.	12.23		14.38
										LI = 50	Very High	Ls = 55	Moderate

for some boreholes.

$$F{S}_L=\frac{CR{R}_L}{CSR}\ast MSF\ast K_{\sigma }\ast K_{\alpha },$$

(1)

$$CR{R}_{7.5}=\frac{1}{34-{\left(N_1\right)}_{60}}+\frac{{\left(N_1\right)}_{60}}{135}+\frac{50}{{\left[10{\left(N_1\right)}_{60}+45\right]}^2}-\frac{1}{200}$$

(2)

$$CSR=\left(\frac{{\tau }_{av}}{{\sigma }_v^{\prime }}\right)\ast d=0.65\ast \frac{a_{max}}{g}\ast \frac{{\sigma }_v}{{\sigma }_v^{\prime }}\ast r_d$$

(3)

In Equations (1)–(3), MSF: magnitude scaling factor, K_σ: a correction factor for the level of vertical effective confining stress, K_α: a correction factor for the level of static horizontal shear stress, (N₁)₆₀: corrected SPT-N value, a_max: horizontal peak ground acceleration, g: gravitational acceleration, σ_v: total vertical normal stress, σ_v′: effective vertical stress, r_d: stress reduction coefficient which is defined as

$$r_d=1.0-0.00765z if z\le 9.15 \mathrm{m}$$

(4a)

$$r_d=1.174-0.0267z if 9.15 \mathrm{m}<z\le 23 \mathrm{m}$$

(4b)

The resilience of soil against liquefaction can be evaluated using the factor of the safety method. However, structural damage caused by liquefaction depends on the intensity of the liquefaction. The liquefaction potential index (L_I), proposed by Iwasaki et al. [56], allows microzonation in terms of liquefaction potential and includes the concept of the factor of safety. The L_I values for the project area are determined by Equation (5)

$$L_I=\underset{0}{\overset{20}{{\displaystyle \int }}}F\left(z\right)w\left(z\right)dz$$

(5)

In Equation (5), F(z) is equal to 1 − FS_L if FS_L < 1, and F(z) is equal to 0 if FS_L ≥ 1. w(z) is 10–0.5 z for z < 20 m and w(z) is 0 for z > 20 m where z is the vertical depth of the midpoint of the soil layer. Iwasaki et al. [56] defined four categories of liquefaction potential: very low, low, high, and very high. The L_I values of some boreholes in the study area are given in Table 2. Accordingly, the liquefaction potential of the project site ranges from 14 to 55 and is categorized as very high.

Due to the limitations of the L_I method, this method was revised by Sönmez and Gokceoglu [57] as the liquefaction severity index (L_s). Equation (6) is used to determine the L_s. The L_s of sand layer on the project site can be classified as moderate to high, with values ranging from 20 to 73.

$$L_s={{\displaystyle \int }}_0^{20}{P}_L\left(z\right)w\left(z\right)dz,$$

(6)

$$P_L\left(z\right)=\frac{1}{1+{\left(\raisebox{1ex}{$F_L$}\!\left/ \!\raisebox{-1ex}{$0.96$}\right.\right)}^{4.5}} for F_L\le 1.411$$

(7a)

$$P_L\left(z\right)=0 for F_L>1.411$$

(7b)

4. Design of Jet Columns

As described in the previous section, the foundation soils of the infills to be constructed in the offshore environment are highly susceptible to liquefaction. For this reason, ground improvement is mandatory. In ground improvement practice, jet grout, dynamic compaction, and deep-soil mixing, stone column methods are used to eliminate liquefaction problems. The improvement works must be applied on the offshore platform. Among the methods mentioned, jet grouting is the most suitable method due to operational reasons. The design of the jet columns was carried out by a Unitek–Vemak Joint Venture [58]. The lowest uniaxial compressive strength and modulus of elasticity values were accepted as 3 MPa and 920 MPa, respectively. The planned jet ground pattern is 2 m × 2 m, while the diameter of those is 80 cm. Considering these design parameters, the ground conditions of the improved soil were calculated by employing the average of weighted areas as shown in Figure 7. Accordingly, the improved and natural shear strength parameters of loose sand and moderately dense sand layers are given in

Table 3. The average shear strength parameters for loose sand and moderately dense sand layers in study site.

	Unimproved			Improved
	c (kN/m2)	Φ (°)	E (kN/m2)	c (kN/m2)	ϕ (°)	E (kN/m2)
Loose sand layer	0	25	4750	38	26	41,852
Moderately dense sand layer	0	30	8250	38	31	44,912

.

Özsoy and Durgunoğlu [59] proposed a method for the seismically safe design of jet grout columns. In this method, the cyclic stress ratio (CSR) is reduced by a CSR reduction factor (SR) that is determined by using Equations (8) and (9). The SR is calculated as 0.0396 for the study site.

$$CS{R}_{design}=CSR\ast S_R$$

(8)

$$S_R=\frac{{\tau }_s}{\tau }=\frac{{\tau }_s}{{\tau }_{avg}}=\frac{{\tau }_s}{\left(1+\left(G_r-1\right)a_r\right)}=\frac{1}{G_r}\frac{1}{\left(a_r+\frac{1}{G_r}\left(1-a_r\right)\right)}$$

(9)

$$G_r=\frac{G_{sj}}{G_s}$$

(10)

$$a_r=\frac{A_{jet}}{A_{imp}}$$

(11)

where

• G_r: the shear modulus ratio,
• a_r: area replacement ratio,
• G_sj: shear modulus of jet-column,
• G_s: shear modulus of soil

The lowest factor of safety against liquefaction was obtained as 1.01 from the Equation (8), and hence the necessary ground improvement was provided. After this stage, a total of 5 trial columns were constructed. Trial columns were constructed at different withdrawal speeds, injection pressures, and different locations, with a water/cement mixture of 1. There are two nozzles with a diameter of 2.2 mm in the jet grout machine, and the trial works were carried out with a rotation speed of 15–17 rpm. Details of the trial columns are given in

Table 4. General information on the trial jet columns.

Jet No.	Length of the Jet-Column (m)	Depth of the Water (m)	Injection Pressure (bar)	Injection Flow Rate (L/min)	Withdrawal Speed (cm/min)	Amount of Mixture (kg)	Amount of Cement(kg)	The Length of Core Samples (m)
TC-19	10.3	7.20	430	54	35	5000	2500	1.5
TC-26	10.1	7.40	440	54	40	4667	2334	10.1
TC-35	10.3	7.40	420	53	40	4400	2200	10.3
TC-37	10.3	7.30	440	54	40	4450	2225	4.5
TC-38	10.4	7.30	420	53	35	4600	2300	10.4

. In addition, Figure 8 shows the borehole works on the trial columns and cores obtained from the trial boreholes. As can be seen from Table 4, cores varying between 1.5 m and 10.10 m were obtained. When checking the jet grout performance, the total core recovery and the uniaxial compressive strength were considered. The total core recovery values were obtained as 85–100% from the trial columns. Therefore, the core quality is sufficient (Figure 8), and the performance of the trial columns is successful. In addition, core samples were obtained for laboratory experiments.

Unit weight tests were carried out according to ISRM [60], sonic velocity testing according to ASTM C597 [61], and uniaxial compression testing according to ASTM C39 standards. The average unit weight values of the X64 and X71 series specimens were 16.94 kN/m³ and 18.04 kN/m³, respectively. This indicates that the cement injected into the natural sand seriously densified the material. This change is seriously reflected in the P-wave velocities, which were 1997 m/s and 2208 m/s for the X64 and X71 series specimens, respectively. The uniaxial compressive strength is the most critical parameter in jet grouting. The values obtained were 2.71 MPa and 4.51 MPa for the X64 and X71 series specimens, respectively. These values are 3–4 times higher than the design value of 1 MPa. In order to control the obtained values, the relationships between unit weight, P-wave velocity, and uniaxial compressive strength were checked. Linear, exponential, power, and logarithmic regression functions were tried, and the function with the highest correlation coefficient was selected. The relationship between P-wave velocity and uniaxial compressive strength (UCS) is shown in Figure 9a. A linear and relatively high relationship was obtained between P-wave velocity and unit weight, as shown in Figure 9b. This is explained by the linear increase in the elastic wave velocity with increasing material density. Finally, the relationship between unit weight and uniaxial compressive strength was investigated (Figure 9c). As can be seen in Figure 9c, a linear relationship with a high-correlation coefficient was obtained between unit weight and uniaxial compressive strength.

When the results were examined, it was found that the X64 series specimens yielded relatively lower parameters than the X71 series specimens. When the jet grouting parameters were considered, almost the same amount of cement was used during the construction of both wells (Table 4). In addition, while the injection pressure was 420 bar in the construction of the column where X71 samples were obtained, the injection pressure was 440 bar in the construction of the column where X64 samples were obtained. However, one of the most important parameters affecting jet columns is soil properties. However, since the site is the seabed, the ground shows homogeneity in terms of geotechnical properties. Therefore, in this evaluation, it will not be an erroneous evaluation to accept the soil properties as constant. Hence, the most important parameter that draws attention here is the withdrawal speed. The 35 cm/min withdrawal speed provided a much higher performance than the 40 cm/min withdrawal speed. However, with both 35 cm/min withdrawal speed and 40 cm/min withdrawal speeds, values well above the desired values were obtained. As a result, it is understood that at least a 420 bar injection pressure, 40 cm/min withdrawal speed, and a 1:1 cement/water ratio meet the necessary conditions in terms of project design.

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:

(a)

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.

(b)

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.

(c)

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 Özturan and Kayahan [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.

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.

Finite element analyses were performed to evaluate the performance of the jet grouting application in the study site for static and pseudo-static conditions. To account for the effects of the earthquake-induced loads on a geotechnical structure, one of the preferred methods is pseudo-static analysis due to its ease of application. In the context of pseudo-static analysis, the dynamic loads are applied as equivalent inertial forces by converting the peak ground acceleration to constants of body forces, and these forces are applied to the whole mesh. The Mohr–Coulomb model was used in numerical modelling by employing the PLAXIS 2D software (https://www.bentley.com/software/plaxis-2d/). The analyses were carried out along the most critical cross section, XX′, shown in Figure 5. A total of 11,008 triangular elements and 88,661 nodes were generated in the mesh configuration of the numerical models (Figure 16). The soil strength parameters given in Table 3 were employed in the analyses. The seismic load was applied in accordance with the PGA of the study site calculated for the 2475-year return period. The results of the finite element analyses are shown in Figure 17 and Figure 18. According to the results, the maximum vertical displacement calculated for the unimproved loose sand layer and moderately dense sand layer was 28 cm and 18 cm, respectively. However, under pseudo-static loading conditions, the maximum vertical displacement for the unimproved loose sand layer and moderately dense sand layer was calculated as 31 cm and 26 cm, respectively. As can be seen from Figure 18, the maximum vertical displacement values significantly decrease for the improved soil with jet grouting columns. The largest vertical displacements were determined as 9 cm and 8 cm, respectively, for the improved loose sand and moderately dense sand layer under the static condition. On the other hand, the largest vertical deformations were calculated as 14 cm and 11 cm for pseudo-static loading condition. Finite element analysis results evidently prove the positive effects of soil improvement. Additionally, it is important to state that the calculated vertical displacement values are within acceptable limits.

6. Conclusions

In general, the offshore structures constructed or to be constructed in active seismic regions are susceptible to liquefaction. During the 6 February 2023 Türkiye Earthquakes, the problem of liquefaction was encountered in the Iskenderun Port. For this reason, it is important to investigate the ground failures sourced from earthquakes in offshore structures to be built in seismically active regions. In this study, a case study is presented that is a typical example for these structures. The main conclusions obtained from the study can be highlighted as follows:

(a)

For geotechnical site characterization, a total of 10 offshore geotechnical boreholes were drilled and detailed seismic reflection profiles were obtained. The detailed stratigraphic sections and their geotechnical parameters of the seabed sediments were obtained. The seabed stratigraphy and their geotechnical parameters are generally homogenous. This feature of the project site decreases the geotechnical uncertainty.

(b)

The seabed sediments in the project area are susceptible to liquefaction. As a result of liquefaction analysis, the factor of safety was calculated as 0.04–0.29 for natural conditions. For ground improvement methods, the jet grout column construction was considered. The targeted uniaxial compressive strength of the jet grout columns was 3 MPa, and the pattern of the columns was designed as 2 m × 2 m. When considering this design, the minimum factor of safety was obtained as 1.01.

(c)

In the first stage of the jet grout construction works, various trial columns were constructed, and the optimum jet parameters were determined. Injection pressure, withdrawal speed, and amount of cement were determined as 420 bar, 40 cm/min, and 2200 kg, respectively. The minimum and maximum uniaxial compressive strength values of the core specimens obtained from the jet columns were found to be 2.71 MPa–4.51 MPa. As a result, the improved parameters considered in the design were achieved.

(d)

Determination of jet grout dimensions constructed in a terrestrial environment is relatively easy when compared to jet grout dimensions constructed in an offshore environment. For this reason, new applicable approaches for this purpose are necessary. Hence, in the study, a new procedure for the determination of jet grout dimensions constructed in an offshore environment is introduced.

(e)

The methodology is based on the shallow seismic reflection method. The whole area is measured by seismic method while the site is in virgin conditions. The measurement results are assessed together with the data obtained from the offshore boreholes. After this stage, jet grout columns are constructed, and seismic measurements are obtained again. The jet grout dimensions are determined by employing the shallow reflection method. The results are checked by boreholes and observed by the divers visually and it is determined that the results are satisfactory.

(f)

While the quality control of jet grouts can be carried out on a certain number of columns with conventional methods, the dimensions of all columns can be controlled with the method proposed in this study. In addition, the method is non-destructive on the columns, it is easier, and faster to apply.

(g)

The most important limitation of the method proposed herein is that the strength parameters of the jet columns cannot be obtained directly. For this purpose, core samples must be obtained from test columns and a limited number of control drillings.

(h)

The methodology is introduced by employing one project. Depending on the development of technology and increase in the number of projects, the method can be advanced, and the reliability of the method can be increased.

(i)

In a numerical analysis, the larger vertical deformation values are determined for unimproved ground, while the vertical deformation values calculated for mitigated ground are significantly reduced. The finite element analysis results obviously show the importance of soil improvement against liquefaction.