2.1. Site Condition
The reclamation project on Segitiga Island, covering an area of 40 hectares, is located in district Manyar, Gresik, East Java.
Figure 2 shows the location of the reclamation and soil investigation. The soil data investigation involves a total of six offshore borings with a depth of 60 m, utilizing standard penetration test (SPT) procedures. Three borehole points (BH-3 to BH-5) are located in the reclamation area, two points (BH-1 and BH-2) are located in the planned dock area, and one point (BH-6) is located in the designed bridge connecting area.
The pipe protection area is located in the BH-5 to BH-6 area, where the construction of a causeway is planned. Based on this, the soil data reference for pipe protection design utilizes the soil data analysis from BH-5 and BH-6.
Figure 3 shows a comparison of the N-SPT values in the BH-5 and BH-6 areas. According to the SPT test results, the data from BH-6 are used in the design planning, considering the most critical outcome. The soil profile at BH-6 consists of a 15-m-thick layer of very soft to soft clay after a 1-m-thick layer of very loose sand. Further down to a depth of 60 m, the soil is composed of sand with a medium-to dense consistency.
2.2. Soil Properties
Soil stratigraphy based on BH-6 data is divided into (1) 1-m-thick very loose sand; (2) 15-m-thick very soft to soft clay; and (3) medium to dense sand down to a depth of 60 m. The soil data parameters are determined using correlations of corrected N-SPT values. The correction for N-SPT values is based on the equations by [
22,
23]. The correction for the N-SPT value due to field procedure is calculated based on Equation (1) from [
22].
Correction of the N-SPT value due to overburden stress based on Equations (2) and (3) from [
23] was calculated as follows
The correction N-SPT value due to the ground water table based on Equation (4) from [
24] the calculation is as follows:
After obtaining the corrected N-SPT values, the correlation of soil parameters is then carried out based on the corrected N-SPT values. The first correlation is performed to determine the physical and mechanical parameters of the soil. Correlation for coarse-grained (cohesionless soil) and fine-graded soil (cohesive soil) is conducted using the [
25] correlation table, as shown in
Table 2.
Next, a correlation assessment is carried out for the Young’s modulus values based on the N-SPT values using the correlation table [
25], as seen in
Table 3.
2.3. Soil Model Parameter
To model the subsoil profile realistically, one must consider the non-linearity of the soft soil layer. There are various models that have been developed to achieve a realistic model for field conditions, including the hardening soil model (HSM), soft soil model (SSM), modified cam clay model (MCC), etc.
In this study, the sand layer soil model uses the hardening soil model (HSM). The hyperbolic elastoplastic model (HSM) takes into account the nonlinearity of the soil by showing how stiffness changes with stress but it does not look at viscous effects like creep and stress relaxation. HSM is based on the Mohr–Coulomb failure criterion and its yield surface can expand due to plastic strain. The hardening soil model uses plasticity theory, unlike the Mohr–Coulomb model, which uses an elasticity model. The hardening soil model considers both shear hardening and compression hardening of the soil. A detailed formulation of the hardening soil model is explained by [
26].
The hardening soil model (HSM) is introduced to characterize the mechanical behavior of soil under loading, unloading, and consolidation phases. The HSS model incorporates the small strain properties of materials, enabling a more precise prediction of soil deformation. Usually, the HSS model utilizes drained soil parameters such as unit weight (γ), drained shear strength (
c′), effective internal angel friction (
φ′), and lateral limit compression (
ES) [
27]. The hardening soil model can be used for modeling different types of soil, including granular and soft soil. Power
m represents the stress dependency of soil stiffness, where for soft soil,
m = 1. The hardening soil model needs three stiffness parameters at a chosen reference pressure (
Pref). There are a (1) triaxial modulus/secant modulus (
E50ref), (2) oedometric modulus (
Eoedref), and (3) unloading–reloading modulus (
Eurref). A series of laboratory tests determine this parameter [
28]. However, some scholars have used statistical and inverse analysis to determine the stiffness parameter for this model [
29,
30]. This is conducted considering the time and cost if only using a laboratory test.
Furthermore, to model the clayey soil in this study, a soft soil model is employed. Some scholars have used the soft soil model (SSM) to analyze the behavior of soft soil [
31]. Soft soil such as very soft clay, normally consolidated clay, silt, and peat, is modeled using the soft soil model (SSM) in finite element modeling (FEM). Unlike Mohr–Coulomb, which assumes linear behavior and constant stiffness of soil throughout soil depth, soft soil developed a nonlinear behavior of soil. Soft soil model considers soil nonlinearity via a linear stress dependency of stiffness. There are two key model inputs that define the compressibility of the soil in isotropic loading, unloading, and subsequent reloading. The first is the modified compression index lambda (
λ) for modeling the compression of soil and the second is the modified swelling index kappa (
κ). This parameter can be calculated automatically in the software using the compression index (
CC) and swelling index (
CS). The calculation formula for
λ and
κ is given by [
32] as
The yield failure criterion of the Mohr–Coulomb theory serves as a basis for the soft soil model yield function, except that the yield function of SSM defines an ellipse with PP, isotropic pre-consolidation pressure defines the length of the ellipse, and M is the height of the ellipse in the p′ plane. M also represents the ratio of horizontal to vertical stress under primary loading and is used to determine the earth pressure at rest (Konc).
Input parameters for the soil modeling in Plaxis 2D Version 21.01.00.479 (build 479), based on the requirements from PLAXIS Version 21 [
33], are displayed in
Figure 4. Each soil layer has a color shown in
Figure 4. This color will be displayed in the Plaxis modeling schematic shown in
Figure 5.
2.5. Construction Sequence
The construction sequence of the embankment for the causeway reclamation area begins with the installation of bamboo pile reinforcement in a configuration of six bamboo piles with a depth of 6 m and a spacing of 1 m. Subsequently, the installation of the pipe barrier is carried out using bamboo piles in a configuration of three bamboo piles with a depth of 6 m, installed continuously along the intersecting area.
After the piles are installed, the next step is the installation of a four-layer bamboo mat adjusted to the bamboo pile installation. Subsequently, a non-woven geotextile is placed on top of the bamboo mat and the limestone embankment can be carried out.
The embankment filling is carried out step by step, allowing the soft soil layers, predominantly very soft to soft clay, to undergo consolidation over time and reduce excess pore water pressure. This will enhance the soil’s bearing capacity [
3,
32,
34]. Based on this, the construction stages will be divided into 12 embankment phases, with each phase implemented over a period of 10 days. The geometry and thickness of each layer will vary and will be checked for the stability of the executed embankment.
2.8. Pipe Protection
The embankment carried out above the pipeline may lead to potential damage to the pipe. Therefore, reinforcement is necessary to prevent the pipe from experiencing excessive axial and lateral loads. The reinforcement is planned to utilize a combination of U-ditch (2000 mm × 2800 mm), concrete footing slab, and corrugated steel plate (CSP), along with additional bamboo piles and bamboo mattresses. The specifications for the reinforcement are presented in
Table 5.
Figure 7 illustrates the reinforcement plan of corrugated steel plate and u-ditch and
Figure 8 illustrates bamboo mattresses reinforcement.
Corrugated steel plate (CSP) reinforcement has various advantages, including its fast construction process and relatively low cost [
36]. Additionally, CSP is lightweight and environmentally friendly [
37]. Typically, CSP reinforcement is used in bridges, highways, and underground passages [
38,
39]. Studies on the behavior of a steel shell during backfilling by [
40,
41,
42] have shown that the strain and displacement values on CSP are higher when subjected to static loading compared to dynamic loading [
40]. In this study, the design of CSP reinforcement is combined with a U-ditch and concrete footing slab to simplify the installation process and enhance the support bearing capacity of the pipeline reinforcement.
2.9. Constitutive Model of Bamboo
The bamboo mattress is composed of a bamboo stick arranged to form a mattress as shown in
Figure 7. The determination of the cross-sectional area and moment of inertia of the bamboo has been studied by [
15,
16]. The total cross sectional area of bamboo that builds the mattress is
. The
can be calculated by equation below [
16], as follows:
Sum of the moment inertia of bamboo is
. The
can be calculated as below [
16], as follows:
Bamboo piles are a bamboo reinforcement that forms the basis of pile foundation reinforcement. Irsyam in [
15] illustrates the concept of determining the spring constant of bamboo pile Cerucuk bambu, as shown in
Figure 9. The diameter of the bamboo is assumed to be uniform, which is 8 cm. The ultimate bearing capacity of bamboo pile on soft soil can be calculated using the equation below.
where
is average circumference of bamboo,
are the undrained cohesion of soil,
are the dept of each soil layer, and
is the sectional area of bamboo pile. The magnitude of soil deformation so that the value of does not move is assumed to be δ = 0.1 ×
d, with 𝑑 as the diameter of the bamboo. The magnitude of the spring constant (
k) can be calculated using the following equation: