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Dieses Kapitel geht der Analyse des Einflusses tiefer weicher Bodenfundamente auf die seitliche Verdichtung von Brückenpfahlgründungen nach. Die Studie konzentriert sich auf die horizontale Verschiebung, das Biegemoment und die Scherkraft von Pfahlgründungen unter unterschiedlichen Belastungsbedingungen. Durch numerische Analysen mit der 3D-Software PLAXIS werden die Auswirkungen des Starts der Pfahlgründung nach der Fertigstellung der ersten und zweiten Pfahlkonsolidierungsschicht verglichen. Die Ergebnisse zeigen, dass ein Baubeginn nach der zweiten Schicht zu deutlich geringeren maximalen Biegemomenten, Scherkräften und horizontalen Verschiebungen führt. Die Studie hebt auch die Verwendung von Bambuspfählen und Flößen als flexible Straßenbettkonstruktion hervor, die zur Stabilität des weichen Bodenfundaments beiträgt. Die Schlussfolgerung betont die Wichtigkeit der Verifizierung des numerischen Modells mit Vor-Ort-Tests als zukünftige Referenz in ähnlichen Projekten.
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Abstract
In order to prevent seawater from flowing back into cities, road construction in Indonesia's coastal areas often requires the construction of breakwaters on the outside of bridges. The area is widely distributed on deep soft foundations, and the foundation soil at the breakwater is reinforced using the preloading method. The road is built on friction prefabricated pipe piles as the bridge foundation. Using PLAXIS 3D to analyze the beneficial effects of bamboo piles and bamboo rafts as working cushion layers on the filling of deep soft soil foundation and the settlement and deformation characteristics of the foundation, and simulating the adverse effects of lateral compression on the bridge pile foundation caused by the soft soil foundation under different loading conditions of the sea embankment. The results show that the flexible working cushion layer of bamboo piles and bamboo rafts is conducive to improving the loading stability of the soft soil foundation, when the second layer of pile is consolidated and stabilized before carrying out bridge pile foundation construction, the lateral compression generated by the soil will be smaller.
1 Introduction
When constructing bridges on deep soft soil foundations, engineering problems caused by geological reasons are not uncommon. Scholars at home and abroad have proposed corresponding research methods based on lateral displacement of soil. According to Poulos [1] discusses the pile-soil interaction relationship within the elastic range, believing that soil pressure is directly proportional to lateral displacement of soil. The pile foundation is treated as an elastic foundation beam for solution, which is only suitable for situations with small horizontal deformation and not suitable for soft soil with high porosity. Springman [2] used model experiments to simulate the stress shape of adjacent pile foundations under lateral pile loading, and based on this, proposed a calculation method for bridge abutment pile foundations under passive diagram action using a triangular assumed soil displacement mode on the pile side. Li et al. [3] studied the effect of soft soil drainage on the lateral soil pressure of passive piles, and obtained that in soft soil foundation pile loading construction, the maximum bending moment of the pile body under slow loading will be 20–50% smaller than that under fast loading, and the lateral soil pressure of the pile will also be smaller. Carrying out pre-loading construction first and then carrying out pile driving operations is beneficial for controlling the displacement of the pile body. Simultaneously controlling the loading rate will also reduce the force on the pile foundation. Huang [4] used three-dimensional finite element method to simulate engineering examples, aiming to improve engineering design through numerical analysis methods. The results have good reference significance for actual construction, but the analysis results are still far from previous research conclusions, and cannot effectively solve on-site problems. Hou [5] used a combination of theoretical calculations, numerical simulations, and on-site monitoring to analyze the maximum horizontal displacement and maximum pile body cracks of pile foundations under original differential loading and optimized differential loading conditions. Jiang et al. [6] used finite element numerical calculation methods to simulate construction and analyze the effects of additional loads on the negative friction resistance, internal force of the pile body, and deformation of the pile foundation. Che [7] conducted numerical analysis on the adjacent area of the existing pile foundation for loading operations, resulting in significant long-term displacement of the pile foundation. Xu [8] calculated through numerical analysis and theoretical calculations that as the road load increases, the neutral point of pile foundation friction gradually moves downwards, and the negative friction and horizontal displacement of the pile foundation significantly increase.
At present, there are few reports on the risks and determination of the loading time for both pile foundation treatment and bridge pile foundation construction in coastal areas with deep soft foundations. Based on a certain highway in Indonesia, studying the lateral compression of bridge pile foundations on deep soft foundations under different loading conditions of cofferdams has certain reference value for similar projects.
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2 Engineering Background
The design scheme of a certain highway project in Indonesia intends to adopt a “bridge embankment separation” scheme, which is a road formed by a sea side water retaining seawall and a landside bridge; Based on the on-site water depth situation, some bridge sections are planned to adopt the “island building and road pile foundation construction” process, with a standard section width of 45.1 m for island building and road backfilling; The seawall adopts the construction technology of bamboo rafts, bamboo piles, and drainage boards for preloading. The total height of the stacking is 9.5–10.0 m, the spacing between drainage boards is 1 m, and the depth of drainage board installation is 23.0–33.0 m. Bamboo stakes are made up of seven bamboo sticks with a length of 8 m tied together into a bundle as one bamboo stake. The bamboo row consists of three bamboo pieces grouped together, with a spacing of 1 m. The upper and lower layers of bamboo are crisscrossed and tied together, totaling nine layers of bamboo; the distance between the seawall and the bridge island is 35–42 m; The top elevation of the backfill for the access road and island construction is 1.6 m. A typical cross-sectional view of the design scheme is shown in Fig. 1.
Fig. 1
Typical cross-sectional view of design scheme (the image is sourced from the design drawings and can be publicly used)
Using large-scale finite element PLAXIS 3D [9, 10] to conduct computational analysis on the stability of seawall loading and the impact of seawall loading construction on the safety of bridge pile foundations. The soft soil foundation adopts the soil hardening HS model, which is based on the hyperbolic relationship between deviatoric stress and axial strain in triaxial drainage tests. Bamboo piles and bamboo rafters are rigidly connected, with bamboo piles equivalent to pile units and bamboo rafters equivalent to board units.
The model calculates a horizontal (vertical to the axis of the seawall) length of 250 m in the area. The foundation treatment form of the cofferdam is preloading: using 9-layer bamboo rows + 8 m long bamboo piles (16 rows on each side, with a spacing of 1.0 m), plastic drainage boards are installed at a depth of 33.0 m, with a spacing of 1.0 m. The seawall is filled in three levels, with the first layer being 3.0 m high, the second layer being 2.0 m high, and the third layer being 3.7 m high.
The total length of the bridge pile foundation is about 62.6 mm, of which the pile length above the surface is 8 m, and the pile length below the surface is 54.6 mm, with a pile diameter of 0.8 m. The spacing between the seawall and the nearby seawall side pile foundation is 35 m. The calculation model is shown in Fig. 2.
Fig. 2
Layout of test area (the image is sourced from our company's genuine PLAXIS 3D software and can be publicly released)
The parameters of the finite element model are obtained from the survey report, and some parameters are obtained from the inversion of measured settlement in the field test section. According to literature research [11], The calculation parameters of bamboo piles and bamboo rafts are shown in Table 1, the design requirements of pipe piles are shown in Table 2, and the soil properties of the model foundation are shown in Table 3.
Table 1
Bamboo pile and bamboo raft parameters
Material name
Stiffness (MPa)
Bamboo raft
1810
Bamboo pile
240
Table 2
Pipe pile parameters
Material name
Stiffness (GPa)
Diameter (m)
Wall thickness (m)
Severe (kN/m3)
Pipe pile
33.8
0.8
0.12
10
Table 3
Foundation soil properties parameters
Depth (m)
Soil name
Natural bulk density (kN/m3)
Modulus (MPa)
Stiffness stress power exponent
C (kPa)
φ (°)
Permeability coefficient Kx = Ky (m/d)
Permeability coefficient Kv (m/d)
0.0–5.0
Oc very soft clay
15.5
0.6
0.5
13
19
6.15 × 10–6
3.07 × 10–6
5.0–12.5
Very soft clay
15.5
0.7
0.5
15
19
6.15 × 10–6
3.07 × 10–6
12.5–27.5
Very soft to medium stiff clay
16.2
1.9
0.5
15
19
6.15 × 10–6
3.07 × 10–6
27.5–53
Very soft to medium stiff clay
15.8
3.8
0.5
15
19
6.15 × 10–6
3.07 × 10–6
53–60
Stiff to very stiff clay
17.5
15.2
0.5
15
25
4.75 × 10–3
4.75 × 10–3
60–80
Very stiff to hard clay
18.0
21.9
0.5
15
30
4.75 × 10–3
4.75 × 10–3
4 Numerical Analysis Result
Due to time constraints, the construction of bridge pile foundations needs to be carried out simultaneously during the embankment filling process. During the process of seawall filling, the soil will undergo vertical and lateral deformation, which may cause deformation or instability of the bridge pile foundation due to lateral soil compression. The determination of the optimal construction time for bridge pile foundation is a key factor in measuring the safety and progress of this project. According to the characteristics of soil consolidation deformation, the impact of layered embankment filling on bridge pile foundations is analyzed in two working conditions:
Working condition 1: After the first layer of the seawall is filled and consolidated, the construction of the bridge pile foundation begins.
Working condition 2: After the second layer of the seawall is filled and consolidated, the construction of the bridge pile foundation begins.
4.1 Total Settlement of Seawall
The consolidation compression caused by the filling of seawall on deep soft foundation is only related to the height of the pile, the thickness of the soft soil layer, and the depth of the drainage plate installation. The construction process of bridge pile foundation will not affect the final settlement of the seawall foundation soil.
According to the calculation results, the final total settlement of the seawall is 2.85 m. The specific settlement calculation cloud map is shown in Fig. 3.
Fig. 3
Cloud map of total settlement calculation for seawall (the image is sourced from our company's genuine PLAXIS 3D software and can be publicly released)
4.2 Horizontal Displacement of the Pile Foundation Closest to the Seawall
The filling of seawall will cause lateral deformation of the soft soil on both sides of the seawall, and the lateral displacement of the soil near the pile will compress the pile foundation and cause horizontal displacement. Different loading processes have different effects on the lateral compression of the pile foundation. Through numerical analysis, the maximum horizontal displacement of the pile foundation closest to the seawall in working conditions one and two is 116 mm and 53 mm, respectively, in the direction away from the seawall side. The horizontal calculation cloud maps of the pile foundation are shown in Figs. 4 and 5.
Fig. 4
Condition two: Horizontal displacement of pile top
4.3 The Bending Moment Value of the Pile Body Closest to the Seawall
The filling of seawall will cause lateral deformation of the soft soil on both sides of the seawall, and the lateral displacement of the soil near the pile will compress the pile foundation and cause bending deformation. Different loading processes will cause different lateral compression of the pile foundation, and also result in different internal forces of the pile body. Through numerical analysis, the maximum bending moment values of the pile foundation closest to the seawall in working conditions one and two were calculated to be 220.2 kN·m and 88.6 kN·m, respectively. The maximum bending moment occurred below the mud surface. The calculation cloud maps of the pile bending moment are shown in Figs. 6 and 7.
4.4 The Shear Force Value of the Pile Closest to the Seawall
The filling of seawall will cause lateral deformation of the soft soil on both sides of the seawall, and the lateral displacement of the soil near the pile will compress the pile foundation, generating a certain shear force on the pile body. Different loading processes will cause different lateral compression of the pile foundation, and also cause different internal forces in the pile body. Through numerical analysis, the maximum shear force values of the pile foundation closest to the seawall in working conditions one and two were calculated to be 101.7 and 40.1 kN, respectively. The maximum shear force occurred at the lower part of the pile body, and the shear force calculation cloud maps of the pile body are shown in Figs. 8 and 9.
For the first time, a flexible roadbed structure with bamboo piles and bamboo rafts has been adopted on a highway in Indonesia, which is conducive to the rapid formation of construction work cushion layer. At the same time, the overall effect of bamboo piles and bamboo rafts is conducive to the stability of soft soil foundation under later loading. By analyzing the adjacent pile foundation construction at different loading stages, the horizontal compression effect generated by the loading process on the pile foundation causes horizontal displacement, bending deformation of the pile body, and shear force of the pile body. After the completion of the second layer of pile consolidation, the construction of the bridge pile foundation begins. The maximum bending moment and maximum shear force experienced by the pile body are 40% of the starting pile foundation construction after the completion of the first layer of pile consolidation. The maximum horizontal displacement of the pile top after the completion of the second layer of pile consolidation is 50% of the starting bridge pile foundation construction after the completion of the first layer of pile consolidation. There is a lack of comparison between on-site measured data and numerical analysis results. In the future, on-site tests should be carried out to verify the accuracy of the numerical model, so as to have a certain reference value in similar projects in the future.
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