3D FE Analysis of Effect of Ground Subsidence and Piled Spacing on Ultimate Bearing Capacity of Piled Raft and Axial Force of Piles in Piled Raft ()
1. Introduction
Piled raft foundations are widely used for civil structures such as in [1-4]. The use of piled rafts in settling soils has been faced with many problems such as the changing in bearing capacity and variation of load sharing between piles and raft, total and differential settlements may be affected.
Some research papers relating to ground subsidence have been published in the literature. Among them, [5-7] pointed out some effects of ground subsidence on bearing capacity and differential settlement of foundation. They also emphasized some cities in which the ground subsidence have occurred with high rate (e.g. Bangkok, Ho Chi Minh, Shanghai, Mexico, etc.). The reason for the ground subsidence comes from the pumping of ground water for water supply.
The objective of this study focuses on investigating that whether the ultimate bearing capacity of piled rafts and the axial forces of piles in piled rafts change or not under the effects of ground subsidence and piled spacing. The research was conducted by using 3D FE analysis with Plaxis 3D Foundation Version 2.0.
In analysis, ground subsidence caused by ground water pumping was simulated by drained condition and normal condition was simulated by undrained condition. Vertical distribution load was applied to the surface of the raft. The effects of ground subsidence and piled spacing were figured and some discussions were given in this study.
2. FE Modelling of Piled Rafts
2.1. Geometry of Foundation
A 2.8 × 2.8 × 0.75 m piled raft with 4 piles was considered in this study. To reduce the calculation time, only one-quarter of the foundation was modeled, using symmetry boundary conditions. To enable any possible mechanism in soft clay and to avoid any influence of the outer boundary, the model was extended in both horizontal directions to a total width of 10 m. Figure 1 shows the geometry of foundation and soil in FE analysis.
2.2. Properties of Soil and Foundation
Only one layer of soft clay was simulated in this analysis. The calculation was done by plastic analysis with effective parameters. Undrained and drained conditions were considered in the analysis. Table 1 shows the properties of soft clay. Young modulus of soft clay was increased
Figure 1. Geometry of foundation and soil in FE analysis (unit: m).
Table 1. Properties of soft clay for FE analysis.
with depth, starting at yref = −0.25 m, and the increment was 300 kN/m2. The properties of piles and raft are presented in Tables 2 and 3 respectively.
2.3. Applied Load
The effect of seflweight of foundation was ignored by assigning the unit weight of raft and piles equal to zero. The foundations was firstly applied a load of 1500 kPa in order to estimate the ultimate bearing capacity.
Based on the result in Table 4, a load of 96 kPa calculated from ultimate bearing capacity of case 1 (for undrained condition) with FS of 2 was applied to all cases to investigate the behavior of the foundation in ground subsidence condition under working load. Figure 2 presents the loading type which was applied in the analysis for both case 1 and case 2. The time considered in the analysis was 18,250 days (or 50 years).
2.4. Details of Simulation
A borehole was used to assign information of soil layer
Table 2. Properties of piles (embedded pile).
Table 3. Properties of raft (floor).
Table 4. Variation of ultimate bearing capacity.
Figure 2. Applied load for case 1 and case 2.
and location of water table. The properties of soil layers in Table 1 were inputted to material data set and the location of water table was defined at the ground surface (y = 0). The raft was simulated by floor element and the properties were given in Table 2. The piles were simulated by embedded piles and the properties in Table 3 were used.
Figure 3 shows the simulation of piled raft foundation in FE analysis. The global coarseness of the mesh in horizontal as well as vertical directions was set to fine. 2D finite element mesh was generated before generating a full 3D mesh. The 2D mesh generation process was based on a robust triangulation principle that searched for optimized triangle and which resulted in unstructured mesh. Large displacement gradients were expected around and under the raft and the piles. Hence, refine cluster was done for the raft and piles to have finer mesh. The 3D mesh composed of 15-node wedge elements was created by connecting the corners of the 2D triangular elements to the corresponding points of the corresponding elements in the next work plane. A total of 4472 elements and 13,044 nodes were created after generating the mesh.
The interfaces (Rinter) were taken as 0.6 for soft clay. Initial stresses were generated by using K0 Procedure in which the default value of K0 was based on Jaky’s formula. Construction stages was followed the type of load (Figure 2) and plastic was used for calculation type. Drained condition in material type was selected for the soil when simulating ground water pumping condition.
3. Results and Discussions
Figure 4 shows load-settlement curves of piled rafts in ground subsidence condition for case 1 (s = 2d) and case 2 (s = 4d). The ultimate bearing capacity of piled raft foundations changed under effect of ground subsidence and piled spacing (Figure 4(a)).