A. INTRODUCTION
An apartment building at Uppsala, Sweden constructed on marine and lacustrine clay has been studied in [1-2]. The apartment has four-stories with a basement and the plan area is approximately 532m2. The foundation consists of a raft supported by 48 timber-concrete composite piles. The raft has a dimension of 38m x 14m in plan and a thickness of approximately 0.4m. The composite piles consist of a 0.3m diameter concrete section for the top 7m spliced with an 18m long timber pile. The diameter of the timber piles is assumed to be 0.3m at the point of the splice and then to reduce with depth. The pile layout of the foundation is illustrated in figure 1. The foundation is subjected to a total vertical load of 30.4MN.
The foundation was instrumented with different types of devices including 12 pile load cells, 10 contact pressure cells, 3 bellows-hose settlement gauges and pore pressure gauges at 5m intervals up to a depth of 35m. Figure 10 shows the locations of instruments for the entire foundation.
The geological profile consists of soft marine clay with a thickness of up to 15m underlain by lacustrine clay with a maximum thickness of 15m. The thickness of each layer varies within the footprint of the building. Field tests including oedometer tests and vane tests have been carried out to determine the geotechnical parameters for the clay layers as illustrated in figure 3. The undrained shear strength is about 30kPa for both clay layers. Based on the oedometer results. The creep strength of the clay was determined by vane tests and results indicated that the undrained shear strength of the top clay layer was reduced by about 25%.
B. MODEL OF PILE-RAFT FOUNDATION
The DeepFND Finite element method is used for the simulation of the raft foundation. The concrete pile cap is simulated with shell finite elements with thickness equal to 0.4m. The piles are modelled as Euler Bernoulli beams with embedded pile springs as interface elements. Both the pile cap and pile concrete is simulated as a linear elastic constitutive law with young modulus equal to Ec=29000Mpa and Poisson ration v=0.2. For the timber part of the composite pile a young modulus equal to E=5000Mpa was adopted.
A soil profile of two different layers with constant stiffness and strength properties is adopted in the analysis, based on the soil test results presented in the previous paragraph. In reality, stiffness properties vary along the depth of every layer as summarized in [2] by the equation of Es(Mpa) = 2 + 0.2z where z is the depth from the surface. As a result, an additional sub layer was included in the soft marine clay layer with the only purpose of varying the stiffness of the layer along the depth. The strength and stiffness properties of the soil layers are illustrated in table I.
The constructed model in DeepFND is illustrated in figure 4a. The mesh discretization generated by the DeepFND software is illustrated in figure 4b.
C. ANALYSIS RESULTS
The analysis results for the piled raft foundation are illustrated in the figures bellow. The deformed shape and contour of displacement of the FEM model is depicted in figure 5 and a comparison of the simulated settlement contour and measured settlement contour are depicted in figure 6. Figure 7 presents the moment diagrams and pile cap settlements on the 2D cap model.
D. CONCLUSIONS
The Finite element analysis method implemented in DeepFND is utilized to capture the settlement of a residential building in Uppsala, Sweden. The finite element method is capable of accurately capturing the flexibility of the slab and piles of the piled raft foundation, the slab and pile to soil interface and nonlinear soil behavior near the foundation.
E. REFERENCES
[1] Jendeby, L. (1986). Friction Piled Foundations in Soft Clay – A Study of Load Transfer and Settlements, Chalmers University of Technology. Goteborg, Sweden, Vasastadens Bokbinderi AB, Gothenburg, Sweden, pp. 6.1 – 6.20.
[2] S. W. Chow and J. C. Small (2006), case studies for piled raft on clay
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