National Academies Press: OpenBook

Design Guidelines for Increasing the Lateral Resistance of Highway-Bridge Pile Foundations by Improving Weak Soils (2011)

Chapter: Chapter 4 - Finite Element Modeling of Single Pile Load Test

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Page 51
Suggested Citation:"Chapter 4 - Finite Element Modeling of Single Pile Load Test." National Academies of Sciences, Engineering, and Medicine. 2011. Design Guidelines for Increasing the Lateral Resistance of Highway-Bridge Pile Foundations by Improving Weak Soils. Washington, DC: The National Academies Press. doi: 10.17226/14574.
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Page 51
Page 52
Suggested Citation:"Chapter 4 - Finite Element Modeling of Single Pile Load Test." National Academies of Sciences, Engineering, and Medicine. 2011. Design Guidelines for Increasing the Lateral Resistance of Highway-Bridge Pile Foundations by Improving Weak Soils. Washington, DC: The National Academies Press. doi: 10.17226/14574.
×
Page 52
Page 53
Suggested Citation:"Chapter 4 - Finite Element Modeling of Single Pile Load Test." National Academies of Sciences, Engineering, and Medicine. 2011. Design Guidelines for Increasing the Lateral Resistance of Highway-Bridge Pile Foundations by Improving Weak Soils. Washington, DC: The National Academies Press. doi: 10.17226/14574.
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Page 53

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51 The soils in the field are all clays typically classified as CL or CH according to the borehole log shown in Figure 3-3(a). The soil is modeled up to a depth of 45 ft. The soil was divided into 13 layers. The top 9 layers are all 2.5 ft thick, the next 3 lay- ers are 5 ft thick, and the last layer is 7.5 ft thick, as shown in Table 4-1. Since all of these 13 soil layers are primarily clays, the soils are modeled with a von Mises model without hardening. A total of 13 sets of material parameters for von Mises type elasto- plasticity were estimated from the lab or in-situ tests. In the finite element model, the elastic Young’s moduli and compres- sion strengths are needed. The elastic Young’s moduli can be estimated from the following relationships: where G is the shear modulus; vs is the shear wave velocity measured from the downhole seismic cone testing; γ is the total soil unit weight that can be estimated by aver- aging from the lab data along the depth; g is the gravity constant; rd is a reduction factor that accounts for the large deforma- tion effect and remolding effect, here a value of 0.25; E is the Young’s modulus in the elastic part of the von Mises model; and The Poisson’s ratio, v, is assumed as 0.45 due to the nearly undrained condition of the clay during the tests. The yield strength in the von Mises yield function is twice the measured undrained strength, as follows: k su= 2 8( ) E v G= +( )2 1 7( ) G r g vd s= γ 2 6( ) The simplified distributions of shear wave velocity and undrained shear strength based on the test data are plotted in Figures 4-1 and 4-2, respectively. The total length of the pile is 46.5 ft and the pile toe depth is 45 ft in the soil. The pile is modeled as a 1-D linear elastic beam-column element. For the steel pipe pile with concrete fill, the composite EI is required. The pile EI is calculated as 1.41 × 107 kip-in2, using a compressive strength of 5150 psi based on compression tests on concrete cylinders at the time of testing. The steel cross-sectional area is computed based on an outside diameter of 12.75 in. and a 0.375-in. wall thickness. Young’s modulus for the pile is 29,000 ksi and the Poisson’s ratio is 0.20. Since the 1-D beam-column element has no physical dimen- sion in the cross-sectional plain, special measures were taken to include the diameter effects of the pile by connecting the soil nodes with pile nodes using radially rigid “spokes,” which also are modeled by a very stiff elastic beam-column element. Non-extension spring elements link the outer ends of the spokes and the soil nodes to model the gapping between the pile and the soil. The compression stiffness of these spring elements is very large but the extension stiffness is zero. To avoid possible numerical instability, a very small extension stiffness value is used in the finite element model. The soil was modeled as 3456 3-D solid element accounting for large deformation and large strain effects. The pile was modeled using 36 1-D elastic beam elements. The pile and the surrounding soil were linked by 468 non-extension spring ele- ments. A total of 4,717 nodes were in the FEM mesh, which is shown in Figure 4-3. The nodes on the outside surface of the cylinder are restrained against horizontal movements. The nodes on the bottom surface are restrained against move- ment in any directions. The movements of the nodes in the middle plane are restricted to embody the load and geometry symmetry. A displacement control method is used for this problem. The node on the pile top is selected for the displacement C H A P T E R 4 Finite Element Modeling of Single Pile Load Test

52 Figure 4-1. Tested, simplified, and model shear wave velocity distribution. Figure 4-2. Tested, simplified, and model undrained shear strength distribution. Table 4-1. Model parameters used for FEM model. Layer # Top Depth (Ft) Bottom Depth (Ft) Thickness (Ft) Su (psf) Vs (Fps) 1 0.0 2.5 2.5 950 416 2 2.5 5.0 2.5 325 389 3 5.0 7.5 2.5 350 357 4 7.5 10.0 2.5 400 338 5 10.0 12.5 2.5 450 355 6 12.5 15.0 2.5 500 425 7 15.0 17.5 2.5 525 495 8 17.5 20.0 2.5 550 565 9 20.0 22.5 2.5 600 550 10 22.5 27.5 5.0 655 500 11 27.5 32.5 5.0 750 500 12 32.5 37.5 5.0 845 500 13 37.5 45.0 7.5 940 500 Notes: Su is undrained shear strength; Vs is shear wave velocity; and Fps is feet per second. 0 5 10 15 20 25 30 35 40 45 0 200 400 600 800 Shear Wave Velocity (fps) De pt h Be lo w Ex ca v at io n (ft) Test Simplified Model 0 5 10 15 20 25 30 35 40 45 0 250 500 750 1000 1250 Undrained Shear Strength, Su (psf) De pt h Be lo w Ex ca va tio n (ft) Unconfined Torvane Simplified Model

control, which is laterally pushed up to 2.5 in. The pile head is considered to be a free-head boundary with no rotational constraint. The static loading step number is 50 with pile head lateral displacement of 0.05 in. per loading step. The simulated pile head load versus pile head displacement and pile rotation are shown in Figures 4-4 and 4-5, respec- tively. The loads have been doubled to account for symmetry. The curves exhibit the conventional hyperbolic shape that would be expected for soft clay and are in good agreement with the test data. This suggests that the single pile in clay under lateral loading can be satisfactorily simulated using the simple von Mises soil model with the above-mentioned parameters. The calibrated parameters for the soil and pile will be used for the later pile group analysis. 53 Figure 4-3. FEM mesh used for analysis of single pile lateral load test. Figure 4-4. Simulated and tested data on pile head load vs pile head displacement. Figure 4-5. Simulated and tested data on pile head load vs pile head rotation. 0 5 10 15 20 25 30 35 0 0.5 1 1.5 2 2.5 3 Pile Head Deflection (in) Pi le H e a d Lo a d (ki ps ) Test Model 0 5 10 15 20 25 30 35 0 0.2 0.4 0.6 0.8 1 1.2 1.4 Pile Head Rotation (Degree) Pi le H ea d Lo ad (k ips ) Test Model

Next: Chapter 5 - Finite Element Modeling of Pile Group Load Tests »
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TRB’s National Cooperative Highway Research Program (NCHRP) Report 697: Design Guidelines for Increasing the Lateral Resistance of Highway-Bridge Pile Foundations by Improving Weak Soils examines guidance for strengthening of soils to resist lateral forces on bridge pile foundations.

The report presents computational methods for assessing soil-strengthening options using finite-element analysis of single piles and pile groups and a simplified approach employing commercially available software.

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