## 3 Plaxis Finite Element Analysis
### Case Study Analyses
#### 3.2.1 Case Study I: Caliche at the Far Location from the O-Cell  
Case Study I is based on the TP-19 Osterberg test, in which the caliche layers existed very close to the ground surface and far from the O-cell. The material properties used in the analysis are presented in Table 2. Two competent caliche deposits were located at depths of 18 and 30 feet with thicknesses of four and six feet, respectively. The O-cell was located below the lower caliche layer at depths of 50 and 80 feet, and the drilled shaft was subjected to the total axial load of 3316 kips. The strain gauges were installed along the shaft length to record the pile displacement. A very fine mesh was used in the procedure of the finite element simulation. Similar to the field load application with the Osterberg test, the load was applied in the simulated model in 15 consecutive loading stages up to 3316 kips. The water table was defined at an 85-foot level, following the boring data to simulate the phreatic conditions to include an undrained state. The results of both loading scenarios, the Osterberg bidirectional test, and a conventional top-down test were analyzed using the load transfer, t-z curves, and load-settlement diagrams. As shown in Figure 7, the soil layers between caliche and O-cell carry more loads in the upward loading compared to top-down loading conditions. More load has been carried by caliche layers in the top-down loading scenario. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multicolumn{8}{c}{**Material Properties of Sensitivity Case Study II**} \\ \hline Parameter & Unit & Concrete & Clayy Sand & Sandy Clay & Cemented Sand and Gravel & Caliche & Stiff Clay \\ \hline Material Model & & Linear Elastic & MC & MC & MC & MC \\ \hline Drainage condition & & Drained & Drained & Drained & Drained & Drained \\ Dry Unit Weight & kly/tk3 & 0.15 & 0.12 & 0.12 & 0.12 & 0.16 & 0.13 \\ Saturated Unit Weight & kly/tk3 & 0.15 & 0.12 & 0.13 & 0.13 & 0.16 & 0.13 \\ Young’s Modulus & kly/t2 & 445,600 & 1000 & 1500 & 3000 & 560,000 & 1000 \\ Poisson’s ratio & & 0.15 & 0.3 & 0.3 & 0.3 & 0.2 & 0.4 \\ Cohesion & kly/t2 & - & 0.8 & 1 & 0.1 & 20 & 0.2 \\ Friction Angle & Degree & - & 35 & 28 & 45 & 35 & 30 \\ \hline \hline \multicolumn{8}{c}{**Interface Material**} \\ \hline Material Model & Unit & MC & MC & MC & MC & MC \\ \hline Drainage condition & & Drained & Drained & Drained & Drained & Drained \\ Dry Unit Weight & kly/tk3 & 0.12 & 0.12 & 0.12 & 0.16 & 0.13 \\ Saturated Unit Weight & kly/t3 & 0.12 & 0.13 & 0.13 & 0.16 & 0.13 \\ Young’s Modulus & kly/t2 & 1000 & 1500 & 3000 & 560,000 & 1000 \\ Poisson’s ratio & & 0.3 & 0.3 & 0.3 & 0.2 & 0.4 \\ Cohesion & kly/t2 & 0.8 & 1 & 0.1 & 20 & 0.2 \\ Friction Angle & Degree & 22 & 23 & 30 & 28 & 18 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Material Properties, Case Study II. The load transfer curve shows that the upward axial load of 6400 kips was decreased to 4500 kips at depths of 19 to 36 feet, while the load transfer curves of the Osterberg tests show a slight decrease of 500 kips within the same length. The finite element analysis showed that the amount of dissipated load in the caliche from the conventional loading scenario was more than three times than the reported results using the top-down test. Since the caliche layer was located at a distant location from the O-cell, the caliche could not be fully mobilized by the O-cell during the Osterberg test. Therefore, the measured load-bearing capacity of caliche could estimate just a portion of the pile's full capacity. Unlike the Osterberg test, a comparable load from the top could mobilize the caliche more, and consequently, more unit shear stress was developed. The nonlinear stress-strain responses of the drilled shafts in the soil containing caliche layers were also investigated using t-z stiffness curves.