Metro Excavation in Taipei, Taiwan

Reanalysis of metro excavation in Taipei with diaphragm wall and steel struts

An 18m deep excavation supported by a 1m reinforced concrete diaphragm wall and steel struts in soft to medium stiff clays, in Taipei was reanalyzed with the DeepEX software program. The excavation was originally presented in a paper by Hsiao-Chou Chao, Richard N. Hwang, and Chung-Tien Chin (Moh and Associates), in the 2010 earth retention conference 3 (Aug 1-4, Bellevue, WA). In the original publication, the authors presented a methodology for correcting inclinometer displacements for walls that are embedded in clays where inclinometers do not extend beyond the wall base. The authors presented an analysis with a popular finite element analysis software program. The purpose of the analysis was to reanalyze the excavation with our software DeepEX and compare results with the presented finite element analysis.

The diaphragm wall was 1.0m in thickness and was supported by five levels of struts. The struts comprised mostly of double H-beams braced at regular intervals. Assumed soil properties are summarized in table 1:

Table 1: Assumed soil properties in deep excavation re-analysis simulation in Taipei

In DeepEX clay strength is modeled with assumptions that can simulate both OC and NC response. In this case, the undrained shear strength Su is treated as a maximum cutoff value, while the exact response is determined from the defined OCR at the borehole level using the peak and constant volume friction angles. For clays, the peak friction angle is used at the OC envelope and helps determine the effective cohesion at each overconsolidated point, while the constant volume shearing angle is used at the normally consolidated region.

Tables 2 and 3 compare strut reactions as predicted from the finite element and DeepEX analysis. Depending on the assumptions made for OCR strut reactions can differ by as much as 40%, but results are still comparable with the reported finite element simulation.

Table 2: Comparison with slightly OC soils                                Table 3: Comparison with mostly NC response

   

Wall displacements are more comparable against measured data when NC response is considered. Concluding, displacements and support reactions are in reasonable agreement with the reported finite element analysis.

The following figure presents the full model in the final excavation stage with DeepEX showing wall bending moments and horizontal wall displacements: