O-Cell Testing and FE Analysis of 28-m-Deep Barrette in Manila, Philippines

Alfaro's Peak is to be a 28-story residential building located in Makati, Manila, Philippines. The loads are high and concentrated, which necessitated supporting the building on deep foundations, penetrating into a residual soil called the Guadalupe Tuff formation encountered at a depth of approximately 15 m. The foundation chosen consisted of a perimeter diaphragm wall combined with rectangularly shaped, 2.4-m2 cross section, barrettes to support interior columns. A static loading test using the Osterberg-cell (O-cell) test method was performed to study the barrette capacity and deformation behavior. This paper describes the O-cell test, summarizes a finite-element (FE) analysis performed to assist interpretation of the results, and indicates foundation design change adopted as a result of the test. The maximum applied O-cell load during the tests was 11,600 kN. The accumulated upward movement of the top plate was about 10 mm. The accumulated upward movement of the bottom plate was 58 mm, corresponding to about 6% of the barrette width. The results of testing and analyses performed show that the shaft resistance (side shear) acting on the barrette is proportional to the effective stress distribution. This means that any design based on the parameters established from the analysis of the test must include the unloading consequence of basement excavation at the site. The FE computations enabled a comparison between the O-cell test and a conventional head-down test, which indicated that the O-cell test results are representative for the behavior of the barrette in a conventional head-down test and gave insight in the overall load-transfer behavior of the barrette. The O-cell test, strain gauge instrumentation, and FE analysis gave reliable results of decisive importance for the design of the barrettes and other foundation units at the site.