As a consequence of all these difficulties, the designer, together with the general contractor and the foundation contractor, decided to undertake a final detailed design for construction, and a new risk analysis of the project. This work included new surveys and in- depth studies of the geology and hydrogeology of the region to eliminate the dewatering effects and the traffic impact during the construction of the plastic diaphragm wall barrier. The success of this project and optimization of the design were only possible because of the dialogue among the client, engineering company, general contractor and foundation contractor. The owner and client is the Metro – Companhia do Metropolitano de Sao Paulo. The station construction is currently underway by the Andrade Gutierrez / Camargo Corrêa JV who are waiting for the TBM to come through before the station is then closed and finished. Geodata Geoengenharia do Brazil provided the basic and executive design, and the foundations, including the diaphragm wall, was constructed by Brasfond Group. The Final Design The client established several construction requirements for the final design, the most important of which are listed below: • Solutions to mitigate the contamination found in the surrounding areas and prevent spreading • Construction systems and methods to accelerate the station’s excavation phase, in order to fulfill the TBM schedule • Keep construction costs unaltered support were both considered as temporary and disposable structures in the original solution. Furthermore, and most important, due to the improved quality of the joints, the plastic slurry wall barrier along Santo Amaro Avenue was eliminated, avoiding the traffic impact during its construction. In the final design, 133 singular (single bite) panels, with dimensions of 1.0 m x 2.80 m (3.3 ft x 9.2 ft) and 30 m deep (98.5 ft) were used to give the external circular geometry for the shafts. At each of the five shaft intersections, four diaphragm panels 36 m (118 ft) deep, forming the shape of an arrow, were constructed simultaneously, aimed at “interlocking” the shafts. This solution offered a way to hold the arch effect of the shafts (in principle acting as big tieback) and avoid having to fully close the circles with panels, and then having to demolish these structures after the excavation. Total surface of the constructed wall was 15,337 m (165,000 ft ) 2 2 considering a secondary panel overlap on each side between 0.4 m (1.3 ft) and 0.65 m (2.1 ft), always aiming to ensure the tightness of the joints due to the curved nature of the shafts. The new design enabled simultaneous excavation of all 5 shafts and reduced construction time from the estimated 555 days to 380 days, thus meeting the new Line 5 schedule, creating a barrier to the contamination and minimizing traffic disruptions. These achieve- ments were reached without increase in costs for the client, consequently meetings all the goals set out by the client. Constructing the Panels Original Design (above) and Final Design (below) Brasfond was contracted to build the diaphragm wall, and the designer chose the hydromill technology for two specific reasons. One was to provide precise verticality control in the execution of the diaphragm wall panels critical for the panel overlap, which guaranteed the insertion of the reinforcement cage in the arrow shaped mega panel. The other reason was that, given the contamination in the area, increased quality of joints between panels through overlap created at the time of excavation of the secondary panel, mitigated seepage of surrounding groundwater into the station and avoided contamination movement. The construction sequence of the panels involved the To address the challenges, Geodata designed a structural diaphragm wall to provide peripheral containment for the station and act as a seepage barrier wall. This new soil retaining solution was proposed and accepted by the client, once it proved possible to build at the same cost of the original solution. A 1 m (3.3 ft) thick structural diaphragm wall was proposed, based on the use of a hydromill, with no changes made in the final layout of either the shafts or its architecture. The new solution avoided dewatering and all its related risks. Moreover, the diaphragm wall was considered to be a permanent structure, whereas the slurry wall and the shotcrete following steps (see page 14). 1) Construction of the guide wall 2) Excavation of the primary panels using a hydraulic grab 3) Inserting the reinforcement cage and 4) Concreting the panel 5) Repeating the methodology above for the next primary panel 6) Drilling the secondary panel overlapping both primaries with the use of the hydromill 7) Lowering the reinforcement cage and 8) Concreting the panel. This sequence was then repeated until all primary panels and secondary panels were constructed. These were all single bite panels for the periphery of the wall except for the arrow shaped mega panel. DEEP FOUNDATIONS • JAN/FEB 2014 • 13
