settling of the church and other nearby buildings. To avoid this risk, the building pit was filled with water, and the remainder of the excavation was continued to a depth of 15 m (49 ft). “The excavators worked on large pontoons and from shore,” de Wit continued, “all of them with access to an advanced GPS control system that stored all the positions of the concrete framework and old and new foundation piles.” The system enabled the excavators to move closely to the underwater obstacles without touching them. After the excavation, more activities Aerial photo of the Market Hall site, taken early October 2011 (Photo Credit: Provast) vibro pile, thus enhancing the tensile performance through soil friction. The pre-stressed square core gives the piles a high strength for both pressure and tensile loads, Hermens added. This pile type made it possible to create a pile with its top at 15 m (49 ft) depth without leaving any obstacles above this level. “The diaphragm wall is a steel com- bined wall built of 1,016 mm (40 in) dia- meter open tubes and intermediate double- sheet piles AZ14-770,” said de Wit. “The depth of the tubes is 25 m (82 ft) and the sheet piles 19 m (62 ft). Both were installed with high-frequency vibrators. Hammers also were used to install the sheet piles.” Recognizing the risk of openings in the joints between the tubes and sheet piles and the resultant openings in the wall that can cause excessive settling of nearby structures, the tubes were installed with a mini- mal horizontal displacement and vertical distortion, de Wit continued. Precautions included measurements of the horizontal connection of the vibrator to the rig. Soil anchors usually are the preferred option for supporting steel walls for such a large building pit, according to de Wit. How- ever, because permission was not granted by all neighboring building owners to place anchors outside the building site, a concrete supporting framework, instead, was designed to support the walls. The framework was built when the excavation reached approximately 5 m (16 ft) below street level, de Wit said. This framework, also called a stamp window, is a grid of concrete beams that runs two ways between the steel combined walls. With this framework, the remaining dry excavation to 7 m (23 ft) could continue without the risk of pushing the steel retaining walls inward. Excavating “In the Wet” The dry excavation to 7 m (23 ft) lowered the water table in the Pleistocene sand layer by 3 m (10 ft), de Wit said. Lowering the water table further could cause excessive took place under water, said Hermens. Commercial divers cleaned the piles and diaphragm walls, and placed the pilecap reinforcements. Next, underwater concrete was poured continuously for 3.5 days over the pilecap reinforcements. Underwater concrete usually is applied without any reinforcement and is used only for the construction phase. Although DHV did three smaller projects with reinforcement in underwater concrete for the end phase, Hermens said, the technique had not been done before on a scale like this. Because building codes in the Netherlands do not address reinforce- ment in underwater concrete for the end phase of a structure, DHV developed a project-specific protocol for design, calcu- lation and construction that was checked and authorized by the Delft University of Technology in the Netherlands. Pumping of the building pit began a week after the concrete was poured, Hermens said. Once the pit was pumped dry, the finishing layer of reinforced concrete was poured, becoming the floor of level 4, the lowest level of the parking garage. The basement levels A worker fills underwater concrete from two mixing trucks (Photo Credit: Provast) will be completed during 2012, followed by the above-ground construc- tion of the Market Hall shops and apartments scheduled for completion during 2014. DEEP FOUNDATIONS • MAR/APR 2012 • 67
