Geotechnical Investigations Geomet conducted 95 electrical cone penetration tests (CPTs); a practice pioneered in the Netherlands during the early 1930s to determine optimal pile length and bearing capacity prior to pile installation. Most of the CPTs were 35 m (115 ft) deep, de Wit said, enough to cover the full depth of the Pleistocene sand layer between 20 m (66 ft) and 35 m (115 ft) deep. Some CPTs were lengthened to 70 m (230 ft) to cover the deeper Kedichem clay and sand layers. “The over-consolidated Kedichem Formation is an important consideration for heavy buildings in Rotterdam,” de Wit noted, “because it can cause settlements of more than 100 mm (4 in).” Because of large upward water pressure, Kedichem settlement for Market Hall will be limited to a maximum of 20 mm (0.8 in) under the arch, he said, and the resultant rise in middle of the arch (the section of the structure without apartments above it) to a maximum of 20 mm (0.8 in). The risk assessment considered the surrounding buildings, including the sixteenth century St. Laurens Church, which has a foundation of short wooden piles, vulnerable to lowering of ground- water tables, de Wit said. Other nearby structures, including a train tunnel and metro station (minimum distance of 20 m [66 ft]), and a school (roughly 6 m [20 ft] distance) built on prefabricated concrete piles, posed low risk. Early Structures and Old Piles The excavation’s biggest challenges were the old foundation piles and other remnants of earlier structures on the building site, said de Wit. Before the 2,500 new foundation piles were installed, archeologists excavated two small pits up to 10 m (33 ft) deep. In one pit, they found a farmhouse built approximately 1200 AD, during the period of the first human habitation of what was then Rotta. Over the centuries, periods of terrain elevation changes and building took place, each leaving construction material and wooden foundation piles in the ground, de Wit continued. In World War II, most of the old city-center of Rotterdam was destroyed, sparing only St. Laurens Church. After the 66 • DEEP FOUNDATIONS • MAR/APR 2012 A cross section of the Market Hall Rotterdam excavation and foundation construction (Photo Credit: Provast) war, a school building and a parking garage with two underground levels were built on the site. When these structures were razed to build Market Hall, their 350 prefabri- cated concrete piles, up to 25 m (82 ft) long, remained in the ground. Removing the old piles was a challenge, de Wit said, given their large footing diameters of up to 69 cm (27 in). Before installing the new piles and diaphragm wall for Market Hall, 35 of the old piles had to be removed. The remaining 315 old piles were cut off to 7 m (23 ft) deep after the dry excavation and then to 15 m (49 ft) deep after the underwater excavation. Pile removal involved drilling a large- diameter steel tube over each pile to a few meters under the pile. Next, the tube and pile were removed by slowly pulling them together while vibrating the tube con- tinuously. During removal, a bentonite- and-cement mortar was injected into the hole. Most piles were removed successfully; a few were broken at the deepest part due to the large footing and were left behind. Pile Installation Cast-in-situ vibro-combi pile was the most advantageous pile type for this project, said de Wit. The vibro-combi pile is suitable for driving up to 32 m (105 ft), from which the last 12 m (39 ft) is in the dense Pleistocene sand. The casing is driven into the ground with the foot cover under it. A pre-stressed square concrete core is placed in the casing, acting like a prefab concrete pile, but is used in combination with a concrete-filled casing. The space between the casing and the square core is filled with grout. The casing is drawn up while ramming. The foot cover remains in the ground. “Vibro- combi pile” was translated from the Dutch term “vibro-combinatiepaal.” The vibro-combi pile gets its name This close-up photo, taken after the building pit was pumped dry, shows the reinforcement bars protruding from the underwater concrete (Photo Credit: Provast) because it’s a combination of the precast pile core used in combination with a concrete-filled casing. This system com- bines the benefits of the precast concrete pile and the vibro pile. The large tensile force that a precast concrete pile can withstand in the shaft is combined with the rougher wall and larger cross-section of a
