Soil engineering and foundations specialist Trevi is carrying out all the foundation works on a unique project in Frankfort, Germany.
FOUR Frankfurt is high-rise building in the heart of Frankfurt. Thanks to its purchase in 2015, “Groß und Partner Grundstücksentwicklungsgesellschaft” laid the foundations for the execution of this mega-project.
This building development includes four new high-rise buildings on a multi-level and multifunctional construction foundation. Thanks to its 228-meter-high office tower, the building is the highest office space in Germany and, with its slightly shorter 173-meter high tower, also ranks as the country’s highest residential tower. Parts of the existing buildings will be kept to comply with historical preservation requirements and integrated into the new construction.
Due to the simultaneous execution of the works in all areas and the relevant logistical needs, which add to the enormous pressure set by the compliance with the time limit, one of the main challenges of the construction process concerns its position in the city center.
The four towers of the FOUR Frankfurt project are supported by a base slab and its perimetral walls. Due to space restrictions, the client decided to build the basements using the “top down” construction method. This means that first the diaphragm wall and the foundation piles are constructed, and then the concrete slab is poured to form the first basement floor.
Subsequently, excavation works take place under that “cover” and the other basements floors are all built in the same way, while the construction of the building can start simultaneously.
Since the distance between the opposite walls of the perimeter would span over 100 meters, prefabricated columns were positioned into the piles to support the slabs. The structural loads of the above-ground structures and of the slabs are directly supported by the construction piles through the columns.
The foundation and excavation protection enclosure work is being carried out using the diaphragm wall method. In this case, two different methods are used.
Generally, the excavation of the panels is carried out using a clamshell. However, due to the strength of the limestone banks, a hydromill is also used, in order to minimize the impact generated by vibrations on the neighboring structures.
Large-diameter bored piles ranging from 1,500 to 2,000 millimeters are used as foundation elements. In the upper part, a temporary casing is used. Then, in the lower part, uncased drilling is carried out using stabilizing bentonite mud, to reach a total drilling length of up to 50 meters.
After reaching the design depth, the reinforcement cage is installed, then, the concrete is poured into the shaft. Since the working level is significantly higher than the foundation one, the reinforcement cage is placed up to 20 meters below the working level.
Due to the selected top-down construction method, it is necessary to position the plunged columns in the piles. Their installation requires precision, since these prefabricated columns are not just temporary supports — but also permanent columns in the basement floors. The job allows a maximum vertical tolerance of 2 centimeters and a maximum horizontal tolerance of 5 centimeters. This led to an innovative system for this project: After reaching the design depth of the pile, the reinforcement cage is installed, and subsequently a collar and a sleeve are positioned around and into the casing. The sleeve serves to guide the precast column and support its assembly.
In the next step, the position of the sleeve is suitably adjusted by using adjusting screws on the collar in compliance with the given tolerance requirements. After the adjustment of the sleeve, the pile is backfilled with concrete using a tremie pipe, and then the prefabricated column is installed, guided by the sleeve, inserting it for 3 meters into the fresh concrete of the pile.
Trevi Group works in special foundations, tunneling, soil consolidation and other soil engineering projects, and builds rigs to serve those markets. The group is also active in the drilling sector, in oil, gas and water, in both products and services. It was established in Cesena, Italy, in 1957 and now has branches in over 80 countries. Brands under the group include Trevi, which specializes in soil engineering; Petreven, the oil drilling division; Soilmec, which produces and develops plant and machinery for soil engineering; and Drillmec, which produces and develops drilling rigs for oil, gas and water. For more information, visit www.trevigroup.com.