U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
The second visit of the study tour was to Cologne, Germany, on June 20 and 21, 2002, for meetings at the German Federal Research Highway Institute (BASt), which is similar to the FHWA Turner-Fairbank research facility. The review involved formal meetings with and presentations by representatives of various departments within the BASt as well as a representative from the Bundesministerium for Verkehr, Bau-und Wohnungswesen (Germany Ministry of Traffic, Construction and Housing). Presentations were also made by representatives of the private sector, including:
On the second day in Germany, the team met with the highway department in Meschede and attended a presentation on its activities, including a roadway construction project where extensive rock slide and cut slope stabilization problems have occurred. We made a field visit to the roadway construction project site (figure 8). The team then made a second site visit to a geosynthetic-reinforced, mechanically stabilized earth retaining wall project, which was constructed with a "green" steel mesh facing using recycled, contaminated construction debris as fill (figure 9). The use of contaminated fill for embankment construction was also encountered in several locations in Belgium (airport and rapid rail).
The current construction of rapid rail lines in the Netherlands, Belgium, Germany, and Italy has been a showcase for innovation. On the way to Germany, the team took the opportunity to visit the Giessen-Oudeker project (a high-speed freight rail project) in the vicinity of Rotterdam, the Netherlands. At this specific portion of the project, the Betuwe route, they were constructing a cut and cover tunnel for the twin rail line below an existing river in a residential area (figure 10). Self-boring micropiles are being used to act as both a tiedown for hydrostatic pressure and vertical support as the trains pass through the tunnel.
In Germany, a process was presented to evaluate which method would provide optimum acceleration considering the total scope and integration with all phases of the project (i.e., how accelerated construction methods fit in with the critical path for project completion). A project study on accelerated construction with regard to the Autobahn A-26 was presented to demonstrate the technique.
The German group presented their experience with a variety of foundation technologies, including geotextile-encased columns, which accelerate dewatering plus improve support; embankments on piles, which eliminates preloading; geosynthetic reinforcement support of embankments on piles or columns; a combined soil stabilization system (CSV), which provides a one-step installation of cement columns; and the drilled-in, self-grouting micropiles used on the project in the Netherlands. Another presentation discussed the use of instrumentation on the compaction equipment to measure dynamic modulus for improvement compaction uniformity and evaluated required effort and use of this equipment for compaction control. Discussions were also held on keeping the designer on board during construction to rapidly resolve issues or modify design and bring the contractor in during design to identify methods to accelerate construction.
As in Sweden, the significant factor in Germany is maintaining traffic during construction, which often drives the construction procedures and has led to innovations in parallel bridge construction. The German group presented information on their current use of prefabricated steel bridges to save time and cost over traditional precast concrete bridges. They also reviewed several bridge reconstruction projects in which the new bridges were constructed adjacent to existing bridges, and then moved into place on the existing alignment after the old bridge was demolished. Total project disruption to traffic: approximately 72 hours.
The following section reviews the accelerated construction methods identified in Germany in relation to the amplifying questions.
In Germany, a more in-depth presentation was made on the use of geotextile-encased columns (or geotextile-coated columns), a technology identified during the Swedish tour. As noted in the section on Sweden, this technology consists of inserting continuous, seamless, high-strength geotextile tubes into soft soil with a mandrel and filling the tube with sand (or fine gravel) to form a column with a high bearing capacity. Displacement methods can be used for installation of the columns in very soft soils, and excavation (e.g., augering) methods can be used in stiffer soils. Construction loads are transferred through the columns onto the underlying natural foundation. In addition, the soil surrounding the columns is improved through consolidation, which further improves the embankment support conditions and decreases secondary settlements. A horizontal layer of high-strength geotextile is placed over the column heads to transfer load between columns. Construction time is saved through direct embankment support, decreasing or eliminating staged construction and surcharge loading. Time for secondary settlements of the soft soil is also decreased. Several case histories were presented, which demonstrated both the construction techniques and the potential for use of this rapid method for stabilizing very soft ground or organic soil. Documentation of these applications was made available (in German) with field measurements. This information requires critical evaluation for instrumentation, especially in relation to design. No design code is currently available. The method is currently designed and installed with the assistance of the proprietary contractor and system supplier. QC and performance are evaluated through detailed instrumentation. The geotextile manufacturer and contractor provide the expertise. The scan team believes that performance could also be evaluated through in situ testing using standard penetration tests (SPT) and/or CPT. Nominal training of 2 to 3 days is required. The system is patented and is available to licensed firms. Contracting is currently performed in Germany on a performance basis.
Column-supported embankment is a conventional technique for construction over soft subgrade that has often been dismissed because of its relatively high cost. However, the Germans have found that this method provides such a significant time savings over preloading and surcharging that the additional cost is often outweighed. This is the fastest embankment foundation stabilization method. Settlement of the embankment is more controlled than with preloading. This is especially important for soft ground embankment widening projects and construction adjacent to other structures. Since conventional piling equipment is used, the technique does not require specialized equipment. There are no patents on the equipment or process, and the equipment is readily available in the United States. The advent of new piling techniques as discussed in this report, some of which use lightweight equipment and improved pile installation, make this application even more attractive. In addition, geosynthetic reinforcement used to spread the load over piles, a recent practice in the United States and Europe, allows for increased pile spacing, thus reducing the number of piles required. A presentation on the design of geosynthetic reinforcements was given in Germany. The column-supported embankment technology uses conventional practice for all aspects of design, specification, and construction such that it can be readily implemented. Design is relatively mature and well developed. Conventional monitoring methods, such as settlement monitoring and field observations of pile installation, are used to control construction. Routine soil sampling and strength testing are used to develop design information and construction performance. Special training is not required for construction crews. Standard performance-based specifications are already in place, and the owner can measure performance by measuring settlement of the embankment.
The CSV soil stabilization system is a flexible foundation system formed by installing small-diameter columns of cement-sand or lime-sand mixture using a displacement auger. The auger runs through a container filled with the dry granular mixture, turning against the drilling rotation and downward movement to transport and compact the dry mix into the ground. This technique applies to most soils because it does not use the in situ soil as a binder. The main limitation is the need for ground water to hydrate the dry sand-cement mix. This one-step process allows for rapid installation of the columns for ground improvement in soft cohesive soils, organic soils, or loose sand (approximately 1 minute of installation time per meter of depth). The installation equipment is low weight and thus easy to mobilize. The benefits of this system include low noise, no vibration, no soil spoils for disposal, and real-time instrumentation for automated installation and documentation.
The installation does require the use of specialized equipment and process, which are both patented. The equipment is available in the United States. Design is based on on-board monitoring and experience backed by field tests. Termination of the column in competent soil is based on crowd pressure. The equipment includes a data recording system for monitoring the crowd pressure, rotary speed, and withdrawal speed. Field quality control is based on the installation of "calibration" columns, which are installed at numerous locations on a site prior to production work. Static load tests using the same installation equipment and lab testing of the cores are performed on the calibration columns. If satisfactory results are achieved, the production columns will be installed using the on-board automatic installation method with the same settings as those used to install the accepted calibration columns. Special training is required for the equipment operator. Helpers can be hired locally, and the crew usually requires up to 3 days of training. Performance-based specifications are used. Research on CSV is currently in process between the U.S. Army Corps of Engineers and the University of Tulane. In Germany, the research and development has been performed by the private sector without government funding as yet. There is a significant amount of related research being performed by an international group dealing with pile-raft foundations, which is discussed in the section on Belgium.
The real-time automatic controlled variable roller compaction and documentation system allows for optimization of compaction rates and real-time quality control. The system works by using accelerometers to monitor the speed of the dynamic wave through the soil induced by the vibratory rollers in order to measure the dynamic stiffness of the soil, which generally increases with higher compaction. Efficient fill densification is achieved via automatic adjustment of compaction energy and the measurement/documentation feedback, eliminating time wasted on compacting areas that are already adequately compacted. This energy variability and efficiency is achieved by the use of two counter-rotating weights in the drum rather than the conventional single, one-directional eccentric weight. The weights rotate in opposite directions and only come together in a common direction in the downward vertical inclination. This eliminates unwanted and wasteful movements in the lateral and upward directions that occur with conventional compaction drums. Internally, the entire counterweight assembly is rotated to adjust the direction of the point where the two weights act together. If the onboard monitoring system determines the soil is compacted to a satisfactory level, it will automatically reduce the vertical component of force at the specific time and location.
In addition, the ability to monitor density improvement during compaction both speeds up and improves the aerial extent of QC. Most importantly, the ability of instrumented compaction equipment to provide 100 percent QC coverage enables the use of performance-based approaches to specifications and the effective implementation of warrantees and guarantees for both earthworks and pavements.
This method does require proprietary, specialized monitoring equipment, but the equipment and process are not patented. The equipment is readily available in the United States and requires nominal operator training. Contracting this technology is based on standard contracting for fill placement. Currently, the proprietary equipment manufacturer has researched the procedure and there is a need for an independent study. Again, this technology is used with normal compaction procedures; therefore, there are no additional environmental concerns to normal compaction activities.
Self-boring and self-grouting hollow bar nails and micropiles (for sheet piling, slope stability, retaining walls), which were reviewed in Sweden and on the project site in the Netherlands, were presented by the developer in Germany. These hollow bars are installed using a one-step installation procedure for drilling and grouting. Eliminating the normal micropile (and nailing) requirements of auger, inserting the micropile (or soil nail), and then grouting, should reduce construction time. There is also some reported improvement in adhesion, which may reduce pile lengths. In this proprietary procedure, the same high-strength steel is used to drill, grout, and reinforce. Specialized equipment is required, but there are no patents on the equipment or process. The equipment is readily available and currently in use in the United States. This technology is commonly used by private industry in several regions of the United States, but it has not made significant inroads into public sector contracting, primarily because of questions concerning corrosion. Corrosion issues need to be addressed; however, the manufacturer has composite galvanization and epoxy-coated systems available. A coextruded material with an outer, noncorrosive material is also under evaluation at this time. Design is conventional and relatively mature, with current design and construction installation guidance covered in FHWA’s guidelines documents. Load tests are used as with conventional micropiles to evaluate minimum cover provided. Skilled and experienced laborers are required and make a big difference in performance. This technology is contracted per rod installed, with contractor confirming performance through load tests. With regard to environmental issues, the technology is relatively quiet and produces low vibrations. Grout is used, and there is some spoil to be disposed.
With regard to micropiles, a significant French research project called FOREVER was supposed to be completed in September 2002. There is also ongoing research at the University of Munich. Corrosion issues are currently being investigated internally by the manufacturer and will require independent evaluation.
Germany presented projects in which replacement bridges were completely “prefabricated” next to the existing bridge, and then moved into final position (jacking or specialized trucks) once the existing bridge was rapidly demolished (see figure 11). In all cases, the time savings were significant, with two structures reported to have been completely replaced (demolishing and moving the new bridge into position) within 72 hours. The opportunity to use this technology depends on site conditions and traffic control requirements. This approach also minimizes impact to the environment.
The Germans have traditionally used precast, prestressed concrete girders. In recent years they started to use steel prefabricated members to reduce weight and increase length of prefabricated segments. Prefabricated construction is suitable for improving speed of construction, reducing traffic interruption, and minimizing environmental impact. Prefabricated elements can be built in an enclosed environment during bad weather conditions. The weight reduction from using two I-girder systems and prefabricated steel replacement bridges makes it easier to move the structure into place after destruction of the old bridge structure. German designers do not have special design consideration for fracture-critical members in a two I-girder bridge system. The lighter weight of the structure also makes use of existing foundations more feasible without significant modification. Seismic concern is not a consideration in German bridge design.
With regard to Eurocode 7, the Germans still use their own design standards, but they have converted a significant portion of their code to the Eurocode limit state design (load and resistant factor design [LRFD] in the United States) format. By the end of 2003, the Germans should have evaluated all issues with the new Eurocode.
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