further insurance against possible settlement, ground movement instrumen- tation was monitored 24 hours a day. At Marmorkirken Station, Trevi constructed 120 T-shaped, permanent hydromill panels to a depth of 41 m (135 ft) for the walls. Due to space limitations – in some locations only 10 m (33 ft) separated the station walls – the crew used a compact Trevi Hydromill HC-03 to construct the diaphragm panels. The trenches were initially filled with bentonite mud to stabil- ize the boreholes, followed by the place- ment of reinforcing steel cages and concrete. While typical hydromill-installed panels have a 50 mm (2 in) overlap to ensure wall continuity, anticipated heating of the concrete during joint preparation prevented the use of conventional methods in Copenhagen. As an alternative, Trevi used its proprietary “Milano Joint,” which shapes the panel joints by using breakable PVC pipes fixed to both sides of the primary panels. The PVC joint forms are The steel reinforcement cages were lowered into the wall excavations in one piece using a Soilmec SC-65 service crane. Concrete used for the panels is designed in a cement-filler-microsilica proportion to maintain a setting temperature under 65°C (150°F) as per Danish code. Throughout the process, a Koden device – using ultrasonic distance measurement – helped measure the shape and deviations of pile boreholes and slurry trenches. The device relies upon ultrasonic distance measurement to detect deviations of pile boreholes and slurry trenches. A precision sensor was inserted along the axis of the borehole to determine and plot the borehole’s exact distance to the wall in one or two directions. Using these measure- ments it was possible to detect and plot local collapses of the borehole walls. Unique Desander Setup SPECIAL LESSONS LEARNED :ISSUE Historical construction sites, like the Cityringen Marmorkirken Station, challenge conventional construction methods. Innovative equipment and techniques must be developed to meet the demands of a limited construction area surrounded by fragile historic buildings. Compact and flexible equipment with quality control monitoring was necessary to meet the daunting regulatory restrictions while maintaining the production schedule. destroyed and the joint surfaces are scratched and cleaned with a special bit during the drilling of the secondary panels, which ensures water tightness. Conversely, in conventional construction, the hydromill over-excavates the cured sides of both primary panels to expose freshly scored surfaces of concrete along the entire edges of the primary panels. The limited space at Marmorkirken required a unique desander setup con-sisting of a Soilmec SMT-500 bento- nite slurry desanding plant and SDM-35 debris management unit. The desander separates sands, clay and organic particles in the bentonite muds and was selected for its small footprint – double stacked storage units, scalper screen and dual hydrocyclone separators are stacked vertically in a four-level configuration. The system features centrifuges in line, rather hour (400 yd /hr ) bentonite output. This the hydromill’s 300 m / 3 than in parallel, to match 3 was necessary because there was no room to store the used bentonite at Marmorkirken, and Trevi couldn’t run the centrifuges overnight to process the stored material due to work hour restrictions. The integrated SDM-35 (along with an SMD-90 mud decanter centrifuge) features an automated, top-loading system, wherein drill cuttings, after being separated from the slurry, are dumped from the second level of the plant into waiting trucks at ground level. Site Restrictions Excessive vibration and noise were concerns due to potential damage to the church and other buildings, which impacted the methodology for constructing the diaphragm panels in the intermittent bands of hard flint rock within the underlying limestone bedrock. A vibratory hammer could not be used because of the excessive vibration, so Trevi utilized a rotary drill and water pressure hammer to bore through the rock. Throughout the operation, the high-strength drill bits were checked daily and repaired/replaced within Trevi’s on-site workshop. Stringent noise restrictions mandated a 5-day work week with a single 7 a.m. until 6 p.m. day shift. Additionally, the owner specified a 70 decibel noise limit during working hours, prompting Trevi to install a Excessive vibration and noise were concerns due to potential damage to the church and other buildings, which impacted the methodology for constructing the diaphragm panels in the intermittent bands of hard flint rock within the underlying limestone bedrock. noise mitigation panel on the SC-100 (used as a service crane) to reduce engine noise. While the machine was supplied with its own noise suppression system, mobile sound barriers were also used to reduce noise levels by a few additional decibels, enabling Trevi to keep the noise below specified requirements. DEEP FOUNDATIONS • MAY/JUN 2014 • 87
