Since 2009, work has been progressing to expand the 100-year-old Holtwood hydroelectric power station on the Susquehanna River, Holtwood, Pa., with the aim of increasing the generating capacity by 125 MW. After carefully reviewing proposals from numerous vendors, PERI USA, Baltimore, was commissioned to supply a wide variety of formwork and shoring solutions for this complex project.

In addition to other products, Variokit heavy-duty shoring towers were used to safely transfer the enormously heavy loads from the slabs of the intake structure. An advantage for the contractor, Walsh Construction, Canonsburg, Pa., was that the planning and supply of the formwork and scaffolding was from a single source so that the systems used were perfectly matched to one another.

The expansion facilities for the power station included an intake structure and a powerhouse. Most of the new facilities’ structural components were characterized by their large dimensions resulting in high loads being imposed on the formwork and shoring systems. For the concrete work, various PERI systems were being used—each optimally adapted to suit both geometry and loads.

Among other products, PERI supplied its TRIO panel formwork system for the north wall of the powerhouse. The panel formwork system was also used for the pier walls of the six bays of the two turbine units. CB 240 climbing brackets were used to support the panel formwork for the two-sided climbing conditions of the project. The high load-bearing capacity of the CB brackets allowed greater bracket spacing and therefore larger climbing formwork units, while at the same time having the ability to carry very high loads.

Walsh Construction also used CB 240 climbing units for the 98-foot high south wall of the powerhouse. The external walls of the powerhouse were poured against rock and used the versatile Vario GT 24 wall formwork elements support by SKS single-sided climbing scaffold platforms. The SKS climbing scaffold system transferred the loads resulting from the fresh concrete pressure by means of walers, compression spindles, strongbacks, and the brackets themselves into the SKS anchors placed in the previous pour. Formwork ties were not required for the one-sided pours.

Heavy-duty shoring for thick slabs

During construction of the intake structure roof, extremely high loads had to be carried to the ground. The slab thickness ranging from 15 feet to around 43 feet required heavy-duty loadbearing shoring. Variokit shoring towers were provided to transfer maximum leg loads of 660 kN into the ground. Three separate towers were aligned in a single row in each of the bays. The six-­ and four­-legged towers were positioned on main beams with a corresponding special wedge connection compensating for the 34% bottom slab slope. For height adjustment at the top of the towers, VST head spindles were used, which allowed hydraulic lowering and lifting under full load.

SLS heavy-duty spindles were used for bracing of the 40-foot-high X 11-foot-wide shoring towers. Thanks to the high rigidity of the towers, the construction team was able to pre­assemble the units on the ground and then quickly position them between the intake walls with the crane. PERI delivered the shoring tower bents to the jobsite pre­assembled so assembly times were kept to an absolute minimum, ensuring the demanding timeline of the project could be met.

The compatibility of the engineering construction kit with the modular scaffold system allowed fast installation of all the required working platforms. Up to 100 feet tall, UP stair towers provided safe access to all areas of the project, and the width of the stair towers enabled site personnel to easily climb and descend in both directions simultaneously.

The intake slab formwork elements also consisted of PERI system components featuring GT 24 lattice girders, SRU steel walers, and RCS rails. The 19-foot-wide formwork elements were pre­assembled on the ground and then flown into place with the crane. After the concrete hardened, the slab formwork was lowered hydraulically, ensuring that the complete striking procedure was carried out easily, safely, and efficiently. Subsequently, the construction team lowered the towers onto roller bearings and pulled them out of the intake structure.

Planned and perfectly implemented

One of the biggest challenges for the contractor is maintaining the tight schedule required to complete the power plant expansion on time, scheduled for completion in spring 2013. A particularly important issue is a smooth execution of all working operations and processes associated with the formwork and shoring operations. Thanks to its comprehensive product range and vast experience, PERI offered the contractor optimized solutions for all the work to be carried out. Apart from materials, the overall solution included all technical documents such as structural calculations, assembly drawings and support, and site assistance.

The optimized, well balanced design of the systems supported the contractor in meeting the challenging time schedule of the project.