Abstract:
Circular shafts are commonly used as initial Support of Excavation for tunneling projects.  Because circular geometry limits large bending stresses typically developed by straight sections; use of circular shafts have significant cost and installation advantages. Moreover, in place of round shafts, and as an alternative to rectangular or square shafts, elliptical shafts may be used to optimize design and still realize the advantages that circular shafts may offer.
Numerous rib and liner plate elliptical shafts have been designed by Jennmar and successfully installed for projects throughout the United States.  Common shaft types include liner plate, driven sheet piling or soldier pile and lagging, slurry walls, or secant piles. The decision to pursue a square, rectangular, or a circular shaft geometry will ultimately influence the type of shaft selected. However, rolled shaft ribs with liner plate or wood lagging, in a circular or elliptical shape, offer the same level of excavation support as the other types, with significant cost and installation benefits.