ELLIPTICAL SHAFTS AS AN ALTERNATIVE TO STANDARD CIRCULAR & RECTANGULAR SHAFTS-Paper

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The paper presents elliptical shafts as a practical alternative to circular, rectangular, and square excavation-support systems, using Baltimore’s SC-940 sewer project as a case study. The project required 21 shafts, 6–12 m deep; 18 used liner plate and ring-beam construction, including 15 elliptical shafts.<br /><br />Elliptical shafts combine two radii to create long, narrow excavations suited to constrained sites. Their steel ribs and liner plates are designed to resist earth and surcharge pressures, with checks for both arch-thrust stress and out-of-plane buckling. Contact grouting and timber blocking transfer ground loads to the ribs and help keep the structure stable.<br /><br />The design was particularly useful where narrow rights-of-way, nearby historic walls, property access, and utilities made conventional shaft arrangements difficult. It allowed driveways and walkways to remain accessible, accommodated utility conflicts within the excavation, required a smaller equipment footprint, and reduced shoring and disposal costs. Incremental excavation also helped crews respond to irregular rock and soil conditions.<br /><br />Construction required careful grouting of boulder cavities and precise alignment of the slender ribs. The varied liner-plate sizes, two rib radii, and changing blocking patterns complicated material organization and slowed installation. Pipe-jacking forces also caused minor shaft distortion, addressed with reinforced mud slabs and additional bracing.<br /><br />Overall, the case study concludes that elliptical shafts can provide cost and access advantages in restricted locations, but their design and installation require close attention to ground conditions, structural stability, grouting, and construction sequencing. Shafts should be evaluated by a qualified designer for their intended use.

Keywords

elliptical shafts

excavation support

liner plate

ring-beam construction

Baltimore sewer project

earth pressure

buckling

contact grouting

pipe-jacking

restricted site