Projects

Geotechnical Services for Boulevard of the Allies

State-of-the-art concept of oblique aerial digital imagery aids in design of rockfall treatments for nearly vertical rock slope.

Project is perched on the crest of Duquesne Bluff about 100 feet above the Parkway East elevation.

Client

Pennsylvania Department of Transportation, District 11-0

Location

Pittsburgh, Pennsylvania

Our Role

Soils/Geologic Studies, Foundations, Geotechnical Investigations, Transportation, Highways.

Data

Completed: 1997

Outcomes

Small diameter micro-piles stabilized structures without demolition/reconstruction
Grouted and spin rock bolts secured existing structures and rock blocks
Pre-cast concrete for the cantilevered roadway slabs expedited construction.

Boulevard of the Allies, a major artery linking the central business district of downtown Pittsburgh to Oakland, Pennsylvania, and the eastern suburbs, is perched on the crest of Duquesne Bluff, a 115-foot high rock slope above the Parkway East elevation.

The presence of Duquesne University, Mercy Hospital, and established businesses adjacent to the inbound westward lanes of the Boulevard forced the designers to extend the four-lane roadway to the slope crest and, at places, cantilever beyond.

What We Did

Gannett Fleming investigated the geotechnical and structural deficiencies of the Boulevard of the Allies and provided the remedial alternatives, construction plans, and specifications necessary to reconstruct this roadway for use as a detour during the closure of the Fort Pitt Bridge and Tunnel. Design drawings were not available for several of the retaining structures present at the site. Therefore, in order to determine the in situ geometry, an innovative geophysical study consisting of seismic topography was used to determine the thickness of the concrete. Confirmed by horizontal coring of the wall, this geometry was used to assess the stability of the existing walls.

Because of the nearly vertical nature of the rock slope adjacent to S.R. 0885, conventional survey would have been impractical, if at all possible. A state-of-the-art concept of oblique aerial digital imagery was used to aid in the design of rockfall treatments. The slope was photographed with a Precision Aerial Survey System Bell 47 Helicopter and a Zeiss RMK TOP15 precision aerial camera. These images were used by Gannett Fleming CADD operators to develop an Intergraph MicroStation topographic file that was used to generate cross sections at any location along the slope. Additionally, a digital photogrammetric mosaic showing existing slope and structural features, as well as proposed treatments was developed. These images were provided in the bid documents and were used by the contractor during bidding and construction.

Key Features

Oblique aerial photography used for geologic mapping
Provided geologic mapping and rock joint studies, including Markland analyses
Rock slope stability evaluations
Implemented rock bolting, rock netting, tied back walls, and buttressing
Geophysical study determined concrete wall and retaining wall thickness
Foundation design used pin piles
Post-tensioned rock-bolt design to stabilize the underlying rock slope.

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