Urban sites present firms with foundation challenges.

New York City's Times Square, for decades a magnet for city residents and visitors from all over the world, has undergone and continues to undergo redevelopment aimed at attracting corporate tenants. In 1985 Mueser Rutledge Consulting Engineers (MRCE), a geotechnical and structural foundation engineering firm located in New York City, performed a preliminary evaluation of the subsurface conditions on four developable sites for the New York City Economic Development Corporation. All four sites have since been developed and are occupied by high-rise office buildings. MRCE was involved in planning, designing, and constructing three of these properties. Two of the projects, 5 Times Square and Times Square Tower, located on respectively the southwest and southeast corners of 42nd Street and 7th Avenue, presented particular challenges.

The developer of the two sites, Boston Properties, Inc., wanted full-basement buildings that would occupy the entire lot footprints to maximize the use of the site. The buildings were developed property line to property line and did not require setbacks. Thus, the buildings designed for the sites were narrow structures with higher than usual loads at the perimeter, requiring innovative foundation solutions.

Times Square Tower--a 48 story highrise--was built on the last of the four development sites in Times Square. The site has active subways on all sides and a busy subway station extends into it, as a result the design and construction of the foundations for the Tower presented multiple challenges that required a collaborative approach between the engineers and the foundation contractor on the project. Through the innovative planning of the design team, the structure was designed to occupy the entire site and maximizes the total rentable area. To accomplish this, existing structures were demolished, debris and rock were excavated to 30 feet below sidewalk level, adjacent subway structures on all four sides of the site were braced, high capacity caisson supports were designed and installed immediately adjacent to subways, and existing foundations were adapted to accommodate new building foundations.

Because subways border the site, shoring and excavation was a major concern. Constructability issues such as vibrations and equipment tolerances were critical as they directly impacted the adjacent subways. Subway service could not be interrupted at any time during construction. To address these issues, MRCE and contractors met early in the design process to establish a project approach that would permit work to proceed in and around all of the adjacent structures without damage and/or interruption to public services.

5 Times Square is a 38 story tower extending to each property line, with two full basement levels. The design and construction of the foundations for this structure was complicated by its close proximity to two operating subways and the historic New Amsterdam theatre. During the design phase, MRCE planned and implemented a subsurface investigation program, provided foundation recommendations as well as excavation support recommendations, and prepared a Historic Preservation Plan to monitor and protect the theatre during construction. MRCE provided the client with practical and innovative solutions to design and construction issues and facilitated approvals from NYCTA and 42nd Street Development.

The new building foundations extend below the basement level of the theatre. Real-time monitoring of these structures while excavating rock with heavy equipment and installing the rock socketed caissons allowed the work to proceed safely and without delay under a tight construction schedule. The two subways remained active throughout the project and regularly scheduled performances of the Lion King continued in the theatre throughout the project construction.

The dense, heavily developed urban setting at both sites forced the geotechnical engineers to rely heavily on historical information in preparing geotechnical reports for the sites. Limitations at the site prompted such foundation innovations as high-strength steel and grout mini-caissons in places where conventional caissons could not be used.

The innovations for the most part came from the foundation contractors, and new approaches of this type are likely to become increasingly common as redevelopment efforts continue in downtown urban areas around the country.

The design and construction of 5 Times Square and 7 Times Square illustrate how early coordination on the part of foundation contractors, construction managers, engineers, owners, and public agencies was critical to the installation of complex foundations in tight locations in a timely fashion and to the successful completion of both projects.

BY TONY D. CANALE, P.E., ASSOCIATE AND GEORGE J. TAMARO, P.E., PARTNER MUESER RUTLEDGE CONSULTING ENGINEERS