Project Description

From 1976 to 1981, Lachel & Associates, Inc. was retained to provide geotechnical, instrumentation, and construction methods evaluations for three sections of the New York City transit system. The routes involved two tunneled sections in Manhattan and two open-cut sections in Queens. All of the work involved overlying glacial drift with grain sizes ranging from silt to boulders more than a meter in diameter. Of particular interest was the subway section advanced through the infamous New York City "Bull's Liver." These highly saturated and flowable silts are particularly troublesome because of their tendency to flow through very small openings under very low hydrostatic head. This flowing ground often results in significant surface subsidence. On this particular project, Lachel was responsible for instrumenting and advising the Contractor over areas susceptible to this flowing ground. These susceptible areas were identified by the Lachel geologists and predictions based on potential settlements were delineated. These zones were later instrumented with various types of geotechnical instrumentation to assure that settlements would not occur and ground would not flow into the excavations. While surface subsidence had been documented prior to Lachel's involvement on the project, not a single incident of significant disturbance occurred after the company was assigned to the project.

Scope of Services

On the East 63rd Street and Archer Avenue sections, the company was also retained by the Contractors to install and monitor instrumentation in what were deemed to be potential areas for loss of ground during construction. The company also installed and monitored rock instrumentation beneath the overburden which detected the existence of high in-situ stresses which resulted in surface convergence of the open cuts of magnitudes approaching four inches. The instrumentation and analysis of these movements were considered to be pioneering at the time they were installed; and they led to significant changes in the way the New York City Transit Authority instrumented and designed their projects.

Between 1980 and 1981, Lachel studied the use of Tunnel Boring Machines in the Manhattan schist, and evaluated the instrumentation for the E. 63rd St. portion. It was particularly important to monitor the effects of subsidence at the surface, given the presence of nearby industrial facilities.

The approximate costs to construct the various facilities were as follows: E. 63rd St. Station - $80 Million; E. 63rd St. running tunnel - $50 Million; 41st Ave. running tunnel - $60 Million; Archer Ave. running tunnel - $50 Million.

Client

New York City Transit Authority