Nondestructive Testing of the New Croton Aqueduct

A nondestructive seismic/sonic testing program was conducted for approximately 19 tunnel miles of the 16 foot diameter brick lined New York City water supply New Croton Aqueduct (tunnel) to:

1. Document the condition/strength of the tunnel liner (brick and mortar).
2. Identify low velocity materials behind the liner that are indicative of low strength poorly cemented rubble, or fractured bedrock.
3. Determine the elastic moduli values of the liner and behind the liner materials.

Seismic/Sonic direct and refracted wave measurements were conducted along three continuous survey lines at the 2:00, 12:00, and 10:00 o’clock positions of the aqueduct.

Liner and behind liner velocity values were the basis for categorizing the condition of the tunnel liner and the “rubble” and rock behind the liner. The sonic velocity values are directly related to the moduli (Young’s, shear, bulk) values as well as Poisson’s Ratio, and are indirectly related to the strength of the concrete which is calculated from the moduli values.

The sonic techniques used for this testing program evaluated tunnel liner and behind the liner conditions at a level well beyond that discerned by unaided human eye (or ear). The sensitivity of the method allows tunnel flaws such as a poorly cemented liner and behind liner materials, voids and/or unconsolidated materials to be identified. The condition of the liner and behind liner materials is assessed by stress wave velocity measurements recorded after a projectile impact is applied to the tunnel surface. Lower than normal compressional and shear wave velocity values are recorded when any of the above mentioned flaws exist.