A two-lane tunnel on a busy highway will be expanded. Reflected seismic waves were used to map structural features in the surrounding rock from the inside of the tunnel. The results of seismic imaging, largely in accord with the geological investigation, provided 3D characterization of the rock mass useful for the expansion project. Parts of the concrete liner of the tunnel had separated from damaged rock. This posed a challenge to conducting the seismic ground characterization from inside the tunnel. The uniqueness of the seismic survey approach was not the use of a small, hand-held source driving a swept frequency signals into the rock which allowed conducting the survey without stopping the traffic; nor was it the use of high sensitivity, broad-band accelerometers for detecting seismic waves in the rock. It was the fiberglass waveguides (1 meter long), anchored to the rock via holes drilled through the liner that introduced the uniqueness. These waveguides provided reliable coupling to the rock mass for sources and receivers mounted inside the tunnel near the inner surface of the liner. The sources and receivers, in interchanging arrays, formed 14 sets of "directional seismic antennae" along the tunnel, each deployed around the liner circumference within the limits of having to leave one lane open to traffic. Each "antenna" was used for imaging its own section of the rock mass around the tunnel. Overlapping ground images were then joined to successfully map the whole length of the tunnel.
A decision was made to widen the east-bound (EB) tunnel for one of the short double tunnels in Colorado. To assure an optimal design and a safe and economical construction, a comprehensive geotechnical investigation of the rock mass around and south of this tunnel was launched. The investigation included using reflected seismic waves for imaging anomalous rock mass conditions associated with possible fractures, faults, weathered rock etc.
The major obstacles for this investigation were:
Elevated attenuation levels particularly for shorter/ higher frequency seismic waves due to often disturbed and fractured rock mass;
Disruption caused by the traffic in the tunnel; and,
Concrete liner blocking access for coupling seismic sources and receivers to the competent parts of the native rock over 1 meter behind the inner surface of the liner.
Using very sensitive accelerometers significantly improved detection of the weakest seismic waves by the digital data acquisition system.
The noise problem associated with the traffic was overcome by using a swept frequency seismic source. At the same time the controlled wide spectrum of the source signal enhanced detection of shorter reflected waves, improving the resolution of seismic imaging. And the access to the native ground for sources and receivers was provided through fiberglass rods (waveguides) anchored to the rock through holes in the liner.