FLOW MEASUREMENT IN TUNNELS BY DYE INJECTION

The design for the MetroWest Water Supply Tunnel Project (MWWST) calls for an extensive grouting program to control water inflows during construction and to reduce potential leakage from the tunnel once it is placed in operation. Measurements of water inflows are integral to this grouting program.

In addition to the inflow data recorded by the tunnel mapping team and periodical walk over surveys by inspectors, a profile of the inflows using they dye injection method was conducted on a regular basis during construction after the lining was completed and during the grouting operation.

Although the method of flow measurement by dye injection has not yet been widely used in tunnels, the principle is perfectly applicable to measure discrete and diffuse inflows in tunnels. The method consists of injecting a fluorescent dye, Rhodamine WT, at a given rate on the tunnel invert. The dilution of the dye is directly proportional to the volume of water flowing onto the invert. Water samples are then taken downstream of the injection point and the dilution of the dye is determined at the site laboratory.

The concentration of the dye in water samples collected is measured with a fluorometer. This readout can measure a range of concentration varying from 100 parts per billion (10-7) to 10 parts per trillions (10-10). When performed according to good practices as established in the field of chemistry, the method may yield data accurate to 0.5%.

Even though such accuracy is not required in tunnel application, it remains that diligence is required to obtain satisfactory results. In practice, the accuracy and the resolution depends on various factors such as an appropriate injected rate and concentration of the dye, the turbidity of the flow and the cleanness of the water.

The results obtained at the MWWST were within 10% of the total inflow measurement by the flow meter set at the portal and also consistent with the observations collected by the inspectors. These results show dye injection to be a useful tool in profiling tunnel water inflows.