Planning for water mitigation

In general, source reduction will, if effectively accomplished, reduce the level of moisture experienced in the grotto walls as well as eliminate the water flow into the grotto. The major drawback is the difficulty associated with identifying the source of the water supply to the aquifer. If site geology and hydrology are complex, it is unlikely that the source of water to the aquifer will be sufficiently reduced by surficial treatments. Water interception, on the other hand, allows for an accurate location of the aquifer and its effective control. An effective, peripheral drainage system will rely on pressure relief much the same way as the subterranean flow was initially attracted to the grotto (Fig. 1). Accordingly, it may be impossible to

Figure 10

Schematic section of a conceptual water-inception system for Yungang.

Control moisture occurring at the wall of the grotto since the grotto will, in effect, remain a pressure relief tube.

Each mitigation system has proved effective, depending on site characteristics. Site geology and hydrology are typically complex, especially in sedimentary formations. The drainage, or source-control, system illustrated in Figures 8 and 9 appears to have a reasonable chance of success. Its

Figure 11

Typical water-interception system used in tunnels.

Installation should reduce the amount of water from the source. The overall effectiveness of any surficial drainage program will be determined only by observing and monitoring the level of subsurface water before and after the installation of surficial flow control devices. Similarly, the grottoes north and west along the cliff face seem to afford an opportunity to experiment with intrusion-abatement programs. Clearly, water intrusion must be abated if the grotto contents are to be preserved. For an appropriate solution to be developed, knowledge and experience must be effectively combined with experimentation.