Water intrusion

Grottoes located in regions where rainfall is considerable and groundwater tables are likely to fluctuate will experience variations in the moisture con-

Figure 1

Schematic cross section through the cliff face at Yungang.

Tent of the rock that constitute the grotto structure. At Yungang, water reportedly flows from the base of grotto walls during the rainy season.

The natural geotechnical process that originally formed the cliff site selected for grotto excavation is both the main source of the problem and a resource for possible solutions.

A sandstone cliff face, as shown in cross section in Figure 1, is usually created by the formation of a river valley. Water will flow through the ground in the direction of the valley floor, especially if the bedding planes slope in that direction. The toe of the cliff will usually coincide with a change in the character of the sedimentary material near the valley floor, which is often less permeable than that of the cliff face. Water will descend to this less permeable stratum and then flow toward the river valley. Before the grotto was excavated, water would, because of the change in overburden pressure, tend to flow out at the intersection of the valley floor and the cliff face, contributing significantly to its recession. With the excavation of the grotto and the associated change in overburden soil pressure inside the grotto, the water intrusion point moved back to the intersection of the grotto floor and wall.

Understanding this water-flow pattern suggests two mitigation strategies: (1) remove or reduce the source of underground water, and/or (2) install an intercept that provides an alternative escape path for the subsurface water.

Alternatives for Cliff Recession Abatement