Page 54 - Geotechnical Engineering Soil and Foundation Principles and Practice

Special Problems with Sedimentary Rocks
Special Problems with Sedimentary Rocks 49

Figure 3.6
Remnants of karst
topography along
the Yangtze River,
China.

ground-penetrating radar, while definitive, is handicapped by low penetration in
wet soils. Cave exploration and mapping by adventurous ‘‘spelunkers’’ may be
used but there are obvious risks, one of the most serious being a rapid rise in the
groundwater table in response to rain running into the sinkholes.

Depths of Caverns
As CO 2 -charged rain water infiltrates vertically down through cracks and chan-
nels, it reaches and seeps laterally toward a nearby stream valley, in the process
eating away at the limestone to hollow out a cavern. A groundwater table is
a muted expression of ground surface elevations, so caverns normally tend to dip
downward toward a neighboring valley where the groundwater may emerge
as springs.

As valleys cut deeper, groundwater levels are lowered and develop cavern systems
at successively lower levels. Vertically oriented channels connecting the differ-
ent levels are appropriately referred to by cave explorers as ‘‘glory holes.’’ Such
stacking aids drainage and exploration of the upper levels, but lowering of the
groundwater table also reduces buoyancy and in effect increases the weight of
the rocks and induces collapse, particularly of shallow caverns where the roof
is thinnest.

Tunneling in Limestone
Tunneling in limestone below a groundwater table can invite precipitous flood-
ing by encountering an open fissure or cavern. This danger is reduced if the
tunnel opening is sealed off and pressurized, but costs are much higher because
without careful control workers may get the ‘‘bends,’’ or nitrogen bubbles in the
blood stream during repressurizing. A temporary seal can be achieved by ground

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