Limestone is the light-colored rock that occurs in ledges that outcrop in roadcuts or along stream valleys. Limestone is quarried and is a valuable aggregate used in engineering. However, most limestone is not satisfactory for such purposes, and limestone bedrock has some peculiarities that can present serious problems for foundation engineers.
Why Call It Limestone?
The name limestone comes from its use to make lime for mortar, which dates from ancient times. The dominant mineral in limestone is calcite, or calcium carbonate, CaCO3. When calcite is heated to several hundred degrees Celsius it decomposes and releases carbon dioxide, CO2, leaving quicklime, CaO. Quicklime retains a pebble shape until it is hydrated by adding water to make hydrated lime, Ca(OH)2.
The name ‘‘quick’’ can be attributed to the heat released by the rapid hydration.
Other Minerals in Limestone
While the most common calcium carbonate mineral in limestone is calcite, another mineral with the same chemical composition accumulates in shells of invertebrate marine animals. This mineral is ‘‘aragonite,’’ which can be much harder than calcite due to crystallite orientations and possible interactions with organic matter. ‘‘Coquina’’ is soft limestone that is composed almost entirely of shells and may be used for surfacing roads. Coral reefs are another example of limestones that are made up of skeletons of marine animals.
Residual Soils Developed in Limestone
Competent limestone rock is suitable for supporting building foundations, but a major goal of exploration is to establish its competence. Weathering of limestone, in contrast to weathering of shale or sandstone, leaves relatively little residue after the calcium carbonate is simply dissolved away. Surface soils on limestone typically are clayey, and incorporate increasing percentages of rock fragments with depth, being transitional to solid rock. Foundation borings in limestone normally are continued deep enough into the rock to ensure that it is continuous downward, and not a disconnected piece of rock or ‘‘float’’ resting in clay.
Clay Pockets in Limestone
Surface water penetrating limestone follows along vertical fractures, creating solution cavities. These often are filled from above with residual soil. The vertical orientation of clay pockets and their random nature is such that they may be missed by exploration drilling borings and come as an unpleasant surprise when excavations are opened for construction of foundations.
The surprise often can be averted by inspecting nearby outcrops and roadcuts. Failing that, geophysical measurements such as earth resistivity or the use of ground-penetrating radar can be quite helpful. Such tests may be performed by geotechnical engineers or by geologists.
A tunneling machine essentially is a horizontal drill that is pushed forward by hydraulic pistons that push against opposing plates expanded against the sides of the completed tunnel boring. Therefore in order for the machine to advance it must have competent rock to push against. Exploration borings were conducted every 50 ft (15 m) along a tunnel route and showed no clay pockets, but the machine found one that was between the borings. The machine was stopped and has no capability for backing up, so a tunnel was started from the other end so that the machine could be pulled forward, and the tunnel completion date was moved back one year. A geophysical survey probably would have prevented this.
Caverns and Sinks
Whereas clay pockets penetrate from the ground surface downward, caverns occur at the level of a groundwater table and remain as open voids until the top caves in. Caverns are difficult to locate as they can readily be missed by borings, and geophysical seismic and resistivity measurements focus on what is there and not what is gone. Ground-penetrating radar would be most useful but the penetration depth is limited, particularly in clay.
Caves are of obvious concern in foundation engineering, as a roof collapse can drop part or all of a building or other structure into the ground without advance warming. The consequences can be devastating. A cavern collapse under a railroad track in South Africa left a train suspended in midair by welded rails.
A cavern that already has collapsed is a sink, and cavernous ground often can be recognized from the occurrence of sinks. Sinks may be obvious, as in Fig. 1, or also can be detected from streams that disappear and have no visible outlet.
Sinks also are a major concern for groundwater supplies, as they are direct conduits leading into aquifers. Whatever falls or is thrown into a sink therefore may be only a step away from the well, and the once-common practice of dumping everything into a sink from bald tires to dead dogs and old automobile batteries must stop.
Sinks also are a challenge for investigators because a floor may only conceal part of a deeper cavern. A hint as to consequences of a brash action may be found in animal bones lying in the bottom of a sink.
Cavernous ground impacts the feasibility of a dam because of the potential for leakage. Sealing of leaky dam foundations and abutments can be attempted by grouting, and it is not unusual for the volume of grout pumped to exceed the volume of the dam itself. Also, groundwater flow through underground channels can wash away the grout as it is injected, in which case asphalt or fibrous materials may be added to try and seal things off. Experience indicates that the prognosis for success is marginal.