Environmental Engineering Reference
In-Depth Information
laboratory facilities. Underlying the limestone and exposed in the floors of many facili-
ties is a black, pyritic shale of Upper Pennsylvanian age. Sulfide alteration of the pyrite
results in swelling, and in subsequent years as much as 3-4 in. (8-11 cm) of floor heave
occurred, causing severe floor cracking as well as cracking of the mine pillars left to
provide roof support (Coveney and Parizek, 1977). Possible solutions are discussed in
Section 10.6.4.
Heaving from the swelling of a black, pyritic carbonaceous shale is reported to have
caused damage to structures in Ottawa, Canada (Grattan-Bellew and Eden, 1975). In some
instances, the concrete of floors placed in direct contact with the shale has turned to
“mush” over a period of years. Apparently, the pyrite oxidizes to produce sulfuric acid,
which reacts with calcite in the shale to produce gypsum, the growth of which results in
the heave. The acid builds up in the shale to lower the pH to an observed value of 3, leach-
ing the cement from the concrete. The phenomenon does not appear to affect the more
deeply embedded footing foundations.
Gneiss and other metamorphic rocks
may contain seams of montmorillonite that can be
troublesome to deep foundations, tunnels, and slopes.
10.6.4
Treatments to Prevent or Minimize Swelling and Heave
Foundations
Excavations
Sections as small as practical should be opened in shales, and water infiltration prevented.
The opening should be covered immediately with foundation concrete, cyclopic concrete, or
compacted earth. The objective is to minimize the exposure of the shales to weathering,
which occurs very rapidly, and is especially important for dams and other large excavations.
Deep foundations
, which generally are drilled piers extending below the permanent zone
of saturation, eliminate the heave hazard. The piers, grade beams, and floors must be pro-
tected against uplift from swelling forces.
Shallow rigid mat or “rigid” interconnected continuous footings
may undergo heave as a
unit, but they provide protection against differential movements when adequately stiff.
Other methods, often unsatisfactory, include:
Preflooding to permit expansion, then designing to contend with settlements of
the softened material and attempting to maintain a balance between swell and
consolidation.
●
Injection with lime has met with some success, but in highly active materials it
may aggravate swelling, since the lime is added with water.
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Excavation of the upper portion of the swelling clays, mixing that portion with
lime, which substantially reduces their activity, and then replacing the soil-lime
mixture as a compacted fill is a costly, and not always satisfactory, procedure.
●
Floors
Floors should consist of a structural slab not in contact with the expansive materials or
supported on a free-draining gravel bed that permits breathing. Infiltration of water must
be prevented.
Pavements
Foundations for pavements are prepared by excavating soil to some depth, which depends
on clay activity and environmental conditions, and replacing the materials with clean
granular soils, with the same soil compacted on the wet side of optimum, or with the same
