Expansive Pyritic Shales
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Transportation Research Record 993 19 Expansive Pyritic Shales DEAN D. DUBRE, MUMTAZ A. USMEN, and LYLE K. MOULTON ABSTRACT shale exhibits the general characteristics of sounder rock (1). Because of its abundance (it con Although much attention has been directed to stitutes about 50 percent of all exposed rock) , shale the solution of problems created by swelling is often used as an engineering material to build soils, relatively little effort has been de fills and embankments, highway bases in certain voted to the solution of problems created by cases, and as a foundation for all types of struc expanding pyritic shales. This is a very tures, including bridges and other transportation real problem that has caused extensive dam structures. Extensive studies have been conducted on age to many structures in the United States the evaluation of many shales for highway use <!-~) , and abroad. In an effort to call attention but little attention has been directed in these to this problem and to the need for further studies to pyritic shales either as a construction research directed at its solution, the or foundation material. current state of the art with respect to The importance of understanding the nature of swelling pyritic shales is presented and pyritic shale is two-fold, First, it is an expansive discussed. The factors and mechanisms re material for which the mechanism of expansion or sponsible for swelling of pyritic shales are swelling is different from what is known for ordi identifiedi methods of testing and evaluat nary shales or soils (10). The swelling phenomena ing the potentially expensive shales are observed in pyritic shales are primarily caused by outlinedi and ways of controlling or elimi the volume changes induced by the chemical altera nating the problem are summarized. A number tion of sulfide minerals. Second, lechate from py of case histories involving experiences with ritic shale can attack concrete and cause deteriora expansive pyritic shales are included to il tion. This phenomenon is known as "sulfate attack" lustrate the potential seriousness of the (11) and also results from the chemical alteration problem and to underscore the need for addi o~minerals, this time leading to volume changes in tional research. concrete. The purpose of this paper is to summarize the ex isting information and knowledge on pyritic shales. It has been reported that the average annual losses Factors and mechanisms of swelling in pyritic shales in this century from floods, earthquakes, hurri and experimental methods of evaluating swelling po canes, and tornadoes have been less than one-half of tential are reviewed. A number of case histories re the damages from expansive soils (.!_) • Geotechnical lated to heaving damage and concrete deterioration engineers who deal with expansive soils or shales a re presented and discussed. Finally, known methods have been faced with the decision of either accept of controlling the problem are described and re ing a certain risk in view of the uncertain response search needs are identified. of foundation materials to changing environmental conditions or choosing safe but generally expensive foundation systems. There has been a general reluc FACTORS AND MECHANISMS OF SWELLING IN PYRITIC SHALES tance to adopt the latter approach because it has appeared that strengthening the foundation can some Primary and Secondary Reactions times cost more than correcting the damages that can result fcom foundation heaving. It is important to Heave observed in certain shales has been identified note, however, that whereas considerable money has to be the result of the oxidation of sulfide miner been spent to control floods, relatively little has als, the most common of which are pyrite and marca been invested to develop and implement methods for site. Pyrite, or ferrous disulfide (FeS2 J, is a controlling or mitigating damages associated with yellowish mineral with a metallic glint that long foundation heaving. ago earned the name of "fool's gold." Pyrite is gen A family of expansive materials that has caused erally regarded as stable or slightly reactive under significant damage to many structures, but has been normal conditions i however, on exposure to dampness widely overlooked, is pyritic shale. In the past for a long period in the presence of air, it oxi damage to structures by expanding pyritic shale was dizes and expands. Pyrite is widespread, being the not readily identified. Consequently, damages were most common iron sulfide mineral. It occurs in rocks often attributed to differential settlement, frost of all types and geologic ages, most often in meta heave, subsidence, or poor construction practices. morphic and sedimentary rock. Marcasite has the same More recently, however, the problems associated with chemical formula as pyrite, but X-ray analysis re expansive pyritic shales have been better recog veals a different atomic arrangement. Marcasite is nized. Spanovich (2) suggests that part of the rea generally gray in color and is more susceptible to son for an increased frequency of problems caused by oxidation than pyrite. However, it is less abundant expanding shale is a result of advances in powerful than pyrite (12). excavation equipment that can produce greater cuts Crystallographically, the oxidation and expansion and expose deeper fresh shale strata, and also as a mechanisms of the sulfide minerals to sulfate are result of the increased use of slab-on-grade con quite readily understood. The pyrite structure is struction (because of economic constraints) without stacking of close-packed hexagonal sheets of sulfide providing crawl spaces to absorb heave. ions with iron occupying the interstices of the sul Shale is a sedimentary rock formed by the compac fide layer. The packing density of this configura tion and cementation processes acting on clay, silt, tion is related to the radius of the sulfide ion, and sand particles. Compaction shale is a transition which is 1.85 angstroms, producing a volume of 26.14 material between soil and rock susceptible to sig cubic angstroms, In the sulfate structure, each atom nificant weathering and slaking, whereas cemented of sulfur is surrounded by four atoms of oxygen in 20 Transportation Research Record 993 tetrahedral coordination. The packing density of the (18) and others (12 ,19-21) state that the growth of corresponding sulfate compounds will depend on the gypsum crystals within shale is the primary cause of radius of the sulfate ion, which is 2.805 angstroms. heave. Resolution of this issue is of practical as This results in a volume of 94. 4 cubic angstroms, well as theoretical importance. If heave is caused which represents an approximate volume increase of by the formation of gypsum, black shales that are 350 percent per packing unit (13). If calcium is essentially free of calcite would be considered safe present, gypsum may be formed, which may give rise from an engineering standpoint because the supply of to an eight-fold increase in volume over the origi calcium ions would be limited and the amount of gyp nal sulfide (14). sum formed would be small. The prima~ oxidation reactions occurring in the The chemistry of the formation of gypsum in py sulfide alteration process are thought to be as fol ritic shales appears to be rather straightforward, lows (15): involving an attack on calcite by sulfuric acid formed by the oxidation of pyrite. The acid reacts 2FeS2 + 2H2 0 + 702 --> 2FeS04 + 2H2 S04 (I) with calcite to produce a calcium sulfate solution ferrous sulfuric that may be transported to the site of crystalliza sulfate acid tion. The alteration products (gypsum) are fre quently located near existing pyrite, which indicate 4FeS04 + 0 2 + 2H2 S04 --> 2Fe2 (S04)i + 2H2 0 (II) ferrous ferric that they precipitate out of solution and begin to sulfate sulfate grow after traversing a short diffusion path (17) . As crystallization proceeds, additional solution is 7Fe2 (S04)2 + FeS2 + 8H2 0--> l SFeS04 + 8H2 S04 (III) brought in from surrounding areas by capillary ac ferric pyrite tion, and the growing crystals force the shale lay sulfate ers apart. This process is analogous to frost heave by ice lenses. Reaction I will normally proceed unaided, al By the use of a scanning electron microscope on though the oxidation of the sulfide may be assisted samples of weathered pyritic shale, Grattan-Bellew by autotrophic bacteria. Reaction II is thought to be and Eden (22) found that gypsum can form in two mor entirely due to the ferrobacillus-thiobacillus bac phologies: (a) bundles of fibers growing normal to teria, because the reaction cannot proceed in an the bedding planes of the shale, and (b) flat, acid environment. This was shown by Goldhaber and blade-shaped crystals growing parallel to the bed Reynolds ( 16) in a series of experiments. Data ob ding planes. Examination of large quantities of tained on samples drawn periodically and analyzed heaved shale has revealed that the major cause of for total sulfur in solution, thiosulfate and sul heave is the growth of the bundles normal to the fate, indicated that the rate of oxidation increased laminations. Why some of the gypsum forms blade like markedly as pH increased, particularly above a pH of crystals while others form bundles is not fully un 7. Reaction III oxidizes more pyrite by reacting derstood. It is speculated that the presence of iron with ferric sulfate, a strong oxidizing agent pro salts and the physical conditions present during the duced in Reaction II. crystallization may modify the morphology of gypsum Gypsum is known to form from the reaction of sul crystals. furic acid with calcite. Jarosite, another main re An interesting argument for the expansion of py action product, is essentially insoluble in water r itic shales caused by the growth of gypsum crystals and forms most readily in an acid environment (12) • is presented by Coveney and Parizek (~.