ROAD BED AND BUILDING HEAVE FROM ALTERATION OF SULFIDE AND SULFATE MINERALS

Too often the geotechnical staff and engineers on highway projects are so concerned with ARD that the potentially deleterious effects of volumetric changes during alteration are overlooked or neglected. During the initial or sulfide induced reaction, pyrite is converted to sulfates such as anhydrite, gypsum, melanterite, rozenite, jarosite and alunite as sulfur bonds in tetrahedral co-ordination with oxygen increase unit volume by ~350%. In a second stage of sulfate induced reactions, sulfate minerals in the presence of Ca convert to gypsum and more hydrated sulfates such as ettringite, copaipite and halotrichite. Although hard data are available for the calculation of molar volume (MV), little is known on expansion pressures. Interaction between sulfuric acid and Ca- minerals (calcite) to form gypsum has been shown empirically to generate a heave pressure on the order of 10,000 psf (479 KPa). Other studies suggest an upper limit of ~2100 KPa, sufficient to heave foundations in a 5 story building.

Most of the case histories involve syngenetic pyrite (mainly framboidal) in Black Shales. For example, gypsum (CaSO₄ •2H₂O) (MV=74.7), and halotrichite (Fe²⁺Al₂(SO₄)₄•22 H₂O) (MV=484), have been identified respectively in differential floor heaves over Marcellus Shale at the Evangelical Hospital, Lewisburg, PA, and Juniata College, Huntingdon, PA. However, diagenetic pyrite may play a larger role in Black Shale expansion than previously recognized.

Addition of limestone as a neutralizing agent may exacerbate the problem by promoting sulfate- sulfate reactions to form thaumasite (Ca₃Si(OH)₆(SO₄)(CO₃)•12H₂O) (MV=329) and ettringite (Ca₆[Al(OH)₆]₂(SO₄)₃•26H₂O) (MV=697) in (a) road beds (Cedar Hills State Park, Texas), (b) destabilizing slopes in a waste rock pile at Questa Mine, New Mexico, or (c) generation of gypsum and iron hydroxide sufficient to block drains in an earth dam embankment near Carington, Derbyshire, England, where liberated CO₂ asphyxiated 4 men in an inspection chamber.