Shales and Other Degradable Materials
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LChapter 21 JOHN L. WALKINSHAW AND PAUL M. SANTI SHALES AND OTHER DEGRADABLE MATERIALS 1. INTRODUCTION time frame that may be as short as only a few hours. Such a rapid breakdown is easily identified by he designers of earthwork must take precau- straightforward laboratory tests. Other rock mate- T tions when the materials at hand cannot be rials show no appreciable change in strength over classified as rock or as soils in terms of their behav- many years. Unfortunately for engineers, the prob- ior in slopes or in civil engineering works in gen- lem materials fall somewhere between these eral. In their in situ form, the geologic formations extremes of rock behavior. may have names or appearances that imply rocklike Predicting the behavior of degradable materials behavior. Once disturbed, however, some of these has been the subject of much research since the formations retain the character of rock, but others early 1960s, and this research thrust continues may degrade to soil-size particles in a time frame today. Much of this research can be tied to the that is relevant tà the long-term performance of construction of large transportation facilities. In slopes built in, on, or with these materials. the United States, the development of the The currently available methods of identifying, Interstate highway system required much higher classifying, and treating these degradable materials cuts and embankments than had been éomrnon in so as to reduce the risk of slope failure are discussed the past. Problem geologic materials that previ- in this chapter. Sedimentary rocks, which consti- ously could be economically addressed by avoid- tute the bulk of degradable materials worldwide, ance, minor mitigation, or maintenance created are discussed first. Other degradable materials, in- the need for new engineering solutions. In Central cluding weathered igneous and metamorphic America the construction of the Panama Canal rocks, are discussed in less detail. Emphasis is resulted in slope problems that continued decades placed on the successful use of these materials in after construction and are also associated with the embankments and on their treatment in the for- properties of degradable materials (Berman 1991). mation of cut slopes. In geologic terms, all of the soils and rocks in the earth's crust are "degradable" 2. GEOLOGICAL CONSIDERATIONS materials, since all materials modify over geologic time. However, on the human time scale, only Degradable materials were not considered in detail comparatively rapid degradation of a strong or hard in the 1978 landslide report, but the basic princi- rocklike material into a weaker soil-like material is ples that govern them were identified: of concern to the designer of stable slopes. Certain sedimentary rocks can exhibit a loss of strength Before one can completely comprehend the that can be of several orders of magnitude within a particular problems of stability, one must under- 555 556 Landslides: Investigation and Mitigation stand the lithology of the physical properties third miscellaneous substances. The principal not only of the rock mass itself but of all the ma- minerals of shales, such as quartz, clay miner- terials in the mass.... A sedimentary rock se- als, and hydrated oxides (such as bauxite and quence, for example, is markedly different from limonite), are formed by the weathering of an igneous series or a metamorphic complex. feldspars and mafic igneous rocks. Some asso- Each particular type is characterized by a certain ciated minerals such as calcite, dolomite, texture, fabric, bonding strength, and macro pyrite, illite, and glauconite are formed during and micro structures. The most important rock and after deposition of the primary minerals. properties are the nature of the mineral assem- (Hopkins 1988, 8) blage and the strength of the constituent min- erals; a rock material cannot be strong if its Unfortunately, many of the shale particles are less mineral constituents are weak or if the strength of the bonds between the minerals is weak. than 1 .im in diameter, and consequently study of (Piteau and Peckover 1978, 194) - their mineralogy is difficult or impossible by sim- ple visual observation. The resulting geologic field The strength of the bonds between the miner- classification of shales does not reliably relate to als is also related to the geologic history of the for- engineering properties. mation of interest. The resulting hardness is Terzaghi and Peck described very clearly the generally due to long-term consolidation under geologic processes that lead to the problem prop- external pressures and not to cementing minerals erties of shales: (McCarthy 1988). Degradable materials can be grouped according to two broad geologic sources, As the thickness of the overburden increases those derived from sedimentary rocks and those from a few tens of feet to several thousands, derived from igneous and metamorphic rocks. the porosity of a clay or silt deposit decreases; an increasing number of cohesive bonds devel- 2.1Degradable Materials from ops between particles as a result of molecular Sedimentary Rocks interaction, but the mineralogical composition of the particles probably remains practically Shales constitute about one-half of the volume of unaltered. Finally, at very great depth, all the sedimentary rocks in the earth's crust. They are particles are connected by virtually perma- exposed or are under a thin veneer of soil over a nent, rigid bonds that impart to the material third of the land area (Franklin 1981). Shales are the properties of real rock. Yet, all the materi- by far the most pervasive and problematic degrad- als located between the zones of incipient and able material. As early as 1948, Taylor stated (53): complete bonding are called shale. Therefore, "Shale itself is sometimes considered a rock but, the engineering properties of any shale with a when it is exposed to the air or has the chance to given mineralogical composition may range take on water, it may rapidly decompose." between those of a soil and those of a real rock. In the American Geological Institute's Glossary (Terzaghi and Peck 1967, 425-426) of Geology, shale is defined as a fine-grained detrital sedimentary rock, formed These two authors further suggested using an im- by the consolidation (esp. by compression) mersion test on intact samples to obtain the relative of clay, silt, or mud. It is characterized by performance of otherwise "identical sedimentary finely laminated structure, which imparts a fis- deposits." As will be discussed in Section 3, this sility approximately parallel to the bedding.... was the direction taken by many researchers of [It is composed of] an appreciable content of clay minerals and detrital quartz. [Shale in- that time. cludes rocks such as] claystone, siltstone, and Just as increasing loads over geologic time play mudstone. (Bates and Jackson 1980, 573) an important role in the interparticle bonding of shale formations, the reverse process, unloading, Referring to Huang (1962), Hopkins noted that has significant effects. During the removal of load, typically, shales are composed of about one- "the shale expands at practically èonstant horizon- third quartz, one-third clay minerals, and one- tal dimensions" (Terzaghi and Peck 1967, 426). Shales and Other Degradable Materials 557 During expansion, the interparticle bonds are bro- chemical. Oilier (1969) described in detail several ken, and joints form at fairly regular spacings. At types of physical weathering, including sheeting or depths on the order of 30 m, the joints are spaced spalling (fracturing parallel to a free surface created meters apart and are closed. Closer to the surface, by erosion, excavation, tunneling, etc.), frost weath- intermediate joints form because of differential ering (extension of fractures by expansion of freez- movements between the blocks. These joints open, ing water), salt weathering (extension of fractures by allowing moisture to penetrate. The increase in the growth of salt crystals), and isolation (partial dis- moisture content may reduce the shear strength, integration of the rock caused by the volume and, if so, new fissures are formed. The final result changes accompanying temperature changes). and slope of any exposed face depend on the inter- Oilier (1969) also described types of chemical particle bonding remaining in the shale formation. weathering, including solution (dissolution of sol- uble minerals, particularly salts and carbonates), oxidation and reduction (chemici alteration of 2.2 Degradable Materials from Igneous minerals to form oxides or hydroxides), hydration and Metamorphic Rocks (incorporation of water to create a new mineral), Because sedimentary rocks (and shales in particu- chelation (leaching of ions such as metals), and lar) are typically formed relatively near the earth's hydrolysis (reactions between minerals and the surface and without the extreme heat and pressure component ions of water). that occur at depth, they tend to be mineralogi- As noted by Macias and Chesworth (1992), cally stable near the surface. Weathering of these chemical weathering, which brings about miner- materials then involves either a reversal of the alogical changes in igneous and metamorphic consolidation pressure or a dissolution of cement rocks, is usually more crucial than physical weath- bonds holding the grains or mineral groups to- ering in defining the strength properties of the gether. In contrast, igneous and metamorphic materials. Physical weathering, however, does rocks are created under temperature and pressure provide avenues for water to enter the rock by the conditions that are drastically different from con- creation and extension of fractures and subse- ditions at the surface. Macias and Chesworth de- quently encourages the more rapid progress of scribed the implications of the difference: chemical weathering by an increase in surface area exposed to water. One might therefore expect that they would Obviously some minerals, and therefore some weather more readily than sedimentary materi- rocks, are more susceptible than others to the als.... Generally however, expectations in this weathering processes described above.