Identification of Problem Laterite Soils in Highway Engineering: a Review
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IDENTIFICATION OF PROBLEM LATERITE SOILS IN HIGHWAY ENGINEERING: A REVIEW M. D. Gidigasu, Building and Road Research Institute, Kumasi, Ghana Some laterite materials are known to be either inferior pavement aggre gates or troublesome highway and earth dam construction materials. How ever, not all laterite soils belong to this group. A criterion for distinguish ing problem laterite soils from non-problem ones for highway construction is proposed. Such a differentiation would be a considerable asset to high way engineers in determining the quality and suitability of doubtful laterite soils for highway and airfield construction. Laterite soils range in per formance from excellent to poor, and in spite of considerable field and laboratory studies on these soils it is still not yet possible to predict ac curately the behavior of all grades and genetic groups of laterite soils. From an engineering viewpoint, a criterion based on significant engineering characteristics, including such genetically inherent properties as sensitivity to drying and remolding, degree of potential swell and self-stabilization, and predominant clay minerals, may be more useful in predicting probable in situ behavior of laterite soils than the existing temperate-zone soil classification systems. •STANDARD methods of identification of temperate-zone soils based mainly on particle size distribution and plasticity are applicable to most laterite soils. For example, the unified soils classification system has been found adaptable to some laterite soils when extended to include subclassifications for the durability characteristics of gravel and sand and for the plasticity characteristics of the clay and silt fractions of gravels and sands (20). Indeed, the extended unified soil classification system may be applicable to all laterite soils "whose different modes of formation, color, mineralogical composition, etc., do not necessarily mean wide differences in engineering properties beyond the scope of presently used test methods and criteria" (120). The major difficulty in trying to develop a satisfaeto1-y identification system for laterite soils appears tc evolve around the so-called problem laterite soils, which may not yield reproducible test re sults using standard laboratory test procedures and whose engineering behavior may not be accurately predicted on the basis of standard classification tests. No criterion appears to be available for distinguishing problem laterite soils from non-problem ones. Such a system would be a considerable asset to soils engineers in determining the quality and suitability of doubtful late rite soils for construction purposes. This review attempts to propose a criterion for separating probiem and non-problem laterite soils for highway const1~uction based on reported significaJ1t geotochnical prop erties. The term geotechnical properties is used here in a broad sense to include not only index and engineering properties but also such characteristics as sensitivity to drying and remolding, degree of potential swell and self-stabilization, and resistance to degradation during and after construction. This is only a broad grouping that would serve as a basis from which, by expansion and amplification within the original framework, the more detailed and specific classi fication system required for various practical problems may be built up. Publication of this paper sponsored by Committee on !:;011 and Hock Properties. 96 97 IDENTIFICATION BASED ON PEDOLOGICAL INFORMATION Geotechnical studies on laterite soils (118) have revealed that similar combinations of soil-forming processes lead to soils of similar index and engineering properties. In surveys for highway materials and in finding sources of satisfactory fill material for embankment construction, the identification of localities in which pedogenic processes are similar has been found useful (~ ~ g 23). Since laterite soils have developed under the influence of pedogenic factors, these factors have been found useful in re gional identification and classification of laterite soils for engineering purposes (47). Indeed, it has been shown (.!Q, 118) that no system of classification nor any attemptto classify most tropically weathered soils can succeed if it is not based on an apprecia tion of tropical weathering processes. Because similar profiles do develop under similar weathering conditions, a useful means of regional description of laterite soils for highway construction would be to group and classify profiles at least as a first ap proximation on a regional and local basis according to the similarities or differences among the important soil-forming factors (49). Depending on the degree of weathering (i.e., decomposition, laterization, and des sication), laterite materials occur in various textural forms ranging from practically unconsolidated soft clays breakable under finger pressure to hard pedogenic rocks. Empirical concepts of degree of hardness have been used (75), and such terms as "soft" and "hard" laterite rocks may be found in the literature, butsuch qualitative expres sions without quantitative relation to any mechanical properties may be of limited value to engineers engaged in the use of laterite soils for construction. Because of the great variety of laterite materials along with the hardening or softening of these materials with changes in environmental conditions (97), it appears that any identification system based on purely morphological concepts isnot likely to be of much use to the soils engineer. However, an engineering classification system based on degree of weather ing and morphological characteristics has been proposed (68) for laterite soils. "It has been used now for some years with considerable success whereas the temperate climate classification has had to be discarded for tropical soils" (68). A major limitation of the pedological classification system is that one material in this system may have a wide range of properties; the engineer desires a classification system based on significant geotechnical properties that fall within such small ranges that the material can be specified by its classification type. However, once geotechnical characteristics have been established for a pedological soil group, these data may always be applicable to soils formed under similar conditions. IDENTIFICATION BASED ON CHEMICAL AND MINERALOGICAL CHARACTERISTICS Fermor (39) was perhaps the first to propose a comprehensive chemical nomencla ture for laterite soils. Martin and Doyne (76) proposed silica-alumina ratio for clas sifi.cation of laterite soils while others (~ 125) proposed silica-sesquioxide ratio; at tempts were also made to relate chemical property to engineering behavior (84). Lukens (73) summarized available data on the chemical composition of laterite soils and grouped these soils chemically. Some empirical correlations were found between the chemical composition of laterite aggregates and their strength properties. For example, Millard (79) reported a fairly good positive correlation between the iron oxide (Fe203) content andthe strength, and Maignien (75) noted that ferruginous materials are stronger than aluminous ones. Apparently ferruginous late rite aggregates would be preferable to aluminous ones for base course construction because they are harder at the same degree of induration (75). However, chemical data are not always avail able; the determination of even therelatively simple silica sesquioxide ratio requires a chemical analysis not likely to be available to the soils engineer. Experience has also shown that this "ratio is by no means a unique indicator of index properties or engineering characteristics required by the engineer" (92). Moreover, available litera ture on laterite soils (75) would indicate that chemical analyses are often too crude to reveal the origin, nature, and composition of laterite soils. The important minerals identified in laterite soils include goethite, hematite, limonite, 98 chlorite, halloysite (hydrated and dehydrated), kaolinite, and allophane (75). Free alumina was identified in the form of gibbsite, boehmite, and other amorphous forms, and combined silica is frequently present as clay, especially kaolinite or clays of the same group, such as halloysite, mainly in amorphous or subcrystalline forms. Mont morillonite and illite types of clays have not been identified in significant amounts. Clay mineral analysis is not normally employed in the routine examination of soils for engineering purposes. However, in the engineering studies of laterite soils (35) consideration of the clay mineral present has been useful in a number of ways: (if in the characterization of different groups of laterite soils having distinct engineering characteristics (~ g 34); (b) in the formulation of local engineering soils classifica tions, taking into account not only the soil but its environment, which affects both the nature of the soil and the engineering consideration; and (c) in the search for basic understanding of engineering properties of some laterite soils and interpretation of their test results (~ ~ ~ 114, 115). Mineralogical composition of the clay fraction seems to be very important from an engineering point of view and can be used as a means of identification and classification. For example, laterite soils containing high percentages of hydrated halloysite, goethite, or gibbsite are known to be problem laterite soils; those containing montmorillonite and illite may have lower strengths, high construction