Assessing Failure Envelopes of Soil–Fly Ash–Lime Blends Nilo Cesar Consoli, Ph.D. 1 ; Lucas Festugato 2 ; Bernardo Scapini Consoli 3 ; and Luizmar da Silva Lopes Jr. 4 Abstract: This study aims to establish the influence of curing time and the amount of lime and porosity in the assessment of the Mohr-Coulomb failure envelope of fly ash–lime-treated soils based on unconfined compressive strength (σ c ) and splitting tensile strength (σ t ) of such materials. Founded on the concept that the σ t =σ c relationship is unique for each specific fine-grained soil, fly ash, and lime blends, it is shown that the angle of shearing resistance of a given lime-treated soil is independent of the porosity and the amount of lime of the specimen and that cohesion intercept is a direct function of σ c (or σ t ) of the improved soil, which depends of the porosity and volu- metric amount of lime of the soil–fly ash–lime blends. Finally, the concepts are tested with success for a sandy soil treated with fly ash and lime at distinct curing time periods, considering moderate to strong cementation levels. DOI: 10.1061/(ASCE)MT.1943-5533.0001134. © 2014 American Society of Civil Engineers. Author keywords: Fly ash; Lime; Failure envelope; Shear strength; Soil stabilization. Introduction The improved characteristics of compacted soils resulting from fly ash–lime utilization may be very important to some infrastructure projects such as foundations and subgrades of roads and railways platforms, engineered fills, and in mitigating liquefaction and expansibility problems (e.g., Brown 1996; Thomé et al. 2005; Buhler and Cerato 2007; Ghosh and Subbarao 2007; Kumar et al. 2007; Daniels and Das 2008; Consoli et al. 2009a; Rao and Asha 2012). Furthermore, the development of alternatives for reusing in- dustrial by-products (e.g., fly ash) mostly brings environmental and economical benefits. Materials such as fly ash, a by-product of coal combustion in thermal power plants, are profusely produced in southern Brazil. However, they have been rarely used for engineer- ing purposes, an overwhelming majority of them being placed in storage or disposal sites. Several methodologies were established in the last decades (e.g., Rogers et al. 1997) to determine the needed amounts of lime required for modification of soil characteristics. Such methodologies intend to establish a threshold value, supposed to chemically satisfy the soil demand for lime, which has been often suggested as the starting content to adopt for construction expediency purposes. Consoli et al. (2011) developed a dosage methodology for the assessment of a target soil–fly ash–lime unconfined compressive strength, where the porosity/lime ratio (η=L iv ) plays a fundamental role. Consoli et al. (2008, 2009a) have shown that the system failure mechanism of shallow foundations bearing on top of cemented soil–fly ash–lime layers usually start up under tensile stresses at the base of the improved layer. This indicates that the tensile strength (σ t ) can be used as a direct mea- sure of the mechanical strength of soil–fly ash–lime blends when used as compacted layers. Consoli et al. (2014a) studied the effect of distinct variables (e.g., lime content, porosity, curing period, and curing temperature) on the splitting tensile strength of soil–fly ash–lime blends. Consoli et al. (2014b) has shown that the splitting tensile strength/unconfined compressive strength ratio is a constant for soil–fly ash–lime blends, varying in porosity, in the amount of lime, and in curing temperature. Consoli et al. (2009b) has shown that the cohesion intercept and the friction angle of artificially ce- mented soils are linked to the porosity/cementitious material ratio of the artificially cemented blends. However, the determination of failure envelope parameters of fly ash–lime-treated soils tradition- ally requires carrying out triaxial tests (e.g., Consoli et al. 2001) among many other complex and time-consuming tests. An alterna- tive methodology to assess linear failure envelope parameters of fly ash–lime-treated soils is suggested in the present work. The con- cept is to carry out basic tests [such as unconfined compression and splitting tensile tests whose equipment (loading machine and prov- ing rings) can be found even under slight laboratory facilities] on sandy soil–fly ash–lime mixtures to determine (based on an alter- native analytical methodology) the cohesion intercept (c) and the angle of shearing resistance (ϕ) under similar conditions of porosity and amount of lime. The methodology was successfully tested for a sandy soil treated with fly ash and hydrated lime at distinct curing time periods, considering moderate to strong cementation levels. Methodology Proposed As presented in Fig. 1, in the Mohr-Coulomb failure theory, the shear strength (τ ) of a given material is assumed to vary linearly 1 Professor, Dept. of Civil Engineering, Federal Univ. of Rio Grande do Sul, Ave. Osvaldo Aranha 99–3 andar, Porto Alegre, RS 90035-190, Brazil (corresponding author). E-mail: consoli@ufrgs.br 2 Assistant Professor, Dept. of Civil Engineering, Federal Univ. of Rio Grande do Sul, Ave. Osvaldo Aranha 99–3 andar, Porto Alegre, RS 90035-190, Brazil. E-mail: lucas@ufrgs.br 3 Research Assistant, Dept. of Computer Science, Catholic Univ. of Rio Grande do Sul, Ave. Ipiranga 6681, Porto Alegre, RS 90619-900, Brazil. E-mail: beconsoli@hotmail.com 4 Director, Multisolos Piles and InSitu Testing, Rua Bento Gonçalves 578/903, Passo Fundo, RS 99010-010, Brazil; formerly, D.Sc. Student at Dept. of Civil Engineering, Federal Univ. of Rio Grande do Sul. E-mail: luizmar_junior@yahoo.com.br Note. This manuscript was submitted on September 18, 2013; approved on May 21, 2014; published online on August 11, 2014. Discussion period open until January 11, 2015; separate discussions must be submitted for individual papers. This paper is part of the Journal of Materials in Civil Engineering, © ASCE, ISSN 0899-1561/04014174(8)/$25.00. © ASCE 04014174-1 J. Mater. Civ. Eng.