Evaluation of the long-term effect of lime treatment on a silty soil embankment after seven years of atmospheric exposure: Mechanical, physicochemical, and microstructural studies

Highlights

• Effect of lime treatment was studied on a 7-year cured silty soil embankment.

• Average strength level of 3.29 MPa resulted due to cementitious bonding formation.

• Uniform moisture distribution and pH > 11 implied the presence of lime effect in the core.

• Close to the surface, atmospheric exposures caused loss of water content and pH.

• Formation of mesopores contributes to the rise of strength and specific surface area.

Abstract

The long-term effect of lime treatment was evaluated on a 2.5% lime-treated experimental embankment after seven years of atmospheric exposure. The evaluation was done by comparison of (i) the mechanical performance of the field sampled specimens with laboratory cured specimens, and (ii) the physicochemical and microstructural properties of the samples from the lime-treated embankment with specimens obtained from an untreated embankment constructed near to it as a reference embankment.

An average Unconfined Compressive Strength (UCS) level of 3.29 MPa was measured in the lime-treated specimens sampled from the core of the embankment. This UCS level was found to be comparable to the UCS of accelerated-cured specimens obtained at a laboratory scale. Thus, such levels of UCS can be expected after long-term in-situ curing. Scanning electron microscope images evidenced the contribution of the formation of cementitious bonding towards such UCS evolution in the lime-treated specimen. The persistence of the lime effect within the core of the embankment was confirmed by the presence of a pH greater than 11. However, a relative decrease in the pH and water content was observed in the upper layer compared to the core of the lime-treated embankment. This indicates that the effect of lime was lost in the upper layer under constant soil-atmosphere interaction and due to the development of vegetation roots. Pore structure observations made by Mercury Intrusion Porosimetry (MIP) and Barrett-Joiner-Halendapore (BJH) methods highlight the formation of smaller pores (diameter < 3000 Å) under lime effect. These smaller pores have contributed towards the evolution of suction in the core-sampled specimens of the lime-treated embankment. This has led to the long-term water retention capacity of the lime-treated soil. BJH was able to detect mesopore-formation (25–500 Å) under the lime effect in a more precise manner compared to MIP. The evolution of mesopores was found to be coincident with the development of strength and specific surface area of the lime-treated soil.

Introduction

Management of natural resources is a critical challenge in any land development project, especially for projects related to earthworks. A cost-effective way to conserve natural resources is to use soil located directly in the land reserved for the project. This makes it essential to improve the engineering properties of the available soil. In this regard, soil improvement by lime is known to be an efficient and economical technique that leads to improved bearing capacity, strength, modulus, etc. (Al-Mukhtar et al. 2012; Bell 1996; Diamond and Kinter 1965; Little 1995; Osula 1996). Additionally, the lime-treated soil structure is also known to be eco-friendly as the material can be entirely reused after the deconstruction of the structure (Hopkins et al. 2007).

Most studies conducted on soil improvement by lime treatment were obtained from laboratory test results (Ali and Mohamed 2019; Lemaire et al. 2013; Verbrugge et al. 2011) while the feedback from field performance is less investigated. It is worth noting that the conditions faced by the lime-treated soil under the laboratory- and field-testing environments are relatively different.

Several studies have reported long-term strength improvement in pavement layers stabilized with lime (Aufmuth 1970; Cardoso and das Neves, 2012; Little 1995; McDonald 1969). Aufmuth (1970) concluded that the in-situ California Bearing Ratio (CBR) value observed in lime stabilized pavement layers significantly rises with age and tends to appear permanent if compared to the CBR value of the same soil without stabilization. McDonald (1969) confirmed the effectiveness of lime treatment through a study made on pavements subjected to low, medium, and heavy traffics flow after about 13 years from construction. This was in terms of improved smoothness or rideability and better structural response as indicated by deflection measurements. Cardoso and Neves (2012) demonstrated the effectiveness of lime in reducing the overall settlement rate of an embankment built with marls. They showed how lime treatment induced a decrease in the secondary consolidation of the marls with increased curing, reduced the swelling potential, and increased its stiffness.

So far, few studies have reported the behaviour of lime-treated specimens sampled from real or experimental earth structures submitted to long-term environmental exposure. Rosone et al. (2018) studied the effect of seasonal wetting and drying cycles for over 18 months on specimens obtained from lime-treated expansive clay soil embankment. The suction value measured at the top layer of the embankment was reported to be three times higher than that measured at 0.45 m depth from the surface. Below 0.45 m, the total suction value stabilized at about 1.40 MPa. The high value of suction measured on the top of the embankment was attributed to evapotranspiration, leading to water loss, a rise of suction, and crack development.

Bicalho et al. (2018) described a similar observation with respect to the rise in suction and water content loss in specimens sampled up to a depth of 0.75 m from the surface of a 2% lime-treated silty clay experimental embankment due to climatic variation. However, with increased curing time, this impact of seasonal variation in the variability of water content and soil suction was minimised, thus indicating the good stability of the lime-treated soil.

The Friant-Kern Canal construction in California, United States, was based on 4% quicklime by weight to stabilize a highly plastic clay soil in permanent contact with water (Herrier et al. 2012; Knodel 1987). More than 40 years after construction, the study evidenced the increased long-term strength, reduction in swelling and shrinkage potential, as well as significant resistance to erosion, thus showing the improved geo-mechanical stability of the structure (Akula et al. 2020).

All these studies have demonstrated the effectiveness of lime treatment over time and, to an extent, the effect of the soil atmosphere interaction on the modification of properties of lime-treated soil. Beyond the general behaviour mentioned herein, more investigation is needed with respect to strength and microstructural modifications induced by long-term lime treatment on structures cured in the open atmosphere.

The purpose of this study was to thoroughly evaluate the influence of lime treatment in an atmospherically cured lime-treated embankment after seven years of exposure to wet environmental conditions. The climatic condition of the construction region can be referred to in Makki-Szymkiewicz et al. (2015), who investigated the embankment at an early age (up to 1 year from construction) to assess its hydraulic performance. They demonstrated that if proper mixing and compaction process is maintained during construction, a permeability of 10⁻⁹ m/s can be obtained, which was similar to the untreated soil. The present study demonstrates an extended investigation regarding the effectiveness of lime treatment in the long-term. The examination of the lime-treated structure was made in terms of strength, physicochemical characteristics, and microstructure, which was lacking in the studies reported above.

The first part of the study focuses on (i) evaluating the UCS of the lime-treated soil, (ii) examining the presence of cementitious bonding within the fabric of the lime-treated soil, and (iii) investigating the long-term effect of lime treatment on water content, suction, and pH. The second part presents the effect of lime treatment on pore structure and specific surface area modifications.