Effects of freeze-thaw cycles on the moisture sensitivity of a compacted clay

Highlights

• Freeze-thaw (FT) cycles generate cracks that are tens of microns in dimension.

• FT cycles reduce soils' water retention capacity and mechanical properties.

• Mechanical properties are less sensitive to moisture changes after FT cycles.

• Models were proposed to predict mechanical properties considering FT influences.

Abstract

This study investigates the influence of freeze-thaw (FT) cycles on the (i) microstructure, (ii) soil water characteristics, and (iii) sensitivity of the volumetric strain and mechanical properties to moisture content for a compacted clay. The mechanical properties herein include resilient modulus (M_R), unconfined compression strength (q_u), and reloading modulus (E_1%) and stress (S_u1%) at 1% strain. Specimens were compacted at optimum moisture content and subjected to different FT cycles (i.e. 0, 1, 3, and 10 cycles). Scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) were performed to reveal the evolution of the microstructure during FT cycles. Specimens were then wetted or dried to different suction (s) and degree of saturation (S_r) values to obtain the (i) soil water characteristics using filter paper method and (ii) M_R, q_u, E_1%, and S_u1% using cyclic and static triaxial tests. Experimental results demonstrate that FT cycles induce cracks that are tens of microns in dimension, which reduce specimens' water retention capacity and the volumetric strain upon wetting and drying. Mechanical properties also reduce significantly and become less sensitive to the s and S_r after FT cycles. A model was used to predict the variation of the M_R, q_u, E_1% and S_u1% with s and S_r for specimens subjected to different FT cycles. An attenuation coefficient χ_FT was proposed to describe the FT-induced degradation in the q_u, E_1%, S_u1%, and M_R. It was found that the χ_FT for different mechanical properties are consistent. The evolution of their χ_FT with FT cycles can be described by a uniform empirical equation. The study presented in this paper is useful for the rational understanding and prediction of the hydromechanical behaviors of compacted clay taking account of the influence of FT cycles.

Introduction

The rational design and analysis of pavement structure require comprehensive understanding and reliable determination of the hydromechanical behaviors of the compacted pavement materials (including granular materials and subgrade soils) taking account of the influence of external environmental factors (Brown 1996; Huang 2004). For instance, conventional static elastic modulus and shear strength are required to evaluate the settlement, bearing capacity, and stability of the pavement during the construction and service stages. The resilient modulus (M_R) describes the elasticity of soils under cyclic loading conditions and is defined as the ratio of the cyclic deviator stress to the resilient strain (Seed et al. 1955). The M_R is widely used in the mechanistic pavement design methods for analyzing and predicting the fatigue cracking failure of pavement surfacing during the service stage (Ng et al., 2013). Additionally, the soil-water characteristic curve (SWCC) is needed for simulating the distribution, migration, and storage of water in compacted pavement materials (Vanapalli et al. 1996; Zapata et al. 2007; Zhai and Rahardjo 2015). The volumetric behavior upon moisture content change (e.g. swelling or collapse) should be considered when problematic soils (e.g. expansive or collapsible soils) present in the pavement's subgrade (Seco et al. 2011; Estabragh et al. 2013; Khemissa and Mahamedi 2014).

Wetting-drying (WD) processes are the most common environmental factors acting on pavement structures (Allam and Sridharan 1981; Sobhan and Das 2007; Burton et al. 2015). Over the years, considerable research attention has been directed towards the effects of WD processes on the mechanical and water retention properties of soils (Hunter 1988; Lloret et al. 2003; Zhan and Ng 2006; Peng et al. 2007; Dörner et al. 2009; Little et al. 2009; Al-Zubaydi et al. 2010; Tang et al. 2011a; Aldaood et al. 2014; Stoltz et al. 2014; Puppala et al. 2016). The (i) irreversible volumetric strain (ε_v) (Chen and Ng 2013), (ii) hydrauli hysteresis (Miao et al. 2002; Likos and Lu 2003) and (iii) crack propagation (Al-Zubaydi 2011; Tang et al. 2011b) that take place during WD processes are the major factors influencing hydromechanical behaviors. Several models are available in the literature for predicting stiffness and strength properties and their variation with moisture content. Comprehensive reviews of these models are available in Guan et al., (2010) and Han and Vanapalli (2016a).

In seasonal frozen regions, pavement materials are subjected to at least one post-compaction freeze-thaw (FT) cycle annually in addition to WD processes, which may induce significant reduction in their mechanical properties. It is recognized that FT cycles considerably change the density, structure, and integrity of soils (Fredlund et al. 1975; Konrad 1989; Chamberlain et al. 1990; Viklander 1998; Hohmann-Porebska 2002; Cui et al. 2014). Chamberlain and Gow (1979), Graham and Au (1985), Eigenbrod (1996) and Lu et al. (2018) reported that the volume and hydraulic conductivity of soils increase while soil's dry density and cohesion decrease after FT cycles.

Jong et al. (1998) suggested the annual evolution of pavement capacity, as shown in Fig. 1. The pavement capacity refers to the pavements' ability to resist distress caused by traffic and environmental loadings and can be indicated by the M_R, elastic modulus (E), or unconfined compressive strength (q_u). The capacity of the post-thawing pavement (point A in Fig. 1) can be 50%–60% less than that before FT cycles (point B in Fig. 1), which is confirmed by many different experimental studies (Culley 1971; Johnson et al. 1978; Cole et al. 1986; Yarbasi et al. 2007; Kamei et al. 2012; Güllü and Khudir, 2014; Wang et al. 2015; Liu et al. 2017). Several models were proposed to describe the evolution of mechanical properties during FT cycles for pavement soils (e.g. Lee et al. 1995; Simonsen et al. 2002; Ren and Vanapalli 2017).

Considerable studies have been carried out to investigate the respective effects of WD processes or FT processes on the behaviors of soils. The in-situ FT cycles remarkably change soils' structure by introducing fissures, cracks, or large pores, which in turn influence soils' behaviors upon WD (Fredlund et al. 1975; Chamberlain et al. 1990; Cui et al. 2014). Such influence is especially important for surficial pavement layers in seasonally frozen regions that are subjected to alternative FT and WD processes in different seasons. However, studies focusing on the influence of FT cycles on the key pavement design parameters (such as the M_R and SWCC) of compacted pavement soils during the subsequent WD processes are rarely reported, albeit their apparent significance to the rational pavement design practice.

This paper investigates the effects of FT cycles on the (i) microstructure, (ii) SWCC and (iii) the sensitivity of stiffness, strength and volume to the changes in moisture content and suction (s) for a compacted clay collected from Heilongjiang Province in China, which is a seasonally frozen region. Specimens compacted at the optimum moisture content (w_opt) were firstly subjected to different FT cycles (i.e. 0, 1, 3, and 10 cycles) and then wetted or dried to different moisture contents. They were thereafter subjected to (i) cyclic triaxial tests to determine the M_R, (ii) unconfined compression tests to determine the q_u and elastic modulus (E_1%) and stress (S_u1%) at 1% strain level and (iii) suction measurement using filter paper method. During the WD and FT processes, the volumetric measurement was performed to trace the volumetric behavior while scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) were performed to reveal the microstructural changes. A simple model was employed to predict the variation of the measured stiffness and strength properties (i.e. M_R, q_u, E_1% and S_u1%) with moisture content and s at different FT cycles. A dimensionless attenuation factor, χ_FT, was introduced to describe the degradation of these stiffness and strength properties with FT cycles.