Model tests of the barrier measures on moisture and salt migration in soils subjected to freeze-thaw cycles

Highlights

• Model tests of moisture and salt corrosion barriers to the buried pipeline were conducted under freeze-thaw cycles.

• The concentration variations of barrier models are elucidated by convection, salt expulsion and diffusion, respectively.

• At the scene, composite measures taking consideration into the freezing front and water table were recommended.

Abstract

The migration and accumulation of chlorine salt in frozen soils will cause corrosion to the buried pipeline and seriously affect its service life. In order to investigate the effect of moisture and salt migration and accumulation on pipeline corrosion, three models of barrier measures were established using similar criteria, namely, non-saline soil replacement model (M1), non-saline soil replacement with impermeable geotextile barrier model (M2), and gravel replacement model (M3). Laboratory model tests were carried out to study the characteristics of moisture and salt migration and the barrier effect of different measures in response to freeze-thaw (F-T) cycles. The freezing depth is 38.6, 29.7, and 43.5 cm in three models. The volumetric moisture contents and salt concentrations are positively and negatively correlated with temperature, respectively. In M1, the maximum moisture contents decrease and increase obviously inside and outside of the upper boundary, with the variation rate of 11.7% and 11.1%. The maximum salt concentrations decrease by 7.7% and increase by 1.31 times, respectively. The concentration variations of the lower boundary are not obvious. They remain essentially constant in M2, i.e., 0.323 and 0.039 mol kg⁻¹. The salt concentration in M3 has a far less reduction rate of 4.0%. Impermeable geotextile barrier and gravel replacement can prevent moisture and salt migration and corrosion to the pipeline. Concentration variation of M1 is primarily influenced by convection, salt expulsion and diffusion, while that of M2 outside the upper boundary is only influenced by convection, and M3 is caused by convection and salt expulsion. A combination of M3 and M2 measures are required when the buried pipeline is located above the freezing front and the water table is shallow. In other cases, M2 measures are chosen. The results can provide theoretical support and technical guidance for more economical and efficient construction and safe operation of the buried pipeline.

Introduction

Saline soils are widely distributed in China's northwest, northeast, and north (Fig. 1a). There are numerous salt species and a large potential for industrial and mining development in the Qaidam Basin region of northwest China, but these are hampered by regional water scarcity and low utilization efficiency. The aforementioned issues can be addressed by constructing a long-distance water transmission project from the Nalinggele River to the towns of Mangya and Lenghu, with the total length of 364 km (Fig. 1b). Nonetheless, a large quantity of chlorine saline soils with high salinity are distributed in conjunction with the project (You et al., 2020). Salt migrates upward and downward with moisture under the influence of diurnal temperature difference and seasonal freezing and thawing, causing serious frost heave, salt expansion, and corrosion to the overlying buried water pipelines.

(In Fig. 1 (a), 1-Coastal saline soil, 2-Saline soil in northeast, 3-Saline soil in Huang-Huai-Hai Plain, 4-Saline soil in Inner Mongolia Plateau, 5-Saline soil in Yellow River Basin, 6-Saline soil in Gansu, Xinjiang, 7-Saline soil in extreme arid region, 8-Saline soil in high latitude and cold regions.)

The high chloride ions concentration is critical for metal corrosion and damage (Tuutti and Service, 1985). The chloride ions migrate to the oxide passive film between metal and oxide and accelerate its dissolution, causing pores to form and pitting damage to occur (Fig. 2). As chloride ions accumulate around the pipeline, the salt concentration and its corrosion capacity increase, resulting in structural integrity loss and a severe reduction in service life. It is an effective method of preventing moisture and salt migration to the pipeline's periphery and thus avoiding corrosion damage. Laboratory model tests with various anti-saline measures are necessary to clarify the characteristics of moisture and salt migration in soil during freeze-thaw (F-T) cycles, and comparing the blocking effect of different measures can provide theoretical support and a reference value for the selection of cost-effective and effective treatment measures on site.

Arid environments have been the subject of numerous studies on moisture and salt migration in soils (Wang et al., 2018; Yin et al., 2021). The impact of temperature and salinity on soil characteristics has gotten a lot of press (Chen et al., 2019). However, in reaction to F-T cycles, particularly freezing conditions, it is a complex dynamic process. In freezing soils, there are various types of migration, including convection, salt diffusion, and expulsion, among others.

Currently, the properties of moisture and salt migration under freezing conditions have been validated by massive amounts of laboratory experiments and theoretical analysis. In response to a temperature gradient, moisture and salt convectively migrate from the warm to the cold end of soils (Qiu et al., 1986; Kelleners, 2020). Convection is the primary driver of moisture migration, whereas both convection and diffusion influence salt migration (Liu et al., 2021). Salt migration is more noticeable in the downward freezing phase than in the upward freezing process (Baker and Osterkamp, 1989). Salt diffusion migration is also triggered by a spatial concentration difference, and salt diffuses from the frozen to the unfrozen zone (Stähli and Stadler, 1997; Bing and He, 2011; Ireson et al., 2013; Chen et al., 2021). As the temperature rises, salt may only slowly diffuse back into the mobile water through larger pores, resulting in a less concentrated salt solution draining from macropores (Larsson et al., 1999; Holten et al., 2018). Furthermore, salt expulsion will occur in freezing soils, with 90% of salt being expelled with a cooling rate of <0.1 °C d⁻¹ (Konard and Mccammon, 1990). The rate of freezing is inversely related to the amount of salt ejected (Kadlec et al., 1988; Baker and Osterkamp, 1989). To describe the fraction of expelled salt near the freezing front, the salt expulsion coefficient was devised (Zukowski and Tumeo, 1991).

For sodium sulfate saline soils, phase change kinetics was taken into account to analyze the migration characteristics of the coupled moisture, salt, and heat, and salts mainly collect along the freezing front, with discontinuous stratified salt crystal areas inside the soils (Koniorczyk and Gawin, 2011; Zhang et al., 2020a, Zhang et al., 2020b; Xu et al., 2020).

Previous works have used the micro model to confirm the microscopic mechanism of salt migration (Galley et al., 2015; Dedovets et al., 2018). Short fingers and long streamers in the desalination of the expanding ice layer can be seen in the internal and interfacial convective processes of salt (Middleton et al., 2016). Salt diffusion and expulsion were the main mechanisms for the movement of salt-rich inclusions in saturated porous media in response to freezing (Chen et al., 2021).

Only a few investigations of moisture and salt migration in soils during F-T cycles have been conducted, and only a few of these were carried out in large-scale model tests or in the field. Moisture and salt migration in soils during freezing and thawing is a complex problem involving the thermodynamics of the soil-ice-salt-water-matrix and phase changes, which can cause severe damage to soils and structures such as frost heave, thawing settlement, salt expansion, and collapse (Xu et al., 1999).

A series of actively cooling techniques have been proved to be effective to keep the thermal stability of embankment, crushed-rock revetment (Lai et al., 2006; Liu et al., 2017), duct-ventilated embankment (Zhang et al., 2006; Qian et al., 2016), and thermosyphons (Wen et al., 2005; Ma et al., 2012), etc. Chemical and physical methods are used to prevent salt swelling in saline soil subgrades (Ding and Chen, 1992; Chen et al., 2006). The chemical method primarily involves adding chloride to the saline soil to convert it into non-saline soil, but it is not suitable for large areas of saline soil due to need to strictly control the additive content. Physical methods include replacement by non-saline soil and laying of partition layers, which include gravel, aeolian sand, and geotextiles (Chen, 2008). The gravel barrier layer is appropriate for strong and over-saline soil sections, but the rise of capillary water and fine particle content must be controlled. Water and salt migration can be effectively blocked by aeolian sand. Secondary salinization, on the other hand, occurs above the aeolian sand barrier in the strong and over-saline soil sections due to significant capillary water action. The impermeable geotextile partition layer can effectively prevent secondary salinization of the soil (Wang et al., 2003), but it is necessary to prevent the filler from puncturing it. Laboratory model tests have validated the effectiveness of various barrier measures, such as crushed-rock layer, aeolian sand, impermeable geotextile, and composite forms, in preventing moisture and salt migration to the upper soil layer and avoiding longitudinal cracks in saline soil subgrades (Ma et al., 2017a, Ma et al., 2017b).

The above research about the migration characteristics of moisture and salt mainly concentrated on the soil surface, and the model tests of prevention measures from frost heave and salt expansion mainly focus on roadbeds. However, the migration mechanism of moisture and salt around the pipeline in buried soil was not involved, especially how does the migration and enrichment of moisture and salt affect the pipeline after barrier measures are taken. As a result, research on the characteristics of moisture and salt migration around the pipeline, as well as the barrier effect of various measures, is required. Long-distance buried water transmission pipelines for the saline soil area in Qaidam Basin are mainly plagued by corrosion issues caused by salt migration and accumulation. The leakage of pipelines caused by salt corrosion will exacerbate the subsidence of the foundation. Consequently, the physical methods of treating saline soil foundations are adopted, taking into account the economics of various treatment methods. Non-saline soil and gravel were chosen as replacement measures, impermeable geotextile is laid as the partition layer between the non-saline soil and saline soil, and permeable geotextile is set as a control to impermeable geotextile and can separate two different soils.

As a result, three saline soil foundation treatment models were developed in this study, namely, non-saline soil replacement and permeable geotextile model (M1), non-saline soil replacement and impermeable geotextile barrier model (M2), and gravel replacement and permeable geotextile model (M3). The indoor model tests were carried out using the similarity principle to study the migration characteristics of moisture and salt under F-T cycles, and the barrier effect of different models was compared. The main factors of the increase in salt concentration around the pipeline are analyzed in conjunction with the salt migration mode and the variation of the cooling rate. Finally, recommendations for the selection of barrier measures appropriate for the site were made, providing a theoretical foundation and technical support for the safe operation of a long-distance buried water transmission project.