CANAL SEGARRA-GARRIGUES

Pertinent background information and findings from previous studies related to the selected technologies are summarized in the following sections.

4.3.1. Macadam stone base layers

Macadam stone base (MSB) layers with large maximum aggregate particle sizes of 75 or 100 mm have been evaluated in several research projects (e.g., Hoover et al., 1981; Jobgen et al., 1994; Less and Paulson, 1977; Lynam and Jones, 1979). The presence of large voids and improved particle interlocking between the large aggregates were believed to help minimize

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freeze-thaw related damage. Visual inspections from 1988 to 1992 showed that MSB layers exhibited the best overall performance and durability compared to asphalt- or biochemical- treated base layers (Jobgen et al., 1994). Less and Paulson (1977) reported that a 200 mm thick MSB layer was the most cost-effective design in Iowa. It was noted in those studies that

construction of the MSB layers was relatively simple and fast, but the MSB layers needed to be constructed on either a prepared subgrade or an existing unpaved road surface to prevent subgrade intrusions. However, most of the reported findings in the previous studies were qualitative, and very little quantitative information is available to date in terms of improvement in the mechanistic properties when using MSB layers.

4.3.2. Chemical stabilizations

White and Vennapusa (2013) provided a review of over 70 technical articles that summarized various chemical stabilizers in the contexts of freeze-thaw durability and guidelines for design and construction. They studied differences in the mechanisms and performance of active and passive chemical stabilizers. Commonly used active chemical admixtures include Portland cement, fly ash, lime, and bentonite, whereas passive chemical admixtures include bitumen, plant processed bio-fuel co-products with varying lignin contents and lignosulfates, and polymer emulsions. In the present study, Portland cement, ASTM Class C self-cementing fly ash (ASTM, 2012), and bentonite were used in the demonstration sections, and their relative performance will be presented herein.

Stabilization by Portland cement and ASTM class C self-cementing fly ash can improve the shear strength, stiffness, and wet-dry and freeze-thaw durability of soils (e.g., Cetin et al., 2010; Johnson, 2012; Parsons and Milburn, 2003; Shoop et al., 2003; Solanki et al., 2013; White et al., 2005a; White et al., 2005b; Zhang et al., 2016). Guidance on selection of chemical stabilizers

based on soil classification and plasticity properties is provided by Chu et al. (1955) and Terrel et al. (1979). The use of self-cementing fly ash for soil stabilization provides environmental

benefits in terms of recycling a waste product, and cost savings relative to other chemical stabilizers. However, the physical properties of fly ash vary significantly between plants, which therefore warrants a detailed laboratory mix design and evaluation to ensure that soil

stabilization is effective (White et al., 2005a; White et al., 2005b).

The freeze-thaw durability of chemically stabilized materials has been extensively studied in laboratory settings, typically by measuring material loss during freeze thaw cycles and/or

unconfined compressive strength/California bearing ratio (CBR) tests following a certain number of freeze thaw cycles. In such studies, Portland cement stabilized materials generally exhibit superior performance relative to other chemical stabilizers (e.g., Henry et al., 2005; Parsons and Milburn, 2003); while observations related to performance of fly ash-stabilized soils are mixed. For instance, Bin-Shafique et al. (2010) reported that fly ash stabilized soils lost up to 40% of their strength due to freeze-thaw cycles, but did not experience significant strength loss during wet-dry cycles. Berg (1998) studied the laboratory freeze-thaw performance of reclaimed hydrated fly ash-activated aggregate materials, and found that they did not survive beyond ten freeze-thaw cycles. However, other studies documented that these materials did perform well, even though they break down under freeze-thaw action (e.g., Li et al., 2008; Parsons and

Milburn, 2003; White et al., 2005b). Khoury and Zaman (2007) investigated the effect of freeze- thaw cycles on aggregates stabilized with cement kiln dust (CKD), class C fly ash, and fluidized bed ash (FBA). Their results indicated that the resilient moduli of the various mixtures decreased with increasing freeze-thaw cycles. Comparisons with control specimens were not provided in

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their study, but it was reported that CKD-stabilized base materials deteriorated faster than fly ash- and FBA-stabilized base materials.

Bergeson and Wahbeh (1990) and Bergeson et al. (1995) documented the use of bentonite (sodium montmorillonite clay) surface treatments as a means for dust reduction of gravel roads in comparison with calcium and magnesium chloride. They noted that the negatively charged surfaces of montmorillonite particles interact with positively charged limestone fines, forming an "electrochemical glue" that can effectively reduce dust and improve the slaking characteristics and stability of limestone-surfaced roads. Bergeson et al. (1995) concluded that calcium chloride treatments are 2 to 3 times more effective than bentonite in the short term, but bentonite is more cost-effective because its bonding capability can last much longer (23 winter seasons) than chloride treatments (34 months).

4.3.3. Geosynthetics

Geotextiles and geogrids have previously been evaluated for mechanically improving the freeze-thaw performance of unpaved roads (e.g., Henry, 1990; Henry, 1996; Hoover et al., 1981; Lai et al., 2012). The geosynthetics are usually placed between the subgrade and base layers to provide separation, reinforcement, and subsurface drainage. Hoover et al. (1981) conducted laboratory freeze-thaw tests and concluded that specimens with an embedded geotextile disc showed lower frost-heave rates and greater cohesion and friction angle values, but decreased stiffnesses relative to control specimens. Based on field experiments, Henry (1990) reported that geotextiles used as capillary barriers can reduce the occurrence of frost heaves by approximately 60%. Henry (1996) also indicated that the performance of geotextiles in reducing frost heave rates depends on the geotextile’s pore size distribution, wettability, and thickness.

Geocomposite materials consist of two geotextile layers and a drainage net, and are typically used as capillary barriers to prevent surface materials from becoming saturated (Christopher et al., 2000; Holtz et al., 2008). Henry et al. (2005) conducted a field investigation and showed that a geocomposite drainage layer can keep upper layers of unpaved roads relatively dry, and

accelerate strength recovery of the systems. Christopher et al.(2000) studied locations for installing geocomposite drainage layers and concluded that layers placed on or within the subgrade were quickest at removing water during spring thawing. Henry and Holtz (2001) also found significant reductions in frost heave when the overlying soil had a degree of saturation below 75%, but the geocomposite could not prevent heave when the degree of saturation exceeded 80%, due to water migrating through a film adhered to the middle geonet layer.