Subgrade layers are an important component of a road pavement; their properties affect the selection of the upper layers’ materials and properties (Austroads, 2008). Subgrade layers with high quality materials minimise the chance of subgrade failure, otherwise however good the quality of the upper layers, the pavement section may still be susceptible to failure (Mokwa and Akin, 2009).
53 2.6.1 Subgrade Soil Characterisation
As demonstrated in sections 2.2.1 and 2.2.2, both methods of pavement design, whether they are empirical or analytical, take into account the characterisation of the subgrade soil. In early pavement designs, the characterisation of the subgrade simply consisted of the classification of the soils; later on strength tests were introduced to characterise the subgrade soil.
Laboratory tests for subgrade soil strength and stiffness characterisation include the California Bearing Ratio (CBR), Resistance value (R-value) and the resilient modulus test. In-situ methodologies also exist and these include: the falling weight deflectometer method, plate load and the dynamic cone penetrometer (Mokwa and Akin, 2009). Subgrade CBR values of more than 10 are necessary for pavement sections which are required to carry anything other than low traffic volumes (Mokwa and Akin, 2009). There is general agreement that subgrade soils with CBR values of less than 10% can deflect excessively under traffic loads, causing the subbase to deflect similarly (Schaefer et al., 2008). More lately analytical pavement design methodologies have moved away from using the CBR to characterise subgrades and specify the use of the resilient modulus instead; this is because the CBR value represents shear strength of the soils, rather than the mechanical properties (Brown et al., 1996). An example is the AASHTO design guide 1986 and its later versions.
2.6.2 Criteria
Most mechanistic methods use vertical strain on the top of the subgrade as a design criterion for limiting permanent deformation (Read and Whiteoak, 2003); MEPDG, 2004; Austroads, 2008). The limiting permanent deformation can be the compressive strain on the top of the subgrade or the rut depth at the surface of the pavement. However, research by Theyse et al. (2006) suggests that total deflection on the top of the subgrade to be a better indicator of
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permanent deformation. Section 7.6 of Chapter Seven presents a detailed discussion of the different performance criteria applied in analytical pavement design procedures.
2.6.3 Moisture in Subgrade
The resilient response of subgrade materials is affected by many factors including, moisture change. Any increase in moisture content tends to result in a decrease in resilient modulus value and a corresponding increase of permanent strain. Further, research by (Frost et al., 2004; Uthus et al., 2006; Soliman et al., 2009) showed that seasonal variation of the moisture in the subgrade affects the resilient modulus value. Increases of moisture resulted in a decrease in the resilient modulus of different types of soils by different percentages; these results are necessary in pavement design procedure. Soils with more than 15% fine fraction are most complex fill as their behaviour significantly is affected by water content before and after construction (Nowak and Gilbert, 2015). According to Nowak and Gilbert (2015) a good management of surface water and ground water is essential for earth fill as the water affects significantly the strength of the soils; especially some types of soils such as silts, which are susceptible to water. Section 4.2.1 of Chapter Four discusses the effect of moisture on the resilient modulus value in detail. The rate of this seasonal moisture change is controlled by the type of the subgrade soil; for example sandy soils reach a wet condition faster than clay soils (Austroads, 2008). Permanent deformation parameters are also affected by variation of moisture content; such that, for the same stress level at a range of moisture contents, the permanent deformation development changes (Lekarp et al., 2000a). Werkmeister et al. (2003) found this difference for Huurman-model (Huurman, 1996) parameters at two moisture contents for one unbound granular base course material.
The MEPDG (2004) uses a sophisticated model: the Enhanced Integrated Climatic Model (EICM), for the impact of climatic condition on pavement and subgrade materials in
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mechanistic-empirical pavement design. Regardless of what level of data input (level 1, 2 and 3) are applied, the MEPDG (2004) incorporates the effect of moisture and temperature in the design procedure. An adjustment factor for adjusting the resilient modulus value is determined from the EICM model, which considers the moisture, suction and temperature factors; see Equation 2.45.
𝑀𝑅 = 𝐹𝑒𝑛𝑣 𝑀𝑅𝑜𝑝𝑡 (2.45)
Where MR is the adjusted resilient modulus value considering environmental conditions; Fenv is the adjustment factor; and MRopt is the resilient modulus value at optimum moisture content.
Then the adjustment factor is determined from Equation 2.46.
𝑙𝑜𝑔(𝐹𝑒𝑛𝑣) = 𝑙𝑜𝑔 𝑀𝑅
𝑀𝑅𝑜𝑝𝑡 = 𝑎 +
𝑏−𝑎
1+𝐸𝑋𝑃(𝑙𝑛−𝑏𝑎+𝐾𝑚 (𝑆−𝑆𝑜𝑝𝑡))
(2.46)
Where a is the minimum of 𝑀𝑅
𝑀𝑅𝑜𝑝𝑡; b is maximum of 𝑙𝑜𝑔 𝑀𝑅
𝑀𝑅𝑜𝑝𝑡; Km is the regression parameter;
and (S-Sopt) is the variation in degree of saturation in decimal.
Gupta et al. (2007) suggested two models to include the effect of soil suction on resilient modulus implicitly and explicitly (Equations 2.47 and 2.48). The models are given by the following: (𝑀𝑟)𝑢𝑠 = 𝐾1𝑃𝑎(𝜎𝑃𝑏 𝑎) 𝑘2 (𝜏𝑜𝑐𝑡 𝑃𝑎 + 1) 𝑘3 + 𝑘𝑢𝑠𝑃𝑎𝛩𝑘(𝑢 𝑎− 𝑢𝑤) (2.47) (𝑀𝑟)𝑢𝑠 = [𝐾1𝑃𝑎(𝜎𝑏−3𝑘6 𝑃𝑎 ) 𝑘2 (𝜏𝑜𝑐𝑡 𝑃𝑎 + 𝑘7) 𝑘3 ] + 𝛼1(𝑢𝑎− 𝑢𝑤)𝛽1 (2.48) Where:
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𝑃𝑎 = atmospheric air pressure 𝜎𝑏 = bulk stress = 𝜎1+ 𝜎2+ 𝜎3
𝜏𝑜𝑐𝑡 = octahedral shear stress = √23 (𝜎1− 𝜎3) for 𝜎2 = 𝜎3 𝑘𝑢𝑠 = constant, linear fitting coefficient
𝛩 = 𝜃𝜃
𝑠 = normalized water content; θ is the volumetric water content; 𝜃𝑠 is saturated
volumetric water content k = fitting parameter 𝑢𝑎− 𝑢𝑤 = soil suction
𝛼1 and 𝛽1 are intercept and slope of the (Mr)us at a given 𝜎𝑏 and 𝜏𝑜𝑐𝑡 versus suction relationship
K1, K2, K3, K6, K7 are regression parameters.
The sources of moisture in a subgrade and pavement system can be one of or a combination of: the seepage from high ground, pavement edge, surface discontinuities and capillary action or vapour movement from high water table (Christopher et al., 2006).
To control or minimise the effect of moisture in a pavement system one of the following approaches may become necessary (Christopher et al., 2006):
Prevent moisture from entering the pavement system; this might be achieved by sufficient
cross and longitudinal slopes to speed up the surface runoff of water and sealing surface cracks to prevent infiltration of water.
Use materials that are insensitive to the effects of moisture; cement and asphalt stabilised
soils for example.
Quickly remove the moisture that enters the pavement system; there are drainage features that
are used to remove moisture from a pavement system such as under drains and ditches. Using stabilisers such as cement can help to ensure the stability of foundation layers in case of the ingress of water into the subgrade from different sources. This research focuses on the second approach in which a variety of agents or additives can be mixed with the soil to increase its resistance to permanent deformation.
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Summary
In this section the importance of the subgrade soils was demonstrated. The compressive strain on the top of the subgrade is used extensively in analytical pavement design procedures therefore the characterisation of the subgrade soils is fundamental in pavement design. Afterwards the effect of the moisture on the behaviour of the subgrade soils was explained and discussed and models that determine the resilient modulus values at different moisture contents were presented. The degree of saturation as discussed in Chapter Four has an effect on the resilient modulus value, this has been verified from experimental work in this research and an equation from the literature was selected for the purpose of the design.