Mechanisms of longitudinal cracks along pavement shoulder in ...

In this paper, unsaturated soil mechanics will beused to explain such mechanism of cracks alonghighway shoulder, involving shrink-swell clay subgrade,in central Thailand. Influence of drying oninitiation of cracks will be explored based on seepageand deformation analyses. The fully saturatedand unsaturated shear strength parameters are usedto determine the stability of slope which exacerbatedby water infiltration, and strength softening.Table __________________________________________________1. Summary of subgrade properties at the test site T1Specific LL PL PI Wn γ d SPTGravity, % % % % t/m 3 Blow/ftGs __________________________________________________2.71 43-53 17-21 26-34 20-27 1.49- 101.69LL = Liquid limit, PL = Plastic Limit, PI = Plasticity Index,Wn = Natural water content, sampled on 13-16 August 2009(during rainy season), γ d = Dry density, SPT (uncorrected)2 TEST SITE2.1 Highway route 357Highway route no. 357 (Suparnburi by-pass) locatedin Suparnburi province, central region of Thailandwas selected as the test site where severe longitudinalcracking along asphalt pavement shoulder wasencountered prior to trafficking. The test sites consistedof two adjacent sections: one section havingsevere cracks (sta. 32+600 to sta. 33+100) designatedas T-1 and another having minor cracks (sta.31+600 to sta.32+100) designated as T-2. This paperhowever focuses on the section T-1 only.2.2 Field investigation and basic propertiesAn extensive field investigation programme (includingborehole/test pits sampling, field vane shear,SPT, Kunzelstab penetration tests) revealed that thefoundation soil consists of stiff clay and very stiffclay to the depth of 5.5 metres below which existsthe medium-dense to dense sand. The subgrade materialis classified as a medium-to-high plasticityclay (CL-CH) with thickness of about 2m. Its basicproperties are summarized in Table 1. The soil mapfrom Department of Land Development (used foragricultural purpose) also indicated that Vertisol existedin shallow depth around the vicinity of thehighway. The depth of cracks at Site T1 (Figure 1)appeared to be at least 48 cm and the longitudinallength varied from 2.3 to 90 metres.The maximum dry density, γ dmax , from modifiedProctor tests was 1.74 t/m 3 and the optimum moisturecontent, OMC, was 13.3%. Therefore, the percentcompaction is 86-97% and the soil, especiallyclose to surface, must have wetted up from its ascompactedstate and perhaps have expanded. Detailedsounding tests using Kunzelstab light weightpenetrometer along the slope of embankment indicatesthat the soft zone ran sub-parallel to the slopeand down to depth of 0.5 to 1 metre, as shown inFigure 3. The cracks thus seemed to be associatedwith shallow movement.0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.50-0.5-1-1.5Distance, m-2Depth, m-2.5-3-3.5-4-4.5-5Figure 2. A typical cross-section of the test section.This highway consists of two carriage lanes ineach direction. The carriage lanes were constructedusing the concrete pavement, while the shoulderswere constructed using the asphalt pavement. Thewidths of inner and outer shoulders are 1.5 and 2.5m, respectively. The side slope is about 2 horizontalto 1 vertical (2:1) and covered with grass on bothsides of the slopes. Figure 2 illustrates a typicalcross-section of the test section.-5.50 5 10 15 20 25 30 35 40 45 50Figure 3. Blow counts (per 20cm) of Kunzelstab tests alongembankment slope (a light weight dynamic penetrometer, withweight of 10kg, and a falling height of 0.5metre)Pore-water pressure distribution in the slope wasmonitored at five locations along the slope cross sectionusing Kasetsart University (KU)-made tensiometersas installed by Jotisankasa et al. (2010)approach. Figure 4 shows the pore water pressureand total hydraulic head in October 2009, close tothe end of rainy season in that year. It can be seen700