In-situ assessment of the stress-dependent stiffness of unbound aggregate bases: application in inverted base pavements

Abstract

Unbound aggregate bases are the primary structural components in many flexible pavements. The response of the unbound aggregate base is critical to the overall performance of the pavement, particularly in inverted base pavements given the proximity of the base to the traffic loads. The behaviour of granular materials such as unbound aggregate bases is inherently nonlinear and anisotropic. An experimental methodology is developed to assess the in-situ stress-dependent small-strain stiffness of unbound aggregate bases under controlled load using wave propagation techniques. CODA wave analysis is used to detect small changes in travel time. The methodology is applied in two distinct case histories of inverted base pavements. Results show that field-compacted granular bases exhibit higher stiffness, lower stress sensitivity and more pronounced anisotropy than laboratory-compacted specimens. The discrepancy in stiffness observed among the two field case histories is primarily attributed to traffic preconditioning sustained by the older pavement. Additional results show that the effect of suction on the stiffness of coarse-grained granular bases is insignificant.

On the thermal sensitivity of unbound granular pavement layers

“…It basically asserts that any pre-existing UGL stresses that control R M (treating moisture-change effects as marginal) are essentially timewise constant as long as the temperature level does not change. This assumption is deemed very realistic for densely packed particulate materials, as demonstrated in discrete element models [50,51], in laboratory compaction experiments on sand [52], in laboratory thermal cycling experiments on dense sand [34], in large-scale laboratory experiments on UGM compaction [53], in field investigations on compactioninduced stresses on retaining walls [54,55], and in application of geophysical exploration techniques to assess the in situ modulus of UGLs [56]. Finally, it is noted that there exists some experimental evidence that well-compacted UGMs under constant temperature exhibit time-dependent mechanical behavior, e.g., creep under constant level of load, and stress-relaxation under constant level of deformation [57].…”

Long-Term Response of Sand Subjected to Repetitive Simple Shear Loading: Shakedown, Ratcheting, and Terminal Void Ratio

“…• Overall, the maximum shear stiffness was almost 10 times higher than the secant stiffness, G max =G pp ≈ 10 (e.g., Schuettpelz et al 2010;Papadopoulos et al 2016). This stiffness ratio reflects the stiffness degradation with strain (the peak-to-peak shear strains during repetitive shear varied between γ pp ¼ 0.001 and γ pp ¼ 0.009, whereas shear-wave measurements imposed very small strains γ < 10 −7 ), as well as potential differences between short-wavelength dynamic measurements and quasistatic shear.…”

Optimal Structure Combination for Inverted Asphalt Pavement Incorporating Cracks in Cement-Treated Subbase

“…Inverted asphalt pavements utilize a well-constructed aggregate base (AB) to reduce the thickness of the asphalt concrete (AC) layer and to effectively prevent the propagation of cracks from the cement-treated subbase (CTB) to the pavement surface ( 1 , 2 ). This pavement structure has been researched and applied in countries such as South Africa and the U.S.A. for several decades and has been shown to have long service life and low life-cycle cost ( 3 , 4 ). As a key component of the inverted pavement, the material design and nonlinear anisotropic properties of AB have been widely studied both in laboratories and in field tests ( 5 ).…”

“…As a key component of the inverted pavement, the material design and nonlinear anisotropic properties of AB have been widely studied both in laboratories and in field tests ( 5 ). Given the close proximity between the AB and traffic loads, stress-dependent stiffness has a significant impact on the mechanistic responses of the inverted pavement and therefore it has been studied by many researchers ( 6 , 7 ). In the mechanistic-empirical (ME) pavement design approach, the mechanistic responses of pavement structures to traffic loads are usually important inputs for models of material damage and distress to pavements used to predict the expected design life ( 8 , 9 ).…”