**Suggested Citation:**"Appendix A. Annotated Bibliography of Influence of Unbound Layers and Subgrade." National Academies of Sciences, Engineering, and Medicine. 2019.

*Proposed Enhancements to Pavement ME Design: Improved Consideration of the Influence of Subgrade and Unbound Layers on Pavement Performance*. Washington, DC: The National Academies Press. doi: 10.17226/25583.

**Suggested Citation:**"Appendix A. Annotated Bibliography of Influence of Unbound Layers and Subgrade." National Academies of Sciences, Engineering, and Medicine. 2019.

*Proposed Enhancements to Pavement ME Design: Improved Consideration of the Influence of Subgrade and Unbound Layers on Pavement Performance*. Washington, DC: The National Academies Press. doi: 10.17226/25583.

**Suggested Citation:**"Appendix A. Annotated Bibliography of Influence of Unbound Layers and Subgrade." National Academies of Sciences, Engineering, and Medicine. 2019.

*Proposed Enhancements to Pavement ME Design: Improved Consideration of the Influence of Subgrade and Unbound Layers on Pavement Performance*. Washington, DC: The National Academies Press. doi: 10.17226/25583.

**Suggested Citation:**"Appendix A. Annotated Bibliography of Influence of Unbound Layers and Subgrade." National Academies of Sciences, Engineering, and Medicine. 2019.

**Suggested Citation:**"Appendix A. Annotated Bibliography of Influence of Unbound Layers and Subgrade." National Academies of Sciences, Engineering, and Medicine. 2019.

**Suggested Citation:**"Appendix A. Annotated Bibliography of Influence of Unbound Layers and Subgrade." National Academies of Sciences, Engineering, and Medicine. 2019.

**Suggested Citation:**"Appendix A. Annotated Bibliography of Influence of Unbound Layers and Subgrade." National Academies of Sciences, Engineering, and Medicine. 2019.

**Suggested Citation:**"Appendix A. Annotated Bibliography of Influence of Unbound Layers and Subgrade." National Academies of Sciences, Engineering, and Medicine. 2019.

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A-1 Appendix A. Annotated Bibliography of Influence of Unbound Layers and Subgrade INFLUENCE OF UNBOUND LAYER CHARACTERISTICS ON FLEXIBLE PAVEMENT PERFORMANCE 1) Modulus of unbound base course on total rutting (1, 2) Sensitivity analyses performed in these literatures show the total rutting decreases by the increase of modulus of the base layer; hence, larger modulus of the base layer promotes the resistance to rutting. 2) Cross-anisotropic modulus of unbound base course on total rutting (3, 4) From this literature, total permanent deformation calculated by anisotropic model is more than that by isotropic model. Thus, cross-anisotropic behavior property of an unbound base course helps the accuracy of calculated permanent deformation. 3) Moisture sensitivity of modulus of unbound base course on total rutting and load-related cracking (5, 6, 7, 8, 9) Many researchers demonstrate that matric suction is a better prediction factor than moisture content as a fundamental stress state variable. Modulus of unbound base course has a high sensitivity in the change of matric suction and therefore, the modulus equation has been modified by entering the suction term into the effective stress or as external stress tensor. 4) Modulus of unbound base course on load-related cracking (10) Modulus of unbound base course should be sufficiently high to avoid the fatigue cracking of overlying asphalt layers. 5) Cross-anisotropic modulus of unbound base course on load-related cracking (11) A finite element program was developed to enter nonlinear cross-anisotropic base material behavior, indicating that cross-anisotropic unbound granular materials have a significant effect on evaluation of stress and strain distribution in unbound base course. In this regard, the cross- anisotropic base materials are modeled with the help of assessment of modulus of base layers. 6) Modulus of unbound base course on thermal cracking (12) Thermal cracking due to the thermal shrinkage of base course materials is aggravated with moisture, the high degree of which decreases the modulus of unbound base layers. 7) Modulus of unbound base course on smoothness (International Roughness Index [IRI]) (13) The change of smoothness (IRI) decreases with the increase of base modulus. 8) Shear strength of unbound base course on total rutting (14, 15, 16, 17, 18) Shear strength plays an important role in affecting total rutting accumulation in unbound aggregate materials. The unbound base materials with low shear strength might correspond to a significantly high shear stress ratio value, which positively affects the total rutting.

A-2 9) Shear strength of unbound base course on load-related cracking (19) Load-related cracking is a major structural distress. A larger shear strength of unbound base course improves the integrality of supporting layers and resistance to load-related cracking. 10) Shear strength of unbound base course on smoothness (IRI) (18, 19) IRI value depends upon the development of distresses, such as rutting, fatigue cracking, and thermal cracking. A higher shear strength of unbound base leads to a higher resistance to rutting and cracking, hence reducing the IRI value. 11) Permanent deformation of unbound base course on total rutting (10) Obviously, total rutting is promoted by the increase of permanent deformation of unbound base course. 12) Permanent deformation of unbound base course on smoothness (IRI) (20) Rutting due to the permanent deformation of unbound base course is a major distress resulting in enhancing the surface roughness. 13) Thickness of unbound base course on total rutting (2) The results of sensitivity analysis show that percent change in rutting reduces with increase of the thickness of the base layer. It is suggested that rutting decreases with an increase of the thickness of the base layer. 14) Thickness of unbound base course on load-related cracking (1, 13) With the benefit from a greater base layer thickness for transferring the load to subbase and subgrade, the tensile strain at the bottom of the AC layer is reduced; thus, the load-related cracking is also diminished. The load-related cracking has a moderate sensitivity to base layer thickness. 15) Thickness of unbound base course on thermal cracking (21) The thicker a pavement is, the longer the thermal cracking will take to propagate with depth of pavement. Thus, a greater thickness of unbound base course would help alleviate the severity of thermal cracking. 16) Thickness of unbound base course on smoothness (IRI) (2) The sensitivity analysis is conducted in this study, which indicates that the percent of change in IRI diminishes with increase of thickness of the base layer. It is determined that the IRI value lessens with an increase of thickness of the base layer. INFLUENCE OF UNBOUND LAYER CHARACTERISTICS ON RIGID PAVEMENT PERFORMANCE 1) Modulus of unbound base course on transverse cracking (jointed plain concrete pavement [JPCP]) (1, 22) The reduction of the base support would increase the transverse crack deterioration rate. The sensitivity analysis in this literature shows that transverse cracking decreases from 0.8 percent to 0.7 percent with the increase in modulus of base layer from 1,250,000 to 1,500,000 psi.

A-3 2) Modulus of unbound base course on faulting (JPCP) (23) High modulus of unbound base course would enhance the level of stabilization that decreases erodibility. Erosion is often the dominant deterioration resulting in faulting of JPCP. Therefore, it is suggested that a low modulus of unbound base course would cause the development of faulting. 3) Modulus of unbound base course on punchouts (continuously reinforced concrete pavement [CRCP]) (24, 25, 26) Loss of structural stiffness due to delamination (the reduction of modulus of unbound base course) would cause partial-depth punchouts. 4) Modulus of unbound base course on load transfer efficiency (LTE) (JPCP and CRCP) (27, 28) LTE is a major factor affecting deflections and denoted as unloaded deflection over loaded deflection. A higher modulus of unbound base layer could minimize the deflection imposed by applied loading, thus improving the LTE. 5) Modulus of unbound base course on smoothness (IRI) (1) According to the sensitivity analysis of IRI value with base layer modulus, the plot in this literature presents that IRI decreases from 93.3 to 92.5 inch/mile with the increase in the modulus of base layer from 38,500 to 42,000 psi. 6) Shear strength of unbound base course on transverse cracking (JPCP) (29, 30) Higher shear stresses due to deformation from compaction and/or trafficking caused immediate reflective cracking at all transverse joints in the eastbound direction of the tested pavements. 7) Shear strength of unbound base course on faulting (JPCP) (23) Low shear strength of unbound base course suggests that small degree of bond of interfaces, which negatively associates with faulting. 8) Shear strength of unbound base course on punchouts (CRCP) (24, 27) When the loss of shear capacity is excessive, which represents the decrease of shear strength of unbound base course, the potential for punchouts may be greater. This literature provides the equation of effect of aggregate wear-out that pertains to the propagation of punchouts distress. 9) Shear strength of unbound base course on LTE (JPCP and CRCP) (23) The equation to determine the shear stress is provided in this literature, which shows that low LTE contributes to the high shear stress. It is inferred that high shear strength results in high LTE. 10) Shear strength of unbound base course on smoothness (IRI) (31, 32) The bond between the slab and subbase is dependent on the shear strength of unbound base layer and affects the erosion damage, which is the main cause of faulting. Faulting could minimize the smoothness of surface pavement so that the increase of shear strength of base layer diminishes the roughness on pavement surface.

A-4 11) Erosion of unbound base course on faulting (JPCP) (23, 27,34) The maximum amount of expected erosion equation is provided in this literature. The erosion rate of unbound base course increases, resulting that faulting life of pavement is diminished. 12) Erosion of unbound base course on punchouts (CRCP) (24, 27, 33) If the development of erosion of unbound base course takes place, the punchout life of pavement is decreasing. Sensitivity analysis of subbase erosion indicates that the punchout is aggravated with high erodibility. 13) Erosion of unbound base course on LTE (JPCP and CRCP) (27, 34) Erosion mechanism is due to the joint deterioration process that is reflected in lower relative stiffness, which causes the low LTE. Therefore, the LTE is promoted by the reduction of the erosion. 14) Erosion of unbound base course on smoothness (IRI) (27, 34, 35) The erosion of unbound base course promoted the probability of faulting and punchouts, which also aggravates the smoothness of the surface layer. 15) Permanent deformation of unbound base course on faulting (JPCP) (36) The permanent deformation of unbound base course has a promoting effect on higher deflection of slab, which is a main factor in the potential for faulting. Hence, the permanent deformation of unbound base course has a positive impact on potential of faulting. 16) Permanent deformation of unbound base course on smoothness (IRI) (20) Rutting due to the permanent deformation of unbound base course is a major distress, resulting in enhancing the surface roughness. 17) Thickness of unbound base course on transverse cracking (1) The sensitivity analyses in this literature determined that transverse cracking promotes as the base layer thickness increases from 4 to 7 inches. Also, minimum transverse cracking is generated from base layer thickness that is more than 7 inches. 18) Thickness of unbound base course on faulting (1) The sensitivity analysis of effect of base thickness on faulting shows that the faulting decreases with high base thickness. In this literature, the use of cement-stabilized base thickness from 4 to 8 inches would show a high resistance to mean joint faulting beyond an allowable level of 0.15 inches. 19) Thickness of unbound base course on punchouts (1) In this literature, it is presented that CRCP punchouts reduces from 30.6 to 28.4 per mile with an increase in the base layer thickness from 4 to 8 inches. Moreover, the base layer thickness should be more than 8 inches to satisfy the acceptable limit of punchouts per mile.

A-5 20) Thickness of unbound base course on LTE (27) Taking advantage of susceptibility thickness of unbound base layers to improve the stiffness of base layers would help increase the LTE. 21) Thickness of unbound base course on Smoothness (IRI) (1) The sensitivity analysis of IRI value with thickness of base layer is conducted, as presented in this literature. The general trend of curve indicates that there is a decrease in IRI value with the increase in the base layer thickness. INFLUENCE OF SUBGRADE CHARACTERISTICS ON FLEXIBLE PAVEMENT PERFORMANCE 1) Modulus of subgrade on total rutting (8) Total rutting is usually caused by the consolidation or lateral movement of the material, the occurrence of which is adversely affected by modulus of subgrade. 2) Modulus of subgrade on load-related cracking (1, 8, 37) Load-related cracking is usually attributed to the change of modulus of subgrade. The low modulus of subgrade would enhance the development of load-related cracking. 3) Modulus of subgrade on smoothness (IRI) (2, 38) The IRI is evaluated from the pseudo-profiles according to specific wavelength, which has a negative relation with modulus of subgrade. 4) Cross-anisotropic modulus of subgrade on total rutting (3, 39, 40) Horizontal modulus of subgrade is reduced by using a nonlinear anisotropic model in subgrade, which leads to larger deformation in vertical direction than that calculated in an isotropic model. The isotropic model may cause a substantial error in predicting the subgrade response. The permanent deformation of subgrade calculated when the subgrade is regarded as the cross- anisotropic materials is more than that obtained using the isotropic subgrade model. 5) Cross-anisotropic modulus of subgrade on load-related cracking (3) The use of a cross-anisotropic model to characterize the subgrade materials improves the accuracy of predicted surface deflection. Cross-anisotropy properties of subgrade affects stress/strain distribution, thus influencing cracking and fatigue life. 6) Moisture sensitivity of modulus of subgrade on total rutting and load-related cracking (41, 42, 43, 44, 45, 46) Many researchers have pointed out that soil suction is a major factor for the prediction of modulus of cohesive subgrade materials. Much research has been conducted to correlate modulus of subgrade with soil suction. A higher soil suction generates a larger modulus of subgrade. 7) Shear strength of subgrade on total rutting (47) The rut model, which is related to the shear strength of material, is proposed in this study. Total rutting decreases as shear strength of subgrade increases.

A-6 8) Shear strength of subgrade on smoothness (IRI) (19) Rutting damage is related to the inadequate shear strength of subgrade. Rutting failure also contributes to the loss of smoothness; thus, the decrease of shear strength of subgrade gives more potential of roughness. 9) Permanent deformation of subgrade on total rutting (47) Total rutting would increase with the development of permanent deformation of subgrade. 10) Permanent deformation of subgrade on load-related cracking (48) The performance equation based on rutting and load-related cracking is given, which shows that the vertical compressive strain at the top of the subgrade has a negative correlation with allowable number of load repetitions. Repeated loading plays a significant role in load-related cracking. Lower permanent deformation of subgrade reduces the probability of load-related cracking. 11) Permanent deformation of subgrade on thermal cracking (12) The thermal cracking is due to the shrinkage of supporting subgrade soils. 12) Permanent deformation of subgrade on smoothness (IRI) (12, 20) Roughness is non-load-related distress due to expansive soils and frost-heaving soils. This distress is accelerated by the effect of the moisture that also strongly promotes the permanent deformation of subgrade. Roughness is increased with the initiation of the permanent deformation of subgrade. INFLUENCE OF SUBGRADE CHARACTERISTICS ON RIGID PAVEMENT PERFORMANCE 1) Modulus of subgrade on transverse cracking (JPCP) (1, 22) Increasing the modulus of subgrade would significantly enhance the LTE for larger crack width, thus decreasing the transverse cracking. 2) Modulus of subgrade on faulting (JPCP) (49) An increase in modulus of subgrade causes a decrease in faulting. 3) Modulus of subgrade on punchouts (CRCP) (24, 25) Punchout is predicted at a high rate with increasing the k-value of subgrade by the AASHTO 2008 approach. However, the modified mechanistic-empirical continuously reinforced concrete (CRC) pavement design is described for a modified punchout model, which indicates that punchout is increased with low k-value of subgrade. 4) Modulus of subgrade on LTE (JPCP and CRCP) (27) The equation to determine the LTE is proposed in this study, which indicates that the relative stiffness is included in the equation and positively affects the LTE. Relative stiffness has a negative correlation with modulus of subgrade. The LTE is increased by the high modulus of subgrade.

A-7 5) Modulus of subgrade on smoothness (IRI) (1) Sensitivity of IRI with subgrade modulus is performed in this literature, which shows that IRI value diminishes with the increase in the modulus of subgrade. 6) Shear strength of subgrade on transverse cracking (JPCP) (19) A higher shear strength of subgrade improves the support capacity of underlying layers. In this respect, the increase of shear strength of subgrade raises the resistance of transverse cracking. 7) Shear strength of subgrade on faulting (JPCP) (36) The shear strength of subgrade layer could yield the maximum interfacial shear stress causing the erosion damage resulting in high deflections. Faulting pertains to erosion-affected distresses. Higher shear strength of subgrade layer improves the resistance to faulting damage. 8) Shear strength of subgrade on punchouts (CRCP) (24) Shear strength governs the degree of probability that delamination can occur. The partial-depth punchout is initiated by delamination and loss of structural stiffness. Full-depth punchout is caused by increased vertical shear stress. Thus, punchout is accelerated with lower shear strength of subgrade. 9) Shear strength of subgrade on LTE (JPCP and CRCP) (27) Aggregate interlock has a benefit from a higher shear strength of subgrade layer. The tight aggregate interlock improves the LTE. The increase of shear strength improves LTE. 10) Shear strength of subgrade on smoothness (IRI) (32) Faulting imposed by erosion damage with low shear strength of subgrade layer affects drive safety and reduction of smoothness of pavement. For this reason, improvement of the shear strength of subgrade layer could increase the resistance to roughness of ride. 11) Permanent deformation of subgrade on transverse cracking (JPCP) (19) Permanent deformation of subgrade has momentum on loss of supporting layers, which could cause the development of transverse cracking. 12) Permanent deformation of subgrade on faulting (JPCP) (19, 50) Faulting is defined as a difference of elevation of slab. The faulting model adopted in AASHTO 2008 contains the DE term, which is denoted as accumulated differential energy of subgrade deformation. Higher permanent deformation of subgrade affects aggravating the development of faulting distress. 13) Permanent deformation of subgrade on punchouts (CRCP) (50) Close crack spacing, large crack width, and poor LTE contribute to punchouts. The effect of permanent deformation of subgrade makes poorer LTE; thus, development of punchouts is also affected by permanent deformation of subgrade.

A-8 14) Permanent deformation of subgrade on LTE (JPCP and CRCP) (24) In this literature, deformation energy is provided, which presents that loss of LTE results from the high deformation energy. The unit of deformation energy is pound per inch. Loss of LTE occurs with high permanent deformation of subgrade. 15) Permanent deformation of subgrade on smoothness (IRI) (20) Rutting generated from the permanent deformation of subgrade is associated with increasing roughness.