Bearing capacity of circular footing on geocell–sand mattress overlying clay bed with void

The potential benefits of providing geocell reinforced sand mattress over clay subgrade with void have been investigated through a series of laboratory scale model tests. The parameters varied in the test programme include, thickness of unreinforced sand layer above clay bed, width and height of geocell mattress, relative density of the sand fill in the geocells, and influence of an additional layer of planar geogrid placed at the base of the geocell mattress. The test results indicate that substantial improvement in performance can be obtained with the provision of geocell mattress, of adequate size, over the clay subgrade with void. In order to have beneficial effect, the geocell mattress must spread beyond the void at least a distance equal to the diameter of the void. The influence of the void over the performance of the footing reduces for height of geocell mattress greater than 1.8 times the diameter of the footing. Better improvement in performance is obtained for geocells filled with dense soil.     

Influence of geosynthetic-interlayers on the performance of asphalt overlays on pre-cracked pavements

The functions of geosynthetic-interlayers in retarding reflection cracking and improving fatigue performance of hot mix asphalt (HMA) overlays in flexible pavements are evaluated in this study. The delamination or debonding mechanisms of the overlays are studied when geosynthetic-interlayers are adopted. A polyester grid coated with polymer modified binder (G1), a woven geo-jute mat (G2), and a bi-axial polypropylene grid (G3) interlayer are examined based on their adhesion properties. A two stage experimental program, in which, during the first stage, the performance of the geosynthetic-interlayers sandwiched between the pre-cracked old pavement and new asphalt layers are evaluated using flexural fatigue testing. A digital image correlation (DIC) technic was employed to record the failure modes and the corresponding tensile strains in the overlay system. During the second stage, the effect of interlayers on the interface bond strength was evaluated with the help of shear and tensile bond strength tests. The results show that the inclusion of interlayers retards the propagation of reflection cracking, however, results in the delamination of overlays. The debonding effect is prominent in G3 interlayers due to their high initial stiffness. Overall, interlayers with high interfacial shear and pull-off tensile bond properties proved effective in controlling the reflection cracking and increasing fatigue life of the overlays.     

Experimental and Modeling Studies of Permanent Strains of Subgrade Soils

Mechanistic-empirical pavement design guide for flexible pavements as per the AASHTO design guide requires characterization of subgrade soils using the resilient modulus (MR) property. This property, however, does not fully account for the plastic or permanent strain or rutting of subgrade soils, which often distress the overlying pavements. Soils such as silts exhibit moderate to high resilient moduli properties but they still undergo large permanent deformations under repeated loading. This explains the fallacy in the current pavement material characterization practice. A comprehensive research study was performed to measure permanent deformation properties of subgrade soils by subjecting various soils under repeated cycles of deviatoric loads. This paper describes test procedure followed and results obtained on three soils including clay, silt, and sandy soils. The influence of compaction moisture content, confining pressure, and deviatoric stresses applied on the measured permanent deformations of all three soils are addressed. A four-parameter permanent strain model formulation as a function of stress states in soils and the number of loading cycles was used to model and analyze the present test results. The model constants of all three soils were first determined and these results were used to explain the effects of various soil properties on permanent deformations of soils. Validation studies were performed to address the adequacy of the formulated model to predict rutting or permanent strains in soils.