Wai Ying Yuk Gehling

SEASONAL VARIATIONS OF A SUBGRADE SOIL RESILIENT MODULUS IN SOUTHERN BRAZIL

The influence of suction on the resilient modulus of a shale residual lateritic soil was analyzed. Laboratory tests were carried out to obtain the moisture content-suction relationship and determine the resilient moduli of specimens submitted to drying, wetting, or wetting-after-drying paths. A supplementary study on the effects of compaction method on soil resilient modulus was carried out. Soil suction was measured in situ, with jet-fill tensiometers installed along test sections built in a pavement testing facility. Test sections were loaded by a traffic simulator, and periodically deflections were measured for modulus backcalculation. Laboratory and in situ results confirmed the consensus that suction remarkably affects soil elastic deformability. Wetting-after-drying paths proved to be an extremely severe condition that may lower resilient modulus up to four times. Static compaction led to resilient moduli higher than those of specimens compacted dynamically or by kneading. In situ results matched reasonably well with laboratory moduli of specimens compacted by kneading and tested at optimum moisture content. In general, the importance of drainage design and maintenance was clearly confirmed. In well-drained pavements, subgrade soils will not be saturated for long periods. Suction will control stress state and soil deformability and guarantee that the pavement will carry the designed traffic before failure.

Analysis of the mechanical response of an artificial collapsible soil

Natural collapsible residual soils are a common occurrence in Brazilian unsaturated deposits. The work presented in this paper is part of a larger project designed to identify the most important parameters controlling the mechanical behaviour of this type of material. A laboratory testing programme has been carried out in artificially unsaturated cemented samples reproducing some of the characteristics of natural collapsible soils. A technique for sample preparation has been developed, which has resulted in specimens with high values of void ratio and various degrees of cementation. In particular, it has been decided to use soil cemented mixtures with expanded polystyrene particles which has led to light samples with low density, meta-stable mechanic structure, and good workability. These artificial samples have been subjected to direct shear and oedometer tests, including conventional and suction-controlled oedometer tests. Oedometer tests have been used to characterize the behaviour of this material and to quantify the potential collapse due to loading and wetting. From the observed behaviour it is suggested that the initial void ratio, cement agent and initial suction are all important factors influencing the potential collapse of the soil. For saturated artificial soil samples the critical state lines in the (v, ln p’)-plane and (q, p’)-plane have also been calculated by using an approximated procedure based on the results of direct shear tests.

Rutting of thin pavements: Full-scale study

Since 1992, the Federal University of Rio Grande do Sul, along with the roads department of the same Brazilian state, has undertaken comprehensive research on weathered basalts, with the purpose of reducing the cost of low-volume pavements. Laboratory studies have led to a criterion (on the basis of the point load test) used to select deposits of these intensely fractured rocks. A traffic simulator was designed and built, and a pavement test facility was constructed on the university campus. A study was done of rutting of thin pavements on which weathered basalts were used as base layers. The traffic simulator applied more than 267,000 axle loads, ranging from 82 to 130 kN, on five full-scale test sections. Two different weathered basalts and three base thicknesses were used. A postmortem evaluation revealed that the base layer contributed the most to rutting. The axle load applications caused the pavement structure to settle and consolidate. A total of 4,148 measurements of rut depth, made at intervals, provided a statistically significant data set. Rutting evolution was shown to depend not only on traffic characteristics but also on pavement structure. With consideration of rutting as a major failure cause in thin pavements and a rut depth of 25 mm as a terminal criterion, load equivalence factors were calculated by means of a reliability analysis.