Behaviour of typical pavement subgrade materials in Hong Kong under portable falling weight deflectometer test

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Behaviour of typical pavement subgrade materials in Hong Kong under portable falling weight deflectometer test

 

Author: Leung, Lai Ming Gordon
Title: Behaviour of typical pavement subgrade materials in Hong Kong under portable falling weight deflectometer test
Degree: Ph.D.
Year: 2015
Subject: Pavements -- Subgrades
Pavements -- China -- Hong Kong -- Testing.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Civil and Environmental Engineering
Pages: 228 pages : illustrations ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2811490
URI: http://theses.lib.polyu.edu.hk/handle/200/8012
Abstract: The major objectives of this research were to improve the understanding of behavior of local pavement subgrade materials under portable falling weight deflectometer (PFWD) test, as well as to make possible recommendations on the utilization of the equipment for mechanistic pavement analysis and design in Hong Kong. In order to fulfill the objectives, investigation works as listed below were accomplished in four key stages: *Resilient behaviour and modulus (MR) of the typical subgrade materials, *Dynamic stresses induced by traffic under typical road pavements, *Stress distributions within the soil mass resulted from the PFWD tests, *Representativeness of LWD moduli (ELWD) to reflect the true properties of the subgrade materials. *Influences of confining pressures to the stress / strain responses and LWD moduli (ELWD) of the subgrade materials, *Correlations between ELWD and MR under the context of Hong Kong. Initial Stage: Resilient behavior of most unbound subgrade materials greatly relies on the surrounding conditions such as stress levels, stress durations, confining pressures and deviator stresses. To facilitate systematic study on these factors and catching up with the trend of many overseas practices, extensive repeated triaxial load (RTL) tests were carried out on a variety of local subgrade materials to study their resilient behaviour. Resilient moduli (MR) of the typical subgrade materials in Hong Kong were obtained and the results were compared with the ones pursued from the PFWD tests at the Final Stage of Study. On the other hand, in order to compare and relate the dynamic stresses induced by ordinary PFWD tests and the ones induced by running traffic under typical road pavements, soil pressure gauges were installed under a widening section of Tolo Highway and the dynamic stresses generated were collected by tailor-designed data acquisition system. Stage Two: Stage Two Study was conducted to gain understanding on the dynamic stress distributions within soil subgrade material placed inside a mass container under LWD tests, with the support by in-situ soil pressure gauges and comprehensive data acquisition system. From the investigation, several important findings were yielded: (1) It was indicated that beyond the distance of 1D (i.e. one times the load plate diameter) from the source of load, the ELWD stays fairly constant, this corresponds to a vertical stress ratio (stress measured by SPG / stress applied by LWD) of about 0.2, (2) The horizontal confining stresses are highest at the location of the source of load and reduce dramatically (in a power form) away from the source, (3) The drop height of mass was found to impose a moderate influence on the ELWD. The increase in drop height of mass can induce increases in confining stresses, and in turn increases in ELWD as clearly indicated by the test results, (4) The horizontal confining pressure has the strongest linear correlation with ELWD, being followed by vertical stress and deviator stress. The test findings were compared and verified by finite element analysis and results between them were found to be reasonably alike.
Stage Three: The purposes of the Stage Three Study were to validate the representativeness of ELWD to reflect the true elastic properties of the typical subgrade materials and to investigate how the surrounding confining pressures induced may affect the strains and subsequent stiffness of a soil column sample. In order to achieve the objectives, the LWD test set-up was modified to mimic the one similar to the set-up of RTL test. The key findings of the Stage Two Study are such as (1) the ELWD produced reflect well on the actual elastic moduli of the subgrade materials, (2) the level of surcharge applied was a significant factor to dictate the magnitude of static confining pressures. In turn, the static confining pressures significantly influenced the dynamic confining pressure magnitudes generated by the LWD tests, (3) the moisture content of the soil was one of the factors that could affect the magnitudes of the dynamic confining pressures, and lastly (4) obvious tendencies of reductions in recoverable strains due to increases in average confining pressures and hence increases in stiffness for the subgrade soils. Final Stage: Correlations between LWD and resilient moduli were investigated under the Stage Four Study (Final Stage). The results justified that the LWD was a useful tool and the LWD modulus was able to form a good yardstick to estimate resilient modulus, which was an essential parameter for mechanistic pavement design. It was found that moduli ratios (ELWD / MR) ranged between 0.74 and 2.36. Softer soil materials tended to have higher moduli ratios (more than 1.0) and stiffer soils have lower moduli ratio (less than 1.0). Through the regression analysis, it was considered that the moduli produced by LWD tests with 50% and 25% of the drop heights were most suitable to form estimations for the resilient moduli of the subgrade materials. Finally two correlation models were established and recommendations were also made for the better utilization of the LWD equipment. The works as covered in the Initial Stage was the first time ever conducted in Hong Kong and the works as contained under the subsequent three stages (Stage 2, Stage 3 and Final Stage) were considered to be highly "original" in a global context.

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