Influence of specimen geometry and grading curve on the performance of an unbound granular material

Van Zyl, Eben Barnard (2015-03)

Thesis (MSc)--Stellenbosch University, 2015.

Thesis

ENGLISH ABSTRACT: This research study investigates the influence of specimen geometry and grading curve, on the performance of a typical South African unbound granular material. The experimental design incorporates three grading curves to evaluate the influence of grading. In addition, to evaluate the influence of specimen geometry, two specimen sizes were included into the experimental design. Laboratory testing consisted of monotonic tri-axial tests to evaluate the shear performance (Cohesion and Friction Angle) and more complex short duration dynamic tri-axial tests to evaluate the load spreading ability/stiffness (Resilient Modulus) of the selected materials. In order to achieve the objectives of this study, a large tri-axial apparatus was needed that could accommodate specimens as large as 300mm ϕ * 600mm high. This would allow a full grading of large aggregate (up to 50mm particle size) to be accurately evaluated. Further development and commissioning of such a large triaxial apparatus therefore formed part of this study. The representative parent material selected for testing consisted of a G2 graded crushed Hornfels stone. The material was dried and sieved into fractions where after it was carefully reconstituted to allow for accurate control of specimen grading during specimen preparation. The three grading curves consisted of two adjusted grading curves (referred to as S19 and G19C), adjusted from the full G2 grading, and the full G2 grading itself (referred to as the Full grading curve). Material property tests, Sieve Analysis, Bulk Relative Density (BRD) and Optimum Moisture Content (OMC) tests were performed to gain an understanding of the material characteristics. Moisture-Density relationship curves were developed to identify a common Moisture Content that, for all three grading curves, would yield a common Dry Density. A Moisture Content of 4.7% was identified that would yield a Dry Density of 2340 kg/m3 for all three grading curves. This density could be achieved for both sizes of specimen preparation apparatuses without damaging material particles. Specimens were compacted using the representative vibratory hammer compaction method, sealed and left for 24 hours to allow redistribution of moisture and initial development of Cohesion. The shear parameters (Cohesion and Friction Angle) were investigated through monotonic tri-axial testing. It was found that Cohesion and Friction Angle are influenced by both grading curve and specimen geometry. Cohesion was found to reduce as the coarseness of the material grading increased (i.e. finer S19 grading yielded higher Cohesion than its coarser G19C counterpart) and the Friction Angle was found to increase with increase coarseness (i.e. finer S19 grading yielded lower Friction Angles when compared to the G19C grading). The influence of specimen geometry was also investigated. It was observed than Cohesion decreased with an increase in specimen size. Friction Angle on the other hand was found to increase with increased specimen size. From dynamic tri-axial test results, it was observed that the Resilient Modulus is influenced by both specimen geometry and grading curve. The influence of specimen geometry however is complex and no constant trend throughout the grading curves tested could be identified. Grading curve however was found to increase the Resilient Modulus for coarser gradings (i.e. coarser G19C vs finer S19). Increased large particle-to-particle contact area yields higher friction within the material specimen, resulting in lower strains induced by higher stresses, i.e. higher Resilient Modulus. It was shown, for both monotonic and dynamic tri-axial tests, that the coarser G19C grading curve yields more representative results to that of the Full grading curve when compared to the finer S19 grading. This was observed for shear and resilient performance properties. Additionally, a simple design case study yields similar trends. In conclusion, material characterisation plays an important role in the design of unbound granular materials (UGM’s). Current laboratory characterisation techniques however used adjusted gradings to limit the effects stemming from the ratio between specimen diameter and maximum particle size. This research has shown that some of the current practices do not best represent the true in-situ grading. It has been shown that both grading curve and specimen geometry influence the performance of UGM’s which, in turn, influences the design of a pavement structure. Therefore, accurate modelling of the true in-situ grading, through testing apparatuses capable of accommodating in-situ gradings, is required.

AFRIKAANSE OPSOMMING: Hierdie navorsingsstudie evalueer die invloed van proefstukgeometrie en gradering, op die gedragseienskappe van ‘n tipiese Suid Afrikaanse ongebinde granulêre aggregaat. Om die invloed van gradering te evalueer, is ‘n eksperimentele ontwerp ontwikkel wat drie materiaal graderings insluit. Verder, om die invloed van proefstukgeometrie te evalueer, is twee proefstukgroottes toegevoeg tot die eksperimentele ontwerp. Monotoniese drie-assige toetse is uitgevoer om die skuifsterkte (Kohesie en Wrywingshoek) van die materiaal te ondersoek. Addisioneel is die styfheid (Veerkragmodulus) van die materiaal ondersoek deur dinamiese drieassige toetse. Om die doelwitte van hierdie studie te bereik was ‘n groot skaalse die-assige toets apparaat benodig wat groot, 300mm ϕ * 600mm hoogte, proefstukke kan akkommodeer. So ‘n apparaat laat toe dat die volle gradering van aggregaat (tot en met 50mm korrels) akkuraat geëvalueer kan word. Daarom vorm die ontwikkeling en opstelling van so ‘n apparaat deel van hierdie studie. Die tipiese Hornfels gebreekte klip, met ‘n G2 gradering, wat ondersoek is, was gedroog en in verskeie fraksies gesif om die akkuraatheid van proefstuk voorbereiding te beheer. Die drie graderings bestaan uit twee aangepaste graderings (S19 en G19C gradering skale), aangepas vanaf die volle G2 gradering, en die vol G2 gradering homself (verwys na as die “Full” gradering skaal). Materiaal gedragstoetse, Sif Analises, Nat Gekompakteerde Relatiewe Digtheid (BRD) en Optimum Vog Inhoud (OVI) toetse, was uitgevoer om die materiaal eienskappe te ondersoek. Om ‘n gemeenskaplike Vog Inhoud en Droë Digtheid, wat vir al drie graderings geld, te vind, is Vog-Digtheid verhoudingskurwes ontwikkel. Vanaf die kurwes is identifiseer dat ‘n Vog Inhoud van 4.7% ‘n Droë Digtheid van 2340 kg/m3 vir al drie graderings sal lewer. Vibrasie kompaksie is toegepas om albei skale van proefstukke te kompakteer waarna die proefstukke vir 24 uur geseel is om vogverspreiding en ontwikkeling van Kohesie toe te laat. Monotoniese drie-assige toetse is uitgevoer om die skuifsterkte parameters (Kohesie en Wrywingshoek) te ondersoek. Die resultate het gewys dat beide gradering en proefstukgeometrie die Kohesie en Wrywinshoek beinvloed. gradering lewer hoër Kohesie waardes i.v.m. die growwer G19C gradering). Die Wrywingshoek is gevind om te verhoog soos die grofheid van die gradering verhoog (m.a.w. die fyner S19 gradering het laer Wrywingshoeke gelewer i.v.m. die growwer G19C gradering). Resultate het verder gewys dat groter proefstukke laer Kohesie en hoër Wrywingshoeke lewer. Daar kan wel gedebateer word dat variasie in materiaal die verandering van die skuifsterkte parameters gee, maar die proefstukvariasie is beperk om sodoende die invloed daarvan onopmerkbaar te maak.. Verder is die verlaging in Kohesie en verhoging in Wrywingshoek, a.g.v. ‘n vergroting in proefstuk grootte, vir albei aangepaste graderings geobserveer. Dit is ‘n moontlike aanduiding dat die verandering nie materiaal afhanklik is nie maar eerder beinvloed word deur die grens toestande tydens kompaksie. Dinamiese drie-assige toets resultate het gewys dat die Veerkragmodulus beinvloed word deur beide proefstuk geometrie en gradering. Daar is gevind dat die invloed van proefstukgeometrie kompleks is, en geen konstante verhouding, wat vir alle toets graderings geld, kon identifiseer word nie. Vir die invloed van gradering is daar gewys dat die Veerkragmodulus hoër is vir die growwer gradering (m.a.w. G19C gradering lewer hoër styfheid as S19 gradering). ‘n Verhoging in korrel-tot-korrel kontak area lewer hoër interne wrywing in die proefstuk wat bydrae tot laer vervorming by hoër spannings, m.a.w. hoër Veerkragmodulus. Baie interessant, vir beide monotoniese en dinamiese drie-assige toetse is gevind dat die growwer G19C gradering, i.v.m. die fyner S19 gradering, die ware G2 (Full) gradering beter verteenwoordig. Hierdie observasie is geldig vir beide die skuifsterkte parameters en weerstands eienskappe. Aggregaat karakterisering is ‘n belangrike deel in die ontwerp van ‘n ongebinde granulêre materiaal laag. Huidige karakterisering metodes gebruik aangepaste graderings sodat resultate nie beinvloed word deur die verhouding tussen proefstuk diameter en maksimum klipgrootte nie. Hierdie ondersoek het gevind dat van die huidige aanpassings nie die ware gradering verteenwoordig nie. Die resultate wys dat beide gradering en proefstuk geometrie die gedrag van die ongebinde granulêre materiaal beinvloed, so ook die ontwerp van ‘n padstruktuur. Daar is dus ‘n behoefte om die ware gradering te ondersoek wat slegs moontlik is met groot skaalse toets apparaat, wat groot klip korrels kan toets. Verder, indien daar ‘n verstandhouding tussen huidige (klein skaalse) toets apparaat en groot skaalse apparaat ontwikkel kan word, kan resultate aangepas word, vanaf die klein skaalse resultate, om die ware materiaal gedrag meer te verteenwoordig.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/96951
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