Thesis (MSc)--Stellenbosch University, 2015. / 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.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/96951 |
| Date | 03 1900 |
| Creators | Van Zyl, Eben Barnard |
| Contributors | Jenkins, Kim Jonathan, Rudman, Chantal, Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | 223 pages : illustrations |
| Rights | Stellenbosch University |
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