Investigation of factors affecting the load carrying capacity of driven solar panel support posts

Uwimana, Aimable (2015-12)

Thesis (MEng)--Stellenbosch University, 2015.

Thesis

ENGLISH ABSTRACT: A number of large “solar farms” have already been built in South Africa and more are under construction. Many of these make use of banks of photovoltaic panels mounted on a frame-work supported on columns known as posts. Various methods of founding are used for these posts. One of the founding methods involves driving the post into a predrilled hole filled with granular material. In South Africa, this method is used in semi-arid areas where the sites are frequently underlain by clacretes or by shallow rock. However, there are no clear design guidelines for such foundations. When the installation is not done correctly and the solar panel support structure is subjected to the action of wind and water, the driven posts could settle or be pulled-out of the ground creating failure. Therefore, an investigation of factors affecting the shaft pull-out capacity of driven post is vitally important. In order to provide a better understanding of load bearing capacity of driven posts in both compression and tension, research has been carried out into the effect of backfill material type, compaction density, compaction moisture content, backfill saturation, post-to-hole area ratio and aging of the backfill. An experimental programme using three types of backfill material, three degrees of compaction, three area ratios and two compaction moisture con-tents was carried out to assess the influence of these variables on axial and oblique pull-out loading capacity of predrilled driven posts. In addition, the moisture content at the time of loading and the effect of aging of the backfill was considered. Each material was evalu-ated in terms of particle size distribution, particle shape, and shearing resistance using di-rect shear tests. The pull-out resistance was evaluated using an Instron load-testing ma-chine on experimental driven posts at half scale. The results from the direct shear tests revealed an increase in shear strength with an in-crease in the degree of compaction and normal stress. It also shows that the crusher dust material has higher shear strength than Malmesbury and Philippi sands. However, shear strength reduces with saturation while dilation reduces with an increase of both normal stress and moisture content. The shaft pull-out capacity results from the experimental model tests confirm the findings of the direct shear tests on material performance. The pull-out resistance increases with both degree of compaction and area ratio. It reduces with saturation. In addition, the pull-out capacity of the post increases significantly with time. Comparing the axial and oblique pull-out capacity, a higher resistance was observed under oblique pull-out loads than for axial pull-out. This helps to explain why posts have been observed to settle under very modest dead loading but are, nevertheless, able to resist significant wind loading on the underside of the panels. This study contributes to understanding the effect of material type, compaction density, area ratio, water content and aging on the shaft pull-out capacity of driven post foundations for solar panels. Practical specifications and technical guidelines are developed to ensure an improved installation of foundations using this predrilled driven post method.

AFRIKAANSE OPSOMMING: ʼn Aantal groot sonkrag plase is alreeds in Suid-Afrika gebou en nog meer word tans ontwikkel. Baie van hierdie plase maak gebruik van banke van fotovoltaïese panele wat op ’n raamwerk gemonteer is. Die raamwerk word ondersteun deur kolomme bekend as pale. Verskeie metodes van fundering word gebruik vir hierdie pale. Een van hierdie metodes behels die indryf van die paal in ’n vooraf-geboorde gat gevul met korrelrige materiaal. In Suid-Afrika word hierdie metode in semi-droë areas gebruik wat dikwels onderlê word deur vlak rots. Daar is egter geen duidelike riglyne vir die ontwerp van sulke fondamente nie. Wanneer die installasie nie reg uitgevoer word nie, en die sonpaneel-struktuur word blootgestel aan die gekombineerde aksie van wind en water, kan die gedrewe pale versak of uit die grond getrek word, wat swigting teweegbring. Om hierdie rede is dit baie belangrik dat die faktore wat die uittrek-kapasiteit van die skag van die gedrewe paal affekteer, ondersoek word. Om ’n beter begrip van die lasdraende vermoë van gedrewe pale in beide druk en uittrekking te bewerkstellig, is navorsing uitgevoer op die effek van tipe invulmateriaal, kompaksie digtheid, voginhoud by kompaksie, versadiging van invulmateriaal, verhouding van paal-tot gat-deursnitarea en veroudering van terugvulmateriaal. ’n Toetsprogram, waarin drie tipes invulmateriaal, drie grade van kompaksie, drie deursnitarea-verhoudings en twee voginhoude tydens kompaksie gebruik is, is uitgevoer om die invloed van hierdie veranderlikes op aksiale en skuinsuittrekweerstand van vooraf-geboorde gedrewe pale, te bepaal. Daarmee saam is die voginhoud tydens belasting en die effek van veroudering van die invulmateriaal ook beskou. Elke materiaal was geëvalueer in terme van partikelgrootteverspreiding, partikelvorm en skuifweerstand deur direkte skuiftoetse uit te voer. Die uittrekweerstand van eksperimentele gedrewe pale was geëvalueer deur gebruik te maak van ’n Instron toetsapparaat. Die toetsmodel was halfskaalgrootte. Die resultate van die direkte skuiftoets toon ’n toename in skuifsterkte met ’n toename in die graad van kompaksie en vertikale druk. Dit het ook getoon dat die vergruiser fynstof ’n hoër skuifsterkte as die Malmesbury en Philippi sande het. Die skuifsterkte neem egter af met versadiging terwyl die dilatansie afneem met ’n toename in beide die vertikale spanning en voginhoud. Die skag uittrekvermoë resultate van die eksperimentele modeltoets bevestig die skuifgedrag van die materiaal. Die uittrekweerstand verhoog met ’n toename in die graad van kompaksie en area verhouding en verlaag met versadiging. Die uittrekvermoë van die pale neem aansienlik toe met tyd. Deur die aksiale en skuinsuittrekvermoë te vergelyk, is ’n hoër weerstand waargeneem met skuinsuittrekbelastings as met aksiale uittrek. Hierdie bevinding verklaar waarom daar al waargeneem is dat pale wat versak onder die invloed van beskeie dooiegewiglaste, beduidende windbelastings aan die onderkant van panele kan weerstaan. Hierdie studie dra by tot ’n beter begrip van die effek van materiaal tipe, kompaksiedigtheid, area-verhouding, voginhoud en veroudering op skag-uittrekweerstand van gedrewe paal fondamente vir sonpanele. Praktiese spesifikasies en tegniese riglyne word ontwikkel om ’n verbeterde installasie van fondasies waarin die voorafgeboorde gedrewe paal metode gebruik word, te verseker.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/98048
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