The Disintegration characteristics of residual granite soil

Mong, Danica (2022-04)

Thesis (MEng)--Stellenbosch University, 2022.

Thesis

ENGLISH ABSTRACT: Plutons of the Cape Granite Suite are largely limited to the south-western Cape with exception of occurrences near George and in Namaqualand (Brink, 1981). The composition and texture of Cape granitoids are very complex and variable, depending on the location of the plutons. Problems associated with residual granite soil from the Cape Granite Suite include the presence of a collapsible grain structure. In partially saturated residual granite soils, quartz particles have colloidal coatings around individual grains, contributing to a seemingly high strength soil, and when saturated under load, the colloidal bridges become unstable, causing particles to pack into a denser state, leading to sudden settlement of the foundation. This research contributes towards classifying and characterising decomposed granite soil, from the Cape Granite Suite, in terms of its physical properties and engineering behaviour. The soil was classified based on its grading properties, Atterberg limits, maximum dry density, and optimum moisture content, and characterised based on triaxial compression test results. The triaxial compression test results provided shear strength properties, volumetric behaviour during shearing, critical state properties, and stress paths. Focus was placed on the influence of water on the behaviour of decomposed granite soil during different stages of the triaxial compression test, and on individual grains and/or aggregated particles which resulted in particle breakage after triaxial shearing. The amount of particle breakage was determined from the relative breakage of particles after triaxial shearing. Classification properties primarily depended on the degree of weathering for decomposed granite soil. The particle size distribution (PSD) shows that the decomposed granite soil ranges from gravel to clay size particles with the greatest amount of particles present in the medium- sand and fine-sand size ranges. Decomposed granite soil has a low plasticity with a typical fines content (<0.075mm) of around 15%. Descriptions for some of the soil properties are given below: • The mineralogy consists of quartz, plagioclase, K-feldspar, mica, kaolinite, and calcite. • SM “poorly graded silty sand” material (USCS), Class A-2-4 material (AASHTO system), and G8 quality “gravel soil” (TRH14 system). • Maximum dry density of 2120.5 kg/m3 by using modified AASHTO compaction effort, and an optimum moisture content of 8.5%. The mechanical behaviour of decomposed granite soil is evaluated through an isotropic compression and swelling test, and triaxial compression tests on saturated, dry, partially saturated, flooded, and air-dried specimens. Saturated results show that the soil conforms to the critical state Modified Cam-Clay model with a friction parameter, M = 1.41, an isotropic normal compression line λ = 0.073 and N = 1.88, and an unload reload line κ = 0.012. Partially saturated specimens (Sr = 20% - 80%) had similar strength and stiffness results as the saturated specimens indicating relatively small pore water suctions, and flooded specimens had a decrease in strength. Air-dried specimens reflected the effects of seasonal wetting and drying. Interparticle bonds gave the soil a cohesive component of strength whereas saturation of the soil caused the collapsible grain structure to collapse and cause a significant decrease in strength. Particle breakage of decomposed granite soil increased with increasing confining pressure and also with the presence of water. CT scans showed that larger particles primarily underwent particle degradation and that the process of particle breakage was mainly due to the separation of particle amalgams and breakage along pre-existing fissures.

AFRIKAANSE OPSOMMING: Plutone van Kaapse graniet is grootliks beperk tot die suidwestelike Kaap met uitsondering op voorvalle naby George en Namakwaland (Brink, 1981). Die samestelling en tekstuur van Kaapse granitoïede is baie kompleks en veranderlik, afhangende van die ligging van die plutone. Probleme wat verband hou met residuele granietgrond van die Kaapse graniet sluit in die teenwoordigheid van 'n swigversakkende korrelstruktuur. Residuele graniet gronde wat gedeeltelik versadig is bevat kwarts partikels wat bedek is met kolloïdale materiaal. Hierdie dra by tot ‘n klaarblyklike hoë krag in die granietgrond en wanneer dit versadig word onder lading, word die kolloïdale brûe onstabiel en veroorsaak dat die partikels saam pak in ‘n digter toestand wat lei tot skielike versakking van die fondasie. Hierdie navorsing dra by tot die klassifikasie en karakterisering van verweerde granietgrond van die Kaapse graniet in terme van die grond se fisiese eienskappe en ingenieurs gedrag. Die grond is geklassifiseer op grond van sy graderings eienskappe, Atterberg grense, maksimum droë digtheid en optimum voginhoud, en gekenmerk op grond van drieassige kompressietoets resultate. Die resultate van die drieassige kompressietoets het skuif sterkte eienskappe, volumetriese gedrag tydens skeer, eienskappe in kritieke toestande en spanning paaie. Fokus is geplaas op die invloed van water op die gedrag van verweerde granietgrond tydens verskillende stadiums van die drieassige kompressietoets, veral op individuele partikels en/of saamgestelde partikels wat gelei het tot partikel breking na drieassige skering. Die hoeveelheid partikel breking was bepaal deur die relatiewe breek van deeltjies na skeer te evalueer. Klassifikasie eienskappe hang hoofsaaklik af van die graad van verwering van die verweerde granietgrond. Die partikelgrootteverdeling-kurwe toon aan dat die verweerde granietgrond wissel van gruis tot klei-grootte partikels met die meeste hoeveelheid partikels teenwoordig in die medium-sand en fyn-sand grootte reekse. Verweerde granietgrond het 'n lae plastisiteit met 'n tipiese fyn stof inhoud (<0,075 mm) van ongeveer 15%. Beskrywings vir sommige van die grondeienskappe word hieronder gegee: • Die mineralogie bestaan uit kwarts, plagioklaas, K-veldspaat, mika, kaoliniet en kalsiet. • SM-materiaal met 'swak gegradeerde siltagtige sand' (USCS), klas A-2-4 materiaal (AASHTO stelsel) en 'gruisgrond' van G8-kwaliteit (TRH14 stelsel). • Maksimum droë digtheid van 2120,5 kg/m3 deur gebruik te maak van gewysigde AASHTO verdigtingspoging en 'n optimum voginhoud van 8.5%. Die meganiese gedrag van verweerde granietgrond is geëvalueer deur middel van 'n isotropiese druk-en-swel toets, en drieassige kompressietoetse op versadigde, droë, gedeeltelik versadigde, oorstroomde en luggedroogde monsters uit te voer. Versadigde resultate het getoon dat die grond voldoen aan die kritieke toestand gewysigde Cam-klei model met 'n wrywings parameter, M = 1.41, 'n isotropiese normale kompressie lyn λ = 0.073 en N = 1.88, en 'n aflaai- herlaai lyn κ = 0.012. Gedeeltelik versadigde monsters (Sr = 20% - 80%) het soortgelyke sterkte- en styfheid resultate gehad as die versadigde monsters wat aandui dat gedeeltelik versadigde monsters relatief klein porie water suiging het. Monsters wat oorstroom is, het 'n afname in sterkte gehad Luggedroogde monsters weerspieël die gevolge van seisoenale benatting en droging. Interpartikelbindings het die grond 'n samehangende komponent van sterkte gegee, terwyl versadiging van die grond veroorsaak het dat die swigversakkende korrelstruktuur in duie gestort het en 'n aansienlike afname in sterkte veroorsaak het. Partikel breuking van verweerde granietgrond het toegeneem met toenemende beperkte druk en ook met die teenwoordigheid van water. CT-skanderings het getoon dat groter deeltjies hoofsaaklik partikel breuking ondergaan het en dat die proses van partikel breuking hoofsaaklik te wyte was aan die skeiding van partikel-amalgame en breek langs bestaande splete.

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