Rock mechanics for construction of the gravimeter vault at the Matjiesfontein Space Geodesy and Earth Observation Observatory

Van Wyk, Peter Ryan (2013-12)

Thesis (MEng)--Stellenbosch University, 2013.

Thesis

ENGLISH ABSTRACT: The suitability of local construction materials for construction purposes is governed by several rock mechanical properties. Strength, durability, performance and petrography of aggregates all influence the decisions engineers make in deciding if the aggregate is suitable and sustainable throughout the lifetime of a structure. This thesis investigates these properties by combining engineering, chemistry and geological disciplines to make informed decisions. The pertaining project for which the research was conducted is the construction of the gravimeter vault at the Matjiesfontein Geodesy and Earth Observation Observatory (MGO) although the research acquired can be used for other projects of a similar nature and other outbuildings at the MGO. Material at and around the site were tested for strength and durability according to certain South African National Standards (SANS). Slake durability was tested as certain rock types tend to slake when exposed to the atmosphere such as tillite of the Dwyka formation and shale of the Karoo Supergroup. Concrete cube strength was determined on cubes containing crushed rock from Matjiesfontein as well as river sand. Cube strength was conducted to analyse performance and to establish a mix design that would be sustainable throughout the lifetime of the project. Petrographic examination using Powder X-ray diffraction (PXRD), X-ray fluorescence (XRF), Scanning Electron Microscopy (SEM) and inspection under a petrographic microscope were conducted. These methods were used to determine if a risk exists for alkali-silica reactivity (ASR) in concrete when the rock types are used as aggregate, particularly if high quartz-bearing rock types such as quartzite of the Table Mountain group were to be used as coarse aggregate. Inspection of thin sections for strained quartz under a petrographic microscope and SEM imaging were important in determining if ASR may occur. The gravimeter vault was constructed using materials sourced mainly from Laingsburg. Tillite satisfied all tests and analyses conducted during the research, making it more suitable for construction than the other materials from Matjiesfontein. The slake durability test indicates that tillite is nearly as durable as quartzite, which is considered the most durable rock type at Matjiesfontein. This is due to atmospheric conditions in the Karoo being very dry with low humidity in comparison to the coast where slaking is known to occur. Slaking properties were most prominent for shale at Matjiesfontein and if excavated, it is recommended to cover the exposed shale with a layer of asphalt or cement to prevent slaking. The 10% fines aggregate crushing test (10%FACT) value for tillite was over the 210 kN prerequisite and the wet-to-dry ratio over 75% making it suitable for road construction according to the 10%FACT. All cube tests reached the desired 40 MPa prerequisite although the mixtures containing local river sand were unworkable. Unlike quartzite and quartzitic sandstone from Matjiesfontein, tillite is low in quartz and has minimal strained quartz. Therefore, no risk exists for ASR if tillite were to be used as aggregate in concrete.

AFRIKAANSE OPSOMMING: Die geskiktheid van plaaslike konstruksiemateriale vir konstruksiedoeleindes word deur sekere rots meganiese eienskappe beïnvloed. Sterkte, duursaamheid, volhoubaarheid en petrografie van aggregaat beïnvloed die besluite wat ingenieurs moet neem sodat die aggregaat aan standaarde voldoen en gedurende die leeftyd van ‘n struktuur volhoubaar is. Hierdie tesis ondersoek die genoemde eienskappe deur ingenieurs-, chemiese- en geologiese dissiplines te kombineer. Die voorgenome projek, waarvoor die navorsing ter sprake is, is vir die konstruksie van die gravimeterkluis by die “Matjiesfontein Geodesy and Earth Observation Observatory (MGO)”, alhoewel die navorsing ook gebruik kan word vir soortgelyke projekte, sowel as die konstruksie van die res van die geboue by die MGO. Materiale van die terrein en die omgewing is volgens sekere Suid-Afrikaanse kodes vir sterkte en duursaamheid getoets. Die blusbaarheid van materiale is getoets omdat sekere materiale, soos tilliet van die Dwyka-formasie en skalie van die Karoo Supergroep blus wanneer dit aan die atmosfeer blootgestel word. Die betonsterkte van kubusse, waarin plaaslike gesteentes en riviersand van Matjiesfontein vir aggregaat gebruik is, is bepaal. Die kubusse is getoets om die sterkte daarvan te analiseer en om ‘n betonmengsel, wat tydens die leeftyd van die projek volhoubaar is, daar te stel. Petrografiese eksaminering deur X-straal difraksie (XRD), X-straal fluoressensie (XRF), Skandeerelektronmikroskopie (SEM) en inspeksie onder ‘n petrografiese mikroskoop is gedoen. Met die doel om die petrografiese samestelling van materiale van Matjiesfontein te bepaal, is hierdie metodes gevolg om te uit te vind of daar ‘n risiko vir alkalie-silikaat reaksies (ASR) bestaan, as die gesteentes in beton gebruik word. Veral gesteentes met ‘n hoë hoeveelheid kwarts, soos kwartsiet van die Tafelberg-groep, is hier ter sprake. Inspeksie van dunsnitte vir gespanne kwarts onder ‘n petrografiese mikroskoop en SEM was belangrik om die risiko vir ASR te bepaal. Die gravimeterkluis is hoofsaaklik met materiale afkomstig van Laingsburg gebou. In vergelyking met die ander gesteentes by Matjiesfontein is tilliet, volgens alle toetsparameters, die mees geskikte gesteente vir konstruksiedoeleindes. Die blusbaarheid van tilliet vergelyk goed met kwartsiet, wat as die sterkste en duursaamste gesteente by Matjiesfontein, beskou word. Die rede hiervoor is die atmosferiese toestande wat baie droër is, asook die laer humiditeit in die Karoo, in vergelyking met gebiede nader aan die kus waar blusting meer algemeen voorkom. Die blusbaarheid van skalie by Matjiesfontein kom algemeen voor. As daar dus uitgrawing in hierdie gebied plaasvind, word dit aanbeveel dat ‘n laag sement of asfalt oorgegooi word om die blusting te verhoed. Die 10%FACT waarde vir tilliet was bo die vereiste 210 kN, asook bo die 75% nat-teenoor-droog-verhouding en daarom is dit volgens die 10%FACT-toets as padboumateriaal geskik. Alhoewel mengsels, wat plaaslike riviersand bevat het, onwerkbaar was, het alle toetsmonsters (kubusse) die vereiste 40 MPa- sterkte bereik. In vergelyking met kwartsiet en kwarsitiese sandsteen, bevat tilliet min kwarts en ook minimale gespanne kwarts. Daar bestaan dus geen risiko vir ASR indien tilliet van Matjiesfontien in die beton gebruik word nie.

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