The engineering properties and road building characteristics of mudrocks, with special reference to Southern Africa

Abstract

English: Mudrocks occur in many parts of southern Africa and large quantities are used in road construction. This study involved research on the "state of the art" of the road construction aspects of mudrocks in southern Africa, as well as the general engineering-geological properties of such rocks and the assessment of the most appropriate tests for classifying mudrocks for road construction purposes. The "state of the art" investigation, done by means of interviews with users of the material, revealed much confusion about the terminology related to these rocks. It was therefore decided to use the term "mudrock" as the all-embracing term with "shale" and "mudstone" indicating fissile and massive varieties respectively. The survey also found mudrock to be an important construction material, especially in the Cape Province and Natal, where it is used up to subbase level. Large differences of opinion exist regarding the general road-building qualities of mudrocks. No particular problem pertaining to construction with mudrock was identified, but the break-down phenomenon on exposure gave rise to most of the concern expressed. It was felt that other tests, additional to the standard ones used in road construction, are necessary to characterize mudrocks. A wide variety of property investigations and tests was performed on 14 mudrocks, sampled as soon as possible after excavation, from different geological formations throughout South Africa. Although the samples were selected to cover a wide range of engineering geological properties, since they were limited in number it cannot be claimed that they are necessarily representative of the whole range of southern African mudrocks. Standard road construction tests showed some samples to be of subbase standard. Plasticity indices were usually low. The addition of four per cent lime generally caused a marked increase in the CBR strength. An "accelerated weathering" test, in which CBR compactions were followed by wet-dry cycles, indicated the different rates of break-down of the samples. Extensive free swell tests on cubes, pretreated differently before immersion in water, showed that the majority of samples expand less than one percent during immersion after oven-drying. tended to break down expanded more. Air-drying increased the expansiveness of the rocks to levels similar to those obtained after oven-drying. The mudrocks were found to absorb water at different rates. In an experiment during which test cubes were subjected to temperature and humidity changes, it was found that the mudrocks were more sensitive to humidity changes, both as far as volume changes and moisture adsorption were concerned. However, even large variations in temperature and humidity were not able to produce a visual break-down such as was observed for some samples when they were immersed in water. The results from classification tests were investigated and correlated to select tests which showed the best possibilities for mudrock classification. It is considered that a road construction material needs to be resistant to crushing, abrasion, and decomposition. The 10 per cent FACT, carried out on dried and soaked samples, is preferred for testing the resistance to crushing. A wet ball mill test was developed for testing resistance to abrasion. This test was more successful in separating samples of different qualities than the Los Angeles abrasion test and the results correlated well with those obtained from an ultrasonic disaggregation test. The sand equivalent test is recommended to determine the presence of deleterious material in the crushed mudrock. The break-down of mudrocks at various rates on exposure is not fully evaluated by the above tests, nor by the standard road construction tests. Two varieties of the break-down phenomenon were observed although there appears to be a gradation between the two processes, i.e. the break-down of the rock into hard fragments (disintegration) and the break-down into silt or clay-sized particles (slaking). It is considered that slaking can be measured by means of the slake durability test, but disintegration should be evaluated qualitatively by performing a five-cycle wet-dry test, using water. Tests, such as the Washington degradation, ethylene glycol soaking, methylene blue adsorption, sodium sulphate soundness, rate of slaking and conductivity, were found to be unsuitable for general classification purposes.

Afrikaans: Modderrots word in verskeie dele van suidelike Afrika aangetref en groat hoeveelhede daarvan word in padbou gebruik. In hierdie studie is die huidige gebruike, sowel as die algemene ingenieurs-geologiese eienskappe van sodanige rots en die beoordeling van die mees geskikte toetse met betrekking tot die klassifikasie van modderrots vir padbou-doeleindes in suidelike Afrika, ondersoek. Die ondersoek na die huidige gebruike wat verband hou met modderrots, gedoen deur onderhoude met gebruikers van die materiaal te voer, het getoon dat daar heelwat verwarring bestaan oar die terminologie wat betrekking het op hierdie rotse. Daar is dus besluit om die term "modderrots" as die allesomvattende term te gebruik met "skalie" en "moddersteen" onderskeidelik as fissiele en massiewe tipes. Die opname het oak getoon dat modderrots n belangrike konstruksie-materiaal is, veral in die Kaapprovinsie en Natal waar dit tot in die stutlaag gebruik word. Daar bestaan groat meningsverskille oar die algemene padboueienskappe van modderrots. Geen besondere probleem behorende tot the gebruik van modderrots in padbou is waargeneem nie maar die verskynsel van opbreek na blootstelling wek die meeste kommer. Daar is gevoel dat, benewens die standaardpadboutoetse, nag bykomende toetse nodig is vir die karakterisering van modderrotse. n Groot verskeidenheid ondersoeke na en toetse van die eienskappe is uitgevoer op 14 modderrotse, waarvan monsters so gou moontlik na uitgrawing uit verskillende geologiese formasies dwarsoor Suid-Afrika verkry is. Alhoewel die monsters gekies is om n wye reeks ingenieurs-geologiese eienskappe te dek, is hulle vanwee die beperkte getal nie noodwendig verteenwoordigend van die hele reeks suider Afrikaanse modderrotse nie. Standaard padboutoetse het getoon dat sommige van die monsters van stutlaaggehalte was. Plastisiteitsindekse was oar die algemeen laag terwyl die byvoeging van vier persent kalk gewoonlik n aansienlike KDV sterktetoename tot gevolg gehad het. n Versnelde verweringstoets waartydens KDV-verdigting gevolg is deur nat-droogsiklusse het die verskillende tempo's van opbreking van die monsters getoon. Uitgebreide vrysweltoetse, uitgevoer op kubusse wat verskillend behandel is voor indompeling in die water, het getoon dat meeste van die monsters minder as een persent swel tydens indompeling na oonddroging. Sommige verweerde rotse en diegene wat geneig was om op te breek het egter meer geswel. Lugdroging het die uitswelvermoe van die rotse verhoog tot op dieselfde vlak as die wat na oonddroging verkry is. Daar is gevind dat modderrotse water teen verskillende tempo's absorbeer. In 'n eksperiment waartydens toetskubusse aan temperatuur- en humiditeitsveranderinge blootgestel is, is daar gevind dat modderrots meer gevoelig is vir humiditeitsveranderinge. Dit geld vir beide volume-veranderinge en vogadsorpsie. Nietemin, kon selfs groot wisselings in temperatuur en humiditeit nie sigbare opbreking te weeg bring nie, soos wat wel waargeneem is by sommige van die monsters by indompeling in water. Die resultate verkry uit klassifikasietoetse is ondersoek en gekorreleer om die mees geskikte toetse vir modderrots-klassifikasie te kies. Daar word geag dat 'n padboumateriaal bestand moet wees teen vergruising, afskuring en ontbinding. Die 10 persent FAVT, uitgevoer op gedroogde en geweekte monsters, word vir die toets van bestandheid teen vergruising verkies. 'n Nat balmeultoets is vir die toets van bestandheid teen afskuring ontwikkel. Hierdie toets was meer suksesvol in die skeiding van monsters van verskillende kwaliteite as die Los Angeles-afskuurtoets, en die resultate het goed gekorreleer met die wat verkry is uit 'n ultrasoniese disaggregasietoets. Die sand-ekwivalenttoets word aanbeveel om die teenwoordigheid van skadelike materiaal in die vergruisde modderrots te bepaal. Die opbreking van modderrots teen verskillende tempo's as gevolg van blootstelling word nie deur bostaande toetse of deur die standaardpadboutoetse geevalueer nie. Twee variasies van die opbreekverskynsel is waargeneem alhoewel dit blyk dat daar nie 'n definitiewe skeiding tussen die twee prosesse is nie; d.i. opbreking van die rots tot harde stukkies (disintegrasie) en opbreking na slik- of kleigrootte partikels (blus). Daar word geag dat blus deur middel van die blusduursaamheidstoets bepaal kan word, maar disintegrasie moet kwalitatief evalueer word deur 'n vyfsiklus nat-droogtoets met water uit te voer. Toetse soos Washingtondegradasie, weking in etileenglikol, metileenblou-adsorpsie, natriumsulfaatgaafheid, tempo van blus en geleidingsvermoe is as ongeskik vir algemene klassifikasie doeleindes bevind.