The influence of geotechnical properties on the performance of gravel wearing course materials

Abstract

Unpaved roads comprise about 72 percent of the national and provincial road network in South Africa and substantially more of the total southern African road network. Significant costs are incurred annually by the authorities on the maintenance of these roads and enormous sums are associated with the cost of using these roads. Specifications for the use of materials for unpaved roads in southern Africa abound, the origin of most of these, however being rather obscure. It would appear that many of them have been transferred from other countries, mostly in the northern hemisphere. The geological materials used for construction in southern Africa, having been subjected to aeons of weathering and minimal major periods of recent glaciation, differ greatly from those in the northern hemisphere. One of the main objectives of this research was to investigate the performance of local geological materials used in wearing courses for unpaved roads and relate this to their geotechnical properties. This involved the sampling, testing and monitoring of 110 sections of unpaved road in the Transvaal and South West Africa over a period of more than three years. A large data base of gravel height, roughness, rut and corrugation measurements and the severity and extent of dustiness, stoniness, potholes, cracks, loose material, slipperiness and trafficability was compiled. The material and performance data were then used to develop performance related specifications and prediction models for roughness progression and gravel loss with the aim of providing improved unpaved roads and consequently reducing the road user and maintenance costs, in the national interest. Some of the savings will obviously be off-set to some extent by slightly increased materials location and construction costs. These costs are, however, shown to be minimal in comparison with the potential savings to the road user, the national economy and the generally improved quality of life of many road users in southern Africa. A good knowledge of the geotechnical properties of potential wearing course materials (which are significantly dependent on the geological origin) is necessary to differentiate good gravels from poor ones.

Ongeplaveide paaie maak ongeveer 72 persent uit van die nasionale en provinsiale padnetwerk in Suid Afrika en heelwat meer van die totale padnetwerk van suidelike Afrika. Groot kostes word jaarliks aangegaan deur die owerhede vir die instandhouding van hierdie paaie, asook enorme kosts wat in verband gebring kan word met die gebruik van hierdie paaie. Materiaalspesifikasies vir ongeplaveide paaie in suidelike Afrika is volop. Die oorsprong van die meeste hiervan is egter twyfelagtig en dit wil voorkom of dit uit ander lande, hoofsaaklik in die noordelike halfrond, afkomstig is. Die geologiese materiale gebruik vir padkonstruksie in suidelike Afrika verskil grootliks van die in die noordelike halfrond. Dit was onderwerp aan eeue se verwering en minimale groot-skaalse onlangse glasiale werking verskil grootliks van die. Een van die hoof doelwitte van hierdie navorsing was om ondersoek in te stel na die werkverrigting van plaaslike geologiese materiale wat gebruik word vir die slytlaag van ongeplaveide paaie en om dit in verband te bring met hul Geotegniese eienskappe. Dit het behels monsterneming en toets van materiale, asook monitering van 110 seksies ongeplaveide pad in Transvaal en Suid-Wes Afrika oor 'n tydperk van drie jaar. 'n Groot databasis bestaande uit inligting oor metings van gruishoogte, ongelykheid, spoor en sinkplaat asook inligting van graad en omvang van stof, klipperigheid, slaggate, krake, los materiaal, glibberigheid en rybaarheid is saamgestel. Die materiaal- en werkverrigtingsdata is toe gebruik om werkverrigtingsverwante spesifikasies op te stel, asook om modelle daar te stel om verswakking in gelykheid, en gruisverlies te voorspel. Die doel is om beter ongeplaveide paaie te voorsien en gevolglik die padverbruikers- en onderhoudskoste in nasionale belang te verminder. 'n Gedeelte van die besparing sal onvermydelik deur effens duurder konatruksiekoste en moontlike langer vervoerafstande van materiaal gekanselleer word. Hiedie koste is egter minimaal in verhouding tot die potensiele besparing vir die padgebruiker, die nasionale ekonomie en die algemene verbetering in lewenskwaliteit van baie padgebruikers in suidelike Afrika. 'n Goeie kennis van die geotegniese eienskappe van potensiele slytlaagmateriale (wat grootliks afhanklik is van geologiese oorsprong) is noodsaaklik om te differensieer tussen goeie an swak gruise.