On-line version ISSN 2309-8775
J. S. Afr. Inst. Civ. Eng. vol.50 n.1 Midrand Mar. 2008
A L Parrock; J S Strydom (visitor); V J Rieger
Triaxial testing of naturally occurring, slightly silty, medium-graded, coarse sand, derived from completely weathered granite (with some gypsum), compacted to 95 % of Mod AASHTO density generated unload/reload Young's modulus E-values of about 300 MPa at a representative confining stress of 90 kPa. This is some 80 % higher than what would usually be expected for this type of G5 material.
Mohr-Coulomb shear strength parameters of c = 15,9 kPa and φ = 51,4° were obtained from the high-quality triaxial tests.
Taking into account the variability of the materials, it is suggested that these be downgraded to c = 12,7 kPa and φ = 48,8° for use as design parameters.
It is common practice to stabilise natural gravel materials to generate weakly cemented sub-base layers. However, the presence of naturally occurring gypsum within the in-situ granite generated concern as negative effects were observed on elements of past construction.
The use of cement as a stabilising agent was eliminated by generating a nearly equally strong layer by mixing the naturally occurring gravels with varying quantities of crushed stone, crusher waste and dune sand, the latter to combat a high plasticity index. The best result was obtained by using a blend of 50 % natural gravel, 30 % crusher waste and 20 % dune sand.
These blended materials, generated unload/reload Young's modulus E-values of some 560 MPa at 95 % Mod AASHTO compaction and a confining stress of 247 kPa. This E-value is very similar to what is thought would be attained for a cracked and hydrated cement-stabilised layer but without the disruptive effects of a lowered confining stress that would be the case when a stabilised layer shrunk and cracked on hydration of the cement stabilising agent.
Keywords: Triaxial testing, unload/reload Young's modulus, confining stress, shear strength, airport pavement, mechanical modification
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During ALAN PARROCK's 34 years of postgraduate experience, he has specialised in the geotechnics and geostochastics of soil and rock masses with particular reference to roads/pavements. Alan was initially exposed to road materials design and construction during his tenure at the Natal Roads Department. His skills were further honed as a researcher in the materials and design branch of the National Institute of Transport and Road Research in Pretoria. He advanced his pavement/surfacing expertise under the world-renowned Professor Carl Monismith of Berkeley, during postgraduate study at Stellenbosch in the early 1980s. Since that time Alan has been involved in various road projects at the specialist level; forensic audits of distressed pavements in Botswana, principal components analysis for binders used on contracts, dynamic creep evaluations and recently, triaxial testing of pavements materials on an airport contract in Namibia.
JACO STRYDOM obtained his National Diploma in Civil Engineering in 1995. As a qualified engineering technician, Jaco has been involved in numerous civil engineering projects in South Africa, Lesotho and more recently in Namibia relating to all aspects of materials investigations, materials designs, laboratory testing and contract supervision. Jaco has extensive experience with the interpretation and application of a multitude of standard specifications relating to civil engineering materials. He has also gained extensive experience in asphalt overlays, rehabilitation of roads, bridges, retaining and drainage structures, buildings and civil structures, and geotechnical Investigations. Recently Jaco did extensive research into and also gained considerable experience concerning the evaluation of pavement materials by means of triaxial testing.
During VICTORRIEGER's nine years of postgraduate experience he has been involved in numerous road and rail construction projects. Victor joined the Transportation Department of Windhoek Consulting Engineers in July 2000. His first major exposure was the design and construction supervision of arterial and collector roads for the City of Windhoek and the design of the Northern Railway Extension Project in Namibia. Victor currently heads the Coastal Office for Windhoek Consulting Engineers and also acts as the client's site representative on the Walvis Bay Airport Project where he and his team are responsible for monitoring the construction quality.