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Journal of the South African Institution of Civil Engineering

On-line version ISSN 2309-8775
Print version ISSN 1021-2019

J. S. Afr. Inst. Civ. Eng. vol.50 n.1 Midrand Mar. 2008




Why stabilise? Using triaxial tests for determining pavement stiffness and shear strength parameters of mechanically modified layers



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



Full text available only in PDF format.




ASTM 2003. Test method 'D 2850 - 03a Standard Test Method for Unconsolidated-Undrained Triaxial Compression Test on Cohesive Soils' as approved on 1 November 2003.         [ Links ]

COLTO 1998. Standard specifications for road and bridge works for state road authorities. Committee of Land Transport Officials.         [ Links ]

De Beer, M, Fisher, C and Kannemeyer, L 2004. Tyre-pavement interface contact stresses on flexible pavements - quo vadis. Proceedings of the 8th Conference on Asphalt Pavements for Southern Africa (CAPSA'04).         [ Links ]

Deer, W A, Howie, R A and Zussman, J 1966. An introduction to the rock forming minerals. London: Longman.         [ Links ]

Duncan, J M and Chang, C-Y 1970. Nonlinear analysis of stress and strain in soils. Journal of the Soil Mechanics and Foundation Division, ASCE, 96(5):1629-1653.         [ Links ]

Duncan, J M, Byrne, P, Wong, K S and Mabry, P 1980. Strength, stress-strain, and bulk modulus parameters for finite element analyses of stresses and movements in soil masses. Report No UCB/GT/80-01 of the Charles E Via, Jr Department of Civil Engineering, Virginia Polytechnic Institute and State University.         [ Links ]

Emery, S J 1985. Prediction of moisture content for use in pavement design. PhD thesis, University of the Witswatersrand, Johannesburg.         [ Links ]

Grabe, P J and Clayton, C R I 2005. Designing railway formations for the future. Civil Engineering, 13(5):16-19.         [ Links ]

Harr, M E 1987. Reliability based design in civil engineering. New York: McGraw-Hill.         [ Links ]

Haupt, F J. 1980. Moisture conditions associated with pavements in Southern Africa. MSc dissertation, University of the Witwatersrand, Johannesburg.         [ Links ]

Hugo, F and Engelbrecht, J C 1982. [In Afrikaans.] The evaluation of the elastic properties of ballast material under cyclic loading. The Civil Engineer in South Africa: Transactions of the South African Institution of Civil Engineers, 23(4):145-157.

Janbu, N 1963. Soil compressibility as determined by oedometer and triaxial tests. Proceedings of the European Conference on Soil Mechanics and Foundation Engineering, Wiesbaden, Germany, 1:19-25.         [ Links ]

Jooste, F J 2004. A re-evaluation of some aspects of the mechanistic-empirical design approach (Paper 122). Proceedings of the 8th Conference on Asphalt Pavements for Southern Africa (CAPSA'04).         [ Links ]

Kondner, R L 1963. Hyperbolic stress-strain response: cohesive soils. Journal of the Soil Mechanics and Foundation Division, ASCE, 89(1):283-310.         [ Links ]

Kondner, R L and Zelasko, J S 1963a. A hyperbolic stress-strain formulation for sands. Proceedings of the 2nd Pan-American Conference on Soil Mechanics and Foundation Engineering, Brazil, 1:289-324.         [ Links ]

Kondner, R L and Zelasko, J S 1963b. Void ratio effects on the hyperbolic stress-strain response of sand. In Laboratory shear testing of soils. ASTM STP 361, Ottawa.         [ Links ]

Kondner, R L and Horner, J M 1965. Triaxial compression of a cohesive soil with effective octahedral stress control. Canadian Geotechnical Journal, 2(1):40-52.         [ Links ]

Lambe, T W and Whitman, R V 1969. Soil mechanics. New York: Wiley.         [ Links ]

Maree, J H 1979. The results of single stage and repeated loading triaxial shear tests on a range of crushed stone materials.         [ Links ] [In Afrikaans.] National Institute for Transport and Road Research, CSIR, South Africa. Technical Report RP/12/78.

Maree, J H 1982. Aspects of the design and behaviour of pavements with granular base layers.         [ Links ] [In Afrikaans.] PhD thesis, Department of Civil Engineering, University of Pretoria.

Packard, R G 1967. Computer program for airport pavement design (SR029.02P). Skokie, Ill: Portland Cement Association.         [ Links ]

Parrock, A L 2007. Personal contact with Dr Herman Wolff.         [ Links ]

Schneider, H R 1997. Panel discussion: Definition and determination of characteristic soil properties. Proceedings of the Fourteenth International Conference on Soil Mechanics and Foundation Engineering, Hamburg, 6-12 September, 4:2271-2274.         [ Links ]

Simpson, B 1995. Limit state design in geotechnical engineering. Proceedings of a seminar introducing Eurocode 7 to South Africa, October.         [ Links ]

Simpson, B 1997. Theme lecture: Some questions about limit state design in geotechnical engineering. Proceedings of the Fourteenth International Conference on Soil Mechanics and Foundation Engineering, Hamburg, 6-12 September, 4:2267-2270.         [ Links ]

Taute, A, Verhaeghe, B M J A and Visser, A T 2001. Interim guidelines for the design of hot-mix asphalt in South Africa. Prepared as part of the Hot-Mix Asphalt Design Project, September.         [ Links ]

TMH1 1979. Standard methods of testing road construction materials. Technical methods for highways. Pretoria: National Institute for Transport and Road Research, CSIR.         [ Links ]

TRH 14 1985 (reprinted 1989). Guidelines for road construction materials. Published for the Department of Transport for the Committee for State Road Authorities.         [ Links ]

Wolff, H 1992. The elasto-plastic behaviour of granular pavement layers in South Africa. PhD thesis, Department of Civil Engineering, Faculty of Engineering, University of Pretoria.         [ Links ]

Wolff, H 1985. Mechanistic analysis of an existing rail track with a bound layer.         [ Links ] [In Afrikaans.] MEng thesis, University of Stellenbosch.



Alan Parrock's
ARQ (Pty) Ltd
PO Box 76379
Lynnwood Ridge 0040
T +27-12-348-6668
F +27-12-348-6669

Jaco Strydom
Namibia Technical Services CC
PO Box 30623
Pioneers Park
Windhoek 9000, Namibia
T +264-61-215-324
F +264-61-215-327

Victor Rieger's
Windhoek Consulting Engineers (Pty) Ltd
PO Box 2484
Windhoek 9000, Namibia
T +264-64-206-401
F +264-64-206-525




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.

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