1. a Council for Scientific and Industrial Research, P.O. Box 320, Stellenbosch 7599, South Africa
2. b Deutsches GeoForschungs Zentrum, Claude-Dornierstr. 1, Gebäude 401, Raum 1.05, Weßling 82234, Germany
3. c Department of Civil Engineering, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa