The orapa untreated kalahari sand base experiment: performance over 30 years and derived material and pavement designs

Abstract

The purpose of this experiment on the Serowe-Orapa road in Botswana constructed in 1989 was to evaluate the performance of untreated, Kalahari fine red sand against similar sand treated with 2,5% bituminous emulsion and ferricrete gravel control sections. After 19 years, 0,3 MESA, two reseals and no patching the test and two control sections were still in a good to very good condition. DCP testing indicated that the sand base course had an in-situ CBR of about 160 and the pavement a total capacity in excess of 1,0 MISA and at least 0,5 MESA by a conservative extrapolation of rut depths, in comparison with about 0,8 MESA of the ETB. The high in-situ strength of the base was due to suction-induced apparent cohesion due to the low moisture contents. A specification derived for such an untreated red sand base course on similar red sand lower layers with untreated red sand sealed shoulders and a double seal on the carriageway for up to 0,5 MESA for a South African Category C, 0,6 for a Category D and 0,8 MESA for a Botswana Category II road over 20-30 years includes the following: GM 1,10-1,20; Passing 425 μm 75-85%; PI on P425 μm NP, on P075 μm 2-40; min. soaked MAASHO CBR 60; min. CBD-extractable Fe 0,3% or 1,5% Fe203 by XRF analysis. Whilst extra care is needed in construction and surfacing, such a pavement has tremendous potential for economical, low-volume, sealed roads in the vast area covered by Kalahari sands in which rock and conventional gravels are scarce.