A performance study on flexible road pavements in Kenya

Abstract

This research study set out to evaluate the pe rrorrnanoe o f some six road test sections locat.ed on in-service bitumen standard roads in and around Nairobi, Kenya. The selected road test sections were:- (L) Thika road test. site ES1, w i t.h a s':;.rface dressing, 305·rom s t ab Ll Lzed mu rram base and 457 mm murram sub-base; (ii) Langata road test site ~S2, with a surface dressing, 229 ~~ crushed stone base and 229 n~ murram sub-base; Mombasa road test site ES asphaltic concrete surfacing, 130 mm crusher-run base, 200 mm crushed stone sub-base and 300 mm improved subgrade; (L v) 1-1ombasaroad test site ES4 I with a 100 mm asphaltic concrete surfacing, 165 mm crusher-run base, 165 nun crusher-run sub- / ~ 150 r.~improved subgrade; (v) Mombasa road test site.ESS, with a lQO mm

packed stone base, 200 rom soft stone subbase and 300 mm improved subgrade; (vi) Mombasa road test site BSf,; •.,.;•.th a H)O r.'.;". asph aLtic concrete surf ac ing, 200 nun hand

packed stone base, 200 mm soft stone subbase and 300 mm improved subgrade. The study also included a laboratory investigation of the strength characteristics of some of the subgrade soils used in the construction of the roads covered in this study. Also included is an analytical study of the influence of flexible pavement geometry and material characteristics on various pavement distress parameters, using variational techniques and a digital computer. Finally, a basis upon which flexible pavement design could be developed in Kenya was also considered. Flexible pavement performance evaluation was done using data collected periodically over a period of 16 months from the following field studies:- (a) pavement rebound deflection measurements using Benkelman deflection beams and 3175 kg dual wheel loads, (b) pavement deflection profile measurements using the same apparatus as in (a) above, (c) pavement settlement measurements using a Wild Ni 3 precise level, (d) cracking measurements and (e) rutting measurements The subgrade soil strength characteristics were investigated using California Bearing Ratio (C.B.R)

and controlled strain triaxial compression tests. The analytical study of the influence of pavement geometry and material characteristics on pavement deflections, stresses and strains in various layers was carried out using the following computer programs, based on the finite-element technique:- (a) PAVE - a program based on a triangular element mesh formulation, (b) PAST - a program based on isoparametric element mesh formulations, (c) ROAD - a triangular element program capable of pavement fatigue cracking simulation, (d) UNIV - a triangular element program capable of elastic modulus variation as a function of stress. The study found that considering the design standards of the road test sections and the traffic traversing them, the t'st sections were in general performing satisfactorily. Pavement. age, traffic and climate were found to be some of the major factors affecting pavement performance. Evidence was found to suggest that for cracked pavement sections',.rebound deflections provided reliable / indications of pavement weakness. This relationship was however not found to hold in the case of rutting. The tangent moduli of the subgrade soils tested

were found to be a linear functio~ of confining pressure. The soils were found to exhibit a peaked CB~ - moist~re cc~tc~t relationship. =vi2enc~ ~~~ found to suggest that the resilience (deformation moduli) characteristics of the subgrade soils tested i~lluenced the performance of the pavemcn~s fou~ded on them, irrespective of the magnitudes of CBRs of thesesoi.ls, thus rendering CBR alone inadequate as a measure of subgrade soil strength. The use of·the finite-element technique in predictive analysis in the field of flexible pavement design and performance monitoring relies heavily on how accurately the behaviour-and strength parameters of the pavement materials can be determined. The technique as used in the analysis presented in this thesis was found to habour great potential as a:l analytical tool for pavement analysis .