*3.1. Validation of Kitwe-Ndola Road*

Results of the measurements after profile analysis and field trenching at Chainages; 60 + 365, 60 + 427, 60 + 437 and 60 + 487 of Kitwe-Ndola Road are shown in Figures 10–17.

**Figure 10.** Transverse profile of 60 + 365 km.

**Figure 11.** Profile of trench measurements of 60 + 365 km.

**Figure 12.** Transverse profile of 60 + 427 km.

**Figure 14.** Transverse profile of 60 + 437 km.

**Figure 15.** Profile of trench measurements of 60 + 437 km.

**Figure 16.** Transverse profile of 60 + 487 km.

**Figure 17.** Profile of trench measurements of 60 + 487 km.

From the profiles it is evident that the rutting at 60 + 365 km and 60 + 427 km resulted from within the asphalt surfacing. This is evidenced by the shoving and upheaval of the asphalt, which is clearly visible in Figures 10 and 12. It can also be seen from Figures 11 and 13 that the profile of the top of the asphalt surfacing is different from that of the top of the base, subbase and subgrade.

The profiles at 60 + 437 km and 60 + 487 km, on the other hand, show that the underlying layers are responsible for the rutting failure. The asphalt profile is almost equivalent to the underlying layers and there is little to no shoving and/or upheaval of the asphalt.

The TPAM was undertaken to verify these observations. Distortion parameters where determined and calculated as outlined in Section 2. For example, for profile Chainage (CH) 60 + 365;


The procedure was repeated for all the profiles on the road (Table 2). It was established that at 60 + 365 km and 60 + 427 km the rutting was in the asphalt while at 60 + 437 km and 60 + 487 km the rutting emanated from the underlying base/subbase. This is consistent with the observed profile shape.


**Table 2.** Kitwe-Ndola Road profile analysis.

The results of the profile analysis were compared to measurements from the trenching. The results showed consistency between the two methods (Table 3).

**Table 3.** Comparison of trench measurements vs. profile analysis.


#### *3.2. Chibuluma and Kitwe-Chingola Roads*

After the validation, the profile analysis was conducted on sections of the Chibuluma and Kitwe-Chingola Roads. The results are shown in Tables 4 and 5, respectively.


**Table 4.** Chibuluma Road profile analysis.

**Table 5.** Kitwe-Chingola Road profile analysis.


#### **4. Discussion**

Results indicated that in all cases, the ratio of area (R) was greater than 0.05 (the limit between HMA and base/subbase failure). This is an indication of the profiles having significant positive areas. It was also determined that the total area (A) was significantly greater than the combination of the critical coefficients ((C1+ C2)/2), as shown in Tables 4 and 5 for Chibuluma and Kitwe-Chingola Roads, respectively. This meets both criteria for HMA failure and thus indicates that the asphalt was mainly responsible for the rutting failure. These results are consistent with the observed shape of the rutted road sections which were relatively deeply depressed and accompanied by upheavals along the sides of the wheelpath. The results are also consistent with recent research suggesting that rutting mainly occurs in the asphalt (surfacing) layer.

These results are an indication of inadequacy in the performance of the asphalt. Zambia, like most developing countries, is still using empirical pavement design methods whose focus is mostly on protecting the subgrade, thus leaving rutting to be assessed during the mix design of the asphalt. The results therefore call for particular attention to be paid to the asphalt mix design and construction in order to avoid/minimize this problem. Some of the measures that should be considered, as suggested in the literature as well, include:

1. Mix design method: The Marshall mix design method used in empirical designs has proven to be unsuitable for present day traffic, as evidenced by the steady increase in rutting problems [2]. A study by Verhaeghe et al. [16] suggested that the Marshall mix design method can be used for lower traffic or non-rut potential situations, but for high traffic or rut potential situations, gyratory-based design methods should be used, possibly coupled with performance tests.


#### **5. Conclusions**

Rutting failure is a common problem on most Zambian roads. The determination of which of the pavement layers is responsible for the failure is the first step in remediating the problem. The objective of this study was to determine the source of rutting failure in some selected sections of road using the TPAM.

It was established on both Chibuluma Road and Kitwe-Chingola Road that rutting failure mostly emanated from the surfacing layers. This is consistent with the observed shape of the rutted sections. The sections of the Kitwe-Ndola Road analyzed indicated a half split with two sections indicating HMA failure and the other two indicating base/subbase failure.

It was also established that the TPAM is an effective way of determining the pavement layer(s) responsible for rutting failure. The method showed very good comparability with the trench method. This indicates that the inferences made in the TPAM criteria are consistent with the traditional methods of rutting failure determination and can therefore perform the same function.

The TPAM is simple and less time consuming than traditional methods. A task which would have taken at least a couple of days was completed within a matter of a few hours. There was also less disturbance to traffic and, most importantly, the proposed method eliminated the need for expensive destructive testing. This tool will therefore be invaluable to road agencies, particularly in developing countries.

**Author Contributions:** Conceptualization, N.C.; Investigation, N.C. and R.L.; Project administration, N.C.; Supervision, N.C.; Writing—original draft, R.L.; Writing—review & editing, N.C.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.
