Manejo Sanitario

Gravel loss and subsequent replacement by re-sheeting is the most significant factor affecting the life cycle operating costs of an unsealed road pavement. Typically a 150 mm thick unsealed wearing course will be lost within 8 to 12 years, after which a new wearing course will be required. The loss of wearing course material on unsealed roads results from:

 traffic abrasion and loss of fine binding material  degradation of stone due to weathering and polishing

 climatic conditions, i.e. wind and rain introducing scouring and erosion  patrol grading loose material to windrows and over-cutting the surface  pavement material selection.

8.3.1 Measuring and Estimating Gravel Loss

The ability to correctly estimate gravel loss is very useful to a manager scheduling resheeting operations because it helps identify where resheeting is required and the amount of material required. However, there is currently little information available regarding actual gravel loss, how much gravel is on the road and therefore what gravel is to be added. It would seem that many practitioners wait for the subgrade to show through before resheeting, which is far too late. Gravel loss can be estimated by:

 monitoring core levels of gravel depth over time

 taking spot levels on various representative sections of roads and measuring annual wear loss

 measuring the rate of loose material generated between wheelpaths (Andrews 2001)  applying a formula, calibrated to local conditions, to estimate loss

 using technology based on ground penetrating radar to measure existing gravel depth  differentiating between the materials used in the base and wearing course.

Ground penetrating radar (GPR)

An innovative approach to estimating gravel loss is the use of GPR technology. The technology uses a pulse of energy fired into the road surface and the time delay of its reflection to calculate the distance to the object and thereby the thickness of different layers in a pavement. GPR equipment mounted on a vehicle can travel at moderate speed. GPR antennas can come in various forms, including above, and close to, the surface. A horn-based antenna, which is an above-surface type, is shown in Figure 8.6.

Source: Giummarra (in press)

Figure 8.6: Ground penetration radar (GPR) with horn antenna

Trials on the suitability of different GPR systems, on both sealed and unsealed roads, have provided favourable results (Giummarra 1998). However, care should be taken when using GPR equipment to ensure that the gravel and subgrade dielectric properties are suitable. An initial test section should be used to ascertain the suitability of this equipment to local pavement conditions.

8.3.2 Predicting Gravel Loss

International studies of the performance of unsealed roads have led to the development of a number of models, in particular relating to World Bank projects in developing countries. These models consider the inter-relationships between construction, maintenance and vehicle operating costs. Factors considered include:

 the impact of gravel loss on resheeting intervention

 the impact of surface looseness on vehicle operating costs (VOC)  the impact of surface roughness on maintenance intervention

 the impact of rut depth on maintenance and re-sheeting intervention strategies  journey time as an indicator of road condition

 traffic volumes (both ways) as an indicator of pavement wear  the impact of climate on surface dust and erosion characteristics 

In the absence of historical data to provide the most accurate determination of future gravel loss from the sheeting material along an unsealed road, various predictive formula have been

developed, three of which are now discussed. Where possible, these models have been adjusted to minimise the number of parameters, example calculations are provided in Table 8.1.

Predictive Model 1: TRRL

The Transport and Road Research Laboratory (TRRL) (now TRL) model was originally based on the results of a study in Kenya and developed by Jones (1984). The model is described by the formula:

GLA = f(0.133(ADT)2/((0.133ADT2 + 50)) x (4.2 + 0.0336ADT + 504MMP2 + 1.88VC)

1

where GLA = annual gravel loss (mm/year)

ADT = average daily traffic in both directions (veh/day) MMP = mean monthly precipitation (metres/month)

VC = gradient (%) for uniform road length f = constant for various gravels

(laterite: 1.3,quartzite:1.5, volcanic:0.96, coral:1.5, sandstone:1.4, calcretes: 2.0-4.5)

Predictive Model 2: HDM-4

The HDM-4 model, as described by Paterson (1987) and can be described as follows: GLA = 12.63 + 0.898(MMP x G) + 3.65(KT x ADT))

2

where GLA (MLA in original formula) = predicted annual material loss (mm/year) MMP = mean monthly precipitation (mm/month)

G = average longitudinal gradient of the road (%) ADT = average daily traffic in both directions(veh/day)

KT = traffic-induced material whip-off coefficient. and

KT = MAX(0, 0.022 + 0.969(KCV/57300) + 0.00342(MMP x P075) – 0.0092(MMP x PI) – 0.101(MMP)) 3

where PI = Plasticity Index

KCV = average horizontal curvature of the road (deg/km) P075 = amount of material finer than the 0.075 mm sieve.

Predictive Model 3: Paige-Green (1990)

This model is described by the formula:

GLA = 3.65(ADT(0.059 + 0.0027N – 0.0006P26) – 0.367N – 0.0014PF + 0.0474P26)

4

where GLA (GL in original formula) = annual gravel loss (mm) Fawcett et al. (2001) ADT = average daily traffic in both directions

N = Weinert N-value (a climate index value)

PF = plastic factor (plastic limit x per cent passing the 0.075 mm sieve) P26 = per cent passing the 26.5 mm sieve.

It was recommended that the particle size distribution be recalculated assuming that 100% was passing the 37.5 mm sieve and that the Weinert N-value (12 x evaporation in the hottest

month(mm)/annual precipitation(mm)) be limited to a maximum value of 11 (Jones, Sadzik and Wolmarans 2001).

Table 8.1: Comparative rates of annual gravel loss

Location: Unsealed Road through Flinders Ranges, South Australia

Predictive Model 1: TRRL Predictive Model 2: HDM-III Predictive Model 3: Paige-Green

Input Parameters: Material f = 1.4 PI = 20, P075 = 22 P26 = 95, P075 = 23, PL = 15, PF = 345 Traffic

ADT = 92 vehicles per day ADT = 92 vehicles per day ADT = 92 vehicles per day Rainfall

MMP = 0.026 metres/month MMP = 0.026 metres/month N = 11

Alignment VC = 0 G = 0, KCV = 0, KT = 0.017

Annual Gravel Loss

GLA = 10 mm GLA = 18 mm GLA = 11 mm

It was noted that the gravel loss along the unsealed road was believed to be between 7 mm and 14 mm per year.