MAINTENANCE MANAGEMENT SYSTEM OF FOREST ROADS

Miklos Kosztka¹

¹ University of Forestry and Wood Sciences, Sopron, Hungary.

Abstract

When designing forest road pavements, they should be designed for a defined lifespan. During the certain lifetime of the pavement, besides continuous repairs, other maintenance work should also be done. For the sake of unified service conditions of the pavement, sometimes as a method of maintenance a thin layer is added, the effect of which is to increase bearing capacity. At the end of the pavement life, at the exhaustion of the pavement's bearing capacity or when there is a significant increase in traffic, the pavement should be strengthened for a redesigned lifespan. So maintenance of the new kind of pavement should start again.

During the complete activity cycle some questions might arise: For what lifespan should the pavement and the strengthened layer be designed? How many times and in what cycles should the road be covered as part of maintenance work? When should the pavement be strengthened to create new road conditions?

On the basis of the model showing bearing capacity changes, the effect of different road maintenance works can be analysed. Therefore, we could draw general rules from which a policy for maintenance management of the pavement can also be developed. By doing so at the time of pavement design the different times that maintenance will be necessary can be estimated and effective materials and energy consumption can be ensured.

Introduction

Pavement design realizing cheap and up-to-date principles for material usage of forest roads can only be accomplished by well-considered pavement management strategies. Using the model of changes in bearing capacity we can examine different pavement management strategies. With the comparison of these, appropriate rules for pavement management of forest roads can be established. Comparing the difference between the needed and the actual expenses for the maintenance of standard conditions of the pavement, the proper strategy and the optimum timing can be chosen.

Complex problems of pavement design and maintenance of forest roads

When designing the pavement of forest roads we have to minimize the construction and maintenance expenses in the long run. On the other hand material consumption and the number of interventions should be cut to the least.

Forest road pavements should be designed for a certain life span. During this life span the damage of the pavement ought to be protected by different pavement management strategies.

At the same time we also guarantee the conditions of fast, safe and economical traffic on the road. The simplest intervention is to repair, by which we only put a stop to faults locally. We do not repair, by doing so, the total status of the pavement, but only the formation of a sort of fault chain can be kept back and averted. Maintenance is a bigger kind of intervention when we build a thin layer on the total pavement surface. By that we create a homogenous surface condition; in addition, we also increase the bearing capacity and life span, respectively. At the end of pavement life span when the bearing capacity has decreased, the pavement should be renewed. At this point a layer with preplanned life span ought to be built onto the existing pavement structure. By this intervention we actually start a new maintenance cycle.

When planning the question is: For what life span should the new pavement and the strengthened layer be designed? When and how many times should the pavement be covered by a thin layer? When should the pavement be strengthened?

An answer to these questions can be obtained from the Pavement Management System (PMS) concerning forest roads which examines the pavement damage and the effect of maintenance and renewal.

The most important maintenance interventions (upkeeping and renewal) are required by the bearing capacity of the pavement. PMS has to be calculated on the basis of these. The objective is the creation of rules to estimate when these interventions are needed.

To do so, changes in bearing capacity over time have to be known. On the basis of a model showing changes of bearing capacity we can compare different road maintenance policies, choose suitable strategies and establish general rules of PMS.

The model of bearing capacity

We have set up the bearing capacity model based on AASTHO experiments and supplementary experiments in Hungary, which were based on methods of designing the pavement of the forest roads. We think this method is easy to follow since it describes clearly the deflection change typical of bearing capacity influenced by the traffic and the inbuilt layers. Consequently the deflection characteristic of pavement bearing capacity for a given traffic is:

log(s₀) = 1.1580.2198 * log(F)

where:

F = total traffic passing through during the projected life span (equivalent standard axles).
s₀ = deflection typical of the pavement's ability to carry traffic.

Supposing this relation is valid at any given time during the pavement life we can accept that with little change the process of decreasing bearing capacity can be described as follows:

log(s₀) = 1.1580.2198 * log(f *e)

where:

f = average traffic per year
e = time until the end of the projected life span

The effect of the new pavement layer on bearing capacity can be expressed from the relation used in designing the strengthening stratum. The needed thickness of the strengthening layer is:

D H_e = B*log(s_e/s_u)

where:

D H_e = equivalent thickness of the strengthening layer

s_e = deflection before interference

s_u = the deflection after interference

B = a constant typical of the pavement (between 65-75), the value of which is going to be 70 in the following analyses.

From this can be derived the bearing capacity after building a new layer:

log (s_u) = log (s_e) D H_e/70

Combining the relations that describe the effect of the new pavement stratum and the bearing capacity decrease we can create the model to describe the change of physical state including all the important parameters:

where:

e_e = life span after the last interference
e_b = time between the last and the planned interference
f₁ = traffic per year after the last interference
f₂ = traffic per year after the planned interference
D H_e = thickness of inbuilt pavement with the planned interference
s - bearing capacity of the pavement after e years of interference
e - time after the last interference.

With this relation we can simulate the effect of different pavement management strategies produced on bearing capacity. We can also work out cost comparisons by calculating and discounting the construction costs of the inbuilt layers.

Analysis of pavement management strategies on the basis of model of bearing capacity change

Using the above relations we have analysed three kinds of pavement management strategies:

· In the first version only the required repairs of pavement were done. After ten years, on the basis of the deflection, the pavement was strengthened for another period often years.

· In the second version only the needed pavement repairs were done until the eighth year of road delivery. At that time and before the end of the projected life span, a thin layer was built on the pavement as a conservation measure.

· In the third version only the required pavement repairs were done until the sixth year of road delivery; then, during the sixth year a layer of D H_e = 6.6 cm thickness was built in as a conservation measure.

After that and up to the end of the sixteenth year only the needed repairs are done. Then, on the basis of the deflection, the pavement has to be strengthened.

The starting data of the analysis are shown in Table 1, the model of bearing capacity change can be seen in Figure 1.

Figure 1. Model in change of bearing capacity

Table 1. Starting data of analysis

The final results of the investigation are summarized in Table 2 from which the following conclusions can be drawn:

· It can be stated that version 2/2, with several interferences, achieves practically the same life span as version three.

· The thickness of the pavement determined by certain strategies at the end of the life span is always bigger than the projected thickness. The proportion of the built-in pavement per year and that of the total cost seems to be most favourable in version 3 and least favourable in version 2/1.

· Version 3 meets more closely the predefined conditions at the end of the project's life span.

Table 2. Technical and financial characteristics of strategies

Version	Life-span	Pavement		Delivering years'		Number of interferences with building number
		Total in-built thickn.	Average in-built thickn.	Total costs	Average costs
	year	ucm	ucm/year	HUF	HUF/year
1	20.0	58.0	2.90	2,359,820	117,991	7
2/1	20.7	71.0	3.43	2,447,729	118,248	6
2/2	27.1	77.8	2.87	2,515,766	92,833	5
3	26.0	62.4	2.40	2,353,964	90,537	3

Version	Pavement measured by the end of life span				Degression of expenses from ideal
	Thickness	Thickness built in yearly	Expenses	Year proportion of expenses	Total expenses	Year proportion
	ucm	ucm/year	HUF	HUF/year	HUF	HUF/year
1	42.9	2.15	2,252,250	112,613	107.570	5,378
2/1	43.1	2.08	2,262,750	109,312	184,979	8,936
2/2	44.3	1.63	2,325,750	85,821	190,016	7,012
3	44.1	1.70	2,315,250	89.048	38.714	1,489

(Costs calculated at 1985 price level)

It can be concluded that:

· When planning new roads, pavements should be designed for the longest life span and they should be maintained by a proper strategy of conservation and strengthening.

· A proper road maintenance policy can be seen in version 3, e.g. before the end of the projected life span we create a homogenous surface by placing a thin layer once or twice both as a conservation measure and a strengthening measure. The pavement is to be strengthened before total collapsing of the road.

Determination of optimum timing of intervention

At this stage the proper strategy is known, but the question is: "At what time should we intervene?" There is no problem when talking about choosing the time of the planned strengthening because this should be carried out at the end of the projected life span or one or two years earlier.

It is not simple to determine the time of building the thin layer as a conservation measure. From an economic point of view this should be done at a time when we are able to influence the projected life span at the lowest cost and in the most effective way.

Therefore, we have to examine how the required costs change during the projected life span and the influence of certain interventions. To estimate this change we have to show the difference between the needed and the actual expenses. This difference is the amount of the unsatisfied demand.

From the point of view of bearing capacity the amount of deflection should be maintained continuously to allow a certain amount of traffic to pass through during a given life span. On the basis of the above, the amount of deflection which shows the bearing capacity changes during a series of years without any intervention can be calculated, as well as the thickness of the strengthening stratum that has to be laid down to ensure the deflection at a certain point in time. The value of the layer to be built in can be also calculated.

The thickness of the layer to build in the "i^th" year before the projected life span:

D He	= thickness of layer to be built in the ith year in order to keep the level (maintenance)
es	= life span marked to keep the level
ei	= time elapsed without interference
f	= average traffic per year
se	= initial deflection that allows traffic during the designed life span.

With full knowledge of pavement thickness to be built in every year it is easy to calculate the expenses to be incurred.

On the basis of this, the model of the three policies of pavement maintenance shows required and actual expenses (Figure 2). It can be seen that the functions "required and actual expenses" are a limiting factor for certain strategies and it results that during the projected life span the least unsatisfied area is realized by version 3.

Figure 2. Model in change of expenses

To define the time of renewals, suggestions should be taken by the curve showing version 3. It can be seen that the thickness of the layer built in the sixth year resulted in a better bearing capacity than that of the new road so more was spent on the road for maintenance. If we built the same layer in the seventh year, an unsatisfied area would remain and the projected life span would also decrease. For this reason, the proper point in time for conservation could be fixed as follows: Under given traffic conditions maintenance should be done when unsatisfied expenses can be eliminated by building in a minimal layer thickness in compliance with the usual construction methods, (In version 3 this optimum point in time would be 6.3 years.) Of course all the expenses should be calculated according to this technique.