Bearing Capacity of Forest Roads on Poor-Bearing Road Subgrades following Six Years of Use

Abstract

The research was conducted on a forest road on the territory of the State Forests in Poland, in the Brzeziny Forest District, where eight test sections with a total length of 422 m were created with different pavement system on a low-bearing soil substrate (clay, silt loam) as part of the road reconstruction in 2016. The bearing capacity of the pavement was evaluated based on the static strain modulus ME MN·m−2 by measuring with a statistic plate (VSS), the dynamic deformation modulus Evd MN·m−2 obtained from lightweight deflectometer measurements, and the elastic deflection of the pavement Us mm evaluated from Benkelaman beam measurements. It has been shown that pavements made of crushed aggregate and common gravel on timber roller substructure maintain good bearing capacity parameters, where the average values of secondary modulus of strain are above 130 MN·m−2, and in the case of pine rollers, this modulus has increased. Pavements on low-bearing soils reinforced with willow brushwood mattresses have low bearing capacity parameters, with averages of 26.09 ≤ MEII ≤ 53.93 and 22.1 ≤ Evd ≤ 39.1 MN·m−2, but the technical condition of the pavement makes it possible to continue carrying out forestry-related transportation. The research confirms the possibility of reinforcing soils with poor bearing capacity with wooden rollers, and in the case of willow mats for roads with light truck movements.

Evaluating the Forest Road Systems Subjected to Different Loadings by Using the Finite Element Method

“…In Romania, the forest roads are restricted to public traffic [4,15,52,53], and are classified into three categories (masterly, principal/main and secondary forest roads), depending on the traffic characteristics and the forested areas they serve [4,15,53]; accordingly, the road systems are usually selected based on the category of road [36]. In addition, most forest roads are made of flexible systems [20,54], which are sized based on the traffic requirements [11,15,17,55]. Gravel surfacing is the most common in Romanian forest roads [3,15,[22][23][24]56], and most of the forest roads were designed for a maximum permissible weight of 25 tones [57], before 1990 [58].…”

“…Due to the distribution of loads over the road surface, there is a high number of options which can be used to place the layers within a road system [3,17,[60][61][62][63]. Undersized road systems are prone to premature failure [56], especially when the traffic occurs in the freezing / thawing periods [11,21,24,25], or during periods of high humidity [11,29,54,64]. It should be noted that the layers have different thicknesses and strengths and are made of materials whose quality may decrease with depth [8,9,54,56].…”

“…Undersized road systems are prone to premature failure [56], especially when the traffic occurs in the freezing / thawing periods [11,21,24,25], or during periods of high humidity [11,29,54,64]. It should be noted that the layers have different thicknesses and strengths and are made of materials whose quality may decrease with depth [8,9,54,56]. According to the literature [3,15], the upper layers should be made of stronger, high-quality materials [65][66][67].…”

“…The gravel pavements have to be well compacted, so that the entire road system be as impermeable as possible, which will prevent the action of climatic factors on the whole system. On the other hand, the layers' succession in terms of materials and thickness should provide a decrease in loading, so at the roadbed it will be less than the ground's load-bearing capacity [3,25,54,68].…”

“…In order to simulate the effect of traffic on road systems, many authors have resorted to the Finite Element Method, also applied in the case of the present research, either to analyze the stress and deformations occurring in the case of roads with non-rigid road systems [59,[71][72][73], or to determine the number of load cycles that asphalt roads can withstand [8,9,60,77], and to observe the impact of multi-axle vehicles on road systems [17,61,67,78], even to highlight the differences that appear in the case of displacements in the depth of road systems with or without geogrids, at various test cycles [68,71]. These interactions can be analyzed both in the static and dynamic domain [11,54,60,61], while understanding the influence of traffic on the support surfaces is essential both for the design of resistant transport ways, as well as to be able to evaluate, later in use, the factors that influence the behavior of constructions over time [2,72].…”