**1. Introduction**

The forested land in Japan occupies an area of 25 million ha, which is 68% of the land, and most of it is located in mountainous areas: i.e., 24.5% on less than 15◦ of slope, 39.1% on 15–30◦, 13.5% on 30–35◦, and 22.8% on more than 35◦ [1]. The geology features of the Japanese islands are complicated and fragile because the islands are on the edge of the Eurasian tectonic plate, under which the Pacific and the Philippines tectonic plates move gradually downward, creating an accretionary wedge [2–4]. The Japan Forestry Agency recommends the following classification for road networks in the forested area. One classification is forest roads with widths of 3.0–5.0 m and, if necessary, retaining structures, such as concrete walls. Standard-sized log trucks (10-t trucks) transport logs on forest roads, which also witness public use. The other classification is spur roads or "forest operation roads" with a road width of around 3.0 m for logging operations; log transportation is achieved by smaller-sized log trucks [5–8]. This is because the low standards of spur roads, such as narrower road width, higher allowable gradient, and smaller curve radius, are suitable for steep and complicated terrain. Most of the forest roads in Japan allow public traffic and, if necessary, are paved while this is not the case in other countries [3,9–12]. The construction of road networks in forested areas in Japan has shifted from forest roads to spur roads. In 2012, the total lengths of forest roads and spur

**Citation:** Suzuki, Y.; Hashimoto, S.; Aoki, H.; Katayama, I.; Yoshimura, T. Verification of Structural Strength of Spur Roads Constructed Using a Locally Developed Method for Mountainous Areas: A Case Study in Kochi University Forest, Japan. *Forests* **2023**, *14*, 380. https:// doi.org/10.3390/f14020380

Academic Editor: Rodolfo Picchio

Received: 19 January 2023 Revised: 9 February 2023 Accepted: 10 February 2023 Published: 13 February 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

roads were 19 × <sup>10</sup><sup>4</sup> km and 12 × <sup>10</sup><sup>4</sup> km, respectively. In 2020, they were 20 × <sup>10</sup><sup>4</sup> km and 20 × <sup>10</sup><sup>4</sup> km, respectively, equivalent to 8.0 m/ha and 8.0 m/ha for forested areas in Japan [13,14]. Therefore, constructing spur roads with low cost and sufficient strength is crucial for sustainable forest management in Japan. Different terms are used for such roads in Japan, i.e., spur road, strip road, forest operation road, forestry operation road, operation road, etc. Hereafter, the term spur road is used in this paper.

Among the various forest practitioners who have developed spur road construction methods for their local properties of the terrain, geology, soil, and forest, Mr. Keizabrou Ohashi, or Oohashi, is the most famous practitioner, whose contribution includes his method of spur road construction and forest management [3,4,15–19]. The essence of his method, termed the Ohashi-type spur road construction method, is as follows: a minimum road width of around 2.0 m for small-sized trucks, 3-t to 5-t class excavators, and 2-t trucks; detailed route selection against collapse concerning terrain, geology, soil type, forest type, etc.; and use of small, round wood material as the retaining material for the fill slope toe and road shoulder [3,4,15–19]. He developed the method in the late 20th century, mainly for the Chichbu geology belt areas combined with decomposed granite areas. The Chichibu belt collapsed, and once it was deposited below sea level, the geologic stratum was not formed and the soil is fine-grained [3]. Owing to fine-grained soils, large landslides with long runouts can result from heavy precipitation [3]. Because fine-grained soils limit the forest road cut slope height and the ability to construct fill slopes [3], the Ohashi method often employs retaining materials for fill slopes.

The method witnessed widespread use in Japan around the 1990s and 2000s, with some modifications to fit the geology and soil of other areas [20,21]. One such variant is the Shimanto spur road construction method developed around the 2000s by a municipal officer of the forestry section in order to make narrow spur roads robust against collapse, even in steep terrain. The Shimanto belt is relatively young and exhibits typical features of accretionary wedges composed of sedimentary rocks [3]. The slopes facing the north or the Japan Sea are generally dip slopes, and those facing the south or the Pacific Ocean are opposite slopes [3]. There are numerous circular slips and deep-seated landslides on the north-facing slopes, and it is risky to construct roads on the dip slopes [3]. For the inclined stratum in the Shimanto belt, which is composed of sedimentary rocks, deep excavation has been conducted for preparing the structural foundation of the road [4]. After deep excavation to ensure the depth required for the road width, ~30-cm-thick blocks of compacted subsoil were piled up [4]. This method is suitable for gravelly soil, which is the main soil type in the Shimanto belt, while a small portion of clay serves as the bondage medium [4]. The surface soil of the natural slope was maintained during construction and used for the surface of the filling slope to provide an early green surface because it contained buried seeds [4]. In some cases, stumps are placed at the fill slope toes with thick roots embedded under the embankment [6]. The road width is limited to 2.5 m considering the usage of 3 t–5 t class excavators and mini-forwarders [22]. A mini-forwarder is a popular machine for small-scale forestry in Japan. It is propelled by rubber tracks and equipped with a winch with a typical loading capacity of 500–1000 kg [23].

Including the co-related principals of these methods, a training manual [24] was compiled by forest engineers in 2010 for on-the-job training of spur road construction operators in government-aided training programs. The book introduces various local methods developed to fit the geological and terrain features of the area. Umeda et al. [21] pointed out that the designated strength of the roadbed cannot always be obtained in certain areas if a method developed for another area is adopted unless the operator completely understands the concept of the method.

Regarding the proper compaction of roadbed construction, Sawaguchi et al. [25] found that the maximum compaction strength can be obtained by less than 10 round trips of an excavator when it is used as a compaction device. Although more retaining walls are required for forest roads, which have higher design standards than spur roads, on steep terrain [26], the following problem has recently arisen: many sections of improperly constructed spur

roads on steep slopes have collapsed one or two years after construction [27]. This problem should be taken into account in construction supervision. Furthermore, companies involved in such unsustainable forest management should be punished by law. Concerning the age-related change effect of road bed strength and bearing capacity, Umeda et al. [21] reported that in areas with low compaction ability soils, there is a spur road construction method that requires three years for completion. This method seems to effectively utilize the age-related change effect of the road strength.

Kochi University Forest adopted the Shimanto method and constructed some routes of spur roads in the 2010s. The forest is in the same prefecture for which the method was developed but in a different geology belt. The present study assesses the performance of the method in terms of the roadbed strength and bearing capacity. We verify the hypothesis that the spur road construction method was properly used to achieve sufficient mechanical strength.
