Some factors influencing bearing capacity of forest road surfaces

In this paper we tried to determine how bearing capacity of forest road surface was influenced by various factors. In order to investigate bearing capacity of forest road surface, 284 measuring points were chosen from ten primary and seven secondary roads, which were specially prepared for this study. There are two methods for measuring bearing capacity: plate bearing test and field CBR test. For the former, Impact Type Bearing Apparatus and for the latter, Drop-Ball CBR Apparatus were adopted in this investigation. Many data were elaborately analyzed according to statistical method. The results obtained from this study are summarized as follows: (1) A highly significant difference of bearing capacity was found between primary and secondary roads. This finding was supported by “K” values (K₃₀) of plate bearing test, as well as in CBR values of Drop-Ball Test. Average “K” values measured on the surface of primary and secondary roads were 19.31kg/cm³ and 14.84kg/cm³ respectively. For the average CBR value on the surface of primary and secondary roads, 47.58% and 18.85% were registered respectively. (2) Except for the average value of “K” on the surface of secondary roads, a significant difference of average bearing capacity on the surface of every forest road was found. (3) There was a significant difference of bearing capacity at the four representative parts of the road cross section: center, rut, shoulder of road and passing place. These could be arranged in order of greatness of K₃₀ and CBR as follows: rut, center, shoulder and passing place (for primary roads), and center, rut, shoulder and passing place (for secondary roads). (4) There was a significant difference of average bearing capacity in different materials composing road subgrade: soil, rock, asphalt and concrete. (5) There were significant variations of bearing capacity of forest road surface depending on the length of time after road construction. For the primary roads, bearing capacity was lowest within a year after road construction, while the highest value of bearing capacity of the secondary roads was observed between 2 and 3 years after construction. (6) Higher values of bearing capacity were observed near the beginning of the primary and the secondary roads. On the contrary, lower values were measured where the test points were separate from the beginning of the road. (7) From the appearance of the forest road surfaces, we classified them into five degrees of road standard. Bearing capacity of the primary road surface increased as the degree of road standard improved. However, in the secondary roads such a tendency could not be observed. (8) “K” values varied according to the factor of moisture content in subgrade soil. The greatest value of “K” was observed on the primary roads where the moisture content of subgrade soil was 30_??_39%. In the case of the secondary roads, another tendency was found: the greater values of “K” measured were proportionate to the lower moisture content. This correlation was highly significant. (9) Bearing capacity on the surface of forest roads was not influenced by the weather while this test was continued.