A forest road has been commissioned, for the construction of which AS “Latvijas valsts meži” (LVM) used an innovative technical solution for the first time: round timber piles to support the roadbed, ensuring that forestry operations can be carried out even in particularly challenging conditions of deep peat.
The “Tēvgāršas ceļa turpinājums” forest road has been commissioned in the Limbaži forest district of the LVM Western Vidzeme region, within the territories of Brīvzemnieki and Braslava parishes in the Limbaži municipality. The total length of the road is 1.6 km, and a 550-metre-long section was built over deep peat with a sapropel interlayer using 4,281 round timber piles. Using an excavator and a special driving head, 5.4-metre and 4.2-metre-long softwood sawlog piles with a thin-end diameter of 20-24 cm were driven into the positions specified in the construction design.
“The challenge of this construction project was the installation of a large-diameter culvert in a drainage ditch. To allow this culvert to be installed in the intended location, steel sheet piling was used. At the installation site of the culvert, the depth of the peat was 4.80 m, meaning it was impossible to install the culvert without removing the peat from this area. To do this, these sheet piles were required to ensure that the embankment would be stable after construction, maintain its load-bearing capacity and not mix with the adjacent peat,” explains Ivars Caune, Forest Road Expert at LVM Forest Infrastructure.
Why was this solution chosen?
Design work for the 2.5 km “Tēvgāršas ceļš” forest road began as early as 2011. While preparing the documentation for the construction design, a black stork nest was discovered on the planned road route. Consequently, the length of the planned road was reduced to 0.907 km. The road construction works were completed in 2014.
The constructed forest road provided access to a forest massif, but did not enable the management of the entire area. Therefore, in 2016, design work for the “Tēvgāršas ceļa turpinājums” forest road was initiated. The route of the road was planned so as not to affect the black stork micro-reserve and other identified environmental values.
During the design phase, engineering surveys revealed that there was peat of varying depths within the 75 m to 800 m range along the planned road route. In 2018, as road construction began, it was found that the road design solution proposed by the author of the construction project, using low-value softwood industrial wood, could not be implemented in the specific unstable soil conditions; therefore, additional engineering research was carried out using ground-penetrating radar. During this investigation, it was discovered that the depth of the peat was significantly greater than indicated during the initial engineering survey.
In 2020, the redesign of the construction project was initiated, with the author of the project proposing various solutions to overcome this section of the road.
“By analysing the proposed solutions, we concluded that we had to use natural construction materials, which is why a road structure on timber piles was chosen. The idea of the construction project’s author was to drive timber piles down to load-bearing soil and then build the road structure on top of these piles. In Latvia, this type of road construction had not been carried out before, but based on the experience of European countries, such structures have been used to stabilise the ground and subsequently build the road surface. Before organising the road construction works, an expert assessment of the construction project was carried out, which confirmed that this solution is safe and viable,” explains Ivars Caune, Forest Road Expert at LVM Forest Infrastructure.
In the repeated procurement for the completion of the forest road, the general partnership "VM VB" was the successful bidder. The contractor began construction in September 2021, planning to finish it a year later, but the war in Ukraine significantly increased the prices of building materials and fuel, so the completion deadline was extended until 31 October 2023.
Monitoring during the construction and operation of the timber pile structure
Parallel to the forest road construction works, the development of a monitoring system began to study changes in the structure during construction and operation. The task of the monitoring project initiated by the project group of the “Forest and Wood Products Research and Development Institute” is to study the durability, changes and future potential of this type of structure.
As part of the research project, equipment and measuring instruments were installed in the roadbed of the “Tēvgāršas ceļa turpinājums” forest road, through which information is collected during construction and over a 24-month period following construction regarding potential horizontal and vertical deformations of the structure, groundwater levels, pressure from the road structure and heavy goods vehicles on the timber piles, as well as other measurements.
“Interim monitoring reports indicate that this type of construction is safe; after the structure is built, there is a small amount of settlement, which is related to the tensioning of the geotextile between the timber piles, after which the settlement of the road structure stops. This type of road structure can be applied in the future in places where there are soils with very low load-bearing capacity and it is impossible to change the road route, because it has proven its viability; the constructed road structure is safe and predictable,” explains Ivars Caune, Forest Road Expert at LVM Forest Infrastructure.
The ongoing monitoring will conclude in the spring of 2024. In the future, LVM will be able to use the results of this development project and the construction experience to select technical solutions for building forest roads in particularly difficult conditions, overcoming local sections with very low soil load-bearing capacity.