Octavian Popescu1 and Emil Untaru1
1 Research Institute for Forest Management, Bucharest, Romania.
The research and experiments performed in Romania concerning road embankment stabilization by means of forest vegetation has had good results. This is especially true in the areas for preventing soil erosion and, to some extent, landslip.
The most efficient techniques in different conditions have proved to be: terraces supported by small fences, stone benches, and vegetatively reinforced terraces.
A variety of techniques in different situations are presented with associated results.
Starting in 1982 the Research Institute for Forestry Management has carried out research work on how to vegetate artificial slopes for future development of different species and types of forest cultures, and the effects of artificial protection cultures.
The research work and experiments have taken place as follows
· research work on the evaluation of forest sites in sub-Carpathian zones at Vrancea and Buzau, in Transfagarasan, Valea Prahovei, Valea Bistritei, Bicaz, Podisul Moldovei, and Dobrogea;
· experiments and culture of tree species in the sub-Carpathian zones at Vrancea and Buzau, in Transfagarasan zones at Dunarea, Black Sea channel;
· research concerning the development of different species and cultures on artificial slopes and their effects on zones in the sub-Carpathian zone at Vrancea, Transfagarasan, Valea Prahovei, Valea Bistritei, Dobrogea.
This paper presents the main results.
Soil analysis should consider the local soil profile and determine the predominant soil horizon present on the finished surface. Cut slopes expose soil or rock horizons in natural sediments. Fill slopes present various combinations of soil, soil and rock or only rock on the finished surface. In order to determine the characteristics of cut and fill slopes the following main factors were considered:
· lithologic subsoil with crumbly rock slopes, slopes with moderate consolidated rocks and compact rock slopes;
· subsoil of cut slopes, mainly cut in soil or bedrock;
· structure content of the soil or sediment;
· stability degree, depending on the slope gradient, soil characteristics and lithologic subsoil directly affecting the cut slopes.
· type of materials mainly soil or mainly rock;
· texture of the soils (sandy to loamy or loamy-clayey to clayey);
· structure type and content of fill material (structureless, low structure, or high structure).
In some cases, other factors should be taken into consideration in order to adopt a suitable solution, for example, noxious substances for plants, pH, CaCO3 content, salinity, spongy character of soil, rock or fill sediment, or moisture content.
The physical-chemical analysis of finished layers generally present a very low content of nutrients, due to the thin horizons of disintegrated and partially altered rock. This results in a low or very low content of assimilable nitrogen for tree growth.
Works for slope consolidation and site preparation for vegetation establishment
Properly designed slope protection and stabilization has to include two components a vegetation-biological and a mechanical structural component.
The effective means of stabilizing bare soil surfaces and preparing soil to be vegetated in different site conditions are:
· narrow terraces of 30° to 40° slopes - 40-50 cm width, fences supported on cut and fill slopes, made up of loess, loams, sand, gravel, clays, marls, mass erodible;
· narrow terraces of 25 to 45° slopes - 40-50 cm width, supported by stone benches of 40 cm width (15 cm underground) on cuts and fills, in formations of soft rocks alternating with hard ones (with stones necessary to execute the benches), for 25°-45° angle slopes;
· terraces of less than 30° slopes - 40-50 cm width, vegetation " reinforced" on sucker and swallow thorn bed, on cuts and fills in formations of marls with sandstone in sessile oak and beech sub-zones, for lower than 30° slopes;
· simple terraces on slopes cut in soil and soft bedrock or fill made up mainly of unconsolidated rock materials and stable soil, partially free of grass, 75 cm width, at slopes lower than 25° and 50 cm width at slopes lower than 25°-35°, 2-3 m apart from axis to axis;
· narrow terraces, 30-40 cm width, 1.5-3 m from axis to axis on cuts and fills in soft rock subsoil or on a surface layer mainly of disintegrated or altered rocks, about 30 cm thickness, intended to belt plantations with swallow thorn (sessile oak and beech sub-zones) and alder tree (in beech and spruce sub-zones) on 30-50° slopes.
It is specified that the artificial slopes on soil where mass movements have occurred need to be stabilized by consolidation works, such as retaining walls, drainage, trenches for rain waters prefab consolidations.
Biotechnical slope stabilization is the combination of vegetative and structural components to prevent certain negative effects. Therefore, it is recommended to use shrubs or grass layers, to avoid ground overcharge and the disturbance to the consolidation works by the rooting system of the trees.
Considering different site conditions the types of plants determine the most appropriate planting technique as follows:
· planting in common holes, 30/30/30 cm on unprepared soil, on terraces, especially on cuts of soil or soft rocks (loess, loams, sands, gravel or mixture of this rock with clays, marls and sandstone's);
· plant culture, grown in polyethylene bags, on high structured ground especially in dry zones (steppe and sylvosteppe);
· planting with vegetal soil, added to hole (10-20 cm) on high structured ground;
· planting with continuous vegetal layer 30 cm thick, on cuts of limestone, clays and hard rocks;
· belt planting on narrow terraces on cuts and fills in soft rock layers, or having as surface layers 30 cm altered rock layer;
· planting in slits on slopes having more than 50 degrees, when it is impossible to plant in belt or common type.
Types of forest cultures and their protective characteristics
The research highlights that the most suitable species for slope afforestation are low and medium grown ones with high capacity of suckering and shooting.
Besides others grown species as acacia, grey and black alders which could be maintained under 7 m by fellings at 8-10 years of age. This avoids the lowering of slope stability and eventual risks for the traffic on transport roads, such as slidings, landslips, tree breaking or falling.
The above-mentioned species present a suitable behaviour and provide the following non-erodable characteristics in the various zones:
· for steppe and sylvosteppe: acacia, Japanese sophora, flowering ash, tree-of-heaven, St. Lucie cherry, oleaster, lilac, indigo bush, smoke tree, dog rose, common hawthorn on cuts mainly profiled in ground and loess or on fills made up mainly of loessoide materials and ground, as well as in slope covering, cut in various rocks, having a continuous vegetal ground layer, maximum 30 cm thick; oleaster on slopes cut in loessoide materials, clays, sands, groundless;
· for quercetum and sessile oak sub-zones: acacia, oleaster, flowering ash, St. Lucie cherry, swallow thorn, lilac, common privet, indigo bush on cuts profiled in clays and sand with low clay content and marls, to which in the access zone of roots there is some ground; acacia, oleaster and swallow thorn in the same conditions but groundless and swallow thorn on subsoil made up mainly of marls and sandstone;
· for beech and common spruce sub-zones: grey and black alder, mainly on intermediate and shadowed slopes; swallow thorn on sunny slopes; elm leafed spiraea scarlet, elder, goat willow on washed slopes mainly in crystalline schist with low contact of accumulated soil in fissures.
Among the arborescent species with satisfactory to good results are: black pine, from sylvosteppe up to beech sub-zones; Sylvester pine from sessile oak sub-zones up to spruce sub-zone; larch and common spruce in spruce sub-zone, especially on fills.
Concerning the behaviour of different types of forest cultures on subsoils of loess or sands with loams and clays with low content of CaCO3, the most suitable are the pure culture of acacia or mixed with flowering ash, St. Lucie cherry, tree-of-heaven, oleaster for steppe to sessile oak sub-zone.
On soil with high CaCO3 on structure content, the most suitable were the culture with flowering ash, St. Lucie cherry, tree-of-heaven, oleaster, mixed with shrubs (50% trees, 5% shrubs) or only shrubs.
On subsoils in marls-sandstone in forest lands, the most appropriate have been swallow thorn, or the mixed black and Scots pine 50 percent and swallow thorn 50 percent.
On slopes in the mountain zone, alder has been used in shadow areas while in sunny ones, swallow thorn.
The most efficient planting diagram has been 1.5 m x 1.5 m and 2 per m for acacia and alder, 1.5 per m for St. Lucie cherry, tree-of-heaven, flowering ash mixed with shrubs, 1.5 per m and 1 per m for shrubs (corresponding to 4 450-10 000 plants per hectare).
For belt planting the distance was 33 cm, resulting in 10 000-15 000 plants per hectare.
Besides the protection effect, the cultures on slopes could be sometimes productive. Alder has a growth rate of 3-6 m/ha/a while for acacia it is 2-4 m/ha/a depending on site conditions. Certain fruit shrubs, such as swallow thorn and common hawthorn, can produce 1 000-3 000 kg/ha/a
The vegetation established on road slopes can also enrich and diversify the forest landscape.