|Proceedings of the seminar on environmentally sound forest roads and wood transport (1998)|
|ITEM 5 - ENVIRONMENTALLY SOUND FOREST ROAD CONSTRUCTION AND MAINTENANCE|
1 National Institute of Wood. Bucharest, Romania.
Most of the forest roads in our country are in the mountain region, characterized by steep terrain, with a dense river network, creeks, glens and springs. This region has abundant rainfall generally above the yearly mean, which favours the instability of slopes and landslides. The above conditions result in a large number of bridges to cross rivers and of culverts to drain waters from ditches and rainwater from slopes. Thus a large volume of works, consisting mainly in the construction of supporting walls, is required for the protection and consolidation of roadbeds. To show the importance of engineering works, such as bridges, culverts, support walls, it should be mentioned that on the existing forest roads we have built:
· over 1200 bridges with a total length exceeding 20 km;
· over 170 000 culverts;
· over 5 million m of supporting walls.
In addition, the above engineering works represent an important safety factor for the traffic and to keep the roads in good condition since it is known that, in case of degradation or destruction, these roads may be out of operation. For these reasons special attention has been given to the strategy to be implemented in the realization of forest roads.
Safety level against flood
Engineering works are designed to resist high floods. The climate of our country is characterized by abundant rainfalls unevenly distributed on the territory all year round, causing floods in periods of high intensity. This condition also occurs when the snow, accumulated in the period November to March, thaws due to a sudden increase of temperatures and makes the river flows increase.
We have a series of regulations concerning the dimensions of engineering works to prevent breakdowns due to high river flows in the flood period.
Forest roads and the engineering works are classified by our existing laws in class IV of importance which establish rules for the dimensions depending on river flows with a probability of 5 percent, i.e. maximum flows occurred every 20 years.
Evolution of construction solutions
Starting in 1952-1953 the concept was introduced for the creation of a permanent forest road network suitable for operation all year round, not only for the transport of wood from the exploitation sites but also for the management of wood resources, the application of specific treatments, hunting, fruit collection, etc.
Construction solutions for engineering works in forest roads developed over time in accordance with the new concepts for forest road building.
So we have passed from construction solutions of a semi-permanent character using wood as basic raw material, with a service life of 5-10 years, to solutions of a permanent character for a service life of 100 years, based on concrete, reinforced concrete, and stonework.
The engineering works include two large categories:
· Artworks - bridges and culverts
· Protection and consolidation of embankments.
Construction solutions for bridges and culverts represent a large range of spans, types, forms, constructions and technologies. So, for the category of culverts with a design span of up to 5 m the following solutions are used:
· pipe culverts;
· slab culverts;
· arched culverts;
· ovoidal culverts.
Pipe culverts have inner diameters of 60 cm, 100 cm, 140 cm and 150 cm. At the beginning these culverts were made of reinforced concrete pipes produced on the site; the length of the pipe was 1 m.
In 1961-1962 we introduced PREMO pipes of pre-stressed concrete with diameters of 160 cm and 100 cm and lengths of 5 m in parallel with BUCOV pipes of centrifugally cast concrete with an inner diameter of 140 cm and a length of 2.5 m.
Progressively the PREMO and BUCOV pipes replaced the prefab pipes.
The pipe culverts are used on a large scale for forest road building due to their advantages over other construction solutions:
· ease to handle;
· fast execution without problems;
· short time of execution;
· rapid putting into service;
· smaller costs compared with other solutions.
The major disadvantage of these culverts is clogging with materials from the slopes, which may put the culverts out of service. This is more severe with diameters of 60 cm.
The culverts include three distinctive parts:
1. foundation of 20-30 cm concrete layer, gravel or broken stone depending on soil type;
2. pipe laid on foundation layer;
3. end connections consisting of tympans and intake chambers.
The water flows in through tympans with flared wings, and through intake chambers in the case of buried culverts.
The culverts are furnished downstream with a tympan and a wall for soil protection against erosion since the waters coming out from the pipe at different speeds depending on the slope (Figure 1).
Recently we have tried the elimination of the tympans and of the intake chambers by extending pipe length and installing an intake chamber in an excavation.
This solution has given good results in the case of roads with short service life such as for extraction of wood from windthrows, temporary crossings and for the rehabilitation of some finished works. Over the long period the solution is not satisfactory because the pipe becomes clogged, the water penetrates under the pipe and causes deformation and finally bursting of the pipe.
We have also tried a protection against the entering of branches or other materials by a grate mounted above the intake chamber but the results were unsatisfactory. The grate clogs and the water cannot enter the tube. The only efficient solution is proper maintenance of the works by removing the materials which obstruct water drainage.
Slab culverts have a clearance span between abutment faces of maximum 4 m. Small culverts with a clearance of 0.5 m and 1 m have been used in 1960-1961 and then replaced by pipe culverts. The culverts with a clearance of 2 to 4 m were first designed and realized with reinforced concrete cast in situ and, starting in 1964-1965 the use of factory-made prefab reinforced concrete was introduced (Figure 2). This solution became definitive in a short period of time mainly for technical reasons and for its advantages:
· high quality;
· ease of realization and reduction of period for putting into service.
The substructure can be made up of concrete foundation, concrete elevation and stonework depending on sources of materials.
Arched and ovoidal culverts have been used until 1963-1964 and, after that, sporadically in the case of high crossings.
Bridges frequently used on forest roads have a single span between 6 m and 18 m and are built according to the following categories:
· slab bridges with a clearance of 6 m, 8 m and 10 m;
· bridges with beams for a clearance of 12 m to 18 m;
· arched bridges with a clearance 6 m to 8 m;
· bridges with a length of more than 18 m.
Slab bridges with a span of 6-8 m were built according to three construction solutions:
1. reinforced concrete slabs cast in situ
2. bridging deck of 4-6 beams with reduced height;
3. bridging deck of prefab reinforced concrete caissons mounted by crane and tied in by concreting of longitudinal joints and by concrete nogging (see Figure 3)
The first two solutions were used beginning from 1952-1953 to 1970-1972 when they were replaced by reinforced concrete caissons.
Beam type bridges built on forest roads in 1952-1953 presented two alternatives:
· bridges with bridging deck of reinforced concrete cast in situ;
· bridges with bridging deck of beams and prefab reinforced concrete slabs (Figure 4).
Bridges with the bridging deck of reinforced concrete cast in situ are modular constructions with a span of 10 to 18 m and consist of beams connected by nogging at the ends, carriageable slab and two pavements.
This solution was used for all the bridges built between 1953 and 1965 up to 1978.
The bridges with the bridging deck of beams and slabs are modular constructions with spans of 14 m, 15 m and 18 m. The bridging deck consists of 4-5-6 prefab reinforced concrete beams, depending on the width of the carriageway, and reinforced concrete slabs. The joining together of these components is realized by reinforcing with concrete the joints between slabs. This solution was applied after 1965 and progressively replaced the bridging decks of cast in situ reinforced concrete.
The abutments of the bridges have the foundation of concrete and the elevation of concrete or stonework with cement mortar.
The embankment connection is normally with wings of the same material as for the abutment. Connection with wall and cone quarter is not used because it is vulnerable to floods. For every category of structure work (bridges and culverts) the same solutions were adopted for long periods due to the fact that National Institute of Wood had elaborated designs and projects which were applied to the whole forest road network realized in respective periods.
For crossing water streams with high flows and for crossing valleys, callows, or pools bridges with a length of over 18 m were built.
The solutions were agreed upon by the authorized bodies on the basis of techno-economic studies. The construction solutions adopted were:
· reinforced concrete web girders;
· pre-stressed concrete girders, cantilevered;
· reinforced concrete framework;
· prefab pre-stressed concrete beams.
The design size of bridges and culverts is in accordance with existing laws:
· 3.50 m carriageway for a single traffic lane;
· 6.00 m for two traffic lanes.
The bridges have two pavements of 0.75 m each.
From the information collected and from an analysis of the structure works during a period of 20-40 years of service of forest roads, the following aspects are worth mentioning:
· Degradation of the substructure and especially of the foundations occurs where footing depth is not observed, by scouring of material under the foundation sole, and where the concrete mark and execution technology are not observed causing breakdown of the concrete cast in the foundation.
· Degradation of bridge metalling which, if not repaired in time. allows water penetration and, by successive freezing and thawing, weakens the structure of the bridges. Albeit with great delay, the repairs have improved the situation so that at present there is no bridge out of operation. Maintenance has a high degree of importance to reduce greatly the cost of repair works for bridges and culverts damaged by truck drivers and when the load limit of trucks is exceeded.
The bridges and culverts are designed for a truck train A 10 and are checked for a crawler S30 or S40. The train A 10 consists of a string of 10 ton trucks (3 ton front and 7 ton back) at a distance of 8 m from each other. In this string a 13 ton truck (3.5 ton front and 9.5 ton back) is introduced at a distance of 4 m from the other vehicles. The calculation is made for a string of trucks on each traffic lane.
The crawler S30 (S40) has a total weight of 30 ton (and 40 ton, respectively) and a chain contact length of 4 m. The calculation is made for a single vehicle and for 1-2 traffic lanes.
Starting in 1980, with the increase of road means capacity, the bridges have been tested also to loads transported by these means (forwarders ATF 25).
In the case of bridges along roads through the forests but used also for other purposes the train is dimensioned in accordance with the category of the respective road (see Figure 5).
Aspects regarding environmental protection that are considered when selecting construction solutions are:
· selection of execution technology which shall not disturb the environment in the short term;
· selection of solutions which shall not alter the drainage of water.
The main measures adopted in the execution technology are:
· prevention from casting of concrete in large amounts since by spreading cement the soil is degraded and, at the same time, large areas are used for storing aggregates, cement, and fuels;
· prevention from utilization of polluting equipment (power generating sets, bulldozers, excavators) which with the leakage of fuel in the rivers can be dangerous for the environment;
· removal of materials accumulated from the foundations;
· use of prefab components for superstructure to avoid execution of scaffolding on the rivers. Execution must be clean and fast.
Landscaping is carried out by using natural stone on slopes and degrading areas.
Among the construction solutions studied to overcome the problems, the following can be mentioned:
· introduction of pipe culverts of prefab components of square section because these have better drainage and lower risk of clogging;
· introduction of prefab components for bridges with larger widths to shorten construction time.
Protection and consolidation of embankments
This type of works is currently used along water streams that must be protected against erosion and consolidated to avoid slippage. The most frequent are (Figure 6):
· supporting walls;
The supporting walls are:
· Heavy support walls of stone and concrete normally used for supporting slopes of loose ground and also for the protection of embankments in areas where the waters have a pronounced erosion effect. If available, stonework is the preferred material.
· Lining walls for protection of slope cuts in the case of stable slopes but formed of easily degradable soils.
· Dry stone walls.
The support walls towards riverbeds have a disadvantage in that footing conditions are often not observed and consequently floods may scour and break the walls.
The gabions consist of wire net baskets containing stones or river boulders. They are used at the basis of the embankment towards the river where, laid on a layer of fascines, protect the embankment against erosion. They are elastic and, in case of scouring, take the shape of the ground with no damage.
Stonefills are the pennings of stone or boulders which, laid at the basis of the embankment towards the river, are the most efficient way of protection in areas where the water has no pronounced erosion effect.
Casings are some boxes of cut logs or prefab components of reinforced concrete containing stone or boulders. The advantages are fast execution, good behaviour and long duration.