Substratum and Foundation

The optimum requirements will have been met if the subsoil has excellent bearing capacity and also can be used as building material, which can reduce the construction costs drastically. Unfortunately, such conditions are rare. Most sites either have been developed already or lack the appropriate dimensions. Geology is differentiated only

Figure 3.12. Standard cross section of a minor rural road. Source: Adapted from [23].

in wide areas. However, development areas are usually smaller. It is improbable that a subsoil with sufficient bearing capacity can be found only by changing the alignment of the road; subsoil treatment measures will be required in any case. Local zones with bad subsoil should be upturned.

The following substrata are more or less suitable for the building of minor rural roads (suitability of other materials can be determined from Table 3.10, as discussed later in Section 3.4.7):

• good substrata with the requisite bearing capacity include limestone, moraine, ballast, alluvium or alluvial deposit, conglomerate, granite, gneiss, schist, or slate (solid rock);

• substrata lacking both sufficient bearing capacity and suitability as building material include opalinus stone or clay, marl, marl slate, decomposed moraine, slope wash, loess loam, molasse debris or soft sandstone, alluvial soil or silt, flysch debris, decomposed sandstone, clay slate, marl slate, and peat.

Cost-conscious construction requires as little displacement of earth (earthwork volume) as possible, from excavation to deposition. The aim is to achieve balance. It is with this in mind that the choice of machines for transport and spreading should be made. If earth displacement over long distances by means of trucks, dumpers, or other transport is unavoidable, preliminary steps should be taken to stabilize the subsoil or to provide the necessary foundation layer. This can result in a considerable increase in cost, however.

Table 3.8. Stability of earthslope






Noncohesive rock





Solid rock



For cost reasons, state-of-the-art bottoming (filling) in accordance with the regulations is rarely possible. The bottoming can be done makeshift fashion with the caterpillar track of the excavation equipment or using pressure from the power shovel. This often results in long-lasting problems of uneven settlement. Therefore, waiting one to two years before fine subgrading of the foundation layer and surfacing with the bituminous layer is recommended. Distinctions must be made among the following types of bottoming:

• Side bottoming is used in the case of mixed profiles with excavation and embankment immediately to full dumping height. To prevent any danger of slipping or shifting it may be necessary to form a slope base and stepping. No real compacting will be necessary.

• Head bottoming is used in the case of short embankments built over gullies and ditches immediately to full dumping height. Again, no real compacting is necessary. Better results will be achieved with the help of excavation equipment.

• Layered bottoming is used only in the case of long embankments. The bottoming should be done in layers, with subsequent compacting.

The compaction requirements for the subgrade surface are contained in several standards of different countries. The requirement that calls for a ME > 150 kg/cm2 and a CBR > 8% in the case of 100% Proctor standard is not achievable, in particular with cohesive ground. More realistic values for simple roads are ME > 100 kg/cm2 and CBR > 3%. These are minimum values, however, for construction to begin, that is, to consider installing and compacting the road body. There must be no mixing of the sand and gravel with the subsoil and, moreover, a suitable foundation must be available for the compacting. By way of improvement, the following measures should be foreseen:

• where the natural water content (wnat) is approximately equal to the optimum water content by compression (wopt), normal compacting is possible.

• where wnat is much greater than wopt, dewatering by airing or lime stabilization is possible.

• where organic soil or peat occurs, a pounded layer, a spur layer, fibrous matting, or stabilization of foreign material can be used in application of the "floating road" principle. (floating road: road structure has to be enough flexible to avoid fracture in case of unequal settlements caused by the underground)

The largest possible slope gradient and the lowest level of safety permissible in terms of stability under load should be preferred, in order to keep earthmoving to a minimum. In the event that the slope slips or is demolished as a result, repair during construction or when the snow next begins to melt in mountainous regions, will be more cost-effective than carrying out the bottoming with fairly level slopes and large cubatures. It would be advisable to model new slopes of earthworks on stable slope gradients in the immediate vicinity and in general to adapt the road to the surroundings. This generally means low slopes. In any case, fairly level slopes are necessary for the activities carried out in agricultural areas. The availability of a number of variations (steep slopes achieved with biotechnical measures, or retaining walls) should make it easier to reach consensus in the search for a cost-effective solution.

The safety of the slope should be an immediate priority (application of maximum safety level). When deciding on the method, attention should be paid to appropriateness to the landscape, good cover of the surface to prevent erosion, thorough rooting for consolidation, and good water runoff. The method and the choice of plants will depend on the purpose, the local conditions, the climate, the danger of erosion, the steepness, and so on. Biotechnical measures often are combined with civil engineering and landscape architecture techniques [26, 30].

Bear in mind that, even as early as the planning stage, few walls and only short ones are necessary for minor rural roads. With simple roads, the supporting structures are mainly deadweight constructions (wood boxes, wire ballast baskets, block bottoming). Wood boxes and wire ballast baskets, in particular, have to be renewed every 10 to 20 years. These can be combined with biotechnical measures. Guidelines for the dimensions of nonreinforced concrete walls are available in a number of countries. State-of-the-art supporting walls are required for greater heights. These are expensive, however, and their design loads are not suitable for minor rural roads.

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