Construction

Construction of the road body will depend on the subsoil (bearing characteristics, uniformity), environmental conditions (hydrological conditions, frost), traffic (frequency of standard single-axle weight), and construction materials (toughness, stability). Whereas proven standard values should govern the construction of dirt roads and rigid-type roads (concrete slabs), the following should conform more to a flexible construction style. In

Table 3.9. Type of road and traffic volume

Frequency of Standard Road Type Single-Axle 8.2-t Load Vehicles

Connecting roads 100,000-150,000

Forestry roads 25,000-50,000 Feeder roads

Agricultural roads 10,000-25,000

Forestry roads 10,000-25,000 Access roads

Agricultural roads 10,000

Table 3.10. Determining the subsoil bearing capacity and the road body construction method in relation to subsoil conditions

Subsoil

Procedures for Determining Subsoil Bearing Capacity

Construction

Fine-grain soils, sand and gravel with many fine particlesa CBR value <10% (scarcely or not at all practicable with trucks)

Sand, gravel with many fine practicles 10% <CBR value <20% (practicable with trucks)

Gravel with few fine particlesc CBR value >20%

Determination of CBR value with the help of a Farnell hand penetrometer

Determination of deflection by means of Benkelman beam

Determination with subsoil bearing capacity not necessary

Determination of the SN strength index by means of the construction formula or construction nomogram

Procedure similar to road body reinforcement: determination of SN with reinforcement diagram for CBR = 10% Choice of a suitable surfacing course a (USCS: CL, CH, ML, MH, OL, OH, SC-CL, SM-ML, GC-CL, GM-ML) b (USCS: SW, SP, SM, SC, GC-CL, GM-ML)

this context, greatly simplified calculation principles should apply (traffic dimension of 100,000 to 200,000 standard single-axle loads, time dimension of 30 to 40 years).

The construction principles can be determined with the help of Table 3.9 (type of road and traffic volume), Table 3.10 (subsoil bearing capacity), and Table 3.11 (regional factors). This will help to determine the construction diagram of Fig. 3.15 (Nomogram) of the SN weighted strength index. The thickness and composition of the road body can be determined on the basis of the coefficient of bearing capacity for different courses and materials, shown in Table 3.12. An example in Fig. 3.16 illustrates this simple construction method.

In determining the covering course for dirt roads, a simple diagram can be of considerable assistance (Fig. 3.17). This permits an objective assessment of the situation when discussing a road's suitability in relation to the landscape.

Table 3.11. Regional factor (R ) for simple roads

Type of road (conditions) R

Roads requiring no snow clearance

(forestry roads, miner rural roads) 1.0

Connecting roads <700 m above see level 1.3 Connecting roads:

Favorable hydrological conditions 1.5

Unfavorable hydrological conditions 2.0

Table 3.12. Coefficient of bearing for the most commonly used building materials

Course Thickness (cm)

Table 3.12. Coefficient of bearing for the most commonly used building materials

Course Thickness (cm)

Construction Materials

a Value

Same Bearing Capacity

Minimum

AB 25/TA 25 (Asphalt pavement)

0.44

0.9

6

MHT B (Hot mix)

0.40

1.0

6

HMT A (Hot mix)

0.30

1.3

6

Bituminous stabilization

0.23

1.8

12

Stabilization with cement

mix-in-place

0.20

2.0

15

mix-in plant

0.30

1.3

15

Stabilization with lime

quick-acting

0.13

3.1

15

slow-setting

0.30

1.3

15

Crushed gravel

0.14

2.9

20

Round grave

0.11

3.6

It should be borne in mind that the construction traffic will already involve a certain number of standard single-axle loads, that is, there is a preloading that somewhat reduces the theoretical mean life of the road. Logging, quarrying, and landfilling activities can likewise put great stress on minor rural road networks. All basic relevant information should be available at the planning stage. If at a later stage there is to be an extra burden, reinforcement of the road must be carefully planned. The following are useful general guidelines:

• Construction site transport «0.8 standard single-axle load per m3.

• Timber transport «0.4 standard single-axle load per m3.

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