I.4.2.7.2 Compaction of road layers

The design procedures assume that the specified material properties obtained from a certified laboratory are satisfied in the field. Insufficient compaction may result in field densities below the minimum required. In such cases, the strength of the material is not fully utilised, and densification or failure may occur under traffic. The following should be considered:

  • Compaction problems may result from material grading deficiencies or poor construction practices
  • Blending of material from different sources to improve the grading and compaction potential of the material may be better than trying to achieve density with excessive rolling.
  • When compacting a layer, the support layer needs sufficient support to act as an anvil, otherwise the compaction energy is transmitted and lost through the pavement structure.
  • The use of impact rollers can improve the strength and support from the subgrade substantially. Hand-held rollers may be inadequate to achieve the required density

Table I.19 gives the minimum compaction standards required for various layers of the pavement structure. For detailed compaction standards for various material, other relevant documents such as TRH 14 (1985) and COTO should be consulted.

Table I.19 : Compaction requirements for the construction of granular and cemented pavement layers (and reinstatement of pavement layers)
Pavement layer/Material Compacted density
Surfacing Asphalt 93% to 94% MVD
Base Crushed stone G1 86% to 88% AD
Crushed stone G2 100% to 102% MDD
Crushed stone G3 98% to 100% MDD
Waterbound macadam 86% to 90% AD
Natural gravel G4 86% to 88% MDD
Cemented (C3/C4) 97% to 98% MDD
BSM 97% to 100% MDD
  Asphalt 96% MVD
Subbase Natural gravel 95% to 97% MDD
Cemented (C3/C4) 95% to 96% MDD
Selected subgrade Soils and gravel 93% to 95% MDD
Subgrade Soils and grave 90% MDD
Fill (cohesionless sand)   90% MDD

Note: MVD = Maximum Voidless Density (see SABITA Manual 35/TRH 8); AD = Apparent Density; MDD = Maximum Dry Density.

I.4.2.7.3 Pavement subgrade

Generally, it is preferable to keep the design of the whole street the same, with no change in layer thickness across the road/street. However, the pavement cross-section may vary if problematic subgrade conditions (i.e. expansive clay) are encountered, hence requiring special treatment. The main subgrade problems that have to be considered include the extreme changes in volume that occur in some soils as a result of moisture changes (e.g. in expansive soils and soils with collapsible structures); flaws in structural support (e.g. sinkholes, mining subsidence and slope instability); the non-uniform support that results from wide variations in soil types or states; the presence of soluble salts which, under favourable conditions, may migrate upwards and cause cracking, blistering or loss of bond of the surfacing; disintegration of cemented bases and loss of density of untreated bases; and the excessive deflection and rebound of highly resilient soils during and after the passage of a load (e.g. in ash, micaceous and diatomaceous soils).

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I.4.2.7.4 Road/Street levels

One of the primary functions of a road/street is to provide access to dwellings and other land uses. To optimise accessibility, the design of road/street levels should consider that road/street levels place some restrictions on rehabilitation and create special moisture/drainage conditions. In some cases, rehabilitation in the form of an overlay may cause a problem, particularly with respect to the level of kerbs and channels, camber and overhead clearances. In these cases, strong consideration should be given to bottom-heavy designs (i.e. designs with a cemented subbase and possibly a cemented base), which would mainly require the same maintenance as thin surfacings and little structural maintenance during the analysis period.

I.4.2.7.5 Service trenches

Trenches excavated in the pavement to provide essential services (electricity, water, telephone, etc.) are frequently a source of weakness in the structural design. This is a result of either inadequate compaction during reinstatement, or saturation of the backfill material. Service trenches can also be the focal points of drainage problems. To minimise problems related to service trenches, compaction must achieve at least the minimum densities specified for various materials. These densities are readily achieved when granular materials are used, but it becomes more difficult when natural materials are used, particularly in the case of excavated clays. When dealing with clay subgrades it is recommended to, if economically feasible, use a moderate-quality granular material as a trench backfill in preference to the excavated clay. The provision of a stabilised “cap” over the backfill may be considered to eliminate settlement as far as possible. Care must be taken not to over-stabilise (i.e. produce a concrete), as this results in significant problems with adhesion of the surfacing and differential deflections causing failure around the particles.

Settlement in the trench, giving rise to standing water and possibly to cracking of the surface, will permit the ingress of moisture into the pavement. Fractured water, sewerage or stormwater pipes lead to saturation in the subgrade and possibly in the pavement layers as well. Alternatively, a trench backfilled with granular material may even act as a subsurface drain, but then provision for discharge must be made. It is, however, generally recommended that the permeability of the backfill material should be as close as possible to that of the existing layers in order to retain a uniform moisture flow regime within the pavement structure.

I.4.2.7.6 Kerbs and channels

Kerbs and channels are important to prevent edge erosion and to confine stormwater to the street surface. Consideration should be given to the type and method of construction of kerbs when deciding on a layer thickness for the base. It is common practice to construct kerbs upon the (upper) subbase layer to provide edge restraint for a granular base. This restraint will help to provide the specified density and strength. Care must be taken to ensure that this type of structure does not “box” moisture into the base course material. In the case of kerbing with a fixed size (i.e. precast kerbing or kerbing with fixed shutters cast in situ), it may be advantageous to design the base thickness to conform with the kerb size (e.g. if the design calls for a 30 mm AG with a 125 mm G4 underlay, and the gutter face is 160 mm, rather use a 130 mm G4).

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I.4.2.8 Cost analysis

This section discusses how doing a cost analysis can assist the designer in selecting the optimal pavement type for a development project. The selection of the final pavement design is based on the life cycle assessment of a number of alternative designs. The purpose of the structural design method is therefore not the selection of the final design, but to provide the designer with a number of design alternatives with the required structural capacity. It should, however, be noted that a cost analysis may not take all the necessary factors into account and it should therefore not override all other considerations. Financial affordability should also be considered. The availability of funds for the initial construction, and the availability of maintenance funds must be considered, as these could influence the final design decision. The designer should consult TRH4, TRH12 and other relevant guideline documents for detailed information on cost analysis.

The main economic factors that determine the cost of a facility are the analysis period, the structural design period, the construction cost, the maintenance cost, the salvage value at the end of the analysis period and the real discount rate. The cost comparison of alternative pavement designs for a specific design case can be done using the Present Worth of Cost (PWOC), the Net Present Value (NPV) or the Internal Rate of Return (IRR) of the initial construction and anticipated maintenance and rehabilitation costs. The PWOC method is briefly discussed next.

I.4.2.8.1 The Present Worth of Cost method

The PWOC method of cost analysis is recommended in this Guide, and should be used only to compare pavement structures in the same road/street category. This is because roads/streets in different categories are constructed to different standards and are expected to perform differently, with different terminal levels of service. The effect these differences have on street user costs is not taken into account directly. Although the economic principles presented in this document refer to flexible pavements, the same economic principles apply for concrete. A complete cost analysis should be done for functional classes/categories U2-B and U3-B roads/streets. For functional classes/ categories U4-C to U6-E roads/streets, a comparison of the construction and maintenance costs will normally suffice. A difference in the economic indicator between two alternative designs of less than 10% is insignificant, and the designs are assumed to be equivalent in economic terms.

The total cost of a project over its life is the construction cost plus maintenance costs, minus the salvage value.

The present worth of costs can be calculated as follows:

PWOC = C + (M1(1 + r)-x1+ ...+ Mj(1 + r)-xj ) + ... - S(1 + r)-z Eq I.9
Where:
PWOC = present worth of cost
C = present cost of initial construction
M1 = cost of the jth maintenance measure expressed in terms of current costs
r = real discount rate
xj = number of years from the present to the jth maintenance measure, within the analysis period
z = analysis period
S = salvage value of pavement at the end of the analysis period, expressed in terms of the present value

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The construction cost should be estimated from current contract rates for similar projects. Maintenance costs should include the cost of maintaining adequate surfacing integrity (e.g. through resealing) and the cost of structural maintenance (e.g. the cost of an asphalt overlay). The salvage value of the pavement at the end of the analysis period can contribute to the next pavement. However, geometric factors such as minor improvements to the vertical and horizontal alignment and the possible relocation of drainage facilities make the estimation of the salvage value very difficult. The choice of analysis period and structural design period will influence the cost of a road/street. The final decision will not necessarily be based purely on economics, but will depend on the design strategy.

(i) Construction costs

The checklist of unit costs should be used to calculate the equivalent construction cost per square metre. Factors to be considered include the availability of natural or local commercial materials, their expected cost trends, the conservation of aggregates in certain areas, and practical aspects such as speed of construction and the need to foster the development of alternative pavement technologies. The potential for labour-based construction also needs to be considered. The cost of excavation should be included as certain pavement types will involve more excavation than others.

(ii) Maintenance costs

There is a relation between the type of pavement and the maintenance that might be required in the future. When different pavement types are compared on the basis of cost, these future maintenance costs should be included in the analysis to ensure that a sound comparison is made. This is critical for the planning of future maintenance activities.

It is important to consider that relaxations of material, drainage or pavement thickness standards will normally result in increased maintenance costs. The type of surfacing and water ingress into the pavement also plays an important part in the behaviour of some pavements. For this reason, planned maintenance of the surfacing is very important to ensure that these pavements perform satisfactorily. The service life of each type of surfacing will depend on the traffic and the type of base used.

Table I.20 gives guidelines regarding the service life that can be expected from various surfacing types. These values may be used for a more detailed analysis of future maintenance costs. Typical maintenance measures that can be used for the purpose of cost analysis are measures to improve the condition of the surfacing and structural maintenance measures applied at the end of the structural design period. Maintenance will also be influenced by the level of distress of the pavement (moderate or severe).

Other road/street-user costs should also be considered, although no proper guide for their determination is readily available. The factors that determine overall road/street-user costs are: running costs (fuel, tyres, vehicle maintenance and depreciation), which are largely related to the street alignment, but also to the riding quality (PSI); accident costs, which are related to street alignment, skid resistance and riding quality; and delay costs, which are related to the maintenance measures applied and the traffic situation on the streets.

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Table I.20 : Suggested typical ranges of period of service of various surfacing types (modified from SAPEM)
Base type Surfacing type
(50 mm thickness)
Typical range of surfacing life (years) per
functional class/category
U2-B, U3-B
(ES1-ES10)
U4-C, U5-D, U6-E
(ES0.003-ES3)
Granular Bitumen sand or slurry seal - 2-8
Bitumen single surface treatment 6-10 8-11
Bitumen double surface treatment 6-12 8-13
Cape seal 10-12 8-18
Continuously graded asphalt - -
Gap-graded asphalt premix - -
Hot mix asphalt Bitumen sand or slurry seal - 2-8
Bitumen single surface treatment 6-10 8-11
Bitumen double surface treatment 6-12 8-13
Cape seal 8-15 8-18
Continuously graded asphalt 8-12 -
Gap-graded asphalt premix 10-15 -
Porous asphalt 10-15 -
Cemented Bitumen sand or slurry seal - -
Bitumen single surface treatment 4-7 5-8
Bitumen double surface treatment 5-8 5-9
Cape seal 5-10 5-11
Continuously graded asphalt 5-10 -
Gap-graded asphalt premix 6-12 -

- Surface type not normally used.

(iii) Real discount rate

When a ‘present-worth’ analysis is done, a real discount rate must be selected to express future expenditure in terms of present-day values. This discount rate should correspond to the rate generally used in the public sector. For public projects, the discount rate used is published by the National Treasury. 8% is recommended for general use. A sensitivity analysis using rates of say 6, 8 and 10% could be carried out to determine the importance of the value of the discount rate.

(iv) Salvage value

If the road/street is to remain in the same location, the existing pavement layers may have a salvage value but, if the road/street is to be abandoned at the end of the period, the salvage value could be small or zero. The assessment of the salvage value can be approached in a number of ways, depending on the method employed to rehabilitate or reconstruct the pavement.

The salvage values of individual layers of the pavement may differ considerably, from estimates as high as 75% to possibly as low as 10%. The residual salvage value of gravel and asphalt layers is generally high, whereas that of concrete pavements can be high or low, depending on the condition of the pavement and the method of rehabilitation. The salvage value of the whole pavement would be the sum of the salvage values of the individual layers. In the absence of better information, a salvage value of 30% of initial construction cost is recommended.

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(v) Optimisation of life cycle costs

The main purpose of the determination of a representative Level of Service (LoS) for a road/street (see Section I.4.2.1) is to illustrate the associated life cycle costs. This identification can enable authorities and decision makers to select a design that will be affordable and upgradable. The costs associated with a typical road/street are made up of design and construction costs, maintenance costs and road/street-user costs. Construction costs are high for high LoS values and low for low LoS values. Maintenance costs, on the contrary, are low for high LoS values and high for low LoS values.

This concept is illustrated in Figure I.15 with typical, present worth-of-cost versus LoS values. The combined cost curve has a typical minimum value between the highest and lowest LoS values. Street-user costs are low for high LoS streets and high for low LoS streets.

Figure I.15: Typical cost versus level of service curve values

I.4.2.9 Construction aspects

I.4.2.9.1 Staged construction and upgrading

Two concepts that need to be considered as part of the life cycle strategy of a street during design are staged construction and upgrading. Although it is difficult to exactly define and completely separate these concepts, certain characteristics may be more typical of one than of the other.

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The aim of staged construction is to spread some of the financial load from the initial construction period to some stage later during the life cycle of the facility. However, right from the onset, the aim is to provide a particular level of service for the duration of the structural design and analysis period of the facility. There may be slight changes in, for instance, the riding quality of the facility, but these should have a marginal influence on the operating cost of the facility to the user. On the other hand, upgrading will normally take place when the demands placed on an existing facility far exceed the level of service the facility can provide. The new facility has to provide a much higher level of service at a much reduced cost to the user. An example would be the upgrading from a gravel to a surfaced street.

Staged construction may be done by adding a final layer, or reworking an existing layer at some stage early during the structural design period of the facility. Most of the money spent during the initial construction of the facility should therefore be invested in the lower layers of the structure, providing a sound foundation to build on in the future.

During the upgrading process, maximum use should be made of the existing foundation provided by the pavement being upgraded. Dynamic Cone Penetrometer (DCP) and Falling Weight Deflectometer (FWD) surveys may provide the information required to incorporate the remaining strength of the existing pavement in the design of the future facility. Special equipment may also be used to maximise the bearing capacity of the in-situ material. With impact roller equipment, it is usually possible to compact the in-situ material to a depth of 600 mm at densities well above those normally specified for the subgrade and selected layers of a pavement, without excavating and replacing any material. This results in few layers or thinner layers being required in the pavement structure.

One of the problems that may be associated with staged construction is the limitation placed on street levels by the other services in the street reserve, particularly the stormwater drainage system. If a system of kerbs, gutters and stormwater pipes is used, it may not be possible to add an additional structural layer to the pavement system at a later stage. In such cases, consideration should be given to initially providing a subbase-quality gravel base, and to rework this layer at some early stage in the structural design period of the pavement by doing deep in-place recycling and stabilisation with cement, bitumen emulsion or a combination of the two. A second problem that requires consideration is the cost of repeated mobilisation on a project. The mobilisation of plant and resources for a light pavement structure in an urban area (usually of short length) is often a significant portion of the total cost.

In general, staged construction spreads the financial burden of construction and is economically more viable than initial full construction. The economics of each project must, however, be considered on merit. It must also be kept in mind that because some of the cost of construction is shifted a few years into the life cycle of a pavement, future budgets must allow for this cost plus inflation.

The details of upgrading from a gravel street to a surfaced street will depend largely on individual projects and will be determined by the bearing capacity of the material on the existing street. As already mentioned, the strength of the existing pavement should be optimally utilised in the new design and if material is imported to the gravel street, the possible utilisation of this material in a future upgrading to a paved street should be kept in mind. The cost analysis for upgrading from a gravel to a surfaced street must at least include the savings in vehicle-operating cost as part of the benefit to the street user. The cost of upgrading should be weighed against the benefits by means of a cost-benefit analysis, expressing the benefits as a ratio to the cost. Software is available for this type of analysis.

I.4.2.9.2 Construction approaches

Construction of streets within settlements has become a highly mechanised process but, over the past few years, the possibility of creating employment opportunities has led to greater use of labour-intensive technologies. Local people are often appointed as subcontractors to established contractors or as contractors for small projects.

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These initiatives have not only contributed to local economic development and the transfer of skills, but have also contributed to the successful completion of construction projects by providing local knowledge and getting buy-in from local communities.

A choice between conventional construction (mechanised) and labour-intensive construction may have some impact on the selection of materials and the structural design of the pavement.

Conventional construction is suited to most new street construction assignments, perhaps with the exception of construction in confined areas. Advantages may include rapid mobilisation and completion, while disadvantages may include limited involvement of the local community.

In order to ensure the maximisation of job creation to the extent that it is economic and feasible, the terms of reference for technical consultants engaged to carry out feasibility studies should require the consultant to examine the appropriateness of designs that are inherently labour intensive; to report on the economic implications of using such designs; and to ultimately design a project based on designs and technology appropriate for construction that maximises labour-intensive methods.

Labour-intensive construction should strive to obtain the standards set for conventional construction. However, the design should ensure that the standards specified are appropriate. This necessitates a critical review of all specifications during the design stage.

All construction activities cannot always be executed by means of labour-intensive methods. This must be recognised in the design. Examples of activities demanding greater mechanisation are the following:

  • Deep excavation (apart from safety considerations, material can only be thrown a certain height by shovel)
  • Excavation and spreading of very coarse material
  • In-situ mixing of stabilising material (cement or lime) effectively into coarse aggregates
  • Application of tar (due to safety considerations)
  • Compaction of thick layers or very large aggregates (e.g. rock fill) with small (pedestrian) rollers
  • Mixing of high-strength concrete
  • Excavation of medium to hard material
  • Haulage by wheelbarrows over long distances
  • Placement of heavy pipes

To investigate the potential for employment creation through construction, it is useful to select the construction activities that have the biggest impact on employment creation (where the contribution of this activity forms a significant part of the project cost and the activity has the potential for employment creation). Table I.21 provides an indication of the relative contribution of the main construction activities to the total project cost. A preliminary cost analysis can be done. It is further necessary to consider using a local plant and materials (i.e. rent plant and purchase material from the community).

The result of this investigation may indicate that some activities cannot be done by means of manual labour, due to construction practicalities or the availability of materials. A more detailed cost analysis can then be done. Make sure that the design can be specified.

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Table I.21 : Relative contribution of main activities
Description % contribution towards project costs
Rehabilitation Paved Gravel
Site accommodation 3 2 4.5
Accommodation of traffic 5 5 4
Clearing and grubbing 0.5 1 2
Drainage 3 3 7.5
Culverts 3 15 11
Kerbs and edging 3.5 8.5 0
Earthworks 6 4 22
Pavement layers 10 14 16
Base 8 10 0
Prime and seal work 35 15 0
Ancillary works 5 4 6.5
Landscaping 2 1 1.5

Table I.22 illustrates the potential of one of these main activities (pavement layers) for using labour-intensive methods.

Table I.22 : Potential of pavement layers for labour-intensive construction methods
Layer Type Potential
Subbase In-situ soil Good
Imported Good*
Stabilised soil Fair, not practical
Base In-situ soil Good
Natural gravel Good
Emulsion-treated gravel Good
Crusher run Fair, not practical
Cement-stabilised gravel Not practical**
Lime-stabilised gravel Fair, good***
Bituminous premix Fair, good
Waterbound macadam Good
Penetration macadam Good
Surfacing Sand seal Good#
Slurry Good
Double seal Good#
Cape seal Good#
Asphalt Fair
Roller-compacted concrete Good
Concrete (plain) Good
Concrete (reinforced) Good
Segmental blocks Good##

Notes:

*
**
***
The suitability of this will depend entirely on the haul distance.
Cement-stabilised gravel is not suitable for labour-intensive methods due to its quick setting time.
Lime-stabilised gravel is more suitable as it reacts and sets more slowly, but achieving an even mix is difficult by entirely manual means and labourers must take extreme care to avoid contact between the lime and skin during application. Protective clothing is essential.

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# In the case of bitumen surfacing, only certain types of emulsion have a non-critical application temperature and are suitable for hand laying.
## Quality control of on-site manufacture is critical.

A further breakdown of possible activities that are labour-intensive in road pavement construction is provided in Table I.23.

Table I.23 : Typical labour-intensive activities
Component Activities
Accommodation of traffic Watering of gravel diversions
Clearing and grubbing As required
Drainage Catch pits and manholes
Excavation of open drains
Lined open drains
Subsoil drains
Culverts Inlet and outlet structures
Excavation of trenches
Installation of lightweight pipes
Manufacture of reinforced concrete slabs, walls and decks
Masonry walls
Kerbing and edging Manufacture of concrete elements
Laying of kerbing and edging
Earthworks Minor earthworks
Pavement layers Crushing of aggregates
Screening of stockpiles
Haulage of materials
Spreading of materials
Removal of oversize materials
Base Construction of labour-intensive base types (waterbound macadam, emulsion-treated base, stabilised or un-stabilised gravel)
Manufacture of paving blocks
Laying of paving blocks
Prime and seal work Hand spraying of prime
Manufacture and laying of slurry
Seals (Cape seal, double seal, single seal)
Ancillary works Fencing
Masonry walls
Gabions
Concrete structures
Painting of road markings
Landscaping Grassing
Planting of trees

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