Características de la población de referencia

When discussing road pavements, it is common to distinguish between flexible and rigid pavements. The pavements of engineered gravel roads and bituminous roads are commonly referred to as “flexible” while concrete pavements are referred to as “rigid”. This section focuses on flexible pavements, as they represent the dominant pavement type for low-volume roads.

The following terms are used for the components of a flexible pavement:

Surfacing: The uppermost layer of the pavement is termed surfacing. The main purpose of the surface course is to provide a watertight seal which protects the layers below and is strong enough to resist the abrasive wear caused by traffic. It normally consists of a bituminous surface dressing or a layer of premixed bituminous material as shown in Figure 5-1.

Road base: This constitutes the main load-bearing layer of the pavement. It normally consists of crushed stone or gravel, or of gravelly soils, decomposed rock, sands, and sand-clays stabilised with cement, lime or bitumen.

Figure 5-1: Structure of a flexible pavement

Sub-base: The layer immediately underlying the road base is called the sub-base which acts as a secondary load-spreading layer. It normally consists of a material of lower quality than that used in the road base such as unprocessed natural gravel, gravel-sand, or a gravel-sand-clay mix. From designconsiderations, this layer may not be required for low-volume roads especially where good subgrades exist. Otherwise, it may serve as a separating layer preventing contamination of the road base by a low strength subgrade material and, under wet conditions; it has an important role to play in protecting the subgrade from damage by construction traffic. Subgrade: This is the upper layer of the natural soil, normally consisting of undisturbed local material, or soils excavated elsewhere and placed as a fill. In both cases it is compacted during construction to give added strength.

5.2 PAvEMENT MATERIAL SELECTION

The design17 _{of the sub-base and base courses on low-volume roads have traditionally been done on the basis}

of importing materials from identified borrow pits without any serious consideration of the suitability of the in-situ materials within the immediate road reserve.

5.2.1 Minimum specifications

The base course in the road pavement needs to be built of material of a certain minimum strength. For low volume sealed roads with traffic levels up to 500 vehicles per day, the appropriate range for the CBR strength for a base course is from 15% to 45% and above (soaked CBR), with a (P.I) between 6 to 12, provided that appropriate drainage is provided. Experience shows that the grading18 _{of the base material is of minor importance in relation}

to its strength.

5.2.2 In-situ material

On most low volume sealed road construction sites, materials hauled over long distances from borrow pits for base/sub-base formation are often almost of the same (if not inferior) quality to spoiled in-situ material within

Surfacing Base

Subbase Subgrade

17 _{Other pavement design analyses including traffic, climatic, and material characteristics are not covered in this guideline.}

18 _{Grading is a way of classifying a soil by determining its grain size distribution. This is done through sieving the soil sample through a set of}

some sections of the road formation. This happens when design engineers do not take the pains to prospect for new suitable material along the alignment but rely on existing borrow pits with sometimes very long haulage distances. This unnecessarily increases the cost of the road due to the excessive material haulage. Pavement costs can be signifi cantly reduced when more efforts are made during design to investigate the use of locally available materials.

It is therefore useful to start pavement material investigations and design of low-volume roads by determining the strength and other characteristics of the in-situ soils. If by chance the local soils are of insuffi cient strength it is worthwhile considering possible improving the material through mechanical or chemical stabilization to meet the minimum design specifi cations thus reducing the need to import foreign material with the benefi ts of:

• Signifi cant project cost savings (up to 20%) on transport cost, depending on the haul distance. • Avoiding potentially useful material being excavated to spoil and thereby reducing project costs.

• Increased progress as adequate road-building material is found next to the road, leading to earlier works completion and additional savings.

• Feasibility of using labour-based work methods for the excavation and placing of sub-base/base thus increasing the labour content of the project.

• Limiting environmental degradation caused by opening large borrow pits

The fl ow chart below illustrates the steps to be followed for cost-effi cient pavement material selection for low- volume sealed roads.

Chart 5-1: fl owchart for pavement materials selection and design

In-situ material investigation: (Along centre line & side drains)

15 < In-situ CBRs < 45% 6 < P.I < 12,

No Does in-situ material meet road base

specifi cations?

Yes

In-situ CBRs > 45% P.I < 12,

Investigate use of in-situ material with appropriate treatment (stabilization/modifi cation) to meet

base specifi cations

In-situ CBRs < 15% P.I < 6

Use in-situ material for road base formation Advantages: • Drastically reduces cost • Labour-based friendly

• Higher speed of construction & outputs

Is cost of treatment higher than importing

from borrow pit?

Discard use of in-situ materials. Import road base material from

borrow pit.

Treat in-situ material to specifi cation and use for road base

No

Yes

5.2.3 Imported material

It is only in cases with very poor in-situ materials (CBR < 15%) and where treatment of such soils is very expensive, that materials need to be imported. The final design decision to import material or improve in-situ soils should therefore be informed by a detailed cost assessment of the in-situ treatment as against the cost of borrow in terms of haulage distances, material quantities, extent of overburden to be removed and the feasibility of excavating materials using labour-based work methods.

5.3 CENTRE LINE SURvEy

The quickest and most economical means of establishing the bearing capacity of the in-situ material along the road alignment is by means of the Dynamic Cone Penetration (DCP) test.

DCP tests should be carried out in the middle of the wet season or just at the end of the wet season. This will give a safety factor in the design.

It is recommended that DCP tests are carried out at 50 metres intervals along the route at three points, the centreline, and at each of the road shoulders. In the case of short lengths of road/street it is useful to perform at least three tests on every 80 – 100 metres. Continuous DCP measurements can be made to a depth of 800 mm or to 1200 mm when an extension rod is fitted.

It is recommended that other field tests, such as soil plasticity and grading are also done at the selected points.

The DCP tests can be used to establish the CBR for in-situ materials along the alignment of either an existing or new road. Chart 5-2 below can be used to determine the CBRs.