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South Africa has a large network of roads. However, the maintenance of all these roads is costly. Highways have high financial implications due to the fact that it is a high-volume road. This means that low-volume roads have to be constructed or upgraded at a lower cost.
Before construction may commence, the road needs to be designed according to specific design systems. This review takes a look at the design principles incorporated into the design of low-volume roads. There are three design systems, namely:
The three design systems will be investigated based on research done by various authors and summarized.
All the researched information will be obtained from various publications within the civil engineering industry. The objective is to create a better understanding of the different design systems in the civil engineering industry.
These three designs systems are basically used to design three types of pavements in South Africa, namely:
Flexible Pavements This is a mutli-layered pavement structure.
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the layers are usually roadbed, subgrade, subbase, base and a surfacing layer.The pavement layers need high shear strength and the lower layers need less shear strength.
This pavement structure requires less layers than flexible pavements. It relies on the concrete slab to provide shear strength.
This pavement structure is stiffer than a flexible pavement but less stiff than a rigid pavement. The surfacing is concrete blocks.
Based on research done, the following information was summarized for the different design systems.
An empirical design is a system based on experience obtained from previous experiments done.
However, this experience is obtained through numerous experiments to ensure that it actually works. These experiments do not have to be scientifically proven in order to prove that it works. Numerous experiments will require a number of observations in order to prove that a relationship exists.
In pavement design, an empirical design method is quite common. These designs can range from the simplest form to the most complex form. Most of the time these designs are based on what was designed in the past. Obviously, the designs must have worked previously.
Typical examples of the Empirical design method are:
The AASHTO Structural number Method: This method was developed between 1955 and 1965. All the test results of the AASHTO road test was observed in Ottawa, Illinois. Basically, the test method systematically quantifies the interaction between road deterioration, traffic and composition of pavement structure on a closed loop test on trucks. The formulae are as follows.
The table below illustrates the advantages and disadvantages of this design method:
Can be used as a check method for other pavement designs methods Developed for conditions and materials which are not standard to South Africa
Dynamic Cone Penetrometer (DCP) Method: This Test method involves basic visual evaluation of the DCP instrument. The DCP instrument is a shaft with a hammer attached to it. The main objective of the DCP test is to measure the in situ soil strength of a soil layer. The method is usually used for light pavement layers. The strength of a layer is determined by measuring the number of blows against the penetration of the DCP apparatus.
The table below illustrates the advantages and disadvantages of this design method:
Simple method to do Human error may influence results
Can be used for new and existing designs Empirical
According to Pavement Interactive, mechanics is the science of motion. In pavement design, this means calculating pavement responses in terms of stresses, strains and deflections. Examples of this calculations would include traffic loading and environmental conditions. Based on the results of these calculations a pavement life will be determined. The empirical factors are ignored such as rutting and cracking.
Below is a diagram of the Mechanistic design approach.
Typical examples of the Mechanistic design method are:
Manual M10 Method: this is the first formal design method for concrete pavements. This design method originates from the AASHTO method for concrete pavements, although it is a mechanistic design method. Manual M10 follows a design sequence in order to determine the design characteristics of the pavement. All the design specifications are determined by using graphs. The sequence is as follows:
The table below illustrates the advantages and disadvantages of this design method:
Easy to use Default values need to be adjusted for specific designs
Updated regularly Ensure that variation in properties are realistic and reflect actua conditions.
Allows different slab support layers
Lockpave: this is a computed design software program. It focuses on permeable concrete paving. The program’s key focus is to calculate strains and stresses occurring near vertical load axis of bottom layers. The application can perform the following functions:
The table below illustrates the advantages and disadvantages of this design method:
There are a range of methods available No rehabilitation design procedures
South African standard input data Computer model only allows limited designs
This design method takes into consideration the design methods of an empirical approach and a mechanistic approach. Where the empirical approach is based on experience of past work will combine with the calculation of pavement responses in the mechanistic approach. This simply means that a designer with many years’ experience can use the knowledge obtained from previous designs and incorporate it into new designs using computer software that calculates mechanistically. The outcome of the results can therefore be analyzed by an experienced designer.
A typical example of the Mechanistic-Empirical design method are:
The South African Mechanistic-Empirical Design Method: This design method was developed in Theyseetal in 1996. It can be used to design four pavement layers:
This method was reviewed and together with a list of other design methods, it forms part of the South African Pavement Design Method (SAPDM).
The table below illustrates the advantages and disadvantages of this design method:
This method is well suited for new and rehabilitation designs Relatively complex and only related to computer applications
Accommodates changes in operating conditions Overly sensitive to variation in the input data.
The transport industry is facing many challenges. A particular challenge is to design low-volume roads that can perform for a required period, under different climatic and loading conditions. In terms of design, the Mechanistic-Empirical design approach is the most favorable when it comes to designing pavements that are reliable and easy to maintain such as a low-volume road.
A mechanistic-empirical approach is advantageous over other design approaches and is summarized as follows:
Using this approach allows for a more realistic design for the determination of a pavement life.
The Principles Incorporated in the Design of Low Volume Roads. (2024, Feb 06). Retrieved from https://studymoose.com/document/the-principles-incorporated-in-the-design-of-low-volume-roads
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